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This thesis describes new signal processing techniques applicable to miniature low finesse Fabry-Perot interferometric sensors. The principle of operation behind the techniques presented resides in the use of the axial modes from a single multimode laser diode to produce a series of phrase shifted interferometric outputs in conjunction with the path imbalance of the sensor microactivity.

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A novel accelerometer based on light modulation has been designed and a prototype device manufactured. The device utilises the change in refractive index brought about by stress induced by the applied vibration. A detailed mathematical analysis of several feasible sensing designs has been performed to aid the design process. A mathematical model has been developed to assess the performance characteristics of the light modulated accelerometer the results of which were also used as a design tool. The prototype accelerometer was tested, from 1 g to 50 g between frequencies of 25 Hz to 2000 Hz, on a vibration system under three modulation schemes. The acceleration response of the device was seen to be linear over the testing range whilst the frequency response dropped off initially and levelled off at approximately 1 kHz. An experimental accelerometer was also assembled on the vibration table so that various materials could easily be tested without having to undergo precise machining. The acceleration and frequency responses showed similar behaviour to those obtained with the prototype accelerometer. However, the actual response levels varied with each material. To aid in the development of the accelerometer the stress-optic and thermo-optic coefficients have been determined for a range of polymeric materials. The stress optic coefficient was determined for polycarbonate, polymethyl methacrylate, polvinyl chloride and araldite epoxy resin using a circular polariscope and two interferometer configurations up to the yield stress of the materials tested. Each material showed a constant coefficient over the testing range. The results obtained using each technique were in good agreement with each other and the limited literature data available. The thermal variation of refractive index was also determined for the same materials. The Abbe refractometer was used for the determination between 5 and 140°C using five wavelength sources and two interferometer configurations using a HeNe laser from -50°C to approximately 30° above the glass transition temperatures. The change in index was found to be linear over the temperature range tested. However, at the glass transition temperature a change in gradient was observed with each material. Two simple mathematical relationships were used to predict the thermo-optic coefficient. These gave values reasonably close to those obtained in experiment.

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International Telemetering Conference Proceedings / October 26-29, 1992 / Town and Country Hotel and Convention Center, San Diego, California / A fiber optic event timing system was developed for the High Speed Test Track at Holloman Air Force Base, Alamogordo, NM. The system uses fiber optic sensors to detect the passage of rocket sleds by different stations along the track. The sensors are connected by fiber optic cables to an electronics package that records the event time to a resolution of 100 nanoseconds. By use of a GPS receiver as the timebase, the event time is stored to an absolute accuracy of 300 nanoseconds. Custom VMEbus boards were developed for the event timing function, and these boards are controlled by a programmable high speed sequencer, which allows for complicated control functions. Each board has 4 electro-optic channels, and multiple boards can be used in a VMEbus card cage controlled by a single board computer. The system has been tested in a series of missions at the Test Track.

Many properties of polymers can be affected by dissolving gases and supercritical fluids at high temperatures and pressures. Solubility and diffusivity are crucial parameters in polymer processing applications that indicates the content of gases and supercritical fluids in a polymer. Hence, different devices for measuring solubility and diffusivity have been researched, but most of the devices used today are very complex, expensive, and requires long experiment time. In this final thesis, the feasibility of fiber optic sensors as measurement devices for solubility and diffusivity of gas/SCF in polymers have been investigated. Many of the polymers used in polymer processing have high refractive index, from 1.40 to 1.60. However, most of the refractive index sensors based on fiber optics only operate in refractive index ranges of 1 to 1.44 because once the surrounding refractive index becomes greater than that of cladding, the total internal reflection is lost and only small portion of the light propagation occurs. This final thesis first reviews the current methods to measure solubility and diffusivity of gases and supercritical fluids in polymers. In addition, different types of fiber optics sensors used for sensing the refractive index are reviewed. Then, the thesis presents cost efficient, but effective fiber optic refractive index sensors, which are the silver nanoparticle coated LPG sensor, uncoated PCF MZI sensor, silver nanoparticle PCF MZI sensor, and the transmission intensity based gap sensor, to sense the surrounding refractive index in the region greater than the cladding, for the future application of solubility and diffusivity measurement. Moreover, future works that would help in sensing solubility and diffusivity of gas in polymers are also proposed. / Graduate

Steam reforming of methanol and hydrocarbon are currently the processes of choice to produce hydrogen. Due to the endothermic nature of these reactions, zones of low temperature are commonly found in reformers. These zones can potentially damage the reformer through thermal stresses. Moreover, the response time and size of a reformer are controlled by the heat available to the reaction. The objective of this thesis is to demonstrate the feasibility of using fiber Bragg gratings as an alternative solution for temperature monitoring in methanol and methane steam reformers. To meet this objective, a sensor array containing seven gratings is placed in a metal-plate test reformer. First, temperature monitoring during methanol steam reforming is conducted in 12 different sets of conditions. The resulting profile of the temperature change along the length of the catalyst captures the zones of low temperature caused by the endothermic nature of the reaction. Several small changes in the temperature profile caused by increasing temperature and/or flow rates were captured, demonstrating the ability to use these gratings in methanol steam reforming. Similar experimental work was conducted to validate the possibility of using fiber Bragg gratings as temperature sensors in methane reforming. Using a regenerated grating array, data was collected for 13 operating conditions. The conclusions arising from this work are similar to those drawn from the methanol steam reforming work. The regenerated FBGs exhibited a behaviour that has not been reported in the literature which is referred to in this thesis as secondary erasure. This behaviour caused some instability in the grating signal and erroneous readings for some operating conditions. Despite this, the grating measurements captured the zones of low temperatures in the reformer and the small changes brought about by increasing the reforming temperature and lowering the steam to carbon ratio. / Graduate

