Ceramic laser materials: Past and present
Highlights
► Notable achievements in solid state lasing using ceramic materials are discussed. ► Lasing performance from various ceramic based lasers are presented. ► R & D efforts for high power lasers towards 100 KW and beyond are also discussed.
Introduction
Since the first discovery of solid-state lasers in 1960 [1], much effort has been focused on developing high quality laser gain materials mainly based on single crystals. Due to high thermal conductivity, chemical stability, ease of machining, and excellence in laser energy–conversion efficiency, crystalline lasers such as yttrium aluminum garnet (Y3Al5O12) (YAG) and Ti:sapphire are widely used in various industrial applications including semiconductor industry, steel industry as well as medical industry. However single crystals are generally grown from the melt, and they suffer from drawbacks such as segregation of the dopant from the host, optical inhomogeneity caused by stress during crystal growth and high cost and low productivity due to high temperature processing. It was not until 1964 that the first solid-state laser fabricated from polycrystalline ceramics using Dy:CaF2 was reported [2]. Since then, a tremendous amount of effort has been made to realize high power lasers suitable for various commercial as well as military applications. In fact, ceramic Nd3+:YAG has been recently fabricated and used to demonstrate 67 kW [3] and >100 kW [4] of output power at 1.06 μm, respectively. Although there are some remaining issues in ceramic lasers such as lack of availability of high quality commercial raw powders and immature technology for making laser-grade perfect ceramics, polycrystalline ceramics are advantageous over single crystals in many ways. The process is simple, cost effective, and typically carried out at lower temperature. More importantly, much higher doping concentrations in ceramics can be obtained without phase segregation as we often observe in single crystals [5]. In this paper, we discuss the notable achievements and progress in solid state lasing using ceramic materials.
Section snippets
History of ceramic laser materials
While the physical and optical properties of ceramic YAG have been improved so that it is now comparable, if not better, than single crystal YAG, the earlier ceramic lasers were of inferior quality and not necessarily made from YAG. The following highlights some key milestones in the development and demonstration of lasing using ceramic materials.
High power lasers: towards 100 KW and beyond
There have been some significant achievements that have led to the output power to increase from 1 KW and break the 100 KW mark. Latham et al. [28] reported 6.5 KW output power from a thin disk laser based on a 200 μm thick 9%-Yb:YAG ceramic active medium with a 1 mm thick undoped YAG cap to mitigate thermal loading (Fig. 12). Several similar thin disk lasers were combined to generate a total of >25 KW output [29].
Three other groups have developed high power slab based lasers as shown in Fig. 13
Summary
It took more than four decades to realize 100 KW output power from polycrystalline ceramic since the first demonstration of lasing in 1964 using Dy2+:CaF2 ceramic. Major technical breakthroughs from various research groups have been reported and these led to today’s success. They include the capability of producing high purity powder, novel ceramic sintering technology and innovation in optics/device design. Other notable achievements including highly doped microchip lasers, ultrashort pulse
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