We present a comprehensive description of electrically-driven vertical-external-cavity surface-emitting diode lasers (VECSELs) at 980 nm, mode-locked by saturable absorber mirrors. A novel partially-integrated time-domain model combines accuracy and flexibility, allowing for a semi-analytical stability analysis of the compound-cavity modes, tracking the mode-locking onset and an optimization analysis. The linear stability analysis of the monochromatic solutions (i. e., the compound cavity modes) indicates that single mode solutions exist and are stable only in a limited current range around threshold. Increasing the current above this current level leads to a multimode solution through a Hopf bifurcation. This bifurcation point is followed by a continuous transition leading from harmonic oscillations to fully-developed pulses that correspond to the mode-locked solution. We obtain stable, fully-developed mode-locked pulses of few tens of picoseconds at 15 GHz repetition rate in good agreement with reported experimental results. We discuss the dependence of the mode-locking regimes on the reflectivity of the distributed Bragg reflectors, spot area of the spatial mode, and number of quantum wells in the emitter and absorber cavities. The optimization analysis reveals that, in order to favor the mode-locking onset, the effective coupling between the emitter and saturable absorber cavities has to be optimized through the standing wave pattern in the composite cavity and spot-area of the spatial modes.
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 either changing the coupling strength or the optical propagation phase. The successive polarization switchings are correlated to the creation of new compound-cavity modes when these parameters are continuously varied. The switching dynamics and the role of asymetries are also discussed.
The performance of an external-cavity mode-locked semiconductor laser is analyzed theoretically and numerically. Passive mode-locking is described using a fully-distributed time-domain model including fast effects, spectral hole burning and carrier heating. We provide optimization rules in order to improve the mode-locking performance, such as reducing the pulsewidth and time-bandwidth product as much as possible. Timing jitter is determined by means of extensive numerical simulations of the model, demonstrating that an external modulation is required in order to maintain moderate timing-jitter and phase-noise levels at low frequencies. The effect of the driving conditions is investigated in order to achieve short pulses and low timing jitter. Our results are in qualitative agreement with reported experiments and predictions obtained from the master equation for mode-locking.
We investigate the synchronization properties of two mutually-coupled semiconductor lasers (SL) in a face to face configuration, when a non-negligible injection delay time is taken into account. Under the appropriate conditions, we derive a thermodynamic potential analog to the one studied by Mork et al. and by Lenstra for a semiconductor laser subject to an optical feedback. In this context, the role that noise and detuning play in the dynamics of the system is clearly identified. When operating in the Low Frequency Fluctuations (LFF) regime, the effect of the detuning on the leader-laggard operation is also analyzed. Finally, we focus on the short intercavity regime and we study the influence of the detuning and the propagation phase on the dynamics of each laser.
We have studied the dynamical behaviour of two semiconductor lasers subject to an optoelectronic bidirectional coupling and optionally to feedback, considering non-zero delay times in the propagation of the signals between both lasers and through each feedback loop in the case the latter exists. Starting from delayed rate equations
for the photon and carrier densities, we have investigated the stability of the fixed points and limit cycles of the system as function of the coupling and feedback strengths, as well as the delay times. From this analysis, quasiperiodic route to chaos and several interesting phenomena like the recently discovered "death by delay"
are predicted for this system.
In this contribution, we analyze the synchronization properties of two chaotic optical field generated by two VCSELs in presence of a saturable absorber. The vectorial nature of the chaotic field requires a continuous control scheme to achieve a high-quality synchronization between the two sources. We proposed a novel encoding scheme, exploiting a two contact VCSEL structure, which allows an easy and direct modulation of the transmitter. The added degree of freedom constituted by the chaos in polarization allows a reduction of the probability error (or enhancement of the transmission capacity) at the receiver.
We develop a model that describes the polarization dynamics of VCSELs with an absorbing region surrounding the active zone. We find that the dynamical behavior of the optical field is organized in four regions: stable linearly-polarized operation, intensity pulsations of a linearly-polarized component, pulsations of both total intensity and polarization, and polarization self-pulsations with constant total intensity. We characterize the four regions by computing the polarization resolved optical and power spectra. The predicted scenario agrees with recent experimental results.
Secondary pulsations are an example of diffusive turn-off transients that can limit the performance of VCSELs in optical communication systems. Secondary pulsations are firstly analysed by using a model where a modal expansion of the electric field is performed. The maximum power of the secondary pulsations and the time at which they appear fluctuate when the spontaneous emission noise is present. A linear relation between the two previous quantities for each individual turn-off event is found. In the single-mode regime, the averaged maximum power during turn-off transients increases when increasing the injection current. However, in the multi-mode regime, the strength of secondary pulsations decreases when increasing the current. Secondary pulsations are also analysed by using a spatio-temporal description of the VCSEL dynamics, where the modal profiles are determined from the distribution of injected carriers and the thermal lens. This model also incorporates polarization effects and a frequency-dependent susceptibility. In this model, the carrier-induced refractive index changes increase the strength of secondary pulsations as compared to that obtained with the modal expansion. It is also shown that the use of ring-shaped electrical contact enhances the strength of secondary pulsations, while it decreases when multi-transverse mode operation is present.
We develop a mesoscopic model of semiconductor dynamics for vertical-cavity surface-emitting lasers which allows us to describe polarization and transverse mode dynamics simultaneously. Within this model, we study the selection processes and the turn-on delay for the switch-on of different transverse modes in gain-guided VCSELs. We consider different active-region diameters, excitation conditions and current shapes. Following the application of the current pulse, transverse modes become excited in a quite definite sequence. After the turn-on, the VCSEL initially switches-on in the fundamental transverse mode, but higher-order transverse modes become excited later. In general, the results obtained are in qualitative agreement with experiments reported recently. Finally, we discuss the current shape dependence on the transverse mode selection at threshold.
In this paper we present numerical and experimental investigations on the synchronization of the instabilities originated by the mutual coupling of two semiconductor lasers in face to face configuration. We have restricted ourselves to the analysis of two lasers with identical parameters and operating at the same frequency. Numerical simulations are based on standard rate equations for each semiconductor laser whereas the mutual injection is modeled by including delayed optical fields. Experiments are performed using almost identical Fabry Perot lasers coupled through the TE component. As soon as the coupling strength is increased we observe fluctuations in the power dynamics that appears synchronized except for a small time lag. This asymmetric operation of the perfectly symmetric system allows to differentiate between leader and laggard lasers. Synchronization properties are studied making use of the synchronization plots and cross-correlation measurements. Extensive investigations of the dependence of the time traces and correlation degree on the coupling strength and current level demonstrate good agreement between numerical and experimental observations.
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.
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