This thesis details the design, fabrication and measurement of an integrated optical Bragg grating filter, operating at a free space wavelength of 1532 nm, based on silicon-on-insulator (SOI) ridge waveguide. Grating-based integrated devices can interact with optical signals in photonic integrated circuits (PIC) in such a way as to selectively transmit, reflect or detect the signals that are resonant with these devices. Channel filters can access one channel of a wavelength division multiplexed signal without disturbing the other channels and are therefore important elements in WDM communications. Resonator filters are attractive candidates because they can potentially realize the narrowest linewidth for a given device size. Device models for this kind of device are developed by using the MATLAB programming language. Coupled mode theory (CMT) for filters, and the effective index method (EIM) which reduces a three dimensional (3D) analysis into two dimensions is used as modeling theoretical background. Computer modeling identifies the effect of device structure on the performance of the devices, and is also used to predict the output characteristics of this kind of device. This provides an understanding of device physics and operation, and a basis for comparison with experimental results. A common fabrication sequence for integrated optical Bragg grating filters based on SOI ridge waveguides is designed, developed and demonstrated. This includes the photomask for optical ridged waveguide, interferometic lithography for grating pattern and high accuracy RIE etching. This work demonstrates Bragg grating as a technology for realizing PIC in SOI material system, and presents the technology required to design, fabricate, characterize, and model these integrated devices. / 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)

The spectral characteristics of fiber grating-stabilized 980nm pump lasers were studied theoretically and experimentally. A new process of fiber tip flattening making quadrangular-pyramid-shaped fiber lens (QPSFL) was successfully developed. This fiber tip flattening process can improve the yield fabricating QPSEL up to 20% compared with the previous research of the process without tip flattening. The QPSFL was used in coupling between the high-power 980nm laser diodes and the single mode fibers (SMFs). A fiber grating was fused at the end of the single mode fiber. The reflectivity of fiber gratings were 6%, 8% and 10%, and the length of the external cavity was about 2.5m. The measured result showed that the stability of temperature to peak wavelength and driving current to peak wavelength were improved 60 and 30 times, respectively. A theoretical modeling which combined the laser rate equation and the grating theory was successfully developed as well. The qualitative analysis of the numerical simulation showed that the peak wavelength would be locked in the reflection spectrum of the fiber grating. On the other hand, we have also finished packaging a butterfly 980nm pump laser module by the laser welding technology.

A fully distributed multi-parameter acoustic sensing technology is proposed. Current fully distributed sensing techniques are exclusively based on intrinsic scatterings in optical fibers. They demonstrate long sensing span, but their limited applicable parameters (temperature and strain) and costly interrogation systems have prevented their widespread applications. A novel concept of acoustic fiber Bragg grating (AFBG) is conceived with inspiration from optical fiber Bragg grating (FBG). This AFBG structure exploits periodic spatial perturbations on an elongated waveguide to sense variations in the spectrum of an acoustic wave. It achieves ten times higher sensitivity than the traditional time-of-flight measurement system using acoustic pulses. A fast interrogation method is developed to avoid frequency scan, reducing both the system response time (from 3min to <1ms) and total cost. Since acoustic wave propagates with low attenuation along varieties of solid materials (metal, silica, sapphire, etc.), AFBG can be fabricated on a number of waveguides and to sense multiple parameters. Sub-millimeter metal wire and optical fiber based AFBGs have been demonstrated experimentally for effective temperature (25~700 degC) and corrosion sensing. A hollow borosilicate tube is demonstrated for simultaneous temperature (25~200 degC) and pressure (15~75 psi) sensing using two types of acoustic modes. Furthermore, a continuous 0.6 m AFBG is employed for distributed temperature sensing up to 500 degC and to accurately locate the 0.18 m long heated section. Sensing parameters, sensitivity and range of an AFBG can be tuned to fit a specific application by selecting acoustic waveguides with different materials and/or geometries. Therefore, AFBG is a fully distributed sensing technology with tremendous potentiality. / Ph. D. / Fully distributed sensing techniques are part of the growing ”Internet-of-Things” trend, as they improve on traditional point sensors by providing spatially distributed measurements. Current techniques for fully distributed sensing are based on fiber optics, and while these techniques are capable of measuring parameters along a lengthy sensing distance, their wider application is constrained by limited applicable parameters and costly interrogation systems. In this research, an innovative, fully distributed, multi-parameter acoustic sensing technology based on acoustic fiber Bragg grating (AFBG) is proposed. AFBG takes advantage of the interaction between an acoustic wave and the periodic structure on the measured material, and uses the spectrum property of an acoustic wave to achieve ten times higher sensitivity than traditional time-of-flight methods. In addition, a fast interrogation method is developed to avoid frequency scan, reducing both the system response time (from 3 min. to <1 ms) and system cost (from $5, 000 to < $500). AFBG can be fabricated using different elongated materials (i.e. waveguides) as acoustic waves propagate along a variety of materials without extensive power loss. In this research, AFBG is deployed on a sub-millimeter metal wire and silica fiber to demonstrate effective corrosion and temperature sensing (25 ∼ 700 ◦C). In addition, hollow tubes are shown to be feasible waveguides for simultaneous temperature (25 ∼ 200 ◦C) and pressure (15 ∼ 75 psi) sensing. Finally a long AFBG is employed for distributed temperature sensing up to 500 ◦C. Wide applicability and low cost suggest that this sensing technology may be a viable approach for fully distributed sensing, contributing to the growing Internet-of-Things movement.

