Optical Communications, 2nd Edition

6.5 DIGITAL SIGNALING WITH HETERODYNE DETECTION

Digital bits can also be encoded directly on the phase or frequency of the laser carrier itself if heterodyne detection is used at the receiver. This would represent a coherent digital system, as opposed to the direct detectior systems of the previous sections. Heterodyning allows the received modulated laser carrier to be translated to a lower RF frequency, where the digital modulation can then be decoded by standard RF decoding methods.

The most common carrier modulations are BPSK and noncoherent frequency shift keying (FSK) [5, 6]. For the laser link, this would correspond to direct phase or frequency shifting of the laser spectral line itself (Figure 6.12a). This would mean that narrow line width lasers must be used for these systems, to separate out the modulations from the laser phase and frequency noise itself. Phase and frequency modulation of laser sources can be achieved by basic phase-retarding circuitry or by piezoelectric control of the laser cavity. Direct heterodyning at the receiver (Figure 6.12b) via a local offset laser mixes to RF, producing the RF carrier with the same modulation. This is then decoded by RF BPSK or FSK detecting circuits, generating the decoded bits.

Figure 6.12. Digital heterodyne system. (a) Encoder. (b) Receiver and decoder.

If a strong local laser field is used for the heterodyning, the photodetected ...