ieee journal of quantum electronics, vol. 48, no. 6, … · systems, yb:yag is the most studied...

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 6, JUNE 2012 827

Development of High EnergyDiode-Pumped Thick-Disk Yb:YAGChirped-Pulse-Amplification Lasers

Brendan A. Reagan, Alden H. Curtis, Keith A. Wernsing, Federico J. Furch, Bradley M. Luther,and Jorge J. Rocca, Fellow, IEEE

(Invited Paper)

Abstract— We discuss the results of work directed towardthe development of high energy (>1 J), high average power,diode-pumped picosecond lasers. The design and operation ofdiode-pumped chirped-pulse-amplification Yb:YAG lasers thatcombine either room temperature or cryogenically-cooled regen-erative amplifiers with cryo-cooled power amplifiers for supe-rior thermal performance and efficient energy extraction arediscussed. Results obtained using thick-disk amplifiers includethe generation of 100 mJ, 5-ps duration laser pulses at 100-Hzrepetition rate, and 1-J pulses of 8.5-ps duration at 10-Hzrepetition rate. The performance of the amplifiers in terms ofpulse energy and bandwidth under a variety of pump conditionis presented.

Index Terms— Diode-pumped lasers, solid-state lasers,ultrafast optics, Yb:YAG lasers.

I. INTRODUCTION

H IGH energy laser systems producing picosecond andfemtosecond pulses are key enabling tools for numerous

applications. Flashlamp pumped chirped-pulse-amplification(CPA) solid state lasers allow the generation of high energypulses with ultrahigh intensity [1-3]. However, the operationof flashlamp pumped high energy lasers is limited to relativelylow repetition rates by thermal issues. Applications such asthe generation of high average power soft x-ray laser radiation[4-6] and the efficient x-ray generation from plasmas cangreatly benefit from compact high energy ultrashort pulselasers that operate at increased repetition rates. Compactlaser-diode-pumped solid state laser systems capable ofproducing high energy pulses of picosecond duration are

Manuscript received November 22, 2011; revised March 9, 2012; acceptedMarch 14, 2012. Date of current version May 1, 2012. This work wassupported by the Engineering Research Centers Program of the NationalScience Foundation (NSF) under NSF Award EEC-0310717 and by previoussupport from NSF Grant 0521649.

B. A. Reagan, A. H. Curtis, K. A. Wernsing, B. M. Luther,and J. J. Rocca are with the National Science Foundation Engineer-ing Research Center for Extreme Ultraviolet Science and Technology,and also with the Department of Electrical and Computer Engineering,Colorado State University, Fort Collins, CO 80523 USA (e-mail: [email protected]; [email protected]; [email protected];[email protected]; [email protected]).

F. J. Furch is with the National Science Foundation Engineering ResearchCenter for Extreme Ultraviolet Science and Technology, and also with theDepartment of Physics, Colorado State University, Fort Collins, CO 80523USA (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JQE.2012.2191535

also of great interest for other applications that include thepumping of optical parametric chirped-pulse-amplification(OPCPA) systems for the generation of ultrashort, highintensity laser pulses [7-10].

Their potential for high average power, high efficiency,and compact size has motivated the development of severaldiode-pumped CPA laser systems in recent years. Most of theeffort has been directed toward Yb-doped materials becauseof their high quantum efficiency, long fluorescence lifetime,and absorption bands in the 900-1000nm spectral regioncovered by commercially available high power diode lasers.For high energy, high average power, diode- pumped CPA lasersystems, Yb:YAG is the most studied material, owing to itsexcellent thermal characteristics [11, 12]. Several laser systemsproducing pulses with more than 100 mJ energy at repetitionrates of 10 Hz or greater have recently been demonstrated[9, 13–17]. Several of these laser systems feature thin-diskroom temperature Yb:YAG amplifiers [13, 16, 18]. Theseinclude lasers that produced uncompressed pulses of morethan 150 mJ energy at 100 Hz repetition rate [13], 200 mJpulses of sub-2ps duration at 10 Hz [9], 25mJ pulses at 3kHzthat were compressed to 1.6 ps [18], and 20 mJ pulses at12.5 kHz that were compressed to 830 fs [10]. A numberof other Yb materials have been recently implemented inCPA amplifiers producing pulses with energies up to 427 mJbefore compression with the most promising being Yb:YLF[19–21], Yb:CaF2 [22, 23], and Yb:KYW [24]. These materi-als have a broader, flatter fluorescent spectrum than Yb:YAGmaking them attractive for sub-ps pulse durations. Howeverthese materials have lower thermal conductivities than YAGmaking the development of high average power amplifiersmore difficult.

