Semiconductor lasers (SL) subject to delayed optical feedback are well-known to exhibit complex emission dynamics. In recent years, research has focused on systems with weak to moderate feedback levels and short external cavities. In this so-called Short Cavity Regime (SCR), the external cavity round trip frequency is larger than the relaxation oscillation frequency of the laser. The major characteristics of the intensity dynamics in the SCR have been successfully described by the well-known Lang-Kobayashi (LK) rate equations [1]. However, general applicability of the model is not given, since it is based on several assumptions. In particular, the spatial extension of the laser is neglected. In addition, only one single mode of the solitary laser is considered. Nevertheless, several features of the dynamics of multimode lasers can be explained within the single mode LK-model or with extended LK-models considering weak coupling of several longitudinal modes. However, if the ratio between the external cavity length and the effective length of the laser cavity becomes small, resonance effects might become relevant for the dynamics. I present results for a multimode SL system subject to moderate optical feedback with resonant mode coupling. In contrast to common (LK-) SCR dynamics, the chaotic dynamics in this system involves an unusually high number of longitudinal modes; this number can exceed 100 leading to an optical bandwidth of approximately 7nm. This dynamics is associated with broad band intensity rf spectra with a bandwidth of ~5GHz.
I will discuss this behavior in the context of feedback induced spectral dynamics and modal overlap leading to strong interactions between the modes, demonstrating how this behavior goes beyond the LK-description. The control over such an extended range of bandwidth offers new perspectives for applications where (in-)coherence properties are of importance.
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