high power fiber lasers current status and future perspectives [invited]

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Richardson et al. Vol. 27, No. 11 /November 2010 /J. Opt. Soc. Am. B B63

High power fiber lasers: current statusand future perspectives [Invited]

D. J. Richardson,* J. Nilsson, and W. A. Clarkson

Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK*Corresponding author: [email protected]

Received July 19, 2010; revised August 27, 2010; accepted August 28, 2010;posted September 10, 2010 (Doc. ID 131957); published October 22, 2010

The rise in output power from rare-earth-doped fiber sources over the past decade, via the use of cladding-pumped fiber architectures, has been dramatic, leading to a range of fiber-based devices with outstanding per-formance in terms of output power, beam quality, overall efficiency, and flexibility with regard to operatingwavelength and radiation format. This success in the high-power arena is largely due to the fiber’s geometry,which provides considerable resilience to the effects of heat generation in the core, and facilitates efficient con-version from relatively low-brightness diode pump radiation to high-brightness laser output. In this paper wereview the current state of the art in terms of continuous-wave and pulsed performance of ytterbium-dopedfiber lasers, the current fiber gain medium of choice, and by far the most developed in terms of high-powerperformance. We then review the current status and challenges of extending the technology to other rare-earthdopants and associated wavelengths of operation. Throughout we identify the key factors currently limitingfiber laser performance in different operating regimes—in particular thermal management, optical nonlinear-ity, and damage. Finally, we speculate as to the likely developments in pump laser technology, fiber design andfabrication, architectural approaches, and functionality that lie ahead in the coming decade and the implica-tions they have on fiber laser performance and industrial/scientific adoption. © 2010 Optical Society ofAmerica

OCIS codes: 140.3510, 140.3580, 140.5680, 060.2320.

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. INTRODUCTIONver the last decade powers produced by fiber lasers have

hown a remarkable increase, by an average factor of 1.7ach year, corresponding to �2 orders of magnitude overhe past decade (Fig. 1), and representing a much steeperncrease than that shown by their bulk solid-state coun-erparts. These increased power levels, with yet higherowers anticipated, are leading to a rapid penetration ofber systems into applications formerly dominated byther lasers. However, the attraction of fiber lasers goesay beyond the capacity to generate raw optical power,

ince they possess a number of other physical attributeshat distinguish them from other classes of lasers andhat differentiate them in terms of functionality, perfor-ance, and practicality. These include the following fea-

ures, all of which have played an important role in estab-ishing the commercial interest and driving the relativelyapid development and practical deployment of fiber la-ers:

• Robust single- (transverse) mode operation in mono-ode fibers, giving—in particular—a significant degree of

reedom from the thermally induced mode distortionsommonly shown by bulk solid-state lasers.

• Broad gain linewidths (up to �20 THz), allowing ul-rashort pulse operation, and a wide wavelength tunabil-ty.

• Availability of high gains, offering the option of mas-er oscillator power amplifier (MOPA) schemes. In fact, aigh power fiber laser system typically consists of a low oredium power master oscillator followed by a high power

ber amplifier.

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• Ultrahigh optical and electrical to optical conversionfficiencies.

• Fully fiberized cavities without the need for carefullignment of free-space components allowing for robustnd compact system designs.

The aim of this paper is to review the progress made toate toward higher power operation of fiber lasers acrossbroad range of operation regimes and to look ahead to-ard the developments and research themes that are

ikely to shape the further development of the technologyver the next decade. The paper is structured as follows.e begin in Section 2 by providing a review of the current

tate of the art in terms of high power continuous-wavecw) operation—as best exemplified by progress in theeld of ytterbium-doped fiber-based laser (YDFL) sys-ems, which are currently the preferred gain media byirtue of their efficiency, broad gain bandwidth, and op-rational wavelength around 1060 nm. In Section 3, weeview progress in pulsed-fiber-laser performance, againargely in the context of YDFLs, but with the emphasiseing to illustrate the broad range of operational modeshat are possible using fiber laser technology. In Section 4,e complete our summary of the current state of the arty describing progress in terms of power scaling of fiberasers operating on other rare-earth (RE) transitions andn particular those that allow the generation of laser lightt important longer nominally eye-safe wavelengthsround 1550 and 2000 nm. Our intention throughouthese sections is not to be rigorously complete in terms ofeviewing the existing state of the art, as the space avail-ble simply precludes this. Rather, we look to providing a

010 Optical Society of America

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B64 J. Opt. Soc. Am. B/Vol. 27, No. 11 /November 2010 Richardson et al.

igh level overview of the technology that will be acces-ible to the non-fiber specialist and identify the key issueshat will likely ultimately limit the performance in differ-nt operational regimes. Furthermore, despite the lengthf this paper, there are many important aspects and ad-ances we have not included, e.g., when it comes to fiberomposition and fabrication. Photodarkening is particu-arly important, and we refer the interested reader to theiterature.

In Section 5, finally, we attempt to look ahead and pro-ide a personal view as to how the technology is likely tovolve in the light of technological advances in relevantnderpinning areas and speculate as to the new fibers,ystem concepts, and architectures required to continueo extend the performance envelope on all fronts over theoming decade. Fiber lasers are optically pumped, andhis makes the development of semiconductor pumpources particularly important. We draw our conclusionsn Section 6.

. STATE OF THE ART—CONTINUOUS-AVE FIBER LASERS

igh power YDFLs have been described in detail in sev-ral publications [1–6], and our aim here is to concentraten the most important points. The first operation of a fi-er laser, in the early 1960s, was demonstrated by Snitzernd co-workers [7–9] and revisited in the 1970s by Stonend Burrus [10], before major interest was rekindled in985 due largely to the efforts of Payne and co-workers11,12]. Their work on neodymium-doped fibers revealedhe potential for optical gains of several decibels per mil-iwatt of absorbed pump power. Consequently, even with a

odest pump power of a few milliwatts, provided byheap and by then readily available laser diodes (LDs),evels of optical gain could be reached that were highlyromising for applications such as optical telecommunica-ion amplifiers. In that context, interest was focused onhe erbium-doped fiber amplifier [13], whose operatingavelength of �1.55 �m falls in the third telecommuni-

ation window. This was of immense importance, becauseo solution existed at the time for this huge application.he impact on telecommunications of this work has beenrofound, leading to the development of the Internet ase know it today. Besides the excitement about telecom-unication amplifiers there was also considerable inter-

st in fiber lasers, as they offer a number of attractive fea-ures when compared to other lasers. Now, nearly 50

ig. 1. Progress in output power from diffraction-limited andear-diffraction-limited fiber lasers. Since 1999, all results areith Yb-doped fibers.

ears after the initial demonstration, and 25 years fromheir renaissance, cw fiber lasers, specifically silica-hostDFLs, have reached 10 kW in the single-mode (SM) re-ime [14]. This is one order of magnitude higher than forther single-mode fiber sources. The attractions and po-ential of Yb-doped fibers had been recognized by Hannand co-workers in 1990 [1,2,15]. These include a broadain bandwidth, with operation from 975 nm [15–18] to180 nm [19] demonstrated in different devices. WhileDFLs work best between 1060 and 1100 nm, laser op-ration even at these extreme wavelengths has beencaled to a very respectable 100 W and above. A tuningange of 110 nm has been achieved at the watt level [20],nd as much as 152 nm at lower power [15].Another attraction is the broad absorption band ex-

ending from 900 to 980 nm, covering wavelengths athich high power pump LDs are at their best. The ab-

orption further extends out to 1050 nm, which allowsandem-pumping (in-band pumping with high-brightnessump sources) [14,21] at wavelengths close to the emis-ion wavelength. Figure 2 shows the absorption and emis-ion spectra of Yb-doped silica fiber. Additional spectro-copic attractions include a long, 1 ms, metastable stateifetime and a simple energy level structure with only twof energy levels, which benefits energy storage and allowshe use of high doping concentrations. In addition, as withther RE dopants, the fiber geometry facilitates low laserhresholds, high gain efficiency, and high gain. These fea-ures have paved the way for power scaling in a wideange of fiber-based devices.

. Power Scaling of Ytterbium-Doped Fiber Sourcest is for high powers, though, that YDFLs are most re-owned. Since 1999, the record-breaking near-diffraction-

imited fiber lasers shown in Fig. 1 have all beenb-doped. Yb’s superior power-scaling properties stem

rom an exceptionally low quantum defect for LD pump-ng at 9xx nm, and thus a low thermal load, as well asigh permissible dopant concentrations and thus highump absorption per unit length. These are both impor-ant factors for power scaling of fiber lasers, with thehermal management greatly further simplified by the fi-er geometry. Nonlinearities and damage are equally im-ortant factors. These are easiest to avoid in cw opera-ion, and consequently the record-breaking lasers inerms of the average power have all been cw.

Three key technologies underpin Yb-doped as well asther high power RE-doped fiber lasers: One is low-loss

ig. 2. Emission and absorption spectrum of ytterbium ions inifferent silica hosts.

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oping with RE ions [7]. The other two are directly re-ated to pumping, reflecting its importance for power scal-ng. They are high power multimode (MM) LDs [22] andouble-clad fibers, which open up the option of cladding-umping [23–25]. Cladding-pumped fiber lasers haveroven to be remarkably scalable and have been the plat-orm for the impressive rate at which the power has in-reased over the last two decades, following the initialemonstration in 1988 [25]. The essence of this double-ladding approach is that the active RE-doped core is sur-ounded by a much larger “inner cladding” (see Fig. 3).he pump beam is launched into the inner cladding andonfined within it by an outer cladding, which surroundshe inner cladding. The pump is then progressively ab-orbed into the core as it propagates along the fiber.hereby, a MM pump can efficiently pump a monomodeore and produce a single-mode fiber laser output. Thus,y means of rather simple and robust configurations (seeig. 4) one achieves a large enhancement of the fiber la-er’s beam brightness compared to that of the pump.ladding-pumped YDFLs allow for up to 6 orders of mag-itude brightness enhancement, and close to 5 ordersave been demonstrated experimentally [3,5,26], al-hough 3–4 orders are more typical. This stems from thearger spatial and angular pump acceptance [representedy the product of area and the square of the numerical ap-rture (NA)] for the inner cladding compared to that ofhe core. Since brightness is the relevant figure of meritor so many applications, the relative ease with which fi-er lasers have achieved this brightness enhancement atigh power has been the stimulant for a widespread andapid increase in efforts on fiber laser development, lead-ng to the improvements illustrated in Fig. 1 and manyther impressive results besides those. As a result, highower fiber lasers are now the leading contenders forany important scientific and industrial applications.From a historical perspective it is worth saying that the

nitial aspirations of cladding-pumped fibers were consid-rably more modest, for example, targeting telecommuni-ation amplifiers at 1.5 �m at the sub-watt level [27]. Theutput power in those days was largely limited by theower and availability of pump diodes with beam qualityood enough for launch into the double-clad fibers. Theow pump brightness favored the use of gain media withow thresholds, including Er:Yb co-doped silica emittingt 1.5–1.6 �m. In these, a high Yb-to-Er concentration ra-io increases the pump rate of the Er ions [28,29]. Nd is aour-level system when emitting at �1060 nm, with anven lower laser threshold. Combined with the relatively

Fig. 3. (Color online) Principle of cladding-pumping.

dvanced state of diodes for the pumping of Nd:YAG at08 nm, this made Nd the dopant of choice for early highower fiber lasers [30].Today’s high power pump diodes are sufficiently bright

o make threshold considerations largely unimportant forost high power fiber lasers. However, there are excep-

ions, with the fiber disk laser being a prominent example31]. This operates with a relatively low pump intensitynd uses a highly MM Nd-doped core to enhance theump absorption efficiency. The first kilowatt-level MM fi-er laser was demonstrated in 2002 using this architec-ure with three cascaded high power gain blocks [31].

Despite this impressive result, Nd-doped fiber lasers inhis wavelength range have largely been sidelined by Yb,ollowing the advent of high-brightness 9xx nm pump di-des. These overcame the hurdle of ytterbium’s higherhreshold and brought out the advantages of a loweruantum defect and, crucially, a higher quench-free con-entration. The demonstration of the first single-mode Yb-oped fiber laser with over 100 W of output power in 199932] served as convincing evidence of these advantagesnd a sign of the progress that was to come. This lasersed a fiber with relatively small inner cladding and core,hich—as one would suspect—brought both the diodesnd the fiber relatively close to their limits, e.g., in termsf diode damage and fiber nonlinearities. Nevertheless,his important demonstration illustrated that the basicechnology was in place to allow for a rapid increase inhe average power by scaling the size of the optical fibernd the pump diode source. Soon thereafter, the cladding-umped fiber laser reached the kilowatt level [33], using aimple YDFL end-pumped by diode stacks. Following re-nements of the large-area core design and fabrication,ingle-mode operation was demonstrated as well [3],omething which would not have been possible with Ndoping. These Yb-doped fibers had inner-cladding diam-ters (for the pump waveguide) of almost 1 mm and coreiameters of around 40 �m [3,5,33]. Thus, the inner-ladding-to-core area ratio was around 400, and the out-ut beam was up to 5 orders of magnitude brighter thanhe diode pump beam. The fiber length was in the regionf 10 m.

Commercial devices at the kilowatt level and aboveend to use an architecture that is different to a free run-

ig. 4. Schematic of a (a) cladding-pumped fiber laser and (b)ber amplifier in free-space end-pumped and all-fiber side-umped configurations, respectively.

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ing laser. Instead of generating all the output power in aingle fiber stage, they often use one or more high powermplifier stages that are seeded by a medium- or evenigh power fiber oscillator. Optical pumping is typicallyngineered around a large number of single-emitter orew-emitter diode sources pigtailed with 100 �m fibers.hese are then combined to achieve high power in a fiberetwork before being launched into the RE-doped fibersing sophisticated fiber arrangements which are oftenroprietary. Examples include SPI Lasers’ approach (GT-ave technology [34,35]) as well as IPGs [36].The highest power diffraction-limited YDFLs have used

et another level of sophistication, namely, tandem-umping. Tandem-pumping, in which one or several fiberasers pump another one [14,21], offers several advan-ages for power scaling. In particular it makes it possibleo pump close to the emission wavelength so that theuantum defect heating can be low resulting in a reducedhermal load. IPG’s 10 kW fiber laser [14] was pumped byDFLs at 1018 nm and emitted at 1070 nm, for a quan-

um defect of less than 5%, which is roughly half of that ofdirectly diode-pumped YDFL. The high pump bright-

ess possible with tandem-pumping also allows for a re-uction in the dimension of the inner cladding required toeach high power levels—it is possible to launch sufficientiode power into a thick but realistic Yb-doped fiber toeach the 10 kW level given the brightness of currenttate-of-the-art pump diodes [5]; however, it is quite chal-enging. This becomes much easier with tandem-umping, although one difficulty is to maintain the highrightness of the pump inside the pumped YDF, i.e., touide the pump in a small inner cladding. This may note strictly necessary, but improves the pump overlap withhe core, which shortens the absorption length. Thus, al-hough direct diode-pumping looks scalable to 10 kW [5],ll double-clad fiber lasers at 3 kW and above have in facteen tandem-pumped, as far as we know.

