7.5 RADIO FREQUENCY CARRIER TRANSMISSION OVER FIBER
In RF wireline communications, a modulated RF carrier is routed over coaxial cable or microwave waveguides. The RF carrier is generally in the mega- or gigahertz range and, therefore, well within the bandwidths of optical fibers. An important topic of interest is that of replacing the RF cabling by optical fiber for routing the RF carrier. This would have the packaging advantage of the narrower fiber, as well as the lower attenuation losses. The approach is to intensity modulate the RF carrier directly on the laser and transmit the optical field over the fiber to the receiver. After photodetection, the RF carrier is recovered. The objective is for the optical link to have as little effect as possible on the RF carrier routing. The conditions for this to occur can be obtained directly from the previous analysis.
Consider the RF-fiber system in Figure 7.9. A modulated RF carrier is filtered by the RF bandpass prefilter centered at the carrier frequency. The output of the filter is the sum of the modulated RF carrier c(t), with power Pc, and the filter output noise n(t), with power Pn = N0Bc, where N0 is the prefilter RF noise spectral level and Bc is the prefilter noise bandwidth. The prefilter output has the RF SNR given by
The combined output waveform x(t)[ = c(t)+ n(t)] is then ...