ultrabroadband infrared solid-state lasers

690 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 11, NO. 3, MAY/JUNE 2005

Ultrabroadband Infrared Solid-State LasersEvgeni Sorokin, Sergey Naumov, and Irina T. Sorokina

(Invited Paper)

Abstract—Ultrabroadband infrared transition metal ion-dopedsolid-state lasers have come of age and are increasingly being usedin trace gas monitoring, remote sensing, telecommunications, oph-thalmology, and neurosurgery. Operating at room temperature,they are stable, versatile, and easy to handle successors to the colorcenter lasers. They are becoming the critical components in opticalfrequency standards, space-based remote sensing systems, and maysoon find application in femtochemistry and attosecond science.

The article reviews the principles and basic physics of thesetypes of lasers, which are distinguished by their ability to sup-port the shortest pulses down to single optical cycle durations andthe ultimately broad tuning ranges. The paper further reviews thestate of the art in the existing diode-pumped sources of broadlytunable continuous wave, and ultrashort pulsed radiation in theinfrared, and provides examples of their successful application tosupercontinuum generation, trace gas measurements, and ultra-sensitive intracavity spectroscopy. Developments in such lasers asCr:YAG, Cr:ZnSe, Cr:ZnS, as well as the recently proposed mixedCr:ZnSxSe1−x laser, are discussed in more detail. These lasersnearly continuously cover the infrared spectral region between 1.3and 3.1 µm.

The gain spectra of these lasers perfectly match and extendtoward the infrared spectra of such established ultrabroadbandlasers, operating at shorter wavelengths between ∼0.7–1.3 µm, asTi:sapphire, Cr:LiSAF/Cr:LiSGaF and Cr:forsterite. This opensup new opportunities for synthesis of single-cycle optical pulsesand frequency combs in the infrared.

Index Terms—Laser, mid-infrared, near-infrared, tunable.

I. INTRODUCTION

R EMARKABLE progress has been achieved in the lastdecade in ultrabroadband transition metal doped crys-

talline continuous wave (CW) and ultrashort pulse lasers, op-erating between 1–3 µm. The driving force for this progresswas rapidly developing application fields in environmental, en-gineering, medical, biological, and chemical sciences, and thestrong demand for compact and versatile sources in telecom-munications (around 1.3 and 1.5 µm) and molecular fingerprint(roughly between 2–5 µm) wavelength regions. Going to theinfrared region also brings an important advantage of rapidlydecreasing Raleigh scattering losses in propagation, which isimportant in telecommunications and for imaging techniquesin turbid media such as optical coherence tomography. Infraredultrabroadband lasers are becoming critical components in op-

Manuscript received December 3, 2004; revised April 19, 2005. This workwas supported by the Austrian Science Fund (projects P14704-TPH andF-016), by the Austrian Ministry of Science, by the Austrian-French exchangeprogram Amadeus, and by the Austrian-Italian exchange program.

The authors are with the Institut fur Photonik, Vienna University of Technol-ogy, A-1040 Vienna, Austria (e-mail: [email protected]).

Digital Object Identifier 10.1109/JSTQE.2005.850255

tical frequency standards, space-based remote sensing systems,femtochemistry, and may even find application in X ray gen-eration and attosecond science. Indeed, the cutoff frequency ofthe high harmonic generation scales as the inverse square oflaser frequency hνmax = Ip+3.17E2/4ω2 [1]. Using the in-frared source allows reaching the required XUV frequency withsignificantly lower intensity sources, as compared to currentlyavailable Ti:Sapphire-based systems, making the high harmonicgeneration closer to real life applications.

At the same time, there exist established technologies forgenerating tunable radiation and even few-cycle pulses in themid-infrared, using the frequency conversion techniques [2].To compete with the available techniques, the solid-state lasersshould be more convenient to operate, more environmentallystable, more efficient, simpler, and more compact in design. Inthis review, we therefore limit our consideration to the systemsthat operate at room temperature and allow direct diode pump-ing. The first part of the paper reviews the principles of ultra-broadband (otherwise called vibronic, which is an abbreviationfrom vibrational-electronic and refers to the phonon-broadenedelectronic transitions) solid-state infrared lasers. The latter aredistinguished by their ability to support the shortest, down tosingle optical cycle, pulse durations as well as ultimately broadsingle line tuning ranges. In particular, the paper discusses issueswhich are specific to infrared lasers: wavelength scaling rules,material issues, and nonlinear optical properties in the infrared.

The second part of the article focuses on the developmentsin such lasers as Cr4+:YAG, Cr2+:ZnSe, Cr2+:ZnS, as well asthe recently proposed mixed Cr2+:ZnSxSe1−x laser. The rapidadvances during the last decade in laser materials and semicon-ductor lasers have led to the development of the two directlydiode-pumped ultrabroadband lasers: Cr4+:YAG [3] operatingaround 1.5 µm, and Cr2+:ZnSe lasers [4], [5] for the 2–3 µmdomain.

In the Cr4+:YAG system, stable self starting generation offew optical cycle pulses and continuum generation spreadingover more than two octaves were achieved. A diode-pumpedCr4+:YAG femtosecond laser has also been demonstrated. Af-ter the Cr3+-doped lasers (LiSAF, LiSGaF, and LiCAF), thisis the second vibronic laser system for which diode-pumpedfemtosecond operation has been achieved.

In the Cr2+-ZnSe system, kW peak power picosecond pulsesand a tuning range of 1100-nm continuous wave regime weredemonstrated. Due to the broadest gain among all solid-statelaser materials, Cr2+-based lasers hold promise for the genera-tion of ultimately short pulses down to single optical cycle. Al-together, the lasers based on Cr3+, Cr4+, and Cr2+ active ions,together with the Tm3+-based lasers [6]–[9], continuously cover

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SOROKIN et al.: ULTRABROADBAND INFRARED SOLID-STATE LASERS 691

Fig. 1. Upper graph: emission cross section spectra of the infrared vibronic ac-tive media. Lower graph: demonstrated tuning ranges of lasers using broadbandcrystalline active media at room temperature. The logarithmic wavelength andfrequency scale, keeping the ∆λ/λ ratio constant, allows correct comparisonof the bandwidths in different spectral regions.

the whole infrared spectral region between ∼0.7–3.1 µm (seeFig. 1) with only a small gap between 1.65 and 1.75 µm. As anoutlook toward future developments, we present recent demon-stration of the first infrared nanocrystalline random lasers. Theseworks open a wide field of unstudied laser physics on thenanoscale, and provide potential for future broadband nanocrys-talline integrated laser devices. Finally, we describe examplesof successful application of these lasers to supercontinuum gen-eration, trace gas monitoring, and intracavity spectroscopy.

A. Historical Perspective

The first tunable continuous wave solid-state lasers weredemonstrated in 1963 [10]. It was a flash lamp pumpedNi2+:MgF2 laser operating around 1.6 µm. A year later, laseroperation of a Co2+:MgF2 and a Co2+:ZnF2 was realizedby the same authors in the wavelength range between 1.75–2.16 µm [11]. Operation of Ni2+ and Co2+ in several otherhosts has also been reported [12], [13]. However, all these lasersrequired cryogenic cooling for operation.

At approximately the same time the color center lasers wereinvented, representing a good alternative in the infrared region.They have low thresholds, relatively high output powers, andare smoothly and broadly tunable, covering the vast wavelengthranges from the visible to the midinfrared [14]. In the singlemode CW regime, color center lasers have extremely narrowspectral line widths; in the mode-locked regime they can pro-vide ultrashort pulses (down to <50 fs [15]). In the seventiesand eighties, the color center lasers, including commercial ones,

found a widespread use in various applications, especially inhigh resolution spectroscopy requiring high spectral or tempo-ral resolution, and in frequency standards. However, they alsorequired cryogenic cooling for CW operation and suffered fromrather low stability of color centers. The room temperature colorcenter lasers exist today in the visible and near-infrared regionsup to 1.3 µm [16], as well as from 1.4 to 1.71 µm [17]. They areall quenched at room temperature and operate only in pulsedmode, motivating scientists to look for alternative broadbandroom temperature lasers.

In the last decade, a new generation of broadband room tem-perature infrared solid-state lasers based on Cr4+ and Cr2+

active ions and operating at wavelengths beyond 1 µm showed agreat deal of promise as tunable infrared sources, and graduallybecome key players in solid-state infrared photonics [18]. Boththese active centers are tetrahedrally coordinated and representthe low crystalline field systems. The optical transitions in suchsystems result in broad vibronic emission bands, which are par-ticularly suitable for tunable and ultrashort-pulsed lasers. Lasersbased on these ions provide a typical tuning range of 20%–30%of the center frequency. Cr4+-doped crystals are suitable forbroadly tunable CW and femtosecond pulse operation in the nearinfrared region between ∼1.1–1.7 µm [19]–[21]. The history ofthe development of Cr4+-doped laser materials started simulta-neously with two active media: with Cr4+:MgSiO4 (forsterite)[26]–[28] operating between 1.1–1.37 µm, and Cr:YAG [22],[23] operating around between 1.35 and 1.65 µm. The rangeof demonstrated Cr4+-doped materials include various gar-nets (e.g., Y3Al5O12 or YAG) [22], Lu3Al5O12or LuAG [24],YSAG [25] etc.; silicates (e.g., Mg2SiO4 or forsterite [26], [27],Y2SiO5 [29], [30]); also germanites (e.g., Ca2GeO4 [31]), aker-manites (e.g., B2MgGe2O7 [32]), tephroite (Mn2SiO4), monti-cellite (MgCaSiO4) and diopside (MgCaSi2O6 [33]), melilites(e.g., Ca2Ga2SiO7 [34]); and apatites (e.g., Ca10 (PO4)6F2

[35]). For extensive reviews on these materials, the reader isreferred to [36] and [37].

Cr:YAG appears to have a greater potential for applicationsdue to better thermooptical and thermomechanical propertiesthan forsterite. More than 2 W in the CW regime and 600 mWin the mode-locked regime is accessible with Cr:YAG [38].The small signal figure of merit value of Cr:YAG crystals, de-fined as the ratio of attenuation at the maximum of absorp-tion to attenuation at the lasing wavelength, was measured tobe between 60 and 120 [3], whereas the same parameter doesnot usually exceed 20–30 in Cr:forsterite [39]. For these rea-sons, direct diode pumping of forsterite is much more difficultthan in Cr:YAG laser, where directly diode-pumped tunable andultrashort-pulsed sources, described in this paper, have beencreated [3], [40], [41].

