The Laser Diode

The light emitting diode of chapter 2 can be modulated to act as the source in an optical communication system. The primary disadvantage of using an LED in such a system is that it emits light over a relatively wide range of wavelengths — the relative width of an LED’s emission is typically around 5 per cent. One effect of this rather broad spectral emission is that because of dispersion in the optical transmission path the modulation bandwidth must be limited, so that different portions of the modulation spectrum do not arrive at different times at the receiver. If an LED which has a peak emission at 600 nm with a 5 per cent spectral width is used as the source in an optical communication system, the bandwidth of the source is 25 000 GHz. Trying to modulate such a source is rather like superimposing modulation on noise.