Optical Communications, 2nd Edition

10.5 EFFECT OF BEAM TRACKING ON DATA TRANSMISSION

While beam tracking is taking place at each receiver in a two-way space link, as in Figure 10.18a, data can be transmitted by directly modulating the tracking beam. After acquisition is completed, the tracking operation holds the focused received field close to the quadrant array center, while generating the tracking error signals. Data can then be transmitted by field modulation, creating a modulated optical carrier, or a pulsed optical carrier, depending on the type of encoding. By summing the outputs of all detectors of the quadrant array at the receiver, a separate data demodulation channel can be generated, as shown in Figure 10.19. In this sum channel, the total power on the array is available for decoding.

The tracking subsystem at each receiver therefore generates the azimuth and elevation correction signals, while the sum channel signal is simultaneously used for data processing. Hence the double-ended tracking can continue to take place while data is transmitted. Pointing errors from transmitter to receiver cause the received beam power to be reduced because of beam pattern loss when operated off boresight. Pointing errors of the receiver shift the diffraction pattern off-center on the quadrant array, causing power losses from photodetection integration in the focal plane. Hence, the tracking operation at each end directly effects data transmission because of these combined power losses. In this section, we examine this ...