However, when speaking about diode pumping, tunability,and ultrashort pulse generation, the highest interest is drawnto the new class of vibronic materials: Cr2+-doped II–VI com-pounds. Cr2+-doped zinc chalcogenides were first suggested inthe mid-nineties by a group of scientists at the Lawrence Liv-ermore Laboratory [42]–[44]. Shortly afterwards, similar Cr2+-doped doped cadmium chalcogenides based on CdSe [45], [46],as well as on CdTe and compounds [47], [48] were proposed.


Since that time, Cr2+-ions have been shown to operate beingincorporated into several binary and ternary II–VI compounds,including ZnSe, ZnS, ZnS1−xSex , CdSe, CdS, Cd1−xMnxTe,and Cd1−xZnxTe. In all these crystals, Cr2+ ions occupy lowcrystalline field tetrahedral sites coordinated by the heavy se-lenide, telluride, or sulphide anions. The low maximum phononfrequency in chalcogenides (compare: 240 cm−1 in ZnSe and850 cm−1 in YAG) leads to the decrease of the nonradiative de-cay rate and increase of the fluorescence quantum yield. At roomtemperature, the latter is close to unity in Cr:ZnSe and is quitehigh in other chalcogenide materials. This provides Cr:ZnSewith one of the lowest pump thresholds among vibronic lasers,and enables efficient room temperature operation. It is not a sur-prise, therefore, that in the last few years Cr:ZnSe has drawn alot of attention as a room-temperature broadly tunable CW laseroperating around 2.5 µm [49]–[51].

Summarizing, the advantages of the lasers based on Cr4+ andCr2+ active ions over other tunable laser systems based on colorcenter crystals or on various nonlinear frequency shifting tech-niques include high-power room-temperature operation retain-ing good beam quality and spectral linewidth, simplicity, stabil-ity, efficiency, and compactness, especially when being pumpedwith the diode lasers. Furthermore, the broad gain bandwidthand high nonlinear response of some laser crystals allows thegeneration of ultrashort pulses. For a review of available broad-band crystalline laser materials for few optical cycle generation,the reader is referred to [21]. Several reviews on Cr4+ [20], [36],[37] and Cr2+ lasers [52], [53] have been published in the lastfew years. These lasers are also suitable for shifting the wave-length further into infrared by using either parametric [2], [54],difference frequency mixing [55], or Raman [56] processes ininfrared transmitting nonlinear crystals.

II. THEORETICAL BACKGROUND

In this section, we describe the origins of the broadband gain,the laser threshold condition, the wavelength scaling rules, andother issues typical for lasers operating on broadband transitionsin the infrared.

A. Bandwidth and Nonradiative Decay

In order to use the term “bandwidth” correctly, we shall keepin mind that it scales with the central wavelength (frequency).For a correct comparison across the wavelength regions, we shalluse the relative bandwidth ∆λ/λ ≈ ∆ν/ν (see also Fig. 1).This value has also the advantage of being directly related to thenumber of optical cycles per ultrashort pulse that uses the givenbandwidth [21].

The broadband gain in the ion-doped solid-state media mayoriginate from different causes: from homogeneous and inho-mogeneous broadening, from overlapping of the spectral lines,and from the electron–phonon interaction. In the ions with op-tical electrons in the inner electronic shells, such as rare-earthions, the linewidths of the individual transitions between theStark sublevels are not very broad, typically of the order of 10cm−1. The broadband gain can be obtained in this case by inho-mogeneous broadening as in disordered crystals [57], [58], or

Fig. 2. Semiclassical configurational coordinate description of the vibronictransition, showing the broadband emission and absorption, excited state ab-sorption (ESA), and nonradiative decay processes.

glasses and fibers, which are beyond the scope of this work. Theinhomogeneous broadening allows reaching bandwidths up to100–300 cm−1. Certain rare earth ions, e.g., Tm3+ or Ho3+,which under the crystal field splitting posses many closelyspaced Stark sublevels, may exhibit strongly overlapping tran-sition bands that can allow continuous tuning over 100–200 nmaround 2 µm (∆λ/λ ∼ 0.1) [6]–[8], [59]–[64].

Yet the electron-phonon broadening of the ions with the op-tical electrons in the outer shell, such as transition-metal ionsor color centers, provides by far the largest bandwidths possi-ble, reaching thousands of cm−1 with ∆λ/λ ≤ 0.4 [21]. TheFrank-Condon diagram in Fig. 2 illustrates the origin of thisbroadening, showing the energy levels of the optical electrons.The energy parabolas correspond to the vibration in one of thenormal modes, represented by the configurational coordinate Q.Strong interaction of the optical electron in the outer shell withthe surrounding atoms results in different equilibrium positionsQ0 and Q′

0 in the lower and upper electronic states, i.e., theparabolas become shifted along the configurational coordinateQ. The electronic transitions (vertical arrows) in both directionsacquire the bandwidth that grows with increasing shift of theparabolas [65]–[67].

While a larger shift of the parabolas is favorable for thebroader gain bandwidth, it also increases a probability of thenonradiative decay channel (dashed arrow in Fig. 2), [65],[68]–[71]. This sets a natural limit to the maximum bandwidthwhich can be shown to be [21]

∆ν

ν0≤ 2.3

√kT

hν0(1)

where hν0 = E0 is the energy distance between the electroniclevels (Fig. 2). Equation (1) predicts that the maximum relativebandwidth of the vibronic transitions scales with wavelength asλ1/2, and this trend can indeed be observed [21]: the midinfraredtransitions in Cr:ZnSe and Cr:CdSe have the largest bandwidthof all known lasers, operating at room temperature.


Fig. 3. Absorption and emission spectra of the laser transition inTm3+:GdVO4 crystal, as well as the tuning range of a laser (open circles) [7].

In transition metal ion doped lasers, there are two mechanismsfor the nonradiative decay. The first is the above-mentioned ther-mally activated nonradiative decay, which is bandwidth depen-dent. The second mechanism is the direct multiphonon process,having the form

1τnr

= Wnr (p)(n(ω, T ) + 1)p (2)

where p = ∆E/hω is the number of phonons with frequency ωnecessary to bridge the energy gap, n(ω, T ) = (exp(hω/kT ) −1)−1 is the mean thermal occupancy in this vibrational mode,and Wnr (p) rapidly decreases with p [65]. The multiphononrelaxation does not depend on the bandwidth of the transition,but rather on the maximum phonon frequency of the lattice. Itsets a limit for obtaining a continuous-wave room-temperaturelaser operation from any laser transition at longer wavelengths.Even the narrowband fixed-wavelength lasers seldom operate atroom temperature beyond 3 µm.

The spectroscopy of transition-metal ions is well under-stood [36], [65]. Due to the very strong interaction with thesurrounding ions, the parameters of the electron states are to alarge extent defined by the crystal structure, so that now it ispossible to speak about “engineering” of the transition-metaldoped laser materials [67]. Another favorable feature of thepurely vibronic transition is that it is a four level type, becausethe phonon relaxation inside both states (waved arrows in Fig. 2)occurs very fast, on the subpicosecond timescale.

To complete the discussion, we note here that Tm3+- andHo3+-doped crystals may also exhibit the vibronic broadeningof their electronic transitions [9], [72], resulting in smooth gainspectra of up to 100 nm width (Fig. 3). The bandwidth of suchtransitions is, however, less than that of purely vibronic transi-tions, because of the shielding of the optical 4f electrons by theouter shells in rare earth ions.

B. Laser Operation and the “Bandwidth Toll”

Even the ideal laser medium (i.e., without lifetime quenching,ESA, and other loss mechanisms) requires a minimum pump in-

tensity to reach the threshold. It follows from the basic principlesthat for optical pumping, the minimum threshold intensity Ith

is proportional [18] to

Ith ∝ αloss

αpump

∆λ

λ0

1λ4

0

(3)

where λ0 is the central wavelength of the emission band, ∆λis the emission bandwidth, αpump is the absorption coefficientat the pump wavelength, and αloss is the loss coefficient at theemission wavelength that may also account for the ground-stateabsorption in quasi-three-level media. The ratio of the latter twois referred to as the figure of merit of the gain medium. Equation(3) assumes that there are no other losses in the cavity exceptthose in the gain medium.

It is clearly seen from (3) that the threshold increase is the“toll” paid for the broad bandwidth. As a result of the highthreshold intensity, CW operation of the broadband media couldnot be obtained under flashlamp pumping, and all successes withCW tunable lasers became possible only with laser pumping:dye lasers under an Ar+-laser pump [73], color center lasers un-der a Kr+ laser pump [74], the first solid-state Cr:GSGG laserunder a Kr+ laser pump [75], followed by Ti:sapphire under anAr+-laser pump [76]. For the same reason, only a few broadbandmaterials ever allowed diode-pumped operation: Nd:glass [77],Cr:LiSAF [78] and Cr:LiSGaF [79], [80], Cr:YAG [3], [40],[41], and recently Cr:ZnSe [4], [5], Cr:CdMnTe [123], andCr:ZnS [127].

Equation (3) reveals also an important wavelength depen-dence of the threshold. The factor λ4 in the denominator ofthe right-hand side of (3) means that the threshold rapidly de-creases for the longer wavelength transitions. This makes di-rect diode pumping of the infrared broadband materials fea-sible [3]–[5], [123], [127]. If we compare otherwise quitesimilar Ti:sapphire (∆λ/λ0 ∼ 0.3, λ0 = 780 nm) and Cr:ZnSe(∆λ/λ0 ∼ 0.37, λ0 = 2450 nm) [21], then the λ4

0 factor makesalmost two orders of magnitude difference in threshold intensity.

In real lasers, the threshold intensity is higher than the onepredicted by (3). First, one should take into account other lossesin the cavity, including the output coupling. Second, there mightexist a number of processes in the active medium itself, suchas nonradiative decay from the upper laser level, upconversion,and excited-state absorption (ESA). Taking these factors intoaccount, we may write the following expressions for absorbedthreshold pump intensity Ith and slope efficiency ηslope [21]:

Ipumpth = (T + L)

hνlas

σemτrad

λlas

λpump

τrad

τ

×(

1 − σlasESA

σem

)−1 (1 +

σpumpESA nth

σpumpabs (Nt − nth)

)

×[1 + cτcσ

lasGSA(Nt − nth)

](1 + αnthτ) (4)

ηslope = ηpumpλpump

λlas

T

T + L

×(

1 − σlasESA

σem

)(1 +

σpumpESA nth

σpumpabs (Nt − nth)

)−1

(5)


where Nt and nth represent the total concentration of active ionsand threshold population inversion, respectively. In these equa-tions, the contributions of different factors have been explicitlywritten out: the output coupler transmission T and other intra-cavity losses L (both logarithmic on a round trip); the Stokesquantum defect λlas/λpump; the fluorescence quantum yieldτ/τrad; and the pumping efficiency ηpump that accounts for thebeam overlap and pump absorption. The contributions of ESAat the laser and pump wavelengths are combined into the twoparentheses in the second row of both expressions, respectively.Finally, the third row of (4) accounts for the ground-state ab-sorption (GSA) and upconversion.

