This PDF file contains the front matter associated with SPIE Proceedings Volume 11356 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

High speed InP lasers have been developed for 400Gbit/s interconnections. Electro-absorption modulator integrated DFB lasers were demonstrated for 106-Gbit/s PAM4 with 0.9 Vpp from 20 to 85°C. 106-Gbit/s PAM4 eye-openings were obtained using directly modulated DFB lasers from 25 to 80°C.

In this paper, we report the integration of sampled grating distributed Bragg reflector (SG-DBR) lasers using an InPbased generic integration platform for the first time. These lasers show 53 nm tuning range in C band from 1533 nm to 1586 nm with 40 dB side mode suppression (SMSR). Threshold current of the laser is 20 mA, and the front facet output power is 8 mW at 100 mA gain current. The Lorentzian optical linewidth amounts to 713 kHz. We fabricated this laser using the standard Multi-Project Wafer processes of the Fraunhofer HHI InP integration platform.

In this conference proceeding, we present a novel technique to control the emission of dual-wavelength lasers. Using a well designed external cavity, we demonstrate that tuning the optical feedback phase allows to efficiently tailor the output power balance between the two wavelengths emitted by the laser. With this technique, a complete switch between one and the other mode can also be achieved and we report a suppression ratio up to 40 dB. Due to its simplicity, the structure can be monolithically integrated easily with the laser itself and offers a precise control of the dual-wavelength emission using a single control parameter.

We show that using dense arrays of InGaAs quantum well-dots enables uncooled high-frequency applications with a GHz-range bandwidth. A maximum 3-dB modulation frequency of about 6 GHz was found. The K-limited maximal frequency of 13 GHz was estimated from the modulation response analysis. The experimental values of the energy-todata reaches 1.5 pJ/bit for the smallest diameter under study (10 μm). A 23 μm in diameter microlaser exhibits open eye diagram up to 12.5 Gbit/s and is capable of error-free 10 Gbit/s data transmission at 30°C without temperature stabilization. Our results demonstrate the potential to achieve miniature high-speed on-chip light sources for optical communication applications using lasers with a diameter of only a few micrometers.

External cavity lasers show a variety of uses, for which quantum well semiconductor lasers are already com- mercially used. Due to the atom-like discrete energy levels, quantum dots exhibit various properties resulting from the three-dimensional confinement of carriers, like high stability against temperature variation, large gain bandwidth, and low-threshold lasing operation. Quantum dots seem to be ideal to address the challenges in the further development of various semiconductor applications, such as high-resolution spectroscopy or broad- band optical communication networks, for which a range of spectral and temporal characteristics is required, for instance a narrow spectral linewidth, low intensity noise or wide wavelength tunability. In this view, exter- nal cavity quantum dot gain chips can be envisoned to replace the current quantum well technology. Using a semi-analytical rate equation model, we successfully analyze both dynamical and noise properties of an external cavity laser made with quantum dot gain medium, operating under strong optical feedback. This paper inves- tigates the turn-on delay, the relative intensity noise, and the frequency noise and compares them to the case without optical feedback. These numerical investigations of an external cavity quantum dot gain chip provide meaningful building blocks for future fabrication research or for developing high performance device such as wavelength-selective components.

We report on broad-area lasers, mode-locked lasers (MLLs), and superluminescent light-emitting diodes (SLDs) based on a recently developed novel type of nanostructures that we refer to as quantum well-dots (QWDs). The QWDs are intermediate in properties between quantum wells and quantum dots and combine some useful properties of both. 1.08 μm InGaAs/GaAs QWDs broad area edge-emitting lasers based on coupled large optical cavity waveguides show high internal quantum efficiency of 92%, low internal loss of 0.9 cm^-1 and material gain of ~1.1∙10⁴ cm^-1 per one QWD layer. CW output power of 14.2 W is demonstrated at room temperature. Superluminescent light-emitting diodes with one QWD layer in the active region exhibit stimulated emission spectra centered at 1050 nm with the maximal full width at half maximum of 36 nm and the output power of 17 mW. First results on mode-locked operation in QWD lasers are also presented. 2 mm long two-section devices demonstrate the pulse repetition rate of 19.3 GHz and the pulse duration of 3.5 ps. The width of the radio frequency spectrum is 0.2 MHz.

