Structural born vibration is the most concern issue for industry. Traditionally, the accelerometer is usually used as the major monitoring device for vibration. As the mechanism getting more and more complexity, more compact, tinier and more lighting, the traditional accelerometers are suffered from the loading effect. Its accuracy of measurement is suspected and cannot match the modern measurement requirement. Hence, the studies of fiber optic vibration sensors become an urgent issue in this era. The reflection wavelength of a fiber Bragg grating¡]FBG¡^is sensitive to the variation of the strain and temperature. Our sensor configuration is made of an interferometer and fiber Bragg grating. The vibration induces a strain of the fiber Bragg grating, and it makes a phase difference between those two light beams in the interferometer. A demodulation circuit is needed to detect the phase difference caused by the vibration. In this project, the aim is focused on the vibration measurement for some complicated rotational machines or structures. A fiber optic accelerometer will be designed and studied as a vibration monitor for the other subprojects. In this the thesis, two kinds of vibration sensor head are designed and studied, the first is a bending loss sensor head and the other is an optic fiber Bragg grating sensor head. The results are narrated as follows¡G¡]1¡^ The dynamic range of the bending loss sensing head is about 50 dB.¡]2¡^The dynamic range of the optic fiber Bragg grating sensing head is 38 dB with test frequency range between 100 ~ 400 Hz, the noise level is around 1.95 ¡Ñ 10-2 rad.

The reflection wavelength of Fiber Bragg Grating is sensitive to the strain and the temperature¡¦s variation. We use Fiber Bragg Grating to be the sensor head and measure the vibration frequency in constant temperature environment. The vibration of object can make the sinusoidal strain to Fiber Bragg Grating, and it will make a little phase difference to the light of the fiber. Using the interferometer and demodulation system, we can measure the phase difference and vibration frequency. Our sensor configuration is made up of imbalance Mach-Zehnder interferometer and Fiber Bragg Grating. The two light of different path need different time to pass through the vibration source, so they make phase difference. We use the demodulation circuit to measure the phase difference causing by vibration and get the vibration frequency. Our experiment structure is a novel configuration of Fiber Bragg Grating vibration sensor. Its intensity of signal is larger than the intensity of original sensor configuration, about 4dB.The novel sensor configuration is easier spread than traditional accelerometer and it is designed of all fiber. The accuracy for measuring low frequency vibration is 99.971%. The Dynamic range of the system is more than 45dB. It is larger than the dynamic range of original sensor configuration, about 9dB. The smallest signal that can be measured is about 0.0075rad.

Abstract The electrical microphone has came to maturity, which has some restrictions on high electromagnetic and wet environments¡CFiber-Optic sensor can improve the problems, because it has better characters in electromagnetic interference and wet environment than the traditional microphone. The structure of Sagnac interferometer is circulator, so the design of head to a sensor has to wind fiber around. Because the minimum radius of winded fiber has a threshold, we can not miniaturize the sensor-head. A typical Mach-Zehnder interferometer has to use high-coherence light source and the length of two arms in equality without any interference, so it is difficult in fabrication. If we make a microphone by FBG and Mach-Zehnder interferometer, and the advantage is that we can use low-coherence light source, and shorten the length of two arms in interferometer. By using the structure, the minimum measured pressure of sound is 0.6 Pa, and the dynamic range is 30dB.

In this paper, the fesibility of using a Fiber Bragg Grating (FBG) as a sensing scheme to detect the underwater acoustic signals is analyzed. When a FBG is disturbed by an underwater sound, the wavelength of the FBG is changed. Therefore, the central spectrum of the reflected light is shifted according to the wavelength change of the FBG. This spectrum can be detected by an imbalanced two-arm interferometer. Its transfer function will be studied. Also, the polarization induced signal fading of those two-arm interferometers will be studied.

A humidity fiber optic sensor based on phase-shifted (PS) Fiber Bragg gratings (FBG) is demonstrated. Compared to the standard FBG sensors, the peak of the PS-FBG slips into 2 narrow peaks and forms a sharp dip in the middle. As a result, the resolution of the measurement will be higher. The sensors used in the experiments were fabricated by coating the PS-FBG surface with a moisture-sensitive polyimide and is based on the strain effect caused by the swelling of the coating after moisture absorption. The same trend seen in a standard FBG sensor can be achieved, but with higher measurement resolution in environments differing by humidity and temperature. This thesis presents simulation and measurement results, including sensitivity and response time, of the PS-FBG sensor approach for humidity sensing, as compared to the standard FBG sensors. Stability and hysteresis are also discussed.

