1 J, 0.5 kHz repetition rate picosecond laserCORY BAUMGARTEN,1,* MICHAEL PEDICONE,1 HERMAN BRAVO,2,3 HANCHEN WANG,1 LIANG YIN,2

CARMEN S. MENONI,2,3 JORGE J. ROCCA,1,2,3 AND BRENDAN A. REAGAN2,3

1Department of Physics, Colorado State University, Fort Collins, Colorado 80523, USA2Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado 80523, USA3XUV Lasers, Inc., P.O. Box 273251, Fort Collins, Colorado 80527, USA*Corresponding author: cbaumga@rams.colostate.edu

Received 24 May 2016; revised 16 June 2016; accepted 17 June 2016; posted 17 June 2016 (Doc. ID 266919); published 14 July 2016

We report the demonstration of a diode-pumped chirpedpulse amplification Yb:YAG laser that produces λ �1.03 μm pulses of up to 1.5 J energy compressible tosub-5 ps duration at a repetition rate of 500 Hz (750 Waverage power). Amplification to high energy takes placein cryogenically cooled Yb:YAG active mirrors designedfor kilowatt average power laser operation. This compactlaser system will enable new advances in high-average-power ultrashort-pulse lasers and high-repetition-rate tab-letop soft x-ray lasers. As a first application, the laser wasused to pump a 400 Hz λ � 18.9 nm laser. © 2016 OpticalSociety of America

OCIS codes: (140.7240) UV, EUV, and X-ray lasers; (340.7480)

X-rays, soft x-rays, extreme ultraviolet (EUV); (140.3480) Lasers,

diode-pumped.

http://dx.doi.org/10.1364/OL.41.003339

The development of picosecond lasers with simultaneouslyhigh energy and high average power is of great interest forapplications in basic research and technology. These applica-tions include the development of compact sources of coherentextreme ultraviolet and soft x-ray radiation [1–3] as well as driv-ing the next generation of high-average-power ultrashort-pulselasers with wavelengths ranging from the ultraviolet to themid-infrared through parametric amplification schemes such asoptical parametric chirped pulse amplification (OPCPA) [4]and frequency-resolved optical parametric amplification(FOPA) [5].

A number of chirped pulse amplification (CPA) [6], pico-second lasers with increased average power have recently beendeveloped [7–17]. This includes the demonstration of a 100 Wthin disk Yb:YAG laser which produced 1 ps pulses of 20 mJ[7], as well as a 250 W cryogenic Yb:YAG amplifier thatproduced 2.5 mJ pulses at 100 kHz [14]. However, the devel-opment of picosecond lasers with simultaneous high energy andhigh average power has been slower. We have previously re-ported a laser system that produced 1 J, 5 ps pulses at100 Hz repetition rate [8,9]. In this earlier demonstration,we exploited the improved thermal [18,19] and spectral

[20,21] characteristics of Yb:YAG at cryogenic temperatures.Other groups have also taken advantage of cryogenic Yb:YAG [10,12–14,16,17], obtaining results that include the re-cent demonstration of 70 mJ picosecond pulses at 1 kHz rep-etition rate [16]. Our 100 Hz laser system was employed todrive plasma-based soft x-ray lasers at 100 Hz repetition rateoperating at a number of wavelengths between λ � 10.9 and18.9 nm, including the demonstration of a λ � 13.9 nm laserwith 0.1 mW average power [1]. The laser was also used tostudy the conversion efficiency and plasma characteristics ofa λ � 6.7 nm laser-produced plasma source for future lithog-raphy [22] and to pump an OPCPA that produced 5 mJ pulsesat λ � 1.6 μm [23]. In this Letter, we report a compact all-diode-pumped λ � 1.03 μm CPA laser based on cryogenicallycooled active mirror Yb:YAG power amplifiers that producespulses with energy of up to 1.5 J at a repetition rate of0.5 kHz, resulting in an uncompressed average power of0.75 kW. The output is demonstrated to have good beam qual-ity and good shot-to-shot stability. The laser pulses are com-pressed in vacuum to durations of about 5 ps FWHM by apair of dielectric diffraction gratings. Finally, we demonstratethe use of this new laser in driving a λ � 18.9 nm, plasma-based soft x-ray laser at the high repetition rate of 400 Hz.