It can be seen from the above expressions that the influenceof these factors is different. While Stokes shift, losses, and ESAcause the threshold increase and impede the slope efficiency,the nonradiative decay, GSA, and upconversion affect only thethreshold. The influence of the upconversion scales with τ , sothat it is practically negligible for the short-living media, i.e.,for all materials considered in this paper. The other factors cannot be disregarded as they cause excess overheating and sig-nificantly influence the usable bandwidth of the ultrabroadbandmedia. Thus, the GSA (reabsorption) causes a narrowing of thegain spectrum on its blue side, as illustrated by Fig. 10.

The ESA is one of the most important limiting factors forthe broadband materials, especially for the vibronic transitions.Fig. 2 shows schematically the ESA process in a transition-metaldoped ion. The higher lying level, to which an ESA transitionmay occur, is in a general case also influenced by the crystallinefield, resulting in a shift of its equilibrium position Q′′

0 alongthe configurational coordinate Q. The ESA spectrum becomesvibronically broadened in much the same way as the main tran-sition. This results in the fact that the ESA, if not forbidden bythe selection rules, often overlaps with the emission spectrum ofthe ion. The ESA thus distorts the gain spectrum, or even com-pletely inhibits the laser operation. The ESA at laser wavelengthturns out to be an important factor that renders many promis-ing broadband materials with otherwise excellent parametersunusable [36].

C. Nonlinearity

One of the important features of ultrabroadband lasers is thepossibility of short-pulse operation. This is an extremely in-teresting and rewarding field of applications that is developingalong with improvements in the technique. Both the generationand the application of the ultrashort pulses rely on nonlinear-optical interactions that occur in the laser medium itself, andduring the propagation in various materials. Techniques likeKerr-lens mode locking (KLM), soliton-like pulse formation,and supercontinuum generation have been discovered and un-derstood on the basis of the visible and near-infrared ultrashort-pulsed lasers. They rely on the third order optical nonlinearity,commonly described by the nonlinear refractive index n2.

In principle, transfer of these techniques to the infrared isstraightforward. One should, however, take into account thewavelength scaling rules, analogous to those discussed abovefor the laser threshold and spectrum bandwidth.

For KLM, the characteristic value is the critical power forself-focusing [130], which is given by Pcrit ∝ λ2/nn2. For sta-ble and efficient KLM operation, the instantaneous power of thepulse should become comparable to this value. As the instanta-neous power of an ultrashort pulse with energy E and durationτ scales as E/τ ∝ E/λ, reaching a given ratio of Pcrit requiresan energy that scales as E ∝ λ3/n2.

If we consider a nonlinear pulse propagation in a fiber, thenthe phase shift is defined by the parameter γ = 2πn2/λAeff =2n2/λw2

0 [187], where w0 is the mode radius. Since the modesize also scales with the wavelength, we obtain the followingscaling rule: γ ∝ n2/λ3.

Both examples make it clear that the transfer of the estab-lished techniques further to the midinfrared range encountersrapidly decreasing effective nonlinearity of the media as fastas λ−3. This infrared challenge has to be taken into accountwhen designing a nonlinear optical device, as we shall see inSection V-A.

III. Cr4+:YAG LASER

Cr4+-doped materials were the first vibronic active media,which enabled ultrabroadband room temperature operation be-yond 1 µm. The strongly vibronic nature of the laser transitionin Cr4+-doped active media (∆λ/λ ∼ 0.20, making two opti-cal cycle pulses feasible), combined with the high nonlinearity(n2 is 6.2 · 10−16 cm2/W in YAG [88] and 6 · 10−16 cm2/Win forsterite [89]) made Cr:YAG and Cr:forsterite attractive forpassive mode-locking. The emission decays with a room tem-perature lifetime of 2–5 µs (∼3 µs in Cr:YAG, which is by afactor of 10 shorter than the corresponding radiative lifetime).

The main spectroscopic features, which distinguish the Cr4+

systems and determine their laser characteristics are: 1) multisiteand multivalent nature of chromium in the majority of the crys-tals; in garnets there are three tetrahedtral sites for Cr4+ ionsthat are distorted along the three orthogonal crystallographicaxes; as a result, only one third of the available tetrahedral Cr4+

ions can effectively participate in the lasing when the pump andthe laser waves are polarized along one of these axes; 2) high(of the order of 10−18 cm2) transition cross sections typical forthe noncentrosymmetric tetrahedral sites, enabling operation atvery low concentration levels of the active ions; 3) excited stateabsorption at the pump wavelength [85], [154], [155]; and 4)nonradiative decay resulting in the decreased quantum yield ofonly 14% in Cr:YAG at room temperature [153] and correspond-ingly high oscillation threshold.

As was outlined in Section I, despite the large variety of ex-isting Cr4+-doped materials, the emphasis of this section willbe made on the Cr4+:YAG crystal, on which the few optical cy-cle operation [38], [146] and ultrashort pulsed operation in thecompact diode-pumped setup [40], [41] have been obtained. Thecompact ultrafast lasers based on these crystals are attractive fora number of applications, mainly in telecommunications (e.g.,for testing fibers or in WDM techniques), but also for remotesensing and trace gas monitoring via overtone molecular spec-troscopy; optical coherence tomography; and precision infraredfrequency measurements.


A. Ultrashort-Pulsed Cr:YAG Lasers

During the last decade, passive mode locking of Cr:YAGlasers has been achieved using various techniques such asKerr-lens mode locking (KLM) [90]–[94], [96]–[99], regenera-tively initiated mode locking [91], synchronous pumping [93],and using a semiconductor saturable absorber (SESAM) [40],[100]–[104], [126]. High repetition rate (up to 4 GHz) femtosec-ond Cr4+:YAG lasers have also been demonstrated [104]–[107].Pulses as short as 43 fs at 200 mW of average power were ob-tained [96], [97]. The pulse duration was limited in Cr4+:YAGby the high third-order dispersion (TOD), which amounted to∼9000 fs3 [96]. Especially in case of the prism dispersion com-pensated Cr:YAG laser, the TOD is significant [38].

Developments in recent years in chirped mirror technology,and better understanding of the physics behind generation of ul-trashort pulses, have led to a significant shortening of the pulseduration down to 20 fs [146] at 400 mW of output power. Theseimpressive results were achieved using only double-chirped mir-rors for dispersion compensation [122] under Nd:YVO4 laserpumping. This pulse duration corresponds to about four opti-cal cycles. More recently, a self-starting KLM Yb-fiber laserpumped Cr4+:YAG laser was realized [38], generating neartransform limited five optical cycle (26 fs) pulses at 250–450 mW output power. The maximum average output power of55 fs pulses was 600 mW, which is the highest reported mode-locked output power for a Cr4+:YAG laser thus far. Both ap-proaches to self starting, pure KLM as well as a SESAM-assistedself-starting, were successfully implemented in the latter study.

It is worth noting that the SESAM-based devices allow the de-velopment of practical self starting sources of ultrashort pulses.The published SESAM designs for Cr:YAG lasers include In-GaAs quantum wells grown on the highly reflecting GaAs/AlAsBragg stacked mirror [149], and an InGaAs/InAlAs quantumwell saturable absorber bonded to a gold mirror [150]. TheSESAM with a gold reflector allowed the generation of 44 fspulses at 65 mW at 1520 nm [102], [103]. Similarly to [96], thepulse duration was limited by the TOD, which is significant andpractically unavoidable in the Cr:YAG femtosecond lasers usingthe prism dispersion compensation technique. Therefore, imple-mentation of the chirped mirrors with the TOD compensationfunction is necessary if one needs to obtain even shorter pulses.Recently, ultrabroadband saturable Bragg reflectors consistingof a GaAs/AlxOy Bragg mirror and an InGaAs/InP quantumwell were used to start 36-fs pulses in the chirped mirror com-pensated Cr:YAG laser [83].

An alternative approach to start femtosecond pulses in thechirped mirror controlled Cr:YAG laser used an ultra-broadbandhybrid dielectric-InGaAs-InP saturable mirror [38]. Recently,this approach allowed generation of the shortest pulses so farfor the SESAM initiated KLM Cr:YAG laser pulses (27 fs) ofonly five optical cycles in duration, as described in Section III-B.

B. SESAM-Mode-Locked Diode-Pumped Cr:YAG Laser

Until recently there existed only one directly diode-pumpedSESAM-mode-locked crystalline femtosecond laser, Cr:LiSAF(Cr:LiSGAF), operating around 0.8–0.9 µm [129], [144]. Since

the development in 1999 of the first directly diode-pumpedCr:YAG laser [3], there was a strong incentive to obtain ul-trashort pulses from this compact broadly tunable over 200-nmlaser [3], [152].

As mentioned above, few optical cycle pulses were obtainedfrom the solid-state [146] as well as fiber-laser-pumped [38]KLM Cr:YAG laser. Although KLM provides the shortest pulsedurations and has even been demonstrated to be self-starting[38], its main drawback is the necessity of a critical alignmentand the requirement of high intracavity power, which is gener-ally difficult to provide in directly diode-pumped lasers. There-fore, implementation of SESAM should greatly simplify con-struction of a directly diode-pumped femtosecond Cr:YAG laser.Along with diode pumping and implementation of SESAM, aconsiderable role is played by the prismless mirror-dispersion-control approach [129], based on chirped mirrors in makingresonator design really compact and stable.

To cope with the bandwidth issue of the SESAM, we imple-mented an alternative approach based on the hybrid InGaAs/InPbased semiconductor saturable absorber mirror combined withthe broadband dielectric mirror, as well as a set of ultrabroad-band chirped dielectric mirrors with the TOD compensationfunction operating in the 1350–1650-nm wavelength range.

