It should be emphasized that this arrangement would yield a nonzero photodetector output of nominally the same magnitude for either “0” or “1.” This is fundamentally different from on-off keying (OOK), in which “0” or “1” is represented by the absence or presence, respectively, of a signal. Taking advantage of this, prior to final digitization of the return signal at “0” or “1,” the output of the “0” photodetector could be inverted, then subtracted from the output of the “1” photodetector to obtain twice the signal-to-noise ratio achievable in OOK.

The receiver subsystem would include Faraday-anomalous-dispersion optical filters (FADOFs), which would reject background light to such a high degree that the system could operate over a long path during daytime. The FADOFs would essentially prevent skylight from reaching the photodetectors while allowing about 80 percent of the signal photons to pass through. Without the FADOFs, it would be necessary to increase the laser power by a factor of 10 for daytime operation.

This work was done by D. A. Hazzard, J. A. MacCannell, G. Lee, E. R. Selves, D. Moore, J. A. Payne, C. D. Garrett, N. Dahlstrom, and T. M. Shay of New Mexico State University for Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to: New Mexico State University Las Cruces P.O. Box 30001 Las Cruces, NM 88003 Refer to GSC-14759-1, volume and number of this NASA Tech Briefs issue, and the page number..

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