High-sensitivity receiver for infrared laser communications

The design of a high-sensitivity wide-bandwidth 10.6-μm heterodyne receiver has been established for space and ground operational use, and key elements for a satellite-to-earth communications link were demonstrated. The receiver includes a cooled HgCdTe infrared mixer diode, a conical scanner for spatial tracking, an acquisition channel for spatial search and station alignment, and an automatic frequency-control channel to maintain a fixed laser frequency offset. The infrared mixer is designed to provide nearly quantum-noise-limited operation over an extended range of mixer temperature, bias voltage, and IF. The resultant experimental receiver has a measured noise equivalent power of less than 10^-19W/Hz over the 15- to 40-MHz IF band for mixer temperatures from 85 to 115 K, and less than W/Hz up to 140 K. Mixer 3-dB cutoff frequencies as high as 420 MHz were measured at a mixer temperature of 125 K. An analysis and engineering equations are given for receiver noise components, noise equivalent power, available mixer conversion gain, mixer transducer gain, and quantum-noise factor in terms of such factors as mixer parameters, quantum efficiency, mixer temperature, dynamic conductance, bias voltage, LO power, and IF amplifier characteristics.