Several groups, including ours, have taken advantage ofthe improved thermal characteristics, higher gain, and reducedsaturation fluence obtained when Yb:YAG is cooled to cryo-genic temperature [11, 12, 25–29] to develop diode pumpedCPA lasers [14, 15, 29–31]. These include an early demon-stration of a cryogenic Yb:YAG CPA in which a diodepumped cryogenically-cooled Yb:YAG regenerative amplifierproduced 8 mJ, 35ps duration pulses at 10 Hz repetitionrate [30]. We have reported an all-diode-pumped system withtwo cryogenic Yb:YAG amplification stages that is capable ofproducing 8.5ps, 1 J laser pulses at 10 Hz repetition rate [14].This system was used to successfully drive a soft x-ray laser

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828 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 6, JUNE 2012

TABLE I

COMPARISON OF Yb:YAG MATERIAL PROPERTIES AT AMBIENT AND

CRYOGENIC TEMPERATURE

Parameter 300 K 77 K Ref.Thermal conductivity (W/mK) 10 90 [12]Thermo-optic coefficient, dn/dT (10−6/K) 7.8 0.9∗ [11]Expansion coefficient, α (10−6/K) 6.14 1.95∗ [11]Peak stimulated emission cross section(10−20cm2)

2.1 11 [25]

Emission bandwidth (FWHM nm) 9 1.5 [25]Saturation fluence (J/cm2) 9.2 1.7 [25]The source for each value is shown in the last column.*Data reported for a temperature of 100 K.

at a wavelength of 18.9nm, the first demonstration of suchlaser with an all-diode-pumped laser system. Additionally,40 mJ pulses before compression were recently generatedat 2kHz repetition rate by a two stage cryogenic Yb:YAGlaser [31]. These pulses were compressed to 15ps durationand used to pump an OPCPA producing very short pulsesat 2.1 μm [7]. Laser systems have also been reported thatcombine a room temperature regenerative amplifier with acryogenically-cooled power amplifier. These include a systemdescribed in detail below, that produced 100 mJ pulses of4.8 ps duration at 100 Hz repetition rate [15], and a laser thatgenerated 23 mJ uncompressed pulses at 5 kHz [28]. A cryo-cooled amplifier that combined of Yb:YAG and Yb:GSAG forincreased bandwidth was also demonstrated to produce 12 mJpulses of 1.6 ps at 5 kHz [29].

II. CRYOGENIC Yb:YAG

Cryo-cooling improves nearly all thermal and laser proper-ties of Yb:YAG. Table 1 shows a comparison of the reportedspectroscopic and material properties of Yb:YAG at liquidnitrogen temperature and room temperature. Most impor-tantly for high average power lasers, the thermal conductivityincreases nearly an order of magnitude when cooled to cryo-genic temperature [12]. Other material parameters that affectthermal behavior, including the thermal expansion coefficientand the thermo-optic coefficient are improved by similarfactors [11]. Furthermore, at room temperature Yb:YAG is aquasi-3 level laser system with nearly 5% of the total Yb3+ions occupying the lower laser level. As a result, a considerableamount of pump energy is required to reach transparency inlarge volume amplifiers. As can be seen from these properties,from a purely thermal-performance point of view Yb:YAG isa significantly better high average power laser material whencooled cryogenic temperatures.

