Multilayer optical learning networks

A new approach to learning in a multilayer optical neural network based on holographically interconnected nonlinear devices is presented. The proposed network can learn the interconnections that form a distributed representation of a desired pattern transformation operation. The interconnections are formed in an adaptive and self-aligning fashioias volume holographic gratings in photorefractive crystals. Parallel arrays of globally space-integrated inner products diffracted by the interconnecting hologram illuminate arrays of nonlinear Fabry-Perot etalons for fast thresholding of the transformed patterns. A phase conjugated reference wave interferes with a backward propagating error signal to form holographic interference patterns which are time integrated in the volume of a photorefractive crystal to modify slowly and learn the appropriate self-aligning interconnections. This multilayer system performs an approximate implementation of the backpropagation learning procedure in a massively parallel high-speed nonlinear optical network

BEAM COMBINING OF FIBER LASERS AND VERTICAL EXTERNAL CAVITY SURFACE EMITTING LASERS USING VOLUME BRAGG GRATINGS

Advances in high brightness laser sources with near diffraction limited beam quality outputs have enabled wide range of applications. However, physical constraints such as material heating and nonlinear effects limit the maximum achievable power of these laser sources. In order to obtain higher power level, beam combining techniques such as coherent beam combining and spectral beam combining are employed to enhance the power and brightness of a single output beam. In this thesis, we investigate various beam combining approaches using a holographic gratings based beam combiner, volume Bragg gratings, for combining the high brightness lasers sources. First, we theoretically study the performance of the beam combiners using the coupled wave equations. Then we conduct beam combining experimental demonstrations of high brightness fiber lasers in both active and passive coherent beam combining schemes using the proposed combiners. Lastly, we explore beam combining methods suitable for high brightness semiconductor lasers with the volume Bragg gratings beam combiners

Electromagnetic Wave-Matter Interactions in Complex Opto-Electronic Materials and Devices

This dissertation explores the fundamentals of light-matter interaction towards applications in the field of Opto-electronic and plasmonic devices. In its core, this dissertation attempts and succeeds in the the modeling of light-matter interactions, which is of high importance for better understanding the rich physics underlying the dynamics of electromagnetic field interactions with charged particles. Here, we have developed a self-consistent multi-physics model of electromagnetism, semiconductor physics and thermal effects which can be readily applied to the field of plasmotronics and Selective Laser Melting (SLM). Plasmotronics; a sub-field of photonics has experienced a renaissance in recent years by providing a large variety of new physical effects and applications. Most importantly, plasmotronics promises devices with ultra-small footprints and ultrafast operating speeds with lower energy consumption compared to conventional electronics. One of the primary objectives of this dissertation is to present an optoelectronic switch termed as Surface Plasmon Polariton Diode (SPPD) for functional plasmonic circuits based on active control of Surface Plasmon Polaritons (SPPs) at degenerate PN+-junction interfaces. In this context, the operational characteristics of the proposed plasmonic device are studied by the self-consistent multi-physics model that couples the electromagnetic, thermal and IV characteristics of the device. The SPPD uses heavily doped PN+-junction where SPPs propagate at the interface between N and P-doped layer and can be switched by an external voltage. Here, we explore the features of SPPD using three different semiconductor materials; GaAs, Silicon and Indium Gallium Arsenide (In0.53Ga0.47As). When compared to Si and GaAs, the In0.53Ga0.47As provides higher optical confinement, reduced system size and faster operation. For this reason, in our dissertation (In0.53Ga0.47As) is identified as the best semiconductor material for the practical implementation of the optoelectronic switch providing high optical confinement, reduced system size, and fast operation. The optimal device is shown to operate at signal modulation surpassing -100 dB and switching rates up to 50 GHz, thus potentially providing a new pathway toward bridging the gap between electronic and photonic devices. Also, the proposed optoelectronic switch is compatible with the current CMOS semiconductor fabrication techniques and could lead to nanoscale semiconductor-based optoelectronics. Furthermore, we have extended the concept of the above optoelectronic switch to design and study a new type of all-optical switch, referred to as Surface Plasmon Polariton Diode (thermal) (SPPDt). The SPPDt operation is governed by a unique optical nonlinearity that exists only for surface electromagnetic waves, i.e. SPPs, propagating at highly doped semiconductor junction interfaces. This dissertation will address the design and characterization of the SPPDt and will bring new insights into the underlying thermo optic nonlinearity. The gained understanding will be applied to design practically feasible devices including logic gates which can bridge the temporal and spatial gap between electronics and optics by providing high switching rates and signal input/output (I/O) power modulation. Enhanced light-matter interactions have further been explored and extended towards tailoring plasmonic resonances due to laser interactions with metal powder beds pertaining to Selective Laser Melting (SLM) processes. This is done by adapting the self consistent model developed for the plasmonic device to better understand the complex electrodynamic and thermodynamic processes involved in SLM. The SLM is an advanced rapid prototyping or additive manufacturing technology that uses high power density laser to fabricate metal/alloy components with minimal geometric constraints. The fabrication process is multi-physics in nature and its study requires the development of complex simulation tools. In this dissertation, for the first time, the electromagnetic interactions with dense powder beds are investigated under full-wave formalism. Localized gap and surface plasmon polariton resonance effects are identified as possible mechanisms toward improved absorption in small and medium-size titanium powder beds. Furthermore, observed near homogeneous temperature distributions across the metal powders indicates fast thermalization processes and allows for the development of simple analytical models to describe the dynamic interplay of laser facilitated Joule heating and effects of radiation and thermal conduction. The Explicit description is provided for important SLM process parameters such as critical laser power density, saturation temperature, and time to melt. Specific guidelines are presented for improved energy efficiency and optimization of the SLM process deposition rates

Tunable mid-infrared light sources based on intersubband transitions

This thesis describes how for the first time, unidirectional operation and coupled ring tuning were realised on a quantum cascade laser material; specifically on a new strain compensated In0.7Ga0.3As/AlAs0.6Sb0.4 grown on InP substrate and operates in pulsed mode in the 3-4 micron hydrocarbon absorption region. Unidirectional ring lasers have the advantages that, in the favoured emission direction, they can have up to double the quantum efficiency of bidirectional lasers and do not suffer from spatial hole burning. In this work, this operation was realised by incorporating an "S"-crossover waveguide into the ring cavity in a manner that it introduces non reciprocal loss and gain in the counter-clockwise (CCW) and clockwise (CW) directions respectively. The measured result showed higher quantum efficiency in the CW. In fact at 1.5 times the threshold current, 90 % of the light was emitted in the favoured CW. On the other hand, the coupled ring quantum cascade laser showed nearly single mode operation, with side mode suppression ratio ~22 dB. Continuous wavelength tuning of about 13 nm was observed from one of these devices, at a tuning rate of approximately 0.4 nm/mA

Advances in Middle Infrared Laser Crystals and Its Applications

In the last twenty years, there has been a growing interest in middle infrared (mid-IR) laser crystals and their application to achieve mid-IR laser radiations, which has benefited from the development of novel mid-infrared crystals and the improving quality of traditional mid-IR crystals. Moreover, these works have promoted the development of related technical applications. This Special Issue of the journal Crystals focuses on the most recent advances in mid-IR laser crystals, from materials to laser sources and applications. It aims to bring together the latest developments in novel mid-IR crystals, improvements in the quality of mid-IR crystals, mid-IR non-linear crystals and mid-IR lasers, as well as the application of mid-IR technology in spectroscopy, trace gas detection and remote sensing, optical microscopy and biomedicine. Aspiring authors are encouraged to submit their latest original research, as well as forward-looking review papers, to this Special Issue