THERMAL OPTIC DISTORTION

Abstract

Abstract : The optical distortion in neodynium doped glass which is induced by pump radiation is discribed. Optical distortion was observed at 6328A and the optical path length was found to be dependent on four primary effects: (1) change in physical length; (2) Change in refractive index due to temperature rise; (3) Change in index resulting from stress; (4) Change in index associated with an excited state population of neodymium ions. The experimental techniques used and the results obtained are presented. Included are measurements of optical path length variations, pump-induced birefringence, change in physical length, change in refractive index, bulk temperature rise, and the deflection of a light beam. The theory of thermal optic distortion was developed for the first time to include Fermat's principle. This approach leads to equations defining both the slope and trajectory of rays through the material. The resulting equations are employed to predict ray refraction, beam divergence, and the optical path length through the material as a function of radius, time, and polarization. Good agreement between theory and experiment is achieved provided a new term is added to the expression for the change in refractive index. This term arises from the fact that the polarizability of the neodymium ion in its excited 4F(3/2) level is different from its value in the 4I(9/2) ground level. The inclusion of this new term in the expression for the change in refractive index implies that large optical distortions can exist in 'athermalized' glass.