Efficient and accurate sensing of various parameters is needed for numerous applications. In this regard, different categories of sensors play a significant role and different applications require diverse sensing mechanisms owing to the operating conditions and field constraints. Among the several sensor methodologies available, optical fiber sensors have found significant attention, because of their advantages such as negligible foot print, small mass, immunity to Electromagnetic Interference, etc. In the category of optical fiber sensors, Fiber Bragg Grating (FBG) sensors have found importance in many fields such as health monitoring of civil structures, environmental monitoring involving gas & humidity sensing, monitoring parameters like pressure, tilt, displacement, etc. In the recent times, FBGs have found applications in biomedical, biomechanical and biosensing fields. A FBG is a periodic change of the refractive index of the core of a single mode optical fiber along its longitudinal axis. The periodic modulation in the index of refraction is obtained by exposing a photosensitive germanium-doped silica fiber to an intense UV laser beam. FBGs, in the basic form, can sense strain and temperature. However, in recent years, several newer sensing applications of FBGs have been demonstrated. Some of the main features of the FBG sensor which qualify them for diverse sensing applications are high sensitivity, large operational bandwidth, multiplexing & multi modal sensing capability, etc. In this thesis work, FBG sensor based devices have been developed for newer applications in bio-medical fields for the measurement of force, flow and temperature. Particularly, novel transduction methodologies have been proposed, in order to convert the measurand parameter into a secondary parameter that can be sensed by the FBG sensor. The evaluation of the force required for a spinal needle to penetrate various tissue layers from skin to the epidural space is vital. In this work, a novel technique for dynamic monitoring of force experienced by a spinal needle during lumbar puncture using Fiber Bragg Grating (FBG) sensor has been developed. The Fiber Bragg Grating Force Device (FBGFD) developed, measures the force on the spinal needle due to varied resistance offered by different tissue layers during its traversal. The effect of gauge of the spinal needle used for the lumbar puncture procedure affects the force required for its insertion into the tissue. The FBGFD developed, has been further utilized for a comparative study of the force required for lumbar puncture of various tissue layers with spinal needle of different gauges. The results obtained may serve as a guideline for selection of suitable gauge spinal needle during lumbar puncture minimizing post puncture side effects on patients. The pulmonary function test carried out using a spirometer, provides vital information about the functional status of the respiratory system of the subject. A Fiber Bragg Grating Spirometer (FBGS) has been developed which has the ability to convert the rate of air flow into a shift in wavelength that can be acquired by the FBG sensor. The FBGS can dynamically acquire the complete breathing sequence comprising of the inhalation phase, pause phase and exhalation phase in terms of the air flow rate along with the time duration of each phase. Methods are adopted to analyse and determine important pulmonary parameters using FBGS and compare these parameters with those obtained with a commercially available hospital grade pneumotachograph spirometer. Thermal imaging is one of the emerging non-invasive neuro-imaging techniques which can potentially indicate the boundaries of a brain tumor. The variation in tissue surface temperature is indicative of a tumor existence. In this work a FBG temperature sensor (FBGTS) has been developed for thermography of a simulated tissue using Agar material. The temperature of the embedded heater which mimics a brain tumor along with the surface temperature of the tissue model, is acquired using FBGTSs simultaneously. Further, the surface temperatures are studied for varying heater temperatures as well as varying positions of the heater in the simulated tissue model. To conclude, FBG based devices have been developed in this work, for applications in biomedical domain, with appropriate transduction methodologies for sensing different parameters such as force, flow and temperature.

Partially coherent interaction of the feedback light with the field in the laser cavity is affirmed with the fiber Bragg grating (FBG) stabilized 980-nm pump lasers, on the contrast of normally accepted totally incoherent state of operation in the “coherence collapse” regime. Coherence parameter y was defined in this paper to identify the fraction of feedback light working coherently. It is shown that y can be determined by fitting the measured power-difference versus pumping-rate curve to the simulation results. Experiments confirm that coherence parameter y decreases while the distance between the FBG and the laser facet increases, and vice versa. While, if the device is kept operating in the “coherence-collapse” regime, y would not change with the amount of feedback. This work will be help to improve the performance of the high power FBG stabilized 980-nm pump laser. / Thesis / Master of Applied Science (MASc)