In addition, the emission and absorption spectra of Yb:YAGare significantly narrowed when cooled to liquid nitrogentemperature, leading to an increase of both the absorptionand stimulated emission cross sections [25, 32]. The increasedabsorption of the pump band near 940nm can be exploitedto allow efficient absorption in a single pass or a fewpasses, eliminating the need for complicated multipass pumpbeam setups in some amplifier geometries. More importantly,the increase of the stimulated emission cross section from2.1×10−20 cm2 to 11×10−20 cm2 [25] lowers the saturation

fluence by the same factor, allowing for efficient energyextraction at non-damaging laser fluence. However, a tradeoffis a reduction in bandwidth. As can be seen in Table 1, thebandwidth of the laser transition decreases to around 1.5 nmFWHM when the gain medium is cooled to liquid nitrogentemperature. While this is not a problem for CW or nanosec-ond laser systems, it limits the minimum achievable temporalpulse width for CPA laser systems. As a result the com-pressed pulse duration from all cryogenically cooled Yb-YAGamplifiers have been longer than 8ps FWHM [14, 30, 31],while a number of room temperature Yb:YAG systems haveproduced amplified pulses with duration in the range of1-4 ps [9, 13].

In this paper we discuss progress in the development ofan all-diode-pumped, high energy (>1J), high average powerCPA laser system. The demonstration of Yb:YAG amplifiersat both cryogenic and room temperature are presented. Thisincludes the combination of a room temperature regenerativeamplifier and a cryogenic multipass power amplifier thatproduces 140 mJ laser pulses at up to 100 Hz repetitionrate with high beam quality [15], that were compressed into100 mJ, 4.8 ps duration laser pulses. This hybrid temperatureconfiguration avoids excessive loss of bandwidth in the pre-amplifier where the largest amplification takes place, while stilltaking advantage of the higher gain and higher thermal para-meters of liquid nitrogen-cooled Yb:YAG in the high poweramplification stage. We also describe a cryogenic regenerativeamplifier operating at a temperature of 110 K that produces8mJ pulses at a repetition rate of 100 Hz. The pulses fromthis amplifier were compressed to 8.5 ps FWHM duration.The performance of these amplifiers in terms of compressiblepulsewidth, energy, and repetition rate are presented below inmore detail than we previously reported [15], for a varietyof pump conditions. Additionally, measurements of a highrepetition rate Joule-level cryogenic Yb:YAG amplifier arepresented that show the potential for the production of 1-2 J,picosecond laser pulses at high repetition rates.

III. ROOM TEMPERATURE AND CRYOGENIC

REGENERATIVE PREAMPLIFIERS

Fig. 1 shows a schematic diagram of the mode-lockedoscillator and stretcher that were used to seed the amplifiersdescribed in the following sections. The Yb:KYW oscillatoris pumped by a 30 W fiber-coupled 980nm laser diode.Similar oscillators have been previously demonstrated [33, 34].The oscillator is passively mode-locked by a 2% saturableloss SESAM and is dispersion-compensated by a homemadechirped mirror. The mirror was designed in a similar manner tothat reported in [35] and has a design group delay dispersionof −2000 fs2. This oscillator produces ∼20nJ pulses at arepetition frequency of 56 MHz with a 6nm FWHM bandwidthcentered at a wavelength of 1032nm. Some of the resultsdescribed here were obtained using an earlier version of theoscillator that was dispersion compensated by a prism pair wasused. However, this does not impact any of the results. Pulsesexiting the oscillator are stretched by a grating stretcher withan Offner telescope that is also shown in Fig. 1, and are used

REAGAN et al.: DEVELOPMENT OF HIGH ENERGY DIODE-PUMPED THICK-DISK Yb:YAG CPA LASERS 829

Fig. 1. Schematic diagram of the diode pumped, mode-locked Yb:KYWoscillator, and the grating stretcher used to produce the seed pulses.CM: −2000 fs2 GDD chirped mirror. OC: output coupler. SESAM: semicon-ductor saturable absorber mirror. LD: 30 Watt, 980-nm, laser diode coupledinto a 200-um optical fiber. TFP: thin film polarizer. FR: Faraday rotator.

Fig. 2. Schematic diagrams of the (a) room temperature and(b) cryogenically-cooled regenerative amplifiers. LD: 940-nm laser diodecoupled into a 600-um optical fiber. TFP: thin film polarizer. FR: Faradayrotator. λ/2: half waveplate. λ/4: quarter waveplate. PC: Pockels cell.

to seed Yb:YAG regenerative amplifiers. The grating stretcheris a 150 mm wide holographic diffraction grating with agroove frequency of 1714 mm−1, and the Offner telescope iscomposed of a 1000 mm radius concave mirror and a 500 mmconvex mirror spaced by 500 mm. The stretcher is double-passed with respect to the normal configuration (the beamis incident on the grating a total of 8 times). It produces astretch of ∼85 ps per Angstrom of bandwidth and supportsabout 2 nm of bandwidth near 1030 nm.

