This PDF file contains the front matter associated with SPIE Proceedings Volume 9134 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

The use of Internet has increased and continues to increase exponentially, mostly driven by consumers. Thus bit rates in networks from access to WDM and finally the computer clusters and supercomputers increase as well rapidly. Their cost of energy reaches today 5-6 % of raw electricity production. For 2023 a cross over is predicted, if no new "green" technologies or "green" devices" will reduce energy consumption by about 15% per year. We present two distinct approaches for access and computer networks based on nanophotonic devices to reduce power consumption in the next decade.

The near-threshold dynamics of a QD and a commercial QW laser are investigated both experimentally and theoretically. Below threshold, the resonance frequency and damping factor of the QD laser exhibit a different behaviour as compared to the QW counterpart. In the near-threshold regime, the intra-dot carrier relaxation is predicted from an empirical pairstates model to have a strong impact on the QD laser’s modulation dynamics. The widespread of experimental values for the damping factor reported in the literature for QD lasers is a further indication that this empirical approach is pushed to the limits in this situation. More accurate microscopic modelling should rely on a separation of electron and hole dynamics.

In recent years, quantum-dot (QD) semiconductor lasers attract significant interest in many practical applications due to their advantages such as high-power pulse generation because to the high gain efficiency. In this work, the pulse shape of an electrically pumped QD-laser under high current is analyzed. We find that the slow rise time of the pulsed pump may significantly affect the high intensity output pulse. It results in sharp power dropouts and deformation of the pulse profile. We address the effect to dynamical change of the phase-amplitude coupling in the proximity of the excited state (ES) threshold. Under 30ns pulse pumping, the output pulse shape strongly depends on pumping amplitude. At lower currents, which correspond to lasing in the ground state (GS), the pulse shape mimics that of the pump pulse. However, at higher currents the pulse shape becomes progressively unstable. The instability is greatest when in proximity to the secondary threshold which corresponds to the beginning of the ES lasing. After the slow rise stage, the output power sharply drops out. It is followed by a long-time power-off stage and large-scale amplitude fluctuations. We explain these observations by the dynamical change of the alpha-factor in the QD-laser and reveal the role of the slowly rising pumping processes in the pulse shaping and power dropouts at higher currents. The modeling is in very good agreement with the experimental observations.

We introduce an analytical approach to the multi-state lasing phenomenon in p-doped and undoped InAs/InGaAs quantum dot lasers which were studied both theoretically and experimentally. It is shown that the asymmetry in charge carrier distribution in quantum dots as well as hole-to-electron capture rate ratio jointly determine laser’s behavior in such a regime. If the ratio is lower than a certain critical value, the complete quenching of ground-state lasing takes place at sufficiently high injection currents; at higher values of the ratio, our model predicts saturation of the ground-state power. It was experimentally shown that the modulation p-doping of laser’s active region results in increase of output power emitted via the ground-state optical transitions of quantum dots and in enhancement of the injection currents range in which multi-state lasing takes place. The maximum temperature at which multi-state lasing exists was increased by about 50°C in the p-doped samples. These effects are qualitatively explained in the terms of the proposed model.

Spin-controlled semiconductor lasers are highly attractive spintronic devices providing characteristics superior to their conventional purely charge-based counterparts. In particular, spin-controlled vertical-cavity surface emitting lasers (spin-VCSELs) promise to offer lower thresholds, enhanced emission intensity, spin amplification, full polarization control, chirp control and ultrafast dynamics. Most important, the ability to control and modulate the polarization state of the laser emission with extraordinarily high frequencies is very attractive for many applications like broadband optical communication and ultrafast optical switches. We present a novel concept for ultrafast spin-VCSELs which has the potential to overcome the conventional speed limitation for directly modulated lasers by the relaxation oscillation frequency and to reach modulation frequencies significantly above 100 GHz. The concept is based on the coupled spin-photon dynamics in birefringent micro-cavity lasers. By injecting spin-polarized carriers in the VCSEL, oscillations of the coupled spin-photon system can by induced which lead to oscillations of the polarization state of the laser emission. These oscillations are decoupled from conventional relaxation oscillations of the carrier-photon system and can be much faster than these. Utilizing these polarization oscillations is thus a very promising approach to develop ultrafast spin-VCSELs for high speed optical data communication in the near future. Different aspects of the spin and polarization dynamics, its connection to birefringence and bistability in the cavity, controlled switching of the oscillations, and the limitations of this novel approach will be analysed theoretically and experimentally for spin-polarized VCSELs at room temperature.

We have studied experimentally and theoretically the nonlinear dynamics of a 1550 nm single transverse mode VCSEL subject to two-frequency orthogonal optical injection. In this type of injection both injected fields have a linear polarization that is orthogonal to that of the free-running VCSEL. We have found different behaviors that include irregular and periodic dynamics in the orthogonal polarization, periodic dynamics in both linear polarizations and a situation in which both linearly polarized modes lock to the most intense injection when its wavelength is close to the free-running laser wavelength. In this study we also analyze the generated high-frequency microwave signal found when the VCSEL is emitting only in the orthogonal polarization. The relative strength of peaks in the optical spectra at the frequencies of both master lasers depends on the behaviour of the VCSEL under single optical injection by the most intense master laser. The peak in the optical spectrum that appears at the frequency of the most intense master laser is larger than the peak that appears at the frequency of the other master laser, providing that there is stable locking when only light from the most intense laser is injected. In this case a significant emission of the VCSEL at the frequency of the most intense master laser is observed. On the contrary, if there is not stable locking when only light from the most intense laser is injected, the magnitude of both peaks becomes similar and a significant emission of the VCSEL at the frequency of the weakest master laser is observed. Good agreement is found between our experimental and theoretical results.

Laser diodes typically behave like damped oscillators: they are generally expected to only show damped relaxation oscillations toward a stable fixed point. In vertical-cavity surface-emitting lasers (VCSELs), the picture appears to be quite different as polarization dynamics can be experimentally observed including bifurcations to self- pulsation and even chaos. Physically, the circular geometry of VCSELs makes the polarization selection very weak and, thus, the additional degree of freedom can enable complex dynamical behavior in the laser diode. Here we report on a new dynamical behavior in a free-running VCSEL: we observe a bistability between two limit cycles oscillating around two distinct elliptical polarization states whose main axes are symmetrical with respect to the polarization at threshold. Although the existence of two symmetric elliptical polarizations and the associated limit cycles are predicted by the San Miguel, Feng and Moloney (SFM) model, the hysteresis cycle observed experimentally highlights the importance of asymmetry in the dynamics from the elliptically polarized states. We demonstrate that this behavior can be accurately reproduced in theory within the SFM framework when taking into account a small misalignment between the phase and amplitude anisotropies of the laser cavity. Our results bring new light into VCSEL polarization dynamics and provide a very good qualitative agreement with the bifurcation scenario predicted by the SFM model.

Emission wavelength setting of 1310nm-waveband VCSELs designed for coarse wavelength division multiplexing (CWDM) 4x10 Gbps fiber-optics transmission can be controlled thanks to the wafer fusion fabrication approach. This approach allows performing the cavity adjustment before bonding the distributed Bragg reflectors (DBRs) to the active cavity of the device. Cavity adjustment was performed by digital etching with nanometer precision and proves to be very effective in compensating for epitaxial growth thickness off-set relative to nominal design and thickness nonuniformity across the wafer. With this fabrication approach we reach on fused VCSEL wafers more than 90% yield of devices that fit the CWDM wavelength slots.

