Time-resolved interferometric technique and measurements of the nonlinear refractive index of laser materials

Abstract

Intensity-dependent changes in the refractive index of optical materials associated with the propagation of intense laser beams cause self-focusing and beam breakup and a resultant loss of focusable energy. For long amplifier chains, such as used in Nd-glass fusion lasers, beam degradation originates in many transmissive elements. These include the lasing medium, lenses, windows, polarizer substrates, Faraday rotators, and nonlinear crystals. The beam degradation is proportional to n2/n(n − 1) for lenses, to n2/n for laser rods, windows, and rotators oriented normal to the beam, and to n2/n2 for laser disks at Brewster’s angle, where n and n2 are, respectively, the linear and nonlinear refractive indices. In all cases low n2 values are desirable. Accurate data on n2 is required for optimum staging of components in high-power laser systems.