Chirp-based direct phase modulation of VCSELs managed by Neural Networks

VCSEL's capacity of direct modulation and its low cost makes this device a feasible cost-effective transmitter for ultra-dense wavelength division multiplexing (uDWDM) metro-access networks using coherent detection. However, performing direct-phase modulation in semiconductors can be complex due to its nonlinear characteristics. This research presents Neural Network (NN) training techniques for Time-Series analysis in order to describe the correlation between the input current given to the device and its output optical phase, using a 1550nm RayCan SM-VCSEL. Main goal is training a NN capable of predicting an ideal optical power signal for a specific phase result achievable by inverse training, that is: optical phase is the neural network input while the optical power is the desired target. The experiment is done in three stages: (i) VCSEL's characterization, (ii) NN training to predict input current knowing optical power, and (iii) NN training to predict optical power from a known optical phase

Machine Learning-based Predictive Maintenance for Optical Networks

Optical networks provide the backbone of modern telecommunications by connecting the world faster than ever before. However, such networks are susceptible to several failures (e.g., optical fiber cuts, malfunctioning optical devices), which might result in degradation in the network operation, massive data loss, and network disruption. It is challenging to accurately and quickly detect and localize such failures due to the complexity of such networks, the time required to identify the fault and pinpoint it using conventional approaches, and the lack of proactive efficient fault management mechanisms. Therefore, it is highly beneficial to perform fault management in optical communication systems in order to reduce the mean time to repair, to meet service level agreements more easily, and to enhance the network reliability. In this thesis, the aforementioned challenges and needs are tackled by investigating the use of machine learning (ML) techniques for implementing efficient proactive fault detection, diagnosis, and localization schemes for optical communication systems. In particular, the adoption of ML methods for solving the following problems is explored: - Degradation prediction of semiconductor lasers, - Lifetime (mean time to failure) prediction of semiconductor lasers, - Remaining useful life (the length of time a machine is likely to operate before it requires repair or replacement) prediction of semiconductor lasers, - Optical fiber fault detection, localization, characterization, and identification for different optical network architectures, - Anomaly detection in optical fiber monitoring. Such ML approaches outperform the conventionally employed methods for all the investigated use cases by achieving better prediction accuracy and earlier prediction or detection capability

Degradation Prediction of Semiconductor Lasers using Conditional Variational Autoencoder

Semiconductor lasers have been rapidly evolving to meet the demands of next-generation optical networks. This imposes much more stringent requirements on the laser reliability, which are dominated by degradation mechanisms (e.g., sudden degradation) limiting the semiconductor laser lifetime. Physics-based approaches are often used to characterize the degradation behavior analytically, yet explicit domain knowledge and accurate mathematical models are required. Building such models can be very challenging due to a lack of a full understanding of the complex physical processes inducing the degradation under various operating conditions. To overcome the aforementioned limitations, we propose a new data-driven approach, extracting useful insights from the operational monitored data to predict the degradation trend without requiring any specific knowledge or using any physical model. The proposed approach is based on an unsupervised technique, a conditional variational autoencoder, and validated using vertical-cavity surface-emitting laser (VCSEL) and tunable edge emitting laser reliability data. The experimental results confirm that our model (i) achieves a good degradation prediction and generalization performance by yielding an F1 score of 95.3%, (ii) outperforms several baseline ML based anomaly detection techniques, and (iii) helps to shorten the aging tests by early predicting the failed devices before the end of the test and thereby saving costsComment: Published in: Journal of Lightwave Technology (Volume: 40, Issue: 18, 15 September 2022

Semiconductor ring lasers for all-optical signal processing

Since the late 1980s there has been a strong interest in exploiting optical bistablity for all-optical signal processing. In this scenario, a novel and promising building block is the semiconductor ring laser (SRL) that exhibits bistability between the counter-propagating cavity modes. This thesis reports on the design, fabrication and characterisation of 1550 nm lasing wavelength SRLs that are intended for applications as all-optical flip-flops and logic elements. Substantial optimisation of SRL design and processing technology is carried out in order to promote unidirectional bistable operation and allow high yield. Fabricated, large size, 150 um - 200 um radius SRLs, show robust unidirectional bistable operation with 30 - 35 dB directional extinction ratio (DER) between the counter-propagating modes, from near threshold up to 5 - 6 times threshold current bias. A significant advantage of the optimised technology is that 98% of the devices per chip show continuous wave (cw) and room temperature lasing with an average 2 - 3mA threshold current dispersion. Switch-on and switch-off times as short as 60 ps and 30 ps were measured, respectively, and reliable 10 Gbit/s flip-flop (FF) operation with external triggering optical pulses was achieved with these devices. Temporal measurements and calculations show that the switching speed of the free running SRL is limited by the carrier lifetime. A monostable device consisting of a SRL and an integrated distributed feedback laser (DFB) source is also presented, and this holding beam (HB) configuration is used to demonstrate all-optical NOT operation with data rates up to 2.5 Gbit/s. Dry etch chemistries for realizing 3.2 - 4.5 um deep waveguides, which show minimal bending losses, are developed and evaluated in order to enable dense integration of SRL devices. In addition, compact, milliwatt output power racetrack shaped cavity designs with radii as small as 10 um are presented. These devices exhibit minimal intra-cavity back-reflections by employing bi-level etching couplers and adiabatic straight to curved waveguide convertors. Finally, these developments provide a more than 150 times footprint reduction compared to large radius devices, whilst also preserving the robust unidirectional operation of their relatives with slightly lower, 20 - 30 dB DER

