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Due to the wide range of applications in structural health monitoring and the advantages of stability and flexibility in harsh environment, the Brillouin scattering based distributed fibre optic sensors have attracted significant attentions in recent decades. These sensors can quantitatively measure the distributed strain and temperature information along the optic fibre. The Brillouin optical time-domain reflectometry (BOTDR) has the advantages of single-end access and simple operation at the construction site, which makes it very popular in civil engineering applications. Furthermore, the demand of the dynamic sensing of strain information is becoming stronger and stronger, along with the rapid developments in some civil engineering fields such as geophysical sciences and the oil and gas industries. Therefore, the dynamic measurement of strain using the BOTDR system is a very practical and promising topic, especially for the civil engineering related industries. In this study, a BOTDR optical system is set up. A high extinction ratio modulator is chosen to overcome the drawbacks of the power leakage caused by the modulator. A high-speed polarization scrambler is used for the stable dynamic measurement of the BOTDR system. The coherent detection is adopted in this setup to boost the scattered Brillouin power from the optical fibre. A method realizing the distributed dynamic strain measurement of BOTDR is proposed and experimentally demonstrated using small gain stimulated Brillouin scattering (SBS) based on the short-time Fourier transform signal processing algorithm. The concept of small gain SBS is put forward for the first time. The backscattered Brillouin power from the fibre under test and the signal-to-noise ratio of the system output are enhanced by the small gain SBS. The input power limits of the BOTDR are discussed, and it is found that the modulation instability is the dominant input threshold for this BOTDR system. The frequency uncertainties of the Brillouin frequency shift, with given pulse durations, fibre lengths and number of averaging, are calculated. The 60Hz sinusoidal strain vibrations on the optic fibre under test are experimentally detected with 4m spatial iv Word Template by Friedman & Morgan 2014 resolution and 5.1MHz Brillouin frequency uncertainty. The scattered Brillouin power is captured with different input optical power, indicating the induced small gain SBS on the optic fibre. Then, an image denoising method using the convolutional neural network (CNN) is applied to the derived Brillouin gain spectrum (BGS) images to enhance the performance of the Brillouin frequency shift detection and the strain vibration measurement of the small gain SBS BOTDR system. By reducing the noise of the BGS images along the length of the fibre under test with different network depths and epoch numbers, smaller frequency uncertainties are obtained, and the sine-fitting R-squared values of the detected strain vibration profiles are also higher. To further improve the detection of the vibration, the BGS images along the time axis are drawn for the fibre section with vibration and denoised by the neural network. With the context of neighbouring spectra over time during the denoising process, the detected strain vibration profiles of the BOTDR system are smoother with the smallest Brillouin frequency uncertainty of 2.32MHz and the sine fitting R-squared values are enhanced with the maximal value of 0.931. After that, the methods to expand the receptive fields of the denoising CNN without deepening the network are introduced to find a way to further denoise the BGS images and boost the strain vibration detection of the BOTDR system. The two main methods of the U-net CNN denoising and the dilated CNN denoising are used for the BGS image denoising respectively. The U-net CNN demonstrates a worse detection of the Brillouin frequency shift, as the local information is lost during the denoising process. On the contrary, the dilated CNN method leads to even better detected frequency accuracies and better sine-fitting r-square values of vibration profiles, compared with the denoising CNN method. By denoising the BGS images along the time axis over the fibre section with vibration, the detected strain vibration profiles are more sinusoidal with the maximal R-squared value of 0.966, and the frequency uncertainty is further improved to 2.21MHz.

