Certain space communications applications have very high link availability requirements. These real-time missions would benefit from laser crosslinks in space that do not experience solar outages when the receiver must view the transmitter against the background of the solar disc. We have examined the performance penalty for a direct detection avalanche photodiode (APD) receiver with the sun in the FOV. The increased signal strength required by the solar background varies with datarate and other system parameters, but typically ranges from 10 to 15 dB. A narrower optical passband centered on a solar Fraunhofer absorption line has been found to offer an order of magnitude decrease in the required signal strength at the receiver. Several options are available to implement this performance improvement, and we summarize the potential and advantages of candidate techniques.
Robert T Carlson,
"Direct Detection Lasercom With The Sun In The Field Of View", Proc. SPIE 0885, Free-Space Laser Communication Technologies, (2 May 1988); doi: 10.1117/12.976547; https://doi.org/10.1117/12.976547