Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2004. / Includes bibliographical references (leaves 194-209). Also available in electronic version. Access restricted to campus users.

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 82-84). Available also in a digital version from Dissertation Abstracts.

No description available.

M.Ing. / The development of fibre optic technology as we know it today, has taken a giant leap forward since the introduction of Internet technology and other telecommunication devices. More recently, the need exists to retrieve and send huge amounts of data fast and efficiently by using fibre optic cable in computer networks, without the need of expensive equipment. Due to the large amounts of data (like video and audio) that has to be sent and received across a computer network at large distances, this dissertation is primarily concerned with the design and manufacturing of low-loss uniform long period fibre gratings for the realization of future optical add/drop multiplexers. An azimuthally symmetric manufacturing method is implemented using several optical components and devices. Several electronic devices are connected to a Mannesmann Rexroth ECODRIVE03 drive controller unit, and a computer control program was written in VISUAL BASIC 6.0 to control these devices during the manufacturing of these unique LPGs. A number of theories and numerical methods such as the coupled-mode theory and the transfer matrix method are applied in the analysis, modelling, and simulation of short/long-period fibre gratings. Simulation results were obtained in MATLAB 6.5 and the code was written in such a way that the end user could simulate any short/long-period grating and obtain the necessary results such as the transmission spectra, time delay and dispersion for uniform fibre gratings. These simulations obtained were a good indication on how the experimental results should look after manufacturing the unique symmetric LPGs.

M.Ing. / Bragg gratings exist due to permanent changes in the refractive index introduced in the fibre through exposure to ultraviolet radiation. Standard telecommunications fibre (SMF-28) is only slightly photosensitive at 248 nm irradiation. A comparative study of different photosensitization techniques is undertaken on the basis of the desired photosensitivity characteristics and efficiency of the technique used. The analytical method is used to determine valuable properties of fibre Bragg gratings. The bandwidth and maximum reflectivity of the fibre Bragg grating is dependant on the change of refractive index and is determined analytically. Obtaining very high reflectivity and bandwidth (~lnm) can only be achieved with highly photosensitive optical fibre where refractive index changes of~ 1 o-3 are possible. The change of the refractive index in the optical fibre is related to the phase change by: A new interferometric measurement technique based on this assumption allows the direct measurement of photosensitivity. This technique is modelled with a modified Layer Peeling algorithm. Standard telecommunications fibre was exposed to high-pressure (26B - 160B) hydrogen for several days. This increased the photosensitivity of the optical fibre significantly. The photosensitivity of the fibre is directly dependent on the hydrogen concentration inside the fibre. Refractive index changes, M ~ 1.3x10-3 were achieved in germanium doped fibre and M ~ 5x10-3 in germanium/boron codoped fibre. The knowledge of the hydrogen concentration inside the fibre is important in studying photosensitivity, transmission losses and the wavelength drift after Bragg grating manufacturing. The diffusion proceeds interstitially with no significant chemical interaction. A hydrogen diffusion model was developed based on the transfer of heat between two objects. Although the photosensitivity phenomenon was discovered in optical fibre more than 20 years ago, no complete physical explanation exist for it at present. We agree that stress relaxation and/or compaction are the main reasons for photosensitivity in optical fibre but also that it is still a complex and multifaceted phenomenon. A study of the thermal decay of fibre Bragg gratings suggests that fibre Bragg gratings written in hydrogen loaded fibre is less thermally stable than gratings written in germanium doped fibre. The analysis of accelerated ageing will predict the thermal stability of the Bragg grating over time.

D.Ing. / This work comprises a study of reflective interferometric fibre optic sensors. The use of Bragg gratings, multilayer quarter-wave stacks, and rugate mirrors for fibre optic sensing applications is discussed in this work. Rugate mirrors are presented in more detail since they form an important part of the research. The control system for an electron cyclotron resonance plasma enhanced chemical vapour deposition reactor was developed for the growth of inhomogeneous dielectric filters. The system is intended to control the growth of films of the required refractive index profile for optical applications on different substrates including fibre ends. The system also includes an automatic microwave tuner. Rugate mirrors deposited directly on optical fibre were used in a dual cavity Fabry- Perot interferometric strain sensor. It was found by computer simulation that reflectances of 40% for the two outer mirrors and 68% for the centre mirror allow the interferometer to have maximum fringe amplitude. The sensor was evaluated experimentally as a strain gauge. The maximum strain applied to the sample was about 0.12% and the corresponding phase change was about 800 radians. The discrepancy between the readings of this sensor and those of a resistive strain gauge, applied to the same structure, did not exceed 5%. Two novel fibre optic sensors were developed during this work: a merged Sagnac- Michelson interferometric sensor for distributed disturbance detection, and a disturbance location sensor using modified Sagnac and Mach-Zehnder interferometers. Both are intended for distributed impact location along the fibre. The magnitude of impact can also be measured with these sensors. The merged Sagnac-Michelson interferometric sensor uses two light sources and a frequency selective mirror to separate the Michelson and Sagnac signals. Birefringence in the fibre was used to bias the Sagnac interferometer to have a sine response. Computer simulations yielded the optimal biasing conditions: the state of linear polarisation of the input light should be rotated by r/4 with respect to the horizontal axis; birefringence in the Sagnac loop must provide retardation of ff/2 with the optical axes rotated by K/4 with respect to the horizontal axis. We verified the sensor concept experimentally. The discrepancy between measured and actual locations of disturbances applied to the fibre did not exceed 2.5 m for a 200 m long sensing loop. The sensor using the modified Sagnac and Mach-Zehnder interferometers makes use of phase modulation and synchronous detection to obtain the sine-biased Sagnac signal. A modified Sagnac interferometer configuration that incorporates an additional coupler and a mirror, allows separation of the Sagnac and Mach-Zehnder signals. Operation of the new configuration was verified experimentally in the system with a sensing fibre of 100 m long. The discrepancy between measured and actual locations of disturbances applied to the fibre did not exceed 2 m.