In the present talk we discuss the application of Template-Assisted Selective Epitaxy (TASE) for the monolithic integration of III-V active photonic devices on silicon. The main concept of TASE relies on the guided growth of III-Vs within a confined oxide template. At one extremity of the template there is access to silicon to start the nucleation, and subsequently it is the template which guides the growth progression. This decoupling of the resulting geometry from the growth mode and substrate orientation, results in a larger processing window as we no longer rely on the growth conditions to tune the geometry, as well as a number of other advantages. A further unique advantage of TASE for silicon photonics applications is that it allows for the truly local integration of III-V material at precisely defined positions, since the location of the III-V may be defined with nm-scale precision in the same lithographic step as silicon passives. TASE was originally developed for electronics, but in recent years we have expanded it to enable several photonic devices. In the present talk, I will discuss our work on GaAs and InP microdisk lasers fabricated by either direct growth or via the use of micro-substrates. These devices show lasing at room temperature around 870 nm with thresholds of about 10 pJ/pulse. We also explore the use of metal-clad cavities for further light confinement.

This work reports on the optical feedback dynamics of InAs/GaAs QD lasers epitaxially grown on silicon operating in both the short and long delay regimes. Both undoped and p-doped QD lasers are considered. Whatever the external cavity length, no chaotic oscillations are observed on both samples as a result of the small α-factor observed in the silicon QD lasers. Despite that, experiments conducted in the short-cavity region raise period-one oscillation for the undoped QD laser. In addition, the transition from the short to long delay regimes can be finely covered by varying the external cavity length from 5 cm to 50 cm, and the boundaries associated to the appearance of the periodic oscillation are identified. In the short-cavity region, boundaries show some residual undulations resulting from interferences between internal and external cavity modes; whereas in the long-delay regime, the feedback ratio delimiting the boundaries keeps decreasing, until it progressively becomes rather in- dependent of the external cavity length. Overall, our results showed that the p-doped device clearly exhibits a much higher tolerance to the different external feedback conditions than the undoped one, seeing that its periodic oscillation boundaries are barely impossible to retrieve at the maximum feedback strength of -7 dB. These results show for the first time the p-modulation doping effect on the enhancement of feedback insensitivity in both short- and long-delay configurations, which is of paramount importance for the development of ultra-stable silicon transmitters for photonic technologies.

The ultra-narrow linewidth diode lasers self-injection locked to high-Q crystalline microresonators are available commercially, providing the linewidth below 1 kHz for various wavelengths and enabling microcomb generation. Here we demonstrate a technique that allows applying this approach for photonic integrated chips containing microresonators and significantly narrow the laser diode (Fabry-Perot or DFB) linewidth due to self-injection locking to the silicon nitride (SiN) microresonator with high Q-factor. Considered laser diodes are CMOS-compatible, as well as integrated microresonators made of silicon nitride. This makes it possible to realize in the future large-scale production of laser devices based on microresonators. We stabilized the InGaAsP/InP Fabry-Perot (FP) laser diode (Seminex, 100 mW, 1535 nm, 20 nm spectral width) and the DFB laser (Nolatech Company, 1550 nm, output power up to 20 mW) by different silicon nitride microresonators with Q-factor higher than one million (LIGENTECH Company). Microresonators with different free spectral ranges (1 THz, 150 GHz, 35 GHz) allowed observing the different regime of operation, single frequency, and multi-frequency, when different laser diode lines are suppressed. The spectral linewidth of each locked line was better than 20 kHz (limited by Ref. laser). The developed technique allowed us to integrate different types of laser diodes with high-Q SiN microresonators and developed a fully integrated optical frequency comb source. We measured spectral characteristics (spectral linewidth, phase noises) of free running and locked states, the stabilization coefficient, and the locking range and compare these values to the theoretical estimations. We discuss requirements for the optical frequency comb generation in such systems and demonstrate measured spectra of optical combs. Also, we discuss possible applications of such system, operating in multi-frequency locking or comb regimes, and demonstrate the application for spectroscopy measurements.

An experimental and theoretical analysis of the effect of optical injection on optical frequency combs generated by gain-switching a single mode laser is performed. Combs with a frequency separation f_R in the GHz range are generated by gain-switching a discrete mode laser (DML). The effect of optical injection on the comb has been analyzed for a wide range of the injected power and detuning of the injected master laser from the DML frequency. A rich variety of nonlinear behaviours is found when the injected power is increased for a fixed value of the detuning. At low injected powers two combs due to the gain-switched laser and the injected field are obtained with the same frequency spacing f_R. As the injected power is increased, the comb from the gain-switched laser locks to the master laser. In the parameter space spanned by detuning and optical injection strength, we obtain several regions of locked combs with a tongue shape around detuning values given by multiples of the frequency spacing of the comb f_R. When injected power is further increased combs with frequency spacing equal to rational fractions of f_R are obtained. We also obtain irregular behaviour with low values of the ratio between the intensity of comb lines and noise level. Numerical simulations are in very good agreement with the experimental results.

