## 4 Review and Status of Cryogenic Laser Technology
### Cryogenic Nanosecond Lasers  
In this section of the manuscript we review the performance of nanosecond cryogenic laser technology. Figure 46 displays the energy/pulse, repetition rate as a function of average power. Table 9 tabulates the corresponding date, energy/pulse, repetition rate, average power, pulsewidth, and reference [30; 135; 147; 148; 149; 150; 151; 152; 153]. A number of techniques are used to generate nanosecond pulses from cryogenic amplifiers, including acousto-optic or electro-optic Q-Switching, seeded regenerative amplifiers, and in some case regenerative amplifiers followed by one or more power amplifier stages. In [147], 6.5 mJ/pulse at 200 Hz and a 10 ns FWHM pulsewidth, 1.5 mJ at 1 kHz with a 10 ns pulsewidth, and 140 mJ at 100 Hz with a 10 ns pulsewidth were reported, respectively. For this system, a seed laser was used to drive a regenerative amplifier that in turn drove a 4-pass power amplifier. The highest average power produced was 14 W. An earlier paper [148] reported the demonstration of 4.6 mJ pulse energy at a 16 kHz repetition rate in an oscillator configuration that incorporated an acousto-optic Q-switch. The average power produced was 74 W and the pulsewidth 175 nm. Another lower average power cryogenic laser that produced 120 mJ/pulse at a 500 Hz repetition rate, 60 W of average power, and a 70 ns pulsewidth is reported in [149] where a cryogenic Yb:YAG oscillator was used to drive a power amplifier. The cryogenic laser group at MIT, the first to report average power >100 W, demonstrated a 114 W average power cryogenic oscillator [30; 150] that produced 22.8 mJ/pulse at 5 kHz, and with a pulsewidth of 16 ns. The highest average power nanosecond cryogenic laser reported to date [135] produced 8.5 mJ pulses at a 40 kHz repetition rate for an average power of 340 W. The pulsewidth produced using an acousto-optic Q-switch was 75 ns. Figure 46: Nanosecond output energy/pulse (blue) and repetition rate (pink) as a function of average power at 1029 nm from demonstrated Yb:YAG cryogenic lasers in the years 2005–2014. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & **Energy/Pulse** & **Repetition** & **Average Power** & **Pulsewidth** & \multirow{2}{*}{**Reference**} \\  & **(m)** & **Rate (Hz)** & **(W)** & **(ns)** & \\ \hline
2005 & 4.6 & 16,000 & 74 & 175 & [148] \\
2010 & 6.5 & 5000 & 114 & 15 & [147] \\
2010 & 1.5 & 200 & 1.3 & 10 & [147] \\
2010 & 140 & 1000 & 1.5 & 10 & [153] \\
2010 & 22.8 & 5000 & 114 & 16 & [30; 150] \\
2011 & 4.5 & 40,000 & 340 & 75 & [135] \\
2013 & 120 & 500 & 80 & 70 & [149] \\
2013 & 10,100 & 1 & 10.1 & 10 & [152] \\
2014 & 3050 & 10 & 30.5 & 10 & [151] \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Reported Cryogenic Q-Switched Yb:YAG Demonstrations From 2005–2011. Two high energy/pulse, low repetition rate cryogenic nanosecond lasers have been reported in the literature. The first [151] achieved 3.05 J/pulse at 10 Hz for an average power of 30.5 W and with a 10 ns pulsewidth, using a seeded regenerative amplifier followed by active-mirror pre-amplifier and power amplifier stages. The largest energy/pulse was demonstrated in connection with the DiPOLE (Diode-Pumped Optical Laser Experiment) project [152]; 10.1 J/pulse at a 1 Hz repetition rate was obtained with a 10 ns pulsewidth and producing 10.1 W of average power. The laser can also produce 6.4 J/pulse at 10 Hz repetition rate. This laser system is innovative in that it uses cryogenically cooled He gas to remove heat from a parallel array of Yb:YAG laser slabs in an arrangement very reminiscent of the Mercury laser project at Lawrence Livermore National Laboratory (LLNL) in the US, where the concept of using stacks of aerodynamically shaped slab holders originated [154]. While we have not specifically discussed the methods of cryogenic-cooling in this article, here we mention that the use of He at cryogenic temperatures to cool laser crystals is in its infancy.