Abstract
We present an experimental and theoretical investigation of the non-linear multimode dynamics of external–cavity VCSELs emitting at 1 and 2.3mm. We account for the stable single–frequency and linearly polarized emission by these laser sources, even in the presence of quantum noise and non-linear mode interactions originating from Four–Wave–Mixing via population pulsations in the quantum-wells. This fact is a consequence of the mode antiphase dynamics. Thanks to the high-Q external cavity configuration, the laser dynamics fall into the oscillation–relaxation–free class–A regime. The characteristic time to achieve single mode emission is ~ 1ms for a 15mm long cavity with an antireflection coated structure and no spectral filter, as for an “ideal” homogeneous gain laser. The side mode suppression ratio is as high as 40 dB, close to the quantum limit. The laser linewidth is at the quantum limit, and is ~ 1Hz at 1mW output. An experimental value <20 kHz has been established. Under standard conditions, without spectral filtering, the optimum cavity length for highly coherent single mode operation is expected in the range 5 to 30mm. Finally, for cavity lengths typically shorter than 5mm, we rather have an “ideal” homogeneous gain class–B laser, exhibiting oscillation-relaxation of the intensity in the 0.1GHz range. These properties contrast with the intrinsic strongly non-linear dynamics of conventional semiconductor lasers.