Integrated heterodyne receiver and spatial tracker for binary FSK communication

In order to simplify system architectures and make efficient use of laser power, space lasercom system designers often try to consolidate the receiver subsystems. In this paper, we present a receiver which uses a single subsystem for both spatial tracking error sensing and communication signal reception. It makes use of an electro-optic crystal as a conical scanner for tracking error measurement, couples the scanned light into a single mode fiber, and uses standard fiber-based heterodyne techniques to derive an intermediate frequency signal. This signal is processed to retrieve both the binary FSK signal and the tracking error signal, as well as an estimate of signal power for use in normalizing the tracking error. The fiber-coupled receiver makes possible a modular architecture, whereby the transmitter, receiver, and telescope subsystems can reside in different parts of the spacecraft. Such an architecture is known to have a number of desirable properties. We present a discussion of the frequency plan, data demodulation, frequency tracking, spatial tracking, and gain control subsystems. Design considerations and experimental results are presented.