Measurement of the coefficient of electronic nonlinearity in optical and laser glass

The electronic nonlinearity coefficient n₂ of transparent dielectrics was determined by direct observation of the self-focusing of smooth single-mode nanosecond laser pulses. The critical self-focusing power for glass of a given type P_cr was found from a series of measurements of the threshold power P_th for different effective specimen lengths L_eff = z_sf, thus eliminating errors associated with the determination of the beam width. The required set of z_sf values was obtained by varying the focusing or defocusing of the radiation on the entrance face of a single specimen. The experimental dependence of P_th^1/2 on (z_sf^–1), enabling P_cr to be determined by the intercept on the ordinate axis, was only linear up to some value of P above which the regular self-focusing pattern was disrupted by the development of transverse perturbations. Data on P_cr and n₂ were obtained for some Soviet glasses (KGSS 1621, GLS 1, LGS 247).