Optical Communications, 2nd Edition

4.5 INTENSITY MODULATED SUBCARRIER SYSTEMS

The previously derived SNR equations pertain to an optical system in which the desired information waveform m(t) is intensity modulated directly onto the optical carrier. An alternative procedure is to make use of an auxiliary subcarrier to carry the information signal. The overall system would appear as in Figure 4.11. At the transmitter the information waveform is modulated onto a radio-frequency (RF) or an intermediate frequency (IF) subcarrier using standard subcarrier modulation formats such as AM, FM, and so on. The subcarrier m(t) is then intensity modulated onto the main optical carrier. At the receiver, the modulated subcarrier signal is first recovered by photodetection of the optical beam. Note that the entire subcarrier spectrum must be within the detection bandwidth of the photodetector, which limits the upper frequency that may be used for the subcarrier. The recovered subcarrier, along with the detector noise, is then fed into a subcarrier demodulation channel to demodulate the information signal. The advantages of such an operation are (1) possible signal-processing improvement obtained during the subcarrier demodulation, and (2) the use of subcarrier multiplexing, in which a group of subcarriers, each carrying separate source information, can be simultaneously sent over a single optical carrier channel.

Figure 4.11. Intensity ...