Cavity quantum optomechanics is a field of investigation which studies the interaction between optical and mechanical degrees of freedom applying the powerful methods of quantum optics. These theoretical methods and experimental techniques have been developed through the second half of the 20th century and gained significant momentum following the invention of the laser. Upon the realisation that coherent manipulation of the motion of mesoscopic, or even macroscopic, mechanical systems was feasible using laser light, interest in cavity quantum optomechanics grew and groups across the globe began working towards the preparation of non-classical mechanical states. The field is now well established, but the preparation of non-classical states of the motion via interaction with optical fields remains challenging to explore, which is believed will open the door to studies of fundamental physics regarding decoherence, the quantum-to-classical transition or may even shed light on the interface between quantum mechanics and gravity. This thesis explores Brillouin scattering in whispering-gallery-mode microresonators. The parametric coupling between high frequency (11 GHz) elastic waves and infrared (1550 nm) light via electrostriction is used to demonstrate several interesting cavity optomechanical phenomena. This thesis contributes to developing this new approach to optomechanics in four key ways. By use of a pair of optical resonances of different transverse structure spaced by the material's Brillouin shift, reaching the cavity optomechanical strong coupling regime was experimentally demonstrated with a fused silica microrod resonator. Operation in this regime is crucial for many optomechanical protocols, importantly optomechanical state-swap between the optical and mechanical modes. A similar resonator was employed to demonstrate measurement-enhanced optomechanical sideband cooling. Here the measurement record of the continuously monitored heterodyne measurement of Brillouin anti-Stokes scattered light is used to reduce the phase-space uncertainty of the mechanical state, effectively cooling the mode beyond the sideband-cooling limit. For this purpose the conservation of Gaussianity of the state (initially a thermal state) under linear measurement (heterodyne detection) is used within a stochastic master equation approach. In another experiment, single-phonon addition and subtraction to a mechanical thermal state was demonstrated, showing a characteristic doubling of the mean phonon number of the mechanical mode. For this experiment a crystalline microresonator of barium fluoride was used, a material which allows reaching superb optical quality factors combined with highest elastic isotropy for a crystalline substance. The non-Gaussianity of the single-quantum subtracted state is demonstrated. Finally, the platform of Brillouin cavity optomechanics with high-frequency phonons in crystalline (barium fluoride) whispering-gallery resonators at cryogenic temperatures is discussed.

Distributed fibre optic sensing (DFOS) is essential for structural health monitoring (SHM) of strain changes induced during the lifetime of a structure. Among different DFOS systems, the Brillouin Optical Time Domain Reflectometry (BOTDR) takes the advantages of obtaining full frequency spectrum to provide strain and temperature information along the optic fibre. The key parameters of distributed fibre optic sensors, spatial and frequency resolution, are strongly linked with the time-frequency (T-F) localisation in the system in three parts: pulse, hardware design and optical fibre. T-F localization is fundamentally important for the communication system, whereas in this study the importance of the T-F localisation to the spatial and frequency resolution, repeatability and the measurement speed are introduced in BOTDR. In this dissertation, the development of DFOS is first introduced, including both traditional methods and new developed designs. The literature review shows the signal to noise ratio (SNR) of BOTDR can be improved by investigating its T-F localisation. In the hardware design, in order to improve the T-F localisation in hardware architecture, a Short-Time Fourier Transform-Brillouin Optical Time-Domain Reflectometry (STFT-BOTDR), which implements STFT over the full frequency spectrum to measure the distributed temperature and strain along the optic fibre, is applied so that the conventional frequency sweeping method can be replaced for high resolution and fast speed measurement, providing new research advances in dynamic distributed sensing. The STFT based BOTDR has better T-F localisation, which in turn provides an opportunity for off-line post signal processing that is more adaptable for fast speed measurements. The spatial and frequency resolution of dynamic BOTDR sensing is limited by the Signal to Noise Ratio (SNR) and the T-F localization of the input pulse shape. The T-F localized input pulse shape can enhance the SNR and the spatial and frequency resolution in STFT-BOTDR. In this study, simulation and experiments of T-F localized different pulses shapes are conducted to examine the limitation of the system resolution. The result indicates that a rectangular pulse should be selected to optimize the spatial resolution and a Lorentzian pulse could be chosen to optimize the frequency resolution, while a Gaussian shape pulse can be used in general applications for its balanced performance in both spatial and frequency resolution. Meanwhile, T-F localization is used for pulse T-F localisation optimisation. A set of Kaiser-Bessel functions is used to simulate different pulse shapes and to compare their parameters in terms of T-F localisation and their Brillouin scattering spectrum. A method using an iterative filtering algorithm to achieve the optimised pulse in terms of T-F localisation is introduced to converge the Effective-pulse Width (TEW) in the time-domain and Effective-pulse Linewidth (FEL) in the frequency domain to identify the fundamental limitations. The optimised pulse can be fitted with a 7th order Gaussian (super-Gaussian) shape and it offers the best experimental performance compared to a Rectangular pulse. The sensitivity of a sensor to strain or temperature variations due to distributed Brillouin scattering is closely related to the power distribution on the Brillouin scattering spectrum which is related to the property of the optic fibre. The performance of a highly nonlinear fibre that can generate a higher Brillouin scattering signal is compared to that of a standard single mode fibre. The results show that much higher SNR of the Brillouin scattering spectrum and smaller frequency uncertainties in the sensing measurement can be achieved by using a highly nonlinear fibre for comparable launched powers. With a measurement speed of 4 Hz, the frequency uncertainty can be 0.43 MHz, corresponding to 10 με in strain or 0.43°C in temperature uncertainty for the tested highly nonlinear fibre. In contrast, for a standard single mode fibre, the value would increase to about 1.02 MHz (25 με or 1.02°C), demonstrating the advantage of the tested highly nonlinear fibre for distributed strain/temperature sensing. Results show that, by using a small effective area highly nonlinear fibre, the strain or temperature resolution can be improved because it generates stronger Brillouin scattering signal with high SNR and high Q factor spectrum, both of which determine the optimal averaging time in a single measurement. In general, the STFT-BOTDR can achieve 1 m spatial resolution, 10 με frequency resolution on a 10 km fibre with measurement speed at about 2.5 kHz.