The layouts of the room temperature and cryogenic Yb:YAGregenerative amplifiers are illustrated in Fig. 2(a) and (b),

Fig. 3. Room temperature regenerative amplifier output pulse energy at100-Hz repetition rate. The three data sets are, for pump pulsewidths of, 1,1.5, and 2 ms respectively. The number of cavity roundtrips before the pulsewas ejected, was optimized for each data point.

respectively. The optical cavities of the two amplifiers aresimilar and are designed to have a cavity mode size of about700 μm FWHM at the location of the laser crystals. The modesize increases to ∼2 mm on the opposite end of the cavity toreduce the laser intensity on the Pockels cell, quarter waveplateand thin film polarizer that are used to couple pulses fromthe stretcher into the amplifier and to eject amplified pulsesout of the cavity. The laser crystal of the room temperatureamplifier is a 1 mm thick × 10 mm diameter 10%-at Yb:YAGused in the active mirror configuration. The crystal is cutwith a slight wedge between the faces of ∼1° to eliminateunwanted reflections in the cavity. The high reflecting face ofthe crystal is soldered to a water-cooled copper heat sink andthe temperature is held at 20 °C. The cavity is designed toallow for two double passes of the pulses through the activeregion of the crystal per cavity single pass, resulting in anincreased gain to loss ratio. The Yb:YAG amplifier crystalis pumped by a 90 W average power 940 nm laser diodearray coupled into a 600 um optical fiber. The laser diode ispulsed to produce square temporal pulses of variable durationand repetition rate. Pump light emerging from the fiber isimaged through a dichroic mirror onto the crystal by a pairf = 100 mm achromatic lenses, producing a relatively uniform600 μm spot. The optics couple the pump light into the crystalwith an efficiency of 65%. A thin film etalon is inserted intothe cavity to tune the laser center wavelength over a fewnanometers with a tolerable loss of energy. This allows usto slightly shift the wavelength to match that correspondingto the peak gain of the following amplification stage that iscryogenically-cooled. The combination of a Faraday rotatorand thin film polarizer is used to inject seed pulses and toextract amplified pulses which are collinear. The regenerativeamplifier is compact, occupying 1×0.3 meter2 of table space.

This amplifier produces pulses of up to 4 mJ energy at100 Hz repetition rate, and energies up to 1.5 mJ at 300 Hzrepetition rate. Fig. 3 shows the pulse energy obtained at


Fig. 4. Pulse energy obtained from the room temperature regenerativeamplifier as a function repetition rate for 0.75 and 1-ms duration 110 Wattpump pulses. The number of cavity roundtrips before the pulse was ejectedfrom the cavity, was optimized for each data point.

100 Hz repetition rate as a function of peak pump powerincident on the crystal. Results for pump pulse widths of1ms, 1.5ms, and 2ms are shown. The output pulse energywas limited to less than 4 mJ to avoid damage in the anti-reflection coated face of the crystal. For each pump power,the Pockels cell timing was adjusted to optimize the numberof cavity roundtrips allowed before the laser pulse is ejectedfrom the cavity. At the highest pulse energies the seed pulsemakes 16 roundtrips in the amplifier. As expected, shorterpump pulse durations result in more efficient amplification,with 1ms pulses having an optical to optical efficiency about50% higher than that obtained with 2ms duration pump pulses.The output pulse energy was very stable with a shot to shotstandard deviation of 27 μJ (0.7%) when the regenerativeamplifier was operated at the highest pulse energy. The outputpulse energy dependence on repetition rate is shown in Fig. 4.With 1ms duration 110W pump pulses, an energy of about 4mJis obtained at repetition rates up to about 100Hz. However,under these pump conditions, the pulse energy deterioratesalmost linearly with repetition rate above 100 Hz, resulting inan energy of 1.7 mJ at 300 Hz. A shorter pump duration of0.75 ms reduces the energy to 2.4 mJ at 100 Hz. However asimilar energy to that obtained with longer pump duration isobtained at 300 Hz repetition rate. The reason for the decaywith increasing repetition rate is most likely heating of thecrystal due to an imperfect thermal contact between the crystaland the heat sink. Fig. 5(a) shows the spectrum of amplifiedpulses exiting the room temperature regenerative amplifier at100 Hz with the etalon tuned to match the measured peak gainwavelength of the cryogenic second stage amplifier describedin the following section. Under these conditions, the pulseshave spectral bandwidth of 0.55 nm FWHM and a measuredtemporal duration of about 450 ps FWHM prior to com-pression. The pulses were compressed by a pair of dielectricgratings with 1740 mm-1 groove density to a duration of 3.6 psFWHM assuming a sech2 pulse shape, as illustrated by the