. Single-Frequency Ytterbium-Doped Fiber Sourceshe fiber lasers considered so far in our discussion, e.g., inig. 1, have been relatively broadband devices with line-idths typically in the 1–10 nm range. In such devices, in

he cw regime, nonlinear scattering—stimulated Ramancattering (SRS) in the first instance—is a weak effect. Its relatively easy to avoid and can often be disregarded,xcept at the highest powers and with long delivery fi-ers. However, some applications benefit from the mucharrower linewidth that can be provided by single-requency sources. One important example is coherenteam combination of multiple high power single-requency fiber sources [37]. This scheme offers the pros-ect of very high powers, and consequently this hastimulated much interest in single-frequency power scal-ng. In the high power regime, a single-frequency fiberource is typically configured as a high-gain MOPA seededy a lower power oscillator as shown in Fig. 5. This con-guration avoids the stability problems of a high poweringle-frequency oscillator and takes advantage of theigh gain, high power, and high efficiency that fiber am-lifiers can provide, all at the same time. Stimulated Bril-ouin scattering (SBS) [38–40] is the dominant nonlinear-ty for such devices [41], particularly at linewidths

maller than �10 MHz. SBS is nominally around 500imes stronger than SRS, making it a severe obstacle forigh power single-frequency fiber sources that needs to beddressed for power scaling. Short fibers with a largeode area (LMA) core design help to suppress nonlineari-

ies, including SBS, and allow scaling to the hundred-att regime [42]. This is a respectable power, but still less

han competing technologies. This means that, withouturther SBS suppression, fibers would be a rather poorhoice for high power single-frequency sources in manyases. However, if SBS can be suppressed then fiber sys-ems offer many advantages. Fortunately, the Brillouinonlinearity is subtler and more complex than the Ramanonlinearity, involving a propagating acoustic wave, and

s amenable to manipulation and indeed suppression.his has been the focus of considerable recent research.here are several options for SBS manipulation and miti-ation, including straining the fiber in order to broadenhe gain bandwidth [43] and signal linewidth broadening44] (if the application allows). The broadening should ex-eed the intrinsic linewidth of SBS to be significant (a fewens of megahertz at 1–1.1 �m), while a linewidth of aew tens of gigahertz may be required to completely sup-ress SBS beyond 1 kW.Since the speed of sound is temperature-dependent, the

ongitudinal temperature variations that occur naturallyn a high power fiber amplifier also help to broaden andhus suppress SBS [45]. Thermal broadening has beenhe most important effect in the progress in single-requency power scaling to date allowing the kilowattevel to be reached [46]. Higher powers still have been ob-ained by applying modest spectral broadening to the sig-al. Thus, 1.7 kW was reached by applying a modest 400Hz phase modulation to the signal and accepting a

lightly degraded beam quality �M2=1.6� [46]. The largerffective mode area of the fiber in this demonstration alsoelped to suppress SBS. This result represents the high-st power high-gain fiber amplifier reported to date, anderves as a convincing demonstration of the ability of fiberources to simultaneously provide high control, high gain,igh efficiency, and high power.Another interesting approach for the SBS suppression

s to manipulate the propagation of the acoustic wave byaking the fiber core an acoustic antiguide in order to re-

uce the acousto-optic (AO) interaction [47–50]. A 10 dBuppression of the Brillouin gain has been realized viahis technique [50], with theoretical optimization of thentiguide suggesting that 15 dB may be possible [51].owever, the challenges of implementing an acoustic an-

iguide in an ytterbium-doped fiber, with all its additionalequirements on concentration, resistance to photodark-ning, etc., should not be underestimated; and so far, thispproach has not yielded better performance than simpleongitudinal thermal broadening.

Fig. 5. Schematic of a typical single-frequency fiber MOPA.

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. STATE OF THE ART—PULSED FIBERASERSne of the great attractions of fiber laser technology, inddition to the high average powers that can be achieved,s the diversity of temporal output properties that can beupported. Due to the very large spectral bandwidthschievable from RE ions in glass, fiber lasers can be builto operate from the cw regime down to pulse durations ofust a few femtoseconds.

. Scaling Peak Power and Pulse Energieso emphasize the flexibility of fiber technology in Fig. 6e show a diagram indicating the fiber-based pulsed gen-ration approaches best suited to generate a given combi-ation of pulse duration and pulse repetition frequencyPRF). Examining the horizontal plane we see that aber-based approach exists for almost any desired combi-ation of performance specifications. For example, the op-ions for low (kilohertz) frequency trains of nanosecondulses include Q-switching [52] and external modulationf a cw laser [53], sub-100 fs operation can be achieved byassive fundamental mode locking at frequencies up to aew hundred megahertz [54,55], and active harmonicode locking can yield picosecond pulses at many tens of

igahertz repetition rate [56]. Additional techniques suchs external cavity compression in fiber [57] or harmonic-assive mode locking [58] can then be used if required toxtend these basic techniques to achieve shorter pulsesnd higher repetition rates, respectively. Time gating us-ng fast modulators can be used to reduce the repetitionates to any desired sub-harmonic of the fundamentalepetition rate. Ultrahigh frequency trains of pulses�40 GHz� can be generated either by time division mul-iplexing of lower frequency sources or by nonlinear opti-al techniques in which a high frequency sinusoidal sig-al (e.g., generated by beating together two lasers withifferent frequencies) is reshaped into a train of pulses athe corresponding frequency using soliton compression ef-ects [59]. The diagram should not be taken too literally;n reality the boundaries between approaches are not well

ig. 6. (Color online) Diagram illustrating (in the horizontallane) fiber-based pulse generation techniques appropriate to aiven operational regime (in terms of repetition rate versus pulseuration). The vertical axis lists the pulse amplification tech-iques required to access particular pulse energy regimes for

emtosecond pulses (OTDM, optical time division multiplexing;ctive ML, active mode locking).

efined, and there are often several options to address aiven pulse duration and PRF range. Nevertheless iterves to highlight the fact that fiber-based approachesllow the generation of a very broad range of pulsedormats—arguably very much wider than for more con-entional and established technologies, such as semicon-uctor or bulk solid-state lasers.The real benefits of the fiber approach though only re-

lly emerge when combining the basic source capabilityescribed above with the use of the fiber MOPA conceptince this allows for significant scaling of both the peaknd, arguably most significantly, average power levels.he challenge with power scaling is to preserve the de-ired seed laser properties during the amplification pro-ess in the face of the various deleterious effects such asber nonlinearity, chromatic dispersion, gain saturation,irefringence, and thermal beam distortions that other-ise degrade the spectral, temporal, and spatial fidelitiesf the signal. Moreover, to ensure reliable system opera-ion, it is obviously necessary to avoid all-fiber and com-onent damage mechanisms. This becomes progressivelyore of a challenge as the peak and average power levels

re increased, resulting in the need to adopt progressivelyore complex mitigation strategies. To emphasize this

act we illustrate the different and progressively morelaborate forms of pulse amplification needed to achievearious levels of pulse energy for femtosecond pulses onhe vertical axis in Fig. 6. Using approaches such as para-olic and chirped pulse amplification (CPA), which wehall describe in more detail shortly, it is possible to boosthe pulse energies achievable from a simple femtosecondber oscillator by more than 6 orders of magnitude: fromhe nanojoule to the millijoule regime.

From the perspective of pulsed fiber systems, the pri-ary performance metrics are output peak power, pulse

nergy, pulse duration, and repetition frequency. The av-rage power is defined by the product of pulse energy andepetition rate, and the pulse energy by the peak power,ulse duration, and detailed pulse shape. The limitationso pulsed system performance are ordinarily set either bynergy extraction limits (generally a dominant consider-tion only in the case of pulses longer than �1–10 ns ase shall discuss later) or, more frequently, by the peakower. With regard to the peak power, the limitation maye defined by

(a) a maximum tolerable degree of nonlinear distortione.g., as characterized by an acceptable nonlinear phasehift induced by self-phase modulation (SPM) or a toler-ble level of energy transfer to other wavelengths by in-lastic scattering due to either SRS or SBS]. Note that thetrengths of all of these nonlinear effects scale either lin-arly (for SPM) or exponentially (for SRS and SBS) in pro-ortion to fiber length and power, and in inverse propor-ion to the mode area [40];

(b) optical damage limits (which for most single-modebers scale in inverse proportion to mode area [60,61]);(c) or, as we enter the megawatt peak power regime, by

elf-focusing (which depends only on the total power flowithin the fiber, and which is typically negligible up until

he peak power exceeds a certain threshold—around 4W at a wavelength of 1060 nm [62]).

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Exactly which of these above limitations dominates in aiven system depends on the pulse duration and the spe-ifics of the laser architecture. However, the importance ofeveloping fibers providing LMAs and high gains per unitength should be obvious. Consequently, this has becometopic of intense research interest over the years. Unfor-

unately, the use of a larger core poses a challenge to theaintenance of single-mode operation and often compro-ises need to be struck between brightness, practicality,

nd stability. The key issues include the practicalities ofending, the presence of increased bend loss, and the ten-ency to higher-order mode (HOM) operation. Moreover,s the larger diameter fiber progressively resembles aulk solid-state laser, so the raft of problems associatedith heat removal, all very familiar to bulk lasers, also

tart eventually to impact fibers. Indeed, one of the mosttriking achievements in the development of high powerber lasers has been the extent to which it has provedossible to reconcile the various conflicting requirements,rawing greatly on the resources of fiber designers andabricators alike. There is still some room for further ad-ances in power levels before these difficulties dominate,ut the success here will depend increasingly on a de-ailed understanding of the limiting mechanisms and ofays to circumvent them. We will return to a more de-

ailed discussion of mode area scaling in Section 5; for theresent suffice it to say that achieving a robust single-ode performance in a core with an effective mode area

arger than 1000 �m2 at 1 �m is currently proving chal-enging within commercial systems (although it is to beppreciated that around 1 order of magnitude largerode areas have been reported in the laboratory using

ighly advanced fiber designs (see Section 5). Note thatne major further advantage of using large-core fibers ishat they provide for greater pump absorption per unitength. This allows for considerably shorter devices andepresents a significant further benefit beyond just re-uced intensities for the mitigation of nonlinear effects.With regard to fiber damage limits due to high peak

ower the standard approach to estimating damagehresholds in the pulsed regime is to use the data andulk damage limits defined in the paper by Stuart et al.or pure fused silica [60]. According to this paper the bulkamage threshold Pmax for pulses with a duration � be-ween 50 ps and 100 ns can be estimated via the followingxpression:

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here X=150 GW ns0.5/cm2=1.5 kW ns0.5/�m2, and � isxpressed in nanoseconds. This result is based on system-tic measurements made at a wavelength of 1.06 �m.his �−0.5 dependence is a result of the fact that for pulses

onger than 10 ps, the damage is thermal (melting/oiling) in origin, with a limiting rate determined by heatiffusion from conduction band electrons to the lattice.or pulses shorter than �10 ps, ablation takes over as

he primary damage mechanism, and a weaker depen-ence on the pulse duration then ensues as shown in [61].e note also that Eq. (1) can be rewritten as EPmax2.25 mJ W/�m4, where E is the pulse energy. This

hows that in this damage-limited regime, a higher peakower has to be traded for a lower pulse energy, and viceersa.

In principle, one might expect the surface damagehreshold of fiber to be similar to that of bulk silica andith appropriate end facet preparation (e.g., laser-

leaving) the observed threshold for fiber does appear topproach this. However, in practice the facets are theeakest points, and depending on the facet terminationuality the surface damage threshold might typically be aactor of 2–10 down on the bulk threshold. In order toitigate against surface damage it is normal to splice

oreless end-caps to the end of the fiber. This allows theeam to expand within the bulk glass prior to reachinghe surface, where the intensities are now substantiallyeduced. Note that few studies have been reported onamage mechanisms in the cw regime where power den-ities for damage are frequently quoted, but with little ifny quantitative justification. The current “wisdom” ishat the cw damage threshold for fibers is around0–20 W/�m2 at 1.06 �m for ytterbium-doped fibers, buthere is a genuine need for systematic studies in this di-ection.

Although we have chosen to provide indicative num-ers for damage thresholds for fiber lasers, we would notike to give the impression that ensuring that the nominalpecification of the laser is such that the intensities ap-ear to be well below these levels eliminates all damageoncerns. This is most certainly not the case. Fiber lasersnd amplifiers are subject to instabilities and modes ofelf-pulsation under certain different operating conditionshich can have catastrophic repercussions. YDFLs arearticularly notorious in this regard. For example, failureo adequately seed an ytterbium fiber amplifier can resultn the formation of intense pulses with energies sufficiento shatter, or even vaporize, the fiber core over consider-ble length scales and to destroy all inline components inhe process (often the pump lasers also). Fiber fuse effects63] can also be of concern, even at relatively modest av-rage powers of just a few watts, unless appropriate cares taken with thermal management. The fiber fuse resultsn a traveling hot spot that slowly propagates along theber destroying the core as it goes. To date little work haseen reported in the open literature on damage mecha-isms within fiber laser systems.To illustrate the favored current approaches applicable

o specific pulse duration regimes, we consider separatelyhe current status of fiber pulse sources in the nanosec-nd and ultrashort (picosecond to femtosecond) regimes.

. Nanosecond Fiber Laser Sourceshe most common and straightforward way to generateanosecond pulses within a fiber laser is to use-switching. This requires the incorporation of some ele-ent within the cavity that switches the cavity loss from

s a high as possible a value, to as low as possible a valuever a suitably short time scale. Both active and passivewitching elements have been used to this effect. Theinimum pulse durations from Q-switched fiber lasers

re just a few nanoseconds, although pulse durations ofhe order or tens to hundreds of nanoseconds are moreypical for cladding-pumped devices.

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Active Q-switching is most commonly achieved in fiberaser cavities using bulk AO devices to modulate the cav-ty loss [64–66]. AO devices have sufficiently fast switch-ng responses (rise time of the order 10–100 ns) and oper-te at sufficiently high repetition rates (from pulse onemand up to 1 MHz or so) to be appropriate for the pumpates and metastable lifetimes [intrinsically0 �s–10 ms, although the actual lifetime is significantlyhortened by amplified spontaneous emission (ASE)] forhe RE transitions of interest. AO devices also provide forery good extinction ratios ��50 dB� and diffraction effi-iencies ��85%� when operating the laser on the first dif-raction order. There have been several reports of-switching using fiber-based AO devices [67,68], rather

han bulk optic components. However, while these mightllow for more compact and lower-loss cavities, the perfor-ance achieved in terms of pulse energy and minimum

ulse duration has generally been inferior to thatchieved with bulk AO components. Passive Q-switchings another attractive option in that it is simple and re-uires no complex drive electronics or expensive activeptical componentry. Various approaches to passively-switched fiber lasers have been demonstrated in the lit-

rature based on a number of different material ap-roaches and which include, for example, the use of crys-als such as Co2+:ZnS [69], Cr4+:YAG [70], andemiconductor saturable absorbers [71]. Unfortunately,he extinction ratios that can be achieved using passiveaturable absorbers, while sufficient to initiate stableelf-pulsation, generally do not allow for sufficient gainold-off to permit the generation of particularly high-nergy pulses, unless longer or cascaded devices are used72].

