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We report what we believe to be record performance for a high average power Yb:YAG cryogenic laser system with sustained output power. In a CW oscillator-single-pass amplifier configuration, 963 W of output power was measured. In a second configuration, a two amplifier Yb:YAG cryogenic system was driven with a fiber laser picosecond ultrafast oscillator at a 50 MHz repetition rate, double-passed through the first amplifier and single-passed through the second, resulting in 758 W of average power output. Pulses exiting the system have a FWHM pulsewidth of 12.4 ps, an energy/pulse of 15.2 μJ, and a peak power of 1.23 MW. Both systems are force convection-cooled with liquid nitrogen and have been demonstrated to run reliably over long time periods.

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We discuss progress towards a kilowatt class CW Yb:YAG cryogenic laser. Cryogenically-cooled crystalline solid-state lasers, and Yb:YAG lasers in particular, are attractive sources of scalable CW output power with very high wallplug efficiency and excellent beam-quality that is independent of the output power. Results are presented for a high power Yb:YAG oscillator that has produced over 550 W of output power with good slope and optical-optical efficiencies while maintaining single transverse mode output. We also describe a new oscillator-amplifier cryogenic Yb:YAG system nearing completion, that will build on the work presented here and result in CW power output of > 1 kW while maintaining near-diffraction-limited beam quality. The oscillator described here consists of a distributed array of seven highly-doped thin Yb:YAG-sapphire disks in a folded multiple-Z resonator. Individual disks are pumped from opposite sides using 100 W fiber-coupled 940 nm pump diodes. The laser system produces a near-diffraction-limited TEM 00 output beam with the aid of an active conduction-cooling design. In addition, the device can be scaled to very high average power in an oscillator-amplifier configuration, by increasing the number and diameter of the thin disks, and by increasing the power of the pump diodes with only minor modifications to the current design. We will present experimental results including output power, threshold power, and slope and optical-optical efficiencies.

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In this paper we discuss a CW Yb:YAG cryogenic laser program that has resulted in the design and demonstration of a novel high power laser. Cryogenically-cooled crystalline solid-state lasers, and Yb:YAG lasers in particular, are attractive sources of scalable CW output power with very high wallplug efficiency and excellent beam-quality that is independent of the output power. This laser consists of a distributed array of seven highly-doped thin Yb:YAG-sapphire disks in a folded multiple-Z resonator. Individual disks are pumped from opposite sides using fiber-coupled ~ 30W 940nm pump diodes.The laser system we have constructed produces a near-diffraction-limited TEM 00 output beam with the aid of an active conduction-cooling design. In addition, the device can be scaled to very high average power in a MOPA configuration, by increasing the number and diameter of the thin disks, and by increasing the power of the pump diodes with only minor modifications to the current design. The thermal and optical benefits of cryogenically-cooled solid-state lasers will be reviewed, scalability of our Yb:YAG cryogenic laser design will be discussed, and we will present experimental results including output power, slope and optical-optical efficiencies, and beam-quality.

We report the demonstration of a heat-fraction-limited CW Yb:YAG laser operating near 77 K with output at 1029 nm, pumped with a diffraction-limited room-temperature CW Nd:YAG laser operating at 946 nm. With a 50% reflectivity outcoupler, the average threshold absorbed pump power was 18.8 mW and the average slope efficiency 91.9%, close to the heat-fraction limited value of 91.5%. Average optical to optical and photon slope efficiencies are 84% and 100% respectively. To the best of our knowledge this solid-state laser is the first to operate at the heat-fraction-limit and demonstrates record slope, photon slope and optical-optical efficiencies for optically-pumped solid-state lasers.