## 1 Introduction and History of Cryogenic Lasers
### The Historical Era  
Good reviews of the use of cryogenic-cooling in the development of solid-state lasers have been presented by members of our group [2], the Massachusetts Institute of Technology (MIT) Lincoln Laboratory laser group [3], and by the laser group at Q-Peak [4]. Of particular note are the works of Keyes and Quist [5] whom used a GaAs diode laser to pump a U\({}^{3+}\):CaF\({}_{2}\) laser (the first diode-pumped laser), wherein both the diode laser and the laser crystal were cryogenically-cooled, the pioneering Ti:Al\({}_{2}\)O\({}_{3}\) work of Moulton [6] which demonstrated increased power output using a liquid nitrogen cooled (at 77 K) crystal, an effect that was attributed to the increased Al\({}_{2}\)O\({}_{3}\) thermal conductivity, and the development of a number of tunable laser materials by Moulton [7; 8; 9; 10]. The pioneering work of Schulz and Henion [11] whom demonstrated 350 W of output power from a liquid-nitrogen cooled Ti:Al\({}_{2}\)O\({}_{3}\) crystal, over 200 times what could be obtained at room temperature, is particularly noteworthy since those researchers were the first to our knowledge to carefully examine the crucial role materials properties such as thermal conductivity, thermal expansion coefficient, and the thermo-optic coefficient (d\(n\)/d\(T\)) play in substantially reducing thermal aberrations and stresses as temperature is lowered from room temperature to 77 K in sapphire. An evaluation of the data available at the time, summarized in Table I of [11], revealed that the thermal conductivity of Al\({}_{2}\)O\({}_{3}\) increased from 0.33 to 10 W/(cm\(\cdot\) K) as temperature was lowered from 300 K to 77 K, a greater than 30 times improvement. The thermo-optic coefficient decreased from 12.8 \(\times\) 10\({}^{-6}\)/K at 300 K to 1.9 \(\times\) 10\({}^{-6}\)/K, a factor of 6.7, while the thermal expansion coefficient decreased from 5 \(\times\) 10\({}^{-6}\)/K at 300 K to 0.34 \(\times\) 10\({}^{-6}\)/K at 77 K, a factor of 14.7. In cooling the Ti:Al\({}_{2}\)O\({}_{3}\) laser to 77 K, the authors were able to demonstrate about a 200 times decrease in the thermo-optical refractive index changes. Other early references of note include the demonstration of a tunable phonon-terminated Ni\({}^{2+}\):MgF\({}_{2}\) laser by Johnson, Guggenheim, and Thomas [12] at Bell Telephone Laboratories in 1963, and the development of a compact liquid nitrogen cooled system that produced CW emission from Nd\({}^{3+}\):YAG laser crystal as well as an ABC-YAG laser crystal [13]. In Europe, an early cryogenic Ho:YAG laser was demonstrated in 1975; an output of 50 W was achieved with liquid nitrogen cooling [14].  
In addition to laser demonstrations, cryogenic cooling has previously been used in spectroscopic solid-state laser crystal investigations to help elucidate the physics, energy level structure, and determine energy level assignments, for many rare-earth and transition metal activator ions. The laser crystal books by Kaminskii [15; 16] are particularly noteworthy, documenting many of the Stark level structures and energy assignments in use today at room temperature, 77 K, and in some cases down to liquid He temperature at about 4 K. Those publications are very useful resources that include historical spectroscopic and laser investigations from around the world, including many from Russia and former Soviet Republics whose many contributions were not fully acknowledged until after the dissolution of the Soviet Union in 1991.  
Early cryogenic laser patents were issued in the United States to Bowness [17] for a liquid nitrogen cooled ruby laser, and to McMahon [18] for a cryogenically-cooled ruby laser. Other early cryogenic work in the United States was performed at Sanders Associates (now BAE Systems, Inc., Arlington, VA, USA), in Nashua, NH.