It has been shown that the maximum energy that cane extracted from fiber lasers and amplifiers is around tenimes the intrinsic saturation energy Esat, given simply by

Esat � h�LAco/��a��L� + �e��L��L = EfluenceAco, �2�

here h�L is the photon energy at the laser wavelength,co is the core area, �L is the spatial overlap factor for the

asing mode with the doped core, and �a��L� and �e��L�re the absorption and emission cross-sections, respec-ively, at the laser wavelength �L [73]. The primary thingo note is that the saturation energy is proportional to theore area. The saturation fluence Efluence of Yb is around.5 �J/�m2. Given our previous comment on the maxi-um practical mode area, this means that pulse energies

f a few millijoules should be achievable in a diffraction-imited beam from fiber lasers based on state-of-the-artMA fibers. This is indeed consistent with the current ex-erimental status in nanosecond Q-switch pulsed fiber la-er systems [65,66]. By reducing the beam quality andorking with larger core structures, pulse energies of

ens of millijoules should be achievable and have indeedeen reported [74]. While these levels of pulse energy mayerhaps be considered small as compared to the jouleschievable from bulk lasers, it is still appreciable and suf-cient for a large number of applications—most notablyarking and precision machining. As a consequence,

iven their many advantages over existing laser technolo-ies, fiber lasers are now being used extensively in thesereas.

Although Q-switching offers the benefit of simplicity, itoes not provide for a great deal of control of the outputemporal pulse shape. As a consequence the great major-ty of nanosecond pulsed fiber systems on the market em-loy a MOPA-based approach in which the seed laser maytself be a Q-switched fiber laser, a directly modulated di-de laser, or an externally modulated cw laser. Note thatuch of the early work on nanosecond MOPAs was actu-

lly conducted on core-pumped erbium-doped fiber allow-ng pulse energies approaching 0.5 mJ and peak powersell in excess of 100 kW [53,75] to be achieved with veryodest ��1 W� levels of pump power. The adoption of

ladding-pumping has subsequently allowed further scal-ng of the average powers, repetition rates, and (to aesser extent) pulse energy [76].

One of the most exciting possibilities of the pulsedOPA is to exploit the rapid modulation possibilities of

ast electro-optic modulators (EOMs) or electro-bsorption modulators (EAMs), or alternatively directlyodulated diodes, to allow adaptive pulse shaping

77,78]. This allows the creation of designer pulse shapesptimized for a particular end application. For example,n our laboratories we have demonstrated a pulsed sys-em capable of generating multi-millijoule shaped pulsest power levels as high as 300 W in an end-pumped highower system [79], and over 100 W within a fully fiberizedystem [80], and have successfully used these in both ma-erials processing and frequency conversion experiments.oth systems were pump power limited, and scaling touch higher average powers and pulse repetition rates

hould certainly be possible. Before moving on it is worthoting that significant results have been made in theanosecond regime with regard to peak power scalingith peak powers approaching 4 MW recently reported

or 1 ns pulses using very large mode area fiber amplifiersased on rod-type fibers (see Section 5) [81]. These peakowers are right at the nonlinear limit imposed by self-ocusing.

. Ultrashort Pulse OperationE-doped optical fibers with their broad gain bandwidthsrovide convenient media for the generation and amplifi-ation of short optical pulses. For example the �100 nmain bandwidth achievable in an ytterbium-doped fiberhould be capable of supporting pulses shorter than 20 fs.s well as providing gain, optical fibers also possess bothonlinearity and dispersion, and these characteristicsave to be considered for short pulse applications. Theseroperties can be varied substantially by fiber design ory the choice of operating wavelength. In certain in-tances, these properties can be used to good effect, for ex-mple, to initiate mode locking. In other instances theyan have a deleterious effect, restricting—for example—he peak power of the pulses that can be generated or, al-ernatively, the pulse quality that can be obtained. Pulsesan be generated either through passive mode-lockingechniques or by active modulation of the intracavityeld. Active techniques are generally more suited to theeneration of picosecond pulses at high repetition rates,hile passive techniques are generally suited to the gen-ration of ultrashort pulses. Note that single cycle pulses

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ith durations as short as 4.5 fs have now been demon-trated using a fiber laser [82].

The limits imposed on the amplification of ultrashortulses are generally defined by peak power considerationsather than energy storage. Typically, spectral distortionue to SPM reduces the quality of pulses and can beaken as the fundamental measure of how much nonlin-arity can be tolerated by a pulse propagating within anmplifier. Nonlinear Kerr effects are characterized by theonlinear coefficient n2 which defines how the refractive

ndex of the core glass material varies with intensity.onlinear refraction then results in short optical pulses,enerating increased optical bandwidth through SPM ashey propagate through a fiber. This degrades pulse qual-ty and limits their utility. A pulse of power profile P�t�enerates an instantaneous phase �t� across the pulsenvelope. In a uniform fiber of length L without loss orain the expression for this phase is given by

�t� =n2kLP�t�

Aeff, �3�

here Aeff is the effective mode area, and k=2� /�, For aulse propagating along an amplifier experiencing a uni-orm logarithmic gain per unit length G, L in Eq. (3)hould be replaced with an effective length Leff (with ref-rence to the amplifier output power) given by

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G�1 − e−GL�. �4�

If we set a maximum nonlinear phase shift of 2� at theulse peak as roughly representative of the maximumPM that can be tolerated, and assume an effective am-lifier device length of, say, 20 cm, then we can estimate aaximum peak intensity of order 100 kW for direct pulse

mplification in a relatively standard 30 �m core LMA fi-er. For a 10 ps pulse this corresponds to a maximumulse energy of order 1 �J and for a pulse of 100 fs around0 nJ. Using state-of-the-art LMA fibers a further order ofagnitude increase in pulse energy and peak power is

ossible using direct pulse amplification—resulting ineak powers of order 1 MW [83,84]. Just as in the nano-econd regime, the peak powers are encroaching upon theelf-focusing limit in the ultrashort pulse regime inimple system configurations.

. Picosecond Fiber Lasersctively mode-locked fiber lasers, gain-switched or mode-

ocked semiconductor lasers, or even cw lasers externallyodulated with fast EOMs or EAMs can all be used to

enerate trains of picosecond pulses. Repetition ratesanging from a pulse on demand through to tens of giga-ertz are possible using an appropriate choice of technol-gy.

These compact sources can then be used as seeds for di-ect amplification to very high average power levels in aber MOPA. For example, we recently reported a 320 Wverage power system based on the direct amplification ofsimple gain-switched 1060 nm semiconductor laser op-

rating at frequencies in the range 0.1–1 GHz [85]. Theutput pulse duration was around 20 ps with peak powersn excess of 20 kW generated. Such sources, offering a

ombination of high peak and average powers in auasi-cw regime, are particularly interesting from theerspective of onward frequency conversion, e.g., to theisible spectral region through second-harmonic genera-ion [86] or supercontinuum generation [87] and to theid-IR using optical parametric oscillators [88]. Using

tate-of-the-art leakage channel fibers directly amplified0 ps pulses with a peak power in excess of 1 MW wereecently reported [84].

. Femtosecond Fiber Laserso get to higher pulse energy levels for femtosecond-OPA schemes it is necessary to implement more ad-

anced external pulse amplification approaches such ashose discussed below. Clearly this adds complexity, andhe pulse energies ultimately achievable are still lowerhan possible with bulk laser approaches due to nonlin-arity and dispersion in the long lengths of fibers in-olved. However, the average power levels achievable,ith reported values in excess of 830 W [89], represent–3 orders of magnitude increase over the existing tech-ology. This can be exploited in a variety of ways, for ex-mple, to increase materials processing speeds and imag-ng rates in several fields of application. We review thewo primary forms of external pulse amplification below.

(a) Parabolic pulse amplification: It has been shownhat by amplifying short pulses in fiber with normal dis-ersion, it is possible to form self-similar pulse solutionshat are characterized by the steady development of a lin-ar chirp across the pulse form, as it is amplified withinhe fiber [90,91]. The self-similar pulses that develop arearabolic in shape, and thus this technique has becomenown as either parabolic or similariton pulse amplifica-ion. The pulse consequently broadens in time as it is pro-ressively amplified, thereby spreading the pulse energyver an extended time window and reducing the overallet peak intensity. At the output of the fiber, these lin-arly chirped pulses can be compressed using an appro-riately dispersive element such as a bulk grating pair orprism-based dispersive delay line. Compression factors

f 10–20 are quite usual. Using this technique, pulse en-rgies approaching 1 �J have been achieved at a wave-ength of �1.06 �m using relatively conventionalb3+-doped LMA fiber amplifiers [92,93], and pulse ener-ies approaching 10 �J (peak powers�50 MW) should ul-imately be possible using state-of-the-art fibers.

(b) Fiber-based CPA: The well-known technique of CPA94] can also be applied to fiber-based laser and amplifi-ation systems [95–97]. In CPA, the short pulses to be am-lified are stretched in time (say from 100 fs and typicallyp to �1 ns) using a highly dispersive element. Thesetretched pulses can then be amplified to high energyithout generating excessive SPM within the fiber ampli-ers, since the peak power is reduced in proportion to thetretching ratio employed. These high-energy pulses canhen be recompressed back to their starting duration, andhus very high peak intensities can be achieved. Therere various options as to the components to be used foroth stretching and compressing the short pulses. Totretch the pulses, one can use a length of dispersiveingle-mode fiber, a bulk grating compressor, or more re-ently chirped fiber Bragg gratings (providing a very com-

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act and attractive option). To compress the high-energyulses at the amplifier output, where the peak power isuch higher, a bulk grating compressor is normally used,

lthough again the use of fiber or volume Bragg gratingsVBGs) or, more recently, hollow-core photonic bandgap fi-ers (PBGFs) has been reported [98]. Using bulk gratingompressors and LMA fiber amplifiers, femtosecondulses with energies in excess of 1 mJ have been obtained96], although pulse energies of �100 �J are more reli-bly achieved. The maximum pulse peak powers reportedrom fiber CPA systems have now exceeded 1 GW [97],nd 10 GW is not an unrealistic target with state-of-the-rt fibers and improved stretchers and compressors. It isorth adding that since the spatial mode quality achiev-ble with fiber amplifier systems is so good, it is possibleo focus beams from fiber CPA systems down to extremelyight dimensions, of say just a few �m2, meaning thateld intensities of order 1016–1017 W/cm2 should ulti-ately be achievable.

. HIGH POWER FIBER LASERS AT OTHERAVELENGTHS

or the many reasons already discussed Yb continues toe the frontrunner for high power fiber lasers. Neverthe-ess, although broad, the emission is restricted to the1–1.2 �m wavelength band, limiting the number of ap-

lications. Although inferior to YDFLs in certain respects,ilica fibers doped with other RE ions also offer a route toigh average power levels in a number of other wave-

ength bands in the near infrared spectral regime around.9 �m out to the short-wavelength mid-infrared regimeround 2.1 �m. See, for example, [99] for a survey of theavelength coverage of different high power fiber lasers.ost of the attention has so far focused on cladding-

umped Er- and Tm-doped silica fibers which providemission wavelengths in the �1.5 and �2 �m bands, re-pectively. Both laser schemes have been scaled to outputower levels in the hundred-watt regime and beyond, butower scaling is much more challenging than for YDFLs,o the maximum output powers achieved to date are someay behind those demonstrated for Yb fiber lasers. Nev-rtheless, Er- and Tm-doped silica fiber lasers have enor-ous power-scaling potential in wavelength regimeshere there is a wealth of applications. In this section, weriefly review the current state of the art for high powerr and Tm fiber lasers, the main challenges for furtherower scaling, and future prospects. We also briefly re-iew progress in power scaling of some of the other popu-ar laser transitions in fiber gain media.

. Er-Doped Fibers at 1.5–1.6 �mr-doped silica fibers provide a route to high power in the1.5–1.6 �m wavelength region corresponding to the

I13/2-4I15/2 transition. In cladding-pumped configura-ions, it has become a standard practice to use a phospho-ilicate core composition with ytterbium added to the cores a sensitizer [27,28,99]. Pump light is absorbed by theb3+ ions, and the Er3+ ions are subsequently excited in-irectly by energy transfer from the Yb3+ ions (see Fig. 7).his allows pumping by high power diode lasers in the10–980 nm band, relaxing the demands on the pumpavelength compared to unsensitized Er-doped fibers

nd, via the use of relatively high Yb3+ ion concentrations,ields a much higher pump absorption coefficient, as re-uired in cladding-pumped fiber configurations with amall core-to-cladding area ratio. The use of a high Yb3+

on concentration in combination with a phosphosilicateost also helps to ensure efficient energy transfer fromb3+ to Er3+ [100], which is essential for operation at highower levels. Cladding-pumped Er,Yb co-doped fiber la-ers can be designed to operate at any wavelength in theand �1530 to �1620 nm [101,102], but operation at thehort-wavelength end of this band is considerably morehallenging in terms of the demands placed on fiber de-ign, pump wavelength, and pump brightness. The maxi-um cw power reported to date for a cladding-pumpedr,Yb co-doped fiber laser is 297 W at 1567 nm [103]. This

aser employed a simple resonator configuration compris-ng a 6 m long Er,Yb co-doped double-clad fiber with a0 �m diameter (0.21 NA) core and a 600 �m diameter-shaped silica inner cladding. Feedback for lasing wasrovided by a perpendicularly cleaved fiber facet and by autted high reflectivity mirror. The fiber was pumped by aiode-stack at �975 nm and yielded 297 W of output at567 nm in a beam with an M2 parameter of 3.9 for1.3 kW of pump. 100 W class single-mode linearly polar-

zed all-fiber sources [104] and wavelength-tunable er-ium fiber sources [105] have also been reported.The Stokes efficiency, and hence the upper limit on the

lope efficiency for this transition, is �62%. However,lope efficiencies for cladding-pumped Er,Yb fiber lasersre typically in the range �40%–45% at best and at highump powers can be much lower due to parasitic emissionASE or lasing) on the Yb transition at �1 �m (e.g.,103]). Indeed, self-pulsed lasing at �1 �m is a commonause of damage to the fiber end facets or pump diodes inigh power Er,Yb fiber lasers and amplifiers. The use of aarefully optimized core design with host composition andr and Yb ion concentrations selected to enhance thenergy-transfer efficiency and raise the threshold forarasitic emission at �1 �m is crucial for power scalingnd makes fabrication of Er,Yb co-doped double-clad fi-ers more challenging than for simpler (singly doped) fi-ers. Additional precautions, such as the use of appropri-te dichroic mirrors to protect pump diodes, can also helpo reduce the risk of damage. Also, operation at elevatedemperatures increases the threshold for Yb parasitic las-ng (due to the increased three-level character) and canlso be used to increase the Er lasing efficiency [106].

. Thermal Limitshile it is certainly true that fiber lasers enjoy a degree

f natural immunity from the effects of heat generation

Fig. 7. Energy level diagram for Er,Yb co-doped silica.

