Double cavity feedback and experimental observation of coherence resonance

We first discuss the effect of isotropic optical feedback from an extremely short external cavity (ESEC) (K. Petermann, 1195, K. Hsu, et al., 1998) on the emission properties of vertical-cavity surface-emitting lasers (VCSELs). When changing the external cavity length a modulation of the total power and the wavelength emitted by the VCSEL is observed with a period of half the emission wavelength (K. Panajotov et al., 2004). By making use of a two modes rate equation VCSEL model we develop a map of bistability to investigate the parametric dependence of polarization properties of VCSELs in such configuration, finding out a periodic dependence of the polarization switching currents on the ESEC length (K. pajanotov, et al., 2004 and M. A. Arteaga et al., 2006). By increasing the external mirror reflectivity we can make this periodic dependence stronger and strongly asymmetric providing the possibility to prevent PS for any injection current and thus, achieving polarization stabilization in VCSELs (M. A. Arteaga et al., 2006). Further numerical simulations with isotropic and non isotropic feedback show how parameters as gain compression coefficients, frequency splitting between the linearly polarized modes, differential gain and the mirror reflectivity, affect the map of bistability (M. A. Arteaga et al., 2006). The theoretical results are supported by extensive experimental mapping (M. A. Arteaga et al., 2006). The switching currents and the hysteresis width can be widely tuned by varying the external cavity length. The amplitude of modulation of the polarization switching current with the external cavity length is experimentally confirmed to be proportional to the external mirror reflectivity, proving its key role in achieving polarization control of such lasers using optical feedback. Moreover, by a proper choice of the optical feedback parameters, the emission can be stabilized in any of the two linearly polarized modes (M. A. Arteaga et al., 2006). Finally, by maki- - ng use of the fine-polarization tuning provided by the ESEC feedback we report on the experimental observation of coherence resonance in a bistable system with delay (M. A. Arteaga et al., 2006). Our system consists of a VCSEL subject to time-delayed optical feedback simultaneously from a long and from extremely short external cavity. Coherence resonance is experimentally proven by analysis of the residence time distribution of the polarization mode-hopping regime and of the signal to noise ratio in the power spectrum.