D.Phil. / This dissertation deals with the fabrication, characterisation, and applications of long-period gratings in optical fibre communications and sensing systems. The aim of this project is to assess long-period gratings as media for active or passive fibre devices, particularly as components for the telecommunications industry. A review of the properties and characteristics of fibre gratings associated with the photosensitivity of germanosilicate fibres is provided, which includes a theoretical analysis of the principles of operation for short-period gratings (fibre Bragg gratings) and long-period gratings. The simulations of the spectral response from these two types of gratings are also presented. A number of long-period grating fabrication methods and techniques, which were reported by some researchers, are reviewed. In this project, the normal long-period gratings and phase-shifted long-period gratings are fabricated by using a line-narrowed KrF excimer laser combined with the metal amplitude mask technique. The metal mask is made of a stainless steel sheet, and the slot width (periodicity) is processed by using high quality photographic tooling. Three normal long-period gratings with different periodicities and one phase-shifted long-period grating can be manufactured simultaneously because there are four metal masks imprinted in one inexpensive stainless steel sheet. The mass-production of long-period gratings becomes possible, and the number of gratings that can be written is limited only by the excimer laser beam or metal mask dimension orthogonal to the fibre axis. The fibres that are used in our experiments are photosensitive optical fibres (PS1500). Long-period gratings can be written directly into these fibres without hydrogenation. Two types of long-period grating devices are investigated and developed for applications in dense wavelength division multiplexing (DWDM)networks: erbium-doped fibre amplifier (EDFA) gain-flattening filters and wavelength-tuneable add/drop multiplexers. Firstly, the transmission characteristics of phase-shifted long-period gratings are simulated theoretically by a combination of the coupled-mode theory and the fundamental-matrix method. It is suggested that a phase-shifted long-period grating device cascaded with another normal long-period grating can be used to flatten the gain spectrum of an EDFA containing three gain peaks. The experimental results show that a broad amplifier with peak-to-peak variations of less than 0.7 dB over 36 nm from 1526 to 1562 nm, which covers the entire C-band of the EDFA, can be realized practically. Next, a wavelength-tuneable add/drop multiplexer is designed and configured. In this device, four identical long-period gratings are assembled on piezoelectric ceramic fibre stretchers. The modelling of the device predicts that 50 ITU DWDM-channel signals could be selected in the wavelength range from 1526.25 to 1563.75 nm with 0.75 nm channel spacing and the cross-talk is less than –39 dB while the total insertion loss is about 0.24 dB. There are some significant advantages of wavelength-tuneable add/drop multiplexing devices over conventional fibre Bragg grating-based devices. (1) There is back reflected light and almost no cross-talk power penalty because the long-period grating couples light into forward-propagating modes. (2) Signal channel isolation is very high due to three stages of coupling mechanisms used in this device, which includes core-cladding, cladding-cladding and cladding-core, efficiently filtering out non-resonant light. (3) The insertion loss of the device is limited only by the separation of two long-period gratings, because there are no losses on non-resonant wavelengths of long-period gratings. Several other applications of long-period gratings in optical sensing systems are also described, and some are experimented on including axial strain sensors, structural bend sensors, temperature sensors, refractive index sensors and chemical concentration sensors.

