Optical Communications, 2nd Edition

7.8 COHERENT COMMUNICATIONS OVER FIBERS

A coherent communications system can be used over a fiber channel with a heterodyning receiver operated at the fiber output. The overall fiber would be similar to that shown in Figure 7.15. Ideally, a heterodyning fiber link would have a lower threshold SNR (lower sensitivity) advantage over a direct detection (noncoherent) system. Of particular importance is the use of the heterodyning laser power at the receiver to overcome the detector thermal noise that limits noncoherent operation. This would permit operation at lower source power levels, or at higher data rates, then in direct detection links. The analysis in Chapter 5 can be directly applied here to determine demodulated SNR or decoding PE when heterodyning is applied.

The difficulty in coherent fiber operation is in maintaining the proper conditions for achieving these advantages. Often auxiliary hardware and higher quality components are needed to insure the necessary heterodyning matching. A laser source must be carefully selected to ensure a suitable narrow linewidth around the carrier wavelength. This is necessary to control the phase noise that will be superimposed on the heterodyned carrier frequency. This means the relatively inexpensive LEDs and laser diodes normally used in pulsed non-coherent systems (where the laser linewidth is not important), are generally not suited for the heterodyned system. Likewise the wavelength stability of the source is also important to be sure ...