Design and performance of the vegetation canopy Lidar (VCL) laser transmitter

Design details of the vegetation canopy laser (VCL) for the NASA multi beam laser altimeter (MBLA) earth orbit mission are presented. The laser is a Nd:YAG Q-switched, diode side-pumped, zig-zag slab design producing 10 ns, 15 mJ pulses at 1064 nm. It employs an unstable resonator as well as a graded reflectivity output coupler with a Gaussian reflectivity profile. A cylindrical lens of undoped YAG collimates the 809 nm pump diode radiation and focuses it into a region ∼ 1.5 mm wide down the 100 mm long Nd:YAG slab. In order to conserve power, a conductively cooled design is employed. The laser is designed to operate over a range of 25°C without active thermal control. Passive cooling is achieved with a large thermal radiator panel consisting of heat-pipes and a change-of-state element. The laser is equipped with a 15× beam expander to limit the output divergence to less than 60 μrad. One discussion will explain the compensation of the thermal lens created in the side-pumped slab and the different treatments of the x and y portions of the z-directed beam in order to obtain an output beam of near circular symmetry. Paraxia modeling was used along with performance data to determine the optimum location of the thermal compensating lens, and how the slab´s thermal lens changed with temperature. This is due to the change in inversion density resulting from the varying overlap of the pump radiation spectral profile with the location of the Nd:YAG pump bands. Performance data as a function of temperature are given. The total number of shots to date and any change in performance as a function of shot count are presented.