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by virtue of their geometry), they are not completely im-une. Indeed, the onset of thermally induced damage or

egradation to the fiber’s outer-coating is one of the mainactors limiting the average output power from cladding-umped Er,Yb fiber sources. In most designs of double-lad fiber, the outer-coating serves both as a protectiveayer and as the low-index layer allowing pump light to beuided in the inner-cladding pump guide. Usually, theuter-coating is a low refractive index fluorinated polymerhat can withstand temperatures up to �150°C–200°Cefore the onset of damage, and 80°C has been suggesteds the limit for long-term reliability [107]. The upper limitn heat deposition density in the core, Ph max, that can beolerated before the onset of coating damage can be esti-ated from [108]

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2

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here Td is the maximum temperature that the coatingan tolerate, Ts is the temperature of the surroundings (oreat-sink), Koc is the thermal conductivity of the outerladding (i.e., the coating), roc is the radius of the outerladding, ric is the radius of the inner cladding, and h ishe heat transfer coefficient. In situations where convec-ive cooling is the predominant cooling mechanism, as isrequently the case in fiber lasers, the second term inquare brackets of Eq. (5) dominates favoring the use ofarger diameter fibers for power scaling as in [103]. Fig-re 8 shows the maximum heat deposition density as aunction of the heat transfer coefficient �h� for a typicalladding-pumped fiber source with ric=200 �m, roc250 �m, and Koc=0.1 W m−1 K−1. It can be seen thath max is strongly dependent on the heat transfer coeffi-ient and hence on the heat-sinking configuration. If wessume that quantum defect heating provides the soleontribution to heating, then the theoretical upper limitn output power PL max for a given single-end-pumped fi-er laser or amplifier before the coating damage limit iseached can be estimated from

PL max �Ph max�abs

�p �q�L

�p − �q�L , �6�

here �L is the lasing frequency, �p is the pump fre-uency, �p is the absorption coefficient for pump lightaunched into the inner cladding, �abs is the fraction ofump light absorbed in the fiber, and �q is the pumpinguantum efficiency (i.e., the number of excited ions in the

ig. 8. Maximum heat deposition density �Ph max� before the on-et of coating damage as a function of heat transfer coefficienth�.

pper laser level manifold for each pump photon ab-orbed. Hence for a typical cladding-pumped Er,Yb fiberaser with an effective pump absorption coefficient of.92 m−1 (i.e., 4 dB m−1), the heat transfer coefficientould need to exceed 300 W m−2 K−1 to allow thermal-amage-free operation at the 100 W power level for theber design above. It is worth noting that an equivalentb fiber laser operating at 1080 nm and cladding-pumpedy a diode source at 975 nm could generate over 0.55 kWefore the onset of thermal damage. Thus, effective ther-al management is crucial for power scaling of Er,Yb fi-

er lasers, and heat generation is currently one of theain factors limiting output power.

. Yb-Free Er-Doped Fibers In-band-Pumped at 1.5 �mn alternative strategy for power-scaling Er fiber lasers

s to pump directly into the upper manifold using a pumpavelength in the �1.45–1.53 �m wavelength region.his has the attraction that the need for Yb co-doping isvoided along with the problems (e.g., ASE, parasitic las-ng, self-pulsing, and even damage) that can arise due toncomplete energy transfer from Yb to Er, resulting in auildup of excited Yb ions. These problems become moreronounced at the high Er excitation densities requiredor the generation of high-energy pulses. Another attrac-ion of this approach is that thermal loading due to quan-um defect heating in the Er fiber is dramatically reducedrom �40% to a few percent. High power diode lasers athe relevant pump wavelengths are now commerciallyvailable from an increasing number of vendors. Their op-ical efficiencies ��35%� and output powers are somewhatower than for pump diodes in the 900–980 nm regime,ut this is partly offset by the lower quantum defectumping cycle. Recent work by Dubinskii et al. [109] dem-nstrated an Yb-free Er-doped silica fiber laser with48 W of output at 1590 nm cladding-pumped by sixber-coupled InGaAsP/InP line-narrowed LD modules at532.5 nm. The optical-to-optical efficiency was �57%ith respect to absorbed pump power. This is far below

he quantum limit, but is nevertheless much higher thanhe optical-to-optical efficiencies achieved in cladding-umped Er,Yb co-doped fibers. Further optimization ofb-free Er-doped silica fibers tailored for high powerladding-pumped laser configurations in combinationith advances in InGaAsP/InP diode laser technology to

ncrease power, brightness, and efficiency (all currentlyuch below the values of 9xx nm diodes) is likely to yieldsignificant improvement in Er fiber laser performance,

articularly so because the low pump absorption of Eralls for high-brightness pumping. This also makesandem-pumping an attractive option. For example,icholson et al. reported a HOM erbium-doped fiber am-lifier pumped by a single-mode fiber Raman laser at480 nm [110]. At 34 W of pump power, this produced 10.7

of average output power at 1554 nm in 100 kHz 1 nsulses. Thus, Er fiber lasers may eventually be able tohallenge the supremacy of YDFLs in terms of outputower.

. Tm- and Ho-Doped Fiber Lasers at 2 �mm-doped silica fiber lasers also offer a route to very highutput power. A particularly attractive feature of Tm-

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oped silica is the very broad emission spectrum for theF4-3H6 transition, spanning the wavelength range from1700 to �2100 nm [111]. This provides wide flexibility

n operating wavelength and serves as an excellent start-ng point for nonlinear frequency conversion to the mid-nfrared. A further attraction of Tm-doped fibers is thebility to scale to much larger mode areas than for Yb-oped fibers while maintaining single-mode operation, byirtue of the longer operating wavelength. This opens uphe possibility of levels of performance in cw and pulsedodes of operation beyond the capabilities of Yb-doped fi-

er sources by virtue of the ability to raise the thresholdor unwanted nonlinear loss processes (SBS and SRS) andatastrophic damage.

Tm-doped silica has three main absorption bands rel-vant to high power operation (see Figs. 9 and 10). Thewo most attractive absorption bands are at780–800 nm and at �1550–1750 nm. The latter pump

and coincides with the emission wavelengths availablerom cladding-pumped Er fiber sources, thus allowingery low quantum defect (in-band) pumping and veryigh slope efficiencies (70%–80%) to be achieved [112],omparable to those demonstrated in YDFLs. A furtherttraction of this pumping scheme is the high brightnessf the Er fiber pump laser, which allows the use of core-umped Tm fiber laser configurations or high core-to-ladding area ratio Tm fiber lasers, giving flexibility inore design and doping levels as well as access to lasingavelengths (�1725 to �2100 nm) across most of the

mission band [112]. Meleshkevich et al. reported a Tm-oped silica fiber laser cladding-pumped by multiple Er fi-er lasers with 415 W of single-mode output at 1940 nmimited by the available pump power [113].

The absorption band at 780–800 nm can be pumpedith commercial high power diode lasers. This has the at-

raction of simplicity compared to the former approach,ut more conventional double-clad fiber designs are re-uired due to the lower brightness of the diode pumpources. Hence the lasing wavelength is generally re-tricted to longer wavelengths in the range1850–2100 nm. The Stokes efficiency for this pumping

cheme is relatively low, �0.4, implying a rather low effi-iency. However, the efficiency can be improved dramati-ally by exploiting the “two-for-one” cross-relaxation pro-ess �3H4+3H6→ 3F4+3F4� [114]. While this has become atandard approach for improving the efficiency in diode-umped Tm-doped crystal lasers for over two decades114], the first demonstration of a cladding-pumped Tm-oped silica fiber with efficiency enhanced by cross-elaxation was only 10 years ago [115]. In this work, the

Fig. 9. Energy level diagram for Tm-doped silica.

ber laser yielded a maximum output power of 14 W at2 �m for 36.5 W of diode pump power at 787 nm using am-doped aluminosilicate double-clad fiber with a Tmoncentration of �1.4% (by weight). Since then dramaticmprovements in the efficiency for cladding-pumped Tmber lasers have been achieved by modifying the coreomposition and Tm doping level to promote the two-for-ne cross-relaxation process while limiting the detrimen-al impact of other spectroscopic processes (notablynergy-transfer upconversion) on performance [116]. Theptimum Tm doping level depends on the mode of opera-ion and desired operating wavelength, but is generallyonsidered to be around 3% (by weight) for most cw laseronfigurations operating toward the long-wavelength endf the emission spectrum [117]. The record slope efficiencyith respect to the absorbed pump power reported for am-doped silica fiber laser cladding-pumped at �0.8 �m

s 74% [116], which compares favorably with the efficien-ies routinely achieved with cladding-pumped Yb fiber la-ers. In practice for higher power Tm fiber lasers based onommercially available fibers, slope efficiencies are some-hat lower ��60%�, but still well above the Stokes effi-

iency. If we assume for a typical high-Tm-concentrationilica fiber laser that the pumping quantum efficiency is1.7–1.8, then from Eq. (5) the upper limit on output

ower before the onset of coating damage is around fourimes lower than for an equivalent YDFL. Clearly, high-m-concentration large-core double-clad fibers have hugedvantages in terms of the ability to mitigate unwantedonlinear processes and catastrophic damage, but effec-ive thermal management is crucial for scaling to veryigh power levels. Moulton et al. recently reported aouble-clad Tm-doped silica fiber laser with 885 W of out-ut at �2040 nm in a MM beam with M2 parameters of 6nd 10 in orthogonal planes [117]. Pump light was pro-ided by two 1 kW fiber-coupled diode pump modules at797 nm. Ehrenreich and co-workers recently demon-

trated an “all-glass” Tm fiber MOPA based on a 50 W Tmber oscillator at 2045 nm and two Tm fiber power ampli-cation stages with over 1 kW output power and with alope efficiency of �53% [118]. This result represents theighest output power reported to date for a Tm fiberource and serves as a very convincing demonstration ofheir power-scaling potential in the 2 �m wavelength re-ime. The combination of power scalability and wideavelength flexibility is a very useful feature and shouldpen up the prospect of many new applications.

Cladding-pumped Tm fibers have also been deployed inore sophisticated MOPA architectures to yieldavelength-tunable and narrow-linewidth output at

Fig. 10. Absorption spectrum for Tm-doped silica.

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ower levels in the hundred-watt regime and above. Pear-on and co-workers recently reported a linearly polarizedingle-frequency Tm fiber MOPA based on a single-requency Tm fiber distributed feedback (DFB) master os-illator followed by three Tm fiber amplifier stages with00 W of output at 1943 nm [119]. Goodno and co-workerseported a single-frequency MOPA based on a DFB diodeaser and four Tm fiber amplifiers with a non-PM (i.e.,on-polarization-maintaining) cladding-pumped final am-lifier that produced 608 W of output [120].This serves once again to underline the potential of Tm

ber sources. It is worth pointing out that while the popu-ar trend for power-scaling Tm fiber sources has been toxploit the two-for-one cross-relaxation process usingommercial diode pump sources at �0.8 �m, this is nothe route that ultimately offers the highest power levels.f efficiency, cost, and complexity considerations are putside, then in-band pumping with multiple high power Erber lasers almost certainly offers the prospect of higherutput power with greater flexibility in operating wave-ength and mode of operation. Thus, with forthcoming ad-ances in fiber component technology for the 2 �m wave-ength regime, we are likely to see further significantmprovements in performance for Tm fiber-based sources.

Extension to wavelengths a little beyond 2.1 �m (lim-ted by material and impurity absorption) can be achievedsing a Ho-doped silica fiber laser operating on the 5I7-5I8ransition [121]. Ho-doped silica offers a very broad emis-ion line extending beyond 2.1 �m and can be pumped di-ectly at �2 �m using a diode laser or a high power Tmber laser [121] or, indirectly, via co-doping with Tm3+ fol-

owed by energy transfer from Tm3+ to Ho3+ [122]. In theatter approach, the Tm3+ ions can be excited using any ofhe pumping schemes described above, and thus Ho fiberasers should be scalable to very high power, complement-ng the wavelength band offered by Tm fiber lasers.

. FUTURE PERSPECTIVESs the previous sections have shown progress in all areasf fiber laser technology over the past 15 years or so haseen staggering with an increase in the average outputower in excess of 60% year on year even in mature areas,nd with much faster growth for less mature devices.rom the perspective of the raw generation of power it islear from the preceding discussions that the fundamen-al average power limits for single transverse mode opera-ion are within sight, and that without major new break-hroughs in mode area scaling/mode control within MMbers and improved thermal and damage management,he record output powers will saturate at the ten to sev-ral tens of kilowatts level from a single core. Likewise,ith regard to pulsed operation again we are relatively

lose to achieving the maximum peak power that mighte achieved from a single core at 1 �m—improved peakowers approaching one order of magnitude higher mighte achieved by extending existing pulsed technology toonger wavelengths and taking benefit from the reducedonlinear effects due to the increased core sizes possible;owever substantial improvements in the peak power willnly be achieved by new concepts for further mode areacaling and by the use of substantially shorter device

engths to reduce the impact of fiber nonlinearity. How-ver that brings forth the conflict between the require-ents for a short fiber for nonlinear mitigation and a longber for thermal management. The self-focusing limithat depends simply on power within the fiber, and there-ore cannot be addressed by a larger mode area, is alsoow within sight and will need to be considered muchore moving forward. Once the single-aperture limits are

ruly reached, then beam combination will be the onlyoute available to extend both cw and pulsed perfor-ance.Given these conclusions we next consider the technical

dvances that need to be pursued to underpin the next de-ade of sustained growth in fiber laser performance—inarticular in terms of brighter pump sources for fiber la-ers, improved power handling (both thermal and opticalowers), extended wavelength coverage, and improvedeam combination technology. It is to be appreciated thathe technological improvements we anticipate can also besed to provide increased options in terms of source con-rol, improved reliability, and flexibility in system design,hich from a commercial perspective is arguably often farore important than improved headline laboratory per-

ormance metrics and can result in significant cost sav-ngs and new market opportunities. Obviously, such ben-fits are often hard to substantiate and to quantify.

. Improved Pump Sources and Pumping Options

. Higher-Brightness Diode Pump Sourceshen considering the future prospects for cladding-

umped fiber lasers, it is worth noting that the dramaticdvances in high power fiber laser technology did not fol-ow immediately after the invention of the LD-pumpedouble-clad fiber by Kafka in 1989 [24]. Indeed the firstemonstration of a 100 W Yb fiber laser was over a decadeater [32]. In truth, as with so many inventions, theladding-pumped fiber laser was born before its time inhe sense that diode pump sources with sufficient outputower and brightness were not yet available. In contrasto the situation for most bulk solid-state laser architec-ures, the requirement for raw pump power must be ac-ompanied by high pump beam brightness to allow effi-ient coupling into the fiber. Thus, in many respects, theramatic rise in power and performance attainable fromber lasers has mirrored developments in diode pumpources in terms of output power and, in particular,rightness. This is likely to remain an underpinningheme also in the future, despite the emergence ofandem-pumping as an increasingly attractive option forigh-brightness pumping. To get a sense of what the fu-ure holds, it is worth quickly reviewing the state of thert in terms of diode pump sources and possible future de-elopments over the next decade to see how these couldenefit fiber laser technology.A number of different diode pump sources have been

mployed to power scale fiber lasers. Diode-bars andtacks offer the attraction of high power-to-package vol-me, but yield rather low brightness (primarily becausef their low fill-factors), and hence require somewhat com-licated free-space optical arrangements to produce out-ut beams compatible with the requirements for cladding-

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umping of fibers. A further drawback of these pumpources is that rather aggressive cooling is needed to copeith the high thermal loading density. Alternatively,road-area (MM) single emitters offer high reliability,imple thermal management, and the highest brightnesser unit emitter area with output power levels up to10 W from �90–100 �m wide emitters in the915–975 nm band. Fiber-based aggregation schemes

mploying multiple fiber-coupled broad-area emitters (seeig. 11) in conjunction with MM fiber combiners [123] al-

ow robust scaling of pump power. Not surprisingly, thisumping architecture has become the architecture ofhoice for many in the field and is at the heart of manyndustrial high power fiber laser products. One of the

ain drawbacks of this approach is that the diode emitterrightness is degraded by more than one order of magni-ude on launching into the MM delivery fiber. This isargely because the M2 parameter parallel to the diodeunction �Mx

2� is typically �15–20 times larger than forhe orthogonal direction, and hence the delivery fiber’sore size and NA must be selected to accommodate thelow-axis beam size and divergence.

