Maria Susana Torre

Influence of optical feedback on the polarization switching of vertical-cavity surface-emitting lasers

We study theoretically the effect of isotropic and polarization-selective optical feedback on the polarization switching of vertical-cavity surface-emitting lasers. We use the framework of the spin-flip model that takes into account the spin degree of freedom of carriers in the semiconductor quantum well. We analyze the polarization switching dynamics for different values of the spin relaxation rate and find a good agreement with recent experimental results . We also study the influence of the external cavity length and find different dynamical behaviors for short and long external cavities.

Influence of the injection current sweep rate on the polarization switching of vertical-cavity surface-emitting lasers

We study the polarization switching of vertical-cavity surface-emitting lasers based on simulations of the spin-flip model. We show that the switching point depends on the ramp signal used to scan the injected current. Fast current ramps enlarge the hysteresis region since the switching point moves to high pump values for increasing injection and to low pump values for decreasing injection. The delay of the bifurcation follows a power law with the slope of the current ramp.

Numerical study of optical injection dynamics of vertical-cavity surface-emitting lasers

We study numerically the transverse-mode dynamics of a vertical-cavity surface-emitting laser with external optical injection. We consider the case in which two transverse modes with parallel polarizations are exited. Varying the strength and frequency of the injected field, we find a rich variety of complex behaviors. We show that by increasing the optical injection strength the laser does not turn into a single-mode laser, as would be expected if the transverse modes have perpendicular polarizations.

Polarization Dynamics of Current-Modulated Vertical-Cavity Surface-Emitting Lasers

We study the dynamics of vertical-cavity surface-emitting lasers (VCSELs) with direct current modulation in the framework of a model for index-guided VCSELs that takes into account two orthogonal linear polarizations. We analyze the effect of current modulation near the polarization switching (PS) of type I, from the high to the low frequency polarization, and near the PS of type II, from the low to the high frequency polarization. We find that the oscillations of the total power are as those of a single-mode laser, unaffected by the underlaying polarization coexistence or polarization competition. We also study the small-signal modulation response in the Fourier domain, for modulation dc values near the PS point. Close to type I PS the response of the total power as well as the response of the orthogonal polarizations has the same functional dependency on the modulation frequency, and can be fitted by the response function of a single-mode laser. Close to type II PS, polarization competition is a significant process at low modulation frequencies. The polarization-resolved modulation response displays features at low frequencies that are not present in the response of the total power, which is as that of a single-mode laser. The dynamics becomes increasingly complex as the modulation amplitude grows, and there is multistability of solutions.

Transverse-mode dynamics in directly modulated vertical-cavity surface-emitting lasers with optical feedback

Vertical-cavity surface-emitting lasers (VCSELs) with optical feedback are known to exhibit different transverse-mode regimes depending on the injection current. Close to threshold a VCSEL operates on the fundamental transverse mode, while for larger injection the dynamics is often multimode, with the optical feedback inducing either in-phase or anti-phase transverse mode oscillations. In this paper, we study numerically the influence of current modulation on these different feedback-induced transverse-mode regimes. The modulation amplitude and period are taken as control parameters. We find that the in-phase and anti-phase regimes are robust under weak modulation. As the modulation amplitude increases, there is a transition to a dynamics governed by the current modulation, where the total output power follows the injection current and there is either single-mode or in-phase multimode behavior. However, the effect of the current modulation depends on the modulation period. Under fast modulation, the laser cannot follow the modulation and the optical-feedback-induced effects are dominant. On the contrary, under slow modulation there is a superposition of modulation and feedback effects, with the total output following the modulated current and an underlying transverse-mode behavior mainly determined by the optical feedback. A resonant behavior was observed for modulation periods close to the internal oscillation period. In this case, current modulation induces pulsing output intensity with single-mode or in-phase multimode behavior.

Dynamical Hysteresis and Thermal Effects in Vertical-Cavity Surface-Emitting Lasers

We study numerically the polarization-resolved light-output characteristic of vertical-cavity surface-emitting lasers (VCSELs) using the spin-flip model for VCSELs extended to take into account thermal effects via a dynamical equation for the active region temperature, including heat dissipation, heating due to nonradiative carrier recombination, and Joule effect. The temperature dynamics is coupled to the carrier and optical field dynamics via a frequency and temperature-dependent gain coefficient. We show that the interplay of thermal effects and injection current variation can result in turn-on and turn-off hysteresis cycles that can be, depending on various model parameters, positive or negative. In the first case, the turn-on occurs at a higher value of the bias current than the turn-off; in a negative hysteresis cycle, the laser turns-on at a lower value of the bias current than the turn-off. These results are interpreted in terms of the interplay of the two time-scales determined by the injection current swept rate and the thermal relaxation rate.

Thermal effects and dynamical hysteresis in the turn-on and turn-off of vertical-cavity surface-emitting lasers

Thermal effects and dynamical hysteresis in VCSELs under dc modulation have been experimentally studied. The results show that the VCSEL turn-on and turn-off currents can display both positive hysteresis and negative hysteresis, depending on the current modulation frequency and on the substrate temperature. Numerical simulations of semiconductor laser rate equations, extended to take into account thermal effects, show a good agreement with the observations.

All-Optical Stochastic Logic Gate Based on a VCSEL With Tunable Optical Injection

We study the dynamics of a vertical-cavity surface-emitting laser with continuous-wave orthogonal optical injection from a tunable laser. We use the dynamical properties of polarization bistability and the interplay with internal or external noise to demonstrate numerically a reliable logic output to two logic inputs encoded in the optical frequency of the injected light. This all-optical configuration is more than ten times faster than the electro-optical implementation and has the advantage of operating at constant injection power.

Transverse Mode Selection in Vertical-Cavity Surface-Emitting Lasers With Optical Injected Signal

The transverse mode selection induced by optical injection in a VCSEL emitting in two transverse modes is analyzed from a theoretical point of view. Different pairs of transverse modes with parallel or orthogonal polarizations are considered. In general, we show that transverse mode selection can be achieved when the two modes have parallel or orthogonal polarizations. The selection is not possible only when both modes have parallel polarizations and the mode with the lowest threshold current has a lower injection rate than the other mode. Different forms of the selection region in the injected power-injection current plane appear depending on the polarization of the transverse modes. When both transverse modes are orthogonally polarized the mode with the lowest threshold is selected when the injected power surpasses a value that depends on the injection current. However when both modes have parallel polarizations the selection region has a completely different shape because the selection of the mode is no longer possible if the injected power or the injected current are large enough. We also show that the modal selection is favoured as the frequency of the injected light becomes smaller than the frequency of the mode to be selected.

Dynamics of Normal and Reverse Polarization Switching in 1550-nm VCSELs Under Single and Double Optical Injection

Polarization switching (PS) in 1550-nm VCSELs under single and double pulsed polarized optical injection is investigated experimentally and in theory. “Normal” and “reverse” PS were achieved, respectively, when the device was subject to single and double optical injection. Speed operation enhancement is observed for “reverse” PS under double polarized optical injection. Also, the minimum injection power requirements for both “normal” and “reverse” PS have been analyzed showing significant differences for the two investigated cases. Furthermore, we have also investigated numerically the influence of important system parameters, such as the normalized bias current, the injection field strength, and the linewidth enhancement factor in the optimal operation conditions for both types of PS. Overall good agreement between theory and experiments is found.