Optical Frequency Domain Reflectometry (OFDR) is an interferometric technique which is capable of interrogating fibers under test (FUT) up to kilometers in length with millimeter resolution[10]. It does so by taking the Rayleigh backscattered light, or Fresnel back-reflected light and combining it with the reference arm to create a beating signal. The beating signal is then Fourier transformed to create a scattering profile of the FUT. Presented in this thesis are 5 novel OFDR configurations that improve the SNR in the spatial domain up to 26dB. As well, 4 new data analysis algorithms are presented that improve the spectral resolution by up to a factor of 40 and spectral SNR by 1.31dB. The FUT's investigated are regular SMF, linear FBG's, and chirped FBG's. With these, the wavelength shift at specific points along the FUT is measured and correlated with temperature changes (with associated applications), longitudinal stress, and torsional stress stimuli.

The light intensity in a focused femtosecond laser pulse can be high enough to initiate non-linear absorption of the radiation in otherwise transparent dielectric media. The highly localized energy deposition into the material results in permanent changes in its chemical and physical properties which can be used in the fabrication of various photonic and microfluidic devices. In this thesis we study the morphology and optical properties of the modification induced by focused femtosecond laser radiation inside fused silica and other inorganic glasses. The characterization of the modified regions is performed using a microreflection refractometry technique and the ultra-high spatial resolution technique of weak selective chemical etching followed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). We demonstrate that the sample irradiation conditions strongly affect the modification morphology. Specifically, for a tight focusing geometry we identify the pulse duration-pulse energy parameter space where three distinct regimes of modification can be achieved. We also address the effects of optical aberrations on the shape, position and the intensity threshold values of the femtosecond laser induced modification. We use the combination of femtosecond laser dielectric modification and selective chemical etching to fabricate high-quality microchannels in fused silica glass. We show a dramatic dependence of the etch rate on the laser polarization and demonstrate that the high differential etch rate inside the modified regions is determined by the presence of polarization-dependent self-ordered periodic nanocracks or disordered nanoporous structures. These nanostructures are much smaller than the wavelength of the femtosecond radiation used for their formation and are the smallest objects ever created by light inside dielectric materials. Exciting potential applications of the nanostructures will be discussed.

This thesis is a collection of studies on a distributed sensor based on Brillouin scattering using polarization maintaining (PM) fibers as well as single mode fibers. With a specially designed optical fiber circuit consisting of two polarizing beam splitters and polarization controllers, PM fibers were successfully used as sensing media for the Brillouin distributed sensor developed at the University of Ottawa. The use of a single axis of a PM fiber minimizes polarization effects and therefore improves the signal to noise ratio of the Brillouin spectrum and hence, the measurement resolution. The strain and temperature dependence of Brillouin frequency, intensity and bandwidth of three types of commercial PM fibers: PANDA, Bow-tie and Tiger were investigated. The main findings from these investigations are as follows: (1) in a PM fiber, the temperature dependence of the Brillouin frequency in the slow axis is the same as that in the fast axes, and so is the strain dependence, (2) the Brillouin spectrum bandwidth is strain and temperature dependent, and (3) PANDA PM fiber shows the greatest promise for use in sensing applications and Tiger fiber is not suitable for Brillouin sensing. The feasibility of simultaneously measuring strain and temperature using PM fibers was examined. The 2nd order partial derivative of the Stokes intensity with respect to frequency and position shows a maximum or minimum at the boundary between two different strained sections. This idea was used to locate the boundary between different strain regions. This provides a spatial location accuracy between 5 cm to 10 cm, which is better than the 20 cm pulse length used in the experiment. The influences of transient phonon (defined as when pulse width is shorter than the phonon relaxation time) relaxation on the Brillouin loss spectrum have been observed and investigated. The transient phonon field lasts longer than the nature phonon lifetime (∼10 ns). This transient phonon relaxation time is pump power and probe ER dependent, and it affects the strain, temperature, and location measurement accuracy.

