Photonic Spiking Neural Networks with Highly Efficient Training Protocols for Ultrafast Neuromorphic Computing Systems

Photonic technologies offer great prospects for novel ultrafast, energy-efficient and hardware-friendly neuromorphic (brain-like) computing platforms. Moreover, neuromorphic photonic approaches based upon ubiquitous, technology-mature and low-cost Vertical-Cavity Surface Emitting Lasers (VCSELs) (devices found in fibre-optic transmitters, mobile phones, automotive sensors, etc.) are of particular interest. Given VCSELs have shown the ability to realise neuronal optical spiking responses (at ultrafast GHz rates), their use for spike-based information processing systems has been proposed. In this work, Spiking Neural Network (SNN) operation, based on a hardware-friendly photonic system of just one Vertical Cavity Surface Emitting Laser (VCSEL), is reported alongside a novel binary weight 'significance' training scheme that fully capitalises on the discrete nature of the optical spikes used by the SNN to process input information. The VCSEL-based photonic SNN is tested with a highly complex, multivariate, classification task (MADELON) before performance is compared using a traditional least-squares training method and the alternative novel binary weighting scheme. Excellent classification accuracies of >94% are reached by both training methods, exceeding the benchmark performance of the dataset in a fraction of processing time. The newly reported training scheme also dramatically reduces training set size requirements as well as the number of trained nodes (<1% of the total network node count). This VCSEL-based photonic SNN, in combination with the reported 'significance' weighting scheme, therefore grants ultrafast spike-based optical processing with highly reduced training requirements and hardware complexity for potential application in future neuromorphic systems and artificial intelligence applications

GHz rate neuromorphic photonic spiking neural network with a single Vertical-Cavity Surface-Emitting Laser (VCSEL)

Vertical-Cavity Surface-Emitting Lasers (VCSELs) are highly promising devices for the construction of neuro- morphic photonic information processing systems, due to their numerous desirable properties such as low power consumption, high modulation speed, and compactness. Of particular interest is the ability of VCSELs to exhibit neuron-like spiking responses at ultrafast sub-nanosecond rates; thus offering great prospects for high-speed light-enabled spike-based processors. Recent works have shown spiking VCSELs are capable of pattern recognition and image processing problems, but additionally, VCSELs have been used as nonlinear elements in photonic reservoir comput- ing (RC) implementations, yielding state of the art operation. This work introduces and experimentally demonstrates for the first time a new GHz-rate photonic spiking neural network (SNN) built with a single VCSEL neuron. The reported system effectively implements a photonic VCSEL-based spiking reser- voir computer, and demonstrates its successful application to a complex nonlinear classification task. Importantly, the proposed system benefits from a highly hardware-friendly, inexpensive realization (a single VCSEL device and off-the-shelf fibre-optic components), for high-speed (GHz-rate inputs) and low-power (sub-mW optical input power) photonic operation. These results open new pathways towards future neuromorphic photonic spike- based processing systems based upon VCSELs (or other laser types) for novel ultrafast machine learning and AI hardware

VCSEL Modeling and Parameter Extraction for Optical Link Simulation and Optimization

A single and multimode vertical cavity surface emitting laser (VCSEL) model and the associated parameter extraction using bandwidth and intensity noise measurements was proposed. To meet the growing bandwidth demand, optical VCSEL links are commonly turning to equalization, and multilevel modulation formats. The need for an accurate physics-based rate equation model is critical for the design and analysis of VCSEL links as we move past 25Gb/s. Single and multimode VCSEL models were able to accurately capture the bandwidth and noise characteristics of VCSELs over a large range of operating conditions. Through spatial mode overlaps, which cause inter and intra mode gain saturation, the multimode VCSEL model uncovered a possible source of positive and negative mode noise correlations recently discovered in VCSELs which affect RIN. Through S-parameter VCSEL characterization and intensity noise measurements, our models could extract and simulate extrinsic and intrinsic VCSEL PAM-4 performance for 100Gb/s+ short reach optical links.Ph.D

