Fourth harmonic generation in BBO and KD*P was investigated using a phase-conjugated Nd:YAG amplifier as a
pump source. The 1.1J, 1.06i source was converted to 0.55J @ 0.53j.t as the pump source for the investigation.
Energy outputs of 250mJ at 0.27ji. were obtained. Control of the angular dephasing error was found to be critical for
good conversion efficiency, particularly in BBO. We measured internal conversion efficiencies of 70% with the
12.7X12.7X2Omm KJYP crystal, and 58% with the 1OX1OX4.6mm BBO. Analysis correlated well with the
experimental results in the case of KD*P, but implied a higher nonlinear coefficient for BBO than is reported in the
literature. An energy imbalance of 15% due to nonlinear absorption was observed for both crystals when they were
tuned for optimum 0.27p. generation. This was found to cause rapid thermal dephasing during 10-20Hz operation.
We conclude that, using either of these materials, careful thermal management is necessary in obtaining efficient
fourth harmonic conversion for higher average power operation.