Semiconductor lasers in advanced interferometric optical-fibre sensors

Semiconductor injection lasers have demonstrated impressive performance in communication systems, including those requiring coherent modulation. Their use in optical-fibre sensors, especially interferometric ones, raises very different questions. These are primarily associated with the fact that, in communications, the signal bandwidth greatly exceeds the source linewidth; whereas in sensors the reverse is true. In the paper the requirements imposed by a range of coherent optical-fibre sensors on the radiation emitted from a typical single-longitudinal-mode semiconductor injection laser are examined. The need for frequency stability and low excess noise is demonstrated and is shown to impose far more stringent conditions on the laser operation than those necessary in a coherent communication system. Finally, the implications of these requirements on the performance achievable in a complex interferometric sensor system are discussed in some detail.