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.

The most promising strategy for reducing the cost of manufacturing polymer matrix composites while improving their reliability is the use of sensors during processing to permit control of the cure cycle based on measurements of the material's internal state. While sensors have been demonstrated that infer the material state indirectly through measurements of acoustic impedance, electrical impedance, or refractive index, sensors that directly measure parameters critical to composite manufacturing, such as resin rheology and resin hydrostatic pressure, would improve characterization of thermoset resins during cure. Here we describe the development of a multifunctional fiber optic sensor that may be embedded in a composite part during lay-up to monitor the state of the polymer matrix during processing. This sensor will output quantitative data which will indicate the viscoelasticity of the thermoset matrix resin. The same sensor will additionally function as a strain sensor following fabrication, capable of monitoring residual strains due to manufacturing or in-service internal strains. / Ph. D.

The main purpose of the optical fiber sensing technology is to detect perturbation of physical fields. By means of some demodulating scheme, we can extract the real signal from those light beams which modified by physical fields. In the thesis, we proposed a configuration of modified Sagnac Interferometer as a sensing system. The optical sensing and demodulation system are exploited separately. Next, we study the advantages and disadvantages of the configuration. Besides, we are also measured the sensitivity and dynamic range. The sensing system used a low coherence light source to reduce cost. This system also improves the shortage of a Sagnac Interferometer which has a blind point in the middle position. In addition, the structure is easily implemented and can detect weak signal in a high noisy water environment. For matching the main structure, we make many kinds of sensing heads for detecting signals under water. We also use the mathematical model as the base of the theory. The dynamic range is 40 dB and the sensitivity is -231.47 dB re V/uPa.

Femtosecond (fs) laser micromachining has been a useful technique either to modify and remove materials or to change the properties of a material, and can be applied to transparent and absorptive substances. Recently high-power fs laser radiation has drawn intensive attention for the induction of refractive index change to fabricate micro-structures in dielectric materials. This thesis studies the optical properties of a novel fiber random grating fabricated by fs laser micromachining technique and extends its applications from optical sensing to random fiber lasers and optical random bit generations. The thesis mainly consists of three parts. In the first part, the physical mechanism behind the fs laser micromachining technique and the fabrication of the fiber random grating are introduced. By employing a wavelength-division spectral cross-correlation algorithm, a novel multi-parameter fiber-optic sensor based on the fiber random grating is proposed and demonstrated to realize simultaneous measurements of temperature, axial strain, and surrounding refractive index. In the second part, Brillouin random fiber laser (BRFL) and Erbium-doped fiber random laser (EDFRL) are introduced, respectively. Firstly, we propose a novel Brillouin random fiber laser with a narrow linewidth of ~860 Hz based on the bi-directionally pumped stimulated Brillouin scattering (SBS) in a 10-km-long optical fiber. A random fiber Fabry-Perot (FP) resonator is built up through the pump depletion effects of SBS at both ends of the fiber. The novel laser is successfully applied for linewidth characterization beyond 860 Hz of light source under test. Secondly, the random grating-based FP resonator is introduced to build up a novel BRFL with narrow-linewidth of ~45.8Hz and reduced lasing threshold. The intensity and frequency noises of the proposed random laser are effectively suppressed due to the reduced resonating modes and mode competition. Finally, the fiber random grating is used as random distributed feedback in an EDFRL to achieve both static (temperature, strain) and dynamic (ultrasound) parameter sensing. Multiple lasing lines with high signal-to-noise ratio (SNR) up to 40dB are achieved, which gives an access for a high-fidelity multiple-static-parameter sensing application. By monitoring the wavelength shifts of each peak, temperature and strain have been simultaneously measured with small errors. The fiber random grating in the EDFRL is also able to sense the ultrasound waves. By achieving single mode lasing with the EDFRL, ultrasound waves with frequencies from 20kHz to 0.8MHz could be detected with higher sensitivity and SNR improvement up to 20dB compared with conventional piezoelectric acoustic sensors. In the third part, we demonstrate that a semiconductor laser perturbed by the distributed feedback from a fiber random grating can emit light chaotically without the time delay signature (TDS). A theoretical model is developed by modifying the Lang-Kobayashi model to numerically explore the chaotic dynamics of the laser diode subjected to the random distributed feedback. It is predicted that the random distributed feedback is superior to the single reflection feedback in suppressing the TDS. In experiments, The TDS with the maximum suppression is achieved with a value of 0.0088, which is the smallest to date.

