A Study of Synchronization Techniques for Optical Communication Systems

The study of synchronization techniques and related topics in the design of high data rate, deep space, optical communication systems was reported. Data cover: (1) effects of timing errors in narrow pulsed digital optical systems, (2) accuracy of microwave timing systems operating in low powered optical systems, (3) development of improved tracking systems for the optical channel and determination of their tracking performance, (4) development of usable photodetector mathematical models for application to analysis and performance design in communication receivers, and (5) study application of multi-level block encoding to optical transmission of digital data

Measuring the Orbital Angular Momentum of Light for Astronomy

While the story of optical orbital angular momentum (OAM) dates back to the development of Maxwell's equations, the study of photon OAM by the physics community begins in earnest in the 1990s, led in part by a paper by Allen et al. describing the independent control of spin and orbital angular momentum in paraxial modes of light. The recognition of the orbital angular momentum of light in astronomy is a much more recent affair. This thesis explores the role of the OAM of light in astronomy and attempts to make the case for the measurement of photon OAM as a new tool in astronomy. Two contributions are made in order to prepare the groundwork for future endeavours: a laboratory assessment of the effectiveness of adaptive optics (AO) systems on atmospheric turbulence when measuring optical OAM, and an initial field test of an instrument measuring the optical OAM spectrum of the sun. Regarding the first study, the author finds that realistic atmospheric turbulence (1'' seeing) severely corrupts any incoming OAM signal at visible wavelengths, in spite of AO correction (<10% power recovered), however results suggest adequate correction at IR wavelengths. In the second study, an instrument to measure the OAM spectrum of a source is constructed and employed to measure the OAM spectrum of local regions of the sun. It represents the first measurement of its kind, distinguishing sunspots by analyzing their OAM spectrum and in addition, demonstrates the improvement of OAM measurements by implementing a lucky imaging routine. Finally, this thesis highlights a new avenue for further study into the measurement of OAM for observational astronomy. A new type of OAM measurement is proposed, capable of measuring rotations in the plane orthogonal to the line of sight. This measurement takes advantage of the rotational Doppler shift, an analogue of the translational Doppler shift, and an OAM interferometer designed to measure the associated phase shift is outlined. A future instrument is also proposed by combining the OAM interferometer with a high resolution spectrograph. This would allow for measurements of both the rotational and translational Doppler shifts, providing information about the three dimensional motion of an object

On non-Gaussian beams and optomechanical parametric instabilities in interferometric gravitational wave detectors

Direct detection of gravitational radiation, predicted by Einstein’s general theory of relativity, remains one of the most exciting challenges in experimental physics. Due to their relatively weak interaction with matter, gravitational waves promise to allow exploration of hitherto inaccessible objects and epochs. Unfortunately, this weak coupling also hinders detection with strain amplitudes at the Earth estimated to be of order 10^−21. Due to their wide bandwidth and theoretical sensitivity, kilometre-scale Michelson style interferometers have become the preferred instrument with which to attempt ground based detection. A worldwide network of first generation instruments has been constructed and prodigious volumes of data recorded. Despite each instrument approaching or having reached its design sensitivity, a confirmed detection remains elusive. Planned upgrades to these instruments aim to increase strain sensitivity by an order of magnitude, commencing the era of second generation detectors. Entry into this regime will be accompanied by an entirely new set of challenges, two of which are addressed in this work. As advanced interferometers are commissioned, instrumental artifacts will give way to fundamental noise sources. In the region of peak sensitivity it is expected that thermal noise in the interferometers’ dielectric mirror coatings will be the principal source of displacement noise. Theory suggests that increasing the spot size of laser light incident on these mirrors will reduce the measured thermal noise. In the first part of this work we examine one method of realising larger spots. By adopting non-spherical mirrors in the interferometers’ arms it is possible to create resonators which support a wide, flat-topped field known as the mesa beam. This beam has been shown to theoretically reduce all forms of mirror thermal noise without being significantly more difficult to control. In this work we investigate these claims using a bespoke prototype mirror. The first results regarding a non-Gaussian beam created in a manner applicable to a gravitational wave interferometer are presented. A common theme among all second generation interferometer designs is the desire to maximise circulating power. This increased power is partnered by commensurately increased thermal perturbations. Since the attractive properties of the mesa beam are produced by the fine structure of its supporting mirrors, it is important that we understand the effects absorption of stored optical power could have on mesa fields. In the second part of this work we report on numerical evaluations of measured thermal noise and mesa beam intensity profile as a function of absorbed power. Increased optical power also has less obvious consequences. As a result of radiation pressure, there exists a pathway between optical energy stored in an interferometer’s arms and mechanical energy stored in the acoustic modes of its test masses. Under appropriate conditions, this coupling can excite one or more test masses to such a degree that interferometer operation becomes impossible. In the final part of this work we determine whether it is possible to mitigate these parametric instabilities using electrostatic actuators originally designed to control the position and orientation of the test masses