This shortcoming can be remedied by reducing thelow-axis divergence of the diodes. This is attractive be-ause the package architecture remains unaltered. Whilet is proved difficult to reduce the emitter size because ofimits on the power density on the facet and the fiber di-meter because of handling issues of very thin fibers,here has been good progress in the reduction of diodeA. Thus, whereas the fiber pigtails support a NA of 0.22,

t is now possible to get pigtailed diodes working at NAs ofess than 0.12 [124]. A more radical approach is to use apatially combined multi-emitter architecture. One ex-mple of such a scheme is illustrated schematically inig. 12. In this approach the individual emitters are col-

imated in the fast-direction by cylindrical microlensesnd then spatially combined using a simple arrangementf mirrors prior to being coupled into the delivery fiber.uccessful implementation of multi-emitter sources re-uires careful design and construction since the tolerancen position of optical components is much more demand-ng than for fiber-coupled single emitters. However, thispproach, in combination with polarization combining,an yield a 10- to 20-fold increase in power and brightnesser delivery fiber.Further increases in power and brightness can be

chieved via the use of wavelength beam combining [125].ne can envisage a spatially combined multi-emitter

ource (similar to the configuration shown in Fig. 12) em-loying VBGs in an external feedback cavity arrangemento select the same operating wavelength of all emittersithin a narrow band (say 0.5 nm). Then multipleulti-emitter sources (each with a different operatingavelength) could be combined into a single beam using

Fig. 11. Fiber-coupled broad-area pump diode.

n arrangement of VBGs [126]. The theoretical upperimit on the number of multi-emitter sources that could beombined in this way depends on their linewidths and onhe useful absorption bandwidth of the RE ion doped fi-er. However, the practical upper limit on the number ofombined sources is also dependent on the residual lossesi.e., due to scattering, absorption, and imperfect antire-ection coatings) of the optical components. For most RE

on pump transitions it should be possible to combine ateast ten multi-emitter sources into a single delivery fibersing this approach. The net result would be over two or-ers of magnitude increase in power and brightness com-ared to a fiber-coupled single-emitter to yield a totalower of over 1 kW per fiber. Clearly, the construction ofuch a source would be very demanding on optical compo-ents and on alignment tolerances; and, thus, when theseractical considerations are factored into the design, theevel of performance achievable may be somewhat lower.evertheless, it is clear that there is enormous scope forn increase in pump power and brightness compared tohat is currently available from high power diodes. Thisill have a huge impact on cladding-pumped fiber lasers,lthough it is certainly possible that tandem-pumping,hich is already in commercial use and provides veryigh pump brightness, will also continue to be the favoredpproach in the future in many cases.The maximum pump power Pp max that can be

aunched, via a single injection point, into a double-cladber using the present generation of broad-area emitterump diodes in conjunction with the various beam combi-ation schemes described above and MM fiber combinerggregation schemes can be estimated from

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ors for the single-emitter diode after fast-axis collima-ion, N is the number of wavelength-combined multi-mitter sources, �coupling is the coupling efficiency throughhe pump beam conditioning and combining optics andnto the active fiber, �NA is the arcsine (NA), � is a factor 1� to account for the need to slightly underfill the fi-er’s aperture and NA to avoid damage to the outer-oating and mounting components, and �p is the pumpavelength. Figure 13 shows the theoretical upper limit

n the launched pump power as a function of the inner-ladding diameter based on current power levels availablerom �100 �m wide broad-area emitters in the 915–975m band (i.e., Ps=10 W), and assuming �NA=0.4,

=0.8, and �=0.8. Three different scenarios are

Fig. 12. Spatially combined multi-emitter diode pump source.

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onsidered. Curve (a) shows the upper limit on theaunched power that can be achieved using fiber-coupledingle emitters. Curve (b) shows the upper limit on theaunched power using spatially combined andolarization-combined multi-emitter sources. Curve (c)hows the upper limit on the launched power for aavelength-combined multi-emitter source with N=10.hus, using the current single-emitter diode technology

n combination with spatial, polarization, and wavelengtheam combination could yield launched pump powershat are two orders of magnitude higher than can rou-inely be achieved today for a given inner-cladding diam-ter and NA. In practice, the pump power that can be ac-ommodated by a given fiber will also depend on thermalamage limits for optical components, MM fiber combin-rs, delivery fibers, and the final active fiber. Thus, theevelopment of “next generation” high-brightness diodeump sources will be very challenging.

. High-Brightness Tandem-Pumpinghere are several attractions of tandem-pumping, andne of them is the high pump brightness. This exceedshat of even the brightest diode pump sources envisagedn the future by at least 2 orders of magnitude. A moreomplex system, a lower overall efficiency, and a limitedange of pump wavelengths may appear to be disadvan-ages of tandem-pumping, as compared to direct diode-umping. However, if we consider high-brightness diode-ased pump sources, these are in themselvesophisticated systems (see Fig. 12) that may well be asomplex as a fiber-laser pump source. The chips used forigh-brightness diodes are also somewhat less efficienthan those of lower-brightness diodes, and spectral beamombination introduces losses, too. Therefore, all told, inhe high-brightness pumping regime, tandem-pumpingith a simple intermediate fiber pump laser can perhapse more efficient than direct diode-pumping, in someases. Furthermore, the possibility of Raman shiftingreatly extends the wavelength coverage of tandem-umping. This can benefit several RE ion laser transi-ions, which are currently more severely affected than Yb-oped fibers by the low brightness of pump diodes atppropriate wavelengths. For example, a Raman-shiftedDFL providing 73 W at 1480 nm was recently used to

ig. 13. Theoretical limit on launched pump power for broad-rea emitter-based pump schemes assuming Ps=10 W, �NA=0.4,coupling=0.8, and �=0.8. The curves show the upper limit on

aunched power that can be achieved using (a) fiber-coupledroad-area emitters, (b) spatially combined and polarization-ombined multi-emitter sources, and (c) ten wavelength-ombined multi-emitter sources.

ore-pump an erbium-doped fiber on a single HOM [110].owever, except for core-pumping, it will be challenging

o maintain the high brightness in the final gain fiber, i.e.,aunch and guide the pump in a small inner cladding.his may not be strictly necessary, but improves theump overlap with the core, which shortens the absorp-ion length. Therefore, to make the most of tandem-umping, it is necessary to optimize the fiber designs andump launch schemes.

. Summary of Likely System Benefitshe benefits in terms of fiber laser performance to be de-ived from improved pump sources depend very much onhe situation. One obvious benefit is the ability to scale toigher average output powers in both cw and pulsedodes of operation, and we remind ourselves that some

perating regimes and some wavelengths demand par-icularly high pump brightness. On the other hand, this isubject to limitations imposed by damage and nonlinear-ty, and it is already now possible to design lasers thateach the damage threshold even operating cw. For those,he benefits of higher pump brightness are less clear. Nev-rtheless, higher pump brightness will also relax the con-traints on the core composition in many cases, allowinghe use of much lower RE ion concentrations with en-anced resistance to photodarkening [127] and reducednwanted spectroscopic loss processes (e.g., energy-ransfer upconversion). This will in turn allow muchasier tailoring of the refractive index profile and RE ionoping profile to facilitate robust single-mode operation inLMA core design, which is important to mitigate both

onlinear degradation and damage. Thus, in many re-pects, pump brightness is still the key to further powercaling. The availability of much higher power and muchigher brightness pump sources will almost certainlyhange current thinking on cladding-pumped fiber de-igns, opening up many new possibilities and yielding sig-ificantly improved performance in all operating regimes.hese developments will further underline the important

uture role to be played by fibers in the high power laserrena.The ability to use much smaller inner-cladding sizes for

given pump power (by virtue of the higher pump bright-ess) will allow the use of much shorter fiber lengths,aising the threshold for unwanted nonlinear processese.g., SBS and SRS). This will benefit fiber sources (e.g.,arrow-linewidth and pulsed) where the upper limit onower is determined by the onset on nonlinear processes,pening up the possibility of a dramatic increase in power.t some point, however, the increased thermal loadingensity will become a problem, as will be discussed below,ut we reiterate that high-brightness tandem-pumpingan be beneficial also in reducing the total thermal load inhe gain fiber.

. Improved Power Handling

. Improved Thermal Managementlthough increased pump brightness brings many directnd indirect advantages to many devices, the use ofhorter fibers with higher thermal loading densities willmphasize the need to improve thermal management.

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ower-extraction per unit length can exceed 1 kW/m [128]n YDFLs, but aggressive cooling is required to avoidamage. With conventional double-clad fibers with a low-ndex polymer coating, the coating is often most sensitiveo a high thermal load. Other thermal effects are nor-ally less of a problem [129]. The coating serves the dual

urpose of guiding the pump light and protecting the fi-er, and this restricts the materials and designs avail-ble. Silicone has been used in the past. It can withstandigher temperatures, but suffers from degradation overime, and the resulting pump NA has been restricted toround 0.4. Teflon (polytetrafluoroethylene) is an interest-ng high-temperature high-NA option. However it is ex-ensive and difficult to apply, and although Teflon coat-ngs are commercially available for passive fibers (e.g.,130]), the benefits for high power fiber lasers have yet toe demonstrated.Today’s preferred low-index coatings (fluorinated acry-

ates), while easily applied during fiber drawing and pos-essing excellent optical properties, are relatively poorrom a thermal point of view. They degrade quickly atemperatures approaching 200°C and long-term reliabil-ty may require operation below 80°C [107]. Further-

ore, their thermal conductivities are poor, e.g., around.24 W K−1 m−1 [107]. Therefore, if special attention isaid to achieving a high heat transfer coefficient h, therst term in Eq. (5) will dominate, and the highest tem-erature reached within the coating, at the interface withhe silica inner cladding, is largely determined by its ownhermal impedance. The temperature difference acrosshe coating depends on the inner-cladding diameter andoating thickness. The use of thin coatings will result in aber with improved thermal resilience. The challenge forhe future lies in realizing a thin coating within the con-traints of providing good optical guidance. We have fab-icated fibers with coating thicknesses of 20 �m, with aeduction in the thermal impedance of nearly a factor of 5ompared to a more conventional coating thickness of00 �m.The temperature difference from the ambient to the

utside of the coating can be substantial, too, and is likelyo dominate the temperature rise seen by the inside of theoating in the case of unforced air-cooling, for example.ore efficient heat transfer arrangements are needed inany cases, ranging from straightforward measures such

s coiling the fiber on a metal heat-sink to immersing it inater, which presents practical problems and long-term

eliability issues. It is also possible to embed the fiber in aetal. This can drastically reduce the overall thermal im-

edance through a conventional heat-sink (cold-plate,ater-cooled heat-sink, etc.), although the different melt-

ng points and thermal properties of the many differentypes of materials involved make this a nontrivial propo-ition. In the example of Fig. 8 above, a heat transfer co-fficient of 104 W m−2 K−1 would be sufficient to make theemperature difference between the outside of the coatingnd the ambient negligible, but a higher value is requiredor a thinner coating.

Thermal management becomes simpler if the require-ents on the polymer coating are relaxed. In so-called all-

lass structures, the waveguides are defined entirely by alass. No light reaches the coating (if the scatter is negli-

ible), so the coating material can be selected to aid ther-al management rather than for pump guiding. This

pens up opportunities for protective coatings made ofhin high-temperature polymers as well as metals. Theacketed air-clad fiber [20,131] is one type of all-glass fiberhat can provide NAs of 0.7 [132] and above, but the sili-a:air mesh that constitutes the low-index outer claddinglso serves as an insulating layer. This increases the tem-erature in the glass, which is normally not critical, butevertheless undesirable. Furthermore, the thermally in-uced stresses in the mesh can be substantial [133] andan conceivably break it. To date, this type of fiber haseen limited to 2.5 kW of output power [134].Instead of a silica:air mesh, a low-index glass can be

sed for the outer cladding. This improves the thermalnd mechanical properties, but the achievable inner-ladding NA is substantially lower than that provided byow-index polymers, so less pump power can be couplednto the fiber. This drawback can be offset by usingigher-brightness pump sources to increase the launchedump power albeit at the expense of increased complexity.t is likely that the next generation of high-brightnessump sources will make all-glass fibers the natural choiceor power scaling, given the benefits to be gained in termsf thermal and mechanical properties and ease of splicingompared to jacketed air-clad fibers.

In addition to the above techniques thermal problemsan also be reduced by decreasing the thermal loadingensity by in-band pumping. This is a key strength ofandem-pumping, which has already proved to be a pow-rful technique for power scaling of YDFLs. For example,PG’s 10 kW fiber laser was pumped by YDFLs at 1018m and emitted at 1070 nm [14], corresponding to a quan-um defect of less than 5% in the final gain stage, which isoughly half of that of a directly diode-pumped YDFL.his result serves to underline the benefits of tandem-umping, which seems likely to remain as the option ofhoice for operation at very high powers.

Nonlinear scattering, rather than stimulated emission,ay ultimately allow for the lowest thermal loads. For ex-

mple, fiber Raman lasers, which have now been scaledeyond 100 W [135,136] and look scalable well into theilowatt regime, can operate with quantum defects of lesshan 4%, in the case of silica pumped at 1060 nm. SBSan also be used for amplification, with a quantum defectf the order of 10−4 in silica fiber. However, high power op-ration presents many challenges, and powers to dateave been limited. For example, an energy of 0.1 mJ waseported in the pulsed regime, in a beam clean-up experi-ent [137]. Finally, fiber-optic parametric amplifiers op-

rate without any quantum-defect heating. Key chal-enges here are related to phase-matching, and the recordower to date is only a few watts [138].

. Improved Optical Damage Thresholdsatastrophic optical damage due to dielectric breakdown

n the core is an increasingly troublesome issue, in par-icular in the case of high-energy nanosecond-scaleulses. As stated earlier, the damage threshold of pulsesn this regime is normally taken to be 1.5 kW ns1/2 /�m2,ith an inverse square-root dependence on the pulse du-

ation [60]. Compared to thermal damage of the coating,

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he options for mitigating catastrophic damage are muchore limited. One option is to use materials with higher

amage thresholds, but there are only few materials withigher damage thresholds than silica. On the other hand,he incorporation of REs and other typical fiber dopants iseported not to adversely affect the damage threshold sig-ificantly [139].It is not clear if higher damage thresholds are realistic.

owever, some recent studies do suggest this possibility.hus, values of around 0.5 TW/cm2=50 kW/�m2 [140]nd even 6 TW/cm2=0.6 MW/�m2 [141] have been sug-ested. Given a self-focusing limit of around 4 MW at060 nm, and that a core area of 200 �m2 is straightfor-ard to achieve, these values are far beyond what iseeded ��20 kW/�m2�. However these damage thresh-lds have not been demonstrated in fibers for reasons thatre partly unknown. Some reasons are readily under-tood, e.g., that the 0.6 MW/�m2 can only be achieved inicrometer-sized spots [141], while larger spots lead to a

ower threshold. This is attributed to propagation andelf-focusing. Also the lower threshold of [140] still re-uired carefully controlled conditions, for example, toliminate facet reflections. Some of these conditions (e.g.,1 �m2 spot size) are not achievable at high powers in aber, which clearly must allow for propagation over ap-reciable lengths with low nonlinear degradation.Less clear is if material imperfections contribute to the

ower damage threshold of 1.5 kW ns1/2 /�m2 that is rou-inely seen in silica fibers. Note here that a 1 �m diam-ter focused spot has a volume of around 1 �m3, which isery roughly 9 orders of magnitude smaller than the coreolume of a LMA fiber. Thus, it seems quite possible thatLMA fiber contains defects which lower the damage

hreshold and which are not readily detected with 1 �m3

amples. A better understanding may open up for higheramage thresholds, when combined with improved fabri-ation techniques, e.g., with higher-purity materials.

