Thesis (BSc. (Hons))--Australian National University, 2002. / Available via the Australian National University Library Electronic Pre and Post Print Repository. Title from title screen (viewed Mar. 28, 2003). "A thesis submitted for the degree of Bachelor of Science with Honours in physics at the Australian National University" "November 2002" Bibliography: p. 73-75.

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<p>This thesis concerns some experimental and theoretical issues in fiber optics. In particular, properties and devices based on photonic crystal fibers (PCFs) are investigated.</p><p>The work can be grouped into three parts. In the first part we use sound to control light in PCFs. The lowest order flexural acoustic mode of various PCFs is excited using an acoustic horn. The acoustic wave acts as a traveling long-period grating. This is utilized to couple light from the lowest order to the first higher order optical modes of the PCFs. Factors affecting the acoustooptic coupling bandwidth are also investigated. In particular, the effect of axial variations in acoustooptic phase-mismatch coefficient are studied.</p><p>In the second part of the thesis we use an electric field to control transmission properties of PCFs. Tunable photonic bandgap guidance is obtained by filling the holes of an initially index-guiding PCF with a nematic liquid crystal and applying an electric field. The electric field introduces a polarization-dependent change of transmission properties above a certain threshold field. By turning the applied field on/off, an electrically tunable optical switch is demonstrated.</p><p>The third part consists of two theoretical works. In the first work, we use relativistic causality, i.e. that signals cannot propagate faster than the vacuum velocity of light, to show that Kramers-Kronig relations exist for waveguides, even when material absorption is negligible in the frequency range of interest. It turns out that evanescent modes enter into the Kramers-Kronig relations as an effective loss term. The Kramers-Kronig relations are particularly simple in weakly guiding waveguides as the evanescent modes of these waveguides can be approximated by the evanescent modes of free space. In the second work we investigate dispersion properties of planar Bragg waveguides with advanced cladding structures. It is pointed out that Bragg waveguides with chirped claddings do not give dispersion characteristics significantly different from Bragg waveguides with periodic claddings.</p>

This thesis concerns some experimental and theoretical issues in fiber optics. In particular, properties and devices based on photonic crystal fibers (PCFs) are investigated. The work can be grouped into three parts. In the first part we use sound to control light in PCFs. The lowest order flexural acoustic mode of various PCFs is excited using an acoustic horn. The acoustic wave acts as a traveling long-period grating. This is utilized to couple light from the lowest order to the first higher order optical modes of the PCFs. Factors affecting the acoustooptic coupling bandwidth are also investigated. In particular, the effect of axial variations in acoustooptic phase-mismatch coefficient are studied. In the second part of the thesis we use an electric field to control transmission properties of PCFs. Tunable photonic bandgap guidance is obtained by filling the holes of an initially index-guiding PCF with a nematic liquid crystal and applying an electric field. The electric field introduces a polarization-dependent change of transmission properties above a certain threshold field. By turning the applied field on/off, an electrically tunable optical switch is demonstrated. The third part consists of two theoretical works. In the first work, we use relativistic causality, i.e. that signals cannot propagate faster than the vacuum velocity of light, to show that Kramers-Kronig relations exist for waveguides, even when material absorption is negligible in the frequency range of interest. It turns out that evanescent modes enter into the Kramers-Kronig relations as an effective loss term. The Kramers-Kronig relations are particularly simple in weakly guiding waveguides as the evanescent modes of these waveguides can be approximated by the evanescent modes of free space. In the second work we investigate dispersion properties of planar Bragg waveguides with advanced cladding structures. It is pointed out that Bragg waveguides with chirped claddings do not give dispersion characteristics significantly different from Bragg waveguides with periodic claddings.

[Truncated abstract] Advanced laser interferometer gravitational wave detectors require an extremely high optical power in order to improve the coupling between the gravitational wave signal and the optical field. This high power requirement leads to new physical phenomena arising from nonlinear interactions associated with radiation pressure. In particular, detectors with multi-kilometer-long arm cavities containing high density optical fields suffer the possibility of 3-mode opto-acoustic interactions. This involves the process where ultrasonic vibrations of the test mass cause the steady state optical modes to scatter. These 3-mode interactions induce transverse optical modes in the arm cavities, which then can provide positive feedback to the acoustic vibrations in the test masses. This may result in the exponential growth of many acoustic mode amplitudes, known as Parametric Instability (PI). This thesis describes research on 3-mode opto-acoustic interactions in advanced interferometric gravitational wave detectors through numerical investigations of these interactions for various interferometer configurations. Detailed analysis reveals the properties of opto-acoustic interactions, and their dependence on the interferometer configuration. This thesis is designed to provide a pathway towards a tool for the analysis of the parametric instabilities in the next generation interferometers. Possible techniques which could be helpful in the design of control schemes to mitigate this undesirable phenomenon are also discussed. The first predictions of parametric instability considered only single interactions involving one transverse mode and one acoustic mode in a simple optical cavity. ... In Chapter 6, I was able to make use of a new analytical model due to Strigin et al., which describes parametric instability in dual recycling interferometers. To make the solution tractable, it was necessary to consider two extreme cases. In the worst case, recycling cavities are assumed to be resonant for all transverse modes, whereas in the best cases, both recycling cavities are anti-resonant for the transverse modes. Results show that, for the worst case, parametric gain values as high as ~1000 can be expected, while in the best case the gain can be as low as ~ 3. The gain is shown to be very sensitive to the precise conditions of the interferometer, emphasising the importance of understanding the behaviour of the detectors when the cavity locking deviates from ideal conditions. Chapter 7 of this thesis contains work on the observation of 3-mode interactions in an optical cavity at Gingin, which confirms the analysis presented here, and also a paper which shows how the problem of 3-mode interactions can be harnessed to create new devices called opto-acoustic parametric amplifiers. In the conclusions in Chapter 8, I discuss the next important steps in understanding parametric interactions in real interferometers – including the need for more automated codes relevant to the design requirements for recycling cavities. In particular, it is pointed out how the modal structure of power and signal recycling cavities must be understood in detail, including the Gouy phase for each transverse mode, to be able to obtain precise predictions of parametric gain. This thesis is organised as a series of papers which are published or have been submitted for publication. Such writing style fills the condition for Ph.D. thesis at the University of Western Australia.

