Wide-band laser communications in space

Candidate wide-bandwidth (1-Gbit/s) satellite laser communications systems are compared on two different bases. First, a comparison is made with projected component technology to establish relative performance between the various approaches based on the fundamental system parameters. From this comparison it appears that the CO2(10.6-μm) system offers a signal-to-noise advantage over the Nd:YAG (1.06-μm) or doubled Nd:YAG (0.53- μm) system for a comparable satellite burden. Second, a comparison is made based upon the concept that launch cost for equivalent systems comprises an optimizing criterion. From this comparison it appears that the launch costs for the CO2and doubled Nd:YAG systems can be similar, but the latter is very sensitive to the projected weight of a large lightweight "photon bucket" receiving aperture. In general, the relative deficiency in signal-to-noise ratio for the Nd:YAG system can only be accommodated through adoption of an open-loop pointing system with an accuracy of 1 μrad or less, as compared to a closed-loop pointing system with relaxed accuracy for the CO2system. The most critical technology problem for the CO2system is that of Doppler compensation. These and other critical technologies for both approaches are listed and discussed.