D.Ing. / Most optical fibre-based ambient refractive index sensors (including individual long-period gratings) rely on spectral attributes obtained in transmission. However, a probe refractometer has been proposed that is based on self-interference of a long-period grating (LPG), thus providing reflectance spectra containing the relevant data. This sensor operates as a Michelson interferometer by virtue of the fact that its constituent LPG acts as both a mode converter and coupler. Its construction is such that optical power coupled into the cladding (when light impinges on the LPG) is reflected at a fibre mirror and returns towards the grating, where it is re-coupled into the fundamental guided mode. Since light waves propagating along the core and cladding material of the fibre cavity beyond the LPG experience different optical path lengths (due to differing mode indices), a phase difference exists between these modes upon recombining at the grating location. This causes interference, which is manifested as a characteristic fringe pattern in the sensor’s reflectance spectrum (analogous to that obtained in the transmission of a twin LPG cascade operating as a Mach-Zehnder interferometer). Research was conducted towards implementing a unique method of temperature compensation in this LPG-based Michelson interferometer. Sensing attributes of individual LPGs were investigated first, with specific emphasis on the temperature characteristics of two different types of host fibre. It was found that LPGs manufactured in conventional ATC SMF-28 fibre (previously hydrogen-loaded to inscribe the grating and annealed after fabrication) and B/Ge co-doped PS1500 fibre from Fibercore exhibited temperature characteristics of opposite polarity. This led to the implementation of a compound-cavity Michelson interferometer whose constituent LPG is written in one type of fibre, while a specific length of the other type of fibre is fusion spliced onto the host fibre section. Experiments verified the success of this temperature-compensation technique, which caused a measured reduction in temperature sensitivity of up to in interferometer phase shift. Measurements of the refractive index of the test substance surrounding the cladding material of the Michelson interferometer’s fibre cavity (and not the LPG itself) could therefore be done without being adversely affected by environmental temperature fluctuations. This was demonstrated experimentally by comparing the interferometer’s phase shift – devoid of temperature-induced effects – due to increasing refractive index of the analyte (as a result of escalating temperature) with index of refraction readings from a temperature-controlled Abbe refractometer. Numerical gradients of linear curves fitted to these results differed by two orders of magnitude less than the resolution of readings obtained from an Abbe refractometer – proof of the success of the temperature compensation technique applied in this LPG-based Michelson refractometer.

I believe art should express things which cannot be easily or entirely explored with words. For me, memory and unconscious mental processes are such matters. Memory is imperfect and impermanent, yet it greatly influences our day-to-day decisions. My memories of fiber art, altered and nostalgic, set me on my path to weaving and the pursuit of fine art. Silent like snowfall: A Retrospective on Memory and Self, explores the memories and concepts behind Silent like snowfall, a woven installation which creates the theoretical space in our minds which houses remnants of memories.

In most industrialized countries, significant future activity in the construction sector will be related to repair and rehabilitation of aging infrastructure. This will require use of durable and high performance repair materials. Among various mechanisms cited for lack of durability in repairs, early-age shrinkage cracking in overlay materials is of utmost importance. Fiber-reinforcement can be used to alleviate some of the issues related to plastic shrinkage. However, quantifying the performance of cement-based composites under restrained shrinkage conditions remains an issue. Various test techniques are available to measure free and restrained shrinkage, but do not simulate the real constraint imposed by the substrate on the overlay. In this dissertation, an innovative test method called the bonded overlay technique is described. An overlay of fiber-reinforced material to be tested is cast directly on a substrate, and the entire assembly is subjected to controlled drying. Cracking in the overlay is then monitored and characterized. During the development of this test method, instrumentation was included to enable measurement of the crack propagation rate using image analysis, evaporation rate, heat development, and strain using embedded sensors. Using the above technique, the effect of mix proportion including variables such as water-cement ratio (w/c), sand-cement ratio (s/c), and coarse aggregate content were studied. An increase in w/c from 0.35 to 0.6 significantly increased the total cracking. Addition of coarse aggregates reduced shrinkage cracking, however, for the range of s/c investigated, no definite conclusions could be drawn. Mixes with 0-20% fly ash and a 7 lit/m3 dosage of shrinkage reducing admixtures indicated no significant reduction in cracking. The influence of fiber geometry on cracking in overlays was also investigated. Fiber types included different sizes of polypropylene and cellulose fibers