Semiconductor ring lasers for high speed communications

The work described in this thesis is aimed at exploring the possibility of optically integrating an OTDM transmitter operating at 4X10Gb/s on an appropriate substrate. It has been shown that such an OTDM transmitter system could be integrated on III-V semiconductor quantum well (QW) substrates if the design of the substrate, the choice of fabrication techniques and the design of the devices are carefully considered. Suitable device structures for the three main kinds of devices involved in OTDM transmitters, namely light source, optical multiplexers (couplers) and optical modulators, have been discussed. Significant progress regarding these aspects, both theoretical and experimental, has been achieved. In this work, it has been intended to investigate all the devices from the integration point of view. This has been reflected in many aspects in the device design and fabrication process. Integration has always been a very important factor to consider in the determination of substrate material structure, device configuration, waveguide structure and fabrication techniques. As a result, the devices developed in this project are suitable for the proposed purpose of an integrated OTDM transmitter system. Investigation into integration techniques has also been carried out. The most important was to introduce bandgap difference on a semiconductor QW material. IFVD technique is studied and produced some encouraging results such as the extended cavity SRL, which integrates an active section with a passive MMI coupler. Vertically coupled waveguide structures have also been invstigated in an attempt to produce extended cavity lasers. The design considerations of extended cavity lasers employing this waveguide structure have been discussed

Energy correlations of photon pairs generated by a silicon microring resonator probed by Stimulated Four Wave Mixing

Compact silicon integrated devices, such as micro-ring resonators, have recently been demonstrated as efficient sources of quantum correlated photon pairs. The mass production of integrated devices demands the implementation of fast and reliable techniques to monitor the device performances. In the case of time-energy correlations, this is particularly challenging, as it requires high spectral resolution that is not currently achievable in coincidence measurements. Here we reconstruct the joint spectral density of photons pairs generated by spontaneous four-wave mixing in a silicon ring resonator by studying the corresponding stimulated process, namely stimulated four wave mixing. We show that this approach, featuring high spectral resolution and short measurement times, allows one to discriminate between nearly-uncorrelated and highly-correlated photon pairs.Comment: 7 pages, 4 figure

Integrated optical technologies for analytical sensing

Recent diversification of the telecommunications industry has resulted in the adaptation of optical materials and their associated fabrication technologies for use in the bioanalytical sensor industry. Flame hydrolysis deposited (FHD) planar silica is one such material. Capable of producing high quality films for optical waveguides, the chemical inertness of the deposited silica makes it an ideal substrate from which to fabricate a biological fluorescence sensor. The aim of the work contained in this thesis was to utilise the FHD silica in optical - fluorescence sensors suitable for use at visible and in particular red wavelengths where several fluorophores can be excited, and background fluorescence from the silica is small. New technologies for producing waveguides have been evaluated in the context of their usefulness in optical sensors, with the intention of producing devices with as few fabrication steps as possible to reduce fabrication time and cost. The design, fabrication and testing of a number of sensor configurations is described, in which optical waveguides were interfaced with microfluidic chambers to provide excitation of a fiuorophore in solution. New waveguide fabrication technologies were used for the first time in sensor systems with integrated microfluidic circuits. Waveguides, written by electron beam densification were evaluated in terms of their performance in splitting an excitation signal into several different components, as would be appropriate for excitation of multiple microfluidic chambers - an 'array sensor'. Both Y-branch waveguides and multimode interference (MMI) splitters were successfully used to split the excitation signal. In addition to electron beam densification, UV irradiation at a wavelength of 157 nm was used to write waveguides in FHD silica. The application of a metal surface mask to define the waveguide structures is described. To allow sensitive detection and identification of fluorophores from FHD silica sensor chips, a single chamber device was successfully interfaced to a system to make time resolved fluorescence measurements, a technique known as time correlated single photon counting (TCSPC). The use of TCSPC allowed measurement of the decay time of the fluorescent dye, by which different fluorescent molecules could be identified, as well as the possibility of low concentration measurements. The research has allowed new technologies for creating waveguides in FHD silica to be adapted for sensing purposes, leading to a platform for creating devices in a number of different configurations

Broadband repeatable <0.025 dB average loss rapid adiabatic based 3-dB coupler in a 45 nm SOI CMOS process