In this paper we report on the progress in the development of modelocked ring lasers that are integrated on a single chip in the InP/InGaAsP material system. With the current optical integration technology it is possible to integrate quantum well optical amplifiers, phase modulators and passive optical components such as waveguides, splitters and spectral filters as standardized building blocks on a single chip. Using such standardized components a number of passively modelocked ring laser devices have been realized in a standardized fabrication process. Results from a few of these devices are presented here. We have observed a record width of the frequency comb from a modelocked quantum well ring laser operating at a 20 GHz repetition rate. The optical coherent comb is centered around 1542 nm and has a 3 dB bandwidth of 11.5 nm. A minimum pulse width of 900 fs was observed. A second device that is highlighted is a modelocked ring laser with a 2.5 GHz repetition rate. Its 33 mm long cavity is fitted onto a chip of 2.2x1.9 mm². One of the goals of this work is to make such designs available in device libraries for use in more complex integrated optical systems using standardized technology platforms.

We present a way to describe mode locking through a master equation, derived from the laser Maxwell-Bloch equations, which take into account coherence in light-gain interaction. Our derivation shows that gain evolution on the fast time scale of the laser pulses must be considered, unlike in Haus master equation. A key point in the derivation is to use an improved adiabatic elimination of the medium polarization which leads to the appearance of the fast evolution of gain. Numerical simulations with parameters suitable for a semiconductor laser show marked differences with respect to Haus master equation in the shape, height and width of the pulses. These effects, which are related to the fast varying part of the gain, are more pronounced the longer is the cavity.

We describe the technique allowing for generation of low-noise wider frequency combs and pulses of shorter duration in quantum-dot mode-locked lasers. We compare experimentally noise stabilization techniques in semiconductor mode-locked lasers. We discuss the benefits of electrical modulation of the laser absorber voltage (hybrid mode-locking), combination of hybrid mode-locking with optical injection seeding from the narrow linewidth continues wave master source and optical injection seeding of two coherent sidebands separated by the laser repetition rate.

In the last decades, various solutions have been proposed to increase the modulation bandwidth and consequently the transmission bit rate of integrated semiconductor lasers. In this manuscript we review some of them, trying to analyze for each one the mechanisms producing the modulation bandwidth extension. A comparison of the various approaches used for in{plane lasers is presented, in terms of both small and large signal modulation responses calculated using a Finite Difference Time Domain (FDTD) numerical simulator.

Laser diodes that emit multiple wavelengths simultaneously are needed in a range of applications including wavelength division multiplexing, high speed optical networks and tera-hertz generation. In this work we report on an integrated approach to obtain multi-wavelength emission from a semiconductor ring laser based on on-chip filtered optical feedback. Semiconductor ring laser have the advantage that they can be easily integrated with other optical components as they do not require mirrors to form the cavity. Moreover, no thermal control of the wavelength emission is needed and therefore the device can be in principle fast. The filtered optical feedback is realized by employing two arrayed waveguide gratings to split/recombine light into different wavelength channels. Semiconductor optical amplifiers are placed in the feedback loop in order to control the feedback strength of each wavelength channel independently. Experimental observations [Khoder et al, Optics. Lett. 38, 2608{2610, 2013] have shown that the effective gain is the key parameter that has to be balanced using the feedback in order to achieve multi-wavelength emission. This can be achieved by tuning the injection current in each amplifier which will change the feedback phase and strength. Numerical simulations using rate equations reproduce the experimental results and show the effects of feedback phase and strength on the multi-wavelength emission.

Phase-conjugate optical feedback (PCF) has been largely used as a way to stabilize and reduce the linewidth of laser emission but is also known to generate complex dynamics including self-pulsation and chaos. In contrast to the large number of theoretical works, there have been only few experiments reporting on nonlinear dynamics from PCF. Most importantly, experiments so far have not addressed the peculiarities of the PCF dynamics in comparison with dynamics observed from conventional optical feedback (COF). We report here experimentally and theoretically on two chaotic dynamics that relate to the peculiar dynamical properties of a laser diode with PCF. First, we find a chaotic dynamics that resembles the so-called low-frequency fluctuations (LFF) of a laser diode with COF, i.e. the output power shows abrupt dropouts at randomly distributed time-intervals followed by a slower recovery. Although the LFF in PCF shows similar statistical properties to those observed in the LFF in COF, they originate from a distinctively different bifurcation scenario. Increasing the PCF strength the laser diode shows successive bifurcations to time-periodic solutions at the frequency of the external cavity and multiples - also called 'external-cavity modes' (ECMs). In contrast to COF the PCF laser system shows no steady state for large enough feedback strength. Following the destabilization of several such ECMs to chaotic attractors, the dynamics shows a transition to a global attractor connecting the chaotic ECMs and that explains the sequence of power dropouts and recoveries. In addition we show how the bifurcations on these self-pulsing ECMs generate dynamics with extreme events, i.e. pulses with peak intensities well above the average value of the peaks in the output power and that show properties similar to the rogue waves in hydrodynamics. This is the first demonstration of temporal extreme events in a time-delayed optical system.

Thanks to the band-gap engineering of quantum confined semiconductor materials and the development of semiconductor-based saturable absorber mirrors, recent years have seen the development of compact and low-cost external-cavity laser diodes generating pulses at several tens of GHz. The physics of the bifurcation leading to selfpulsation leads however to an intrinsic limitation: the fundamental repetition rate is fixed to and limited by the externalcavity round-trip time. By contrast, we demonstrate here that an external-cavity diode laser may generate fundamental self-pulsating dynamics at harmonics of the external-cavity frequency, when a phase conjugate mirror replaces the conventional mirror. As is known from theory, a laser diode with phase conjugate external feedback supports a single stationary solution that bifurcates to self-pulsating dynamics of increasing frequency when increasing the amount of light reflected back to the laser diode. The self-pulsation frequency then increases in step of the external-cavity frequency as one increases the feedback strength. We provide here the first experimental evidence of such harmonic external-cavity fundamental self-pulsation. As a proof-of-concept, we generate experimentally a self-pulsating dynamics at twice and three times the fundamental external-cavity frequency using an edge-emitting laser with a self-pumped ring-cavity photorefractive phase conjugator. Numerical simulations also predict stable higher harmonics.

We investigate the symbolic dynamics of an excitable optical system under periodic forcing. Particularly, we consider the low-frequency fluctuation (LFF) dynamics of a semiconductor laser with periodically-modulated injection current and optical feedback. We use a method of symbolic time-series analysis that allows us to unveil serial correlations in the sequence of intensity dropouts. By transforming the sequence of inter-dropout intervals into a sequence of ordinal patterns and analyzing the statistics of the patterns, we uncover correlations among several consecutive dropouts and we identify definite changes in the dynamics as the modulation amplitude increases. We confirm the robustness of the observations by conducting the experiments with two different lasers under different feedback conditions. The results are also shown to be robust to variations of the threshold used for detecting the dropouts. Simulations of the Lang-Kobayashi (LK) model, including spontaneous emission noise, are found to be in good qualitative agreement with the observations, providing an interpretation of the correlations present in the dropout sequence as due to the interplay of the underlying attractor topology, the periodic forcing, and the noise that sustains the dropout events.

In Europe a number of technology platforms for generic integration are being created for photonic integrated circuits (PICs); in Silicon, in passive dielectrics, and in Indium Phosphide. Such platforms are on the brink of commercialization, they offer a range of calibrated building blocks from which application specific PICs can be built and allow simplified, reduced cost access to a standardised technology, but presently only InP based platforms allow the integration of optical gain blocks; the essential feature of a semiconductor laser. The wavelength is constrained by the platform, usually C-band, but in the near future we expect other wavelengths in the 1.3μm-2.0μm range will be addressed. A frozen platform technology may not seem an ideal starting point for novel laser research but for what may be appear to be lost in epitaxial and process flexibility, much more is gained through a new-found ability to build up complex circuits quickly to deliver new and interesting laser based functionality. Building blocks such as reflectors (a distributed Bragg reflector (DBR) or a multimode interference reflector (MIR)), an amplifier section, and passive waveguides, can be built up by designers into integrated semiconductor lasers of a wide variety of types. This ready integration of novel sources with other circuit functionality can address a wide range of applications in telecoms, datacoms, and fibre based sensing systems. In this paper we describe a number of recent developments on generic InP-based platforms ranging from the fabrication of simple Fabry-Perot lasers, through tuneable DBR lasers, multi-wavelength comb lasers, picosecond pulse lasers and ring lasers.