The Design and Optimisation of Quasioptical Telescopes

The main focus of this thesis is the analysis and optimisation of systems that operate in the terahertz and submillimetre wavebands. Analysis is carried out on the MBI and ALMA interferometers, and on the HIFI instrument on the Herschel Space Observatory (HSO). MBI is a novel instrument designed to test the technique of bolometric nterferometry. ALMA is a 50 element hetrodyne interferometer, currently being constructed in Chile. It is demonstrated that in both MBI and ALMA, the coupling of the signal to the detector horns may be improved by imposing a phase on the field from the sky; a number of possible configurations are considered. HIFI is a high resolution spectrometer, one of three instruments on the HSO. Simulations of the lens antennas used to detect the radiation in the higher frequency channels in HIFI were carried out. A number of methods used to determine the phase centre of the beam from lens antennas are described, and use is made of the same optimisation techniques as for the MBI and ALMA work. As the beams propagating in these systems can be analysed accurately using the paraxial approximation, Gaussian Beam Mode Analysis can be used to simulate the field, and is the main analytical tool used here. Methods of beam shaping are investigated to gain an insight into how coupling may be increased in these systems, and also to design Diffractive Optical Elements (DOEs) for use at terahertz and submillimetre wavelengths in general. The standard methods of the Gerchberg-Saxton Algorithm, and unidirectional optimisation using Differential Evolution and Simulated Annealing are applied to design DOEs. A novel approach using Gaussian Beam Mode Analysis is described. Here, the Gaussian Beam mode coefficients describing a field are optimised to achieve a desired amplitude distribution at a specified plane or planes. This approach was found to achieve highly optimal results, and has a number of benefits over the other methods

Nonlinear microscopy for failure analysis of CMOS integrated circuits in the vectorial focusing regime

This thesis focuses on the development of techniques for enhancing the spatial resolution and localisation precision in the sub-surface microscopy for failure analysis in semiconductor integrated circuits (ICs). Highest spatial resolutions are obtained by implementing solid immersion lenses (SIL), which provide unsurpassed numerical aperture (NA) for sub-surface microscopy. These high NA conditions mean that scalar diffraction theory is no longer valid and a vectorial focusing description should be applied to accurately describe the focal plane electric field distribution. Vectorial theory predicts that under high NA conditions a linearly polarised (LP) light focuses to a spot that is extended along the electric field vector, but radially polarised (RP) light is predicted to form a circular spot whose diameter equals the narrower dimension obtained with linear polarisation. By implementing a novel liquid-crystal (LC) radial polarisation converter (RPC) this effect was studied for both two-photon optical-beam-induced current (TOBIC) microscopy and two-photon laser assisted device alteration (2pLADA) techniques, showing a resolution and localisation improvement using the RP beam. By comparing images of the same structural features obtained using linear, circular and radial polarisations imaging and localisation resolutions both approaching 100 nm were demonstrated. The obtained experimental results were in good agreement with modelling and were consistent with theoretically predicted behaviour. Certain artefacts were observed under radial polarisation, which were thought to result from the extended depth of focus and the significant longitudinal field component. In any application these effects must be considered alongside the benefits of the symmetric field distribution in the focal plane. While SIL sub-surface microscopy offers unmatched spatial resolutions, it is prone to being severely degraded by aberrations arising from inaccurate dimensions of the SIL, imprecise substrate thickness or imperfect contact between SIL and substrate. It is in this context that techniques to identify and even mitigate aberrations in the system are important. A simple approach is demonstrated for revealing the presence of chromatic and spherical aberrations by measuring the two-photon autocorrelation of the pulses at the focal plane inside the sample. In the case of aberration free imaging, it was shown both theoretically and experimentally that the planes of the maximum autocorrelation amplitude and shortest pulse duration always coincide. Therefore, the optics of the imaging system can be first adjusted to obtain the minimum autocorrelation duration and then the wavefront of incident light modified to maximise the autocorrelation intensity, iterating this procedure until the positions of minimum pulse duration and maximum autocorrelation amplitude coincide

