The theory of stretching and compressing of short light pulses by the chirped volume Bragg gratings (CBG) is reviewed based on spectral decomposition of short pulses and on the wavelength-dependent coupled wave equations. The analytic theory of diffraction efficiency of a CBG with constant chirp and approximate theory of time delay dispersion are presented. Based on those, we performed comparison of the approximate analytic results with the exact numeric coupled-wave modeling. We also study theoretically various definitions of laser beam width in a given cross-section. Quality of the beam is characterized by the dimensionless beam propagation products (?x???_x)?? , which are different for each of the 21 definitions. We study six particular beams and introduce an axially-symmetric self-MFT (mathematical Fourier transform) function, which may be useful for the description of diffraction-quality beams. Furthermore, we discuss various saturation curves and their influence on the amplitudes of recorded gratings. Special attention is given to multiplexed volume Bragg gratings (VBG) aimed at recording of several gratings in the same volume. The best shape of a saturation curve for production of the strongest gratings is found to be the threshold-type curve. Both one-photon and two-photon absorption mechanism of recording are investigated. Finally, by means of the simulation software we investigate forced airflow cooling of a VBG heated by a laser beam. Two combinations of a setup are considered, and a number of temperature distributions and thermal deformations are obtained for different rates of airflows. Simulation results are compared to the experimental data, and show good mutual agreement.

In this thesis, we explore the four aspects of fiber Bragg grating sensors: mathematical modeling of Fiber Bragg Grating response/spectral characteristics, fabrication using phase mask, application and interrogation. Applications of fiber Bragg gratings, also known as in-fiber gratings, with emphasis on their sensing capabilities, interrogation of an array of sensors and their performance in structural health monitoring scenario are documented. First, we study the process of photosensitivity phenomenon in glasses, in particular GeO2:SiO2 glasses. For mathematical modeling we consider the 1-D refractive index profile along the propagation axis of an optical fiber drawn from the preform of such glasses. These 1-D index structures exhibit a bandgap for propagation along the fiber axis. We show how the bandgap is dependent on the two structural parameters: index periodicity and effective refractive index. The mathematical model provides the characteristics of three sensor parameters -resonance wavelength also known as the Bragg wavelength (λB ), filter bandwidth (ΔλB ), and reflectivity (R). We show that the evolution of the index structure in germanosilicate glasses is dependent on the inscription parameters such as exposure time, intensity of the laser used for inscribing, the interference pattern, and coherence of the laser system. In particular, a phase mask is used as the diffffacting element to generate the required interference pattern, that is exposed on the photosensitive fiber. We present a mathematical model of the electromagnetic diffraction pattern behind the phase mask and study the effect of the limited coherence of the writing laser on the interference pattern produced by the diffracting beams from the mask. Next, we demostrate the sensing capabilities of the fiber Bragg gratings for measuring strain, temperature and magnetic fields. We report linearity of 99.7% and sensitivity of 10.35pm/◦C for the grating temperature sensor. An array of gratings assigned with non-overlapping spectral windows is inscribed in a single fiber and applied for distributed sensing of structural health monitoring of an aircraft’s composite air-brake panel. The performance of these sensors is compared with the industry standard resistance foil gauges. We report good agreement between the two gauges (FBG and RSG). In some applications it is more desirable to know the spectral content, rather than the magnitude of perturbation. Fiber Bragg gratings sensors can be used to track events that occur in a very small span of time and contain high frequencies. Such applications demand very high speed wavelength demodulation methods. We present two interrogation techniques: wavelength-shift time-stamping (WSTS) and reflectivity division multiplexing (RDM). WSTS interrogation method employs the multiple threshold-crossing technique to quantize the sensor grating fluctuations and in the process produces the time stamps at every level-cross. The time-stamps are assembled and with the a priori knowledge of the threshold levels, the strain signal is reconstructed. The RDM methodology is an extension of the WSTS model to address multiple sensors. We show that by assigning unique reflectivities to each of the sensors in an array, the time-stamps from each of the sensors can be tagged. The time-stamps are collected by virtue of their corresponding pulse heights, and assembled to reconstruct the strain signal of each of the array sensor. We demonstrate that the two interrogation techniques are self-referencing systems, i.e., the speed at which the signals are reconstructed is instantaneous or as fast as the signal itself.

No description available.

No description available.

