  FM-to-AM conversion can cause many negative effects (e.g., reducing of margin
against damage to the optics, and etc.) on performances of third-harmonic
conversion system. In this letter, the FM-to-AM conversion effect in
third-harmonic generation is investigated both at and away from the retracing
point of type-II KDP crystal. Obtained results indicate that the FM-to-AM
conversion can be suppressed effectively when the crystal works at the
retracing point.
