Thermal distortion of diffraction-limited optical elements

Recent developments of laser materials have advanced the state of the art to a point where the optical quality of many of these materials is approaching the diffraction limit. Using such components in a laser does not necessarily guarantee the generation of diffraction-limited laser beams. One of the severe problems is thermal distortion introduced in the optical cavity by the flash lamps. Ruby and glass lasers require a minimum of 0.6 J of heat deposition per joule of population inversion. Typical figures are 4-6 J per joule of population inversion.(1) Nonuniformities in the deposition of this heat cause optical distortions which virtually preclude diffraction-limited laser operation even if the materials themselves are of diffraction-limited optical quality. This paper will investigate these thermal effects in detail, and the relative sensitivity of a variety of materials to nonuniform energy depositions will be discussed. Water close to its point of maximum density and a certain special type of glass known as Pockels glass will be shown to have properties of particular interest for use in diffraction-limited lasers.