This work reports on numerical analysis of the nonlinear dynamics of a semiconductor laser subject to an optical injection with a frequency comb. We identified the fundamental difference between the single mode injection and frequency comb injection. By varying the injection parameters (detuning and injection ratio), the results show that the slave laser is able to reproduce the injected frequency comb (new comb) in different parameters regions. The appearance of the new comb depends on the number of injected comb lines for fixed comb properties.

In this work, self-mode-locking of 100 GHz mode-locked pulses from a single-section InP quantum-dash-based laser chip whilst employed in external cavity geometry at 1550nm is investigated. The chip is operated at a forward current marginally above its monolithic operation's lasing threshold. Ultrashort pulses with 1 ps pulse- width were obtained by compensating the chirp by a single mode fiber (SMF).

We present results on the investigation of the dynamics of wavelength switching in a monolithically integrated widely tunable semiconductor ring laser for application in swept source optical coherence tomography. In this application wavelength switching within several tens of nanoseconds is desirable to reduce motion blur artefacts during imaging. The device under test is realized in an InGaAsP/InP platform, operates around 1530 nm wavelength and has been shown to have a tuning range over 50 nm. Both measurements and simulations of the wavelength switching behavior of the laser are presented. Tuning is achieved using voltage controlled electro-refractive phase modulators with a response faster than 1 GHz and negligible residual thermal tuning. The fastest switching strategy, of the three that we compare in our simulations, is shown to be the one that relies on rapid power-off of the origin wavelength. The longitudinal cavity mode position, that has an important impact on the switching time and switching stability, is shown to be hard to predict after switching due to gain-phase coupling in the amplifier.

Multi-wavelength lasers – i.e. devices that can emit simultaneously on different wavelengths – represent an interesting opportunity for terahertz and telecom applications. In this context, implementing such devices with a generic process appears to be a key requirement to fully benefit from the development of these new platforms for Photonic Integrated Circuits (PICs). However, the genericity of the process comes at the cost of certain limitations in terms of design or performances. Through InP Multi-Project Wafer (MPW) runs, we have developed compact dual-wavelength laser (DWL) designs by taking advantage of the properties of Distributed Bragg Reflectors (DBRs). We use two detuned DBRs as narrow-bandwidth frequency-selective mirrors on one side, and, on the other side, we close the laser cavity with either a broad bandwidth multi-mode interference reflector (MIR) or a third DBR with a broader bandwidth than the first two. While each design successfully shows dual-wavelength emission, their emission properties significantly differ from one to another. In this contribution, we characterize the different laser designs and focus on the DBRs themselves. In particular, we study their characteristics when a current is applied or the temperature is changed. Next, we investigate and analyse their impact on the emission properties of the dual-wavelength lasers. Thus, we provide further insight on the relevance and potential of the proposed DWLs and highlight key points for the tailoring of these devices for a given application.

In single-mode vertical-cavity surface-emitting lasers (VCSELs) the frequency difference between the two orthogonal modes, which is defined by the birefringence present in the cavity, is the key factor to enable ultrafast polarization dynamics in spin-lasers. This could be a promising alternative to overcome the bandwidth limitations in short-haul data transmission. Therefore, controlling the birefringence is indispensable to utilize the full potential of the polarization dynamics. Splittings of around 100GHz were realized with an on-chip approach by integration of a surface grating in an oxide-confined AlGaAs-based VCSEL. In this paper we present further details of the parameter search process using a three-dimensional vectorial optical VCSEL electro-magnetics (VELM) model. We also show the geometrical properties of the processed grating structure.

Vertical-cavity surface-emitting lasers (VCSELs) are commonly used in optical data communication mainly for short-haul transmissions in data centers. Spin-VCSELs can be a promising solution in order to overcome the bandwidth limitations of conventional VCSELs by utilizing the spin and polarization instead of current and intensity. Recently, their polarization dynamics have been enhanced to resonance frequencies of more than 200 GHz by implementing a large amount of birefringence into the laser cavity. For future applications onchip solutions to control the birefringence are preferred. For this purpose, a keyhole-shaped mesa-structure on standard wafer material for an 850nm oxide-confined AlGaAs-VCSEL is used. A variable heating current is driven into the semiconductor ridge connected to the mesa at a constant pump current. This creates an asymmetrical heat gradient. Here we investigate the polarization behaviour in a spin-VCSEL with thermally induced birefringence. We analyze the hysteresis in the heating and pump current of the sample to identify optimized working points near the polarization switching points.

