Wide-band laser communications in space

Abstract

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 CO_2 (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 CO_2 and 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 CO_2 system. The most critical technology problem for the CO_2 system is that of Doppler compensation. These and other critical technologies for both approaches are listed and discussed.