From green laser pointers to sophisticated surgical instruments, frequency-doubled solid-state lasers have found countless successful applications during the past several decades. Now, by combining the mature technology of frequency doubling with the emerging technology of distributed-feedback fiber lasers, scientists at the University of Aarhus in Denmark have opened what they believe are several exciting new commercial possibilities.For one thing, because the linewidths of distributed-feedback lasers are very narrow, the new frequency-doubled lasers can have exceptionally narrow bandwidths, which is an important consideration in spectroscopic and other scientific applications. Moreover, because the fundamental wavelength of a distributed-feedback fiber laser can be varied with different rare-earth dopants, frequency-doubled and -quadrupled wavelengths across much of the visible and ultraviolet spectrum can be obtained readily.For its demonstration, the group started with an ytterbium-doped germanosilicate fiber laser from Koheras A/S of Birkerød, Denmark. The laser’s narrowband distributed feedback restricted its linewidth to less than 35 kHz (averaged over 125 μs) at a wavelength of 1088 nm. They frequency doubled the laser’s output in a 15-mm-long LiNbO3 crystal supplied by Fujian Castech Crystals Inc. of Fujian, China. Thanks to seven percent magnesium doping of the LiNbO3, the scientists avoided the photorefractive damage associated with intense green light in that crystal.Figure 1. Scientists achieved a 55 percent conversion efficiency (1088 nm to 544 nm) by resonating the fundamental wavelength in an external cavity. Reprinted with permission of Optics Letters.Because extracavity frequency-doubling efficiency of continuous-wave lasers is inherently low, the scientists resonated the fundamental wavelength through the nonlinear crystal with a bow-tie resonator (Figure 1). The resonator’s two concave mirrors focused the 1088-nm power to an ∼28-μm waist in the nonlinear crystal, and an electronic feedback loop locked the LiNbO3 cavity frequency to the fiber laser’s. With a little more than 1.5 W of fundamental power incident on the bow-tie resonator, they observed a second-harmonic output at 544 nm of 845 mW, corresponding to an external conversion efficiency of 55 percent. The LiNbO3 crystal was oriented in the external cavity for type-I phase matching with light propagating at 90° to the optic axis, so there was no walk-off between the fundamental and second-harmonic beams in the crystal. As a result, the excellent beam quality of the fiber laser was preserved in the second-harmonic output. When they measured the 544-nm beam quality according to ISO 11146, the scientists found its M-square value was 1.05.Figure 2. Another bow-tie cavity resonated the 544-nm second harmonic from Figure 1 through a BBO crystal to generate 272 nm with 15 percent conversion efficiency.This high-quality beam was ideally suited for further frequency conversion into the ultraviolet, a feat the investigators accomplished with another bow-tie resonator (Figure 2). In this case they used beta-barium-borate (BBO) as the nonlinear crystal and again locked the external cavity’s frequency with a feedback loop. BBO must be angle-tuned to achieve phase matching at 272 nm, so walk-off occurred between the fundamental and the second harmonic, and the quality of the 272-nm beam was somewhat degraded. With a 544-nm input of 730 mW, the scientists observed a 272-nm output of 115 mW, for an external conversion efficiency of 14 percent.Optics Letters, Feb. 1, 2007, pp. 268-270.