Abstract

Absorption of the pump and harmonic beams in nonlinear frequency conversion processes leads to heating of the nonlinear medium and a spatially varying thermal dephasing profile. For high-average-power fundamental beams this thermal dephasing can produce significant losses in power conversion efficiency to second harmonic light. The conversion efficiency is examined for Gaussian temporal pulses; circularly symmetric beams with uniform or Gaussian transverse intensity profiles are considered. For most pulse rates and nonlinear crystals of interest, the thermal dephasing is essentially steady in time. There are two cases of interest: equal and unequal absorption coefficients. For identical absorption coefficients, the thermal and optical problems are uncoupled. In this case it is shown that the conversion efficiency can be a strong function of the radius at which there is zero thermal dephasing. Optimal dephasing radii and conversion efficiency as a function of nonlinear drive and thermal dephasing are calculated for both spatial profiles. For unequal absorption coefficients, the thermal and optical problems are fully coupled. An iterative numerical procedure to solve the problem has been developed. Two cases of interest (fundamental absorption much greater than second harmonic absorption and vice versa) are compared with the equal-absorption-coefficient case.

© 1992 Optical Society of America