A schematic of the kilowatt-class CPA laser system is shownin Fig. 1. The power amplifiers are seeded by pulses producedby a diode-pumped, SESAM-mode-locked, prismless, Yb:KYW oscillator and are stretched by a grating pulse stretcher.The seed pulses are first amplified by a room-temperature activemirror regenerative preamplifier and are subsequently furtheramplified in two cryogenically cooled Yb:YAG active mirroramplifiers, to be finally compressed by a vacuum grating pulsecompressor. Amplification in the water-cooled Yb:YAG regen-erative amplifier results in the production of ∼1 mJ pulses at500 Hz repetition rate. This amplifier uses a 0.5 mm thick,10%-at Yb:YAG active mirror, which constitutes a compromisebetween the thermally efficient thin-disk geometry and simpli-fied pump absorption. The Yb:YAG disk is pumped by∼200 W pulses of 200 μs duration from a fiber-coupled λ �940 nm semiconductor laser diode module. The stretchedpulses suffer gain narrowing during amplification, and theirpulse duration becomes 270 ps. We have compressed thesepulses to ∼2 ps pulse durations. However, the bandwidth is

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0146-9592/16/143339-04 Journal © 2016 Optical Society of America

reduced when the wavelength is tuned to match the peak gainof the subsequent cryogenic amplifiers. A Pockels cell and a pairof crossed polarizers are used to improve the pulse contrast priorto further amplification.

The millijoule-level pulses exiting the laser frontend are am-plified to 100 mJ by a four-pass (where one pass is defined asthe laser pulse traversing the gain medium, reflecting from theHR coating, and traversing the gain medium again) cryogeni-cally cooled Yb:YAG preamplifier. Here, the active mirrorgeometry is again used, in the form of a 5 mm thick, 2%-atYb:YAG slab mounted onto a cryogenically cooled head insidean evacuated chamber. The back face of the indium wire sealedactive mirror is cooled by forced convection with flowing liquidoxygen cooled to 77 K by a nitrogen cryogenic heat exchanger.This results in thermal gradients which are mainly longitudinalto the beam path within the slab and allows us to surpass theheat flux which can be removed via boiling heat transfer with-out the use of a heat spreader. This preamplifier is pumped bythe combination of two 400 W fiber-coupled laser diode mod-ules which are imaged into a ∼4 mm spot on the Yb:YAG ac-tive mirror by achromatic lenses. The pulses make four passesthrough the active region where they are amplified to a maxi-mum energy of 100 mJ at 500 Hz repetition rate, as shown inFig. 2(a). The amplified pulses have a bandwidth of nearly0.4 nm FWHM centered near λ � 1.03 μm [Fig. 2(b)], whichsupports compression to sub-5 ps duration.

The pulses from this preamplifier are subsequently injectedinto the final stage of amplification, consisting of an eight-passamplifier comprising two custom-designed active mirror ampli-fier slabs mounted on a single cryogenic cooling head, which issealed with indium wire, in an evacuated enclosure. The twoamplifier slabs were designed to have a large lateral size to thick-ness ratio that allows for efficient cooling and large pump area,thereby reducing the deleterious thermal effects of thermal

lensing, stress, and depolarization. The slabs are composedof a 2 mm thick by 30 mm × 30 mm square, 3%-atYb:YAG slab that is surrounded by a 10 mm wide Cr:YAGcladding optically bonded along the perimeter for mitigatingamplified spontaneous emission (ASE) and parasitic lasing.As in the previous amplification stage, the entire back facesof the slabs are cooled by direct contact with a flowing,subcooled liquid at 77 K.