This thesis details the first successful phase mask assisted inscription of a retro-reflecting fiber Bragg grating (FBG) with an ultrafast laser source. Ultrafast FBG fabrication is demonstrated in the traditional Ge-doped silica fiber, pure silica fiber, and sapphire crystal fiber. Successful grating fabrication in pure silica and sapphire indicates that, unlike with traditional UV sources, a wide range of materials can be modified with ultrafast laser radiation. A multiple beam interference model is developed that can accurately simulate the femtosecond and picosecond pulse induced interference patterns in the cladding and core regions of the Ge-doped silica fiber. The model is then extended to simulate pulse propagation through the phase mask and investigate the overlap of the diffracted orders. It is found that at extended distances from the phase mask the diffracted orders no longer overlap, resulting in a pure two-beam interference pattern as the +/-1 orders contain most of the incident energy. These predictions are then compared with experimental observations. Hydrogen loading of standard Ge-doped SMF-28 fiber is shown to reduce the grating formation threshold significantly. The existence of two ultrafast induced grating types is demonstrated and their annealing and scaling behavior is investigated. As the index change profile of phase mask assisted ultrafast gratings differs from their UV induced counterparts a Rouard's method model is developed to model the spectral response of these gratings. The spectral response is shown to be consistent with the higher nonlinear intensity dependence of the induced index change. In addition a saturable growth rate is demonstrated and the impact of the incomplete overlap of the induced index change with the core region of the fiber is considered.

Disaster prevention in civil infrastructures requires the use of techniques that allow temperature and strain measurements in real time over lengths of a few meters to tens of kilometres. The distributed Brillouin sensor (DBS) technique has the advantage to combine all these characteristics. The sensing mechanism of the DBS involves the interaction of two counter-propagating lightwaves, the Stokes and the pump, in an optical fibre. Spatial information is obtained through time domain analysis. The sensing data are recorded from the measurement of the pump depletion. We explore the benefits and the drawbacks of this approach and show that there is a power range for which the sensing performances are optima. To achieve that goal, Brillouin fibre generator (BFG) and amplifier (BFA) were studied leading to the derivation of a threshold definition for the BFA, which is the configuration of the DBS. Within that context, numerical and analytical models describing the stimulated Brillouin scattering (SBS) interaction are introduced and validated experimentally. Even if pump depletion is carefully controlled, the Brillouin spectrum shape, and hence the sensor performance, still depend on the sensing parameters such as power, pulse and fibre characteristics. We use a signal processing method grounded in the physics of Brillouin scattering. An analytical approximation, valid for the optimum sensing region, reconstructs the Brillouin spectrum distribution from input sensing parameters and measured data. These data are obtained with a spectrum analysis methodology, based on three original tools: the Rayleigh equivalent criterion, the lengthstress diagram, and the spectrum form factors. This methodology has been successfully used on experimental spectra. The DBS and the signal processing approach were then used to monitor the structural changes in steel pipes and in a composite column, all subjected to heavy loads. The DBS measured the strain distribution of those structures while they were stressed. The DBS provided detailed information on the structure's health at local and global level, associated with deformations, cracks and buckling. This work demonstrates that the DBS is capable of extracting critical information useful to engineers: engineer's experience and judgement in conjunction with appropriate data processing methods make possible to anticipate structural failures. Keywords: Brillouin scattering, optical fibres, SBS threshold, Brillouin generator, Brillouin amplifier, gain saturation, pump depletion, sensor performance, spatial resolution, frequency resolution, signal processing, structural health monitoring, distributed Brillouin sensor, strain measurement.

This thesis includes studies of polarization effects in two main research areas of optical fiber technology: optical fiber communication systems and optical fiber sensors. Polarization of light in optical fiber is sensitive to environmental disturbances. On the negative side, this results in complex measurement processes and errors in communication systems caused by dynamic polarization mode dispersion (PMD) and polarization dependent loss (PDL). On the positive side though, it also results in the possibility of developing a distributed optical fiber vibration sensor. For the purpose of fast polarization measurement for high bit-rate communication systems, a new PDL vector method was proposed based on the equation of motion in Stokes space. It is capable of providing accurate PDL measurements while requiring less measurement steps compared with other available techniques. We had performed a PMD field test, and found the fastest PMD change in submarine fibers under the Caribbean Sea. With long measurement duration (>24h) on one pair of fiber, correlations between polarization effects and tides were reported for the first time. A histogram of the differential group delay (DGD) data and an auto-correlation function of state of polarization (SOP) and DGD were validated by theoretical fittings. The average and fastest drift time for both SOP and DGD was found to be ∼3min and less than 15s, respectively. Polarization effects were then utilized as a sensing parameter to detect and locate external disturbances along the optical fiber. A system based on polarization optical time domain reflectometry (Polarization-OTDR) technique was developed in order to pinpoint the disturbances as well as to give events' frequency information. For the first time, a fully distributed optical fiber vibration sensor has been demonstrated in a 1km fiber link with 10m spatial resolution and 5kHz maximum detectable frequency. Moreover, by our proposed spectrum analysis, multiple simultaneous events with the same frequency components can be identified.

