Noise of semiconductor lasers with external feedback

Semiconductor lasers are employed in optical transmission systems as transmitters. Information carrier is the emitted light whose intensity is modulated by means of the injection current. In this connection, the noise related to the light intensity becomes an important factor. In the case of good lasers, this noise is so low that it can be generally disregarded. However, disturbances of laser operation can cause an increase in the normally low level of laser noise, and consequently, a deterioration in transmission quality. The present investigation has the objective to show the causes of this disturbing effect. Attention is given to the emission of laser diodes without external reflection, the effect of external reflections at a short distance from the laser, and the effect of external reflections at a long distance from the laser.