UV LASER FOR RAYLEIGH BEACON

Ground-based satellite-imaging requires adaptive optics to mitigate atmospheric distortion. These systems require the use of a laser guide-star in the upper atmosphere to produce a point source which can be used to measure the atmospheric distortion to the target of interest. Closed-loop correction requires sampling the atmospheric phase distortion at ~10x the Greenwood frequency, nG, and at sufficient spatial resolution. Rayleigh beacons are most efficient in the ultraviolet due to the lambda-4 scaling of the scattering cross-section and absorption in aircraft windows. With nG, scaling as lambda-6/5, operation in the UV will require an imaging system operating at a Pulse Repetition Frequency(PRF) ≥10 kHz. Simulation of the Rayleigh return for military telescopes exhibiting an aperture of ≤2 m suggests that UV laser systems operating from 343 to 355 nm should produce a pulse energy >50 mJ at a PRF variable from ~2 to >10 kHz. High Pulse Energy UV lasers exhibiting an average power of 500 W and this pulse energy do not exist today. At TP Engineering (TPES), our initial product was a 100 kHz femtosecond laser producing 60 W at 1030 nm, 36 W at 515 nm and 20 W at 343 nm. Delivery of this product resulted in a subsequent contract to increase the output of this laser to >1 kW at 1030 nm, ≥ 500 W at 515 nm and >250 W at 343 nm with a PRF variable from 50 to 200 kHz. While this laser cannot meet the specific requirements for the Rayleigh Beacon, minor modifications of the laser head operated as the amplifier in a Master Oscillator/Power Amplifier architecture can. Specifically, we will develop a higher pulse energy master oscillator employing an electro-optically modulated fiber laser followed by a Photonic Crystal amplifier to enable a PRF variable from ~2 to 15 kHz and a pulse duration of 5-8 nsec. A diode-pumped, solid-state pre-amp increases the pulse energy to enable 200 mJ per pulse extraction from the power amplifier at a PRF variable from 2 to 10 kHz. High-average power Third Harmonic generation produces the 50 mJ per pulse in the UV. Phase I will execute a trade study to determine the best performing option of Nd or YbYAG as the gain material resulting in 355 or 343 nm at the third harmonic, respectively. Phase I will then conclude with the selected option matured into a Preliminary design complete with a validated model and simulation of a compact, rugged system capable of providing 50 mJ at 3w and a PRF of 10 kHz (500 W in the UV). Phase II will construct an operating prototype (Brassboard) that meets all specifications.