Origin and Future of Polycrystalline Ceramic Lasers

Since highly efficient laser oscillation by Nd:YAG ceramics has been reported, various types of transparent ceramics have been developed and also power scaling has been performed. Theoretical clarification on why polycrystalline ceramics having numerous structural defects (grain boundaries) could generate highly efficient laser oscillation is very important, and it will be a road map for the development of polycrystalline laser gain media for the future. The sintering aid was useful for reducing optical loss of ceramics but further improvements in performance were obtained by the development of a laser gain medium without using sintering aids and as a result a slope efficiency of 74% with an output power of 1.8 kW was obtained. This material had 5.5 kW output as a maximum and its power density exceeded 1 MW/cm
³
. Forming composite gain media by ceramic technology is very important for further power scaling and sophistication of laser functionality. In this report, advantages of bonding technology to form composite gain media are described. Also, by removing the grain boundary completely from the ceramic material, high quality single crystal can be synthesized. The slope efficiency of laser oscillation from 2.4% Nd:YAG polycrystal and single crystal showed high values of 57% and 63%, respectively.