The concept of stay-in-place (SIP) structural formwork has the potential to simplify and accelerate the construction process to a great extent. Fibre-reinforced polymer (FRP) SIP structural formwork offers further potential benefits over existing formwork systems in terms of ease and speed of construction, improved site safety and reduced long-term maintenance in corrosive environments. However, it is not without its limitations, including primarily the possibility of a lack of ductility, which is a key concern regarding the use of FRP structural formwork in practice. This thesis presents the findings of an experimental and analytical investigation into a novel FRP SIP structural formwork system for a concrete slab with a particular emphasis on its ability to achieve a ductile behaviour. The proposed composite system consists of a moulded glass fibre-reinforced polymer (GFRP) grating adhesively bonded to square pultruded GFRP box sections. The grating is subsequently filled with concrete to form a concrete-FRP composite floor slab. Holes cut into the top flange of the box sections allow concrete studs to form at the grating/box-section interface. During casting, GFRP dowels are inserted into the holes to further mechanically connect the grating and box sections. An initial experimental investigation into using GFRP grating as confinement for concrete showed that a significant increase in ultimate strength and strain capacity could be achieved compared to unconfined concrete. This enhanced strain capacity in compression allows greater use of the FRP capacity in tension when used in a floor slab system. Further experimental investigation into developing ductility at the grating/box-section interface showed that the proposed shear connection exhibited elastic-‘plastic’ behaviour. This indicated the feasibility of achieving ductility through progressive and controlled longitudinal shear failure. Following these component tests on the concrete-filled grating and the shear connectors, a total of six (300 x 150 x 3000) mm slab specimens were designed and tested under five-point bending. It was found that the behaviour of all specimens was ductile in nature, demonstrating that the proposed progressive longitudinal shear failure was effective. A three-stage analytical model was developed to predict the load at which the onset of longitudinal shear failure occurred, the stiffness achieved during the post elastic behaviour and, finally, the deflection at which ultimate failure occurred. Close agreement was found between experimental results and the theory.

<div>Fabrication of nanoscale highly periodic structures is a vital capability for research on quasicrystals, directional and specular selective emitters, and plasmonics. Laser interference lithography is a maskless lithography process capable of producing patterns with high periodicity over large areas, and is compatible with standard optical lithography processing. In this work, a Lloyd's mirror lithographic interferometer is simulated, built, and tested. Featuring a HeCd CW laser at 325 nm, spatial lter, and vacuum stage, it is capable of generating patterns with a sub-100 nanometer half pitch, over a large area (approximately 8 cm²), with minimal distortion, in a single exposure; with 2D patterns possible using multiple exposures. The interferometer features a compact sliding enclosure, simple alignment and operation, and quick adjustments to the desired period. One-dimensional and two-dimensional patterns were generated and matched well with simulation.</div>

The development of Laser-Induced Thermal Grating Spectroscopy (LITGS) for diagnostics of combusting and non-combusting flows is described. The first use of LITGS to provide in situ calibration of 2-Dimensional temperature distributions generated using Two-Colour Planar Laser-Induced Fluorescence (TC-PLIF) is reported. Time-resolved measurements of temperature distributions in a firing GDI optical engine obtained by TC-PLIF were made during the compression stroke and calibrated to the absolute temperature scale by simultaneous LITGS measurements. The accuracy and precision of the temperatures derived from LITGS data are evaluated using alternative methods of data analysis - Fast Fourier Transform and Fitting to theoretical models of the experimental data. The relative merits of the two methods are examined for analysis of weak LITGS signals obtained under engine conditions of low pressure and high temperature. The combined TC-PLIF and LITGS system was demonstrated by performing repeated single-shot measurements for 1 in every 10 four-stroke cycles showing excellent correlation of the temperatures derived from both techniques. Direct measurement of the effect of 'charge cooling', of order 5 K, for operation with direct injection is reported. Inhomogeneous temperature distributions were observed during the compression stroke for fired operation with Port Fuel Injection (PFI) and also with Gasoline Direct Injection (GDI). The effects of varying the relative concentrations of toluene and iso-octane in the two-component fuel were investigated. Extension of the LITGS technique to multi-point measurements along a 1-D line is described. By recording signals from 4 points on separate detectors using a fibre-coupled photodiode array the limitations of Streak Cameras used previously for 1-D LITGS measurements were overcome. Demonstration of principle experiments are reported in which simultaneous 4-point measurements were made with 1 mm spatial resolution and a precision of 0.7 % in temperature gradients in gas flows and in boundary layers at surfaces.