Integrated master-oscillator power amplifiers driven under steady-state injection conditions are known to show a complex dynamics resulting in a variety of emission regimes. We present experimental results on the emission characteristics of a 1.5 μm distributed feedback tapered master-oscillator power-amplifier in a wide range of steady-state injection conditions, showing different dynamic behaviors. The study combines the optical and radio-frequency spectra recorded under different levels of injected current into the master oscillator and the power amplifier sections. Under low injection current of the master oscillator the correlation between the optical and radio-frequency spectral maps allows to identify operation regimes in which the device emission arises from either the master oscillator mode or from the compound cavity modes allowed by the residual reflectance of the amplifier front facet. The quasi-periodic occurrence of these emission regimes as a function of the amplifier current is interpreted in terms of a thermally tuned competition between the modes of the master oscillator and the compound cavity modes. Under high injection current of the master oscillator, two different regimes alternate quasi-periodically as a function of the injected current in the power amplifier: a stable regime with a single mode emission at the master oscillator frequency, and an unstable and complex self-pulsating regime showing strong peaks in the radio-frequency spectra as well as multiple frequencies in the optical spectra.

We present DFB laser diodes emitting in the 76x nm wavelengths range and focus on design and fabrication of the integrated Bragg gratings. Grating functionality is obtained with a periodically patterned GaAs_0.75P_0.25 layer with a thickness of 13 nm. We applied scanning transmission electron microscopy using a high angle annular dark-field detector for the analysis of the buried grating structures and for the improvement of the etching and regrowth conditions. Ridge waveguide DFB lasers with optimized gratings and production process show single mode emission with intrinsic linewidths below 10 kHz. Coated 1.5 mm long ridge waveguide DFB lasers emit stable over 5000 hours at a constant power of 100 mW.

We simulate and analyze how beam quality improves while being amplified in edge emitting broad area semiconductor amplifiers with a periodic structuring of the electrical contacts, in both longitudinal and lateral directions. A spatio-temporal traveling wave model is used for simulations of the dynamics and nonlinear interactions of the optical fields, induced polarizations and carrier density. In the case of small beam amplification, the optical field can be expanded into few Bloch modes, so that the system is described by a set of ODEs for the evolution of the mode amplitudes. The analysis of such model provides a deep understanding of the impact of the different parameters on amplification and on spatial (angular) filtering of the beam. It is shown that under realistic parameters the two-dimensional modulation of the current can lead not only to a significant reduction of the emission divergence, but also to an additional amplification of the emitted field.

For high-power semiconductor lasers, asymmetric reflectivities of facets are employed in order to improve slope efficiency. Cavity lengths of these laser diodes have been increased to better distribute heat in order to improve output power. However, these two methods result in an inhomogeneous longitudinal profile of photon density, which leads to a nonuniform gain profile and is typically referred to as longitudinal spatial hole burning (LSHB). In this work, we developed a model to self-consistently calculate the longitudinal photon density distribution, carrier density distribution, and gain distribution in a high-power semiconductor laser. The calculation is based on modified rate equations, and a finite difference method is used to solve the differential equations. Newton’s method is employed to obtain final results with residual error below 10^-6. The impact of LSHB was analyzed with different parameters, and we demonstrate that LSHB is expected to limit the maximum achievable output power of semiconductor lasers having cavity lengths in excess of several mm. The results are expected to be useful in the optimization of high-power semiconductor laser designs.

A master oscillator power amplifier (MOPA) system for the generation of ns-pulses with high peak power, narrow spectral line width, and stabilized emission wavelength will be presented. The master oscillator is a distributed feedback (DFB) ridge waveguide (RW) laser. The tapered amplifier consists of one RW section and one flared gain-guided section. The DFB laser is operated in continuous wave mode and emits at 973.5 nm with a spectral line width below 10 pm. The RW section of the amplifier acts as an optical gate. The tapered section amplifies the generated optical pulse. An optical peak power of 15.5 W for a pulse width of 8 ns is obtained. The emission wavelength remains constant at all output power levels of the MOPA system for a fixed current into the DFB laser. The spectral power density of the ASE is 37 dB smaller than the lasing spectral power density. The spectral line width is smaller than 10 pm, limited by the resolution of the optical spectrum analyzer.

Generating high power laser radiation with diode lasers is commonly realized by geometrical stacking of diode bars, which results in high output power but poor beam parameter product (BPP). The accessible brightness in this approach is limited by the fill factor, both in slow and fast axis. By using a geometry that accesses the BPP of the individual diodes, generating a multi kilowatt diode laser with a BPP comparable to fiber lasers is possible. We will demonstrate such a modular approach for generating multi kilowatt lasers by combining single emitter diode lasers. Single emitter diodes have advantages over bars, mainly a simplified cooling, better reliability and a higher brightness per emitter. Additionally, because single emitters can be arranged in many different geometries, they allow building laser modules where the brightness of the single emitters is preserved. In order to maintain the high brightness of the single emitter we developed a modular laser design which uses single emitters in a staircase arrangement, then coupling two of those bases with polarization combination which is our basic module. Those modules generate up to 160 W with a BPP better than 7.5 mm*mrad. For further power scaling wavelength stabilization is crucial. The wavelength is stabilized with only one Volume Bragg Grating (VBG) in front of a base providing the very same feedback to all of the laser diodes. This results in a bandwidth of < 0.5 nm and a wavelength stability of better than 250 MHz over one hour. Dense spectral combination with dichroic mirrors and narrow channel spacing allows us to combine multiple wavelength channels, resulting in a 2 kW laser module with a BPP better than 7.5 mm*mrad, which can easily coupled into a 100 μm fiber and 0.15 NA.

We present result of optical studies on InAs/GaIn(As)Sb/InAs type II quantum wells predicted for the active region in interband cascade lasers, and further for laser-based gas sensors operating at room temperature in a broad wavelength range of mid infrared. Using photoreflectance spectroscopy supported by electronic structure calculations we determine the oscillator strength of the fundamental optical transition in structures with GaIn(As)Sb material of various compositions hole confinement layer. We show that incorporation of arsenic into this layer can affect several crucial properties significantly like transition wavelength and its probability, but also the structural material quality affecting the radiative efficiency. Also, by using photoluminescence we investigate one of the crucial parameters for the performance of interband cascade lasers, the spectral emission width of type II quantum wells constituting the laser active region.

We present the optimization of the carrier injection, heat flow and optical confinement aimed at single mode operation in anti-guiding long-wavelength VCSELs and VCSEL arrays. The analyzed structure incorporates InP/AlGaInAs quantum wells within an InP cavity. The cavity is bounded by GaAs/AlGaAs DBRs. The tunnel junction is responsible for carrier funneling into the active region. The air-gap etched at the interface between cavity and top DBR provides the confinement of the lateral modes. To rigorously simulate the physical phenomena taking place in the device we use a multi-physical model, which comprises three-dimensional models of optical (Plane Wave Admittance Method), thermal and electrical (Finite Element Method) phenomena. In the analysis we investigate the influence of the size of single and multiple emitters and the distance between the emitters in the case of the VCSEL arrays. As a result, we illustrate the complex competition of the modes and determine the geometrical parameters favoring specific array modes in the considered designs and compare the designs with respect to mode discrimination.