Synchronization and application of delay-coupled semiconductor lasers

The work in this thesis is focused on the complex dynamics of semiconductor laser (SL) devices which receive time-delayed feedback from an external cavity or are delay-coupled with a second semiconductor laser. We investigate fundamental properties of the dynamics and study the utilization of transient complex dynamics of a single SL arising from delayed feedback and external signal injection for a neuro-inspired photonic data processing scheme. Based on experiments and numerical modelling, we investigate systems of two coupled SLs, gaining insights into the role of laser and coupling parameters for the synchronization characteristics of these systems. We link certain features of the synchronization dynamics, like intermittent desynchronization events, to the underlying nonlinear dynamics in the coupled laser system. Our research thus combines both fundamental insights into delay-coupled lasers as well as novel application perspectives

High Speed Human Action Recognition using a Photonic Reservoir Computer

The recognition of human actions in videos is one of the most active research fields in computer vision. The canonical approach consists in a more or less complex preprocessing stages of the raw video data, followed by a relatively simple classification algorithm. Here we address recognition of human actions using the reservoir computing algorithm, which allows us to focus on the classifier stage. We introduce a new training method for the reservoir computer, based on "Timesteps Of Interest", which combines in a simple way short and long time scales. We study the performance of this algorithm using both numerical simulations and a photonic implementation based on a single non-linear node and a delay line on the well known KTH dataset. We solve the task with high accuracy and speed, to the point of allowing for processing multiple video streams in real time. The present work is thus an important step towards developing efficient dedicated hardware for video processing

Synchronization and application of delay-coupled semiconductor lasers

The work in this thesis is focused on the complex dynamics of semiconductor laser (SL) devices which receive time-delayed feedback from an external cavity or are delay-coupled with a second semiconductor laser. We investigate fundamental properties of the dynamics and study the utilization of transient complex dynamics of a single SL arising from delayed feedback and external signal injection for a neuro-inspired photonic data processing scheme. Based on experiments and numerical modelling, we investigate systems of two coupled SLs, gaining insights into the role of laser and coupling parameters for the synchronization characteristics of these systems. We link certain features of the synchronization dynamics, like intermittent desynchronization events, to the underlying nonlinear dynamics in the coupled laser system. Our research thus combines both fundamental insights into delay-coupled lasers as well as novel application perspectives. In order to explore the capabilities of a single SL with delayed feedback, we follow the concept of reservoir computing (RC) based on delay systems. In particular, we study two different tasks, which are computationally hard for traditional computing concepts. We explore several feedback configurations, data injection methods and operating regimes of the laser and identify the task-dependent optimal operating conditions. Our work demonstrates the potential of simple photonic setups and the RC concept for future computational paradigms. Furthermore, we study the synchronization properties in systems of two delay-coupled SLs with relay. We explore the consequences of asymmetries in this basic setup for the dynamics and synchronization properties. One key question is, how synchronization decays or is lost, which is of significant importance for applications in chaotic communications schemes and key-exchange protocols. We follow an event-based approach and connect changes in the synchronization levels for varying operating parameters or varying mismatches to the onset and characteristics of desynchronization events. Our results regarding synchronization levels and synchronizability underline the significance of symmetry and matching parameters for the identical synchronization of delay-coupled oscillators. We apply our findings regarding the possibility for identical synchronization to develop and implement an experimental method to identify determinism in the chaotic dynamics of a SL with delayed feedback. Our method is based on zero-lag synchronization of the laser with a twin system. We focus our investigation on power dropouts in the Low Frequency Fluctuations regime of a SL since they represent distinct dynamical features whose origin had been controversially discussed in the past. Our method can be adapted in principle to other nonlinear delay systems which exhibit intrinsic noise to test for traces of determinism. Our work is of general relevance for research in nonlinear dynamics, as many of our results and methods can be adapted for other delay systems and provide general insights into the characteristics of delay-coupled systems.Konstantin Hicke reconoce con gratitud la ayuda financiera concedida por el Govern de les Illes Balears (Conselleria d'Educació, Cultura i Universitats) para la formación de personal investigador. La ayuda ha sido seleccionada en el marco de un programa operativo cofinanciado por el Fondo Social Europeo. Tesis realizada en el Instituto de Física Interdisciplinar y Sistemas Complejos, IFISC (CSIC-UIB) para optar al título de Doctor, en el Programa de Física del Departamento de Física de la Universitat de les Illes Balears.Peer Reviewe