Fig. 5. (a) Spectrum of 3.6-mJ pulses exiting the room temperatureregenerative amplifier at 100-Hz repetition rate with the etalon tuned tomatch the cryogenic peak gain wavelength. These pulses have a bandwidth of0.55-nm FWHM and were compressed to 3.5-ps FWHM duration (sech2 fit).(b) Second harmonic generation autocorrelation of these pulses after com-pression. The solid curve is a sech2 fit to the data.

second harmonic generation (SHG) autocorrelation shown inFig. 5(b).

We have also developed a cryogenically cooled Yb:YAGregenerative amplifier, shown in Fig. 1(b). In this amplifier, a2 mm thick, Brewster-angle, 5%-at Yb:YAG crystal placed ina small evacuated chamber is cooled to cryogenic temperaturesby a closed-cycle helium cryostat. The temperature of thecrystal is adjusted by a small electric heater in the coldfinger to avoid excessive line narrowing. To obtain the resultspresented here, the cold finger containing the crystal wasmaintained at a temperature of about 110 K. The crystal ispumped by the same 940nm laser diode, however the pumpspot was reduced to ∼400 μm diameter to account for theincreased pump area caused by the angle of incidence. Thepump light enters the chamber through a dichroic mirror sealedto the chamber, and the laser pulses enter and exit the chamberthrough Brewster windows. The output pulse energy obtainedfrom this amplifier as a function of pump power is shown inFig. 6. The results correspond to a pump pulse duration of2 ms. As can be seen from this figure, ∼8mJ pulses were


Fig. 6. Pulse energy obtained from the cryogenic regenerative amplifier ofFig. 2(b) as a function of peak pump power for several repetition rates. Thepump pulsewidth was 2 ms.

Fig. 7. Second harmonic autocorrelation of compressed pulses from thecryogenic regenerative amplifier. The solid line is a least squares sech2 fit ofthe data. The pulses have a duration of 8.5-ps FWHM assuming a sech2 pulseshape.

obtained at repetition rates up to 100 Hz. The repetition rateof the amplifier was adjusted up to 100 Hz with virtually nochange in the beam quality or energy obtained. We have notattempted to operate the amplifier at higher repetition rates.The amplifier was very stable, with a shot to shot standarddeviation of less than 1%. Operation at higher pulse energieswas observed to result in sporadic optical damage to the lasercrystal and dichroic mirror.

A comparison of Figures 3 and 6 show that the higher gainof cryogenically-cooled Yb:YAG leads to 2x increase in theoutput pulse energy for equivalent pump energy in about halfof the number of trips through the gain material. However, thespectrum of the amplified pulses from the cryogenic amplifieris narrowed to about 0.2nm, which limits the compressedpulse duration to 8.5 ps FWHM, as can be seen from theautocorrelation shown in Fig. 7. This also reduces the stretchedpulse duration to about 185 ps FWHM. The spectrum iseven narrower when the amplifier is cooled to liquid nitrogen

Fig. 8. Schematic of the cryogenic multipass amplifier.

temperature. Operation at a temperature of 77K did not allowcompression to less than 14 ps FWHM.