The damage threshold of 1.5 kW ns1/2 /�m2 applies toulk damage, but similar values are possible also for theurface. However, the likelihood for surface defects andmpurities, e.g., from polishing compounds [140], is con-iderable, and this can lead to a much lower surface dam-ge threshold. We expect the standard end-capping tech-ique applied today to be used increasingly. This reduceshe power density at the facet. One seemingly attractiveption for effective end-capping is to “erase” the core athe end of the fiber by diffusion of dopants or collapsing ofoles in the case of air:glass microstructured fibers. Addi-ional benefits of end-capping include better thermalroperties, reduced feedback into the guided modes, thevoidance of a high-intensity standing-wave pattern athe facet, and simpler coating of end-facets in some con-gurations. The deposition of anti-reflection, high-eflection, and dichroic coatings is likely to increase in theuture. The benefits of this will be modest in some cases,ut this is nevertheless a natural future progression.A larger mode area is another possibility, which we will

iscuss further in conjunction with nonlinear degrada-ion. We note here that even with the lower damagehreshold of [60], i.e., 1.5 kW/�m2 for 1 ns pulses, theelf-focusing limit would be lower than the damage limitor mode areas above 3000 �m2. It is also worth pointing

ut that the requirements on the mode distribution foronlinearity and damage mitigation differ somewhat: formost) nonlinearities, it is the effective area that matters,hile for damage the peak modal intensity is more of a

oncern. It is possible to have very large effective areas,hile still reaching a high peak modal intensity, e.g., in

he case of a fiber operating on a HOM. Furthermore,ith MM fibers, the possibility of hot spots due to modal

nterference is an issue, either as a transient with signalsf low temporal coherence or as a more long-term effectith signals of higher temporal coherence. Mode-attened designs can be used to reduce the peak modal

ntensity [142], but these do remain susceptible to modalnterference.

The temporal stability of pulses is important, too. Thus,ulses with spikes (e.g., from a laser with a few longitu-inal modes lasing simultaneously) have a lower damagehreshold than pulses without spikes [140]. MOPAs arettractive in this regard since they can be seeded withell-controlled pulses; the desired pulse shape and con-

rol are easier to achieve with low power oscillators.

. Improved Nonlinearity Managementvoiding nonlinear effects in fiber is one of the most criti-al issues for many high power fiber laser systems. Forxample, SRS is a major limiting factor for current nano-econd MOPAs and will become an increasingly limitingactor in cw systems as power levels increase beyond 10W. SBS provides a limit to the maximum single-requency power that can be generated in a fiber MOPAnd thus will be a key issue in power scaling for coherenteam combination schemes. SPM is currently the primaryimiting factor for most ultrashort pulse systems. Tech-iques to manage nonlinear effects either through fiberesign or pulse shape control are thus essential.

. Mode Area Scalings mentioned previously, increasing the effective moderea within the fiber provides the most effective way ofinimizing the nonlinearity—reducing both the peak in-

ensities and the device length due to improved pump ab-orption (for fixed inner-cladding dimension). Maintain-ng mode quality as the core area is scaled up is ofrimary importance, and a number of strategies tochieve this have been pursued over the years. Figure 14llustrates various fibers relevant to this section. Initiallyfforts focused on increasing the core diameter while re-ucing the NA accordingly in order to maintain pureingle-mode operation [53]. However, as the NA is loweredhe guidance gets progressively weaker, meaning thatight is more readily lost from the core as the fiber is bent,nd this limits the minimum NA that can be used in prac-ice to 0.04–0.05. Assuming a NA of 0.05, single-mode op-ration is obtained for a core diameter of up to 16 �m andcorresponding effective area of around 200 �m2 for aavelength of 1.064 �m. Using this approach a tenfold or

o improvement in nonlinearity per unit length can be ob-ained relative to the core sizes typical of more conven-ional single-mode RE-doped fibers as used, for example,ithin telecommunications.Larger mode areas can be achieved by relaxing the

trict single-mode fiber requirements and working with

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tep-index fibers at much higher V-values, such that theber is itself capable of supporting MM operation, but en-ineering the fiber device such that is operates on just aingle transverse mode [143–146]. The lowest-order (LP01ode) is preferred in most instances, but there is growing

nterest in operating on a single HOM instead [147] sincehese can have interesting dispersion characteristics;nd, furthermore, they may be more robust to subsequentode-coupling due to perturbations than the fundamen-

al mode. This in turn suggests that larger mode areasan be used. A downside with the use of HOMs is that of-en they exhibit localized regions of high field intensitycross the core that can result in problems with damaget lower values of effective area than might otherwise bexpected. There is potential to avoid this by appropriatengineering of the HOM profile by control of the core re-ractive index profile and geometry, although little haseen done to date in this area.Single-mode operation of a MM core makes it necessary

o establish means to preferentially excite, amplify, and/orttenuate specific optical modes of the fiber while at theame time ensuring that scattering between modes as theignal propagates through the amplifying fiber is mini-ized [143]. Much work has focused on using differential-

ain by selective RE doping across the core [144,145],ifferential-bend-loss by controlled bending of the fiber146], and resonant coupling of higher-order bound coreodes to leaky modes (e.g., in chirally coupled core fibers

148]) to favor propagation of the fundamental mode overOMs. Using such approaches, a further order of magni-

ude or so improvement in the mode area relative to theow-NA SM case can be obtained in certain circumstances.urther improvements by a factor of 5–10 may be achiev-ble by exploiting microstructured fiber technology, inarticular endlessly single-mode designs to obtain ultra-ow NAs in rigid rod-type structures [149,150] (where theigidity is required to minimize mode-coupling and/or ex-ess bend-loss and mode distortion due to bending) or

ig. 14. Different LMA fiber designs for the mitigation of non-inearity. (a) Rod-type fiber incorporating a microstructuredair:glass) core and high-NA inner cladding [150]. (b) Air-coreBGF offering ultralow nonlinearity and high damage thresh-lds for beam delivery [161]. (c) Leakage channel fiber in whichhe leakage loss for high-order modes is arranged to be muchigher than that of the fundamental [151]. (d) Solid PBGF offer-

ng PM guidance (due to stress rods placed close to the core) andn-fiber distributed spectral filtering to shape gain profiles andliminate SRS [165].

eakage channel fibers where differential confinement orend-loss loss can be used to favor fundamental mode op-ration [151]. Using these approaches mode areas in theange 5000–10,000 �m2 at 1.06 �m have been reportedn the laboratory. It is however to be appreciated that con-rolling the refractive index of the core in RE-doped ver-ions of these fibers is extremely challenging. Often someorm of nano-structuring of the core [89,150] is required tovoid the doped regions of the glass (which are likely toave a refractive index higher than the surroundingilica), perturbing the underlying (weak) guidance mecha-ism intended to provide single-mode operation. Such re-ractive index perturbations can severely compromise theode area and ultimate mode quality that can be

chieved. Note that most of the largest mode areas re-orted in the literature are for passive fibers whereoping-induced refractive index perturbations, thermaltability, and transverse gain competition under satu-ated laser operation are not an issue. Exactly how farne can push these more exotic active fiber designs inractice will require further study and better quantifica-ion moving forward.

Another intriguing approach to LMAs worthy of men-ion is the use of gain guiding in an index antiguide [152],hich when pumped provides optical confinement in a

ingle gain-guided transverse mode. The basic principlesave been validated, and single-mode operation in very

arge cores has been reported. While physically interest-ng, the practicality of the approach remains to be dem-nstrated given the very high gains per unit lengtheeded (and the associated heat dissipation required fornything like high average power operation). Neverthe-ess further work in this area is worthwhile—particularlyn the context of pulsed systems.

While the progress in mode area scaling appears mostmpressive the issue of whether the approaches providinghe largest claimed mode area will ever be made practical,iven the concerns of robustness, mode-stability, manu-acturability, packaging, and reliability of pump couplingemain a concern. Certainly few if any commercial sys-ems operate with fibers providing mode areas in excess of000 �m2 at 1.06 �m. Significant further research intohe whole area of mode scaling with a particular focus onmproving practicality and high power operation iseeded; however at this point it is not clear that substan-ial further progress will be made in terms of single-modeingle-aperture emission, although clearly increasedode areas will be achievable by extending the concepts

escribed above to longer-wavelength transitions (i.e., oneight anticipate a factor of 2–4 increase moving from 1 to�m).It is however worth noting the possibility of multimode

nterference (MMI) operation. A tantalizing prospect ofMI devices is that they may offer a route to higher self-

ocusing limits in fibers with low modal dispersion, asell as larger core areas. MMI entails a large MM core

hat operates on several modes, but these are phased suchhat they interfere at the output into a free-spaceiffraction-limited beam or a single-mode beam of aingle-mode fiber. If mode-selective feedback is arrangede.g., when provided through a single-mode fiber), thishasing can be automatic, with the wavelength adjusting

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o minimize the cavity losses and thus maximize the cou-ling into the single-mode fiber [153]. It is also possible tose electronic control of the modal content and phases154], although this requires separate control for both po-arizations to work well, and this has yet to be demon-trated.

Tandem-pumping appears to have an interesting roleo play in mode area scaling, too. This is related to theigh excitation densities and gain that can build up in re-ions that are not well saturated by the desired mode,nd which can lead to mode instabilities and mode qualityegradation in LMA fibers. Tandem-pumping helps toontrol the excessive excitation and gain [155] since theaximum excitation and gain are much lower when

umping close to the emission wavelength. Note here thatsmall inner cladding helps to reduce the absorption

ength in this case, so even if one can get the right wave-ength with direct diode-pumping, the brightness woulde inadequate for close area ratio cladding-pumping. Asn example, Fig. 15(a) shows the Yb excitation level cal-ulated across a uniformly Yb-doped phosphosilicate coreor five different pump wavelengths. The fiber has a0 �m diameter step-index core with a NA of 0.06. Thesere typical parameters for low-NA large-core fibers thate fabricate. This core supports 12 LPmn modes at a sig-al wavelength of 1080 nm. The fiber is saturated by aignal at this wavelength propagating in the unperturbedundamental mode of the fiber. Furthermore, the pumpaveguide area is adjusted to yield a pump intensity that

eads to an average excitation level across the signalode of 3.0%, which is appropriate in phosphosilicate fi-

er for a signal wavelength of around 1070–1080 nm156]. Thus, the gain for the signal in the fundamentalode is the same for the curves in Fig. 15(a). However,

he gain for HOMs is much higher for shorter pump wave-engths due to the high excitation level at the edge of theore. Figure 15(b) shows the influence of the pump wave-ength on the gain of HOMs at the signal wavelength. Itlso shows the highest gain of any HOM at the gain-peakavelength, which varies with excitation level. The gain

s calculated from the excitation distributions in Fig.5(a), using a concentration of Yb ions of 2�1026 m−3.he gain obtained for pumps in the 910–980 nm range ex-eeds 6.2 dB/m for some HOMs at the gain-peak wave-ength. This gain is much higher than the 0.95 dB/m weet for the saturating fundamental mode signal. Such aigh gain difference is very problematic and can lead toarasitic lasing (e.g., in a catastrophic self-Q-switchingode) or otherwise self-saturation from ASE. Tandem-

umping at a longer wavelength resolves this inconsis-ency and facilitates further mode area scaling.

It is interesting to compare tandem-pumping close tohe emission wavelength to other options for controllinghe spatial gain distribution. In contrast to tandem-umping, these are based on controlling the spatial dis-ribution of either the pump or the dopant. One possibilitys to confine the RE ions to regions where the desired sig-al mode has a high intensity. In the case of a conven-ional Gaussian-like mode, the RE ions will then be con-ned to the central region of the core [144,145], which iselatively straightforward, but confined doping will beore difficult for modes with complex distributions such

s HOMs. Furthermore, modes can suffer distortion inent fibers [157], which complicates the use of confinedoping. It also reduces the pump absorption and signalain due to the reduction in the RE-doped volume. An-ther possibility is to confine the pump to regions wherehe intensity of the desired signal mode is high [158,159].his is most easily done with single-mode pumping andas also been demonstrated with HOMs [110]. However,ingle-mode pumping is a severe restriction for brightnessnhancement and power scaling. Furthermore, tandem-umping close to the emission wavelength makes the gainore uniform longitudinally as well, whereas the other

pproaches do not.We have seen above that mode area scaling is critical

or addressing damage and nonlinear degradation, buthis is limited by imperfect mode selection, mode-oupling, and gain saturation effects in active fibers. Inhe high power regime, thermally induced guiding can

ig. 15. (a) Fractional Yb excitation level for an ideal step-indexore with a diameter of 50 �m and a NA of 0.06. All signal pho-ons are in the fundamental mode, and all pump photons arevenly distributed across the fiber cross-section. The numbers ofump and signal photons are the same, spontaneous emission isegligible, and the pump area is adjusted for an average frac-ional excitation of 0.030 for the fundamental mode, correspond-ng to a signal gain of 0.95 dB/m. (b) Fundamental mode—LP01solid horizontal line) and HOM (dashed curves) gains at the sig-al wavelength, and highest gain of any HOM at any wavelengthlong-dashed curve) under different pumping wavelengths withladding area adjusted for the same fundamental mode signalain of 0.95 dB/m [155].

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lso limit the mode area scaling. Light propagation in fi-er lasers, as defined by the refractive index distribution,as been viewed to be largely immune to thermal effects.owever, at very high power levels and for laser transi-

ions with a large quantum defect, thermally inducedhanges in the refractive index profile can lead to strongerode confinement (i.e., for thermo-optic coefficientn /dT�0). This leads to lower thresholds for unwantedonlinear loss processes (SRS and SBS) and catastrophicamage for the fundamental mode. It can also lead to aegradation in beam quality by virtue of parasitic excita-ion of HOMs. The maximum thermal load per unitength that can be tolerated before thermal guiding be-omes an issue and is inversely proportional to the moderea [129]. Thus, in this limit, the ratio of the fiber lengtho the mode area will be fixed [160], which in turn implieshat a larger core can no longer be used to mitigate SRSnd other (third-order) nonlinearities. Under these as-umptions, if SRS is the most important nonlinearity, onean show that the maximum power PSRS-thermal becomes160]

PSRS-thermal = 4��L�Aeff� laser KsilicaGtot

Aco2� heat

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nd the effective area, �L /Aeff�opt. This becomes

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ith the parameters in [160], this ratio becomes.87 nm−1. This can be compared to the ratio of the Ray-eigh length to the beam area at the focus of a diffraction-imited Gaussian beam. This ratio becomes n /�L1.37 �m−1 for a laser wavelength of 1064 nm and a re-

ractive index n of 1.46. This is some 2800 times smallerhan �L /Aeff�opt. We conclude, therefore, that in this ulti-ate limit, a waveguide is necessary for realizing the ge-

metry that leads to the optimal balance between nonlin-ar (SRS) and thermal effects, as determined byundamental material parameters. Furthermore, mostopants produce a larger thermal load than Yb, whichtrengthens the importance of a fiber waveguide for thosether dopants.

