Optical waveguides for coherent optical control of phased array antennas

In coherent optical architectures for phased array antennas, the frequency and phase of the microwave carrier at each element are determined by optical heterodyning two laser signals of differing frequency. Semiconductor diode lasers can be frequency-stabilized with sufficient precision so that their difference frequency can serve as a stable microwave carrier.1 Another approach is to divide a single laser beam into two paths and frequency shift one of them using acoustooptic or electrooptic devices. For a phased array, both laser signals must be distributed among a large number of photodetectors. one for each antenna element. The optical signal distribution can use free-optics, as in the Fourier optics scheme first proposed by Koepf,4 or it can make use of integrated optical devices2 in which power splitting, phase shifting, and power recombining all take place in a single solid state device. The principle advantage of the latter is that, unlike free-space optical systems, the path delays can easily be controlled and stabilized to the required sub-micron tolerances.