As in the case of many other semiconductor nanostructureddevices, the development of infrared InP-based saturable ab-sorbers has been less rapid than the GaAs-based devices. Apartfrom the relative complexity of processing InP-based devices,the main reason for this is the difficulty in fabricating high re-flectivity semiconductor Bragg mirrors, lattice matched to InP.Because of the small refractive index difference between InP andlattice matched InGaAsP (or InGaAlAs), more than 50 pairs ofλ/4 layers are required to get a sufficiently high reflectivity.Such large number of relatively thick layers may lead to defectformation and presents both epitaxy and processing challenges.There is a need for alternative mirror technology that takesadvantage of the InGaAs/InP system while providing broad re-flection bandwidth. The latter can be provided by the dielectricmirror technology. The idea of hybrid technology was first suc-cessfully realized in VCSELs, and involved two dielectric topand bottom mirrors [148].

The InP-based absorber structure consists of the In0.53Ga0.47

As quantum well layer, absorbing at ∼1.55 µm and located be-tween the λ and λ/4 layers of InP. In the development of ourfirst structure, we followed the design suggested in [151]. TheInP/InGaAs/InP heterostructures were grown by low tempera-ture molecular beam epitaxy technique. The crystalline qualityof the grown surface was permanently controlled by the reflec-tive high energy electron diffraction technique, providing layeraccuracy of about ±1.5%. The active layers of the structurewere grown on the stop layer in the following sequence: the λlayer of InP, the 6-nm-thick In0.53Ga0.47As layer, and the λ/4layer of InP.

In order to use the quantum well on the base of InP/InGaAs/InP heterostructure as the mirror with saturable absorp-tion in the laser, the heterostructure was additionally coated bythe dielectric mirrors, the high reflector (HR) from one side, andthe antireflection pair (AR) from another side (Fig. 4). The HR


Fig. 4. The physical structure and electric field distribution in the SESAM.The InGaAs absorber layer is 6-nm thick and is located at the maximum of thestanding wave pattern. The AR layers are to the left, and the broadband HRmirror is to the right of the semiconductor structure.

coating of nine pairs of quarter wavelength Nb2O5/SiO2 wassputtered from the side of the λ/4 InP layer. The absorptioncharacteristics of the structure were further corrected via thedeposition of an antireflection coating. The broadband antire-flection coating, consisting of a HfO2/SiO2 stack, was sputteredonto the InP/InGaAs/InP heterostructure from the side of InPλ-layer. The overall reflectivity was more than 99.5% over the220-nm band at λ = 1500 nm. In the Cr:YAG laser, the second-order GDD is small in comparison with the TOD (Fig. 5). Thechirped mirrors must therefore provide a large amount of theTOD in a wide wavelength range. In addition, the chirped mir-rors must provide very low reflection and scattering losses be-cause of the low gain of Cr:YAG under diode pumping. Fig. 5shows the designed GDD and reflectivity curves of the mirrors.To comply with the above requirements, we used as many as 51layers with thicknesses varying from 150 to 960 nm. The GDDof the chirped mirrors is very sensitive to the accuracy of thelayer thicknesses, so that the final design is optimized to providethe lowest error sensitivity. The upper graph of Fig. 5 illustratesthe error sensitivity of the GDD: even with the technological er-ror of only 1 nm (less than 0.7% of the thinnest layer) the GDDerror can exceed 100 fs2. The overall thickness of the coatingwas 13.5 µm and the average physical penetration depth of lightinto the layer structure changed from ∼3 µm at 1300 nm to∼9 µm at 1700 nm. Using the standard electron beam coatingtechnique would have led to prohibitively high scattering lossesat such thick structures, and we used the ion assisted sputteringtechnique to keep the loss figure low.

The femtosecond laser experimental setup consists of the1-cm-long Brewster-cut Cr4+:YAG crystal absorbing more than60% of the pump radiation at 1 µm (2-cm-long crystal in caseof Yb-fiber laser pumping). The resonator included three highreflector (HR) mirrors with radii of curvature R = 100 mm, theSESAM mounted on a heat sink, two chirped mirrors (Fig. 6),and the output coupler, varying between 0.2%–0.5%. Using thediode-pumped Yb-fiber laser as a pump source and a SESAMas a starting mechanism for KLM mode locking, the laser pro-

Fig. 5. Design of the chirped mirrors and dispersion compensation of theCr:YAG laser. Upper graph: the calculated reflection curve and GDD range(assuming a 1 nm layer thickness error) of a single chirped mirror. Lower graph:the measured reflection curve for four bounces of the chirped mirrors, the roundtrip dispersion for YAG crystal, and the calculated total intracavity dispersion.

Fig. 6. Experimental setup of the KLM-and SESAM-mode-locked Cr4+laser.In the KLM setup, the laser is terminated by a flat high reflector at the right handside. To obtain SESAM mode locking, the SESAM with a focusing mirror isplaced instead, as shown in the inset. OC: output coupler. CM: chirped mirror.

vided stable near transform limited background free pulses withspectral width 120 nm and pulse duration of 27 fs (Fig. 7) at therepetition rate of 150 MHz and output power of 20 mW.

In case of direct diode pumping the laser diodes coupled bythe dichroic wavelength coupler were collimated by the twocylindrical lenses and focused into the crystal by a sphericallens with the 80-mm focal distance. The maximum pump powerincident on the crystal was 7 W. With the pump laser diodes, thesame laser provided stable background free pulses with spectralwidth of 41 nm and pulse duration 62 fs with the time-bandwidthproduct of 0.329 at 15-mW output power from a 0.2% outputcoupler. The advantages of SESAM mode locking over the pure


Fig. 7. The spectrum (upper graph) and the autocorrelation signal (lowergraph) of the ultrashort pulse. The pulse duration corresponds to the Fouriertransform of the spectrum, implying that the pulse is essentially chirp-free.

Kerr-lens mode locking are the reliable stable and clean trans-form limited pulses without a CW component. These resultscan undoubtedly be improved, given the higher quality of theSESAM absorption layers.

C. Directly Diode-Pumped KLM Cr:YAG Laser

While the SESAM mode-locked lasers represent a practicalway of generating self starting ultrashort pulses, it is the KLMtechnology that ultimately leads to the shortest few-cycle pulses.It is therefore interesting to look for ways to create the KLM-mode-locked diode-pumped Cr:YAG laser.

There have been a number of implementations of KLMCr:YAG lasers, using good quality beams of Nd- or Yb-dopedsources. The current state of the art includes sub-20-fs pulsegeneration [146], output power up to 600 mW [38], and 4 GHzpulse repetition rates [147]. It has yet to be demonstrated, how-ever, whether laser diodes with their lower beam quality wouldbe able to support the necessary intracavity power and modediscrimination.

Our experiments used a modification of the previously de-scribed laser setup where the short crystal was replaced by a

Fig. 8. Experimental setup of the KLM diode-pumped Cr4+:YAG laser design.Mirrors M1 and M2 are 100-mm radius of curvature (ROC) high-reflective (HR)mirrors, CM1 and CM2 are chirped mirrors, and OC is the output coupler withtransmission 0.2%.

20 mm long one. Besides higher absorption, this allowed adjust-ment of all the pump diodes, and the crystal. Careful adjustmentof the pump mode and crystal position is important in KLMsystems. With the shorter crystal, such adjustment resulted in asituation such that the diodes on both sides of the crystal endedup being imaged one upon another. With the low concentrationof Cr4+ ions and easy absorption saturation, the Cr:YAG crys-tal provided insufficient isolation, and one of the diodes wouldeventually be damaged. The longer crystal also made necessarymore bounces from the chirped mirrors, which did not causeproblems due to the very low losses of the sputtered coatings(Fig. 5). The final cavity setup with ten chirped mirror bounceson a round trip is shown in Fig. 8. The laser used 100-mm ra-dius of curvature focusing mirrors, and resonator arms 70 cmlong. To increase the intracavity power, a 0.2% output couplerwas used [41]. The laser was first optimized in CW operation,where it demonstrated output power of 110 mW at 8 W of powerincident on the crystal, the threshold pump power being about1 W. With the birefringent quartz plate, the CW laser wavelengthcould be tuned from 1412 to 1569 nm [Fig. 9(a)]. Subsequently,the longitudinal position of the crystal and the position of oneof the concave mirrors were adjusted to obtain strong modula-tion in the laser output. When adjusted for the KLM operation,the laser yielded 60 mW in the CW regime (multimode) and30 mW in the mode-locked regime. After starting by movingone of the end mirrors, the laser remained in the mode-lockedregime for at least 10 minutes without any purging or dust pro-tection. The pulse duration was 65 fs and the spectral width was39 nm (Fig. 9). In Fig. 9(a), one can see that the pulse spec-trum maximum is shifted by 70 nm to the red relative to theCW tuning curve maximum. Similar to the case of the Yb-fiberpumped Cr:YAG laser, this shift can be attributed to the jointaction of stimulated Raman scattering, third-order dispersion,and wavelength-dependent losses in the resonator [87].

The mode locking threshold was 6.5 W of the incident pumppower. The laser remained in single pulse regime up to 8.3 W ofpump power, and the average output power was 10 mW. Abovethis value, the pulse splits up into several pulses with equalamplitude, arbitrarily spread over the time span ∼10 ps. Theaverage output power in this regime was 30 mW with a pulseduration of 68 fs. According to our theoretical analysis, it isnecessary to increase the modulation depth and output couplingin order to widen the single pulse operation region [95]. As


Fig. 9. (a) Spectrum and (b) autocorrelation of a 65-fs pulse from diode-pumped KLM Cr:YAG laser. Tuning curve of CW laser is presented by dotted line.Output coupler transmission is shown in gray [41].

discussed above, this can be readily achieved using the narrowstripe pump diodes.

We have compared the mode locking performance of thediode-pumped KLM laser with the same cavity pumped by aYb-fiber laser. At the pump power of 8 W, we could obtain58-fs pulses with spectral width of 43 nm and 120 mW ofaverage output power. The reason for the lower efficiencyof the diode-pumped setup is the poor overlap of the cavitymode with the pump beam, especially in the tangential plane.Shortening the resonator arms to 50 cm allowed us to increasethe output power (135 mW in CW), but it was difficult to startthe mode locking because of the worse mode discrimination inthe saggital plane.

The Yb-fiber pumped system allowed improvement in thefollowing directions: 1) the output power can be increased us-ing a higher output coupler and 2) the pulse can be shortened byadjusting (reducing) the dispersion. In the diode-pumped sys-tem, using the output coupler with 0.5% transmission did notprovide significant improvement of the output power because ofthe high threshold, but resulted in lower intracavity power, mak-ing the starting of KLM impossible. In order to achieve shorterpulse duration, the intracavity dispersion was optimized usingdifferent numbers of reflections from the chirped mirrors. Un-der Yb-fiber laser pumping, the optimum amount of dispersionwas found to be 8 reflections from chirped mirrors (−250 fs2 at1530 nm). In this regime, the laser is close to the mode lockingstability region [84], and it requires relatively high pump ratesfor stable KLM pulses. Under diode pumping, the mode lock-ing with this dispersion could be initiated, but it was stable onlyfor a few seconds. A greater amount of second order dispersionincreases mode locking stability at the cost of longer pulse du-ration. We have found experimentally that ten reflections fromchirped mirrors (−400 fs2 at 1530 nm) provide the best stabilityin the diode-pumped regime.