We report on an optically pumped vertical-external-cavity surface-emitting laser diode coupled by a high reflectance volume holographic grating (VHG). The major feature of such a system is very narrow linewidth and high degree of power scalability, required for pumping upper energy levels of a gain medium. The cavity design lends itself to scaling to 2D VECSEL arrays suitable for low cost, high brightness pump sources.

We report on the development of a pulsed high-power frequency doubled vertical-external-cavity surface-emitting laser (VECSEL) with a peak output power of 14 W and emission spectrum near 588 nm. The semiconductor gain chip was grown by molecular beam epitaxy and comprised 10 GaInAs quantum wells. The gain structure was designed to be antiresonant at 1180 nm. The fundamental wavelength was frequency doubled to the yellow–orange spectral range using a 10-mm long critically phase matched lithium triborate nonlinear crystal, situated at the mode waist of the V-shaped laser cavity. The emission spectrum was narrowed down to FWHM of < 0.2 nm by employing a 1.5 mm birefringent filter and a 100-μm-thick etalon inside the cavity. By directly modulating the pump laser of the VECSEL, we were able to produce pulse widths down to 570 ns with average and peak output power of 81 mW and 14 W, respectively. The repetition rate was kept constant at 10 kHz throughout the measurements. The maximum peak power obtained was pump power limited. In comparison, at the same coolant temperature, a maximum of 8.5 W was achieved in continuous wave. The maximum optical-to-optical conversion efficiency (absorbed peak pump power to peak output power) was calculated to be 20–21 %.

We study the nonlinear dynamics of semiconductor micropillar lasers with intracavity saturable absorber in the excitable regime. The excitable regime is characterized by an all-or-none type of response to an input perturbation: when the perturbation amplitude is below the excitable threshold, the system remains in its quiet, stable state; when the perturbation exceeds the excitable threshold, a calibrated response pulse is emitted. It is believed to have great potential for fast neuromorphic optical processing, in addition to being also interesting for the study of nonlinear wave propagation. Fast excitable, neuron-like, dynamics is experimentally evidenced with response times in the 200ps range. We also show the presence of an absolute and a relative refractory periods in this system, analog to what is found in biological neurons but with several orders of magnitude faster response times. The absolute refractory period is the amount of time after a first excitable pulse has been emitted during which it is not possible to excite the system anymore. The relative refractory period is the time after a first excitable pulse during which an inhibited response is emitted and has been often overlooked in optical systems. Both these times are of fundamental importance regarding the propagation of stable excitable waves, and in view of designing spike-time based optical signal processing systems. The experimental results are well described qualitatively by a simple model of a laser with saturable absorber.

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.

Following an overview of modeling of (longitudinal) multimode semiconductor laser dynamics, we analyze in detail a model proposed in 2006 to explain deterministic, phase-locked modal alternation, experimentally observed a decade ago. Through a stability analysis, we prove that the numerically obtained electromagnetic field evolution, interpreted as an explanation of the experiments, is nothing more than an extremely long transient, so long as to be hardly identifiable in an entirely numerical approach. Comparison with a model we have recently derived, which predicts a phase instability (Benjamin-Feir-like) compatible with the experimental observations, highlights the crucial ingredient for the dynamics. The wide spectrum of unstable eigenvalues accompanying the phase instability plays the role of an equivalent noise in a fully deterministic description, thus reconciling the heuristic models which could qualitatively reproduce the experimental observation either with deterministic equations in the presence of mode-coupling, or through stochastically driven modal decompositions.

We consider a broad area vertical-cavity surface-emitting laser(VCSEL) subject to injection and to time-delayed feedback. We present analytical and numerical analysis of the dependence of the drift instability threshold and on the feedback strength, feedback phase, and carrier relaxation time. we demonstrate that due to finite carrier relaxation rate the delay induced drift instability can be suppressed to a certain extent. We give analytical estimation of the soliton velocity near the drift instability point which is in a good agreement with numerical results obtained using the full model equations.

We investigate a control technique, known as rocking, for the formation of phase bistable patterns of light by means of spatially periodic injection in a broad area nonlinear optical system. More precisely, we consider a vertical-cavity surface-emitting laser (VCSEL). The spatial periodic injection or spatial rocking is found to convert the initially phase-invariant oscillatory system into a phase-bistable pattern forming one. We investigate the role of carrier lifetime on the efficiency of rocking in a broad-area VCSEL structure. This simple and robust device received a special attention owing to advances in semiconductor technology. We show that the regime where rocking works depends strongly on the ratio between the time scales associated with the electric field and the carrier density. The size of the rocking region depends on the ratio between the time scales.

We report what we believe is the first demonstration of a temporal soliton bound state in semiconductor disk laser. The laser was passively mode-locked using a quantum dot based semiconductor saturable absorber mirror (QD-SESAM). Two mode-locking regimes were observed where the laser would emit single or closely spaced double pulses (soliton bound state regime) per cavity round-trip. The pulses in soliton bound state regime were spaced by discrete, fixed time duration. We use a system of delay differential equations to model the dynamics of our device.

We consider a broad area vertical-cavity surface-emitting laser (VCSEL) subject to optical injection. We experimentally investigate the spontaneous formation of a Cavity Soliton (CS) in a medium size (80μm diameter) VSCEL. CSs are generated and erased when sweeping optical injection power and proper frequency detuning between the master laser and the VCSEL is set.

Electro-absorption modulated 10G and 25G DFB lasers (EML) are key components in transmission systems for long reach (up to 10 km) and extended reach (up to 80 km) applications. The next generation Ethernet will most likely be 400 Gb/s which will require components with even higher bandwidth. Commercially available EMLs are regarded as high-cost components due to their separate epitaxial butt-coupling growth process to separately optimize the DFB laser and the electro-absorption modulator (EAM). Alternatively the selective area growth (SAG) technique is used to achieve different MQW bandgaps in the DFB and EAM section of an EML. However for a lot of applications an emission wavelength within a narrow wavelength window is required enforcing a temperature controlled operation. All these applications can be covered with the developed EML devices that use a single InGaAlAs MQW waveguide for both the DFB and the EAM enabling a low-cost fabrication process similar to a conventional DFB laser diode. It will be shown that such devices can be used for 25Gb/s and 40Gb/s applications with excellent performance. By an additional monolithic integration of an impedance matching circuit the module fabrication costs can be reduced but also the modulation bandwidth of the devices can be further enhanced. Up to 70Gb/s modulation with excellent eye openings can be achieved. This novel approach opens the possibility for 100Gb/s NRZ EMLs and thus 4x100Gb/s NRZ EML-based transmitters in future. Also even higher bitrates seem feasible using more complex modulation formats such as e.g. DMT and PAM.

The behaviour of a dual state quantum dot laser undergoing optical injection is analysed. Depending on the injection current the free-running operation of the slave can be ground state only emission, both ground state and excited state emission or excited state emission only. Complete suppression of the excited state in the dual state lasing regime via sufficiently strong optical injection of the ground state is presented. Injection induced switching between two longitudinal modes of the ground state is also presented in the ground state only regime.

We present an external cavity diode laser which is frequency coupled to a stabilized He-Ne Laser with optical phase locked loop technology. For long term frequency stability the diode laser is locked with a fixed frequency offset to a stabilized He-Ne Laser at 633nm. A commercial PLL chip and the piezo-drive of the diode laser are used for precise frequency control. With this simple technique we develop a diode laser source which reach the frequency stability of a stabilized He-Ne Laser and still provide an optical output power of 5 mW or more. This high optical output power enables applications like multi axis homodyne interferometry with just one laser source.

