5.2 HETERODYNE SIGNAL-TO-NOISE RATIOS

The modulated heterodyne carrier generated from photodetection can now be filtered out to recover the modulations. The overall system following the photodetector is shown in Figure 5.4a. A bandpass filter F(ω) is used to recover the modulated carrier, which is then RF demodulated to produce the information signal. The filtered output spectrum then appears as

images

where Si(ω) is the photodetected output current spectrum, and N0c is the spectral level of the output thermal noise current. The filter is generally tuned to the bandpass characteristic of the desired signal spectrum. If we assume a wideband optical detector [H(ω) = e] and a flat filter function [F(ω) = 1] over the bandwidth of the heterodyned signal, the filtered signal power after heterodyning is

images

Figure 5.4. (a) Postdetection filtering. (b) Output filtered spectrum (strong local field).

images

where images is the time-average signal intensity of the heterodyned carrier in Eq. (5.1.10). Since ImA is the heterodyned carrier power, we can use Eq. (5.1.13) to write this in terms of the received laser power ...

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