Fabrication and performance of broad-area high power narrow linewidth semiconductor laser diodes

This thesis presents the theoretical and experimental examination of broad-area, high-order, distributed feedback (DFB) grating, semiconductor laser diodes for the goal of manufacturably attaining longitudinal mode reduction. Through coupled-mode theory, the coupling coefficient, the reflectivity, and the laser emission linewidth resulting from a refractive index grating are predicted. A comparison is made between lasers with coated and uncoated facets, demonstrating that the impact of DFB gratings is enhanced by facet coating. Both surface-etched and epitaxially buried DFB grating lasers emitting at 15xx nm using designs with varying grating order and fill factor, are fabricated at the Holonyak Micro and Nanotechnology Laboratory cleanroom using standard i-line optical lithography. Alternative lithography techniques are proposed without reliance on small feature definitions by electron-beam lithography. The buried grating lasers are characterized in terms of threshold current, emission wavelength, and spectral linewidth. Under pulsed operation, the threshold currents of lasers diodes 30 μm wide and 2 mm cavity length are found to moderately increase using the DFB gratings. The spectral linewidths measured for buried grating lasers are less than control lasers (lacking gratings), demonstrating a reduction of longitudinal modes with high-order gratings, compatible with optical lithography. The most successful grating designs have 50% fill factor and the measured emission linewidths are consistent with theoretical estimates. This work has demonstrated 1.5 µm laser diodes with spectral narrowing arising from longitudinal mode reduction via DFB gratings, fabricated using designs and techniques amenable with high volume production. While the grating designs demonstrated in this work have not been optimized with respect to optical loss, high-order gratings appear promising for future high-power laser applications

Semiconductor Laser Dynamics

This is a collection of 18 papers, two of which are reviews and seven are invited feature papers, that together form the Photonics Special Issue “Semiconductor Laser Dynamics: Fundamentals and Applications”, published in 2020. This collection is edited by Daan Lenstra, an internationally recognized specialist in the field for 40 years

Cavity Ring-Down Spectroscopy of Trace Components in Gas Mixtures for Breath Analysis and Environmental Applications

Cavity ring-down spectroscopy (CRDS) is a direct absorption technique, which provides high detection sensitivity of gas, liquid or solid phases. By using high reflectivity mirrors, the effective absorption path length can be increased up to the hundred-kilometer range. In this work, a single mode tunable CW laser source was used to achieve high sensitivity detection with a narrow line width. The light source is a distributed feedback (DFB) diode laser. By changing the temperature and the current of the diode laser, the output wavelength was tuned across the absorption line peaks of carbon dioxide, methane (COv2, CHv4) and acetone (CHv3COCHv3). In particular, the main goals of this study are the optimization of the detection sensitivity of the isotope ratio (^13C/^12C) of indoor COv2 and CHv4, exhaled acetone analysis for diagnosing diabetes and studying of the gas content of natural water. In this study, first, the ^13C/^12C ratio of COv2 and CHv4 for room air will be discussed. Second, the results on acetone absorption spectrum for diabetic and non-diabetic people will be presented. Third, a membrane gas separation system and spectroscopic analysis of gas content will be described

Photonic Technology for Precision Metrology

Photonics has had a decisive influence on recent scientific and technological achievements. It includes aspects of photon generation and photon–matter interaction. Although it finds many applications in the whole optical range of the wavelengths, most solutions operate in the visible and infrared range. Since the invention of the laser, a source of highly coherent optical radiation, optical measurements have become the perfect tool for highly precise and accurate measurements. Such measurements have the additional advantages of requiring no contact and a fast rate suitable for in-process metrology. However, their extreme precision is ultimately limited by, e.g., the noise of both lasers and photodetectors. The Special Issue of the Applied Science is devoted to the cutting-edge uses of optical sources, detectors, and optoelectronics systems in numerous fields of science and technology (e.g., industry, environment, healthcare, telecommunication, security, and space). The aim is to provide detail on state-of-the-art photonic technology for precision metrology and identify future developmental directions. This issue focuses on metrology principles and measurement instrumentation in optical technology to solve challenging engineering problems