A technique has been developed for the measurement of pulse trains demonstrating a dynamical behaviour (i.e. not ideally periodic). Existing techniques in this area (e.g. FROG, SPIDER or other heterodyne methods) require very stable pulse trains, or large averaging times, and so are limited when applied to even slowly varying pulse trains. The technique presented involves mixing the comb under test (CUT) with a reference optical frequency comb (OFC) which has a known spectral intensity profile. Mixing these signals on a photodiode results in a series of radio frequency (RF) beat tones. The phase properties of these beat tones can be used to measure the spectral phase between adjacent modes in the CUT, allowing the full complex spectrum of the CUT to be measured simultaneously with one single real time oscilloscope acquisition. With the spectral properties of the comb known, the pulse train can be reconstructed in the temporal domain. By applying this technique to very small sections of the beating signal ( tens of nanoseconds), a time resolved picture of the pulse train behaviour can be obtained. Dynamic signals generated in a LiNbO3 modulator driven by a modulated RF signal have been measured. This technique is well suited to studying the combs produced by mode-locked semiconductor lasers. Quantum dot mode-locked laser combs can be characterised, and pulse train instabilities measured.

Quantum dot lasers have been shown to have greatly enhanced stability in the feedback configuration thanks to a high damping of the relaxation oscillations and they display different dynamics to those of conventional semiconductor lasers. For high feedback levels in conventional devices one obtains Low Frequency Fluctuations: sharp dropouts in intensity and subsequent gradual build-ups. Standard low frequency fluctuation-like traces are conspicuous by their absence in studies of feedback with quantum dot devices. We experimentally examine single mode quantum dot lasers at high feedback levels with a long delay and observe regular pulse-trains with a period equaling the external cavity round-trip time where each pulse features a distinctive broad trailing edge plateau. The distinctive pulse shape is very similar to the recently published strong pulse-asymmetry in two-section, passively mode-locked quantum dot lasers where this asymmetry was shown to result from the creation of different modal groups. We attribute the pulses in our experiment to the same phenomenon: each pulse corresponds to a simultaneous excitation of a number of the external cavity modes. We consider a model tailored specifically for quantum dot lasers with strong optical feedback and find it reproduces the experimentally observed trains extremely well.

The electro-optical efficiency of semiconductor vertical-cavity surface-emitting lasers (VCSELs) strongly depends on the efficient carrier injection into the quantum wells (QWs) in the laser active region. However, carrier injection degrades with increasing temperature which limits the VCSEL performance particularly in high power applications where self heating imposes high temperatures in operation. By simulation we investigate the transport of charge carriers in 808 nm AlGaAs multi-quantum-well active layers with special attention to the temperature dependence of carrier injection into the QWs. Experimental reference data was extracted from oxide-confined, top-emitting VCSELs. The transport simulations follow a drift-diffusion-model complemented by a customized, energy-resolved, semi-classical carrier capture theory. QW gain was calculated in the screened Hartree-Fock approximation with band structures from 8x8 k.p-theory. Using the gain data and by setting losses and the optical confinement factor according to experimental reference results, the appropriate threshold condition and threshold carrier densities in the QWs for a VCSEL are established in simulation for all transport considerations. With the combination of gain and transport model, we can explain experimental reference data for the injection efficiency and threshold current density. Our simulations show that the decreasing injection efficiency with temperature is not solely due to increased thermionic escape of carriers from the QWs. Carrier injection is also hampered by state filling in the QWs initiated from higher threshold carrier densities with temperature. Consequently, VCSEL properties not directly related to the active layer design like optical out-coupling or internal losses link the temperature dependent carrier injection to VCSEL mirror design.

In this paper we present results of computer optical simulations of VCSEL with modified high refractive index contrast grating (HCG) as a top mirror. We consider the HCG of two different designs which determine the lateral aperture. Such HCG mirror provides selective guiding effect. We show that proper design of aperture of HCG results in almost sixfold increase in cavity Q-factor for zero order mode and a discrimination of higher order modes.

Development of a reliable, selective, sensitive, technique for atmospheric trace gas concentrations monitoring is a critical challenge in science and engineering. Tunable single-frequency laser in the 2.3 to 3.3µm wavelength range, working in a continuous regime at room temperature can be used for absorption spectroscopy to identify and quantify several gases such as methane (greenhouse gases) and ethylene (food-processing) which are studied in the IES. We report here on the design and fabrication of 1st to 4th order distributed-feedback (DFB) antimonide-lasers diodes in the 2.3 to 3.3µm wavelength range. This process is applied to all studied structures grown by molecular beam epitaxy (MBE) on GaSb substrate. Electromagnetic modeling helps us to determine the Bragg grating period as well as the global geometry of the structure in order to optimize both modal discrimination and optical power of the lasing mode. The grating is defined by holographic lithography. Two DFB laser diode designs are proposed and investigated in parallel: -Side wall corrugation DFB: A corrugation on the lateral sides of the ridge waveguide is transferred by both wet and dry on a hard mask followed by a Cl2/N2 dry etching in the III-V heterostructure. -Buried DFB: The MBE growth is stopped at the top of the active region. Then the Bragg grating is etched by Ar sputtering . A MBE regrowth process is performed allowing the growth of the upper cladding layer. Next chemical etching of the mesa is done with fluoro-chromic acid. Si3N4 isolation and evaporation of ohmic contacts ends those processes. Finally we will show the results on the fabrication and characterization of the devices. This work is supported by the ANR NexCILAS international project, ANR MIDAS project, NUMEV labex and RENATECH national Network.

100-Gb/s coherent systems based on polarization-division multiplexed quadrature phase shift keying (PDMQPSK), with aggregate wavelength-division multiplexed (WDM) capacities approaching 10 Tb/s per fibre, are being widely deployed due to the benefits provided by coherent detection. The stringent linewidth requirements for lasers used in these systems only allow optical sources with certain phase noise characteristics to be employed. A major challenge is therefore to produce lasers with the requisite performance at low cost. Discrete mode laser diodes (DMLDs) can be designed for narrow linewidth emission and present an economic approach with a focus on high volume manufacturability of monolithic semiconductor lasers. In this paper the performance of a 112 Gb/s long-haul optical transmission system employing PDM-QPSK is investigated using a range of transmitter lasers with linewidth values ranging from 100 kHz to 5 MHz. A linewidth of 100 kHz was obtained from an external cavity laser (ECL) and linewidths ranging from 200 kHz to 5 MHz were obtained from DMLDs. The performance of the system is analysed through experimental measurements and simulations performed by Virtual Photonics Incorporated Transmission Maker (VPI™). Results are presented for back-to-back operation and after transmission through G.654 pure silica core fibre (PSCF) at distances up to 6930 km.

The aim of this paper is to address some of the aspects of thermal management of QCLs. Results include electrical and spectral characterization of the devices. Results show shift of QCL emission mode towards lower wavenumbers during the pulse. Characteristics were registered at different temperatures of operation and driving conditions. Registered shift rates depend on operating temperature, being the highest at room temperature. Based on spectral tuning results, temperature increase rates for different modes of operations were evaluated, delivering information on thermal dynamics of investigated devices.

The effect of the length of wide tunable AMQW laser on its output power is demonstrated in this paper. The InP based AMQW laser is custom designed and fabricated to have a large tuning range. The experimental data showed that the output power profile is not following the traditional behavior of laser diodes. The data showed that the laser length can be categorized in three categories, below transitional cavity length (BTCL), above the transitional cavity length (ATCL), and at the transitional cavity length (TCL). The BTCL and ATCL lengths are not suitable for wide tunable applications however the TCL length is suitable for wide tunable applications and the power profile is closer to the traditional power profile of laser diodes. This type of AMQW laser diodes can be used in many applications.