IV. HIGH ENERGY CRYOGENIC THICK-DISK Yb:YAGAMPLIFIERS

Fig. 8 shows the schematic of a 4-pass power amplifierbased on liquid nitrogen cooled Yb:YAG. A 4.5 mm thick2%-at Yb:YAG crystal in the active mirror configuration is sol-dered to a liquid nitrogen-filled copper heat sink. All edges ofthe square profile crystal were optically bonded to a Cr4+:YAGcladding to prevent parasitic lasing. Cr:YAG is index matchedto Yb:YAG and has high absorption at the laser wavelengthpreventing any significant feedback of spontaneous emissionand parasitic lasing. This thick-disk geometry combined withthe high thermal conductivity at cryogenic temperature allowsfor excellent thermal behavior while limiting the transversegain. Furthermore, in contrast to thin disk amplifiers [13, 16,17, 24, 33], this thick disk active mirror geometry allowsfor nearly full absorption of the pump beam in a singledouble-pass, significantly simplifying the pump geometry. Thecrystal assembly is mounted in vacuum onto a homemadeliquid nitrogen Dewer with capacity of about 3 liters whichis sufficient for uninterrupted operation for several hours.The crystal is pumped through a dielectric multilayer-coatedwindow by a fiber-coupled laser diode capable of deliveringpowers of up to 500 W. The pump light exiting the 600 μmfiber is imaged into a ∼4 mm spot on the crystal by asingle 25.4 mm diameter, 35 mm focal length achromatic lens.Millijoule laser pulses from the room temperature regenerative


Fig. 9. Small signal single-pass gain of the cryogenically-cooled multipassamplifier as a function of peak pump power at 100-Hz repetition rate. Thepump pulse duration was 1.5 ms.

Fig. 10. Energy obtained from the cryogenically-cooled multipass amplifieras a function of peak pump power at 100-Hz repetition rate for pump pulsedurations of 1.5, 1.0, and 0.7ms. The amplifier was seeded with 2.4-mJ pulsesfrom the room temperature regenerative amplifier discussed above.

amplifier described above pass through a pair of crossed calcitepolarizers with a Pockels cell in between to isolate the twoamplification stages and remove small pre-pulses that leak outof the regenerative amplifier. Following this isolation the laserpulses make 4 passes through the laser crystal.

Fig. 9 shows that the small signal single pass gain at 100 Hzrepetition rate reaches a value of 4.5 at a pump power of400 W. Fig. 10 shows the measured output pulse energyobtained at this repetition rate as a function of peak pumppower for pump pulses of three different pulse durations:0.7ms, 1.0ms, and 1.5ms. In this measurement the seed pulsesfrom the first amplification stage had an energy of 2.4 mJ afterthe isolation polarizers. An output pulse energy of 140 mJ wasobtained at 100 Hz repetition rate with an average pump powerof 70 W resulting from 470 W peak power, 1.5 ms duration

Fig. 11. Energy from the cryogenically-cooled multipass amplifier as afunction of peak pump power for four different repetition rates. The pumppulse duration was 1.5 ms and the amplifier was seeded with 2.4-mJ pulsesfrom the room temperature regenerative amplifier discussed above.

-100 0 100 200 300 400 500200

250

300

350

400

450

500

Distance (mm)

Bea

m R

adiu

s (µ

m)

Mx2 = 1.012My2 = 1.054

(a)

(b)

Fig. 12. M2 measurement of pulses exiting the cryogenic amplifier at 100-Hzrepetition rate along with (a) far-field and (b) near-field images. This profilewas measured by focusing the beam exiting the amplifier with a 600-mmfocal length lens and measuring the 4σ beam width.

pump pulses. The amplifier is very stable with a shot to shotstandard deviation of 0.3% at 100Hz, which is slightly betterthan the shot to shot deviation of the regenerative amplifierseeding the amplifier, this is possible because of saturationin the second amplifier. At the maximum output energy theamplifier has an optical to optical efficiency of 20%, whichexceeds that reported for room temperature Yb:YAG ampli-fiers producing similar energies [9, 13]. Furthermore, higherefficiency could be achieved by increasing the fraction ofabsorbed pump power through double-passing the pump beamor increasing the Yb doping percentage. The well behavedthermal response of this amplifier can be seen in Fig. 11,which shows the output energy obtained as a function of peakpump power for repetition rates ranging from 10Hz-100Hz.At repetition rates up to 80 Hz an energy of about 152 mJis obtained, with no thermal effects noticeable. At 100Hzrepetition rate, there is a slight reduction of the amplifiedenergy to 140 mJ, which is due to a reduction of the gaincaused by localized heating. The amplified pulses have very


