Although many lasers and LEDs are able to produce outputs in the visible light band, most current optic fibre telecommunication systems (Fig. 41.3) use signals of wavelengths 0.8 μm to 1.3 μm, both in the infrared band. These are still called optical systems: even though the signals cannot be seen they are transmitted in exactly the same way as visible light signals. The main reason why engineers wanted to be able to modulate coherent light was to take advantage of the tremendous bandwidths which could be carried by these very high-frequencies. The figures below are typical figures, indicating the orders of magnitude involved.
Table 41.1 Carrier Wave
Fig. 41.3 Basic principles of fibre-optic communication