We combine the evanescent field (EF) sensing mechanism with the fiber loop ringdown (FLRD) sensing scheme to create FLRD-EF sensors. The EF sensor heads are fabricated by etching the cladding of a single-mode fiber (SMF), while monitoring the etching process by the FLRD technique in real-time, on-line with high control precision. The effect of the sensor head dimensions on the sensors' detection sensitivity and response time are investigated. The EF scattering (EFS) sensing mechanism is combined with the FLRD detection scheme to create a new type of fiber optic index sensor. The detection limit for an optical index change is 3.2×10-5. This is the highest sensitivity for a fiber optic index sensor so far, without using any chemical-coating or optical components at the sensor head. A new type of index-based biosensor using high sensitivity FLRDEFS technique to sense deoxyribonucleic acid (DNA) and bacteria (Escherichia coli) is created.

Distributed optical fiber sensors offer unprecedented advantages, and the most remarkable one is the ability to continuously measure physical or chemical parameters along the entire optical fiber, which is attached to the device, structure and system. As the most recently investigated distributed optical fiber sensors, phase-sensitive optical time domain reflectometry (φ-OTDR), Brillouin optical time domain analysis (BOTDA) and Brillouin dynamic grating-optical time domain reflectometry (BDG-OTDR) techniques have been given tremendous attention on the advantage of quantitative measurements ability over high sensitivity and absolute measurement with long sensing distance, respectively. However, the accompanying limitations in terms of static measurement range, acquisition rate, laser frequency drifting noise, and spatial resolution limitations in these techniques hinder their performance in practical applications. This thesis pays particular attention to the above three distributed sensing techniques to explore the fundamental limitations of the theoretical model and improve the sensing performance. Before presenting the novel sensing scheme with improved sensing performance, an introduction about distributed fiber optical sensing, including three main light scattering mechanisms in optical fiber, the recent advancements in distributed sensing and key parameters of Rayleigh scattering- and Brillouin scattering-based sensing systems. After that, a study on the theoretical analysis of large chirping rate pulse generation and the theoretical model of using chirped pulse as interrogation signal in φ-OTDR, BOTDA and BDG-OTDR systems are given. In the disruptive experimental implementations, the sensing performance has been improved in different aspects. By using a random fiber grating array as the distributed sensor, a high-precision distributed time delay measurement in a CP φ-OTDR system is proposed thanks to the enhanced in-homogeneity and reflectivity. In addition, a simple and effective method that utilizes the reference random fiber grating to monitor the laser frequency drifting noise is demonstrated. Dynamic strain measurement with a standard deviation of 66 nε over the vibration amplitude of 30 με is achieved. To solve the limited static measurement range issue, a multi-frequency database demodulation (MFDD) method is proposed to release the large strain variation induced time domain trace distortion by tuning the laser initial frequency. The maximum measurable strain variation of about 12.5 με represents a factor of 3 improvements. By using the optimized chirped pulse φ-OTDR system, a practical application of monitoring the impact load response in an I-steel beam is demonstrated, in which the static and distributed strain variation is successfully reconstructed. To obtain an enhanced static measurement range without a complicated database acquisition process, a photonic approach for generating low-frequency drifting noise, arbitrary and large frequency chirping rate (FCR) optical pulses based on the Kerr effect in the nonlinear optical fiber is theoretically analyzed and experimentally demonstrated by using both fixed-frequency pump and chirped pump. Due to the Kerr effect-induced sinusoidal phase modulation in the nonlinear fiber, high order Kerr pulse with a large chirping rate is generated. Thus the static measurement range of higher order Kerr pulse is significantly improved. Chirped pulse BOTDA based on non-uniform fiber is also analyzed, showing a high acquisition rate that is only limited by the sensor length and averaging times due to the relative Brillouin frequency shift (BFS) changes are directly extracted through the local time delays between adjacent Brillouin traces from two single-shot measurement without frequency sweep process. BFS measurement resolution of 0.42 MHz with 4.5 m spatial resolution is demonstrated over a 5 km non-uniform fiber. A hybrid simultaneous temperature/strain sensing system is also demonstrated, showing a strain uncertainty of 4.3 με and temperature uncertainty of 0.32 °C in a 5 km non-uniform fiber. Besides, the chirped pulse is also utilized as a probe signal in the Brillouin dynamic grating (BDG) detection along the PM fiber for distributed birefringence variations sensing. The strict phase-matching condition only enables part of the frequency components within the chirped probe pulse to be reflected by BDG, giving an adjustable spatial resolution without photo lifetime limitation. The spatial resolution is determined by the frequency chirping rate of the probe pulse.