Coherent fiber-optic code division multiple access receiver using an interferometer

Recently spread spectrum code division multiple access (CDMA) has been investigated for fiber-optic local area network, since CDMA communication is a possible means to achieve asynchronous multiple access without delay, and optical fibers are wide-bandwidth. In a typical fiber optic CDMA communication system, the received optical pulse sequence is correlated with the stored address in the decoder. The signal processing operations can be performed on an incoherent or a coherent basis. In incoherent optical CDMA, the phases of the pulse sequences are not important, and they are summed on the basis of power rather than electric field. A relatively low autocorrelation peak, when the network is multiple-accessed, and cross correlation of other users, lowers the signal-to-noise ratio and increases bit error rate. In coherent CDMA, we still has the same basic network and temporal (envelop) pattern for the impulse responses as in incoherent CDMA but now in the optical correlation process the pulses are summed up coherently. Coherent CDMA has potential advantages from the point of view of power budget, side lobes, and signal-to-noise ratio. There are substantial practical difficulties when we try to implement it due to the environmental influence. We proposed a novel receiver system, the conventional incoherent matched filter followed by post signal processing block, where signal processing is performed coherently.