Fundamental theoretical insights into the fine structure of electromagnetic fields in photonic crystals are developed, by examining the singular nature of field representations in linear systems, and fundamental design paradigms are established in this meta-material system. Photonic crystals are optical meta-materials that permit the transport of electromagnetic energy that can be tailored by modifying the underlying periodically structured dielectric profile. Propagation is characterized by the dispersion surface through the Bloch states, where frequency wave vector relations characterize system response. A renewed appreciation for the richness of the dispersion relations has led to a deeper consideration of the modal structure itself and its inherent relationship to symmetry. This has led to a group theoretic treatment of the fundamental system space symmetries expressed through local representations of the singular character of the Bloch mode and its vortex states. Central to this study is an analysis of the local energy transport, which has uncovered the existence of optical vortices centered about phase singularities. Further investigation into the energy transport properties at the local level (i.e., far below the scale of the wavelength) reveals optical features never before explicated in the photonic crystal community. It is shown that the electromagnetic mode structure bears the hallmarks of singular optics, whereby the field becomes characterized by singularities---that is, spatial locations where mathematical descriptions of optical properties become undefined. Optical vortices, revealed by energy circulation about a singular point, are expressed by global system space symmetries and the particular character of the dispersion surface. Vortices are the local field magnitude response to their associated phase singularities. In this study, the locations of optical vortices in real space are determined using phasor geometry, and the symmetry rules for their existence are established with group theory. They are categorized into symmetry and accidental singularities, constraining their locations in reciprocal space. Further research on the vectorial character of the local electromagnetic field has uncovered the rich nature of polarization singularities in these periodic microstructures. A group theory representation is used to express its complementarity polarization singularity representation, where fundamental transformation operations permitted by the system's space group are quantified. As a result, the entire electromagnetic field may be determined from the fundamental domain of the system's space group, using derived transformation properties of the local state of polarization. Furthermore, it is shown that local symmetry requires the electromagnetic field to become singular at particular points, known as Wyckoff positions. The reduction of the electromagnetic field to a fundamental domain, the location of the optical singularities, and the transformation properties of the local state of the electromagnetic field allow one to determine the major features of the sub-wavelength structure of the electromagnetic field from fundamental symmetry principles. In each the case, these fundamental principles were applied to Bloch modes derived in the vanishing dielectric contrast limit. Additional confirmation of theoretical predictions is supported by simulations of high-dielectric-contrast, purely two-dimensional, photonic crystal Bloch modes, in which Maxwell's equations are solved directly using the Finite Element Method (FEM). Finally, studies of optical vortices and polarization singularities within dimensionally confined photonic crystal slabs are studied with three-dimensional FEM simulations, in order to inform the design and fabrication of such structures for future experimental confirmation of the phenomena and possible photonic crystal optical singularity applications.

In today's technology an increasing use is made of materials consisting of thin layers with thicknesses in the range of a few nanometers. Applications of these types of materials can be found in integrated circuits, magnetic heads and tapes, solid-state lasers, X-ray mirrors and coated window glass, etc. They are used for their mechanical, magnetic, optical and/or electrical properties. These properties are related to the structural parameters of these materials, such as the elemental composition, thickness, roughness and number of layers in the material. The first part of this thesis explores both the thickness of layers and the number of layers in x-ray multilayer mirrors by using the Luus-Jaakola optimization procedure in order to achieve a desired reflectivity. The reflectivity of multilayer coatings shows a strong dependence on the thickness of each layer and the number of layers. In the first chapter we present and modify the Luus-Jaakola algorithm to design optical th in-film systems by optimizing the thickness of the layers as well as the number of layers. We show that the optimization model can improve the results using fewer layers of material with the Luus-Jaakola method for the design of x-ray multilayer mirrors in both the angular and spectral domains. Numerical results indicate that the proposed approach performs very competitively with other approaches. It has become possible to design optical coatings with fewer layers. This opens a new opportunity for the design of high quality optical coatings. We then apply this technique to help in the design of multilayer mirrors for a target application, such as an astronomical grazing-incidence hard X-ray telescope. These designs are compared with other author's results. Further, we present a computational study of a modification of the Luus-Jaakola method that could be used in multilayer mirrors with more than two components. Then we apply this method to determine the optical constants, thickness and root mean square roughens of the layers. Finally, in the last part of the thesis we present optimization results of phase-only sampled fiber Bragg gratings by using the modified Luus-Jaakola method. We show that it is able to produce a high number of channels with less refractive-index modulation, high reflectivity, as well as requiring fewer segments.

