Abstract

A motivation for achieving high fourth-harmonic-generation (FHG) conversion efficiencies in glass laser-fusion systems lies in the increased laser-target coupling efficiency at shorter wavelengths. To investigate the factors involved in obtaining high-conversion efficiencies using KDP, we set up a laser system consisting of a Nd:YLF oscillator and glass amplifier. We apertured the output Gaussian beam to 1.7 mm to obtain a flat-topped spatial profile. The system produced peak intensities of up to 8 GW/cm² in 150-ps Gaussian pulses at the fundamental (1.054-μm) wavelength. Conversion to the second harmonic was achieved in a separate KDP crystal using type II phase matching. With this system, we achieved 80% internal second-to-fourth harmonic conversion efficiency in a type I 1-cm thick KDP crystal. We believe this is the highest second-to-fourth harmonic conversion efficiency obtained to date in KDP and attribute it to careful aperturing of the beam to obtain a flat spatial profile.

© 1988 Optical Society of America