Resonators with Variable Internal Lenses

Abstract

In solid state lasers the active medium exhibits the properties of a thermally induced lens when the material is pumped. The refractive power is brought about by a combination of heat generation due to absorption of pump and laser radiation and the flow of heat to the outer periphery due to cooling. For a laser rod this leads to a temperature distribution which is parabolic with respect to the radial position r if the thermal conductivity is constant and the medium is illuminated homogeneously by the excitation source (Fig 12.1). This results in a similar radial dependence of the index of refraction:

$$n(r) = {n_0}\;(1\; - \;\gamma {r^2})$$

((12.1))

where n ₀ is the index of refraction at the center. A bundle of parallel rays incident on the laser rod is focused by the thermal lens for γ > 0 since the optical path length n(r)ℓ inside the medium is longer for rays being closer to the center of the rod.