Abstract

High power UV radiation is very attractive for many industrial applications including ultrahigh-density optical disk mastering, photolithography, material processing, marking, as well as for medical surgery and science¹. While the excimer laser is the leading candidate for the UV source in these applications, fourth harmonic generation (FHG) from solid state lasers, with their compactness and availability of diode-laser pumping, offers a viable alternative to excimer lasers. For solid state lasers, efficient frequency conversion methods must be used to generate high UV power. For low power high-repetition-rate lasers the most promising approach is to use external resonant cavities.^2-4 in this paper we report the use of a single resonant optical cavity to generate the fourth harmonic from the fundamental. The idea is to generate the second harmonic and the fourth harmonic both in one cavity consisting of mirrors that are highly reflecting at the second harmonic only. Hence the fundamental IR radiation is coupled into the cavity simply by a dichroic mirror. Energy is out-coupled from the cavity in the form of fourth harmonic UV radiation. The need for impedance matching is thus eliminated. This idea was put forward by Giordano and Matone for FHG from mode-locked lasers.^5,6 in that case, in order to gain high conversion, it was necessary to match the length of the ring cavity to the IR laser length of 3.96 m with tight tolerance. The small beam size on the crystals led to thermal distortion and complex spatial mode pattern for the UV output. In this paper, we demonstrate that high UV conversion can be achieved from nanosecond IR pulses of energy in the mJ range using a single resonant ring cavity of arbitrary length and consisting of flat mirrors that are highly reflecting at the second harmonic. Using a Q-switched Nd:YAG laser running at 30 Hz with 213 mW of average IR power, we obtained 84 mW in the fourth harmonic at 266 nm. This corresponds to an internal IR to UV conversion of 39.5% (see figure 1). The output UV beam had a near-TEM₀₀ mode with a 95% fit to a Gaussian profile. Enhancement of >65 by the ring over single pass UV generation was measured.

© 1996 Optical Society of America