THERMALLY-INDUCED STRAIN AND BIREFRINGENCE CALCULATIONS FOR A ND-YAG ROD ENCAPSULATED IN A SOLID PUMP LIGHT COLLECTOR

Calculations and experimental measurements of the thermally induced st rain and birefringence are presented for a diode-pumped Nd:YAG rod tha t is encapsulated in a prismatic pump light collector A numerical mode l is developed to determine the spatiotemporal stress-induced strain d istribution across the prism, index-matching fixant, and laser rod, an d the birefringence that arises from the stress-induced strain within the laser rod. Calculations of the birefringence are compared with pol arscopic measurements and display good agreement. Support for the rod on all sides is provided by the prism and fixant, and the distribution and degree of the stress-induced strain (and birefringence) within th e laser rod are therefore influenced by the geometry and composition o f the prism and fixant. These strains are thermomechanical in origin a nd are primarily a function of the elastic modulus of the fixant and t he temperature of the system. Such stress-induced strains are addition al to those strains that are produced from temperature gradients acros s the laser rod and result from the laser rod being constrained from e xpanding. Collectors utilizing index-matching fluid as the encapsulant display the smallest measure of birefringence relating to the tempera ture gradients in the rod. However, for collectors utilizing solid fix ants (with significant elastic modulus), an increase in the birefringe nce results. In this case collector designs that have the laser rod lo cated in a symmetrically shaped prism are effective in reducing the no nuniform pressures on the sides of the rod and therefore the birefring ence. (C) 1996 Optical Society of America