The motivation of this research is to meet the growing demand for the measurand high-resolution, high-spatial resolution, attenuation insensitive and low-cost quasi-distributed temperature and strain sensors that can reliably work under harsh environment or in extended structures. There are two main drives for distributed fiber sensor research. The first is to lower cost-per-sensor so that the fiber sensors may become price-competitive against electrical sensors in order to gain widespread acceptance. The second is to obtain spatial distribution of the measurand. This dissertation presents detailed research on the design, modeling, analysis, system implementation, sensor fabrication, performance evaluation, sensor field test and noise analysis of a quasi-distributed intrinsic Fabry-Perot interferometric (IFPI) fiber sensor suitable for temperature and strain measurement. For the first time to our knowledge, an IFPI sensor using a different type of fiber spliced in between two single-mode fibers is proposed and tested. The proposed sensor has high measurement accuracy, excellent repeatability, a large working range and a low insertion-loss. It requests no annealing after the sensor is made, and the sensor is calibration-free. The sensor fabrication is low-cost and has a high yield rate. The goal for this research is to bring this sensor to a level where it will become commercially viable for quasi-distributed sensing applications. / Ph. D.

In this report, detailed and systematic theoretical and experimental study of low-finesse extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensors together with their signal processing methods for white-light systems are presented. The work aims to provide a better understanding of the operational principle of EFPI fiber optic sensors, and is useful and important in the design, optimization, fabrication and application of single mode fiber(SMF) EFPI (SMF-EFPI) and multimode fiber (MMF) EFPI (MMF-EFPI) sensor systems. The cases for SMF-EFPI and MMF-EFPI sensors are separately considered. In the analysis of SMF-EFPI sensors, the light transmitted in the fiber is approximated by a Gaussian beam and the obtained spectral transfer function of the sensors includes an extra phase shift due to the light coupling in the fiber end-face. This extra phase shift has not been addressed by previous researchers and is of great importance for high accuracy and high resolution signal processing of white-light SMF-EFPI systems. Fringe visibility degradation due to gap-length increase and sensor imperfections is studied. The results indicate that the fringe visibility of a SMF-EFPI sensor is relatively insensitive to the gap-length change and sensor imperfections. Based on the spectral fringe pattern predicated by the theory of SMF-EFPI sensors, a novel curve fitting signal processing method (Type 1 curve-fitting method) is presented for white-light SMF-EFPI sensor systems. Other spectral domain signal processing methods including the wavelength-tracking, the Type 2-3 curve fitting, Fourier transform, and two-point interrogation methods are reviewed and systematically analyzed. Experiments were carried out to compare the performances of these signal processing methods. The results have shown that the Type 1 curve fitting method achieves high accuracy, high resolution, large dynamic range, and the capability of absolute measurement at the same time, while others either have less resolution, or are not capable of absolute measurement. Very different from SMF-EFPI sensors, MMF-EFPI sensors with high fringe visibility usually are more difficult to obtain in practice because the fringe visibility of a MMF-EFPI sensor is much more sensitive to gap-length change and sensor head imperfections. %Previously, only geometric-optics are available to analyze MMF-EFPI sensors which approximate the light in MMF as rays propagating in different directions. Geometric-optics theory has fundenmental limitations because it is approximate and only valid for limited conditions. Moreover, geometric-optics theory is not capable of poviding the exact fringe pattern which is important in the signal processing of white light MMF-EFPI sensor systems. In this report, Previous mathematical models for MMF-EFPI sensors are all based on geometric optics; therefore their applications have many limitations. In this report, a modal theory is developed that can be used in any situations and is more accurate. The mathematical description of the spectral fringes of MMF-EFPI sensors is obtained by the modal theory. Effect on the fringe visibility of system parameters, including the sensor head structure, the fiber parameters, and the mode power distribution in the MMF of the MMF-EFPI sensors, is analyzed. Experiments were carried out to validate the theory. Fundamental mechanism that causes the degradation of the fringe visibility in MMF-EFPI sensors are revealed. It is shown that, in some situations at which the fringe visibility is important and difficult to achieve, a simple method of launching the light into the MMF-EFPI sensor system from the output of a SMF could be used to improve the fringe visibility and to ease the fabrication difficulties of MMF-EFPI sensors. Signal processing methods that are well-understood in white-light SMF-EFPI sensor systems may exhibit new aspects when they are applied to white-light MMF-EFPI sensor systems. This report reveals that the variations of mode power distribution (MPD) in the MMF could cause phase variations of the spectral fringes from a MMF-EFPI sensor and introduce measurement errors for a signal processing method in which the phase information is used. This MPD effect on the wavelength-tracking method in white-light MMF-EFPI sensors is theoretically analyzed. The fringe phases changes caused by MPD variations were experimentally observed and thus the MFD effect is validated. / Ph. D.