It is interesting to note that the results obtained in the diode-pumped system are comparable to those demonstrated in thepreviously described SESAM-mode-locked system. However,the relatively low output power was caused by the losses inthe SESAM rather than by the pump overlap problems, andthe Yb-fiber pumping in the prismless configuration could notsignificantly improve the output power [40]. If low loss broad-band SESAMs are available, the corresponding diode-pumped

systems may seriously compete with the KLM systems, andalso at sub-50-fs pulse durations. Certain experiments can beperformed with the setup in its current state, as described inSection V-A.

IV. Cr2+-DOPED II–VI LASERS

One of the most important developments of the last decade inbroadband diode-pumped lasers was the discovery of the newclass of the Cr2+-doped zinc chalcogenides [42]–[44], and inparticular of Cr:ZnSe as a broadly tunable continuous wavelaser operating around 2.5 µm [49]–[51]. This is because thepreviously existing tunable midinfrared (mid-IR) lasers had ei-ther limited tunability, e.g., various Tm-doped lasers operat-ing around 2 µm or Er-doped lasers around 3 µm, or had tobe cryogenically cooled, e.g., a Co:MgF2 laser operating be-tween 1.6 and 2.4 µm. As has been discussed in Section I, themajority of the known vibronically broadened laser transitionsin the infrared are quenched at room temperature due to thehigh probability of multiphonon relaxation processes. The lowmaximum phonon frequency in chalcogenides leads to a de-crease of the nonradiative decay rate and an increase of fluores-cence quantum yield, enabling room temperature operation upto 3.4 µm. For a detailed account of all available chalcogenidelasers, refer to [18], [53]. Consideration of Cr2+-doped lasers inthis section will be limited to 1) the Cr2+:ZnSe and Cr2+:ZnSlasers since they have demonstrated the highest performancein room temperature, continuous wave, tunable, diode-pumped,and mode-locked regimes and 2) to the presentation of the novelCr2+:ZnSSe material.

The ultrabroad gain bandwidth of these new laser crystals,which is comparable and even exceeds that of Ti:sapphire(Fig. 1), allows ultrabroad continuous tuning spanning morethan 1000-nm wavelength range, as well as generation of the ulti-mately short pulses down to only one or two optical cycles. Suchpulses in the midinfrared spectral region can be used as uniquediagnostic tools for investigation of numerous transient pro-cesses on the femtosecond scale. The broadly tunable ultrashortpulsed lasers in this wavelength range are also attractive for suchapplications as remote sensing, environmental monitoring, mid-IR free space communications, and optical frequency standards,as well as optical coherence tomography [108], ophthalmology,


and dermatology in medicine [109], [158]. They can be also beused for pumping mid-IR OPOs to produce even longer wave-lengths [110].

A. Cr:ZnSe and Cr:ZnS Lasers

The remarkable properties of ultrabroadband Cr2+:ZnSecrystal, such as the high emission cross section σem = 1.3 ×10−18 cm2, the negligibly low ESA [4], the fairly good chemi-cal and mechanical stability, and the thermal conductivity closeto that of sapphire gives this material enormous potential as alaser medium for diode-pumped few-cycle mid-IR lasers andamplifiers. Cr2+:ZnSe, as well as other Cr2+-doped II–VI com-pounds, possess two important features: 1) the existence of thechemically stable divalent Cr2+dopant ions with no need forcharge compensation and 2) a tendency to crystallize in tetra-hedral coordination. As it was explained above, the tetrahedralcoordination results in high transition cross section, short ra-diative lifetime, and smaller crystal field splitting, placing thecentral wavelength further into the IR. A broad absorption bandin this crystal is centered around 1.8 µm [111]–[113], and theemission is located between 2–3.4 µm [113]–[117].

Since the first experimental demonstration [43], [44], thelaser performance of Cr:ZnSe has greatly improved, includ-ing demonstration of direct diode pumping [4], [118], [119],[123], active [120], [121] and passive [121], [206] mode lock-ing, continuous-wave operation spanning several hundreds ofnanometers in the mid-IR [44], [50] at close to the quantumlimit slope efficiency (>60% [4], [49]–[51], [118]) and powerlevels in CW regime in excess of 1.8 W [124] in TEM00 mode.In a pulsed regime, 18 W of output power at 30 W absorbedpower pumped by the Q-switched Tm:YALO laser at 7-kHzrepetition rate, as well as 65% slope efficiency (59% optical-to-optical efficiency), has been recently demonstrated [201]. In thisexperiment tunability between 2.1–2.85 µm was achieved at upto 10 W output power. Based on the analysis of the mechani-cal, thermal, spectroscopic, and laser properties of Cr:ZnSe, theoutput powers over 10 W in CW regime and several Watts inthe mode-locked or amplifier regime are envisioned.

Cr:ZnSe material is also suitable for the thin disk laser design.The lifetime quenching does not exceed 25% up to the concen-tration levels of 1× 1019 cm−3 [125], corresponding to >10cm−1 peak absorption coefficient, a typical figure for Yb:YAGthin disk lasers. In the pulsed mode, Schepler et al. have demon-strated in the thin disk configuration 4.2 W of output power at10 kHz repetition rate [203], [204]. The laser yielded up to 1.4 Win continuous-wave mode. The pump wavelength of 1.89 µmrequired relatively thick samples for good absorption. An opti-mized Cr:ZnSe thin disk laser design with reduced disk thick-ness and proper pump wavelength should be able to producemuch higher output power in a good transversal mode.

To this point, the following record parameters were alsodemonstrated from this laser: 1) the broadest tuning bandwidthof 1100 nm between 2000 and 3100 nm in CW regime (Fig. 10);2) narrow linewidth 600 MHz operation without any intracav-ity etalons [53] as well as single longitudinal mode operationwith 20 MHz linewidth, using intracavity etalons [202]; 3) 350

Fig. 10. Continuous wave tuning of a Cr:ZnSe laser, using a broadband mir-ror set (circles) and an infrared mirror set (squares). The effective gain curveis computed from the fluorescence and absorption cross sections at real ionconcentration Nt and threshold inversion nth.

nm tuning range at 65-mW output power in the diode-pumpedregime (450 nm in Cr:ZnS [106]); and 4), the active [120], [121]and passive mode locking [121] with pulses as short as 4 ps at80 mW and 400 mW of output power, respectively. The n2

value of 1.7 × 10−14 cm2/W at 1.8 µm in Cr:ZnSe is a factorof 50 higher than in Ti:sapphire [206], making the nonlinear-optical mechanisms of mode locking and pulse shorteningfeasible [145].

Very recently the first passively mode-locked Cr:ZnSe laserproducing 11-ps pulses using an InGaSb based SESAM wasrealized [205]. Further optimization of the dispersion compen-sation in this laser should lead to stable self starting femtosecondpulses. The symmetry properties of the Cr-doped chalcogenides(especially Cr:ZnS) produce the pronounced second-order non-linearity [53], which is absent in such crystals as Ti:sapphire.The interaction of the cascaded second-order nonlinearity withthe third-order nonlinearity is another interesting research fieldin connection with ultrashort-pulsed Cr:ZnSe and Cr:ZnS lasers.

The most impressive results have been obtained thus far usingthe Cr2+:ZnSe crystals. However, there exist other promisingCr2+-doped crystals. One of these is Cr2+:ZnS [43], [44]. Thiscrystal has remained less studied as a laser material due tothe lack of good optical quality single crystals. Having similarspectroscopic properties to Cr:ZnSe, Cr:ZnS crystal is knownto have a larger bandgap (compare 3.84 eV in ZnS and 2.83 eVin ZnSe [131]), better hardness, a higher thermal shock pa-rameter (compare 7.1 and 5.3 W/m1/2 in Cr:ZnS and Cr:ZnSe,respectively [43]), and the lower dn/dT (46 versus 70 ×10−6 K−1 [43]) than Cr:ZnSe. In addition, the temperaturequenching of the Cr:ZnS lifetime starts at lower temperaturesthan Cr:ZnSe, which might be a serious disadvantage, especiallyin CW applications. With proper cooling, however, the powerhandling capability of this material should be on par or betterthan that of Cr:ZnSe, making Cr:ZnS attractive for high powerapplications. Our experiments with equally doped Cr:ZnS andCr:ZnSe (e.g., with the same thermal load per unit length)showed that Cr:ZnS performed at least as well as Cr:ZnSe. Thepulsed laser operation of Cr:ZnS laser has first been reportedin [43], [44], and [132]. The spectroscopic study and the firstcontinuous wave operation was recently reported in [143].Using Er-fiber pumping up to 700 mW at room temperaturetunable over 700 nm (between 2.1–2.8 µm), CW operation was


Fig. 11. Comparison of the absorption (upper graph) and fluorescence (lowergraph) spectra of Cr:ZnSe, Cr:ZnS, and Cr:ZnSSe [141]. The emission spectraare corrected for the detector and spectrometer response.

demonstrated [127]. Tuning over 400 nm between 2250–2650nm in the directly diode-pumped configuration [127] as wellas an Er-fiber pumped CW microchip laser at 2320 nm wererecently demonstrated [133]. An advantage of Cr:ZnS is theshift of the absorption peak by about 100 nm to the blue(Fig. 11), allowing a convenient pumping of this material withavailable 1.6-µm telecommunications diodes [127].

Another important issue is broadening the operation rangeof the Cr2+-doped lasers, especially beyond 3-µm wavelength.This could be obtained by using other II–VI compounds with alarger lattice constant and hence lower crystalline field. Forexample, hosts such as CdSe [45], [46], CdTe [134], andCdMnTe [135]–[137] also allow room temperature operationwith Cr2+ ion. Tuning up to the record 3.4 µm in the pulsedregime was demonstrated in Cr:CdSe [138]. In this way, it makessense to also consider the mixed ternary and quaternary com-pounds that would provide both a control over the central wave-length and additional inhomogeneous broadening of the spec-trum, as will be shown in the next section.

