A diode-pumped Cr:LiSAF laser for UAV-based water vapor differential absorption lidar (DIAL)

An autonomous, compact, high-energy, injection seeded, diode-pumped, tunable Cr:LiSAF laser for a water vapor differential absorption lidar (DIAL) system deployed on a high-altitude unpiloted airborne vehicle (UAV) has been developed. A unique laser resonator, consisting of a Cr:LiSAF slab pumped by 8 high-power diode bar stacks in a total internal reflection configuration, provides high extraction efficiency and good laser beam quality. Output pulse energies of >35 mJ at 834 nm and 25 mJ at 816 nm, with pulse widths of ~90 ns at 6 Hz pulse repetition rate, were obtained. A linewidth of less than 0.01 cm ^-1 was achieved by injection seeding with an 816 nm DFB diode laser. Using a photo-acoustic cell (PAC), an innovative seeding system was utilized to lock the DFB laser wavelength to a water vapor absorption line and to consistently seed the slave laser at a desired wavelength within the absorption line. A high spectral purity (>99%) and excellent tuning performance were demonstrated. Water vapor profiles measured with a DIAL system utilizing this laser compare well with radiosonde data taken before and after the measurements