We exploit the transient dynamics of a nonlinear photonic system to perform useful computation. This is achieved within the framework of reservoir computing. State of the art implementations in photonic hardware are evolving towards simple architectures. With nonlinearities present in either the reservoirs input or output layer, even a linear photonic cavity makes for a potent reservoir. However, when targeting all-optical reservoir computers (coming from opto-electronic systems), commonly used non-linearities in opto-electronic conversion equipment, such as modulators and photodiodes, can no longer be relied on. Therefore, optical nonlinearities must be considered. In this work, we numerically and experimentally investigate a delay-based reservoir implemented in standard single mode optical fibers. Our setup is coherently driven and exploits the optical Kerr nonlinearity, which is present throughout the reservoir’s extent (i.e. the fiber ring cavity), to operate as a state-of-the-art photonic reservoir. A set of systems was considered, with different combinations of linear and nonlinear input and output schemes. And we have been able to quantify the effects of different nonlinearities in the system on its reservoir computing performance. Experimental data shows the positive effects of the distributed Kerr nonlinearity on both the linear memory capacity and nonlinear computational capacity of our reservoir computing system. We find a broad range of power levels where this distributed nonlinear effect improves the reservoirs performance. Moreover, we find that the exploitation of this optical nonlinearity in the reservoirs bulk allows for state-of-the-art reservoir computing performance without relying on opto-electronic nonlinearities elsewhere in the system.

As demand towards cloud-based services and high-performance computations grows, it imposes requirements on data center performance, and efficiency. Taking advantage of the mature CMOS process technology, and the fact that silicon is the basic material of electronics industry, silicon photonics makes possible production photonic integrated circuits that satisfy these requirements. Here we explore the short-cavity hybrid laser consisting of a III-V amplifier integrated with a silicon photonic crystal (PhC) cavity reflector by so-called butt-coupling approach. The laser possesses great stability characteristics meeting the criteria for data center interconnect applications. The PhC reflector having a Q-factor of 10⁴ at the lasing wavelength 1535 nm can be considered as a narrow-bandwidth filter. The laser demonstrates single mode and eventless operation without any dynamics on the background, and smooth radiofrequency spectrum without evidence of relaxation oscillation frequency. The latter fact is beneficial for many applications, and indicates extremely high damping in PhC laser, where the photon cavity lifetime is greatly improved by the high-Q PhC cavity reflector. We confirm our experimental observations by theory based on delay differential equation model for a single-section semiconductor laser. We reveal the effective damping of the laser, when the detuning between the filter peak and the laser cavity mode is small, and the imaginary parts of the model eigenvalues equal zero. It is possible to undamp the relaxation oscillations forcing self-Q-switched operation in the laser owing to the cumulative action of the alpha-factor and the narrow filter. In conclusion, we experimentally and theoretically demonstrated that relaxation oscillations can be suppressed in the short-cavity semiconductor laser with a narrow intracavity frequency filter. Additionally, on the basis of our analysis we expect the undamping of relaxation oscillations, and self-pulsations when the cavity mode is detuned from the filter peak frequency. The results might be useful for applications in data communications.

In high power regimes, broad area semiconductor lasers usually suffer from poor beam quality due to their asymmetric beam divergence, large beam quality factor (M²) and from the absence of any intrinsic filtering mechanism that can be integrated inside the cavity. In this work, we present a compact photonic crystal spatial filter, fabricated by periodically modulating refractive index media on a glass substrate using tightly focused femtoseconds laser. This filter work by deflecting the higher angular field components in a given frequency range. We demonstrate the spatial filtering effect when placed in an extended cavity configuration for a single BAS emitter, with transverse width of 400-μm and cavity length 1500-μm. We report a decrease of the laser M² value along the slow axis with the introduction of the photonic crystal inside the cavity, together with a brightness enhancement by a factor of 1.5 compared to that of an unfiltered case. These results were compared with those obtained in the far-field domain, with a conventional spatial filter consisting of an intra-cavity slit.

As sources of short, high-amplitude light pulses, self-pulsing lasers are central to many applications, including telecommunications and neuromorphic photonic computing. We consider an excitable semiconductor micropillar laser subject to delayed optical feedback. The microlaser alone displays an all-or-none response to external perturbations. In the presence of feedback, a first excitable pulse can regenerate itself after a delay time, thus resulting in a train of pulses with repetition rate close to the delay time. Several pulse trains can be triggered and sustained simultaneously. Although they can seem independent on the timescale of the experiment, recent work showed that all pulsing patterns correspond to very long transient towards one of the stable periodic solutions of the system. Only stable solutions corresponding to equidistant light pulses in the feedback cavity were observed. We demonstrate experimentally and numerically that stable periodic solutions corresponding to non-equidistant pulses can also exist and be stable. A bifurcation analysis of a suitable mathematical model unveils the conditions on the timescales of the gain and absorption variables for such solutions to exist. We show that the long-term timing between non-equidistant pulses is fixed by the system parameters and does not depend on the initial timing between the pulse trains. Moreover, the bifurcation analysis demonstrates that, for a given number of coexisting pulses in the feedback cavity, only one configuration is stable, corresponding either to equidistant pulses or non-equidistant pulses with a fixed interpulse timing. The latter originates from a period doubling bifurcation and can also be viewed as a symmetry breaking phenomenon of the time-shift symmetry sustained by the system. Our results provide a better understanding of pulsing dynamics in an excitable laser with delayed feedback. Because the only ingredients here are excitability and feedback, we believe our results may be of interest beyond the scope of laser dynamics.

