Parameters Affecting the Doubling Efficiency

Second-harmonic conversion depends on parameters that are related to the laser source, such as power density, beam divergence, and spectral linewidth, and parameters associated with the harmonic generator, such as the value of the nonlinear coefficient and angular, thermal, and spectral sensitivity to deviations from the exact phase-matching condition.
The most critical laser parameters are power density and beam divergence. In general, one finds that Nd :YAG lasers have sufficiently narrow linewidths for efficient harmonic generation. In these lasers attention is focused mainly on obtaining a diffraction limited beam, i.e., TEM₀₀-mode operation. Nd : glass lasers and tunable lasers require longitudinal mode selection for efficient harmonic conversion. However, the large Nd : glass lasers used in fusion research employ Nd : YLF oscillators which automatically solve the spectral linewidth problem.
The most important crystal parameters are the nonlinear index and the angular sensitivity which are listed in Table 1 for several important materials.

TABLE 1. Properties of important nonlinear materials (data are for 1064 to 532 nm conversion).