Digital Image Correlation (DIC) is a technique which uses images taken before and after deformation to determine displacement and strain data over the surface of the sample. In order to obtain this data for both in-plane as well as out-of-plane direction, multiple views of the sample are required. Typically, this is accomplished using multiple cameras, but it is possible to use diffraction gratings to bend the light coming from the specimen in order to allow a single camera to capture multiple views. This technique is referred to as Diffraction Assisted Image Correlation (DAIC) and has been previously demonstrated at room temperature. This work expands this method for use at high temperatures by incorporating the use of ultraviolet (UV) lights for illumination and filtering out the light in the visible spectrum. This increases the temperature at which useful images can be captured by reducing the glow that specimens produce at elevated temperatures. When not filtered out, this glow saturates the camera sensor making DIC impossible. This new technique is referred to as Ultraviolet Diffraction Assisted Image Correlation (UV-DAIC).

Soft X-ray spectroscopy is a useful observational tool, offering information about high-temperature (10⁶ -- 10⁷ K) astrophysical plasmas and providing useful characterizations of a number of energetic systems, including accreting young stars, cosmic filaments between galaxies, and supermassive black holes. In order to yield high resolution spectra with good signal-to-noise, however, soft X-ray spectrometers must realize improvements in resolving power and effective area through the development of high performance gratings. Off-plane reflection gratings offer the capability to work at high dispersions with excellent throughput, and are a viable candidate technology for future X-ray spectroscopy missions. The off-plane geometry requires a customizable grating meeting distinct fabrication requirements, and a process for producing gratings meeting these requirements has been developed. These fabricated gratings have been evaluated for performance in terms of resolution and diffraction efficiency. Furthermore, these gratings have been conceptually implemented in a soft X-ray spectrometer, the Off-Plane Grating Rocket Experiment (OGRE), whose optical design provides a template for future missions to achieve high performance within a small payload envelope.

Les travaux portent sur les multiplexeurs optiques fonctionnant à mi-IR pour la source à large bande dans l'application de détection de gaz. Deux configurations ont été étudiées: réseau de guides d'onde (AWG) et réseau concave planaire (PCG). Premièrement, le principe du fonctionnement a été compris afin de développer une solution analytique pour le champ de sortie en utilisant une approximation gaussienne du champ et de l'optique de Fourier. Ensuite, un outil de simulation semi-analytique de la réponse spectrale pour les deux configurations de multiplexeur a été développé dans MATLAB. La distribution normale des erreurs de phase a été introduite dans le modèle semi-analytique AWG, ce qui nous a permis d'étudier la corrélation entre l'écart-type des erreurs de phase et le niveau de diaphonie de la réponse spectrale AWG. AWG à 5,65 μm a été fabriqué à partir de la technologie SiGe / Si à l'aide de l'outil MATLAB pour le calcul des paramètres de conception et de l'outil P.Labeye pour le calcul de la géométrie AWG. Les dispositifs avec des paramètres légèrement variables ont été caractérisés: AWG1 avec guides d'ondes de 4,6 μm et MMI de 9 μm; AWG2 avec guides d'ondes de 4,6 μm et MMI de 11 μm; AWG3 avec guides d'ondes de 4,8 μm et MMI de 9 μm. Des mesures des dispositifs sur la puce 36 (centre de la plaquette) et sur la puce 32 (côté de la plaquette) ont été effectuées et analysées. Les mesures de température de AWG2 et AWG3 (puce 32 et puce 36) aux points cinq points de température ont montré une dépendance linéaire du déplacement spectral avec la température qui a une bonne corrélation avec les prédictions de simulation. / The work focuses on optical multiplexers operating in mid-IR for broadband source in gas sensing application. Two configurations were studies – arrayed waveguide grating (AWG) and planar concave grating (PCG). First, principle of operation was understood in order to develop analytical solution for output field using Gaussian approximation of the field and Fourier Optics. Then, semi-analytical simulation tool of the spectral response for both multiplexer configurations was developed in MATLAB. Normal distribution of phase errors was introduced to semi-analytical AWG model, which allowed us to study the correlation between standard deviation of phase errors and the level of crosstalk of AWG spectral response. AWG at 5.65 µm was fabricated based on SiGe/Si technology using the MATLAB tool for design parameters calculation and P.Labeye’s tool for AWG geometry calculation. Devices with slightly varying parameters were characterized: AWG1 with 4.6 µm waveguides and 9µm MMI; AWG2 with 4.6 µm waveguides and 11µm MMI; AWG3 with 4.8 µm waveguides and 9µm MMI. Measurements of devices on chip 36 (center of the wafer) and chip 32 (side of the wafer) were performed and analyzed. Temperature measurements of AWG2 and AWG3 (chip 32 and chip 36) at points five temperature points showed linear dependence of spectral shift with the temperature which has a good correlation with simulation predictions.