Excitable lasers that mimic neuronal activity can be building blocks of ultra-fast neuro-inspired information processing systems, which can revolutionize the fields of optical signal processing, optical computing and artificial intelligence. To implement such photonic neurons, we must first identify low-cost and energy-efficient lasers whose excitable dynamics mimics neuronal dynamics. Here we conduct an experimental study of the dynamics of a diode laser with optical feedback from an external cavity. We focus on the response of the laser when is under weak periodic perturbations that are implemented via direct modulation of the laser pump current. We consider sinusoidal and pulsed waveforms. The dynamics of the laser intensity is compared with the dynamics of the membrane potential of a neuron, which is simulated with the FitzHugh-Nagumo (FHN) model. The analysis of the statistics of the time intervals between dropouts of the laser intensity (optical spikes) and of the time intervals between the neuron's action potentials (or spikes) unveils similarities, and also differences. The analysis of the spike rate reveals a variation with the amplitude and with the frequency of the external signal that is similar for both, the laser and the FHN neuron. Therefore, in terms of the spike rate, the laser response to a weak periodic signal mimics the response of a FHN neuron, and thus diode lasers with optical feedback can be used to implement photonic neurons. A drawback of this implementation is the length of the feedback cavity, which prevents on-chip integration.

We study delay-based photonic reservoir computing using a semiconductor laser with an optoelectronic feedback. A rate-equation model for a laser with an optoelectronic filtered feedback is used. The filter allows only high-frequency signals to pass through the feedback loop. The delay-differential equation model consists of three equations for the normalized electric field intensity I(t), the carrier density N(t); and the filtered intensity signal IF (t). The stability boundaries which correspond to the Hopf bifurcation condition are determined analytically, showing multiple Hopf bifurcation branches in the dynamics, and the parity asymmetry with relation to the feedback sign. We use the Santa Fe time-series prediction task to evaluate the performance of reservoir computing. Our objective is to determine location of the optimal operating point defined as corresponding to minimal normalized means square error (NMSE) and relate it to the stability properties of the system. We use 3000 points for training and 1000 for testing, number of virtual nodes is chosen in regard to the relaxation oscillation frequency. Single-point prediction of the chaotic data is performed. Input signal is determined by the chaotic waveform having n sampling points, and three cases are investigated: prediction of n + 1 ,n + 2 or n + 3 sampling point. The best NMSE value order of 10^7 for n + 1 point prediction task is obtained in the absence of feedback and the rapid increase in NMSE is observed in the vicinity of Hopf bifurcation without regard to the feedback sign. On the contrary, the minimum values of NMSE for n + 2 and n + 3 point prediction task correspond to the Hopf bifurcation, and only for the positive feedback. We discuss whether the parity asymmetry can explain strongly asymmetric reservoir computing results.

Micropillar lasers with integrated saturable absorber (LSA) have been demonstrated to show important neuromimetic properties such as ultrafast excitable behavior, temporal summation, relative & absolute refractory periods and spike latency. In this work we study a LSA with self delayed connection which can show regenerative spiking. Such delayed self connections called as autpases are found in living nervous systems in regions essential for memory. The characteristic response of a single LSA is a 200 ps wide pulse emitted in response to an optical perturbation above the excitable threshold. In the presence of an external feedback, a single above threshold perturbation will trigger a train of pulses. Additional perturbations are capable of retiming an existing pulse train and adding or removing a pulse train, thus enabling all optical control of spikes. If the system is perturbed under correct conditions several times during the external cavity roundtrip time τ, we can create several coexisting pulse trains in the cavity which are essentially the input perturbation repeating with the time period τ. This can be seen as an optical buffer for spikes. However, in the long term these seemingly independent pulses interact and converge towards a stable solution. This final stable solution is based on the input sequence, thus this long term behavior can be seem as an associative memory. Numerical simulations of the Yamada model with delayed feedback and noise are presented and found to be in good agreement with the experimental observations. All the observed dynamics of spike interaction and convergence can be explained solely based with the internal dynamics of carriers.

