Polarization mode hopping in vertical-cavity surface-emitting lasers, induced by optical feedback

Vertical-cavity surface-emitting lasers have shown similar sensitivity to optical feedback as conventional edge-emitting lasers, but new interesting phenomena can he observed due to the coexistence of two linearly polarized (LP) fundamental modes. We report on new dynamic effects in VCSELs induced by polarization insensitive optical feedback from a distant mirror, namely the appearance of low frequency random hops between the two LP modes in a nominally stable LP solitary laser. This behavior resembles that of the mode hopping in a solitary VCSEL close to its polarization switching point. However, a careful observation shows that superimposed on the low frequency polarization mode-hopping, fast oscillatory behavior at a frequency close to the external-cavity frequency appears. A complementary study of the polarization resolved optical spectra reveals jumps between several peaks identified as external cavity modes. We analyze the dynamics using a two-mode rate equation model with delay and noise. We numerically observe polarization mode-hopping in good qualitative agreement with our experimental findings. In particular, the low-frequency hops are complemented with fast oscillations at a frequency close to the external-cavity one and the calculated optical spectra reveal the presence of a limited number of ECMs in each LP-comb. This indicates that the dynamics is created by the interplay of noise, bistability and optical feedback. We will further discuss the effect of noise on delayed bistable laser systems in the context of new dynamical concepts, like coherence resonance and stochastic resonance.