Efficient all-solid-state UV lidar sources: from 100s of millijoules to 100s of microjoules

Abstract

Sandia National Laboratories has developed high-energy all-solid-state UV sources for use in laboratory tests of the feasibility of satellite-based ozone DIAL. These sources generate 320 nm light by sum-frequency mixing the 532 nm second harmonic of an Nd:YAG laser with the 803 nm signal light derived from a self-injection-seeded image-rotating optical parametric oscillator (OPO). The OPO cavity utilizes the RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle. Two configurations were developed, one using extra-cavity sum-frequency mixing, where the sum-frequency-generation (SFG) crystal is outside the OPO cavity, and the other intra-cavity mixing, where the SFG crystal is placed inside the OPO cavity. Our goal was to obtain 200 mJ, 10 ns duration, 320 nm pulses at 10 Hz with near-IR to UV (1064 nm to 320 nm) optical conversion efficiency of 25%. To date we've obtained 190 mJ at 320 nm using extra-cavity SFG with 21% efficiency, and >140 mJ by intra-cavity SFG with efficiency approaching 24%. While these results are encouraging, we've determined our conversion efficiency can be enhanced by replacing self-seeding at the signal wavelength of 803 nm with pulsed idler seeding at 1576 nm. By switching to idler seeding and increasing the OPO cavity dimensions to accommodate flat-top beams with diameters up to 10 mm, we expect to generate UV energies approaching 300 mJ with optical conversion efficiency approaching 25%. While our technology was originally designed to obtain high pulse energies, it can also be used to generate low-energy UV pulses with high efficiency. Numerical simulations using an idler-seeded intra-cavity SFG RISTRA OPO scaled to half its nominal dimensions yielded 560 μJ of 320 nm light from 2 mJ of 532 nm pump using an idler-seed energy of 100 μJ.