Surface Micromachined Widely Tunable VCSEL and OAM-Filter for Optical Data Transmission

The implication of wavelength division multiplexed passive optical network (WDM PON) is becoming more evident as the traffic demands of the mobile network operators keep increasing. It offers a cost-efficient solution to handle the bandwidth and latency requirements of the mobile fronthaul. The key component of such a WDM-PON system is a centralized wavelength-controlled tunable laser. The biggest challenge up to now is the lack of low-cost wideband 1550 nm tunable lasers with 10 Gbit/s transmission capacity. In the first part of this work, a widely-tunable microelectromechanical system vertical-cavity surface-emitting laser (MEMS VCSEL) is developed. The cost-efficient, directly-modulated laser can be utilized for 10Gbit/s transmission over relevant reach. It also offers simplicity for wideband autonomous tuning. The device is suitable for applications including hot backup and fixed wavelength laser replacement for inventory reduction. Within the framework of this work, a PECVD-deposited MEMS distributed Bragg reflector (DBR) mirror is surface-micromachined on top of a short-cavity active VCSEL structure. The MEMS-DBR consisting of SiNx/SiOy dielectric materials has a very high reflectivity with wide stopband. Wavelength tuning is realized by the electrothermal actuation of the MEMS electrode. The fabrication steps of the MEMS aiming for large volume production is discussed in detail. A comprehensive static and dynamic characterizations of MEMS VCSEL including far-field, linewidth, polarization behavior, modulation capacity and relative intensity noise is presented. The effect of the temperature change on its tuning behavior as well as on the static and dynamic performance is investigated. The obtained wavelength tuning range of more than 100 nm covers the complete telecom C-band (1530–1565 nm) and part of L-band (1565–1625 nm). A small-signal amplitude modulation bandwidth of up to 8.35GHz is demonstrated for the center emission wavelength around 1550 nm. This enables to implement a directly-modulated MEMS VCSEL based back-to-back link at 10Gbit/s data transmission for 76 nm tuning range. Also, quasi error-free 10Gbit/s transmission over 40 km standard single-mode fiber for a tuning range of more than 60 nm validates its potential for the above mentioned novel WDM-PON system. Apart from optical communication, the scope of this tunable source is investigated in applications such as dispersion spectroscopy and tunable terahertz (THz) signal generation. Experimental validation of multi-species dispersion spectroscopy using MEMS VCSEL is presented for the first time in this work, where concurrent detection of acetylene (C2H2), hydrogen cyanide (HCN), and carbon monoxide (CO) is demonstrated. The second part of the work constitutes demonstration and experimental validation of a novel optical component called MEMS orbital angular momentum (OAM) filter. The filter consists of a micro-sized spiral phase plate (SPP) which is integrated to the MEMS-DBR of a Fabry-Perot optical filter by means of direct laser writing. The onchip devices are suitable for distinguishing different OAM modes for a broad tuning range around 1550 nm emission and considered as a compact, robust and cost-effective solution for simultaneous OAM- and WDM optical communications. The utilization of the OAM modes as an additional orthogonal basis of information carriers in both free space and optical fiber communication systems potentially enhances the transmission capacity tremendously. Four devices with OAM orders of 0 (i.e., no SPP on MEMS), 1, 2 and 3 have been investigated. They are capable of generating/receiving the OAM beam of corresponding order over a continuous tuning range of more than 30 nm, for which the designed SPPs work with high mode purity. The system performance is evaluated by multiplexing two wavelength- and two OAM channels. Error-free free-space transmission at 10Gbit/s suggests that OAM-filters functioning over a wide wavelength range could be employed as an additional degree of freedom for increasing the capacity of free-space communication to a great extent