Time-resolved spectral dynamics of semiconductor lasers with optical feedback in the regime of low-frequency fluctuation

Summary form only given. The dynamical behavior of semiconductor lasers with optical feedback is a subject of interest since weak optical feedback may induce bistability, oscillations and chaos, while for moderate feedback levels the laser power displays low-frequency fluctuations which lead to an increased RIN and to a dramatic broadening of the emission spectrum. In order to analyze the origin of the pulsing occurring at times shorter than the delay time, we conducted spectrally resolved measurements by horizontally dispersing the laser beam with a 0.3 m grating monochromator. The physical interpretation can be based on a typical multimode instability. The system is essentially controlled by the delay time, which fixes a resonance frequency at /spl tau//sup -1/. For one laser mode, the external cavity modes tend to lock in phase during the build-up of the average total intensity, thus leading to pulses. If the delay is long enough the active material can recover before the next pulse is generated, and another laser mode is able to operate and it will grow from the background. The external cavity modes inside the bandwidth of this "new" operating mode tend also to lock in phase, but this locking phase will in general be different from that of the previously existing mode. When two or more modes synchronize, competition for the gain leads to an instability which gives origin to the drop of the total intensity and to fast and strong variations of the material properties. As a consequence, there is a shift in frequency and later operation at other unrelated laser modes. The process then initiates again.