## 4 Review and Status of Cryogenic Laser Technology
### Cryogenic Ultrafast (Picosecond and Femtosecond) Lasers  
In this final section of our paper, we discuss the application of cryogenic laser technology to enhancing the performance of ultrafast lasers operating in the picosecond and femtosecond regimes. Cryogenic cooling has been implemented to enhance the performance of Ti:Al\({}_{2}\)O\({}_{3}\) in recent years [31; 33; 34; 35], for example, and even commercial companies have begun to incorporate the technology. The enhancement of Ti:Al\({}_{2}\)O\({}_{3}\) ultrafast systems at present incorporates closed cycle Gifford-McMahon or pulse tube coolers to reduce thermal focusing, usually in the pre-amplifier stage of a chirped-pulse-amplification (CPA) system. While the thermal properties of such systems are improved, dramatic improvements have not been demonstrated, partly because sapphire already has a large thermal conductivity at room temperature. State-of-the-art Ti:Al\({}_{2}\)O\({}_{3}\) CPA systems have been limited in average power for quite some time to <100 W. Orders of magnitude increases in the average power capability of Ti:Al\({}_{2}\)O\({}_{3}\) however may be achieved through the development of powerful new cryogenic pump lasers, providing kilowatt average power capability. Several current efforts to produce picosecond high average power cryogenic lasers at the 1 J/pulse level and greater at kHz repetition rates, discussed later in this section, provide validation to designs capable of producing high average power nanosecond cryogenic frequency-doubled green lasers with high efficiency, to be used as pump sources for high-average-power CPA Ti:Al\({}_{2}\)O\({}_{3}\) lasers. At the present time green pump sources are typically limited to 10-30 Hz repetition rates, although lower peak power systems can operate at kHz repetition rates as well. To produce HAPP Ti:Al\({}_{2}\)O\({}_{3}\) systems with high peak and average powers, however, it is likely that all Ti:Al\({}_{2}\)O\({}_{3}\) amplifiers as well as their pump lasers will need to be cryogenically-cooled. A second application for cryogenic laser technology for ultrafast lasers is to build strong joule-level pumps operating with kHz repetition rates, to drive optical parametric chirped-pulse-amplification (OPCPA) ultrafast systems, recently described as third-generation femtosecond technology [156]. Parametric amplification has garnered much attention recently because it generates only small amounts of heat and is capable of providing very broad gain bandwidths allowing the production of few-cycle systems. The development of borate based nonlinear crystals such as BBO and LBO, materials with high damage thresholds, has considerably aided the development of OPCPA femtosecond lasers. OPCPA has already produced femtosecond laser performance, and is presently limited by currently available pump sources. The need for better pump sources has been discussed in the literature [156; 157], with near universal agreement that high average power pumps with very good beam-quality and pulse durations near ~1 ps and operating in the near-infrared regions are required. Near-infrared sourcessuch as Yb operating near 1030 nm are expected to provide fundamental, second, and third harmonic pumping at high average power, and the recent intensive development of Yb based cryogenic laser sources seem well-positioned to address this need. In Table 10, we show reported demonstrations of cryogenic Yb:YAG, Yb:YLF, and Yb:Y\({}_{2}\)O\({}_{3}\) in the time period 2003 through 2015, and the output parameters that were achieved.