The maximum power according to Eq. (8) is muchigher than what has been reached to date, and there aref course a number of additional constraints on the fiberaser power, e.g., available pump power, thermal coatingamage, and optical damage. Those constraints suggesthat a long fiber with a large core should be used. How-ver, the reduction in mode area in a bent fiber also placespractical upper limit on the core diameter, and it is clear

hat these additional constraints may well limit theower much below the value of 36.6 kW suggested by Eq.8). In particular, as discussed in [160], the core size re-uired to avoid optical damage at 36.6 kW may be pro-ibitively large.Equations (8) and (9) can be used in the pulsed regime,

s well, following slight modifications to account for theore severe nonlinear effects typical for pulsed devices atgiven average power. In the conventional SRS-limited

ase, the average power becomes

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Aco2� heat

dn

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hile the optimum value �L /Aeff�opt is given by

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��L

�Aco2� pulse� heat

dn

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Aeff� laser Ksilica gR, �11�

here �pulse is the pulse duty cycle. We see that the opti-al ratio �L /Aeff�opt is proportional to the square root of

he pulse duty cycle. Therefore, the difference betweenhe optimal waveguide and a diffracting geometry be-omes smaller for lower duty cycles and disappears com-letely for duty cycles of the order of 10−6.Single-frequency amplifiers are also limited by nonlin-

ar scattering, in this case SBS. Using Eq. (10) with theaman parameters replaced with the corresponding Bril-

ouin parameters, a maximum power limit of 1.86 kW wasound in [160]. The corresponding value of �L /Aeff�opt be-omes 178 �m−1, which is significantly smaller than foraman-limited cw case operation, but still much larger

han for a diffracting beam, which points to advantages ofwaveguide also for high power single-frequency ampli-

cation. We note finally that in-band tandem-pumpingas a role to play also in this context of ultimate powercalability, in that it helps to reduce the thermal load andhus allows for further power scaling (Eq. (8) above and160]).

. Reducing the Material Kerr Nonlinearitytepping back from the regime of very high average pow-rs where thermal limits need to be considered, the mostromising route to substantially increased nonlinear andamage thresholds for single-mode operation is to exploitollow-core fiber technology (PBGFs [161] and Kagome fi-ers [162]). Such fibers offer extremely low values of opti-al nonlinearity per unit length—as low as one thou-andth of those of conventional solid fibers by minimizinghe modal overlap with the glass structure. While this islearly of great interest for passive fiber delivery, for la-ers the obvious challenge is to find a way to achieve sig-ificant gain when the modal overlap with the solid re-ions of the fiber is so low. One possible solution could beo move away from RE doping and to use a gaseous gainedium within the fiber core. Indeed gas-based lasers

ased on Raman scattering in hydrogen have now beeneported (albeit pumped with high-brightness pulsedingle-mode lasers) [163]. We can only hope that as

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igher-brightness pump laser technology emerges, andump coupling technology for microstructured fibers im-roves, that new concepts and fiber designs better able torade off reduced nonlinearity for signal gain will emerge,nd that these will allow hollow-core fiber to find applica-ion in active high power laser devices. Note that the usef hollow-core fibers also allows light to be transmitted inegions in which conventional solid fibers are strongly ab-orbing, e.g., in the ultraviolet (UV) and mid-IR. This alsoas implications for external frequency conversionchemes in these spectral regions and provides importantaser delivery options across extended wavelengths.

. Mitigation of Stimulated Inelastic Nonlinear Scatteringy Fiber Designiven a certain mode area, fiber length, and desired

trength of elastic Kerr-based nonlinear interaction, it isften necessary to control the level of inelastic nonlinearcattering which is an undesirable parasitic effect inany instances. SRS suppression is an area in which pho-

onic bandgap concepts and antiresonant guiding phe-omena already look set to play a significant role. In this

nstance the fact that the PBGFs guide light only over aimited spectral bandwidth can be exploited to provide aarge distributed inline loss at the Raman Stokes shifthile presenting minimum loss at the signal wavelength.ere, solid PBGF variants can be used (allowing a solid

ore and thus incorporation of cladding-pumpable RE ions164,165]). Significant increases in the SBS threshold

ight in principle also be possible using such an approachith optimized fiber designs and material choices, al-

hough this is much more difficult because of the smalltokes shift and the strength of SBS and may more easilye implemented using inline blazed gratings [166]. Notehough, as already mentioned in Subsection 2.B, that sig-ificant work has also taken place these past years on de-eloping acoustically engineered fibers providing a mucheduced Brillouin response [47–51], and it is conceivablehat such SBS suppression concepts might ultimately beimultaneously incorporated in the design of SRS sup-ressed fibers to provide additional functionality and op-ortunities to control the relative strength of differentnd sometimes competitive nonlinear effects. Finally, ithould be mentioned that it is also possible to engineer in-and dispersion using PBGF effects which again is ex-remely important when managing the impact of opticalonlinearity on short optical pulses.

. Mode Area Scaling Using Multiple Apertures—Beamombinationespite the exciting prospects for further power scalingiscussed above, at some point the upper limit on powerrom a single aperture will be reached. Scaling outputower in cw or pulsed mode beyond this limit can bechieved via beam combination. Well-established featuresf active fibers such as low cost, modular design, reliabil-ty, high efficiency, good beam quality, and high controlnotably in MOPAs) are very attractive for beam combina-ion, and there has been good progress to date. Here, ourbjective is limited to outlining the many different beamombination schemes in a fiber context and discussing

heir scalability and suitability for different operating re-imes. For technical details, we refer to a recent journalssue on the topic [37].

Beam combination can be divided into two types, inco-erent and coherent beam combination, and can be ap-lied to multi-core fibers and multiple single-core fibers.here are furthermore two versions of incoherent beamombination, the so-called spatial beam combination andpectral (or wavelength) beam combination. There arelso two types of coherent beam combination distin-uished by the method of phase control, i.e., active or pas-ive. Polarization multiplexing is the simplest method forncoherent beam combination, but offers at best a factor of

increase in power (and brightness), so often ignored iniscussions about the pros and cons of the various beamombinations schemes. The usual division into operatingegimes applies as well, i.e., cw, high-energy pulses (nano-econds), and ultrashort pulses (femtoseconds to picosec-nds).

The potential benefits of beam combination are clear.irst, it provides a scalable route to increase the effectiveore area, thereby reducing nonlinear effects within theber elements of the system for a given combined field in-ensity or power. It also reduces the corresponding in-ber peak intensities which can be important from a dam-ge perspective depending on the operating regime.oreover, if the approach of separate fibers is used the

verall heat load can be distributed over a number of fi-ers easing thermal management. However, such benefitsome at a considerable cost in terms of complexity andtability of the system, and much more work will be re-uired to try and develop simpler and more practicaleans to affect beam combination.The simplest, and already quite widespread, beam com-

ination technique is spatial beam combination (some-imes referred to as beam stacking) in the cw regime. Theutputs from a number of spatially incoherent fiberources are combined into a single beam in a fiber com-iner or a free-space combiner, or are simply focused intocommon focal point. Fundamentally, this is not different

rom the diode stacking illustrated in Fig. 12, and it offersroute to virtually any desired power level albeit at the

xpense of poorer beam quality and a small reduction inrightness. This approach is used for industrial lasers upo 50 kW of output power [124], and even after combina-ion, the beam quality can still be sufficiently good for de-anding industrial and defense applications (e.g., short-

ange directed energy [167]).Spatial beam combination can also be used for

anoscale-scale high-energy pulses. In this case, the onlydditional complication is pulse timing. This suggestshat a multi-path MOPA configuration consisting of manyarallel high-energy amplifiers seeded by a commonulsed laser should be used. A pulse duration of 1 ns cor-esponds to 20 cm of fiber propagation, so the fiberengths need to be adjusted on this scale to synchronizeulses from all channels. For ultrashort pulses (say 1 ps),he length matching becomes more critical with a toler-nce of the order of 0.2 mm. At such tolerances, thermallynduced variations of the refractive index need to be con-idered. For example, with a fiber path of 10 m, a tem-erature change of 2 K is enough to change the optical

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ath length by 0.2 mm. Differences in temperature be-ween different arms on this scale are easy to envisage, sorecise control of the temperature or, more directly, of theptical path length is likely to be required for incoherentombination of picosecond-scale pulses amplified in sepa-ate fiber arms.

The path-length difference due to different fiberengths as well as to temperature differences may beasier to manage with a multi-core fiber, allowing the dif-erent channels to be amplified in different cores. The fi-er geometry and heat-sinking arrangement is importantor minimizing temperature differences between cores. Aibbon fiber with a linear array of cores should be attrac-ive for this purpose, but the thermal limit on averageower will obviously be lower than for an arrangementith separate fibers.Spectral beam combination is a more sophisticated in-

oherent technique, in which several beams of (typically)imilar beam quality but at different wavelengths areavelength-multiplexed into a single combined beamith beam quality similar to that for an individual (con-

tituent) beam. Thus, the spatial brightness can be in-reased significantly via this approach. The wavelengthsf the different emitters can either be determined by con-entional means (e.g., MOPAs seeded at the desiredavelengths) or the beam-combination arrangement cane located within a common external feedback cavity. Inhe latter case, the cavity automatically selects the cor-ect wavelength for each emitter.

The elements for combining (i.e., wavelength multi-lexing) are crucial for spectral beam combination.iffraction-gratings, VBGs, interference filters, and po-

arization combining followed by wavelength-dependentolarization rotation are popular multiplexing elements.ith a diffraction-grating, the beam combination can be

erformed in a single step with the incident angles of theifferent beams arranged such that all the diffractedeams emerge in a single direction. In addition, theeams to be combined need to be incident on the samepot on the grating and lie in a plane. This is a goodatch for ribbon fibers, although the number of cores,

nd therefore the scalability, will always be quite limitedn a single multi-core fiber configuration. The other com-ination approaches involve cascaded combination witheams arriving from different directions, so they seemetter suited for combination of conventional single-corebers.The upper limit on power attainable via this approach

epends on power handling and cascadability of the com-ining elements, obtainable spectral channel density,ower and linewidth achievable from the individual fibermitters, and the bandwidth over which they can operate.ooking forward, it seems safe to assume that in the cwegime, linewidths of a small fraction of a nanometerhould be achievable at the multi-kilowatt level and pos-ibly beyond from near-diffraction-limited Yb-doped fiberources. Therefore, it appears that the combining ele-ents rather than the fiber sources will determine howany fiber sources that can be combined and the ultimate

imit on power. This is particularly so when one considershat high power radiation can be generated with variousinewidths all the way from 1 to 2 �m, using a combina-

ion of Yb-, Er-, and Tm-doped fiber sources as well as Ra-an sources, and perhaps using a combination of course

nd fine wavelength multiplexing. Indeed, the wave-ength agility of fiber sources is a prime attraction forpectral beam combination.

Still, the power levels used to date appear to be far be-ow what the best multiplexing arrangements can handle.or example, a normal surface relief grating should allowver 100 Yb-doped fiber sources to be combined to a powerf 100 kW [168], and probably well beyond, limited by fi-ancial constraints rather than technical ones for theoreseeable future.

In the pulsed regime, spectral beam combination of in-ividually wavelength-locked lasers requires careful con-rol and synchronization of pulses from the constituent fi-er sources. This is a practical issue, but will becomencreasingly challenging to address as the number ofources to be combined increases and pulse durations de-rease. To date, only four channels have been wavelength-ultiplexed in the nanosecond regime [169]. It may be

etter to use a beam-combined multi-wavelength cw seedaser and a modulator to generate pulses followed byavelength-demultiplexed amplification and subsequent

pectral recombination. See reference [168] for a cw ex-mple of a similar configuration (albeit without interme-iate multiplexing and demultiplexing).As for beam stacking, combining of ultrashort pulses

ill be more challenging due to the required path-lengthatching. Furthermore, the bandwidth of ultrashort

ulses needs to be considered as well, given the limitedhannel bandwidth as well as spatial and/or temporal dis-ersion effects. Therefore spectral beam combinationooks less than ideal for ultrashort pulses.

Coherent beam combination is quite different in manyespects. In passive phasing, a number of gain-fiber armshare a common cavity, but in contrast to the intracavitypectral beam combination, the combining element is notavelength-selective, and the gain fibers all operate at

he same wavelength. Furthermore, instead of the gainrms operating independently as they do for spectraleam combination, the combining arrangement actuallyouples the gain arms to each other. The passively phasedaser array can then be understood to be operating on aingle super-mode of the multiple fibers. It is possible toesign the cavity so that it selects a desired super-modey ensuring that this has the lowest loss of all possibleuper-modes. This super-mode is then coupled to a single,ormally diffraction-limited, mode at the output of theavity. In the low power regime, a single-mode fiber cane used for this purpose, and an arrayed cavity can beormed by cascading conventional single-mode fused-fiberouplers [170]. At higher powers, non-waveguiding ap-roaches are needed, e.g., employing a MM fiber structure171], an external reflector setup in a Talbot or self-ourier configuration [172], or a Dammann grating [173].ome of these arrangements, notably the Talbot cavity,re also attractive for phasing of multiple cores in a singleber [174].In order to avoid excess loss for the desired (lowest-

oss) super-mode, the round-trip phases in all gain armsust be the same, bar for multiples of 2�. Herein lies the

hallenge with passive phasing, as this phase is not con-

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rolled. Nevertheless, due to a degree of spectral freedomf the phased array, the phase condition can largely beulfilled up to array sizes of approximately ten [175–177],s the array finds a wavelength that minimizes the loss ofhe super-mode. Furthermore, it is possible to design cavi-ies with relaxed sensitivity to phase errors [178], al-hough the power scalability of this approach, and indeedhe ability to achieve effective passive phasing, remainso be determined.

An array of the order of ten gain fibers, or cores, doesepresent significant scaling but is still quite limited. Weote however that in contrast to other combinationchemes, important aspects of the physics of passivehasing are not well understood, and there may be oppor-unities for utilizing optical nonlinearities [179,180], forxample, to phase-lock the arms and scale to larger ar-ays. However it should be appreciated that there are sev-ral nonlinear effects in a fiber with different temporalharacteristics, i.e., the Kerr nonlinearity, electrostric-ion, thermal effects, and gain-inversion effects accordingo the Kramers–Kronig (KK) relation. These nonlineari-ies have to be better understood if they are to be ex-loited for phase-locking. On the other hand, in a multi-ore fiber, conventional linear distributed couplingetween cores can also be used for phase-locking [179]. Toate passively phased fibers lasers with as many as 16ores have been demonstrated using this approach [174].

It is interesting also to consider to what extent a largeraser bandwidth (as determined by the gain medium andhe cavity) allows for a larger array due to the higherrobability of finding a wavelength with a small phase er-or. In fact, this relation is not very strong, allowing—rguably—only for logarithmic growth in the array size. Aetter approach may be to use a small array of perhapsve arms, restricted to a narrow wavelength range, andhen to spectrally beam-combine a number of such arraysor further power scaling.

As it comes to pulsing, although a passively phased ar-ay could be mode-locked in theory this would appearery difficult in practice. This would preclude ultrashortulse operation. Also nanosecond-scale operation wouldppear to be problematic due to the nonlinear phasehifts that occur at high peak powers. Intriguingly,hough, promising results have been reported, albeit withnly two channels [181]. A simple, passive, and scalableolution in this area would be very interesting since beamombining in the pulsed regime is generally quite chal-enging and complex.

Active phasing is the engineering approach to coherenteam combination. Here, the light from a single seedource is split and amplified in several arms, and then co-erently recombined. The combination can take place inome element such as a Dammann grating [182] or a self-maging waveguide [183]. Alternatively, the beams can betacked side by side in a phased array, which then forms aiffraction-limited beam (subject to fill-factor limitations)nd which can also be controlled (steered) to a degree. Inhis case, there is no discrete combining element. Activehasing does involve difficult challenges, but the phaseontrol problem appears to be solved (see, e.g., [184]). Thetability of the mounted array does remain an issue, buts being addressed and is not a long-term concern. Also

BS is being addressed: active phasing becomes easierith highly coherent sources, as this allows for longerath-length differences between arms of multiples of 2�.owever, high temporal coherence can lead to SBS, but

here has been very good progress in SBS suppressionnd power scaling of narrow-line sources in recent years,s discussed in Subsection 2.B.Active phasing is very attractive from a scalability

oint of view. Although stringent (e.g., � /20) phase controls required, this tolerance is independent of the number oflements.