The focusing of multimode laser diode beams is probably the most significant problem that hinders the expansion of the high-power semiconductor lasers in many spatially-demanding applications. Generally, the ‘quality’ of laser beams is characterized by so-called ‘beam propagation parameter’ M², which is defined as the ratio of the divergence of the laser beam to that of a diffraction-limited counterpart. Therefore, M² determines the ratio of the beam focal-spot size to that of the ‘ideal’ Gaussian beam focused by the same optical system. Typically, M² takes the value of 20-50 for high-power broad-stripe laser diodes thus making the focal-spot 1-2 orders of magnitude larger than the diffraction limit. The idea of ‘superfocusing’ for high-M² beams relies on a technique developed for the generation of Bessel beams from laser diodes using a cone-shaped lens (axicon). With traditional focusing of multimode radiation, different curvatures of the wavefronts of the various constituent modes lead to a shift of their focal points along the optical axis that in turn implies larger focal-spot sizes with correspondingly increased values of M². In contrast, the generation of a Bessel-type beam with an axicon relies on ‘self-interference’ of each mode thus eliminating the underlying reason for an increase in the focal-spot size. For an experimental demonstration of the proposed technique, we used a fiber-coupled laser diode with M² below 20 and an emission wavelength in ~1μm range. Utilization of the axicons with apex angle of 140deg, made by direct laser writing on a fiber tip, enabled the demonstration of an order of magnitude decrease of the focal-spot size compared to that achievable using an ‘ideal’ lens of unity numerical aperture.

This research investigated the critical slowing down in polarization switching (PS) of vertical-cavity surface-emitting lasers (VCSELs). The experiments were performed by step-function current injection in two types: step-up and stepdown. In the case of step-up and step-down, the relationship between relaxation time and final current in this experiment resembles critical slowing down (CSD). The critical currents of two step-function current experiment are compared. The PS in this experiment is a static case. We also find that the divergence of relaxation time follow a power law. These results contribute to the understanding of the mechanism of CSD in VCSEL's PS (VPS).

The enhancement of the low frequency gain response of a microwave phase shifter based on slow light in a bulk reflective semiconductor optical amplifier (RSOA), by using forced coherent population oscillations (FCPO), is experimentally demonstrated. FCPO is achieved by simultaneously modulating the input optical power and bias current. The beat signal gain improvement ranges from 45 to 0 dB over a frequency range of 0.5 to 2.5 GHz, thereby improving the noise performance of the phase shifter. Tunable phase shifts of up to 40º are possible over this frequency range.

We report on measurements of the working parameters of a 1550-nm single-transverse mode vertical-cavity surface-emitting laser (VCSEL), including those that describe the polarization behavior of the device. Simple expressions for the laser linewidth and laser power as a function of the bias current are used in a first stage. High resolution CW optical spectrum and intensity noise spectrum measurements are performed to apply this technique. Current-induced polarization switching (PS) is observed in our device when increasing the bias current, from the higher to the lower frequency polarization mode. A minimum in the absolute value of the effective dichroism is observed at the PS point. The value of the effective birefringence has a discontinuity at the current at which PS is observed. PS characteristics are used for the extraction of the parameters describing the polarization behavior of the VCSEL.

Reflective semiconductor optical amplifiers (RSOAs) have shown promise for applications in WDM optical networks and in fiber ring mode-locked lasers. Polarization insensitive SOAs can be fabricated using tensile-strained bulk material and a rectangular cross section waveguide. The introduction of tensile strain can be used to compensate for the different confinement factors experienced by the waveguide TE and TM modes. There is a need for models that can be used to predict RSOA static characteristics such as the dependency of the signal gain on bias current and input optical power, the amplified spontaneous emission spectrum and noise figure. In this paper we extend our prior work on non-reflective SOAs to develop a static model that includes facet reflections. The model uses a detailed band structure description, which is used to determine the wavelength and carrier density dependency of the material gain and additive spontaneous emission. The model and includes a full geometrical description of the amplifier waveguide, including the input taper and the position dependency of the TE/TM confinement factors. The amplified signal and spontaneous emission are described by detailed travelling-wave equations and numerically solved in conjunction with a carrier density rate equation. The model uses material and geometric parameters for a commercially available RSOA. The versatility of the model is shown by several simulations that are used to predict the SOA operational characteristics as well as internal variables such as the amplified spontaneous emission and signal and the carrier density.

Polarization switching (PS) between linear polarizations of a vertical-cavity surface-emitting laser (VCSEL) can appear when this device is subject to orthogonal optical injection. In this type of injection the injected field has a linear polarization orthogonal to that of the free-running VCSEL. In this situation interesting nonlinear dynamics appear, one of which is the existence of an injection-locked solution for which the two linear polarized modes of the VCSEL lock to the master laser frequency. This situation has been theoretically predicted and corresponds to an elliptically polarized injection-locked (EPIL) state. In this paper we report an experimental investigation of the dynamics of a long-wavelength single-transverse mode VCSEL subject to orthogonal optical injection. The free-running VCSEL emits a linearly polarized beam in the so called “parallel” direction. The polarization of the injected light is perpendicular to this state and is termed “orthogonal” polarization. We observe the EPIL state when the frequency of the orthogonal injected light is near the frequency of the parallel polarization. The spectral feature of the EPIL state is verified and the power of each polarization is measured. The EPIL region is measured in the frequency detuning-injected power plane. As current decreases, the frequency detuning range for the EPIL to exist is narrower and shifts toward the negative frequency detuning. Periodic dynamics in which both polarizations oscillate with a frequency very close to the relaxation oscillation frequency is found above the upper boundary of the EPIL region. Below the lower boundary of the EPIL zone, periodic dynamics is found only in the parallel polarization.

A time-domain solver for the response of a Semiconductor Optical Amplifier (SOA) relying on multigrid numerical techniques and a wideband steady state material gain coefficient is presented for the first time. Multigrid techniques enable the efficient solution of implicit time discretization schemes for the associated system of coupled differential equations, namely the carrier rate equation in the time domain and the signal amplification in the spatial domain, which in turn enable accuracy- instead of stability-restricted time-discretization of the signals. This allows lifting off the limitations of an equidistant spatio-temporal grid for the representation of the incoming signals adopted by traditional explicit SOA models, releasing an adaptive stepsize controlled solver for the dynamic SOA response with dense timesampling under a rapidly varying SOA signal output and scarce time-sampling when negligible changes are observed.

Simulating light propagation in anisotropic dynamic gain media such as semiconductors and solid-state lasers using the finite difference time-domain FDTD technique is a tedious process, as many variables need to be evaluated in the same instant of time. The algorithm has to take care of the laser dynamic gain, rate equations, anisotropy and dispersion. In this paper, to the best of our knowledge, we present the first algorithm that solves this problem. The algorithm is based on separating calculations into independent layers and hence solving each problem in a layer of calculations. The anisotropic gain medium is presented and tested using a one-dimensional set-up. The algorithm is then used for the analysis of a two-dimensional problem.

Semiconductor ring lasers are semiconductor lasers where the laser cavity consists of a ring-shaped waveguide. SRLs are highly integrable and scalable, making them ideal candidates for key components in photonic integrated circuits. SRLs can generate light in two counterpropagating directions between which bistability has been demonstrated. Hence, information can be coded into the emission direction. This bistable operation allows SRLs to be used in systems for all-optical switching and as all-optical memories. For the demonstration of fast optical flip-flop operation, Hill et al. [Nature 432, 206 (2004)] fabricated two SRLs coupled by a single waveguide, rather than a solitary SRL. Nevertheless, the literature shows that a single SRL can also function perfectly as an all-optical memory. In our recent paper [W. Coomans et al., Phys. Rev. A 88, 033813, (2013)], we have raised the question whether coupling two SRLs to realize a single optical memory has any advantage over using a solitary SRL, taking into account the obvious disadvantage of a doubled footprint and power consumption. To provide the answer, we have presented in that paper a numerical study of the dynamical behavior of semiconductor ring lasers coupled by a single bus waveguide, both when weakly coupled and when strongly coupled. We have provided a detailed analysis of the multistable landscape in the coupled system, analyzed the stability of all solutions and related the internal dynamics in the individual lasers to the field effectively measured at the output of the waveguide. We have shown which coupling phases generally promote instabilities and therefore need to be avoided in the design. Regarding all-optical memory operation, we have demonstrated that there is no real advantage for bistable memory operation compared to using a solitary SRL. An increased power suppression ratio has been found to be mainly due to the destructive interference of the SRL fields at the low power port. Also, multistability between several modal configurations has been shown to remain unavoidable.