Orientador: Hugo L. Fragnito / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-09T05:15:29Z (GMT). No. of bitstreams: 1 DaineseJunior_PauloClovis_D.pdf: 4977952 bytes, checksum: 68cd4c3ce792cf4994babe5cce9b77a9 (MD5) Previous issue date: 2006 / Resumo: Esta tese apresenta estudos experimentais e teóricos sobre o processo de espalhamento Brillouin em Fibras de Cristal Fotônico. Formadas por um núcleo sílica pura e uma casca micro-estruturada (sílica e ar), estas fibras permitem o confinamento óptico e acústico em regiões da ordem do comprimento de onda. Como resultado, a interação acústo-óptica apresenta características radicalmente diferentes daquelas observadas em meio bulk ou em fibras convencionais. Investigamos experimentalmente ambos co- e retro-espalhamento Brillouin. Observamos que quando o diâmetro do núcleo é ~70 % do comprimento de onda óptico no vácuo, o espectro de retro-espalhamento espontâneo apresenta múltiplos picos, os quais atribuímos a famílias de modos acústicos guiados no núcleo da fibra. Além disso, o limiar de retro-esplhamento Brillouin estimulado aumenta por um fator ~5 quando o diâmetro do núcleo é diminuído de 8 .m para 1.22 .m , resultado da natureza complexa dos modos acústicos no núcleo, contendo fortes componentes de deslocamento longitudinal e transversal. No caso de co-espalhamento, realizamos medidas de espalhamento espontâneo e de excitação impulsiva de ondas acústicas utilizando pulsos ópticos de alta intensidade, através do efeito de eletrostrição. Estes experimentos nos possibilitaram observar o confinamento transversal de ondas acústicas no núcleo da fibra fotônica. Desenvolvemos um modelo analítico para a interação acústo-óptica, aproximando o núcleo da fibra como um cilindro de silica suspenso no vácuo, sem a presença da casca. Este modelo nos permitiu entender a física envolvida no processo e também explicar qualitativamente as observações experimentais. Modelos numéricos mais sofisticados foram utilizados para o cálculo dos modos acústicos e óptico suportados pela estrutura completa da fibra fotônica, os quais nos permitiram explicar mais precisamente as observações experimentais. Finalmente, realizamos cálculos numéricos da estrutura de bandas da região micro-estruturada, demonstrando a presença de bandas proibidas (ou gaps fonônicos) para as ondas acústicas / Abstract: This thesis presents experimental and theorethical studies on Brillouin scattering in Photonic Crystal Fibers. With a pure silica core surrounded by a microstructed cladding (silica and air), these fibers allow the confinement of both acoustic and optical waves in sub-wavelength regions. The result is a radically different acousto-optic interaction from what has been observed in bulk media or conventional fibers. We investigate experimentally both forward and backward Brillouin scattering. We observed that for core diameters of around 70% of the vacuum wavelength of the launched laser light, the spontaneous Brillouin signal develops an unusual multi-peaked spectrum, these peaks we attribute to several families of guided acoustic modes. At the same time the threshold power for stimulated Brillouin scattering increases five-fold when the core diameter is reduced from from 8 .m to 1.22 .m , as a consequence of the complex nature of the acoustic modes, each with different proportions of longitudinal and shear strain, strongly localised to the core. In the case of forward scattering, we performed measurements of the spontaneous scattering and also of impulsive excitation of acoustic waves using high intensity optical pulses, through the effect of electrostriction. These experiments allowed us to observe the transverse confinment of acoustic waves in the core of the photonic crystal fiber. An analitic model for the acousto-optic interaction was developed by approximating the core of the photonic fiber by a circular strand of glass in vaccum, initially neglecting the presence of the micro-structured cladding. This simple model allowed us to understand the physics involved in the scattering process and also to qualitatevely explain our experimental observations. Numerical models were then implemented to calculate the acoustic and optical modes of the actual photonic fiber structure, and we were able to explain more precisely our observations. Finnally, we performed numerical calculation of the band structure of the micro-structured region, demonstrating the presence of prohibited gaps for the acoustic wave (phononics band gaps) / Doutorado / Física / Doutor em Ciências