The ability to produce narrow optical pulses has been extensively investigated in laser systems with promising applications in photonics such as clock recovery, pulse reshaping, and recently in photonics artificial neural networks using spiking signal processing. Here, we investigate a neuromorphic opto-electronic integrated circuit (NOEIC) comprising a semiconductor laser driven by a resonant tunneling diode (RTD) photo-detector operating at telecommunication (1550 nm) wavelengths capable of excitable spiking signal generation in response to optical and electrical control signals. The RTD-NOEIC mimics biologically inspired neuronal phenomena and possesses high-speed response and potential for monolithic integration for optical signal processing applications.
We review the dynamics of VCSELs that experience both Polarization-Selective Feedback (PSF) and Crossed- Polarization Reinjection (XPR). Different regimes of regular pulsation were found. For strong enough XPR levels, the VCSEL emission in each of its linearly-polarized components displays a square-wave modulation which regularity is greatly enhanced by small levels of PSF. Such a square-wave is in antiphase for the two polarizations, and it turns out to be stable and robust over broad intervals of current. The frequency of the square-wave is determined by the length of the XPR arm. For weak levels of PSF and XPR, the VCSEL emits a regular train of short optical pulses arising from the locking of the modes in the PSF cavity. The frequency of the pulse train is stable on short time scales, but it wanders with a characteristic time scale of hundreds of roundtrips in the PSF cavity. The experimental results are successfully explained by an extension of the Spin-Flip Model that incorporates gain saturation and the effects of PSF and XPR.
We study experimentally the dynamic properties of a fully integrated high power master-oscillator power-amplifier emitting at 1.5 μm under continuous wave and gain-switching conditions. High peak power (2.7 W) optical pulses with short duration (~ 110 ps) have been generated by gain switching the master-oscillator. We show the existence of working points at very close driving conditions with stable or unstable regimes caused by the compound cavity effects. The optical and radio-frequency spectra of stable and unstable operating points are analyzed.
We have analyzed experimentally and theoretically the modal properties of a semiconductor ring laser and the
wavelength jumps that occur in connection with directional switching above threshold. A transfer matrix analysis
allow us to explain the transfer function measurements when amplified spontaneous emission in the cavity is
accounted for. Moreover the transfer matrix analysis permits to determine the threshold condition for the laser
modes, which split in two branches due to the symmetry breaking imposed by the output coupler and output
waveguides. The wavelength jumps displayed by the device above threshold are interpreted with the frequency
splitting and threshold difference between these two branches of solutions, together with the redshift of the
material gain.
The optical spectrum of monolithic Semiconductor Ring Lasers (SRLs) is measured simultaneously for both lasing
directions with a grating-based OSA, in the regimes of bidirectional and unidirectional operation. In the unidirectional
operation regime the SMSR is larger than 25 dB, and the directional extinction ratio (i.e., the ratio of the power emitted
in the two opposite directions) is larger than 20 dB. The influence of the current injected in the active output waveguides
that act as SOAs is outlined. In the unidirectional regime the linewidth of the SRL is measured by an heterodyne
technique, revealing linewidth values around 2 MHz.
We develop a rate equation model describing the polarization switching phenomenon in vertical-cavity surface-emitting lasers taking into account the major underlying physical origins of this behavior: spin flip relaxation effects, temperature variations and both residual strain in the quantum well and stresses externally applied to the device. To include effects of temperature and stress or strain, we describe the optical material properties of the quantum well by way of a recently derived analytical approximation for the optical susceptibility of uniaxially stressed quantum-well lasers at low temperatures. We review the influence of temperature and stress on the polarization-dependent gain and the linewidth enhancement factor. Combining this information with cavity anisotropies and spin carrier dynamics, we present a model that provides a unified overview of the polarization switching phenomenon. By way of a linear stability analysis, the polarization mode stability is discussed and compared with earlier experimental results.
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.
Cavity solitons are stationary self-organized bright intensity peaks which form over a homogeneous background in the section of broad area radiation beams. They are generated by shining a writing/erasing laser pulse into a nonlinear optical cavity, driven by a holding beam. The ability to control their location and their motion by introducing phase or amplitude gradients in the holding beam makes them interesting as mobile pixels for all-optical processing units. We show the generation of a number of cavity solitons in broad area vertical cavity semiconductor microresonators electrically pumped above transparency but slightly below threshold. The observed spots can be written, erased and manipulated as independent objects. We analyze experimentally the cavity solitons domain of existence in the parameter space and how their characteristics are affected by inhomogeneities and impurities of the vertical cavity devices. A theoretical model, keeping into account the devices characteristics, reproduces numerically the experimental observations with good agreement.