All-optical spiking neural networks would allow high speed parallelized processing of time-encoded information, using the same energy efficient computational principles as our brain. As the neurons in these networks need to be able to process pulse trains, they should be excitable. Using simulations, we demonstrate Class 1 excitability in optically injected microdisk lasers, and propose a cascadable optical spiking neuron design. The neuron has a clear threshold and an integrating behavior. In addition, we show that the optical phase of the input pulses can be used to create inhibitory, as well as excitatory perturbations. Furthermore, we incorporate our optical neuron design in a topology that allows a disk to react on excitations from other disks. Phase tuning of the intermediate connections allows to control the disk response. Additionally, we investigate the sensitivity of the disk circuit to deviations in driving current and locking signal wavelength detuning. Using state-of-the-art fabrication techniques for microdisk laser, the standard deviation of the lasing wavelength is still about one order of magnitude too large. Finally, as the dynamical behavior of the microdisks is identical to the behavior in Semiconductor Ring Lasers (SRL), we compare the excitability mechanism due to optically injection with the previously proposed excitability due to asymmetry in the intermodal coupling in SRLs, as the latter mechanism can also be induced in disks due to, e.g., asymmetry in the external reaction. In both cases, the symmetry between the two counter-propagating modes of the cavity needs to be broken to prevent switching to the other mode, and allow the system to relax to its initial state after a perturbation. However, the asymmetry due to optical injection results in an integrating spiking neuron, whereas the asymmetry in the intermodal coupling is known to result in a resonating spiking neuron.

Semiconductor lasers subject to external feedback are known to exhibit a wide variety of dynamical regimes desired for some applications such as chaos cryptography, random bit generation, and reservoir computing. Low-frequency fluctuations is one of the most frequently encountered regimes. It is characterized by a fast drop in laser intensity followed by a gradual recovery. The duration of this recovery process is irregular and of the order of hundred nanoseconds. The average time between dropouts is much larger than the laser system characteristic time-scales. Semiconductor ring lasers are currently the focus of a rapidly thriving research activity due to their unique feature of directional bistability. They can be employed in systems for all-optical switching, gating, wavelength-conversion functions, and all-optical memories. Semiconductor ring lasers do not require cleaved facets or gratings for optical feedback and are thus particularly suited for monolithic integration. We experimentally and numerically address the issue of low-frequency fluctuations considering a semiconductor ring laser in a feedback configuration where only one directional mode is re-injected into the same directional mode, a so-called single self-feedback. We have observed that the system is very sensitive to the feedback strength and the injection current. In particular, the power dropouts are more regular when the pump current is increased and become less frequent when the feedback strength is increased. In addition, we find two different recovery processes after the power dropouts of the low-frequency fluctuations. The recovery can either occur via pulses or in a stepwise manner. Since low-frequency fluctuations are not specific to semiconductor ring lasers, we expect these recovery processes to appear also in VCSELs and edge-emitting lasers under similar feedback conditions. The numerical simulations also capture these different behaviors, where the representation in the phase space of the carriers versus the round trip phase difference gives additional insight into these phenomena. This proceedings paper gives a short overview of the results of L. Mashal et al. [L. Mashal et al., IEEE J. Quantum. Electron. 49, 790, 2013].

The phase transition in the polarization switching (PS) of vertical-cavity surface-emitting lasers (VCSELs) was recently reported to be a second-order phase transition (SOPT). However, some features of this phase transition indicate that the VCSEL’s PS (VPS) is different from the traditional SOPTs. Most of the phase transition investigations of the laser employ the laser’s intensity as the order parameter. In Landau’s paradigm, that parameter evolutes from zero to non-zero values, or vice versa, during SOPTs, corresponding to a transition between a disordered phase and an ordered phase. Nevertheless, in the VPS, the laser’s intensity remains constant before and after the PS, revealing an order-to-order transition. Furthermore, the laser’s transverse modes cannot transfer to each other through continuous deformations in geometry. That feature attributes a topological characteristic to the laser’s transverse modes. The spatial coherence of the laser also implements a globally geometric characteristic to the laser’s output. Accordingly, there are two similarities between the VPS and quantum phase transitions (QPTs) with topological order. First, both of them belong to the orderto- order phase transitions. Second, in both transitions, two ground states are orthogonal, and are degenerate at the critical point. This paper investigated the analogy between the QPT with topological order and the VPS, exploring that the VPS has a potential to simulate the QPTs of other physical systems.

An Ising simulation is used to interpret the phase transition in the polarization switching (PS) of vertical-cavity surfaceemitting lasers (VCESLs) in this paper. From a point of view of spatial coherence, a simulation with Gaussiandistribution interaction shows a weak first-order phase transition for disorder-to-order transition due to the inhomogeneous interaction in space. For the order-to-order transition in the VCESL’s polarization switching (VPS), the Ising simulation with an external field could give an appropriate description to understand the interaction in VPS and suggest VPS is a first order phase transition (FOPT). Furthermore, via comparing with the Ising model with an external field, the interaction in VPS system should be strong enough to make whole system be in a spontaneous order state. Moreover, there is an injected signal related to injected current in VPS system and playing a role as external field in Ising model. This injected signal could cause the two degenerate states separate into a metastable state and a stable state. The last results which is modulating iteration times of Ising simulation with an external field indicates that the variation of the PS currents regarding the modulation frequency is a dynamical result. This investigation would give numerous contributions for understanding the phase transition and the interaction in VPS’s system.

With this paper we investigate the system-level performance of VCSELs, parameterized with true experimental LI-VI data and dynamic characteristics of state-of-the-art VCSELs with 3 dB modulation bandwidth at 15 GHz, and propose their deployment as high-speed multi-level optical sources in a mid-range active optical cable (AOC) model for performance prediction of a rack-to-rack interconnection. The AOC architecture combines a 6-element 1550 nm VCSEL array, each directly modulated with 40 Gbaud PAM-4 data, with a wavelength division multiplexer (WDM), in order to implement a parallel link with aggregate traffic of 0.48 Tb/s. Transmission reach exceeded 300 m by deploying a two-tap feed forward equalizer filter at the electrical VCSEL driver. Bit Error Rate (BER) measurements and analysis were carried out in MATLAB. In practice, the thermal behavior and basic operational characteristics of the VCSELs fabricated by the Technische Universität München (TUM) were used to study the thermal performance and operational range of the complete AOC model. The VCSELs were initially operated at 20°C and BER measurements showed power penalties of 1.7 dB and 3.5 dB at 300 m and 500 m of transmission distance respectively for all 6 data channels. System performance was also investigated for elevated operating temperatures of the VCSEL module and the additional system degradation and BER penalties introduced by operation at 50°C and 65°C were also investigated for transmission distances of 300 m and 500 m.

