On the Mitigation of Dead Time With Photon-Counting Detector Arrays for FSO Inter-Satellite Links

In December 2024, NASA's Deep Space Optical Communications (DSOC) Psyche Mission beamed laser encoded data from nearly 494 million kilometers away to the Hale Telescope at the Palomar Observatory housed in Caltech. This feat could not have been achieved without the deployment of highly sensitive photon-counting detectors that can register as small as a single photon at the receiver. In the photon-counting regime, an array of detectors offers significant advantages instead of a single detector, particularly, to mitigate dead time, which is an inherent rate-limiting factor associated with photon counting detectors. Our results reveal that by optimizing one important system parameter---the signal spot size on the focal plane array---significant symbol error rate minimization and channel capacity maximization can be achieved. Using a programmable liquid lens, the Gaussian spot size can be dynamically adjusted in real time---based on the detected signal and noise photons during the observation interval---to optimize system performance under dead time.