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

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

State-of-the-art InAs/GaAs quantum dot material for optical telecommunication

This thesis reports on the characterization of the state-of-the-art In(Ga)As/GaAs quantum dot (QD) material grown by molecular beam epitaxy for optical telecommunication applications. A wide variety of characterization methods are employed to investigate the material properties and characteristics of a number of QD-based devices enabling future device optimization. The motivation that prompted this study was predicated mainly upon two technological advantages. First, that the QDs gain spectra exhibits a symmetric gain shape and thus the change of refractive index with respect to gain is negligible at the lasing wavelength. This is therefore expected to result in a zero or a very small linewidth enhancement factor (LEF), which is desirable for instance, for high-speed modulation purposes where frequency chirp under modulation, which is directly proportional to the LEF, may be substantially reduced. Second, the fact that not only QDs exhibit a damped frequency response attributed to the carrier relaxation dynamics but also as the resilience of a laser to optical feedback is inversely proportional to the fourth power of the LEF, QD lasers are expected to demonstrate a relatively higher feedback insensitivity. This bodes well for operating these devices isolator free, which would be greatly cost-effective. The absorption and gain spectra of the QD active material are investigated in chapters 2 and 3, respectively. The LEF of QD lasers at a range of temperatures is studied in chapter 3, which confirms the expectation for the first time for In(Ga)As/GaAs QD lasers from -10 oC to 85 oC. Subsequently, the findings of chapters 2 and 3 are employed in chapter 4 with an electro absorption modulator device in mind which would be able to operate with chirp control. In chapter 5, the modulation response of QD lasers is investigated through examining the relative intensity noise (RIN) spectra in the electrical domain. The resilience of the devices to optical feedback is subsequently studied through the RIN characteristics at a range of temperatures. Chapter 6 provides a summary of the thesis findings and possible future works that may be carried out as continuation to this project, which fell outside of the remit of this work

Advanced Photonic Sciences

The new emerging field of photonics has significantly attracted the interest of many societies, professionals and researchers around the world. The great importance of this field is due to its applicability and possible utilization in almost all scientific and industrial areas. This book presents some advanced research topics in photonics. It consists of 16 chapters organized into three sections: Integrated Photonics, Photonic Materials and Photonic Applications. It can be said that this book is a good contribution for paving the way for further innovations in photonic technology. The chapters have been written and reviewed by well-experienced researchers in their fields. In their contributions they demonstrated the most profound knowledge and expertise for interested individuals in this expanding field. The book will be a good reference for experienced professionals, academics and researchers as well as young researchers only starting their carrier in this field

Small business innovation research. Abstracts of completed 1987 phase 1 projects

Non-proprietary summaries of Phase 1 Small Business Innovation Research (SBIR) projects supported by NASA in the 1987 program year are given. Work in the areas of aeronautical propulsion, aerodynamics, acoustics, aircraft systems, materials and structures, teleoperators and robotics, computer sciences, information systems, spacecraft systems, spacecraft power supplies, spacecraft propulsion, bioastronautics, satellite communication, and space processing are covered