No description available.

A class of low nonlinearity dispersion-shifted and dispersion-flattened fibers for broadband and long haul applications is presented. The refractive index profiles of these fibers assume a depressed-core multi-clad geometry in order to achieve effective-areas much larger than those in conventional optical fibers. A systematic approach for designing large effective-area dispersion-shifted fibers, using a reference W-index profile to initiate the design, is presented. Transmission properties, including effective-area, mode-field-diameter, dispersion, dispersion slope, cutoff wavelength, and bending, microbending and splice losses are evaluated for several design examples. To ascertain that the proposed fibers can be practically fabricated, the effects of varying fiber dimensions and indices on effective-area, mode-field-diameter and dispersion are assessed. It is shown that there is a trade-off between effective-area and mode-field-diameter and, generally, larger effective-areas are associated with larger mode-field-diameters. In other words, less signal distortion due to fiber nonlinearity (larger effective-area) is associated with higher power loss due to bending of fiber (larger mode-field-diameter). Thus, a large effective-area and low bending loss are conflicting requirements. A parameter Q is defined as a performance indicator, considering effective-area and mode-field-diameter. Dispersion-shifted single-mode fiber designs with effective-areas of 78 μm² to 210 μm² and the corresponding mode-field-diameter of 8.94 μm to 14.94 μm, dispersion less than 0.07 ps/nm.km, and dispersion slope of about 0.05 ps/ nm².km are presented. Numerical simulations for propagation of pulses in few designed fibers are performed.Designs of large effective-area dispersion-flattened fibers are also presented, for the first time we believe. These fibers provide large effective-area and low dispersion over an extended range of wavelengths. For our design, over the wavelength range of 1.48 μm < λ < 1.58 μm, the effective-area is 75 μm² to 100 μm², while the dispersion remains below 0.7 ps/nm.km. / Ph. D.

In this research a suspected "gray" area in computer network performance that might exist between the network's optimum performance and complete network failure was investigated. Changes in performance were studied as seen by the application layer of the network as attenuation was introduced at the physical layer of the network. The network performance was measured for four groups of tests running over TCP and UDP. Various scripts emulating different network applications were used. Ultimately the performance was evaluated at different optical power levels, based on throughput, transaction rate, response time, and lost data. Additionally, the bit error rate characterization of the attenuated optical fiber link was analyzed. Ultimately, performance of the fiber optic ATM-based computer network as seen by the user was correlated with optical power degradation introduced in the physical layer. A decrease in performance was found as attenuation was increased, even in the range of "normal" operating conditions. As expected, TCP, although not immune to soft failures, performed better than UDP. A description of the investigation, tests, scripts, methods and results are included. / Master of Science

This research provides a method of using a mixed powder in tube approach for producing and characterizing large quantities of highly oriented, high aspect ratio semiconductor nanowires in an inherently safe and contained manner. This work modifies the previously used mixed powder method to produce significantly smaller features below 100nm in diameter. For the first time SiGe alloys are produced in optical fiber from a mixture of the two powders across the entire compositional range. A discussion of the properties of silicon and germanium and their alloys is given with emphasis on the differences between properties at the bulk scale and at the nanoscale. The limitations of silicon and germanium for photonic applications, due to their indirect band gap nature, is removed when these materials are reduced to the nanoscale. A brief discussion of ways that these properties can be modified is given with size, composition, and strain all being viable factors of control. The optical and electrical properties of these nanowire arrays is evaluated as a function of the size, number of wires, and composition. A clear dependence between size and quantity of wires was observed with respect to composition. The nanowires were found to have complex interactions with light showing high absorption as well as unique transmission characteristics. Arrays of these fibers were able to create a measurable photocurrent and provide potential uses for detection of light and other photonic applications. An understanding of the etching necessary to both expose these nanowires for analysis as well as completely remove them from the glass matrix was developed. Etch rates in these areas was observed to be higher than reported etch values. Etching with dilute solutions was found to allow removal of the wires cleanly and allow recovery of them for other applications. / Master of Science / This research provides a method of using a mixed powder in tube approach for producing and characterizing large quantities of highly oriented, high aspect ratio semiconductor nanowires in an inherently safe and contained manner. These wires are over 1000 times smaller than thickness of a human hair are made using traditional fiber drawing methods or pulling at high temperatures. These fibers differ from traditional optical fibers in that they are produced from a tube filled with powder instead of a solid glass rod. This is similar to the same method used to produce wires in other materials such as copper. The use of the glass to contain the semiconductor material allows us to increase the temperature it is pulled at above the melting point. The liquid material is then drawn into the very small sizes using pores in the glass powder it is mixed with. This allows these wires to be produced in much longer lengths, larger quantities, and easier than previous methods. These nanowires are produced from silicon and germanium, which are two of the most important materials currently used in electronics. These semiconductors are used in most electronics, solar cells, and LEDs that are used in everyday life. Silicon and germanium while very important materials have limitations in photonic applications, interactions with light. The properties of the materials for these applications can be improved by reducing them in size to the nanoscale. The wires produced in this research were evaluated to determine if they possessed the more ideal properties. The wires were found to have detectable photocurrent, electricity generated from light. This is the primary property that is needed in solar cells. The wires produced in this method are an important early step to improving solar cells efficiency and reliability. These v wires have benefits over other forms of silicon because they are produced with protective glass coating in a single step.