Telecommunication networks based on optical fiber technology have become a major information-transmission system, satisfying the growing demand for bandwidth due to increased internet traffic and other applications such as video on demand, etc. Fiber Bragg gratings (FBGs), in recent years, have emerged as critical components for enabling high-capacity transmission since their response can be tailored to meet the needs of specific applications. FBGs are currently the focus of intense research interest in both the fiber communications and sensing fields. Optical fiber Bragg grating structures in single-mode fiber (SMFBGs) have been studied extensively since the discovery of photosensitivity in germanium-doped silica fiber. They have been used in numerous applications ranging from wavelength-selective filtering in wavelength-division-multiple-access (WDMA) systems to temperature and strain sensing. To a lesser extent, Bragg gratings in multimode fibers have also received attention because of easy coupling with light sources. Most of the MMFBGs related research work has demonstrated the formation of a Bragg grating in a graded-index MMF and briefly reported the measured transmission spectrum. So far, there are few theoretical studies on Bragg gratings in multimode fibers. In this thesis, we investigate Bragg gratings in multimode optical fibers both theoretically and experimentally. A comprehensive numerical model for MMFBGs has been established and the corresponding computer simulation software (MMFBG simulator combined with mode solver) developed. The optical properties of MMFBGs were systematically studied for the first time using our own MMFBG numerical software package. It effectively assists the design modeling for MMFBG-based optical devices. Bragg gratings in multimode fiber were also investigated experimentally. Our theoretical simulation results show good agreement with experiments and offer the insightful explanations for the underlying physics of the device. First, the guided modes were modeled and simulated for step index multimode fibers and graded index multimode fibers with emphasis on parabolic fiber structure. These are popular, standard and commercially available MM fibers, and employed throughout our experiments. This allows us for the simulation of fiber characteristics such as cut-off wavelength, mode effective index, propagation constants and optical field distribution. It also allows for calculation of mode coupling coefficients by overlap integral between any chosen guided modes. Therefore, it serves as a powerful model for the design and analysis of optical fibers. Second, the generalized MMFBG coupled mode theory formalism is derived. The physical mechanism of the behavior of MMFBGs is studied and discussed. The general solution to the MMF Bragg grating problem is achieved by Runge-Kutta, Newton-Raphson and shooting numerical methods. Our theoretical treatment, in particular, offers the advantages which can deal with not only self-coupling but also more complicated cross-coupling interactions and can solve arbitrary large number of mode coupling problems throughout the entire spectra simultaneously for multimode FBGs, thus allowing for a precise and quantitative study of MMFBGs. Such an intensive multimode fiber Bragg grating physical modeling and simulations have not been reported previously. It provides an effective means for the design and analysis of optical fiber devices based on Bragg gratings. Third, the optical properties of multimode FBGs were studies experimentally. Numerical predications of the grating spectral characteristics under fabrication and experimental condition are calculated. The results of the numerical calculations are compared with experimentally measured spectra of multimode gratings written by ultraviolet irradiation of deuterium-sensitized fiber with grating reflectivities ranging from 78% to 99.39%. Good agreement is obtained between the theoretical simulations and the experimental results. Thus, we provide quantitative explanations for the observed experimental phenomena. These explanations give both physical insight and a more complete understanding of the nature of the interaction between the wave propagation and multimode fiber gratings. Furthermore, the spectral simulation of the actual experiments prepares a theoretical guidance for the advanced experimental investigation and also presents a step toward MMFBG device design. Finally, the optical properties of MMFBGs were also studied theoretically. To our knowledge, this is the first detailed analysis and thorough investigation on grating characteristics in MMF. It is demonstrated that the transmission and reflection spectra of fiber Bragg gratings in multimode optical fibers strongly depend on the length of grating, index modulation, period of grating, mode excitation condition and physical structure of MMF. The simulation results allow us to deeply comprehend and visualize the more sophisticated behavior within a multimode fiber grating, and will also allow us to confidently predict and evaluate the performance of more complex structure MMFBGs. It provides the fundamental principles for designing the targeted spectrum performance and settles the theoretical rationale for realizing the practical applications. Overall, the comprehensive numerical model and MMFBG solver package developed in this thesis opens a clear and broad window for understanding MMFBG mechanisms from the physical point of view. Various simulation results and spectral characteristics have been researched and discussed under both ideal and experimental conditions for the purpose of experimental analysis and device design. The results of our study indicate that a new class of potential applications based on MMFBGs can be expected in optical fiber sensors and advanced communication systems. / Thesis / Master of Applied Science (MASc)

Invention of silica based optical fiber not only led to revolution in communication but also provided fundamental basis for many research areas. One example area is distributed optical fiber sensing, which has been attracting research interests for decades. Optical fiber sensors are immune to electromagnetic interference, and resistant to corrosion and can endure harsh environment so they have found applications such as structural health monitoring, intrusion detection and oil downhole measurement. Significant research efforts have been paid to fiber sensing area, many techniques have been developed and some of them have been successfully demonstrated, however achieving both high-speed and long-range is still under intensive research. This dissertation proposes and demonstrates a technique with the capability of simultaneous long-range and high-speed sensing by employing serial ultra-weak fiber Bragg gratings (UW-FBGs) and dispersive components. Various factors which have influence on the system performance, including wavelength resolution, spatial resolution and sensing rate, are analyzed. Different types of light sources and dispersive units were designed and a sensing system was built. With this system, both static and dynamic response were measured, and a sensing link consisting of more than 2000 UW-FBGs was successfully measured at the speed of 20kHz. The noise sources of the system were also theoretically analyzed and experimentally measured. This demonstrated sensing technique can be applied to long range temperature and strain sensing. / Ph. D.

Theoretical and experimental studies of real-time interrogation of fiber Bragg grating (FBG) sensors based on chirped pulse compression with increased interrogation resolution and signal-to-noise ratio are presented. Two interrogation systems are proposed in this thesis. In the first interrogation system, a linearly chirped FBG (LCFBG) is employed as the sensing element. By incorporating the LCFBG in an optical interferometer as the sensor encoding system, employing wavelength-to-time mapping and chirped pulse compression technique, the correlation of output microwave waveform with a chirped reference waveform would provide an interrogation result with high speed and high resolution. The proposed system can provide an interrogation resolution as high as 0.25 μ at a speed of 48.6 MHz. The second interrogation system is designed to achieve simultaneous measurement of strain and temperature. In this system, a high-birefringence LCFBG (Hi-Bi LCFBG) is employed as a sensing element.