and one type of glass fiber (volume fraction ranging between 0-0.4%). Glass fibers at a small dosage of 0.1% were the most efficient fiber and completely eliminated cracking. Of the two field projects considered: one was a plaza deck at the UBC Aquatic Center, where cellulose fibers were used, and the second at the UBC ChemBioE building, where polypropylene fibers were used in slabs-on-grade. Both overlays were instrumented with strain sensors, data from which were monitored over the Internet. Results clearly indicated that fibers reduced the strain development in fiber-reinforced overlays when compared to un-reinforced overlays. An energy-based fracture model was proposed to predict maximum crack widths and in a second study, an equation was proposed to correlate early-age shrinkage and flexural toughness of cellulose fibers. In both models, a reasonable correlation with the test data was observed. In addition, factorial design method was used and a mathematical model was proposed to correlate different variables such as w/c, s/c, and fiber dosage. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

This thesis proposes and develops a novel optic configuration, Kerr phase-interrogator, which investigates the phase-shift between two sinusoidally modulated optical signals (SMOS) utilizing Kerr effect. The Kerr phase-interrogator gives birth to an entirely new technique for measuring the phase-shift between two light-waves. Taking advantage of all-optical signal processing, ultrafast responses, and being free from the coherent properties of a laser source, the Kerr phase-interrogator based technique for measuring the phase-shift is a promising novel approach for monitoring and sensing applications. The thesis begins with theoretically demonstrating the operation of Kerr phase-interrogator. As the core of optical process occurs in Kerr phase-interrogator, nonlinear interactions between two SMOSs in the Kerr medium are theoretically analyzed utilizing the models of nonlinear phase-modulation and four-wave mixing (FWM). The phase-modulation-based model is intuitive and allows for conceptual understanding of the operation of the Kerr phase-interrogator. However, this model does not account for the impact of chromatic-dispersion (CD) of the Kerr medium on the operation of the Kerr phase-interrogator. Compared with the former model, the FWM-based model is essential for acquiring insight into Kerr phase-interrogator, and can explain the CD impact of the Kerr medium. The analytical solution of the power of the first order sideband as a result of the nonlinear interaction is obtained in both theoretical models. The obtained solution shows sinusoidal dependence of the power on the phase-shift of the SMOSs. Utilizing this sinusoidal dependence, the phase-shift of two SMOSs can be acquired by measuring the power of the first-order sideband. Birefringence and CD are critical factors that affect the nonlinear interactions and thus impact the operation of Kerr phase-interrogator. In this work, vector analysis is performed on the nonlinear interaction between two SMOSs in a Kerr medium with randomly varying birefringence, and the effect of polarization-states of SMOSs on the operation of Kerr phase-interrogator is investigated. Impact of CD of Kerr medium on the operation of Kerr phase-interrogator is theoretically investigated using theory of FWM and is experimentally verified. Four typical applications, which comprehensively reflect the advantages of Kerr phase-interrogator, are proposed and experimentally demonstrated in this thesis. First, we present a novel approach for measurements of CD in long optical fibers using a Kerr phase-interrogator. The Kerr phase-interrogator measures the phase variation of a SMOS induced by CD in a fiber under test as the laser carrier wavelength is varied. This approach takes advantage of all-optical signal-processing based on Kerr effect to acquire the phase variation, and consequently removes the requirement of complex electrical signal-processors in existing techniques of CD measurement. CD measurement for several fibers is experimentally demonstrated. Second, a novel temperature sensor that utilizes temperature dependence of reflection group-delay in a linearly chirped fiber Bragg grating is presented. The reflection group-delay of chirped grating changes with temperature leading to a variation in the phase of a SMOS reflected from the grating. A Kerr phase-interrogator converts the phase-variation into power variation allowing for temperature sensing with a resolution of 0.0089 oC and a sensitivity of 1.122 rad/oC. Third, a Kerr phase-interrogator is applied for implementation of real-time CD monitoring. CD induces a phase-shift between two SMOSs carried by two different wavelengths. A Kerr phase-interrogator converts the phase-shift into power variation and CD monitoring is achieved by measurement of the power variation in real time with a resolution of 0.196 ps/nm. This application takes advantages of ultrafast response of Kerr phase-interrogator and achieves the real-time monitoring. Lastly, a novel approach for incoherent optical frequency-domain reflectometry based on a Kerr phase-interrogator is presented. The novel approach eliminates the limitation of finite coherent length of the light source, and achieves measurement of long-range distance beyond the coherent length of the light source. Long-range detection of reflection points as far as 151 km at a spatial-resolution of 11.2 cm is experimentally demonstrated.

