Optical Communications, 2nd Edition

8.3 OPTICAL MULTIPLE ACCESSING

Multiple accessing refers to the communication operation of having multiple signals propagate over the same link and yet be individually separable at a receiver. The separability is provided by transmitting the signals with inherent addressing formats superimposed on the signal in some manner. A specific source signal can be recovered by tuning, or aligning, the receiver to the addressing format. All other signals, having an undesired address, will be rejected (partially or totally) by the address recovery procedure. The resulting recovered signals containing the proper address will then be separated out, in the presence of detector noise and any interference from the signals that were not totally rejected. This latter interference is referred to as “multiple access” noise, or simply “crosstalk.” Crosstalk can take on various forms, can appear noiselike in nature, or can appear with definate signal structure.

Optical signal addressing can take on various forms. The most popular are summarized below.

8.3.1 Wavelength Division Multiplexing

Each source is addressed by assigning it a unique wavelength band. All information is transmitted by a laser source using a modulation bandwidth confined to the wavelength assigned to it. The modulation can be noncoherent (intensity modulation) or coherent and involve digital or analog data. Receiver separability is achieved by frequency tuning (predetection optical filtering) to the desired band. Signals in other ...