## 1 Introduction and History of Cryogenic Lasers  
The history of the development of cryogenic lasers may roughly be divided into two distinct eras, which we refer to as the historical and the modern. The historical era covers the early use of cryogenic technology to effect the demonstration of new solid-state lasers, and was focused on the spectroscopic and lasing properties using flashlamp optical pumping, while in the modern era, the advent of diode optical pumping and the increased understanding of the physical properties of laser crystals has led to a comprehensive approach to the development of cryogenic lasers. The comprehensive approach embraces not just the spectroscopic and lasing properties, but includes all of the physical properties needed to successfully design and demonstrate advanced cryogenic laser devices. Such properties include mechanical, thermal, thermo-optic and optical properties such as the index of refraction as well as the crystal dispersion properties. Other defining features of the modern era are the maturation of open and closed cycle cryogenic cooling methods and the rapid development and availability of powerful diode laser sources for pumping solid-state lasers.  
In 1991, the first demonstration of a liquid nitrogen (LN\({}_{2}\)) cooled diode-pumped Yb:YAG laser was reported by Lacovara _et al._[1], and we rather arbitrarily define the year of publication of that paper as the start of the modern era, because it combined cryogenic-cooling with a diode-pumped Yb:YAG laser crystal. Some may argue or imply that the development of cryogenic lasers in the modern era is really nothing new, simply because cryogenically-cooled lasers were first demonstrated in the early years of the development of solid-state lasers and there exist a number of early cryogenic laser patents. Such arguments however ignore the reality that during the historical period all cryogenic lasers were flashlamp-pumped with concomitant large heat generation in the solid-state laser crystal, significantly reducing the effectiveness of cryogenic-cooling. In addition, the modern era has as its goal the demonstration of cryogenic laser technology whose average power scaling is well-understood and is firmly grounded in the detailed measured properties of a number of key laser host physical parameters. The long-sought goal of laser physicists, engineers, and designers to produce near diffraction-limited output lasers while operating at very high average powers and efficiency, and whose output parameters are invariant to average power, is now being realized. It is important to emphasize and acknowledge the countless individual contributors whom have made this goal a reality, spanning both the historical and modern periods of together almost seven decades. It is gratifying that the promise of cryogenic lasers, articulated in an article one decade ago [2] and another 8 years ago [3], is beginning to be realized. Yet it is also exciting to contemplate that we have only just begun to realize the vast potential of cryogenic lasers, and that could not be more true than in the nascent field of cryogenic ultrafast lasers.  
This article discusses cryogenic laser technology, with a particular emphasis on the application of that technology to the development of ultrafast lasers. The intersection of cryogenic and ultrafast laser technologies will undoubtedly provide powerful new laser sources capable of producing extraordinary associative peak and average powers in the near future. We refer to these new lasers as HAPP (High Average and Peak Power) lasers.