High-power broad-area diode lasers (BALs) exhibit chaotic spatio-temporal dynamics above threshold. Under high power operation, where they emit tens of watts output, large amounts of heat are generated, with significant impact on the laser operation. We incorporate heating effects into a dynamical electro-optical (EO) model for the optical field and carrier dynamics along the quantum-well active zone of the laser. Thereby we effectively couple the EO and heat-transport (HT) solvers. Thermal lensing is included by a thermally-induced contribution to the index profile. The heat sources obtained with the dynamic EO-solver exhibit strong variations on short time scales, which however have only a marginal impact on the temperature distribution. We consider two limits: First, the static HT-problem, with time-averaged heat sources, which is solved iteratively together with the EO solver. Second, under short pulse operation the thermally induced index distribution can be obtained by neglecting heat ow. Although the temperature increase is small, a waveguide is introduced here within a few-ns-long pulse resulting in significant near field narrowing. We further show that a beam propagating in a waveguide structure utilized for BA lasers does not undergo filamentation due to spatial holeburning. Moreover, our results indicate that in BALs a clear optical mode structure is visible which is neither destroyed by the dynamics nor by longitudinal effects.

This Conference Presentation, “Passive coherent beam combining in an interferometric semiconductor laser cavity,” was recorded for the SPIE Photonics Europe Digital Conference

In this manuscript, we study the influence of various parameters on the dynamics of a semiconductor laser with phase-conjugate feedback. We report the chaos bandwidth of its chaotic states, and simultaneously the frequency of its self-pulsing modes. We demonstrate that both indicators are unaffected by the delay length at any given feedback strength. A higher pumping current enhances both indicators, as does an increase of the carrier lifetime. We further investigate the influence of the linewidth enhancement factor (α-factor ): restabilization occurs for higher values of feedback strength, thereby enhancing the system's limit of attaining higher values of the chaos bandwidth. The effective bandwidth is observed to follow the general trend of the chaos bandwidth with an exception of transition dynamical solutions. We show that the PCF system is able to generate high-frequency and broadband chaos without any additional active optical element. Because of the high quality of chaos induced by the PCF system, delayed laser systems are strong candidate to generate high-performance chaotic signals.

A semiconductor laser having an emission wavelength of 852 nm is subjected to phase-conjugated feedback. The phase conjugation is implemented using a Rh-doped BaTiO₃ photorefractive crystal of the dimension 5mm × 5mm × 5mm. A combination of several phenomena including light fanning, total internal reflection and four-wave mixing occur in tandem inside the crystal to generate the phase conjugate of the light entering the crystal. Our recent works have established that such configuration exhibits significantly enhanced chaos bandwidth with high spatiotemporal complexity at varied feedback strengths, compared to its conventional optical feedback counterpart in a long external cavity setup. The presented work studies the systematic progression of spatiotemporal complexity in a long-length (≈1.5 m) external cavity setup as a function of the feedback strength. System outputs at varied operating conditions are found to be highly complex with PE upwards of 0.9 and chaos BW in the order of several GHz. Such complex outputs have relevance for applications such as security-based communication and random-bit generation. These system outputs are shown to be dynamically diverse ranging from wide-band chaos to time-periodic oscillations of the output signal corresponding to a higher multiple of the external cavity frequency. The analyses performed in the temporal and frequency domain use several diagnostic tools including permutation entropy, continuous wavelet transform, and statistically calculated chaos bandwidth to unveil and dissect the evolution of the system's characteristic temporal signatures. Thus, we comprehensively determine the true chaotic nature of the system outputs in the spatiotemporal domain.

We propose a compact solution to spatially regularize and temporally stabilize edge emitting (broad area) semiconductor lasers. The scheme relays on the local asymmetries generated by a non-Hermitian potential with a central symmetry axis to manage the flow of light, which stabilizes the radiation while enhancing and localizing the generated laser beam. We introduce a harmonic modulation on the pump with a central symmetry axis, and a complex refractive index modulation that generates a local PT-symmetry within the laser. The local PT-symmetry creates an inward mode coupling that concentrates the light generated in the active layer on axis. We perform a comprehensive analysis of all the modulation parameters and find regimes of simultaneous temporal stability and light concentration. The present approach produces two in one: light localization into a narrow beam emission and the control over the spatiotemporal dynamics, improving the laser performance.

