BiB3O6 Generates 990-mW Second Harmonic with 52 Percent Efficiency
Nonlinear optics has proved to be a valuable tool for extending the tuning range of fixed-wavelength lasers into new spectral regions. When outputs in the visible and ultraviolet are desired, two nonlinear crystals, β-BaB2O4 (BBO) and LiB3O5 (LBO), are the traditional choices, but recent experiments conducted at the Institut de Ciencies Fotoniques in Barcelona, Spain, indicate that a lesser-known crystal, BiB3O6 (BIBO), may be superior in some cases.
Numerous investigators have evaluated the material in both intracavity and external frequency-doubling configurations, but it seems to shine brightest with low-power continuous-wave or high-repetition-rate lasers. In these cases, its higher nonlinearity gives it an edge over other crystals. (Nonlinearity is less important with high-power lasers because second-harmonic conversion efficiency scales with fundamental power. Thus, a high-power laser can achieve acceptable conversion efficiency even with less-nonlinear crystals.)
The researchers in Barcelona measured BIBO’s nonlinearity to be 3.7 pm/V versus ~1 to 2 pm/V for BBO and LBO. Even a small difference in these values is important because second-harmonic conversion efficiency is proportional to the square of the nonlinearity.
But high nonlinearity is not the only requirement for an efficient frequency-doubling crystal. In the low-power regime, where long crystals and tight focusing must be used, the crystal also must have a low walk-off when it is angle-tuned and a high angular acceptance to allow for tight focusing into the crystal. According to the data collected in the recent experiments, BIBO also satisfies these requirements.
In their evaluation of the material, the scientists illuminated a 10-mm-long BIBO crystal with 2.4-ps pulses from a Mira 900 Ti:sapphire laser from Coherent Inc. of Santa Clara, Calif., that was tunable between 740 and 900 nm and pumped by a Coherent Verdi V10 CW diode-pumped laser at 532 nm. The crystal was aligned for type-I phase matching; i.e., fundamental in extraordinary polarization, second harmonic in ordinary. The incoming average power was ~1.9 W, and the peak power of the ~25-nJ pulses was ~10.4 kW. The BIBO faces were antireflection-coated, so virtually all the incident power entered the crystal.
Figure 1. Blue light scattered from a 990-mW beam illuminates an optical table.
The investigators observed the best results by focusing the fundamental radiation to a 50-μm spot in the nonlinear crystal (Figure 1). With the Ti:sapphire laser tuned to 820 nm, the BIBO crystal converted as much as 54 percent of the fundamental power to the second harmonic (Figure 2). The greatest blue (410 nm) output was 990 mW from a fundamental power of 1.9 W. The scientists believe that the saturation in efficiency, visible in the figure, resulted from the onset of two-photon absorption in the nonlinear crystal.
Figure 2. With the Ti:sapphire laser tuned to 820 nm, the BIBO crystal converted as much as 54 percent of the power to the second harmonic in a single pass through the crystal. ©OSA.
The second-harmonic output was highly stable, with a peak-to-peak variation of less than 2 percent. The researchers tuned the second harmonic from 370 to 450 nm by tuning the Ti:sapphire laser and simultaneously adjusting the phase-match angle of the BIBO crystal. They maintained the second-harmonic power at a level of more than 0.5 W across the entire spectral range, and the conversion efficiency was above 50 percent.
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