1. Optical modulation

Transmitting electrical information (signals) by optical means requires electrical/optical transducers at the transmitting end and inverse optical/electrical transducers at the receiving end, to restore the signal. The first electrical/optical conversion operation is called optical modulation. In effect, the signal is transported on the optical carrier, and this is accompanied by a spectral transposition, which we will now examine.

The advantage of semiconductor laser diodes (cf. § 2.1) for telecommunications or radar applications is that they can be easily modulated:

• the optical intensity emitted at the laser output is modulated by an external modulator;

• or more simply, the current flowing through the laser is modulated, resulting in a modulation of light in optical...