Intensity-dependent refractive index in a nonresonant cw Raman laser that is due to thermal heating of the Raman-active gas

The refractive index of H2 is shown to decrease linearly as a function of Stokes power and, to a much lesser extent, pump power in a nonresonant cw Raman laser. The dominant source of the index shift is shown to be thermal and significantly larger than dispersion associated with the Raman resonance. A steady-state theoretical model based on internal heating that is due to inelastic Raman scattering events accurately describes the observed behavior. With this model, frequency pulling of the Raman cavity resonance and phase distortions of the intracavity Gaussian beam are predicted for various levels of generated Stokes power.