Researchers at the Institut für Laser Physik at Universität Hamburg in Germany have set a record for blue CW power generated from a diode-pumped solid-state laser, using a new nonlinear crystal, BiB3O6 (BiBO), to frequency-double the 946-nm line of the Nd:YAG (Figure 1). The intracavity frequency-doubled laser produced up to 2.8 W of 473-nm light when it was pumped with 21 W of 808-nm radiation from a diode laser, an approach that may lead to more efficient, less expensive blue lasers in the commercial market.Figure 1. Researchers at Universität Hamburg have set a record for the generation of blue light from a diode-pumped solid-state laser. The 946-nm laser's mirrors had low reflectivity at 1064 nm to suppress the Nd:YAG's stronger line at that wavelength. The 946-nm transition terminates on the upper Stark-split branch of the ground state, which is approximately 0.7 percent populated at room temperature. This quasi-three-level behavior causes laser photons to be reabsorbed by the ground level in the unpumped regions of the laser rod. To prevent this reabsorption, the researchers matched the longitudinal pump beam very closely to the intracavity laser mode.Type I phase-matching in nonlinear crystal requires that the laser be linearly polarized. But because thermal gradients in the laser rod are circularly symmetric, the rod's thermal birefringence is in polar coordinates. That is, the fast and slow axes have different orientations at different locations across the rod's cross section, so linearly polarized light passing through the rod becomes depolarized. This intracavity depolarization loss can seriously diminish the output power.Other researchers had shown that placing a quarter-wave plate between the laser rod and the back mirror will cause the worst parts of the depolarization to be reversed on a second passage through the rod. The Hamburg group borrowed this simple trick to boost its laser's output by more than 25 percent.Figure 2. The laser produced 4.6 W of 946-nm radiation in the initial configuration (a). With an intracavity frequency-doubling crystal of LBO, BBO or BiBO (b), the setup yielded 1.5, 2.1 or 2.8 W of 473-nm light, respectively. In the arrangement used in the experiment, the laser produced 4.6 W of polarized 946-nm output in the configuration that included a 3.3 percent output coupler for that wavelength (Figure 2, a). When the output mirror was replaced with a high-reflection mirror for both the IR and blue wavelengths and a BiBO crystal was placed in the resonator (Figure 2, b), blue output of 2.8 W was obtained through the second curved folding mirror, which was highly transmissive at 473 nm.The group also experimented with two better-known nonlinear crystals in this second configuration. A 10-mm-long LiB3O5 (LBO) crystal produced 1.5 W, and an 8-mm-long ß-BaB2O4 (BBO) crystal produced 2.1 W. The 10.4-mm-long BiBO crystal produced the record-setter.