Laser Diodes For Free-space Optical Communications

High optical power semiconductor lasers with demonstrated long operating life are an enabling technology for spaced based optical communication links. These links, most commonly connecting two satellites (crosslinks) have become the baseline approach for many satellite systems. This paper will review work in both single mode lasers and partially coherent arrays that have applications in space based systems. Partially Coherent Lasers Evanesently coupled arrays of laser diodes allow high optical power at the expense of multiple spatial mode beams. Coupling these lasers to multi-mode fibers allows a convenient way to circularize and displace the beam. In the late 1980’s we at SDL space qualified a six element array in an hermetically sealed fiber optic package. This device delivered modest power (1 8mW) into a 70 micron core multiple mode fiber. While the optical performance was modest the reliability requirements were stringent. We have collected in excess of 25,000 hours of testing on a population of these lasers without a single failure. Projected life for these devices is in excess of 200,000 hours. Today reliable multimode lasers are available at much higher power and brightness. The beacon laser for the European Space Agencies SILEX program for example couples over 400 mW into an 80 micron fiber1. Even higher power and brightness is reliably obtainable with newer laser designs and direct projection of the light without the brightness loss associated with fiber coupling. Coherent Sources The high data rates and long distances for data crosslinks often drive the design to single spatial mode sources. The inherit advantage over the multimode sources discussed above is the much narrower beam divergence through the optical system for a given final lens size. Reliable single spatial mode lasers based simple single waveguides are commercially available in the optical power range of 100 mW to 150 mW. Projected life for these lasers is in excess of 100,OOO hours. For direct detection links, optical power in the far field can be increased beyond the performance of the single device with polarization and/or wavelength multiplexing. Significantly higher optical power can be expected from the next generation of monolithic single mode laser diodesthat are presently in development. High optical quality beams in the 0.5 W to 1.0 W range have been demonstrated from both edge emitting and surface emitting master oscillator power amplifier (MOPA) devices as well as antiguide arrays. With further reproducibility and reliability demonstrations these devices will form the basis for the next generation of space based cross links.