Laser diode characteristics with external optical feedback

A considerable linewidth narrowing may be achieved with optical feedback. On the other hand, optical feedback may introduce tremendous instabilities. By considering single-mode laser diodes the basic phenomena are reviewed. The main conclusions are: (a) In order to reduce the feedback sensitivity of single-mode laser diodes, the linewidth enhancement factor α should be as low as possible. (b) As long as the external cavity length is of the order of only a few mm, no coherence collapse occurs, thus enabling relatively stable narrow linewidth devices