The front face of the Yb/Cr:YAG assembly is opticallybonded to a 3 mm thick undoped YAG cap that provides struc-tural integrity and further reduces ASE. Each amplifier slab ispumped by a λ � 940 nm, 6 kW, 60 bar laser diode array.These pump lasers were pulsed to produce 500 μs durationpulses, and the output was reshaped and homogenized to forma 16 mm diameter, nearly flat-top pump spot on each slab. Theunabsorbed pump light is relay imaged back onto the activeregion by a spherical mirror. This allows for absorption ofgreater than 90% of the pump radiation, while avoiding an in-creased doping concentration that has been measured to reducethe thermal conductivity of Yb:YAG at cryogenic temperatures[19]. The pulses are injected into the amplifier through a thinfilm polarizer (TFP) at the input of the amplifier. The seedpulses make one bounce of each slab before the beam heightis changed via a periscope and then proceeds to make one more

Fig. 1. Schematic of the diode-pumped high-energy CPA lasersystem that includes fiber-coupled laser diodes (FC LD), a thin filmpolarizer (TFP), and a periscope (P). Regen, regenerative amplifier.

Fig. 2. (a) Measured amplified pulse energy at the output of thecryogenic preamplifier as a function of pump energy at 500 Hz rep-etition rate. A maximum energy of 100 mJ is obtained. (b) Measuredspectra at the output of the preamplifier (blue) and final amplifier(red). The pulses have an amplified bandwidth of 0.36 nm FWHM.

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bounce of each slab. After these bounces, a λ∕4 wave plateplaced before a 0° send-back mirror rotates the polarization90 deg, allowing for the beam to retrace the same path andreflect from the TFP to exit the amplifier. Figure 3(a) showsthe measured laser pulse energy at 500 Hz repetition rate asa function of the total pump energy. A maximum laser pulseenergy of 1.5 J (750 W average power) was obtained with a totalpump energy incident on the Yb:YAG slabs of 4 J. This results in

an optical-to-optical efficiency of ∼37%. Our simulations of theamplifiers predict an accumulated B-integral at 1.5 J of about0.7. As can be seen from the M 2 measurement of Fig. 3(b),the laser displayed good beam quality with values of M 2 ≈ 1.3.Very slight thermal lensing was observed that we compensatedwith the addition of a telescope between the main amplifier andgrating pulse compressor. The effective focal length of the ther-mal lens was determined to be longer than 200 m for a singlepass of the amplifier. No observable depolarization occurred.Figure 3(c) shows the measured pulse-to-pulse energy variationsover 30 min of continuous operation. During this period, thelaser fired nearly 1 million consecutive shots with a standarddeviation of 0.75% over the entire run. This variation is closeto the specified 0.5% shot-to-shot repeatability of the energy me-ter used for this measurement. We have typically operated thelaser for several hours at a pulse energy of 1.4 J at 500 Hz rep-etition rate. As can be seen in Fig. 2(b), the fully amplified pulseshave a bandwidth of about 0.36 nm FWHM, which supportscompression to sub-5 ps pulse durations. Following amplifica-tion, the beam is magnified and collimated by a Galilean tele-scope to a diameter of 40 mm, creating a footprint of 22.5 cm2

on the first grating. This is compressed in vacuum by a pair of1740 mm−1 dielectric gratings with a total path length of about5300 mm between the gratings. The compressor has an overallefficiency of 72%. Figure 4 shows a second-harmonic generation(SHG) autocorrelation of the compressed pulses. This measure-ment was made with the pulses traversing the full system andwith the cryogenic preamplifier stage operating at full power(100 mJ), where the vast majority of the gain narrowing occurs.The SHG autocorrelation shows a nearly sech2 pulse shape andcorresponds to a pulse duration of 3.8 ps FWHM.We routinelyoperate the system at 5 ps pulse duration. Furthermore, no beamdegradation has been observed after compression, and the com-pressed beam’s quality is evidenced by our ability to form a high-aspect-ratio line focus and generate a soft x-ray laser (SXRL)at 400 Hz.

