Since the very beginning of fiber-optic telecommunication systems, the main effort was focused on increasing transmission capacity. This was done first by increasing the signal modulation speed from 155 Mb/sec to 622 Mb/sec, 2.5 Gb/sec, 10 Gb/sec, and recently to 40 Gb/sec. The total available bandwidth of standard optical fibers is about 20 THz. As it is impossible for a single-wavelength laser to utilize such a large bandwidth, schemes based on multiplexing have been developed. There are several techniques of multiplexing. The two techniques applied in photonic processors are (1) time-division multiplexing (TDM) and (2) wavelength-division multiplexing (WDM).