6
OPTICAL DIGITAL COMMUNICATIONS
In the analog transmission of information, the primary objective is to transmit a source waveform from the transmitter to the receiver with as little distortion as possible. In Chapters 4 and 5, this distortion was measured in terms of a postdetection signal-to-noise ratio (SNR) following photodetection. Another way of sending information is by digital transmission, in which the desired message is converted to binary symbols (bits) and transmitted as modulated light fields. The receiver now has the task of detecting, or decoding, the transmitted symbols from the received optical field. The performance criterion is based on the accuracy of this symbol decoding rather than on the SNR considered previously. The optical digital system is the objective of study in this chapter.
The transmission of the digital bits over the optical link can be done on a bit-by-bit basis (binary encoding) or on a bit word basis (block encoding). We first consider binary encoding, then extend to block coding later in Section 6.6.
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