Title of Paper: High Efficiency Remote Sensing Laser Technology
Principal Author: Dr. Renny Fields
Abstract: The Aerospace Corporation in collaboration with the Coherent Semiconductor Group is developing high-brightness laser-diode pump-modules to greatly increase the electrical efficiency of solid-state lasers used for space-based remote sensing. The team has devised new diode bars, diffraction limited lens arrays, and a mount structure that will support volume production of 1-mm pitch stacks of quasi-continuous wave (QCW) arrays within a pump module. The general approach relies on efficient collimation of arrays of 100-200 single mode emitters with matching diffraction limited Gallium Phosphide micro-lens arrays. Parametric studies of diode laser emitter width (2.5, 3.5 & 4.5 microns) versus beam quality, total power and reliability are being traded against the effective divergence resulting from cumulative micron-level mounting errors. To date we have demonstrated 70 watts peak from several arrays at 50% electrical efficiency. We have achieved far field spot sizes 1.5 times the diffraction limit from GaP collimated arrays. A non-optimized lens array transmitted 70% of the energy to the final spot which should lead to a > 15% wall plug Nd:YAG laser. As we gradually improve the systems engineering of the mount, we are also optimizing the diode laser re-growths, facet passivation, lens collection efficiency and lens through-put. Ultimately we believe we can deliver collimated pump photons to the laser mode with 50% wall plug efficiency. We are currently constructing a 10-bar stack that will be capable of 500-600 watts peak power collimated from a 1-cm2 aperture. Though not optimized for heat removal, the primarily copper structure should easily sustain a 5% duty factor. A 50 mj q-switched Nd:YAG laser is currently being developed using the first collimated pump module.