A Short Cavity Semiconductor Laser System: Dynamics Beyond the Lang-Kobayashi Model

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.