There are an increasing number of applications for high power lasers in medicine, industry and the military. Fiber lasers present an attractive alternative to traditional laser types;however, when operating at high power levels, fiber lasers are subject to detrimental nonlinear effects. Stimulated Brillouin scattering (SBS) is a nonlinear acousto-optic interactionwhere a stimulated acoustic wave scatters light, which limits the laser output. This thesispresents a theoretical and experimental framework for Brillouin scattering in continuouswave and pulsed beam regimes. Stimulated Brillouin scattering was observed in opticalfibers in stationary and transient beam excitations and Brillouin frequencies are presentedfor different optical fibers with pulsed beam widths ranging from 15 ns to 500 ns.

Optical resonators are the basis of any experiment where it is desirable to confine light. The most basic optical resonator, a Fabry-Perot etalon, can be as simple as a colorful layer of oil over water, or as complicated as the parallel mirrors in the Laser Interferometer Gravitational-Wave Observatory. When looking for new phenomena, it is often useful to increase intensity. One example is optomechanics which was born after dissipation in resonators was reduced to a level where deformation caused by light pressure became significant. Intensity is related to the circulating power over the transverse area of the mode. Whispering gallery resonators can be ideal as they offer quality factors over 100 billion and can confine light to a transverse mode area smaller than a wavelength squared. While previous work in optomechanics relied on the force of light on the device walls, here we use the force of light on an acoustical density wave. We demonstrate a new family of optomechanics based on Brillouin scattering. Brillouin scattering is most widely known as a loss mechanism in telecom applications in which stimulated scattering limits the usable power. Here we show that stimulated Brillouin scattering of light from sound can be used as an optomechanical actuation mechanism for high frequency (11 GHz) acoustical vibrations. Owing to ultrahigh optical and mechanical quality factors, we are able to excite mechanical vibrations in Silica whispering gallery resonators at microWatt input powers and gained for the first time access to mechanical WGM in microresonators. While WGMs were first studied as mechanical phenomena in domed cathedrals, the term is currently used to describe optical modes despite the fact that light does not whisper. Here I enabled access to real (acoustical) whispering gallery modes in microresonators which further enabled us to transform Brillouin scattering into a cooling process. The combination of forward stimulated Brillouin scattering and backward stimulated Brillouin scattering allowed excitation of modes from 50 MHz to 11 GHz in frequency, and enabled for the first time reversing the energy transfer direction in the Brillouin process to allow cooling.

This dissertation addresses how variations in the particle-matrix interface, particle size, geometry, and volume fraction affect the mechanical properties of layer-by-layer (LBL) assembled thin film materials. Films are primarily assembled using the linear-growth LBL method, which results in a layered structure with distinct interfaces. Particles with rod-like, sphere-like and plate-like geometries are incorporated in between polymer layers at various volume fractions. The volume fraction is adjusted by using different polymers and precursors with variable polymer concentration. In addition, one set of films is assembled using exponentially grown LBL, which results in the formation of diffuse interfaces, to study the role of interface definition. In these films, fumed silica aggregates are added during deposition to mark the location of the original interfaces. The relationship between structure and mechanical properties is studied using Brillouin light scattering. In LBL composites with rod-shaped nano-fillers, the in-plane elastic modulus increases continuously as a function of particle loading, whereas the out-of-plane modulus tails off or even reaches a maximum. The decrease in rigidity in this direction is due to the lack of continuous wetting of fibers by the polymer. Incorporation of spherical fillers results in a larger out-of-plane than in-plane modulus, which is due to the more effective particle-polymer-particle connections and load transfer as a direct consequence of the deposition process. Porosity in these films affects the in-plane Load transmission more prominently. For LBL nano-composites with plate-like particles, the beneficial effect of adding reinforcing particles tapers off because of the transmission of shear forces between particles becomes less efficient with thinner polymer layers. In the latter, structural relaxation occurs more rapidly and they respond more compliantly as a result. Diffuse interface between LBL layers result in the orientation of polymer chains in the film growth direction. As a result, the out-of-plane elastic modulus is higher than the in-plane modulus, and transport of small molecules across layers is facilitated. Addition of nano-particles neither improves nor deteriorates mechanical properties in these films. This work is the first of its kind to provide these new insights into the mechanics of layered thin films.