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Reliability Assessment The reliability of an optical fiber is of paramount importance in communications applications—long links represent large investments and require high reliability. There will, of course, always be unforeseen reliability problems. Perhaps the most famous such example was the fiber cable design on the first transatlantic link—the designers had not quite appreciated the Atlantic shark’s need for a high-fiber diet. The sharks, apparently attracted by the scent of the cable materials, made short work of the initial cable installations.

For homodyne detection, the signal envelope carries twice the energy, and 4ηHET R2PS PLO SNR = ᎏᎏ2 2e RPLO Bn + σ T (36) For a given modulation scheme, homodyne detection will therefore be twice as sensitive as heterodyne. Modulation Formats. The modulation formats appropriate for coherent optical communications can be summarized as follows: 1. Amplitude-Shift Keying (ASK). This technique is simply on-off keying (similar to simple intensity modulation) but with the important constraint that the frequency and phase of the laser be kept constant.

The noise contributions from amplifiers other than the first stage are generally second-order corrections. Figure 19 shows a schematic of the receiver and relevant noise sources. Table 3 gives expressions for, and definitions of the important physical quantities which determine the receiver sensitivity. Figure 20 illustrates two possible configurations for the detector/amplifier combination. Of these, the integrating front end is the simplest (particularly for high-frequency operation) but tends to be slower than a transimpedance amplifier with an equivalent load resistance.