Dilute-Anion III-Nitride Semiconductor Materials and Nanostructures

In this dissertation, the work focuses on the development of the dilute-anion III-Nitride based semiconductor for device applications in visible and deep ultraviolet (UV) spectral regime. First-Principle Density-Functional Theory calculations are employed for the investigation of optoelectronic properties of the dilute-anion III-Nitride semiconductors, which includes the understanding of alloy band structures and related band parameters. Among the dilute-anion III-Nitride semiconductor material class, dilute-As GaNAs, dilute-P GaNP and dilute-As AlNAs are extensively studied in this work. The findings show that the incorporation of anion-content in the GaN or AlN alloy will result in significant changes in the electronic properties, leading to unique features as compared to the conventional III-Nitride alloys such as InGaN and AlGaN alloys. Specifically, the investigation in the electronic properties of dilute-As GaNAs and dilute-P GaNP alloys result in suppression of interband Auger recombination process – a known efficiency-limiting issue in the InGaN quantum well (QW) light emitting diode devices., Further analysis are performed to design novel active region nanostructure of InGaN / dilute-As GaNAs interface QW for visible light emission. The analysis tindicate significantly enhanced spontaneous recombination rate and optical gain across the visible spectral regime from blue to red by using InGaN / dilute-As GaNAs interface QW, as compared to conventional InGaN QW. In the case of dilute-As AlNAs semiconductor, the analysis shows that the incorporation of minute amount of As-content in the AlN alloy will result in the switching of crystal field field split-off band with the heavy hole / light hole band, potentially solving the valence band crossover issue persisting in the AlGaN deep ultraviolet light emitting devices.In addition, extensive studies have been focused in the development of Auger recombination model taking into account the interface roughness in the QW, and analytical solutions for direct Auger recombination processes including interband Auger process for semiconductors. Specifically, the developed Auger model with interface roughness are important to provide intuitive insight of the role of Auger recombination process in the semiconductor devices employing nanostructures

1.3micron quantum dot lasers and superluminescent light emitting diodes

The work described in this thesis involves the development of gallium arsenide based quantum dot lasers and superluminescent light emitting diodes (SLEDs) emitting around 1.3 pm. Initially, the improvement in overall temperature characteristics of a 5 DWELL 1.3 pm quantum dot laser is described through a change in the fabricated device design incorporating a shallow ridge etch and selective gold electroplating. This improved fabrication technique allowed higher temperature ground state operation of the laser and an improvement of 10K in the characteristics temperature at a temperature range higher than 60°C. Later a novel method to broaden the emission spectrum of a SLED by incorporating different amounts of indium in different wells of a DWELL structure is proposed and described. For this device 85nm broad emission spectrum is obtained along with 2.5mW of CW output power at room temperature. Further modification of this structure resulted in a SLED with >8mW CW output power and a 95nm wide, flat emission spectrum at room temperature. In the last part of this thesis a new growth mechanism is described to improve the overall performance of lasers, SLEDs and mesa diodes. For the laser structures lower threshold current density, higher efficiency and lower transparency current densities are observed while the SLEDs emitted >40mW CW output power at room temperature which linearly increased with drive current. Also the mesa diodes exhibited lower reverse leakage current and higher breakdown voltages

Investigation and Analysis of Phenomena in the Far-Infrared Region

The far-infrared (IR) frequency range, or more specifically the terahertz (THz) frequency range, of the electromagnetic spectrum has been comparatively underutilised in the field of solid-state vibrational spectroscopy. This was historically owing to a lack of coherent sources and detectors but now stems from the challenges involved with the interpretation of the resultant spectra as the excited motions in this frequency range consist of complex motions that must be deciphered with the aid of computational simulation using methods such as Density Functional Theory (DFT). This, in turn, gives rise to its own challenges as the accurate simulation of large organic molecular crystals can be computationally expensive. In DFT, weak intermolecular bonds such as H-bonds are poorly represented and an empirical correction is often included to account for them. These are called dispersion corrections and an investigation into the appropriate dispersion correction for α-Lactose Monohydrate (α-LM) was conducted. This molecule was chosen owing to its uncommonly sharp absorption peaks in this frequency range. DFT uses the harmonic approximation to calculate vibrational mode frequencies but this will inevitably remove important anharmonic effects, such as thermal expansion, from the calculation which may reduce correlation between calculated and experimental spectra. The Quasi-Harmonic Approximation (QHA) allows for thermal expansion to be incorporated into the system by applying the harmonic approximation to a range of volumes. This was used to calculate the thermodynamic properties and temperature dependent mode properties of α-LM. This in turn was used to calculate the THz absorption spectrum of α-LM at 300 K. The primary source of THz radiation for broadband spectroscopy is photoconductive switches. These are excited with femtosecond laser pulses that are focused onto a gap between two electrodes. An investigation into the appropriate gap-size for a photoconductive switch being excited by a 150 mW fibre laser was carried out, with the appropriate gap-size being determined to be 20 µm