## 4 Review and Status of Cryogenic Laser Technology
### Cryogenic Ultrafast (Picosecond and Femtosecond) Lasers
While we have not discussed a number of promising Yb based laser materials in this paper, choosing to concentrate here only on significant high average as well as peak power results at cryogenic temperatures, it is clear that Yb lasers are an active and vibrant research topic today, and that in the near future other newer laser materials will be used to advance the state-of-the-art. \begin{table}
\begin{tabular}{l l l l l l l l l l l l l l l} \hline \hline \multirow{2}{*}{**Year**} & **Crystal** & \multirow{2}{*}{\(\mathbf{E_{p}}\) (m)} & \multirow{2}{*}{\(\mathbf{\
u_{q}}\) (Hz)} & \multirow{2}{*}{\(\mathbf{\
u_{W}}\)} & \(\Delta_{\mathbf{\lambda_{c}}}\) (mm) & \multirow{2}{*}{\(\mathbf{\delta_{T_{B}}}\) (\(\mathbf{\rho_{s}}\))} & \multirow{2}{*}{\(\Delta_{\mathbf{\lambda_{s}}}\) (mm)} & \multirow{2}{*}{\(\mathbf{\delta_{T_{B}}}\) (mm)} & \multirow{2}{*}{\(\mathbf{\delta_{T_{B}}}\) (mm)} & \multirow{2}{*}{\(\mathbf{\delta_{T_{B}}}\) (mm)} & \multirow{2}{*}{\(\mathbf{\delta_{T_{B}}}\) (mm)} & \multirow{2}{*}{\(\mathbf{\delta_{T_{B}}}\) (m)} & \multirow{2}{*}{**Ref**} \\  & **Type** & & & & & & & & & & & & \\ \hline
2003 & Yb-YAE & \(30\times 10^{-3}\) BC & 20 & 3.6 & 27.0 & 1017 & 0.097 & 15 & 0.101 & 5 & 0.304 & 0.795 & 11.64 \(\times 10^{6}\) & [155] \\  & \(\sigma\) & \(18\times 10^{-3}\) AC & 20 & 3.6 & 27.0 & 1017 & 0.097 & 15 & 0.101 & 5 & 0.304 & 0.795 & 11.64 \(\times 10^{6}\) & [155] \\ \hline
2007 & Yb-YAG & \(0.296\times 10^{-3}\) & \(80\times 10^{2}\) & 24.0 & 1.5 & 1029 & 1.038 & 1.45 & 10.7 & 1.33 & 11.7 & 11.7 & \(25.6\times 10^{6}\) & [158] \\ \hline
2007 & Yb-YAG & \(0.99\times 10^{-3}\) BC & \(20\times 10^{3}\) & 19.8 & 1.5 & 1029 & 1.038 & 1.45 & 10.7 & 1.33 & 11.7 & \(11.7\) & \(84.6\times 10^{6}\) & [158] \\ \hline
2007 & Yb-YAG & \(8\times 10^{-3}\) BC & \(0.075\) & 1029 & & & & & & & & & \\ \(\text{SHG}\) & \(7.5\times 10^{-3}\) AC & \(10\times 10^{3}\) & 0.019 & 1.5 & 515 & 1.038 & 17.0 & 0.992 & 0.5 & 3.1 & \(25.0\) & \(217.5\times 10^{6}\) & [159] \\ \hline
2008 & Yb-YAG & \(3.7\times 10^{-6}\) & \(78\times 10^{6}\) & 287 & 1.5 & 1029 & 1.038 & 0.4 & 3.89 & 0.3 & 5.2 & 5.5 & \(0.67\times 10^{6}\) & [160] \\ \hline
2009 & Yb-YAG & \(3.1\times 10^{-6}\) & \(78\times 10^{6}\) & 240 & 1.5 & 1029 & 1.038 & 0.4 & 3.89 & 0.3 & 5.2 & 8.0 & \(0.39\times 10^{6}\) & [161] \\  & \(\text{SHG}\) & \(1.8\times 10^{-6}\) & & 130 & & & & & & & & & \\ \hline
2009 & Yb-YAG & \(1.45\times 10^{-6}\) & \(10\times 10^{-3}\) BC & 10 & 1.5 & 1029 & 1.038 & - & - & - & - & 8.5 & \(0.12\times 10^{2}\) & [162] \\ \hline
2010 & Yb-YAG & \(40\times 10^{-3}\) BC & \(2\times 10^{3}\) & 80 & 1.5 & 1029 & 1.038 & 0.24 & 6.49 & 0.10 & 15.0 & - & \(2.1  
Figure 47 shows a plot of the energy/pulse and repetition rate as a function of laser average power, using the values found in Table 10, for both fundamental and second-harmonic wavelengths. The energy/pulse varies from typically microjoules at the higher repetition rates (MHz) to large energy/pulse for low repetition rates. The largest energy/pulse obtained is about 1.02 J at a 10 Hz repetition rate [162]. For intermediate repetition rates in the kHz regime, some mJ of energy/pulse are obtained. The average power associated with low repetition rates and large energy/pulse tend to have average powers <20 W, while kHz repetition rates and intermediate mJ energies tend to be less than 110 W. The highest average powers tend to be associated with MHz repetition rates and lower (\(\upmu\)) energies per pulse. The highest average power, 758 W, was obtained by our group, producing 15.2 \(\upmu\)J/pulse at a 50 MHz repetition rate [132]. The highest green ps average power obtained at cryogenic temperatures was also demonstrated by our group [164]. Figure 48 shows the peak power obtained as a function of average power, again using data from Table 10. Fundamental and second-harmonic values are displayed. The highest peak powers are produced by YbYLF based devices and the lowest by high repetition rate, low energy/pulse lasers.