Nonlinear Crystal Generates 5 W of CW Deep-UV

Breck Hitz

The tight lock that excimer lasers had enjoyed for many years on the market for high-power ultraviolet sources has been loosened somewhat in the past several years by the advent of enhanced nonlinear optics that can convert the output of solid-state lasers into the ultraviolet spectral region. These new solid-state sources often are pulsed to take advantage of the high peak powers that boost nonlinear conversion efficiencies. Recently, however, new nonlinear crystals have been exploited by several research groups to develop CW solid-state sources in the ultraviolet.

The CLBO crystal in the bow-tie resonator generated 5 W of CW 266-nm radiation from a single-frequency 532-nm input of 9.6 W.

Now, a team from Cyber Laser Inc.'s R&D group and from Keio University, both in Yokohama, Japan, has reported generation of 5 W CW at 266 nm by twice frequency-doubling the 1.06-µm radiation of a Nd:YVO₄ laser. The researchers believe that this power is three times greater than had been reported previously. Such a laser could have applications in disk mastering, semiconductor mask inspection and fiber Bragg grating fabrication.

The experimental arrangement is shown in the accompanying figure. The CW green laser was a diode-pumped, single-frequency, intracavity-doubled Nd:YVO₄ laser that is capable of producing 10 W at 532 nm with an M² beam quality of ~1.1.

Two spherical lenses coupled its output into a bow-tie cavity containing the nonlinear crystal, which was a 5 × 5 × 15-mm CsLiB₆O₁₀ (CLBO) crystal. The cavity was resonant at 532 nm, and a servo controller driving a piezoelectric-mounted mirror ensured that the bow-tie cavity length was resonant with the single-frequency green laser.

The scientists maintained the Brewster-cut CLBO crystal at 140 °C to prevent hygroscopic deterioration and oriented it with respect to the green power for type I phase matching. They coupled the ultraviolet power out of the cavity through a mirror that had 96 percent transmission at 266 nm.

They observed a CW output as great as 5 W at 266 nm when the incident green power was 9.6 W. They calculated that 6.1 W of ultraviolet light was generated in the crystal, but that this level was reduced to 5 W in the output beam by Fresnel reflection within the CLBO crystal (which was Brewster-cut for the 532-nm light but not for the orthogonally polarized ultraviolet light) and by the 4 percent reflectivity of the output mirror. They believe that the overall conversion efficiency of 52 percent is the highest reported for the fourth harmonic of a neodium laser.