Fig. 13. (a) Spectrum of laser pulses exiting the cryogenic multipass amplifierat 100-Hz repetition rate. These pulses have a bandwidth of 0.35-nm FWHM.(b) Second harmonic autocorrelation of the pulses with a sech2 fit. Thesepulses have a 4.9-ps FWHM duration assuming a sech2 pulse shape.

good beam quality, as can be seen from Fig. 12, which showsnear-field and far-field images of the beam along with ameasurement of the M2 factor. The measurement for both axesyields values of M2<1.1. However the beam is slightly ovalresulting from the asymmetrical thermal and gain profile ofboth the first and second stage amplifiers.

The spectrum of the amplified pulses exiting the multipassamplifier is shown in Fig. 13(a). During amplification the spec-trum narrows to 0.35 nm FWHM due the narrow bandwidthof Yb:YAG at cryogenic temperature. The full energy pulses(140 mJ) were compressed with 72% efficiency into pulses of100 mJ energy with a 4.8ps FWHM duration (sech2). A SHGautocorrelation trace of the amplified pulses accompanied bya sech2 fit is shown in Fig. 13(b).

The output energy of the 100 mJ-level amplifierdiscussed above can be further increased to the joule-level byincorporating an additional amplification stage. We have previ-ously demonstrated 10 Hz operation of the first diode-pumpedCPA laser capable to generate 1 Joule picosecond pulses [14].

Fig. 14. Measured single-pass small signal gain of the Joule-level amplifieras a function of peak pump power for 10-, 30-, and 50-Hz repetition rates. Thepump pulse duration was 1.5 ms, and the pump beam diameter was 17 mm.

This compact system comprised two stages of amplification:the cryogenic regenerative amplifier described above, and aliquid nitrogen-cooled amplifier consisting of two 5.5 mmthick, 2%-at Yb:YAG active mirror crystals mounted on asingle liquid nitrogen-cooled finger. The crystals were pressedinto the copper heat sink with indium foil and each crystalwas pumped by a 3.5 kW peak power laser diode array.Six passes through each crystal resulted in the generationof 1.45 J laser pulses at 10 Hz repetition rate. The outputwas compressed to generate 1 J pulses of 8.5 ps FWHMduration. No thermal lensing or thermal depolarization wasobserved when the Yb:YAG amplifier was operated up to50 Hz repetition rate. However, at this increased repetitionrate the energy was significantly reduced due to lower gainresulting from localized heating. To improve the thermal inter-face we soldered the laser crystal directly to the copper heatsink. This significantly improved the crystal cooling as can beseen from Fig. 14 which shows the results of measurementsof single-pass small signal gain as a function of peak pumppower for a crystal at 77 K with a pump pulse duration of1.5 ms. The pump light was imaged into a fairly uniformspot 17 mm in diameter. A single-pass gain of greater than2.5 was achieved which corresponds to an energy storageof about 2 J. The gain of the amplifier is observed to beinsensitive to the repetition rate for frequencies up to 50 Hz.However, at cryogenic temperatures, the mismatch in thermalexpansion between the copper heat sink and the Yb:YAGlaser crystal over the large area necessary to produce highenergy pulses degraded the laser beam quality. Reducing thethermal deformation of the laser crystal by using crystal mountmaterials with more similar thermal expansion coefficients toYAG, such as CuW or invar, could allow the generation ofJoule-level pulses at greater than 50 Hz repetition rate.

The 1 J picosecond pulses produced by the CPA Yb-YAG laser system operating at 10 Hz repetition rate weresuccessfully used to demonstrate the first all-diode-pumpedsoft x-ray laser [14]. In this demonstration, the infrared laserpulses were focused to form a line of ∼35 μm FWHM width


on a molybdenum target, resulting in a hot dense plasmawith the conditions necessary for laser amplification at 18.9nm wavelength in the 4d1S0→4p1P1 transition of nickel-likemolybdenum ions. Operation of CPA diode-pumped lasersat increased repetition rate will enable the development ofcompact high average power soft x-ray lasers.

