Optical feedback is introduced into a diode laser by returning some portion of the
optical output back into the device. This is shown schematically in figure 1.1. The
introduction of such feedback has been found to have dramatic and varied effects on the
operating characteristics of the solitary diode laser. Feedback can be disadvantageous, as it
may cause unwanted instabilities in the laser output, or advantageous, as under certain
conditions it can improve various features of the solitary laser, such as increasing the side
mode suppression and narrowing the linewidth