We demonstrate a 75 µm-long rapid adiabatic coupler (RAC) with an average insertion loss <0.025 dB/coupler and an average power splitting ratio of 50±1.09% over 40 nm bandwidth and 68 reticles across a 300 mm 45 nm SOI CMOS wafer.Accepted manuscrip

Laser Ablation for Polymer Waveguide Fabrication

An increase in interconnection density, a reduction in packaging sizes and the quest for lowcost product development strategy are some of the key challenges facing micro-optoelectronics design and manufacture. The influence of high-density, small-sized products has placed significant constraints on conventional electrical connections prompting various fabrication methods, e.g. photolithography, being introduced to meet these challenges and ameliorate the rapidly changing demand from consumers. While high-power solid state lasers are fundamental to large scale industrial production, excimer laser on the other hand has revolutionised the manufacturing industry with high precision, easy 3D structuring and less stringent production requirements. Micro-structuring using excimer laser, best known as laser ablation, is a non-contact micro- and nano-machining based on the projection of high-energy pulsed UV masked beam on to a material of interest such that pattern(s) on the mask is transferred to the substrate, often at a demagnified dimension with high resolution and precision. The use of mask with desired patterns and beam delivery system makes the fabrication in this case accurate, precise and easily controllable. The first part of this chapter introduces the fundamentals of laser technology and material processing. In the second part, optical interconnects as a solution to ‘bottlenecked’ conventional copper interconnections is introduced with emphasis on excimer laser ablation of polymer waveguides and integrated mirrors. Key research findings in the area of optical circuit boards using other techniques are also briefly covered

MULTI-MODE AND SINGLE MODE POLYMER WAVEGUIDES AND STRUCTURES FOR SHORT-HAUL OPTICAL INTERCONNECTS

Single mode and multi-mode polymer optical waveguides are a viable solution for replacing copper interconnects as high speed and large bandwidth short-haul optical interconnects in next-generation supercomputers and data servers. A precision laser direct writing method is implemented for producing various single mode and multi-mode polymer waveguide structures and their performance is evaluated experimentally showing agreement with theoretically developed models. The laser direct writing method is the optimal solution for low-rate cost-effective prototyping and large area panel production. A single mode polymer waveguide bridge module for silicon to glass optical fibers is designed, modeled, fabricated, and measured. The bridge module is designed for waveguide pitch control and low coupling loss from high-density silicon photonic interconnects within CMOS devices and optical silica fibers for long-haul low-loss transmission. A fan-out structure using waveguide S-bend structures is utilized to perform pitch control. Optical coupling within the bridge module is achieved through a novel polymer taper structure to reduce the numerical aperture mismatch between silicon waveguides and silica fibers. Research and development has been implemented into the theoretical understanding and experimental assessments of solving practical interconnect challenges for commercial realization of polymer waveguides

Tailorable and Broadband On-Chip Optical Power Splitter

Featured Application Silicon photonics, Quantum integrated photonic circuits, Photonic integrated circuits. Abstract An on-chip optical power splitter is a key component of photonic signal processing and quantum integrated circuits and requires compactness, wideband, low insertion loss, and variable splitting ratio. However, designing an on-chip splitter with both customizable splitting ratio and wavelength independence is a big challenge. Here, we propose a tailorable and broadband optical power splitter over 100 nm with low insertion loss less than 0.3%, as well as a compact footprint, based on 1x2 interleaved tapered waveguides. The proposed scheme can design the output power ratio of transverse electric modes, lithographically, and a selection equation of a power splitting ratio is extracted to obtain the desired power ratio. Our splitter scheme is close to an impeccable on-chip optical power splitter for classical and quantum integrated photonic circuits.11Ysciescopu

Binary particle swarm optimized 2 × 2 power splitters in a standard foundry silicon photonic platform

Compact power splitters designed ab initio using binary particle swarm optimization in a 2D mesh for a standard foundry silicon photonic platform are studied. Designs with a 4.8  μm×4.8  μm footprint composed of 200  nm×200  nm and 100  nm×100  nm cells are demonstrated. Despite not respecting design rules, the design with the smaller cells had lower insertion losses and broader bandwidth and showed consistent behavior across the wafer. Deviations between design and experiments point to the need for further investigations of the minimum feature dimensions

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