Thermal lens in elastic and viscoelastic plates

A theoretical analysis is made of the formation of a thermal lens in a thin plate due to changes in the refractive index and bending of the surfaces as a result of nonuniform heating by high-power laser radiation. Both the elastic and plastic behavior of the plate material is studied. The first case is investigated using an analytic solution of the decoupled thermoelasticity problem and the second case is studied using an elastic–viscoelastic analogy. The parameters of the thermal lens are determined and a detailed investigation is made of the dynamics of the lens effect. A comparison is made between surface (due to changes in the thickness and deformation of the surfaces) and bulk (due to changes in the refractive index) lens effects which are found to be of comparable order of magnitude. The maximum radiation levels which can be transmitted by a plate in the presence of optical distortions not exceeding the diffraction level, are estimated for many materials used in infrared engineering. The results can also be used to describe the deformation of the surface of a thin plane mirror exposed to high-power laser radiation.