PHASE DISTORTIONS IN SUM-FREQUENCY AND DIFFERENCE-FREQUENCY MIXING INCRYSTALS

We shaw that if two waves are incident on a quadratically nonlinear cr ystal, with the third wave generated entirely within the crystal, a ph ase-velocity mismatch (Delta k not equivalent to O) leads to intensity -dependent phase shifts of the generated wave only if there is walk-of f, linear absorption, or significant diffraction of at least one of th e waves as well as significant energy exchange among the waves. The re sult is frequency broadening and wave-front distortion of the generate d wave. Although the induced phase distortions are usually quite small , they may be significant in applications that require high spectral r esolution or pointing accuracy.