Josep Mulet

ON/OFF phase shift keying for chaos-encrypted communication using external-cavity semiconductor lasers

Synchronization phenomena of two chaotically emitting semiconductor lasers subject to delayed optical feedback are investigated. The lasers are unidirectionally coupled via their optical fields. Our experimental and numerical studies demonstrate that the relative optical feedback phase is of decisive importance: a characteristic synchronization scenario evolves under variation of the relative optical-feedback phase mediating cyclically between chaos synchronization in conjunction with coherent fields, and uncorrelated states in conjunction with incoherent fields. As a key result, we propose, and numerically demonstrate, a novel ON/OFF phase shift keying method opening up new perspectives for applications in communication systems using chaotic carriers.

Chaos shift-keying encryption in chaotic external-cavity semiconductor lasers using a single-receiver scheme

In this letter, we numerically show that chaos shift-keying (CSK) encryption can be achieved by using a single receiver, thus providing a better performance when comparing with the traditional CSK scheme based on two receivers. We analyze the rate equation model for two unidirectionally coupled single-mode external-cavity semiconductor lasers operating in a chaotic regime. The message is encoded in the emitter by slightly varying its injection current. We find that under appropriate conditions, the receiver laser synchronizes to the chaotic emitter, filtering the encoded message and allowing message extraction.

Synchronization scenario of two distant mutually coupled semiconductor lasers

We present numerical and experimental investigations of the synchronization of the coupling-induced instabilities in two distant mutually coupled semiconductor lasers. In our experiments, two similar Fabry–Perot lasers are coupled via their coherent optical fields. Our theoretical framework is based on a rate equation model obtained under weak coupling conditions. In both experiments and simulations, we find (achronal) synchronization of subnanosecond intensity fluctuations in concurrence with asymmetric physical roles between the lasers, even under symmetric operating conditions. We explore the synchronization of these instabilities with respect to the coupling strength and the injection current. We demonstrate the existence of a critical coupling strength, below which synchronization is lost; however, dynamical instabilities persist. Our model correctly reproduces the observed dynamical features over the entire investigated parameter space. We provide an intuitive explanation of the appearance of the achronal solution by analysing the dynamics of the injection phases of the optical fields.

Spatio-temporal modeling of the optical properties of VCSELs in the presence of polarization effects

We develop an optical dynamical model for vertical-cavity surface-emitting lasers (VCSELs) which describes, in an unified way, polarization and spatial effects. The model is based on equations for the lateral dependence of the slowly-varying amplitudes of the optical field in both circular polarizations, and equations for the carrier density in both spin orientations. This provides a natural generalization of the spin flip model for the description of polarization properties of VCSELs extensively used in the literature. In its present form, the model assumes given functional dependence of the guiding mechanisms (built-in refractive index and thermal lensing) as well as the spatial dependence of the current density. We investigate the transverse mode behavior of gain-guided, bottom and top-emitter VCSELs by implementing the model with an analytical approximation to the susceptibility of quantum-well semiconductors. We demonstrate that the stronger the thermal lens, the stronger the tendency toward multimode operation, which indicates that high lateral uniformity of the temperature is required in order to maintain single mode operation in gain-guided VCSELs. We perform analytical calculations of the threshold curves in both types of VCSELs. Also, close-to-threshold numerical simulations show that, depending on the current shape, thermal lensing strength and relative detuning, different transverse modes can be selected.

Mode-locking dynamics in electrically driven vertical-external-cavity surface-emitting lasers

We develop a novel description of electrically driven vertical-external-cavity surface-emitting semiconductor lasers (VECSELs) mode-locked by saturable absorber mirrors. Our approach is based on an analytical solution of the bidirectional traveling-wave equations for fundamental transverse mode operation. The resulting time-domain equations describe the evolution of the electric fields and carrier-densities at the quantum-well layers of the emitter and absorber structures which are coupled through delayed boundary conditions. For the design considered, we obtain stable mode-locked pulses of few tens of picoseconds at 15-GHz repetition rate in agreement with recently reported experimental results.

Bistable polarization switching in mutually coupled vertical-cavity surface-emitting lasers.

We theoretically investigate the polarization-resolved dynamics of two vertical-cavity surface-emitting semiconductor lasers that are mutually coupled through coherent optical injection. We find a sequence of bistable polarization switchings that can be induced by changing either the coupling strength or the optical propagation phase. The successive polarization switchings are correlated with the creation of new compoundcavity modes when these parameters are continuously varied.

Combined effects of semiconductor gain dynamics, spin dynamics and thermal shift in polarization selection in VCSELs

We discuss mechanisms of polarization switching (PS) in Vertical Cavity Surface Emitting Lasers (VCSELs) within a mesoscopic approach based on an explicit form of a frequency- dependent complex susceptibility of the QW semi-conductor material. Cavity anisotropies, spin carrier dynamics and thermal shift of the gain curve are also taken into account in this framework. For large birefringence we find a PS due to thermal shift. For small birefringence we find a different PS, from the high-gain to the low-gain polarization state, that occurs at constant temperature. We characterize polarization partition noise in terms of power spectra. Transverse effects for PS in gain guided VCSELs are also considered.

Characterization of Gain-Switched Pulses From 1.55-

We report on short optical pulse generation by gain-switching (GS) a low-cost commercial vertical-cavity surface-emitting laser emitting at 1.55 μm. The dependence of pulse characteristics on GS parameters is investigated and analyzed. Pulses with duration of 55 ps and time-bandwidth product between 0.91 and 2.2 are obtained at repetition rates between 1 and 3 GHz.

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We implement a dynamic model that describes the polarization behavior in vertical-cavity surface-emitting lasers that contain an absorbing region surrounding the active zone. We find four regions of qualitatively different behavior: stable linearly polarized operation, intensity pulsations of a linearly polarized component, pulsations of both total-intensity and polarization components, and polarization self-pulsation with constant total intensity. We characterize the four regions by computing the polarization-resolved optical and power spectra. The predicted behavior agrees with recent experimental results.

m VCSEL

The performance of an external-cavity mode-locked semiconductor laser is investigated both theoretically and experimentally. The optimization analysis focuses on the regimes of stable mode locking and the generation of sub-picosecond optical pulses. We demonstrate stable output pulses down to one picosecond duration with more than 30 dB trailing pulse suppression. The limiting factors to the device performance are investigated on the basis of a fully-distributed time-domain model.We find that ultrafast gain dynamics effectively reduce the pulse-shaping strength and inhibit the generation of femtosecond optical pulses.