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.

This thesis reports the research and industry-related works carried out from the development of a fibre-optic strain sensor system for Civil Engineering applications. A sensor system consists of a number of core components, including the sensing element, interrogation/demodulation, multiplexing, signal processing and hardware equipment. In the process of development, a number of issues have been identified and investigated, which resulted in the improvement of the system performance, as well as the proposal of new techniques for the sensor system. First, an improved demodulation technique for a type of sensor, namely the fibre Fizeau interferometer (FFI), is presented. The technique is based on the improvement of the Fourier transform peak detection method, which suffers severely from the poor resolution and accuracy of finding the sensor cavity length. The improvement over the original method has been compared and verified through simulations and experiments. Second, a simultaneous demodulation technique for multiplexed FFI and fibre Bragg grating (FBG) sensors using the discrete wavelet transform (DWT) is proposed. Third, a multiplexing technique using amplitude-modulated chirped FBGs and the DWT is proposed. These two proposed techniques have been demonstrated experimentally through strain measurements. The strain resolution, crosstalk and limitations are investigated. In addition, simultaneous quasi-static strain and temperature sensing of different metal plates are performed. Fibre-optic sensors have found numerous applications in different areas. In this thesis, the use of FBG sensors in Civil Engineering applications is demonstrated in four experimental studies, including: (i) long-term measurement of drying shrinkage and creep of structural grade concrete; (ii) simultaneous measurement of shrinkage and temperature of reactive powder concrete (RPC) at early-age; (iii) measurement of coefficients of thermal expansion of cement mortar and RPC; and (iv) field-trial on the strain monitoring of the world?s first RPC road bridge. In addition, the experimental and practical issues of using FBG sensors are considered.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 88-91). Available also in a digital version from Dissertation Abstracts.

No description available.