High power vertical cavity surface emitting laser (VCSEL) arrays with multiple emitters have been receiving remarkable attention currently due to their emerging applications in consumer market such as 3D sensing illumination laser source in mobile devices as well as in automotive LIDAR applications. Failure mode analysis will help provide useful information for VCSEL array design and process improvement. However, using solely general physical failure analysis techniques is insufficient. The challenge of failure mechanism study is how to locate and capture the small physical defects in the early stage since it may randomly occur in the entire active region of the emitter This work developed 3D transmission electron microscopy (TEM) method, that is, planar-view TEM together cross-section TEM, to investigate failure mode phenomenon in this kind of high power VCSEL arrays. Overstressed reliability testing intentionally create failure in VCSEL arrays where dim emitters are found. Optical microscope images can’t see any abnormality while infrared microscope can catch small ‘mouse-bite’ abnormality at oxide aperture. Planar-view TEM method is developed to isolate the target dim emitter and trim away most of the top and bottom DBR layers to keep the oxide layer and active region to thin enough where 200KV electron beam can penetrate planar-view lamella. The whole oxide aperture is achieved and scanning TEM images clearly show the ‘flower-like’ oxide blasters at oxide aperture periphery. It is from further oxidation of the oxide tip. Cross-section TEM reveal the oxide layer morphology where the further oxidation layer from oxide tip is thinner than the original oxide layer. The oxide tip further oxidation is possibly due to non-reaction steam in existence in the oxide causing the second oxidation of oxide layer during overstress test. This work demonstrate that 3D TEM method is good technique to catch small physical failure features in VCSEL arrays, which will help to analyze failure mode in high power VCSEL arrays for 3D sensing application.

The performance of distributed feedback (DFB) polymer lasers was adjusted by changing the cavity coupling. The microcavity of DFB polymer lasers was fabricated by two-beam multi-exposure interference lithography. With the different angles between two gratings, the coupling strength of the cavity modes was controlled. Reducing the coupling strength of cavity modes, the threshold of the polymer lasers decreased. According to the experimental results, the azimuthally polarized output was changed by tuning the cavity coupling. This method provides a facile way to realize low threshold and azimuthally polarized DFB polymer laser.

The commercial availability of laser diodes emitting in the blue spectral region offer excellent opportunity for using them in the detection of nitrogen dioxide (NO₂) gas. Nitrogen dioxide, which is one of the main air pollutants, has strong light absorption cross-section in the blue spectral region. In this paper, a tunable blue diode laser in Littrow external-cavity configuration is investigated. The output power, spectral line-width and tunable range are measured. The results will be used to select a suitable external cavity diode laser (ECDL) system based on a multimode blue laser diode to be employed for the detection of NO₂ gas.

In this work we propose a laser diode pulse modulation algorithm suitable for implementation in microcontrollers. This algorithm overcomes the speed limitations in microcontroller based laser pulse drivers. A feed forward signal is added to a proportional integral and differential (PID) feedback signal after a settling time. The initial feed forward control gives a fast rise time (<5 μs) and the PID feedback ensure immunity to drift for large pulse widths. This algorithm enables a settling time 10 times faster than the PID settling time. The controller is self-learning and updates the feed forward estimator based on the settled feedback control output. The use of pulse width modulation (PWM) for digital to analog conversion scales well to high power laser diodes due to its high efficiency.

To investigate the origin of the deep-notch in the modulation response of multi-mode vertical cavity surface emitting lasers (VCSELs) that occurs at low frequencies, the carrier transport and the mode intensity nonuniformity effects are considered in this work. The model that includes the carrier transport effect is a possible candidate to explain the deep-notch behavior, as it introduces a first-order low-pass filter to the pure intrinsic transfer function. Nevertheless, the expected dynamic characteristics of this filter for high-performance VCSELs, disfavor the transport effect as a possible candidate. Therefore, to fully rule out its impact, we optimized the transport theory by first considering both the transverse and longitudinal carrier transport effects, and second by including the frequency dependence of the transport factor into the derivation of the VCSELs’ dynamic transfer function. Taking all these factors into consideration, however, did not result in a significant improvement in the resulting transfer function form and order. Thus, a second effect, which is the mode intensity nonuniformity effect, is favored as a possible candidate to explain the deep-notch phenomenon. In this effect, the spatial nonuniformity of the transverse optical mode in the radial direction results in concentric disk-like regions with different dynamic properties under small signal modulation. At low driving currents, the model predicts a notch ahead of the relaxation oscillation frequency. This notch is caused by a low frequency roll-off resulting from the spatial nonuniformity of the optical transverse mode. The proposed model introduces a ratio containing a pole and a zero to the pure intrinsic transfer function and can very accurately fit the deep-notch in the measured modulation response. Furthermore, this fitting results in the extraction of very reliable performance indicators. A second topic that is also presented in this work is about the dynamics of the mode resolved modulation response. To achieve a higher bandwidths, it is necessary to study the mode ensemble resolved modulation characteristics of the intrinsic dynamics. Based on the novel carrier-reservoir splitting approach, analytical expressions for the dynamics in the two spatially separated central and peripheral regions are derived.

