Jackson S D, Piper J A
Appl Opt. 1996 Mar 20;35(9):1409-23. doi: 10.1364/AO.35.001409.
Calculations and experimental measurements of the thermally induced strain and birefringence are presented for a diode-pumped Nd:YAG rod that is encapsulated in a prismatic pump light collector. A numerical model is developed to determine the spatiotemporal stress-induced strain distribution across the prism, index-matching fixant, and laser rod, and the birefringence that arises from the stress-induced strain within the laser rod. Calculations of the birefringence are compared with polarscopic 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 the laser rod are therefore influenced by the geometry and composition of the prism and fixant. These strains are thermomechanical in origin and are primarily a function of the elastic modulus of the fixant and the temperature of the system. Such stress-induced strains are additional to those strains that are produced from temperature gradients across the laser rod and result from the laser rod being constrained from expanding. Collectors utilizing index-matching fluid as the encapsulant display the smallest measure of birefringence relating to the temperature gradients in the rod. However, for collectors utilizing solid fixants (with significant elastic modulus), an increase in the birefringence results. In this case collector designs that have the laser rod located in a symmetrically shaped prism are effective in reducing the nonuniform pressures on the sides of the rod and therefore the birefringence.
本文给出了封装在棱柱形泵浦光收集器中的二极管泵浦Nd:YAG棒的热致应变和双折射的计算结果与实验测量值。建立了一个数值模型，以确定整个棱镜、折射率匹配固定剂和激光棒上时空应力诱导应变的分布，以及激光棒内应力诱导应变产生的双折射。将双折射的计算结果与偏光测量结果进行了比较，两者吻合良好。棱镜和固定剂对棒提供全方位支撑，因此激光棒内应力诱导应变（和双折射）的分布和程度受棱镜和固定剂的几何形状及成分影响。这些应变源于热机械作用，主要取决于固定剂的弹性模量和系统温度。这种应力诱导应变是激光棒上温度梯度产生的应变之外的额外应变，是激光棒受约束无法膨胀导致的。使用折射率匹配流体作为封装材料的收集器，与棒内温度梯度相关的双折射量最小。然而，对于使用固体固定剂（具有显著弹性模量）的收集器，双折射会增加。在这种情况下，将激光棒置于对称形状棱镜中的收集器设计，能有效降低棒侧面的不均匀压力，从而减小双折射。
