0.85 Micron Solid State Laser Material Evaluation. Part 3

This report describes the final phase of the program for the development of a 0.85 micron optically pumped laser material: Er(3+):YLF. Laser operation is obtained at room temperature in this material via stimulated 4 S 3/ 2 to 4 I 13/2 transitions. Er:YLF is a true four-level laser. The relationship between growth parameters and feed purity and their effects on crystalline quality were investigated. Dramatic improvement in optical quality was obtained in growth runs using argon as the furnace cover gas. Preliminary evidence of further improvements in crystalline quality (comparable to Nd:YAG) resulted from the use of recrystallized feed. The physical properties of YLF are reviewed and the results applied to a calculation of the thermal loading at fracture of a YLF rod uniformly heated and cooled at the surface. The calculated value is 11 watts/cm at thermal fracture corresponding to predicted output power of 7.6 watts/cm. Spectroscopic studies focused on the effects of increased Er(3+) concentration on laser efficiency. The temperature and concentration dependence of the lifetime of the upper laser level were measured. In flashpumped operation at room temperature an optimum concentration of approximately 5% is predicted. Comparative measurements show significantly improved laser efficiency with Er(3+) concentration over the range 2-3.5%. Improved laser performance was observed using rods grown in argon. Prepared in cooperation with Massachusetts Inst. of Tech., Crystal Physics Lab. See also Part 2, AD0917091. Sponsored in aprt by DARPA.