Researchers Dennis Lowenthal and Jon Seamans of Aculight Corp. (Bellevue, WA) have demonstrated conversion of 1064-nm output from a Nd:YVO4 laser to 1.5 and 3.6 µm using an optical parametric oscillator (OPO) based on periodically poled lithium niobate (PPLN). The pump source—a Spectra-Physics Lasers (Mountain View, CA) FC-bar diode-pumped laser—provided an output of 0.5 W with a pulse-repetition frequency of 20 kHz and is currently used by the US Air Force for laser radar imaging. The PPLN crystal was fabricated by Deacon Research (Palo Alto, CA). The demonstration is believed to be the first of high-repetition-rate, low-pump-energy conversion in a commercially fabricated PPLN crystal.
The device exhibited a low threshold energy of about 12.5 µJ in a 10-mm-long PPLN crystal. The material has a period of 29.3 µm, is 0.5 mm thick, and 1.0 cm long. The largest signal output—at 1.5 µm—with single-pass pumping, was 120 mW with 510 mW of pump power in a symmetric cavity. At 100 mW, beam quality of the PPLN OPO signal output was found to have an M2 value less than 1.3. Experimental data of threshold and slope efficiency were in good agreement with a time-dependent OPO model that includes diffraction.
According to Lowenthal, cavity length and absolute beam sizes strongly affect the OPO performance. Experiments to date have been limited by the availability of mirror mounts and cavity-mirror curvatures required to reduce the cavity length. Lowenthal notes that shorter cavities as well as longer PPLN material can be used and will improve the efficiency and reduce the threshold. The work was supported by Eglin Air Force Base under a Phase I SBIR (Small Business Innovation Research) program.