Finally, maybe one of the most interesting advantages ofCr:ZnSe is the availability of technologically developed andlow cost polycrystalline material. The existing technologies ofproducing ceramic ZnSe, such as a chemical vapor deposition(CVD) method or the hot-press method of powders, result inhigh optical quality low cost substrates of arbitrary size. Re-cently, the first ceramic directly diode-pumped CW tunable andactively mode-locked laser has been developed [139], [142].With proper optimization, a directly diode-pumped femtosecond

TABLE IMAIN SPECTROSCOPICAL DATA OF Cr:ZnSSe IN COMPARISON

WITH Cr:ZnSe AND Cr:ZnS [141]

ceramic laser could be created. This would be a most practicalsource of few-cycle light pulses.

B. Laser Using a Solid Solution of Cr2+:ZnSe and Cr2+:ZnS

Having described the comparative merits of both Cr:ZnSeand Cr:ZnS laser crystals, we now consider the possibilitiesopened by the well known technology and crystal field designpossibilities of the ternary II–VI compounds. Our aim is thedevelopment, spectroscopic, and laser investigation of a novelmixed crystal, Cr2+:ZnSxSe1−x .

Undoped solid solutions of ZnSxSe1−x [140] are being usedas substrates for epitaxial growth of blue emitting diodes, aswell as active media for e-beam longitudinally pumped lasers.In this work, we realized diffusion doping of this crystal grownby seeded chemical vapor transport. Based on the Raman andinfrared absorption spectra, we determined the content of ZnSx

to be 0.42 (crystal composition ZnS0.42Se0.58). The 4× 3×1.5 mm crystal plate was placed in a clean quartz ampouletogether with metallic high purity Cr powder. The ampoule wasevacuated and sealed off. The doping was obtained by leavingthe ampoule in an oven at a temperature of about 825 ◦C fortwenty days.

The results of the absorption and room temperature lumines-cence measurements are given in Fig. 11 and summarized inTable I. As seen in Fig. 11, the high-quality absorption due topredominantly Cr2+ions could be obtained in this crystal withpeak absorption coefficient of 9.5 cm−1 in the maximum ofthe absorption band around 1.69 µm. The room-temperaturelifetime was measured to be 3.7 µs, which is close to the corre-sponding value measured in concentrated Cr2+:ZnS [127] andCr2+:ZnSe, as are the corresponding values for absorption andemission cross sections. However, emission bandwidth is no-ticeably broader than in Cr:ZnS or Cr:ZnSe and is peaked at thesame wavelength as in Cr:ZnSe. Thus, Cr2+:ZnSxSe1−x repre-sents an interesting alternative to pure selenides and sulphides.There are many reasons for studying these crystals, including:1) the possibility of optical and nonlinear property variation bychanging the chemical composition and lattice parameter of thecrystal—indeed, the bandgap of Cr2+:ZnSxSe1−x could be in-creased by ∼0.4 eV by varying x between 1–0.42, leading tothe decrease of the third-order nonlinearity and 2) the largerlattice parameter relative to Cr2+:ZnS leads to the shift of theemission spectrum by∼100 nm toward infrared (Fig. 11). At thesame time, the Cr2+ absorption peaks around 1.68 µm, allowingfor convenient pumping with the available Er-fiber and telecomdiode lasers around 1.6 µm [127]. The only disadvantage may be


Fig. 12. Output characteristics of the Cr:ZnSSE laser [86].

the higher maximum phonon frequency in this crystal (compare∼350 cm−1 in ZnS0.42Se0.58 [141] with ∼250 cm−1 in ZnSe).Similar to Cr2+:ZnS, this may lead to the more rapid onset ofnonradiative decay in this crystal relative to Cr2+:ZnSe.

In laser experiments, we used a 1-mm-thick polished plateof polycrystalline Cr:ZnS0.42Se0,58 in the conventional three-mirror configuration (for experimental details see, e.g., [127]).The sample absorbed ∼38% of the Er-fiber pump radiation at1.607 µm. Without additional cooling, the laser operated at roomtemperature in continuous wave regime, producing ∼30 mW ofoutput power at 3% output coupling with 170-mW thresholdpump power.

These results could be further improved using the Co:MgF2

laser at 1.67 µm. The laser output characteristics are given inFig. 12. The threshold was measured to be less than 80 mWof absorbed power at 2% output coupling. In the similar cavity,Cr:ZnS exhibits 210 mW and Cr:ZnSe a few tens of milliwattsthreshold. Without additional cooling, the laser operated at roomtemperature in the continuous wave regime, producing∼50 mWof output power with 4% output coupling at 600 mW of incidentpump power. This corresponds to slope efficiency of 24% withrespect to absorbed pump power, taking into account the pumpreflection from uncoated surfaces of the lens and the sample.The laser operated at three wavelengths simultaneously (2430,2455, and 2480 nm) due to the intracavity etalon effect, with2480 nm being the strongest laser line.

Using a dry fused silica Brewster prism as a tuning element,we were able to demonstrate tunability over ∼560 nm: from2099 to 2658 nm (Fig. 13). In order to provide a fair comparison,a tuning curve of the polycrystalline Cr:ZnSe sample in similarconditions is given. The tuning range of Cr:ZnS0.42Se0,58 sig-nificantly exceeds that of the Cr:ZnSe on the short wavelengthside. The long wavelength cutoff for both samples was due tothe water vapor absorption in the cavity, as shown by the airtransmission curve.

C. Cr2+:ZnSe and Cr2+:ZnS Random Nanolasers

As noted, the Cr2+-doped laser materials are characterizedby high gain and an intrinsically low lasing threshold, as wellas by such remarkable spectroscopic features as the absence of

Fig. 13. Tuning of the ceramic Cr:ZnSSe laser. For comparison, tuning curveof the ceramic cr:ZnSe is given [86].

excited state absorption and negligible nonradiative decay atroom temperature. It is, therefore, not a surprise that materialsuch as Cr2+:ZnSe successfully operates not only as a singlecrystal, but also in the ceramic form. The ceramic Cr2+:ZnSe hasbeen demonstrated in tunable, diode-pumped, and even mode-locked regimes [53], [194], [195]. The active media were ob-tained in this case by diffusion doping of metallic chromium intothe ceramic ZnSe. Along with several other techniques of pro-ducing ceramic ZnSe, the latter is often obtained by hot pressingthe micro- and nanocrystalline ZnSe powder. A somewhat odd(but not without good reason) question arises as to whether itwould be possible to get laser action from Cr2+:ZnSe, or anyother Cr2+-doped II–VI compound in the powder form.

Indeed, random powder lasers is a hot topic in modernphotonics research. For extensive reviews on this subject, see[188]–[190]. This type of laser has been extensively studiedsince the first proposal in 1966 by Ambartsumyan et al. [191]of lasers with nonresonant feedback, and demonstrated in ZnOby Nikitenko et al. [192] and in Nd3+:LaMoO4 by Markushevet al. [193]. During the last decade, a great variety of randompowder lasers have been developed, all of them emitting in theUV to near infrared wavelength range. Recently, we reportedthe first eye-safe midinfrared ion-doped semiconductor randomlasers based Cr2+:ZnSe [196] and Cr2+:ZnS [197] powders op-erating around 2.4 µm and 2.3 µm, correspondingly.

Experimentally, several samples of Cr:ZnSe and Cr:ZnS pow-ders made by mechanically grinding the Cr:ZnSe single crys-tal with a concentration of Cr2+ ions varying between 5 ×1018 cm−3 and 2 × 1019 cm−3 have been studied. The aver-age size of the nanoparticles in different samples ranged fromhundreds of nanometers to tens of micrometers (Fig. 14). Eachglass ampoule with the powder with inner diameter equal to 1mm and outer diameter equal to 1.5 mm was illuminated by 15-ns sub-millijoule pulses of an OPO near the absorption peak ofCr2+ at 1780 nm in the case of Cr:ZnSe powder, and at 1700 nmin the case of Cr:ZnS powder, in order to measure the lifetimedependence on pump pulse energy. In the spectral studies, thesamples were pumped at 1594 and 1570 nm, respectively. The


Fig. 14. A scanning electron microscope image of the Cr:ZnSe powder [196].

pumping beam was focused on the powder into a spot size be-tween 0.7–1.1 mm.

At pump energy flux comparable to the absorption saturationflux Jsat of bulk Cr:ZnSe and Cr:ZnS (i.e., between 0.3Jsat–0.5Jsat), we observe the dramatic shortening of the emissionlifetime (Fig. 15), the threshold-like behavior of emission in-tensity (Fig. 15), and the radical narrowing of the emissionspectrum at gain peak (Fig. 16). The maxima of the narrowedspectra at 2400 nm and 2300 nm correspond to the gain maximaof both crystals, and are shifted 100 nm from each other. Thespectral linewidth in Fig. 16 is limited by the resolution of theapparatus. To avoid leaking of the pump light, the spectra inFig. 16 have been recorded under shorter wavelength pumpingas compared to the data in Fig. 15. Due to the different absorp-tion, the apparent oscillation threshold energy density is not thesame in the two figures. Threshold pump energy density as lowas ∼20 mJ/cm2 could be observed in Cr:ZnSe and a factor of1.5 higher in Cr:ZnS, which corresponds to the higher thresh-old of Cr:ZnS in the bulk form [127]. An interesting feature ofthis new class of midinfrared random lasers is a remarkably lowthreshold. In fact, the threshold pump intensities in the powderand bulk samples are comparably low (4–6 kW/cm2 in powderversus 3–4 kW/cm2 in bulk samples), and significantly lowerthan in such undoped semiconductor random lasers as ZnO,where the threshold pump intensity Ith is ∼ 80 MW/cm2 [198].This makes the Cr2+-doped ZnSe random lasers very attractivefor real applications.

Finally, it should be noted that the stimulated emission inCr2+:ZnSe and Cr2+:ZnS powders is eye safe and eye safe-pumped. This opens a broad range of applications for mid-infrared random nanolasers in aero- and space technologies,marking and identification, search and rescue, etc. The demon-strated extremely low laser threshold in both lasers renders con-tinuous wave operation in these powder materials feasible. Re-cently, a sensitization of induced radiation in these crystals inthe presence of charge transfer processes [5], [53], [209] hasbeen observed, allowing to pump the upper laser level of Cr2+

Fig. 15. Upper graph: decay time dependence on pump energy in theCr2+:ZnSe powder (excitation wavelength 1780 nm, pump spot diameter 0.7mm). Lower graph: emission intensity versus pump energy for Cr2+:ZnSe andCr2+:ZnS (excitation wavelength 1780 nm, pump spot diameter 1.1 mm) [197].

through the charge transfer mechanism. This phenomenon opensthe way toward electrically pumped active ion-doped nanocrys-talline ZnSe lasers.

