Theoretical Study Of Beam Transformations By Volume Diffraction

Laser beams can be manipulated by volume diffractive elements in addition to conventional optical elements like mirrors, lenses, and beam splitters. Conventional optical elements can be described by applying the basic laws of reflection and refraction at the surfaces of the elements. Even diffraction by surface gratings utilizes relatively simple mathematics. This is to be contrasted with the volume diffraction, which requires coupled wave theory in the slowly varying envelope approximation (SVEA) to obtain accurate results. Efficient spatially distributed diffraction of laser beams is possible due to the high coherence of laser light, and it occurs at specific resonant Bragg conditions. This research work is inspired and driven by the successful development of recording technology for robust, high-efficiency volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass. Mostly VBGs of the reflective type are discussed in this dissertation. Starting with an analysis of electro-magnetic wave propagation in layered media, we have reformulated Fresnel and volume reflection phenomena in terms of a convenient parameter – strength of reflection. The influence that the different non-uniformities inside a VBG have on its spectral properties has been examined. One important result of this work is the proposal of moiré VBG and the derivation of an analytical expression for its bandwidth. A multiplexed VBG used as a coherent combiner is discussed as well. Beam distortion via transmission through and/or reflection by a heated VBG due to residual absorption is analyzed

Integrated polarisation rotators

The ability to control and manipulate the state of polarisation of optical signals is becoming an increasingly desirable feature in numerous applications including integrated optical circuits, semiconductor optical amplifiers (SOAs) and optical communication systems. This thesis introduces the design, optimisation, fabrication and operation of two novel integrated reciprocal single-section passive polarisation converter devices based upon mode-beating. The converter designs consist of asymmetric profiled waveguides, which were realised in a single reactive ion dry-etch process step. An in-situ custom built sample holder was utilised to place the samples at a predetermined angle to the incoming ions, which resulted in waveguide profiles with sloped sidewalls. This fabrication technique also allowed the incorporation of adiabatic taper sections within the device design. The converter section waveguide profile of the first design consists of two sloped sidewalls. Devices realised on a GaAs/AlGaAs material structure achieved a converted transverse magnetic (TM) polarisation purity of 81.4% at a device length of 30 μm for a transverse electric (TE) polarised input signal at an operating wavelength of λ = 1064 nm. The convention used is that TE refers to light polarised in the plane of the wafer and TM refers to light polarised perpendicular to the plane of the wafer. The total optical loss imposed by this device was evaluated to be 1.72 dB. This design was also used for the monolithic integration of a passive polarisation converter incorporated within a Fabry-Perot semiconductor laser diode on an unstrained GaAs/AlGaAs double quantum well heterostructure material system. A predominantly TM polarised optical output from the converter facet of greater than 80% is demonstrated for a converter length of 20 μm at an emitting wavelength of 867.1 nm. The about 1.4 mm long fabricated device has a current threshold level of 100 mA and a side mode suppression ratio (SMSR) of 25 dB. The second converter design is based on the modification of an already existing stripe waveguide structure. The converter section is defined by applying the above mentioned angled dry-etch process on a certain length of the stripe waveguide. The fabricated asymmetric waveguide core profile consists of a sloped undercut. A TM polarisation purity of 90% at a device length of 55 μm for a TE polarised input signal at an operating wavelength of λ = 1064 nm was achieved at the output. The total optical loss imposed by this device was evaluated to be 0.47 dB

Research Studies on Advanced Optical Module/Head Designs for Optical Disk Recording Devices

The Annual Report of the Optical Data Storage Center of the University of Arizona is presented. Summary reports on continuing projects are presented. Research areas include: magneto-optic media, optical heads, and signal processing

Etude et développement de matériaux micro/nano structurés pour l’ingénierie des bandes interdites dans les dispositifs électro-acoustiques à ondes de surface

This work concerns the study of micro/nano structured materials for the engineering of band structures in the field of elastic waves. We were interested in particular to the integration of these materials in electro-acoustic devices and the study of the interaction with the surface acoustic waves.The approach is to carry out the simulation using the finite element method to calculate the band structures and the transmission spectra. We studied the effect of geometrical and elastic parameters of micro-pillars on acoustic branches representing surface modes. Then we discussed the effect of the symmetry of the arrangement on the polarization of the surface modes. We also investigated the effect of the symmetry on the sensitivity of surface modes with the variation of temperature.Experimentally, we have developed interdigital transducers on a piezoelectric substrate of LiNbO3. We have fabricated various phononic crystals composed of nickel micro-pillars, obtained by electrodeposition. The transmission spectra were measured by a network analyzer and compared with the theoretical results.Besides the phononic crystals based on nickel pillars, some other periodic micro/nano structures were also involved in this work, such as two dimensional materials based on self-assembled magnetic nanoparticles and nickel nanowires electroplated through nano-porous alumina membranes.Ce travail porte sur l’étude de matériaux micro/nano structurés permettant l’ingénierie des structures de bande dans le domaine des ondes élastiques. Nous nous sommes intéressés en particulier à l’intégration de ces matériaux dans les dispositifs électro-acoustiques et l’étude de l’interaction avec les ondes acoustiques de surface.La démarche consiste à mener des simulations par la méthode des éléments finis, pour calculer les structures de bande et les spectres de transmission. Nous avons étudié l’effet des paramètres géométriques et élastiques des micro-plots sur les branches acoustiques représentant les modes de surface. Nous avons ensuite discuté l’effet de la symétrie de l’arrangement sur la polarisation des modes de surface. Nous avons également étudié l’effet de la symétrie sur la sensibilité des modes de surface à une variation de température.Sur le plan expérimental, Nous avons élaboré des transducteurs inter-digités sur un substrat piézoélectrique de LiNbO3. Nous avons intégré divers cristaux phononiques composés de micro-plots de Ni, obtenues par électrodéposition. Les spectres de transmission ont été mesurés à l’aide d’un analyseur de réseau et comparés aux résultats theoriques.En dehors des cristaux phononiques basés sur des plots du nickel, d’autres structures ont également été présentées dans ce travail, incluant des matériaux bidimensionnels à base de nanoparticules magnétiques auto-assemblées et des nanofils du nickel électrodéposés à travers des membranes nano-poreuses d’alumine

Research studies on advanced optical module/head designs for optical devices

A summary is presented of research in optical data storage materials and of research at the center. The first section contains summary reports under the general headings of: (1) Magnetooptic media: modeling, design, fabrication, characterization, and testing; (2) Optical heads: holographic optical elements; and (3) Optical heads: integrated optics. The second section consist of a proposal entitled, Signal Processing Techniques for Optical Data Storage. And section three presents various publications prepared by the center