Multimode Resonators, Insensitive against Thermal Lensing

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Abstract
Thermal lensing of the active medium inside a laser resonator leads to a dependence of the beam divergence on the optical pumping power. Two effects influence the multimode divergence: the fundamental mode divergence θ 0 and the mode order m. By using a suitable resonator set-up the variations of the two quantities with pumping power can be arranged to offset each other so that the resultant beam divergence remains constant. When the condition for insensitivity is derived the resulting constant divergence is found to be θ ma = R/d i, where R is the radius of the limiting aperture of the active medium, and d i is the distance from the mirror Si to the principal plane of the active medium. Experiments with Nd-YAG/glass lasers have confirmed these theoretical results.

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