In this study, we report on a methodology based on reverse and forward current-voltage curves (I-V) and on Degree of Polarization (DoP) of electroluminescence measurements on 980 nm laser diodes chip-on-submount (CoS) for the improvement of screening tests. Current-voltage curves are performed at reverse bias up to breakdown voltage (V_BR) using both a high current accuracy (< 1 pA) and high voltage resolution (< 10 mV) at different submount-temperatures (20-50°C). The DoP of luminescence of such devices, related to strains in materials and effect of shear strain on the birefringence, is calculated from the simultaneous measurement of TE (L_TE) and TM (L_TM) polarized light emissions. We observe that application of high reverse voltages occasionally produces significant micro-plasma (MP) pre-breakdown on reverse I-V characteristics as recently observed in InGaN/GaN LEDs and assumed to be a response of electrically active defects. Comparisons between breakdown voltages and number of MP, and changes of leakage current at low forward voltage (< 0.1 V) are considered. DoP measurements are also analyzed versus temperature. Finally the usefulness of these measurements for effective screening of devices is discussed.

We investigate different scenarios leading to simultaneous time-delay concealment both in the intensity and the phase dynamics generated from semiconductor ring lasers (SRLs) subject to delayed feedback. Under appropriate conditions, we found that the delay signature can be eliminated both in the intensity and the phase dynamics of SRLs with cross-feedback even when subject to long delayed feedback. For SRLs with self-feedback configuration, we also found that the concealment of short delay time is possible. The fact that such delay signatures can be eliminated in SRLs subject to short feedback opens the possibility of implementing secure communication schemes and random number generators on chip.

We report experimental bifurcation diagrams (BDs) of an external-cavity semiconductor laser (ECSL). We have focused on the case of the ECSL biased just above threshold to moderate and subjected to feedback from a distant reflector and observed a sequence of bifurcations involving bifurcation cascade as well as intermittency between multiple coexisting attractors. More importantly, we reiterate: the results map out, for the first time to our knowledge, detailed BDs of the ECSL as a function of feedback strength for various external cavity lengths and currents, thus covering a significant portion of parameter space. We have grounded our discussion in extensive theoretical studies based on the Lang-Kobayashi equations and simulated BDs in accordance with our experimental results.

During the last five years, optical chaos-based random bit generators (RBGs) attracted a lot of attention and demonstrated impressive performances with bit rates up to hundreds of Gbps. However all the suggested schemes use external injection schemes (optical injection or feedback) to turn the lasers into chaos, hence strongly increasing setup complexity. On the other hand, we reported that a laser diode can generate a chaotic output without the need for external perturbation or forcing, hence unveiling a highly simplified way to generate an optical chaos at high frequency. However the low dimension and limited number of positive Lyapunov exponent casted doubts about its direct use for chaos-based applications. Here we make a proof-of-concept demonstration for a Random Bit Generator based on polarization chaos. We therefore suggest a highly simplified RBG scheme using only a free-running laser and small-bandwidth acquisition electronics and demonstrate convincing performances with bit rates up to 100 Gbps without unusual or complex post-processing methods. We link these performances to the double-scroll structure of the chaotic attractor rather than the bandwidth of the dynamics, hence bringing new light on the importance of chaos topology for chaos-based applications. In addition our scheme exhibit a strong potential as it enables a low-cost and/or integrated in parallel on-chip scheme.

Random bit generation (RBG) with chaotic semiconductor lasers has been extensively studied because of its potential applications in secure communications and high-speed numerical simulations. Researchers in this field have mainly focused on the improvement of the generation rate and the compactness of the random bit generators. In this paper, we experimentally demonstrate the existence of two regimes of fast RBG using a single chaotic laser subjected to delayed optical feedback: the first one is based on the extraction of all min-entropy contained in each random sample, and the second one is to demonstrate a possibility of increasing the generation rate by extracting 55 bits from each variable.

In this work, we study theoretically the dynamical behavior of two semiconductor ring lasers (SRLs). One is subject to negative optoelectronic feedback and the other laser is subject to incoherent optical feedback. Relying on asymptotic methods, we are able to reduce the original set of five equations used to describe the dynamical behavior of SRLs with negative optoelectronic feedback (SRL-NOEF) or incoherent optical feedback (SRL-IOF) to two equations and one map with time delay valid on time-scales longer than the relaxation oscillations (ROs). The equations of the reduced models turn out to be the same for both systems. As we vary the feedback strength, the devices under consideration in this work display both continuous wave operation and a period-doubling route to chaos. The two counter-propagating intensities of both systems exhibit in-phase chaotic behavior for small delay times comparable to the period of relaxation oscillations. For delay times significantly longer than the period of ROs, the two counter-propagating modes show in anti-phase chaotic oscillations. Moreover, for long delay times, we find that the counter-propagating intensities of both systems depict the same dynamical behaviors when their feedback strengths are increased.

Fully integrated semiconductor master-oscillator power-amplifiers (MOPA) with a tapered power amplifier are attractive sources for applications requiring high brightness. The geometrical design of the tapered amplifier is crucial to achieve the required power and beam quality. In this work we investigate by numerical simulation the role of the geometrical design in the beam quality and in the maximum achievable power. The simulations were performed with a Quasi-3D model which solves the complete steady-state semiconductor and thermal equations combined with a beam propagation method. The results indicate that large devices with wide taper angles produce higher power with better beam quality than smaller area designs, but at expenses of a higher injection current and lower conversion efficiency.

A self-consistent model of a GaAs-based 850 nm coupled-cavity vertical-cavity surface-emitting diode laser is presented. The analyzed laser consists of two identical AlGaAs cavities with GaAs quantum wells, separated with 10 pairs of middle DBR. The current apertures are realized by ion-implantation for the top cavity and selective oxidation for the bottom. To accurately simulate the physical phenomena present in the CW regime of the analyzed device, we use a multi-physical model, which comprises self-consistent Finite Element Method (FEM) thermo-electrical model. The numerical parameters have been found by the calibration based on experimental results. We have analyzed and shown the influence of the driving voltages on the temperature distribution within the analyzed structure and current densities in both cavities.

GaSb-based semiconductor diode lasers are promising candidates for light sources working in the mid-infrared wavelength region of 2-5 μm. Using edge emitting lasers with ridge-waveguide structure, light emission with good beam quality can be achieved. Fabrication of the ridge waveguide requires precise etch stop control for optimal laser performance. Simulation results are presented that show the effect of increased confinement in the waveguide when the etch depth is well-defined. In situ reflectance monitoring with a 675 nm-wavelength laser was used to determine the etch stop with high accuracy. Based on the simulations of laser reflectance from a proposed sample, the etching process can be controlled to provide an endpoint depth precision within ± 10 nm.

We are implementing an electro–thermal simulation tool to optimize the characteristics of GaAs-based vertical- cavity surface-emitting lasers (VCSELs). For this purpose it turned out to be necessary to revisit basic material parameters. In this paper we elaborate on the composition, carrier density, and temperature dependencies of the electron mobility of Al_xGa_1−xAs semiconductors. We present the principles of the pragmatic quasi-three- dimensional (q3D) device model and show selected results.

In order to stabilize the signal delivered by an optoelectronic oscillator (OEO) [1-5], it is necessary to lock the signal of the laser on the resonance. The laser wavelength must be stabilized onto one of the resonator’s resonances to be able to maintain a stable performance of the oscillator. We first present the Pound Drever hall method that has been used to realize this setup. As an alternative method, we have also investigate another technique based on the use of acousto-optic cells (AOC). It is presented on part 3 of this paper.

Semiconductor passively mode-locked lasers are of broad interest due to their potential applications as sources of ultra-short, high frequency light pulses. In spite of the complex dynamics of such devices, a relatively simple delay differential equation model can reproduce the manifold modes of operation experimentally observed. Using such a model we investigate the modes of operation of passively mode-locked lasers. We calculate key model parameters from experimentally measured quantities and thus are able to reproduce experimentally observed features, such as the onset of fundamental mode-locking, pulse width and repetition rate. Despite the simplicity of the gain model used within our approach, nano-structured lasers, such as quantum-dot lasers, can be effectively described. This enables us to make predictions about device behavior in dependence of operational parameters and allows for device optimization.