In this manuscript we analyze the modal dynamics of multimode semiconductor quantum-well lasers. Modal switching is the dominant feature of the devices analyzed and it obeys a highly organized antiphase dynamics which leads to an almost constant total intensity output. For each active mode a regular switching at frequencies of
few MHz is observed. The activation order of the modes follows a well defined sequence starting from the lowest wavelength (bluest) mode to the highest wavelength (reddest) mode, then the sequence starts again from the bluest mode. Using a multimode theoretical model and a simpler phenomenological model we identify that four wave mixing is the dominant mechanism at the origin of the observed dynamics. The asymmetry of the susceptibility function of semiconductor materials allows to explain the optical frequency sequence.
In this manuscript we analyze experimentally the noise effects on the modal dynamics of semiconductor lasers. Modal switching is the dominant feature of multimode semiconductor lasers. In bulk lasers, two modes coexisting for the same values of the parameters switch under the action of the noise present into the system (mode-hopping). In this kind of laser we give evidence of stochastic resonance of the mode-hopping under modulation of the laser pump. In quantum-well lasers the modal dynamics is radically different: it consists of a periodic switching among several longitudinal modes, following a well determined sequence from the bluest to the reddest mode in the optical spectrum. The switching dynamics of the modes obeys to a perfect antiphase leading to an almost constant total intensity output.
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.
We show experimentally that semiconductor lasers with a double cavity or with an injected signal behave dynamically as excitable media. We perform experimental tests in order to characterize excitable pulses. We also present experimental evidence of coherent resonance as the amount of noise is increased in the system.
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.
Moderate feedback levels in edge emitting semiconductor lasers may cause different unstable regimes of operation. One of them is known as the regime of Low Frequency Fluctuations. We performed an experimental study of this regime on a picosecond time scale using a single-shot streak camera. Our measurements show the fine temporal structure of the laser emission in this regime and reveal the multimode nature of laser operation.
We study the polarization behavior of birefringent multi- transverse mode index-guided Vertical-Cavity Surface- Emitting Lasers during CW and modulation operation. The model we use incorporates the (alpha) -factor, different carriers populations associated with different magnetic sublevels of the conduction and heavy hole valence bands in quantum well media, spin-flip relaxation processes and cavity anisotropies. The effect of carrier diffusion and spatial hole burning on the polarization behavior within the fundamental mode regime is also considered. We find that spin-flip dynamics and the (alpha) -factor are crucial for the selection of the polarization state corresponding to a given injection current. We also analyze the direct-current polarization modulation response for both the small and large signal modulation regimes. In both cases the current is modulated around the polarization switching (PS) current, but in the large signal modulation regime the current goes from below threshold to above the PS current. A frequency doubled polarized output is obtained under large signal modulation for modulation frequencies smaller than 10 MHz. At greater frequencies (around 100 MHz) transient mode polarization partition noise becomes important. At the greatest considered frequency (1 GHz) no PS is observed. Finally the effect of multiple transverse mode operation is addressed by considering the two lowest order modes: LP01 and LP11.
Dynamical behavior of a four-stripe array of semiconductor lasers is modeled in time and across the transverse spatial dimension for different values of the injection current, various current and carrier diffusion rates, and different spacings of the stripes, including the limit in which the lasing regions merge giving a single broad-stripe laser. Various stages of the dynamics are revealed as a function of the coupling strength, ranging from quasi-independent operation of the oscillators under weak coupling of the oscillators through chaotic modulation under moderate coupling and finally into the formation of stable or modulated broad area transverse modes under strong coupling.
Transverse pattern dynamics in the output of a laser are studied by detailed numerical solutions of the full set of Maxwell Bloch equations. The effects of different parameters on the behavior of the solutions are investigated. Our numerical results reveal a rich variety of behaviors in the pattern evolution, phase vortex formation and spatiotemporal dynamics in the output of a laser beam.
Transverse pattern dynamics in the output of a laser are studied by numerical solution of the Maxwell Bloch equations. It was found that simple patterns involving many radial and angular modes can be simply described in terms of the motion and interaction of defects, singularities in the complex field. The complexity of the patterns and dynamics increase as the Fresnel number of the resonator is increased and as the mode spacing, gain, and cavity detuning are adjusted to bring more modes within the range of frequencies where gain exceeds loss.
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