Low frequency fluctuations and multimode operation of a semiconductor laser with optical feedback

We experimentally investigate low frequency fluctuations (LFF) in a Fabry-Perot semiconductor laser with optical feedback from an external mirror. During LFF, the time resolved optical spectrum shows that many longitudinal modes of the solitary laser enter into the transients. After each LFF event, the excited solitary-laser modes recover similarly. However, the recovery for the power in each mode is much slower than the recovery of the total power. The intermode exchange of energy during the recovery indicates that a single-longitudinal mode description of such LFF behavior will not capture important underlying dynamics. The relevance of multimode dynamics is confirmed in a feedback experiment where the external mirror is substituted by a diffraction grating. (C) 1998 Elsevier Science B.V.