Wavelength reuse techniques for bidirectional ultra-wide band (UWB) over fiber (UWBoF) networks are presented. The downstream optical signal from the central station (CS) is reused for upstream data transmission with the original data erased at the base station (BS). Two wavelength reuse schemes for the generation of a clear optical carrier at the BS are theoretically analyzed and experimentally demonstrated. In the first scheme, the wavelength reuse is based on phase-modulation to intensity-modulation (PM-IM) conversion and destructive interferencing using a polarization modulator (PolM) and a fiber Bragg grating (FBG). A theoretical analysis is performed which is verified by an experiment. In the second scheme, the wavelength reuse is based on injection locking of a Fabry–Pérot laser diode (FP-LD) and polarization multiplexing. The UWB injection signal contributes to better BERs of both downstream and upstream services and a lower power penalty cause by the wavelength reuse of the whole system than the baseband signal. A bidirectional point-to-point transmission of over a 25-km single-mode fiber (SMF) using a single wavelength from the CS in each scheme is demonstrated.

Given the rapid aging of the worldâ s population, improvements in technology for automation of patient care and documentation are badly needed. This project is based on previous research that demonstrated a â smartâ bed that can non-intrusively monitor a patient in bed and determine a patient's respiration, heart rate and movement without intrusive or restrictive medical measurements. The â smartâ bed is an application of spatially distributed integrating fiber optic sensors. The basic concept is that any patient movement that also moves an optical fiber within a specified area will produce a change in the optical signal. A statistical mode (STM) sensor and a high order mode excitation (HOME) sensor were previously investigated, based on which the author developed the present design including both modal modulation approaches. Development was made in both hardware and software for the combined STM/HOME sensor: a special lens system was installed allowing only the high order modes of the optical fiber to be excited and coupled into the sensor; computer-processing method was used for handling output from the dual STM-HOME sensor, which would offer comprehensive perturbation analysis for more reliable patient monitoring. Experimental results of simulating human body breathing and heartbeats by periodic mechanical perturbations are also presented, and the relative advantage and drawbacks of the two modal modulation approaches are discussed. / Master of Science

Narrow linewidth tunable fiber lasers have become an important tool in research and in-field applications thanks to their high beam quality, great spectral performance, compact structure and environmental robustness. Many methods have been introduced and developed throughout decades to fulfill the need for rapid wavelength adjustment of these fiber lasers. While maintaining a high-level spectral performance, the ease of manipulation and cost effectiveness of the tuning operation are considered beneficial for the lasers used in real world, and therefore, they are the main focus of the work presented in this thesis.                                        An accurate tuning method with narrow linewidth and compact configuration was achieved in the first work. A polarization independent semiconductor optical amplifier (SOA) was used as the gain medium in a unidirectional fiber ring cavity with a circulator connected to a 6-meter long chirped fiber Bragg grating (CFBG). The laser wavelength was chosen by setting the modulation frequency of the SOA the same as the harmonics of the fundamental repetition rate of the light reflected at a specific point on the CFBG. Careful management of the drive current and pulse width helped to generate laser light of narrow linewidth (less than 0.03 nm) and with low power variation (1.46 dB) over a tuning range of 40 nm.                                                      One example of the application of a tunable fiber source was discussed in the second work. An efficient Erbium Ytterbium fiber amplifier, which is seeded by a distributed feedback laser, was designed for continuous-wave differential absorption light detection and ranging (CW DIAL) of atmospheric CO2-concentration. It had a linewidth of 3 MHz, a tuning range of 2 nm over the CO2 absorption peaks at 1.572 μm and an output power of 1.3 W. Wavelength tuning is achieved by adjusting the drive current to the seed laser. Results from the initial CW DIAL testing demonstrate that this tunable fiber source meets the high demands for range resolved atmospheric CO2 monitoring.                                                                                  