Cryogenically Cooled Ytterbium Pump Lasers
The further development of attoscience and strong-field physics critically depends on the development of high-energy high-repetition-rate few-cycle OPCPA systems, that in turn require robust high-energy high-average-power pump lasers. However, the requirements on pump sources for OPCPA are much more delicate than that of CPA because parametric amplification with high efficiency while maintaining good beam quality is only possible with good spatio-temporal characteristics of the pump beam. The development of a high-average-power picosecond pump source that can be synchronized with seed beams at either high or low repetition rate is therefore one of the most important challenges for future OPCPA technologies and their applications. Besides the OPCPA applications, the large-average-power picosecond lasers can be widely used for driving other nonlinear optical processes such as frequency conversion.
Over the last years, high-power high-repetition-rate picosecond laser technologies have been developed both on the basis of fiber and bulk amplifiers [1-3]. Cryogenically-cooled Yb:YAG lasers have proven to be an excellent high-power and also high-energy laser technology for average power scaling because of its good thermo-optic properties, small quantum defect, and low saturation fluence. At cryogenic temperatures (77 K), Yb:YAG has an emission bandwidth of 0.8 nm, suitable for picosecond pulse amplification. A high-power cw Yb:YAG laser with output powers up to >450 W  and a picosecond amplifier at tens of kHz with 24 W of average power  have been demonstrated.
To develop this technology for OPCPA pumping and other applications, we have teamed up with the group of T. Y. Fan from MIT Lincoln Laboratory. Using a cryogenically-cooled cw Yb:YAG amplifier developed at MIT Lincoln Laboratory, we demonstrated the amplification of 5.5-ps pulses at a repetition rate of 78 MHz to 287 W of average output power, which is one of the highest average power picoseconds pulse sources at MHz repetition rates ever demonstrated . More recently we developed a cryogenically-cooled regenerative amplifier and power amplifier combination, that can generate 40mJ pulses at 2kHz repetition rate . The system is shown in Fig. 1.
The amplification result is shown in Fig. 2. A maximum power of 80 W (40 mJ) at 2-kHz repetition rate is obtained at 9-W seed power from the RGA with a slope efficiency of 30% and ~320-W pump power. Currently, the output power is limited by the damage of dewar windows at ~85 W. The cw amplification results using 12-W and 6-W cw seed power (blue dotted and red dashed lines in Fig. 2) clearly indicate that the achievable output power is only limited by the damage threshold of dewar window and other amplifier optics.
In our recent effort, we further upgraded the cryo-cooled Yb:YAG laser by adding the third amplification stage. As a result, we are able to amplify the pulse energy to 54 mJ and compressed output energy of 40 mJ. The upgraded multi-pass amplifier stage of the pump laser is shown in Fig. 3. Correspondingly, the output energy of 2-µm pulses from our 2-µm OPCPA was increased to more than 2 mJ. HHG into the water-window region is in progress using a high-pressure gas cell.
High-energy cryo-Yb:YAG thin disk amplifiers
Laser driver of the future will be able to deliver energetic ultrafast pulses at high repetition rates energizing nonlinear crystals through efficient OPCPA thus producing the IR output to drive HHG in gas jets to produce unprecedented levels of coherent X-ray radiation and other nonlinear processes. High energy bursts of pulses at lower macro-pulse rate with demanding high average power output is useful for laser-based scientific instruments where the count-rate dictates the duration of the experiments. Modern Yb3+-doped materials have been demonstrated as excellent gain media for laser amplification. The small quantum defect minimizes the heat load while the availability of high-brightness pump diodes allows for pumping with high intensities. For average power, the thin-disk geometry is one technique that provides for enhanced cooling. Advances in high average power amplifiers of high beam quality have come from operating Yb3+-doped materials at liquid nitrogen temperatures demonstrating their thermo-optical advantage as excellent gain media in the amplification of short pulses . Combining these two techniques should enable high average power and beam quality in CW laser systems, however, high energy pulsed operation would not be possible because amplified spontaneous emission (ASE) limits the useful gain that can be stored in the thin-disk geometry. A variant of the thin disk: the composite-thin-disk (CTD) pioneered by Zapata  resolves difficulties with high average power pumping, enables aperture scaling by mitigating ASE, and is robust resisting thermally induced deformations. Combined with cryogenic operation we pose here, will bring about a new class of HEP-HAP-DiLiLA technology to power the laser systems in the applications we are pursuing.
The HEP-HAP-DiLiLA experimental setup is shown in Fig. 4. Fitted with an ultrafast chirped master oscillator and grating compressor, this laboratory prototype will drive OPCPA crystals with 100 mJ, 10 ps pulses at 100-200 Hz when completed. A second stage to bring the pulse energy to 1 J is planned. The insert in the left panel of Fig. 4 shows the heart of the technology: a diode pumped cryogenic composite-thin-disk that we estimate could surpass the performance of traditional thin-disks.
Experimental gain data collected with an uncapped thin-disk (C7) and capped thin-disk (CTD1) both have matched our ASE-code predictions satisfactorily (Fig. 5). A dramatic increase in stored energy is expected. Using a 2-mJ regen amplifier as the seed, the first multi-pass amplifier will bring the pulse energy to 100 mJ. The planned second-stage power-amplifier will operate at liquid nitrogen temperature to bring the pulse energy to 1-Joule before being compressed with dielectric-coated gratings. To this end, we are applying novel concepts to address some of the traditional laser system problems in our engineering design to develop a high energy pulsed high average power diffraction limited laser amplifier (HEP-HAP-DiLiLA) that we hope will enable scientific and industrial applications now latent and benefit those existing today with enhancements in processing speed.
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9. L. E. Zapata "Edge-Facet Pumped, multi-aperture, thin-disk laser geometry for very high Average Power Output Scaling” U. S. Patent 6,834,070 B2 December 21, 2004. Inventor: under DOE contract No. W-7405-ENG-48 with the University of California.