A generalized optical communication link is illustrated in Fig. 2.1. The information to be transmitted to the receiver is assumed to exist initially in an electrical form. The information source modulates the field generated by the optical source. The modulated optical field then propagates through a transmission channel such as an optical fiber or a free-space path before arriving at the receiver.
The receiver may perform optical processing on the incoming signal. The optical processing may correspond to a simple optical filter or it may involve interferometers, the introduction of additional optical fields, or the use of an optical amplifier. Once the received field is optically processed it is detected. The photodetection process generates an electrical signal that varies in response to the modulations present in the received optical field. Electrical signal processing is then used to finish recovering the information that is being transmitted.
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