To conclude, two narrow linewidth tunable fiber lasers have been demonstrated based on different tuning mechanism. The rapid and accurate tuning operation with low output power variation is achieved in both works. The great spectral properties of these fiber sources make them powerful tools to be used in applications such as optical communication, remote sensing, spectroscopy, optical coherence tomography, and many more. / Smalbandiga avstämbara fiberlasrar har blivit viktiga instrument inom forskning och andra tillämpningar på grund av deras höga strålkvalitet, goda spektrala egenskaper, kompakthet och stora miljötålighet. Under de senaste decennierna har flera metoder introducerats och utvecklats för att kunna göra snabba våglängdsjusteringar för sådana lasrar, med bibehållande av gott spektralt uppförande. Billiga, smalbandiga, enkelt avstämbara lasrar är viktiga för praktiska tillämpningar, och arbetet i denna avhandling har handlat om att utveckla nya sådana och studera deras egenskaper. I det första arbetet togs en precis avstämningsmetod med smal linjebredd tillsammans med en kompakt utformning fram. En polarisationsoberoende optisk förstärkare av halvledartyp (SOA) användes som fotonkälla i en fiberring-kavitet med en fiberoptisk cirkulatorn kopplad till ett 6 meter långt fiber- Braggitter, ett s.k. ¨chirpat¨, eller kvittrat fiber- Braggitter,( CFBG). Laservåglängden valdes genom att sätta modulationsfrekvensen för SOA-förstärkaren till önskad resonansfrekvens i kaviteten. Endast ljus som reflekterades i en bestämd punkt i CFGBgittret förstärks då, vilket svarade mot den önskade våglängden. Med noggrann inställning av drivström fick man då en smal pulsbredd (mindre än 0,03 nm), avstämbart över ett intervall på 40 nm och med bara en liten variation i effekten (1,46 dB). I det andra arbetet utvecklades en annan avstämbar fiberbaserad laserkälla för att användas i en ny gasmätningsmetod av atmosfären. Metoden utnyttjar kontinuerligt ljus och differentiell absorptions-LIDAR (CW DIAL), där man mäter återspritt ljus från den önskade molekylen på, eller vid sidan om en absorptionslinje hos gasen. Från resultaten kan man sen räkna ut den lokala koncentrationen av gasen med hög spatiell noggrannhet. Lasersystemet bestod av en Yb-Er-fiberförstärkare matad från en smalbandig, avstämbar diodlaser (DFBlaser). Systemet hade en linjebredd på 3 MHz, en avstämbarhet på 2 nm avpassat för absorptionstopparna för CO! vid 1,572 µm, och en uteffekt på 1,3 W. Våglängden kunde ändras genom justering av drivströmmen till DFB-lasern. Systemet användes för att mäta CO! med den nya tekniken för första gången och resultaten visade att den avstämbara fiberkällan motsvarar de höga krav som ställs på monitorering av CO! i atmosfären. Sammanfattningsvis har två avstämbara fiberlasrar med smal linjebredd, baserade på olika avstämningsmekanismer, demonstrerats. För båda lasrarna kan en snabb och noggrann avstämning uppnås, med liten variation i uteffekt. Dessa fiberkällors goda spektrala egenskaper gör dem till kraftfulla verktyg för användning i olika tillämpningar som t.ex. optisk kommunikation, fjärranalys, spektroskopi och optisk koherenstomografi (OCT).

Random fiber lasers, a new type of fiber laser that uses disordered medium to provide distributed feedback, have drawn considerable interest in the photonics community over the past ten years. Stimulated Brillouin scattering (SBS), with a typical narrow spectral width of ~100 MHz, provides an important gain mechanism for random fiber lasers. Brillouin random fiber laser (BRFL) has shown excellent advantages in generating highly coherent photons and in ultrasound sensing. However, the accompanied large intensity noise in BRFLs hinders its further performance improvement and practical applications. In order to design a low noise BRFL, it is important to explore the fundamental physics behind BRFL and study its output characteristics. This thesis focuses on the study of random lasing mechanism in BRFL, which lays the foundation for the demonstration of a low noise BRFL. The main research results and contributions are as follows: (1) In order to understand the dynamic noise properties of BRFLs, the properties of the acoustic wave generated by BRFL, including its intrinsic spectral width, intensity dynamics, distributed spectrum and distributed intensity statistics are characterized for the first time. The characterization method is based on the SBS enhanced polarization decoupled four wave mixing process, where the pump wave, Stokes wave, probe wave and reflected probe wave are coupled through the fiber density variation induced by the acoustic wave. It is demonstrated that the intrinsic spectral width of the acoustic wave in the Brillouin gain fiber depends on the spectral convolution of pump light and Stokes light. Stochastic behaviour is introduced to the intensity dynamics of the acoustic wave when the linewidth of the pump light (or the Stokes light) is larger than several MHz. The distributed spectra of the dynamic grating are determined by the birefringence of the Brillouin gain fiber, which have maximum change on the order of 10-7 to 10-6 when the BRFL is on operation. Different proportion of optical rogue waves are detected at high gain position and low gain position near the lasing threshold, proving the nonlinear