File:DSOC's Downlink Detector Prototype PIA25840.tif

DSOC will test key technologies that could enable high-bandwidth optical, or laser, communications from Mars distances. Bolted to the side of the spacecraft and operating for the first two years of Psyche's journey to the asteroid of the same name, the DSOC flight laser transceiver will transmit high-rate data to Caltech's Palomar Observatory in San Diego County, California, which houses the 200-inch (5.1-meter) Hale Telescope. The downlink detector converts optical signals to electrical signals, which can be processed and decoded.

The detector is designed to be both sensitive enough to detect single photons (quantum particles of light) and able to detect many photons arriving all at once. At its farthest point during the technology demonstration's operations period, the transceiver will be up to 240 million miles (390 million kilometers) away, meaning that by the time its weak laser pulses arrive at Earth, the detector will need to efficiently detect a trickle of single photons. But when the spacecraft is closer to Earth and the flight transceiver is delivering its highest bit rate to Palomar, the detector is capable of detecting very high numbers of photons without becoming overwhelmed. Because data is encoded in the timing of the laser pulses, the detector must also be able to determine the time of a photon's arrival with a precision of 100 picoseconds (one picosecond is one trillionth of a second).

Figure A is a close-up of the active area of the detector (inside the small square at the center of the photograph), where the photons from deep space are collected. The active area measures about 0.0126 inches (0.32 millimeters) across. It has 64 channels (which are connected to the gold contact strips surrounding the black inner square) capable of collectively detecting more than a billion photons per second.