Advances in Trans-dimensional Geophysical Inference

This research presents a series of novel Bayesian trans-dimensional methods for geophysical inversion. A first example illustrates how Bayesian prior information obtained from theory and numerical experiments can be used to better inform a difficult multi-modal inversion of dispersion information from empirical Greens functions obtained from ambient noise cross-correlation. This approach is an extension of existing partition modeling schemes. An entirely new class of trans-dimensional algorithm, called the trans-dimensional tree method is introduced. This new method is shown to be more efficient at coupling to a forward model, more efficient at convergence, and more adaptable to different dimensions and geometries than existing approaches. The efficiency and flexibility of the trans-dimensional tree method is demonstrated in two different examples: (1) airborne electromagnetic tomography (AEM) in a 2D transect inversion, and (2) a fully non-linear inversion of ambient noise tomography. In this latter example the resolution at depth has been significantly improved by inverting a contiguous band of frequencies jointly rather than as independent phase velocity maps, allowing new insights into crustal architecture beneath Iceland. In a first test case for even larger scale problems, an application of the trans-dimensional tree approach to large global data set is presented. A global database of nearly 5 million multi-model path average Rayleigh wave phase velocity observations has been used to construct global phase velocity maps. Results are comparable to existing published phase velocity maps, however, as the trans-dimensional approach adapts the resolution appropriate to the data, rather than imposing damping or smoothing constraints to stabilize the inversion, the recovered anomaly magnitudes are generally higher with low uncertainties. While further investigation is needed, this early test case shows that trans-dimensional sampling can be applied to global scale seismology problems and that previous analyses may, in some locales, under estimate the heterogeneity of the Earth. Finally, in a further advancement of partition modelling with variable order polynomials, a new method has been developed called trans-dimensional spectral elements. Previous applications involving variable order polynomials have used polynomials that are both difficult to work with in a Bayesian framework and unstable at higher orders. By using the orthogonal polynomials typically used in modern full-waveform solvers, the useful properties of this type of polynomial and its application in trans-dimensional inversion are demonstrated. Additionally, these polynomials can be directly used in complex differential solvers and an example of this for 1D inversion of surface wave dispersion curves is given

Vers l’observation du bruit quantique de la pression de radiation dans un interféromètre suspendu : l’expérience QuRaG

The existence of gravitational waves (GW) is one of the most interesting predictions of the theory of general relativity of Einstein. The experimental discovery of GW would be an important test of the theory itself. In addition, the detection of GW will open a new window of observation especially in those regions of the Universe inaccessible to electromagnetic observations. Interferometers, as Virgo are the most promising devices for the detection of GW. Currently, the sensitivity of these detectors is not yet sufficient to have a detection rate of few events/year. Therefore, an intense experimental program to improve the sensitivity is underway. Specifically, the sensitivity of the next generations of GW detectors, at low frequencies, will be limited by the effect of the radiation pressure (RP) on the suspended mirrors. This phenomenon not yet observed experimentally in the ground based GW detectors band, is currently the subject of a very active research field. My work presented here aims at building a detector for studying quantum effects of RP in GW detectors: the QuRaG experiment. It will consist of a suspended Michelson interferometer where each arm will be a high finesse Fabry-Pérot cavity, in which only the end mirror will be further suspended and then sensitive to the RP noise. During my PhD I have actively participated to the R&D of all QuRaG subsystems. Therefore, the work that I have done deals with various aspects of the project whose related problems belong to different domains of physics. My work described in this manuscript demonstrates that QuRaG is realizable and that it will be able to observe the RP noise in the expected frequency range.L'existence des ondes gravitationnelles (OG) est l'une des prédictions les plus intéressantes de la théorie de la Relativité Générale d'Einstein. La découverte expérimentale des OG serait donc un test important de la théorie elle-même et permettra d'ouvrir une nouvelle fenêtre d'observation en particulier dans les régions de l'Univers inaccessible à l'observation électromagnétique. Les détecteurs interférométriques, comme Virgo, sont les dispositifs les plus prometteurs pour la détection d’OG. Actuellement, leur sensibilité n'est pas encore suffisante pour avoir un taux d'observation de quelques événements/an. Un intense programme expérimental pour l’améliorer est en cours. Particulièrement, les prochaines générations de détecteurs d'OG, aux basses fréquences, seront limitées par l'effet de la pression de radiation (PR) sur les miroirs suspendus. Ce phénomène, pas encore observé expérimentalement, est l'objet d'un champ de recherche très actif. Mon travail ici présenté vise à la construction d'un détecteur pour l'étude des effets quantiques de la PR dans les détecteurs d’OG: QuRaG. Il sera constitué d'un interféromètre de Michelson suspendu dont chaque bras sera une cavité Fabry-Pérot de très haute finesse, dans laquelle seulement le miroir de fond sera suspendu et sensible au bruit quantique de la PR. Durant ma thèse j'ai participé activement au R&D de tous les sous-systèmes de QuRaG. Par conséquent, le travail que j'ai fait porte sur divers aspects du projet dont les problématiques appartiennent à différents domaines de la physique. Mon travail présenté ici démontre que QuRaG sera réalisable et qu’il observera le bruit de la PR dans la bande de fréquences attendue