V. APPLICATIONS OF ULTRABROADBAND LASERS

In this section we consider some recent demonstrations uti-lizing the bandwidth capability of infrared vibronic lasers thathave potential to become established techniques for real-worldapplications. In particular, we shall describe low-threshold su-percontinuum generation, trace gas analysis, and high sensitivityspectroscopy. There also exist a number of other applications,such as optical coherence tomography [156], which will not bediscussed here, as they are adequately described elsewhere.

A. Supercontinuum Generation With Cr:YAG Laser

Infrared spectral supercontinuum (SC) generation is ofgreat interest for numerous applications such as frequencymetrology [170], femtosecond pulse phase stabilization [171],ultrashort pulse compression [172], optical coherence tomogra-phy [173], and high resolution spectroscopy [174]. In the near in-frared wavelength range around 800 nm, photonic crystal fibers(PCFs) [175] and tapered fibers [176] provide extremely broad


Fig. 16. Emission spectra of the Cr2+:ZnSe powder (upper graph) andCr2+:ZnS powder (lower graph). The experimental conditions and pump ener-gies are marked on the graphs [196], [197].

SC generation at low input energy (≤1 nJ). This is possibledue to the small fiber core areas and the proximity of the zerodispersion wavelength (ZDW) to the pulse central wavelength.Especially, the strong and controllable contribution of the holeystructure (for the PCFs) or of the surrounding air (for the ta-pered fibers) to the fiber dispersion [177], [178] allows precisetrimming of the fiber dispersion properties.

However, this approach to SC generation does not scale auto-matically with the wavelength. First, we should note the intrin-sic λ−3 dependence of the effective nonlinearity, as discussed inSection II-C. Second, the first ZDW in silica PCFs and taperedfibers typically lies around ∼800 nm [179]. For the infrared SCgeneration, it is therefore necessary to use the second ZDW offibers with core diameters of 1–1.5 µm [180]. In this case, how-ever, we encounter much higher third-order dispersion (TOD)of inverse sign (Fig. 18). As a result, one expects significantspectral restructuring, in particular, due to an unusual interplaybetween the TOD and the stimulated Raman scattering. Addi-tional potential problems are confinement loss and launch prob-lems resulting from the small core sizes ≤λ, and the need foran adiabatic transition region between single-mode and taperedsectors of the fiber [207].

A way to compensate for the intrinsic λ−3 dependence ofthe effective nonlinearity may be to use fibers with very lowdispersion (dispersion shifted fibers) that would allow longerinteraction lengths. Using conventional silica based fiber tech-nology with a GeO2-doped core, one can achieve values of thenonlinear coefficient γ = 21 W−1km−1 [181]. Such fibers with1–5 m length and with relatively low dispersion have been used

Fig. 17. Cross section of the SF6 fiber with 4.5-µm core size. The inset showsthe enlarged core area.

for supercontinuum generation with input energies of 500 pJ[182].

Another way would be to increase the effective nonlinear-ity by using the fiber material with high nonlinear index n2.Recently, fibers from SF6 glass with n2 value of 2.2 × 10−15

cm2/W have been manufactured [183] (Fig. 17). By varyingthe core size in the PCF structure it is possible to position thefirst ZDW in the 1–2-µm range. This allows optimization ofthe dispersion for the given pump pulse wavelength, and allowsutilizing an efficient spectral broadening due to so-called high-order soliton fission mechanism [184]. The high refractive indexn ≈ 1.76 gives a large core to cladding index gradient that pro-vides an efficient fiber-mode confinement in the infrared evenfor the small fiber core sizes, reducing the effective mode areaand increasing the nonlinear coefficient γ.

Experiments with the fibers of 2.5-µm core diameter havedemonstrated SC generation using an ultrashort pulse opticalparametric oscillator (375 pJ pulses with 100-fs width, SC span-ning 0.7–2 µm at −30 dB level) [183], the Er-fiber oscillator-amplifier system (octave-spanning SC excited by 200-pJ pulseswith 60 fs width) [185], and Cr4+:YAG laser (≈40-pJ 60-fspulses) [186].

At 1.5-µm wavelength, the calculated effective mode areas[187] increase from 3.7 to 9.4 µm2 for the lowest modes of thePCFs with 2.5- and 4.5-µm core sizes, respectively. With theseeffective mode areas, the nonlinear coefficient becomes γ = 280W−1km−1 and 98 W−1km−1 for the 2.5- and 4.5-µm cores,respectively. This is a significant improvement over the γ = 21W−1km−1 value for the highly nonlinear silica fiber [181].

The applicability of these fibers as structures for continuumgeneration depends not only on their effective nonlinear proper-ties, but also their dispersion, which has been calculated from thedependence of the effective indexes on the wavelength (Fig. 18).The dispersion characteristics of the PCF are significantly moreappropriate for SC generation in the IR range than those of thetapered fibers [207]. The decisive factor here is the small valueof the TOD that preserves spectral homogeneity and coherence.

Simulation of the pulse propagation in a nonlinear fiber ofthis type predicts [207] that there exist optimal propagationlengths for the smooth and broad spectrum of the pulse. By


Fig. 18. Group-delay dispersions for fundamental modes of the silica TFsand lowest order modes of the soft glass PCFs with different core sizes. Filledregions show the spectral range of Ti:sapphire and Cr:YAG lasers. Tangents tothe dispersion curves in the operational points visualize the TOD.

choosing the right propagation length, it is possible to obtaina practically flat 3-dB coherent spectrum (Figs. 19, 20). Thespectrum is a superposition of a broadened spectrum in azero-order mode and a fraction of power in a first-order mode.With only 35 mW of launched power, it is possible to obtaina spectrum that covers the 1.25–1.63-µm range (Fig. 19). Thealternative approach using the Raman broadening in a holeyfiber requires a 15-W Yb fiber amplifier to generate a flatspectrum in the 1.12–1.33-µm range [208].

B. Trace Gas Analysis

The monitoring and analysis of gases at low concentrationhas become an essential environmental, medical, industrial, andchemical issue. Numerous techniques and instruments focus ondifferent aspects of gas sensing such as sensitivity, selectiv-ity, multicomponent capability, and sample preparation require-ments. Optical measurements offer some unique advantageswith respect to these features. In particular, the optical mea-surements can be performed without direct contact, on a num-ber of components simultaneously, are specimen-independent,etc. To fully exploit these advantages, the laser source shoulddemonstrate broad tunability in the wavelength range of po-

tential specimens, sufficiently narrow oscillation linewidth forgood selectivity, and power for higher sensitivity.

The main absorption features of many relevant gases lie inthe midinfrared (2–15 µm) region with overtones and combi-nation vibrational-rotational bands in the near-IR (0.8–2 µm)spectral range. The atmospheric window between 2–5 µm is es-pecially interesting, because it is characterized by the presenceof the strong fundamental vibrational absorption lines of at-mospheric constituents, vapors and other gases. Those includewater vapor (H2O), filling the whole range between 2.5 and3 µm with maximum around 2.7 µm; carbon monoxide (CO)with strong features around 2.3–2.4 µm; carbon dioxide (CO2)absorbing around 2.7–2.8 µm; nitrous oxide (N2O), having sev-eral absorption features all through the 2–4-µm range; and manyother species. Detection of low concentrations of these and othermolecules, constituting air pollutants or greenhouse gases for thepurpose of environmental diagnostics or even the human breathfor the purpose of medical diagnostics, is currently done us-ing laser systems, which are based mainly on nonlinear opticalconversion techniques and include optical parametric oscilla-tors, difference frequency generators, and tunable semiconduc-tor lasers [157], [158]. Except for the quite complicated andexpensive optical parametric oscillators, these sources providelow power output of the order of 1 mW or less and a narrowtuning range.

A broadly tunable laser such as Cr:ZnSe, would provide aninteresting alternative with important advantages: operation di-rectly in the wavelength range of interest, and significant outputpower. For registration, we adopted the photoacoustic scheme,which is also widely specimen- and wavelength-independent[159], [160].

The source was the tunable Cr2+:ZnSe laser [50], pumpedby an Er3+-fibre laser at 1607 nm. The tuning is achieved by atandem of CaF2 or fused silica prisms by rotating the end mirror.With a single broadband mirror set, a wavelength range between2000–2937 nm (3405–5000 cm−1) can be covered (Fig. 10).With special infrared optics, it is possible to extend the tuningrange to 3100 nm (Fig. 10) and probably even farther, as the longwavelength cutoff is still defined by the mirror transmission.

The typical experimental results are presented in Fig. 21.Using the same source it was possible to perform measure-ments in two different wavelength regions: 2.3 and 2.9 µm.The laser output power of 100–500 mW is orders of magni-tude higher than the powers achieved with difference frequencygeneration in the same wavelength range. This allows sensitivemeasurement using even the relatively weak absorption linesin the spectrum. Using the certified gas mixtures, one can cali-brate the sensitivity of the whole setup and perform quantitativemeasurements [159]. The minimum detectable absorbance at apower level of 300 mW was 1.6 × 10−5. The resulting mini-mum detectable concentrations for a number of gases are plot-ted in Fig. 22, [159], [161]. For many important molecules,the detection limit lies well in the parts-per-billion (ppb)region.

The linewidth of the laser is also an important factor, as itdefines the selectivity of the technique. It was measured usingthe gas lines themselves and was found to be ∼0.2 cm−1 with


Fig. 19. Supercontinuum generation in the the SF6 fiber. (a) Dependence of the output spectrum on fiber length and (b) on the launched power.

Fig. 20. Smooth supercontinuum spectrum, generated by a Cr:YAG laser inthe SF6 fiber.

fused silica prisms, and ∼1.2 cm−1 with CaF2 prisms, dueto their lower dispersion [159]. This linewidth is dominatedby the thermal and mechanical instability of the setup used,because the short-term linewidth was measured to be less than0.02 cm−1 [53].

The Cr:ZnSe laser is thus a versatile laser source that can beused for trace gas measurements in the whole range between2–3.1 µm. The detection limits achieved allow multicomponentmeasurement both for monitoring and detection of trace speciesin ambient air and at working places.