In the pulsed regime, the high energies targeted foranosecond pulsed sources may well lead to difficult-to-ontrol variations in nonlinear phase shifts between dif-erent arms, although this will be less of an issue foronger pulses of, say, 100 ns duration. Even then, though,he change in KK-related change in the refractive index is

concern, and it clearly requires very precise control ofhe system to ensure that the nonlinearities in differentrms are balanced. Coherent combination of ultrashortulsed sources, although again requiring precise path-ength matching (i.e., the number of 2� phase differencesolerated is relatively small), could conceivably be simplern that the pulse energy is smaller and CPA can be used toeep the peak power down. Once again, a single multi-ore fiber is attractive for ultrashort pulses, in that itelps to keep the path-length differences small [185]. Asn illustration of the advantages that this can bring, con-ider the 16-core fiber laser, passively phased in a Talbotavity reported in [179]. This had an effective area ex-eeding 5000 �m2 for operation on the fundamentaluper-mode at 1.06 �m. Applying these approaches withltra-large mode area fiber designs, e.g., leakage channelbers [186], to allow operation at effective areas well be-ond 10,000 �m2 will be increasingly important movingorward—most particularly for pulsed laser systemshere managing the high peak powers is the primary is-

ue and thermal effects are secondary.To summarize, although it seems unlikely that all of

hese alternatives will be pursued successfully in the fu-ure, it is clear that some of them will be, providing dif-erent advantages for different applications, considerableiversity, and a greatly expanded range of options for theaser engineer to choose from. Although the requirementsn the gain fiber differ for different combination architec-ures, to reduce complexity it still looks attractive to de-elop standardized fiber building blocks that can be usedor several architectures.

. Nonlinear Mitigation through Pulse Shape Controlerr effects are ultrafast and therefore manifest them-

elves instantaneously across a pulse as it is amplified.onsequently, if one can accurately shape an opticalulse, it becomes possible in theory to control the evolu-ion and consequent impact of SPM. In this regard vari-us simple pulse forms are of interest—for example, rect-ngular pulses simply generate a uniform phase shiftcross their envelope under the influence of SPM, and thenly frequency-chirp (which compromises pulse quality) isenerated at the leading and trailing edges. If these edgesre sufficiently abrupt, then the relative impact of SPMan be modest, particularly if the chirped frequency com-

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onents can be filtered away at the system output. Para-olic shaped pulses generate linear chirps under the in-uence of SPM that can subsequently be removed usingispersive elements such as grating and prism pairs. Inrinciple, with adequate temporal control SPM can beitigated reasonably well in many instances of practical

nterest [187], leaving SRS as the primary nonlinearity toddress. However, even here, pulse shaping can be gain-ully used. For example, square optical pulses experienceniform SRS gain across their envelope which either cane exploited to ensure that maximum pulse energy is ex-racted from an amplifier before the onset of SRS [73], orndeed can be used as an efficient pulsed wavelength con-ersion mechanism [188].

A number of pulse shaping technologies are now readilyvailable, and in the coming decade we envisage increas-ng use of these in advanced fiber laser systems in ordero mitigate, or indeed exploit, fiber nonlinearity. In termsf fixed-response (and potentially low-cost) pulse shaperse highlight superstructured fiber Bragg grating [189]nd long period grating [190] technology which have beensed for many years in telecommunications applicationso perform complex pulse shaping functions (including theeneration of both square and parabolic pulses). In termsf adaptive filters we point to products such as the Wave-haper from Finisar Inc. based on liquid crystal technol-gy which provides for signal phase and amplitude con-rol with very good ��5 GHz� spectral resolution [191].O and conventional spatial light modulator based de-ices as used extensively with Ti:sapphire-based femto-econd laser systems are also becoming commerciallyvailable at relevant fiber laser wavelengths.We believe that the combination of pulse shaping along

ith nonlinearity/dispersion engineered fibers will offerany interesting device and system applications in the

orthcoming decade and will provide a rich opportunityor significant further innovation. Obviously, once devel-ped for single-core operation these concepts shouldeadily be extendable to the multi-aperture regime to al-ow further power scaling there also.

. Adaptive Fiber Laser Systemsith the emergence of adaptive pulse shaping technology

apable of operating over extended time scales from theemtosecond to effectively the cw regime and the relativeexibility and transparency of MOPAs, the possibility ofreating adaptive, yet practical and affordable, laser sys-ems capable of optimizing their output parameters touit a given application comes to mind. Indeed, consider-ble work on developing sources with pulse shapes opti-ized for particular applications, such as the color-arking of metals, has already been undertaken. We

elieve that this is just the tip of the iceberg and that thisill be an increasing theme of fiber laser research in the

oming decade. One can envisage intelligent self-learningasers, for example, receiving information back on cuttinguality, or speed, and automatically adapting their outputharacteristics over relatively wide operating ranges us-ng suitable learning algorithms to optimize applicationerformance. One can even envisage incorporating someorm of spatial mode optimization into the laser design—or example, some applications require ring type or flat-

op beams, which may be obtained by suitable mode-ngineering or post-processing of the fiber facet. The usef a wavelength selective mode converter at the fiber out-ut with wavelength tuning of the input seed laser mightven allow for a degree of dynamic control. The wholerea of wavelength agility at the seed laser coupled withavelength selective elements within a MOPA systempens up a host of new device opportunities.

. Other Wavelengthshe methods and concepts we have discussed so far arearticularly relevant for lasers that operate close to ther-al, nonlinear, and optical damage limits. However, an-

ther priority is to extend the wavelength coverage ofigh power fiber sources. The challenges here can be simi-

ar to those for Yb power scaling, but they can also beuite different; but regardless, it is apparent that im-roved operation on other laser transitions is sure to fol-ow in due course. Indeed, recent advances in power scal-ng of Tm-doped silica fibers to kilowatt power levels inhe �2 �m wavelength regime bear strong testimony tohis, and further advances are to be anticipated here.overage of much of the near infrared band from �0.9 to2.1 �m can be achieved using RE ion transitions in

ilica and non-silica fibers supplemented where necessaryy Raman shifting. The advent of silica fibers doped withismuth [192] (a so-called poor metal rather than a RE)as opened up the difficult-to-access wavelength regimerom �1140 to �1500 nm, complementing the spectralange available from RE-doped fibers. Power levels areuite modest at the moment [193], but the prospects forcaling to much higher power using in-band pumpingchemes based on the use of one or more Yb fiber pumpasers look promising. It is also worth mentioning therospects of other dopants, e.g., lead and chromium. Aange of dopants have been investigated extensively inhe past, with limited success, but advances in fabricationechniques such as nano-crystals [194] can be used toodify and control the local environment of the active

enters. In this way, all transitions demonstrated in crys-als can conceivably be realized in fibers as well.

Operation on laser transitions in the visible and mid-nfrared bands is much more challenging, and as a conse-uence both output power and spectral coverage areather limited. At wavelengths beyond �2.2 �m, the at-enuation coefficient for silica increases dramatically, andence other glass hosts (e.g., fluorides [195], tellurites196], and chalcogenides [197]) with superior infraredransmission are required. These glasses also benefitrom lower phonon energies than silica, but fabrication ofow-loss fibers (active and passive) is an obvious challenges is the fabrication of associated fiberized components.uch of the work on mid-IR fiber lasers has focused ono-doped ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) [198]nd Er-doped ZBLAN [199] lasers operating in 2.70–2.83nd 2.85–2.95 �m bands, respectively. Development ofhese lasers has been driven mainly by the prospect of aange of applications in the biomedical area exploiting thetrong absorption band in water at �3 �m. To date, theaximum output power reported for a cladding-pumpedr:ZBLAN fiber laser is 24 W for 166 W of diode pumpower at �975 nm [200]. The combination of low effi-

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iency, high thermal loading density, and the relativelyoor thermo-mechanical and optical damage properties ofhe mid-IR glass hosts compared to silica will make fur-her power scaling very challenging.

An alternative strategy for extending the range of op-rating wavelengths to the mid-IR is offered by the hybridber-bulk-laser architecture. In this approach, a highower fiber laser operating in the near-IR wavelength re-ime is used as a high-brightness pump source for a bulkrystal laser. There are a number of fiber-bulk-laser com-inations that can yield output in the mid-IR regime. Oneotable example is Tm- or Er-fiber-laser-pumped Cr:ZnSe201], which offers access to a broad range of operatingavelengths in the �2–3 �m band [202]. Also, a rela-

ively new material, Fe:ZnSe, offers laser emission in the.95–5.05 �m band [203]. As with many bulk laser con-gurations thermal loading and its deleterious effectsepresent a major challenge to power scaling, particularlyn laser configurations such as these where quantum de-ect heating is high.

In many respects the most straightforward way to ex-end the range of operating wavelengths to the visible andid-IR bands is via external nonlinear devices (i.e.,

econd-harmonic generators, optical parametric oscilla-ors and amplifiers, and both difference frequency andum generators). However, the pump requirements for ef-cient nonlinear conversion such as high power, narrow

inewidth, and single polarization are not always easy toatisfy simultaneously in a fiber geometry at high averageower and may result in significant added complexity tohe fiber system. Tailoring fiber designs and systems toeet these needs is very much an ongoing research activ-

ty, motivated by the prospect of access to virtually anyavelength in the UV-visible [204,205] and near mid-IR

206] wavelength regimes at high average power toomplement the spectral coverage offered by fiber lasershemselves in the near IR. Already, using this approachulsed Yb fiber MOPAs have been frequency-doubled toenerate over 80 W of green at 530 nm [207]. Increasingber laser power levels, spectral densities, and wave-

ength coverage for different pulse duration regimes isure to figure as a major area of research activity for theoreseeable future, yielding—via nonlinear frequencyonversion—light at wavelengths spanning the soft x-rayo terahertz regimes.

. CONCLUSIONSn conclusion, we have reviewed the current state of thert in fiber laser technology in relation to both parametricerformance and practical considerations such as thermalnd damage management. Our review has highlightedhat, in both continuous-wave (cw) and indeed manyorms of pulsed operation, the results for ytterbium fiberaser are close (within perhaps a factor of 2–3), of signifi-ant fundamental limits for single-mode operation usinghe existing technology. However, we note that plenty ofcope remains for translating knowledge and experiencedained here to high power lasers based on the less devel-ped Er-, Tm-, Ho-, and Nd-based transitions—indeed,iven the recent progress, it will be interesting to seehether the ultrahigh efficiencies obtainable in ytterbium

eans that this RE system will win out in terms ofchieving the highest possible power levels or whetherhe larger mode areas achievable in thulium will proveecisive despite the lower power efficiency.Looking forward to the next decade we have speculated

s to how developments in the existing underpinningump, fiber fabrication, and component technologies areikely to impact the development of fiber laser technologynd have given some personal thoughts as to how newnd emerging forms of fiber and new system architecturesnd concepts may extend the performance range and ap-licability of the fiber approach. Extending the mode sizehile obtaining robust performance is arguably the most

ritical issue in power scaling both pulsed and cw lasers,nd radical innovation will be required here if the rate ofower growth from single-aperture lasers enjoyed thisast decade is to be maintained long into the next.learly, single-modedness is not essential for all applica-

ions of high power lasers, and therefore more systematictudy and understanding of the power scaling limits ver-us trade-offs in terms of thermal/damage management,ode quality, and stability are also required for MM fi-

ers, not least in developing a better understanding of thempact of mode competition and relative modal excitationevels on pointing stability and damage thresholds. Inushing to the highest possible power levels, it is to be ap-reciated that ensuring long-term reliable operation isikely to be highly challenging, and it may well be that theractical limit for reliable commercial systems will al-ays be a significant factor down on the record levels set

n the laboratory. Hopefully, this down-rating will beinimized as greater understanding and appreciation of

ossible failure modes under high power operation of fi-ers (and associated components) are developed. Clearly,mprovement in the power handling of isolators, pumpombiners, gratings, optical coatings, and indeed the fi-ers themselves will be essential to the continued com-ercialization of the technology as power levels continue

o increase.In terms of pulsed laser performance improvements in

arge mode area fibers (for reduced device lengths, nonlin-arity, and susceptibility to damage) are likely to lead tourther increases in the pulse energies and peak powershat can be derived from fiber systems. 100 mJ-switched and multi-millijoule femtosecond systems areithin sight by exploiting emerging techniques for nonlin-arity and dispersion control. The use of adaptive pulsehaping techniques in fiber MOPA systems are also com-ng to the fore—promising designer pulse shapes at highverage powers for pulses with durations spanning theillisecond to femtosecond regimes. The possibility of us-

ng spatial mode shaping, of either diffraction-limitedeams or indeed heavily multimode (MM) beams, pro-ides further exciting possibilities. Initial results in thisirection are beginning to emerge using both bulk- andber-based mode shaping approaches. Combining bothemporal and spatial beam shaping in fiber MOPAs pro-ides perhaps one of the most intriguing and exciting op-ortunities for future research, leading to the prospect ofOPA-based laser sources providing adaptive control of

patio-temporal laser fields at high peak and averageowers. Combining such advanced lasers with suitable

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eedback and adaptive algorithms should lead to lasershat will self-optimize their outputs to suit particular endpplications—greatly enhancing process efficiency andontrol. Clearly many challenges lie ahead in attemptingo realize such concepts in practice; however opportuni-ies to make significant in-roads in this direction clearlyxist and will certainly be exploited in the coming decade.

Once the single-aperture power limits are reached thenhe only real option for further power scaling will be toook at beam combination. As we have reviewed, numer-us technical approaches exist including coherent or inco-erent, active or passive, and multi-core fibers or multipleingle-core fibers. One should expect substantial activityn these areas in the coming decade, and significantrogress should be anticipated in both cw and pulsed re-imes.

With options for improved wavelength coverage af-orded by new glass types and advances in external fre-uency conversion from the soft X-ray to terahertz re-imes, the future opportunities for fiber laser researchnd further commercialization appear very bright indeed.

CKNOWLEDGMENTShe authors are deeply indebted to their many colleagues,ollaborators, and sponsors for many useful discussionsnd interactions over the years which have undoubtedlyelped inform and shape this article. To acknowledgeach would be impossible in the space available; howevere would like to thank in particular Professor Davidayne, Professor David Hanna, Professor Michalis Zer-as, and Professor David Shepherd, and Dr. Jayantaahu, Dr. Yoonchan Jeong, Dr. Christophe Codemard, Dr.onathan Price, Dr. Andrew Malinowski, and Dr. Shaif-ullam for particularly helpful discussions during the timef preparation of this manuscript. In addition, we grate-ully acknowledge the United Kingdom Engineering andhysical Sciences Research Council (EPSRC) for sus-ained funding over many years for our work in the areaf fiber laser and amplifier technology.

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www.finisar-systems.com/download_53KDmt10500001-1050004-WaveShaper-Family-product-brief-RevB.pdf



high power fiber lasers current status and future perspectives [invited]

Science

nical digest series

cal digest series

pulse duty cycle

refractive index prole

volume bragg gratings

heat transfer coefcient

leakage channel bers

bulk grating compressor