In this thesis work, a systematic quantitative study has been undertaken, on the performance of etched fiber Bragg Grating (FBG) sensors in the investigation of surface molecular adsorption in real-time; it is shown that the limit of detection (LOD) of FBGs etched below 2 microns diameter, is better compared to prominent optical label-free molecular sensing techniques such as Surface Plasmon Resonance (SPR). Novel fiber optic sensors based on FBG and etched FBG with various nano materials (polyelectrolytes, carbon nanotubes, hydrogel, metals and chalcogenides) coated on the surface of the core or cladding, have been proposed for sensing multi parameters such as pH, protein, humidity, gas, strain, temperature, and light etc. Besides being reproducible and repeatable, the proposed methods are fast, compact, and highly sensitive. A novel optical instrument has also been developed to measure angular deviation, binocular deviation and refractive index of glass slabs, and liquids, based on a shadow casting technique. This method uses the deviation in the geometrical shadow cast by a periodic dot pattern trans-illuminated by a distorted light beam from the transparent test specimen relative to a reference pattern.

Several sensor technologies have been developed and experimented over the last few decades to cater various needs of medical diagnostics. Among these, fiber optic sensors, in particular, Fiber Bragg Grating (FBG) based sensors have attracted considerable attention due to their inherent advantages such electrical passiveness, immunity to Electro Magnetic Interference (EMI), chemical inertness, etc. The present research work focuses on design, development and validation of FBG sensor based devices for measurement of certain healthcare parameters in the context of foot function/gait cycle, cardiac and breathing activity, nostril dominance, hand grip/wrist angle force function, etc. The experimental work presented here emphasizes on the effectiveness and competitiveness of the FBG devices developed, in comparison with standard tools such as Accelerometer, Load cell, Electronic Stethoscope, Electromyogram and Dynamometer. In the field of human balance, stability and geriatrics, two independent FBG devices namely, Fiber Bragg Grating based Stability Assessment Device (FBGSAD) and Optical Sensor Ground Reaction Force measurement Platform (OSGRFP) have been designed, developed and experimented for postural stability assessment and gait analysis respectively. The result of these studies have significant implications in understanding of the mechanism of plantar strain distribution, identifying issues in gait cycles, detecting foot function discrepancies, identifying individuals who are susceptible to falls and to qualify subjects for balance and stability. In the field of ergonomic assessment, Fiber Braggs Grating based Hand Grip Device (FBGHGD) is designed and developed for the measurement of hand grip force which helps in the understanding of several important biomechanical aspects such as neuromuscular system function, overall upper-limb strength, vertebral fracture, skeletal muscle function, prediction of disability, incapacity, mortality and bone mass density (forearm, skeletal sites, spine, hip etc.). Further as an extension of this work, the FBGHGD is used for measurement of force generated by the wrist in different positions of the flexion and extension which relates to the wrist muscle activity and its enactment. In the field of cardiac activity monitoring, a novel, in-vivo, non-invasive and portable device named Fiber Bragg Grating based Heart Beat Device (FBGHBD) is developed for the simultaneous measurement of respiratory and cardiac activities. The work involves designing FBGHBD, validating its performance against traditional diagnostic systems like electronic stethoscope, exploration of its clinical relevance and the usage of FBGHBD in studies involving normal persons and patients with myocardial infarction. The unique design of FBGHBD provides critical information such as nascent morphology of cardiac and breathing activity, heart rate variability, heart beat rhythm, etc., which can assist in early clinical diagnosis of many conditions associated to heart and lung malfunctioning. Further, the scope of this work extends towards evaluating several signal processing algorithms and demonstrating a suitable signal processing architecture for real-time extraction of heart beat and respiratory rates along with its nascent morphologies for critical health care application. In the area of breath monitoring, a Nostril Pressure and Temperature Device (NPTD) is designed and developed which aims at simultaneous, accurate and real-time measurement of nostril air flow pressure and temperature to aid in clinical diagnosis of nasal dysfunction and associated nose disorders. The results of NPTD can offer certain vital features like breathing pattern, respiratory rate, changes in individual nostril temperature/pressure, nostrils dominance, body core temperature etc., which can assist in early clinical diagnosis of breathing problems associated with heart, brain and lung malfunctioning. Since the research work in this thesis involve experiments engaging human subjects, necessary approvals from the ethical committee is obtained before the experiments and required ethical procedures are followed during all the experimental trials.

