Stress-induced birefringence arising from vacuum loading in the spatia
l filter lenses of high-energy laser systems is responsible for laser
beam depolarization which may decrease the frequency conversion effici
ency significantly. In order to predict these effects, we have carried
out coupled mechanical-optical calculations. At first, we validated o
ur predictions with experimental results. Then a parametric study allo
wed us to explain the influence of the geometrical characteristics of
the lenses. Our main result is that symmetric lenses lead to minimum b
irefringence.