amplification of the SBS process. (2) In order to study the mode selection mechanism of the distributed random feedback and explore new physics phenomenon in BRFLs, the conventional Rayleigh scattering fiber in BRFL is replaced by the artificially controlled random scattering medium. First, weak FBG array with random spacing offers distributed feedback with varied length, which demonstrate the longitudinal mode filter function of the distributed random feedback. Single longitudinal mode operation of BRFL is realized by using appropriate length of the FBG array. Then, scattering from random fiber grating (RFG) with varied grating period is used to provide feedback for BRFL. The enhanced backscattering strength from RFG improves the slope efficiency of BRFL to 29.3% and reduces the lasing threshold to 10.2 mW. By calculating the correlation of the intensity fluctuation spectra from trace to trace, the correlation of two traces is found to be dependent on the specific two chosen traces, demonstrating the replica symmetry breaking phenomenon in photonics. (3) RFG with relatively large refractive index modulation shows potentials in improving the performance of the BRFL. In order to investigate the working mechanism of the RFG, optical frequency domain reflectometry (OFDR) with spatial resolution of 8 μm is employed to characterize the property of RFG. The backscattering strength and spectral response of RFG is highly related to the degree of randomness of RFG. Theoretically, entropy is introduced to build a quantitative relationship between the degree of randomness and backscattering strength of the RFG based on the transfer matrix method. A linear relationship between the average reflectivity of the RFG in dB scale and sub-grating’s entropy is found. Further, based on a polarization maintaining RFG, a low noise BRFL is proposed and demonstrated. Compared to Rayleigh scattering, the polarization maintaining RFG can tolerate environmental perturbation, leading to a 20 dB intensity noise suppression of the BRFL in the low frequency domain from 10 Hz to 1 kHz. (4) The dynamic properties of the slowly varying frequency drift of a dual-wavelength BRFL in polarization maintaining fiber are characterized. Two principal lasing peaks in each polarization are enabled by the combined distributed Rayleigh scattering and the Brillouin gain provided by the polarization maintaining fiber with large birefringence. Polarization dependent and polarization independent spectral variations are studied in the dual-wavelength BRFL due to the environmental perturbation and gain competition. The probability distribution of the lasing frequency exhibits a dip near the mean frequency that is caused by the spectral hole burning. By calculating the matrix of the Pearson correlation coefficient, the internal correlations between different part of random fiber laser spectra are found, which enhances the understanding of the fundamental physics of random lasing process.

Indiana University-Purdue University Indianapolis (IUPUI) / The use of carbon-fiber reinforced composite materials is not novel in the field of motorsports industry. Their use in collapsible structures for crashworthiness is however not fully understood and predicted. Due to the complex failure mechanisms occurring within the material, the energy absorbing capacity cannot be easily pre dicted. The need to understand their contributions in crashworthy structures is thus of great importance. Furthermore, failure of carbon-fiber composites is highly depen dent on the geometry of structure. Problems arise in both experimental and numerical modelling of these structures. Although many explicit FEA codes exist, they often include experimental parameters that need to be calibrated through either coupon tests or actual crash tests. As composite structures become more commonly used in automotive industry, it is necessary to set some guidelines to successfully model and simulate composite crashworthy structures. The numerical modelling was done in LS-DYNA Enhanced composite damage MAT54. The material properties were configured using experimental coupon tests. The tests were conducted on square composite tubes. The Specific Energy Absorption (SEA) of the tubes were calculated through several coupons. As SEA is a function of geometry, it was necessary to conduct tests with similar geometry as seen in nosecone. MAT54 was chosen to simulate both crush and crash simulations due to its capability to simulate element level crushing. Furthermore, various modifications within the material model, improve its accuracy to determine composite failure. The research utilizes the characterization of material inputs in MAT54 by con ducting quasi-static compression tests on simpler but similar geometry. By utilizing inputs, a zonal optimization was conducted on the nosecone geometry. The number of layers, layer orientations and ply thicknesses were varied to vary the energy absorbed per zone. The deceleration of the vehicle can thus be controlled, and the weight of the structure could be reduced.