As the first application of this high-repetition-rate CPAlaser, we used it to drive a plasma-based high-repetition-rate,λ � 18.9 nm soft x-ray laser operating on the 4d 1S0 →4p1P1 transition of Ni-like Mo. Using a setup similar to thatpresented in [1], we focused ∼1 J temporally shaped pulses into

Fig. 3. (a) Measured pulse energy exiting the final amplifier as afunction of total pump energy at 500 Hz repetition rate. A maximumenergy of 1.5 J was obtained with a total pump energy incident on theYb:YAG slabs of 4 J. (b) M 2 measurement of the amplified pulsesmeasured with an output pulse energy of 1.4 J at 500 Hz repetitionrate. The inset image shows the beam profile at focus. (c) Measuredpulse energy over 30 min of continuous operation. The mean energy is1.41 J with a standard deviation of 0.75% over the entire run.

Fig. 4. Second-harmonic autocorrelation after pulse compression.The solid trace is sech2 fit with a width corresponding to a pulseduration of 3.8 ps FWHM.

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a ∼30 μm wide ×4 mm FWHM long line focus onto the sur-face of a solid molybdenum target. The laser pulses impingedonto the target at a grazing incidence angle of 29 deg to makeuse of refraction within the plasma to efficiently deposit thepump laser energy into a region of reduced density gradientsand optimum density for amplification [24,25]. The shapedpump pulses consisted of a few nanoseconds long ramp thathas the purpose of forming a plasma ionized to the Ni-like ion-ization stage, followed by a main plasma heating pulse of about6 ps FWHMduration. The short plasma heating pulse creates atransient population inversion and a sufficiently large gain forthe ASE to reach gain saturation in a single pass through theplasma amplifier. On-axis spectra of the plasma emission wererecorded using a grazing incidence flat-field diffraction gratingand an x-ray-sensitive CCD with thin Al filters to reject visible/ultraviolet plasma spontaneous emission and scattered pumplaser light. Figure 5 shows a single-shot spectrum with thedriver laser operating at 400 Hz repetition rate in which stronglasing at λ � 18.9 nm is observed. We also made runs of sev-eral thousand shots of the λ � 18.9 nm laser operating at400 Hz to demonstrate the viability of this method for creatinga tabletop high-power SXRL.

In summary, we have developed an entirely diode-pumpedCPA laser based on cryogenically cooled Yb:YAG active mirroramplifiers that produces pulses of up to 1.5 J at 500 Hz rep-etition rate. The amplified laser output can be compressed toyield Joule-level laser pulses of sub-5 ps duration. To the best ofour knowledge, these results constitute the highest averagepower reported to date for a Joule-level, ultrashort-pulse laser.The cryogenic cooling technique employed is scalable and isenvisioned to enable kilowatt average power short pulse lasers.This laser will enable compact sources of coherent short wave-length radiation with unprecedented average power, as well asmake possible a plethora of other applications.

Funding. U.S. Department of Energy (DOE) (DE-SC0011375); Office of Science, Basic Energy Sciences (BES)(DE-FG02-4ER15592); National Science Foundation (NSF)(1509925).

Acknowledgment. We acknowledge the contributions ofMark Woolston, Morris Schmidt, Zachery Roberson, BrandonCarr, Ryan Evans, and Gregory Hoffman. The SXRL work wassupported byDOEOffice of Science, Basic Energy Sciences (BES).

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Fig. 5. Single-shot, on-axis spectrum of Mo line focus plasma showingstrong lasing on the 18.9 nm transition of Ni-likeMo driven by the diode-pumped CPA laser operating at 400 Hz repetition rate. The top imageshows the raw CCD image, and the bottom trace is a vertical integration.

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