C. Intracavity Laser Spectroscopy

Another method of ultrasensitive spectroscopic absorptionmeasurements, where the gain bandwidth plays a crucial role,is time resolved intracavity laser spectroscopy (ICLAS) [162].The highest sensitivity obtained in this method is due to thevery long effective propagation path (many kilometers) that can

be achieved at the initial stages of laser operation. At the sametime, the initially broad spectrum continues to narrow duringthe laser evolution, so that the spectral coverage and long ab-sorption path have to be traded off. It can be shown that the laserbandwidth at time t after the onset of oscillation follows the rule∆λ ∝ ∆λ0 × t−1/2 [162]; i.e., it is proportional to the initialbandwidth ∆λ0 of the gain spectrum. Reversing this relation,we may see that for a given final bandwidth, the effective absorp-tion path length scales quadratically with the initial bandwidth:labs = ct ∝ ∆λ2

0/∆λ2. The material bandwidth thus becomesa crucial parameter in intracavity spectroscopy.

Only a few experiments with ICLAS using broadband sourcesin the infrared were performed: atmospheric spectra were ob-tained in the 2636–2640 nm region using the KCl:Li FA (II)color center laser [163]; in the 2035–2055 nm region using theCo:MgF2 laser [164]; and in the 1770–1950 nm region using theTm-doped fiber laser [165]. Quite recently, the Cr:ZnSe laserhas been used for the intracavity spectroscopy in the 2410–2460nm region [166], and a Cr4+:YAG laser has been applied in the1350–1610 nm range [167].

However, the extreme sensitivity of the intracavity spec-troscopy becomes a problem if it is used in the infrared region,where air constituents possess main and combination absorp-tion lines. As a result, in most of the above quoted experiments,the spectra were completely oversaturated by the atmosphere,predominantly CO2 and water vapor.

Fig. 23, where only water lines are seen, illustrates this situ-ation. This is despite the fact that Cr4+:YAG laser was purgedfor 10 hours by dry nitrogen and then sealed, yet the spec-trum is still obscured by the water vapor. In this configuration,the experiment serves as a water vapor detector with a 0.2-ppb


Fig. 21. Gas absorption using the tunable Cr:ZnSe laser: Spectrum of methaneand carbon monoxide around 2.3 µm (upper graph) [159]; spectrum of CO2

and water around 2.9 µm (lower graph) [161]. All measurements performed atnormal pressure and room temperature.

Fig. 22. Minimum detectable concentrations for a number of gases usingthe photoacoustic registration. The horizontal bars show the tuning ranges ofCr:ZnSe and Cr:CdSe lasers.

detection limit. To avoid water absorption, one can make useof the large available bandwidth and tune the laser to anotherspectral region, e.g., by a pellicle. Fig. 24 presents three slicesfrom a time resolved spectrum, recorded around 6550 cm−1

(1.53 µm) with the cavity in the open air. Despite the muchlower absorption cross section, water vapor still dominates thespectrum, obscuring any other useful information.

Recently, another ICLAS experiment using a Cr:ZnSe laserin the 2.5-µm region under Er-fiber laser pumping has been

Fig. 23. Two time components (32- and 195-µs generation times) extractedfrom a time-resolved spectrum of the dynamics of a Cr4+:YAG laser, operated ina sealed box after 10 h of dry nitrogen purging. The apodized spectral resolutionis 0.07 cm−1. Due to the long effective absorption path length, the spectra looktotally saturated by the residual water vapor [167].

Fig. 24. Atmospheric absorption spectra around 1.53 µm at up to 28-km pathlength. The spectral resolution is 0.36 cm−1. Despite the lower water absorptionat this wavelength range, the water vapor lines still dominate [167].

performed [168]. To avoid water absorption, the whole laserexcept the pump source has been placed into a sealed box,which was evacuated (Fig. 25). The box was then filled with agas of interest at pressures between 0.1–70 mbar, to avoid thecollisional broadening. The recording and retrieval procedure,involving the high resolution Fourier-Transform spectrometer,has been described in [169].

Fig. 26 shows the laser gain curve and the reflection curvesof the mirrors in the cavity. A typical ICLAS spectrum of CO2

is shown, spanning over 100 nm bandwidth, at 2.6-km effectivepropagation distance.

Fig. 27 shows a portion of the CO2 spectrum, correspond-ing to 4.9 km of effective propagation distance. The effective


Fig. 25. Schematic diagram of the time-resolved FT spectrometer. The dashedrectangle shows the vacuum chamber [168].

Fig. 26. Fluorescence spectrum of Cr:ZnSe (gray) and spectral losses dueto the output coupler (OC) and five high reflectors (HRs) on a round trip. Atypical output spectrum at 8.9-µs delay (2.6-km effective propagation distance)is shown in black, with the CO2 absorption lines [168].

Fig. 27. Doppler limited spectrum of CO2 at 4.9-km propagation distance[168].

absorption is as high as 90% for the strongest lines, correspond-ing to a sensitivity of 6 × 10−8 cm−1. The explored spectraldomain is the location of the two weak vibration-rotation bands2ν3 − ν2 (Fig. 27) and 2ν3 + ν2 − 2ν2. The maximum absorp-tion of the line profiles reaches 100% with a pressure-absorptionpath condition equal to 66 mbar and 30 km, respectively. Itis worth noting that previously, these spectra could only bedetected by astronomic measurements in the atmosphere ofVenus [199], [200] which is 96.5% CO2.

Concluding this section, we note that ICLAS spectroscopy inthe infrared region can be successfully used for measuring anddetecting gas constituents with extreme sensitivity. The Cr2+-doped lasers such as ZnS, ZnSe, CdSe, etc., provide additionalflexibility, allowing one to set the observed region from 2.2 tobeyond 2.8 µm by selecting the proper material. In order tomake use of these possibilities and obtain useful information

in the infrared region, special care has to be taken to eliminatenatural constituents of the atmosphere, especially water vapor.

VI. CONCLUSION

Rapid advances during the last decade in laser materialsand semiconductor lasers have led to the development of thetwo major directly diode-pumped ultrabroadband and ultrashortpulsed lasers operating around 1.5 and 2.5 µm: Cr4+:YAG andCr2+:ZnSe lasers.

The Cr4+:YAG laser operates in the few-cycle ultrashort-pulsed regime and supports directly diode-pumped operation,tunable as well as mode-locked. In the diode-pumped regime,the laser generates 65-fs pulses at 30-mW output power. Com-bined with the novel highly nonlinear PCF fibers, this al-lows generation of smooth and coherent supercontinuum in the1–2-µm region with threshold energies of the order of 100 pJ.

The Cr2+-doped II–VI lasers represent a prospective fam-ily of lasers, which currently master applications such as highsensitivity and high resolution spectroscopy, trace gas moni-toring, and remote sensing. Their unsurpassed bandwidth ex-ceeding 1000 nm suggests a bright future for few-cycle pulseand frequency comb generation, optical standards, etc. More-over, similar upcoming materials in this wavelength region, e.g.,Cr2+:ZnSSe, promise even broader bandwidth for ultrashortpulse generation with extended tunability ranges.

ACKNOWLEDGMENT

The authors would like to express their sincere gratitudeto all colleagues and collaborators, without whose participa-tion this work would have been impossible: V. Kalashnikov(TU Vienna); A. Di Lieto and M. Tonelli (Pisa University);N. Kuleshov, V. Levchenko, and V. Scherbitsky (InternationalLaser Center Minsk); E. Vinogradov (Institute of Spectroscopy,Troitsk); A. Shestakov (JSC POLUS); R. Kanth Kumar andJ. Knight (University of Bath); and C. Fischer and M. Sigrist(ETH Zurich). Special thanks go to N. Picque and G. Guelachvili(University Paris Sud, Orsay), not only for their collabora-tion in the Cr:ZnSe ICLAS experiments, but also for com-municating their unpublished results on Cr:YAG intracavityspectroscopy.

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Evgeni Sorokin was born in Moscow, Russia, in1962. He received the M.S. degree in physics andmathematics from the Moscow M. V. LomonosovState University in 1986, and the Ph.D. degree intechnical physics from the Vienna University of Tech-nology, Vienna, Austria, in 1994 for spectroscopy andlaser properties of disordered crystals.

In 1986, he joined the research staff of the Gen-eral Physics Institute of the Russian Academy ofSciences, working on high-temperature Raman spec-troscopy of solids and melts, while also teaching at

the Moscow Physics and Technology Institute. Since 1992, he has been with

the Quantum Electronics and Laser Technology Group of the Vienna Universityof Technology, and since 1999, he has been an Assistant Professor with thePhotonics Institute of the same university. His current research interests includephysics of diode-pumped tunable and ultrashort-pulsed lasers based on novelrare-earth and transition-metal-doped crystals, as well as spectroscopic applica-tions of the ultrabroadband lasers.

Dr. Sorokin is a Member of the Optical Society of America.

Sergey Naumov was born in 1976 in Moscow,Russia. He received the M.Sc. degree in 1999 from theMoscow Institute of Physics and Technology (StateUniversity), Russia, and the Ph.D. degree from Vi-enna University of Technology, Vienna, Austria fordevelopment of the directly diode-pumped mode-locked Cr:YAG laser.

Currently he is doing his post doctoral research onhigh energy Ti:sapphire oscillator at the Max-Planck-Institute for Quantum Optics in Garching, Germany.

Irina T. Sorokina was born in Moscow, Russia, in1963. She received the M.S. degree in physics fromthe Moscow Lomonosov State University in 1986 andthe Ph.D. degree in laser physics from the GeneralPhysics Institute of the Russian Academy of Sciences(GPI), Moscow, in 1992. Her doctoral study was fo-cused on the electron excitation energy transfer pro-cesses from Cr to Nd, Tm and Ho ions in scandiumgarnet crystals. She obtained the Habilitation degreein laser technology and quantum electronics in 2003.

Since 1986, she has been a Research Staff Scien-tist at the GPI. In 1989, she spent a few months as a Visiting Scientist withthe Quantum Electronics Department of the Vienna University of Technology(TU Wien), Vienna, Austria. She is currently with the same department of theTU Wien, where she was a Lise-Meitner Fellow of the Austrian National Sci-ence Fund (FWF) from 1992 through 1994, and a Hertha-Firnberg AssistantProfessor from 1999 through 2003. Since 1999 she has been the Head of theSolid-State Lasers Group at the Photonics Institute, TU Wien. Her current re-search is focused on the development and characterization of the novel broadband crystalline lasers, femtosecond pulse generation, nonlinear optics, andphysics of interionic processes in laser crystals.

Dr. Sorokina chaired a number of international conferences. She is a mem-ber of the Austrian Physical Society, the Optical Society of America, and theAustrian Association of Scientists Wissenschaftsforum.

ultrabroadband infrared solid-state lasers

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