Robust, Efficient, Optical-Damage-Resistant, 200 mJ Nanosecond Ultraviolet Light Source for Satellite-Based Lidar Applic
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Robust, Efficient, Optical-Damage-Resistant, 200 mJ Nanosecond Ultraviolet Light Source for Satellite-Based Lidar Applications Darrell J. Armstrong and Arlee V. Smith Lasers, Optics, and Remote Sensing Department Sandia National Laboratories Albuquerque, NM 87185-1423, U.S.A. ABSTRACT Conventional wisdom contends that high-energy nanosecond UV laser sources operate near the optical damage thresholds of their constituent materials. This notion is particularly true for nonlinear frequency converters like optical parametric oscillators, where poor beam quality combined with high intra-cavity fluence leads to catastrophic failure of crystals and optical coatings. The collective disappointment of many researchers supports this contention. However, we’re challenging this frustrating paradigm by developing high-energy nanosecond UV sources that are efficient, mechanically robust, and most important, resistant to optical damage. Based on sound design principles developed through numerical modeling and rigorous laboratory testing, our sources generate 8–10 ns 190 mJ pulses at 320 nm with fluences ≤ 1 J/cm2 . Using the second harmonic of a Q-switched, injection-seeded Nd:YAG laser as the pump source, we convert the near-IR Nd:YAG fundamental to UV with optical-to-optical efficiency exceeding 21%. INTRODUCTION Tunable [1] and fixed-frequency [2] UV sources based on nanosecond optical parametric oscillators (OPO’s) pumped by the harmonics of Nd:YAG lasers have been reported in the literature, with Refs. [1] and [2] being two typical examples. In these sources, the OPO is pumped by the Nd:YAG second harmonic at λ = 532 nm to generate a signal wave with 600 nm λsignal 1064 nm that is subsequently sum-frequency mixed with additional pump light to reach UV wavelengths. The crystal used for sumfrequency-generation (SFG) can be placed outside the cavity (extra-cavity) or inside the cavity (intracavity), with the latter providing the advantage of higher intra-cavity signal fluence for greater SFG efficiency. UV pulse energies for these sources cover a broad range from about 10 µJ up to perhaps 30 mJ, with overall conversion efficiencies (Nd:YAG fundamental to UV) of about 10% or lower. Although the source in Ref. [1] is versatile because of its broad tunability and moderate UV pulse energy of ∼ 20 mJ, a remote sensing application such as satellite-based differential absorption lidar (DIAL) for measuring stratospheric ozone concentration can be satisfied by two fixed-wavelength UV sources, but requires pulse energies 200 mJ. Because electrical power and pump-laser resources are limited by the constraints of satellite deployment, a suitable single-frequency UV source must be highly efficient. Unfortunately high-energy, high-efficiency UV-SFG has proven difficult because it requires very good beam quality from the OPO and from the Nd:YAG laser, and both are notorious for exactly the opposite – very poor beam quality. With poor beam quality, an attempt to increase the UV energy by using more pump light will likely result in
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