Sensors have become essential elements in human life for safe and comfortable existence in the ever demanding world. Various technologies over decades have contributed in their own way fulfilling innumerable sensing requirements. The discovery of optical sensor technologies has revolutionized the sensing field due to their inherent advantages. Among the large number of fiber optic sensor technologies, FBG based sensors have become widely known and popular within and outside the photonics community and has seen a prominent rise in their utilization. This thesis explores the use of FBG sensors for a wide range of applications scanning across a variety of engineering and medical applications, in the areas of civil engineering, biomechanical engineering, aerospace engineering, geoengineering, etc. It also deals with newer methods of packaging FBG sensors for the measurement of specific engineering parameters like strain, temperature, pressure, displacement and vibration. In the field of civil engineering, FBG sensors are employed for strain sensing on a prism and furthermore tested on a full size brick wallet. During this study, emphasis is made on substituting traditional sensors by specially packaged FBG sensors with the intent of either enhancing the sensing system’s performance or in merging/uniting the inherent advantages of FBG sensors. In the area of biomechanics, a novel sensor methodology using FBG sensors, for measuring surface strains generated on the skin of the calf muscle during various leg exercises is proposed. This methodology is used to address one of the most critical and life threatening issues in long distance air travel, namely the Deep Vein Thrombosis. Further, a FBG sensor based plantar sensing plate, is designed and developed, to measure plantar strain distribution in foot and also to analyze the postural stability. In the field of aerospace engineering, FBG sensors are used for addressing two of the most vital issues; Structural Health Monitoring (SHM) and direct measurement of pressure and temperature on the surface of an aircraft under hypersonic wind flow. Carbon Fiber Composite coupon level testing is carried out to obtain a generic strain calibration factor for the FBG sensor. Further, FBG sensors are exploited for the direct measurement of absolute temperature and pressure on the leeward surface of blunt cone at hypersonic wind speeds. In the domain of geoengineering, the feasibility studies have been undertaken to use a FBG as a seismic sensor and as a bore-well characterizing sensor. A novel FBG seismic sensor package is developed using a single FBG sensor to pick up the seismic waves propagating through the ground generated from earthquakes and ground tremors. Further, FBG sensors are used for measurement of temperature profiles in a bore-well to delineate and characterize the behavior of fractures during seasonal climatic changes. To summarize, the present thesis demonstrates a comprehensive experimental study which bring out the utility of FBG sensors in a variety of challenging applications.

No description available.

Tese de mestrado integrado. Engenharia Mecânica. Faculdade de Engenharia. Universidade do Porto. 2010

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.