V. CONCLUSION

Efficient diode-pumping of cryogenically cooled Yb-basedamplifiers have great potential for increasing the repetitionrate of high energy CPA lasers. The combination of a roomtemperature Yb:YAG preamplifier with cryogenically cooledpower amplifiers of the same material was demonstrated toallow for high average power generation of high energy laserpulses with sufficient bandwidth to support compression to sub5-ps duration. A compact two amplifier laser configuration thatuses this hybrid temperature scheme generated 140 mJ laserpulses at 100 Hz repetition rate that were compressed into100 mJ pulses of 4.8 ps duration. The addition of a high powercryogenic Yb:YAG amplifier will allow further amplificationto 1-2 J at 100Hz repetition rate. In progress towards thisgoal, the generation of 1 J pulses of 8.5 picosecond durationat 10 Hz repetition rate was demonstrated in a system thatused two cryo-cooled amplifiers. These compact and efficienthigh power picosecond laser systems can be expected to havea significant impact in applications such as the pumping ofhigh average brightness table-top soft x-ray lasers, the efficientx-ray generation from plasmas, and in pumping of opticalparametric amplifiers for the generation of high intensity laserpulses of femtosecond duration.

ACKNOWLEDGMENT

We acknowledge the contributions of C. Baumgarten.

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Brendan A. Reagan received the B.S. and M.S. degrees in electricalengineering from Colorado State University, Fort Collins, in 2004 and 2008,respectively, where his thesis work focused on developing capillary dischargecreated plasmas to extend the wavelength range and efficiency of highharmonic generation soft X-ray sources. He is currently pursuing the Ph.D.degree in electrical engineering at Colorado State University.

His current research interests include the development of high energydiode-pumped, short pulse lasers, and the generation of high repetition ratesoft X-ray lasers.

Alden H. Curtis received the B.S. degree in physics from OglethorpeUniversity, Atlanta, GA, and the M.S. degree in electrical engineering fromColorado State University, Fort Collins.

He is currently working with the National Security Technologies, a UnitedStates Department of Energy Contractor, as an Optical Diagnostics Engineer.His current research interests include high energy density physics, laserengineering, and radiation detection.

Keith A. Wernsing received the B.A. degree in english from California StateUniversity, Long Beach, in 2003. He is currently pursuing the Ph.D. degreein electrical engineering with Colorado State University, Fort Collins.

His current research interests include the development of diode-pumpedlasers, and the development of high average power soft X-ray lasers forapplications.

Federico J. Furch received the Licentiate degree in physics from theUniversity of Buenos Aires, Buenos Aires, Argentina, and the Ph.D. degreein physics from Colorado State University, Fort Collins, in 2004 and 2010,respectively.

He is currently an Associate Researcher with Max Born Institute, Berlin,Germany.

Bradley M. Luther received the B.S. degree in chemistry from Texas A&MUniversity, College Station, and the Ph.D. degree in physical chemistry fromColorado State University, Fort Collins.

He is currently a Research Scientist with the NSF Center for EUV Scienceand Technology at Colorado State University, Fort Collins.

Jorge J. Rocca (SM’80–M’83-SM’94–F’00) is a University DistinguishedProfessor with the Department of Electrical and Computer Engineering andthe Department of Physics, Colorado State University, Fort Collins. Hisgroup demonstrated the first gain-saturated table-top soft X-ray laser usinga discharge plasma as gain medium, and later extended the wavelength ofbright high repetition rate table-top lasers down to 8.8 nm using laser-created plasmas, also achieving full phase coherence. He and his collaboratorshave used these compact soft X-ray lasers to perform nano-scale imaging,dense plasma diagnostics, nano-scale material studies, and photochemistryexperiments. His current research interests include the development andphysics of compact soft X-ray lasers and their applications, and the studyof plasmas, subjects in which he has published more than 200 peer reviewjournal papers.

He was an NSF Presidential Young Investigator. He is a fellow of the Amer-ican Physical Society and the Optical Society of America. He was the recipientof a Distinguished Lecturer Award from IEEE in 2008, the Schawlow Prizein Laser Science from the American Physical Society in 2011, and the WillisE. Lamb Award for Laser Science and Quantum Optics in 2012.

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