Realizing that vertical cavity surface emitting lasers (VCSELs) are continuously breaking higher bandwidth limits, it is essential to understand their basic material constituting properties and extract their physical characteristics, in order to fine tune their high-speed performance. Throughout the past decade, the device performance was continuously optimized towards higher bandwidths, faster data rates, and efficient operation. Therefore, for a successful further optimization of their dynamic characteristics, the extraction of a reliable set of their physical parameters becomes indispensable. Consequently, the main objective of this work is to provide accurate physical parameter values of cutting-edge high-speed VCSELs. The extraction process of these set of parameters is based on the novel intrinsic and extrinsic dynamic models that were recently developed by our research group. For the intrinsic dynamics, the advanced split carrier reservoir multimode model is employed. Furthermore, the pure intrinsic modulation response is de-embedded from the total measured device response using our recently proposed novel parasitic-network model. Moreover, the extraction of these device physical parameters is based on the device’s performance indicators, such as the relaxation oscillation frequencies and damping coefficients obtained by fitting the intrinsic model to the measured modulation data. Furthermore, since these performance indicators can be expressed in terms of a combination of these physical parameters, a precise estimation of their values and their possible ranges, along with a carefully designed fitting process, is crucial. Consequently, for extracting a reliable set of data, the accurate prior estimation of these parameters was conducted based on the device physical structure and reported material properties, leading to the establishment of an accurate set of parameters along with their possible ranges. These final calculations are based on simple device geometrical considerations, reported physical and material properties and device static performance measurements. Finally, the estimated values and their ranges are further validated by comparing them to ones in standard literature.

Due to circumstances beyond the presenter's control, and audio recording was not possible for this presentation.

Deep learning for object detection offers the advantage of very low electrical power requirements but with the potential of a very large computation bandwidth due to the ability of Fraunhofer diffraction to perform correlation operations. However, many of the current designs of deep learning networks are not easily implemented in the optical domain. In this paper we develop a modified version of the deep learning library, Keras, that can accurately model optical systems. This allows the discovery of the optimal weights by calculating them on a realistic optical system. Noise sources, speckle models, and calibration errors can be accounted for. The effect of using readily realisable filters such as nematic liquid crystal phase only spatial light modulators is investigated. The effect of multiplexing a number of correlations in order to replicate the Conv2D multiple channel input is assessed. The effect of an optically implementable bias and activation functions are examined and compared to the state-of-the-art software implementations. We show that object recognition can be achieved using spatial light modulator technology and give comparable results to digital implementations.

One of the promising future ways of computing is using principles similar to the human brain work mechanism. Neuromorphic photonics makes it possible to create computational elements with properties similar to the principles of the biological synapse. Neuromorphic computers can overcome the von Neumann bottleneck fundamental limitation of existing computing systems. In a current study, we demonstrate a neuromorphic properties, observing on photoconductive structures based on nanocrystalline ZnO, WO3, In2O3 triggered by presynaptic light spikes with the 405nm wavelength. Photoconductive structures based on ZnO, WO3, In2O3 were deposited as a 100–200 nm thick film on the surface of the chip. Excitatory post-synaptic current value was measured for different excitation pulse durations. The excitatory post-synaptic current caused by a pair of presynaptic light spikes was studied for different delay times between pulses. The ability of these structures to act as biological synapses like high-pass temporal filtering function was demonstrated by measuring post-synaptic current when exposed to a series of 30 consecutive presynaptic light spikes. Our photoconductive semiconductor structures have two different relaxation mechanisms. Due to this, the structures possess short-term and long-term photoconductivity memory. To demonstrate the ability of our samples possesses long-term memory, we studied the semiconductor photoconductivity relaxation values after light exposure during 500 seconds. The memory level after light exposure were stored over an hour. The studied photoconductive structures showed the presence of a spike reaction properties, the effect of amplitude and frequency filtering, short-term and long-term memory, and they are looking promising for use as elements of neuromorphic photonics.

The effect of intermediate catalysis with Ag on structural and near-band-edge-emission properties of ZnO was studied. It was found that the regrowth of ZnO on top of (110)-oriented ZnO structure coated with Ag results in the change of ZnO crystallographic characteristics and appearance of stimulated emission, which was observed under nanosecond photoexcitation. It was assumed that the excitation of lasing became possible due to formation of c-axis oriented ZnO crystals, which can serve as laser microcavities.

Publisher’s Note: This paper, originally published on 1 April 2020, was replaced with a corrected/revised version on 27 July 2020. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.