Wide band gap dielectrics remain transparent for low intensity near-infrared light. However, when light intensity increases, highly nonlinear absorption takes place. The absorption occurs due to the transfer electrons from the valence to the conduction band in the dielectric. We investigate the physics of this ionization process and the material changes induced in the sample by multiple femtosecond laser pulses. We show how transmission relates to the ionization prbabi1ity in high density material. By controlling various experimental parameters we obtain qualitatative and quantitatative information about ionization and different mechanisms involved. Analyzing transmission for a variety of pulse durations, we obtain the field dependent collisional ionization rates for fused silica. Using elliptically polarized light, we probe the sub-laser-cycle dynamics of the ionization. The variation in the ionization rate within a laser cycle is translated, through the differential absorption between the major and the minor axes of the polarization ellipse, into the polarization changes of the transmitted light. In crystalline samples we observe a dependence of the ionization on the alignment of the field with the lattice. The modulation in transmission with the alignment angle of the crystal is analyzed using Fourier transformation and it provides the information on the lattice symmetry. We find that the directionally dependent electron effective mass plays an exponentially dominant role in defining the ionization probabilities. Chemical changes inside the focal volume accumulate over many laser shots. We show that these material changes lead to a localized increase in the ionization, providing a feedback mechanism that leads to the formation of nano-structures in the interaction region. With longer exposure time, defocusing micro-lenses are formed in the bulk. These micro-lenses change the divergence of the laser beam as well as the pulse energy required for the ionization, therefore, leading to enhanced transmission. We observe this effect in a variety of transparent dielectrics. In crystalline quartz we find a novel phenomenon associated with repeated ionization. The permanent changes in the sample create favourable phase matching conditions for strong second harmonic generation.

This thesis includes studies of improving the performance of the distributed fiber optic sensor based on Brillouin scattering, especially focusing on spatial resolution and frequency resolution. A novel distributed Brillouin sensor based on offset locking of two distributed feedback lasers has been developed. With offset locking technique, the beat frequency of two lasers can not only be locked together but also be tuned over a wide range of more than 1GHz. Moreover, a lock-in amplifier has been applied to lock the bias drift of the electro optic modulator, which ensures the probe pulse stability and constant signal-to-noise ratio during the sensing measurement. With the offset locking based distributed Brillouin sensor, a 1m spatial resolution and 0.5°C temperature resolution has been achieved. High spatial and frequency resolution Brillouin sensor with phase locking technique has been studied. A theoretical model dealing with laser source phase locking has been derived. Simulation and experimental results have been used to prove the ability of improving the spatial and frequency resolution with phase locking technique. Improving the spatial and frequency resolution with different pulse pair technique has been explored. This technique can obtain high spatial resolution (<1m) and high frequency resolution (<35MHz linewidth) simultaneously along several kilometer long sensing fiber, which is a promising candidate in the field of structure health monitoring. The ultimate measurement accuracy with different pulse pair Brillouin optical time domain analysis technique has been investigated and it is determined by the combination of the system bandwidth, system signal-to-noise ratio, absolute pulse width, pulse width difference, and the rise/fall time of the pulse. A Brillouin optical time domain analysis sensor based on optical differential parametric amplification technique has been proposed and realized to obtain high spatial and frequency resolution at the same time without increasing the measurement time. Anew theoretical model dealing with optical differential parametric amplification technique has been derived. An experiment is successfully realized to validate the proposal in a polarization maintaining fiber with a spatial resolution of 0.5 m and a strain accuracy of 6 muepsilon.