A source of hot spots in frequency-tripled laser light

Laser light, frequency tripled to the ultraviolet (UV), shows numerous, strongly modulated hot spots in the target plane for the OMEGA laser used in laser-fusion experiments. However, at the fundamental frequency in the infrared (IR), the hot-spot modulation is relatively small. High-resolution measurements of the intensity and phase in the laser near field, combined with computer modeling of beam propagation to the target plane, show that the intensity fluctuations are dominated by the near-field phase errors. The appearance of hot spots in the UV and not in the IR are found to be the result of phase-error tripling caused by the frequency conversion.