A technology for fabricating fiber optic pressure sensors is described. This technology is based on intermediate-layer bonding of a fused silica ferrule to a patterned, micro-machined fused silica diaphragm, providing low temperature fabrication of optical pressure sensor heads that can operate at high temperature. Fused silica ferrules and fused silica diaphragms are chosen to reduce the temperature dependence. The fused silica diaphragms have been micro-machined using wet chemical etching in order to form extrinsic Fabry-Perot (FP) interferometric cavities. Sol-gel is used as an intermediate-layer for both fiber-ferrule bonding and ferrule-diaphragm bonding at relatively low temperature (250 °C). The pressure sensors fabricated in the manner can operate at temperatures as high as 600 °C. The self-calibrated interferometric-intensity-based (SCIIB) technology, which combines fiber interferometry and intensity-based sensing method into a single sensor system, is used to test and monitor the pressure sensor signal. The light returned from the FP cavity is split into two channels. One channel with longer coherence length can test the effective interference generated by the FP cavity, while the other channel with shorter coherence length can get signal proportional only to the source power, fiber attenuation, and other optical losses. The ratio of the signals from the two channels can compensate for all unwanted factors, including source power variations and fiber bending losses. [11] / Master of Science

Conventional fiber optic interferometric sensors employing a monochromatic source prove to be inadequate for applications requiring absolute, real-time value of magnitude and direction of the applied perturbation. This limitation can be overcome by using a broadband light source to extract unambiguous information from the sensor in the wavelength domain. Several variations in the signal processing techniques for white light interferometry are discussed and compared in terms of resolution, bandwidth and cost. A detailed analysis is made of the principle of operation and basic features of the commercially available absolute sensing system. This compact system is self calibrating, has a 100 micro-strain Ole) strain and 2°C temperature resolution and is ideal for applications in environments where the parameter to be measured is static or quasi-static. High finesse Fabry-Perot cavities are employed to obtain almost an order of magnitude sensitivity improvement over conventional low finesse cavities. The principle of white light interferometry is extended to absolute axial stain and temperature sensing in two-mode, elliptical-core fibers. Other novel applications, such as to operation of photo induced refractive index gratings and fiber characterization are proposed. / Master of Science

Since the invention of Laser (in 1960) and low loss optical fiber (in 1966) [1], extensive research in fiber-optic sensing technology has made it a well-defined and matured field [1]. The measurement of physical parameters (such as temperature and pressure) in extremely harsh environment is one of the most intriguing challenges of this field, and is highly valued in the automobile industry, aerospace research, industrial process monitoring, etc. [2]. Although the semiconductor based sensors can operate at around 500oC, sapphire fiber sensors were demonstrated at even higher temperatures [3]. In this research, a novel sensor structure is proposed that can measure both pressure and temperature simultaneously. This work effort consists of design, fabrication, calibration, and laboratory testing of a novel structured temperature compensated pressure sensor. The aim of this research is to demonstrate an accurate temperature measurement, and pressure measurement using a composite Fabry-Perot interferometer. One interferometer measures the temperature and the other accurately measures pressure after temperature compensation using the temperature data from the first sensor. / Master of Science

Spectral interferometry, also known as spectral-domain white light or low coherence interferometry, has seen numerous applications in sensing and metrology of physical parameters. It can provide phase or optical path information of interest in single shot measurements with exquisite sensitivity and large dynamic range. As fast spectrometer became more available in 21st century, spectral interferometric techniques start to dominate over time-domain interferometry, thanks to its speed and sensitivity advantage. In this work, a dual-modality phase/birefringence imaging system is proposed to offer a quantitative approach to characterize phase, polarization and spectroscopy properties on a variety of samples. An interferometric spectral multiplexing method is firstly introduced by generating polarization mixing with specially aligned polarizer and birefringence crystal. The retardation and orientation of sample birefringence can then be measured simultaneously from a single interference spectrum. Furthermore, with the addition of a Nomarski prism, the same setup can be used for quantitative differential interference contrast (DIC) imaging. The highly integrated system demonstrates its capability for noninvasive, label-free, highly sensitive birefringence, DIC and phase imaging on anisotropic materials and biological specimens, where multiple intrinsic contrasts are desired. Besides using different intrinsic contrast regime to quantitatively measure different biological samples, spectral multiplexing interferometry technique also finds an exquisite match in imaging single anisotropic nanoparticles, even its size is well below diffraction limit. Quantitative birefringence spectroscopy measurement over gold nanorod particles on glass substrate demonstrates that the proposed system can simultaneously determine the polarizability-induced birefringence orientation, as well as the scattering intensity and the phase differences between major/minor axes of single nanoparticles. With the anisotropic nanoparticles' spectroscopic polarizability defined prior to the measurement with calculation or simulation, the system can be further used to reveal size, aspect ratio and orientation information of the detected anisotropic nanoparticle. Alongside developing optical anisotropy imaging systems, the other part of this research describes our effort of investigating the sensitivity limit for general spectral interferometry based systems. A complete, realistic multi-parameter interference model is thus proposed, while corrupted by a combination of shot noise, dark noise and readout noise. With these multiple noise sources in the detected spectrum following different statistical behaviors, Cramer-Rao Bounds is derived for multiple unknown parameters, including optical pathlength, system-specific initial phase, spectrum intensity as well as fringe visibility. The significance of the work is to establish criteria to evaluate whether an interferometry-based optical measurement system has been optimized to its hardware best potential. An algorithm based on maximum likelihood estimation is also developed to achieve absolute optical pathlength demodulation with high sensitivity. In particular, it achieves Cramer-Rao bound and offers noise resistance that can potentially suppress the demodulation jump occurrence. By simulations and experimental validations, the proposed algorithm demonstrates its capability of achieving the Cramer-Rao bound over a large dynamic range of optical pathlengths, initial phases and signal-to-noise ratios. / PHD / Optical imaging is unique for its ability to use light to provide both structural and functional information from microscopic to macroscopic scales. As for microscopy, how to create contrast for better visualization of detected objects is one of the most important topic. In this work, we are aiming at developing a noninvasive, label-free and quantitative imaging technique based on multiple intrinsic contrast regimes, such as intensity, phase and birefringence. Spectral multiplexing interferometry method is firstly introduced by generating spectral interference with polarization mixing. Multiple parameters can thus be demodulated from single-shot interference spectrum. With Jones Matrix analysis, the retardation and orientation of sample birefringence can be measured simultaneously. A dual-modality phase/birefringence imaging system is proposed to offer a quantitative approach to characterize phase, polarization and spectroscopy properties on a variety of samples. The high integrated system can not only deliver label-free, highly sensitive birefringence, DIC and phase imaging of anisotropic materials and biological specimens, but also reveal size, aspect ratio and orientation information of anisotropic nanoparticles of which the size is well below diffraction limit. Alongside developing optical imaging systems based on spectral interferometry, the other part of this research describes our effort of investigating the sensitivity limit for general spectral interferometry based systems. The significance of the work is using Cramer-Rao Bounds to establish criteria to evaluate whether an optical measurement system has been optimized to its hardware best potential. An algorithm based on maximum likelihood estimation is also developed to achieve absolute optical pathlength demodulation with high sensitivity. In particular, it achieves Cramer-Rao bound and offers noise resistance that can potentially suppress the demodulation jump occurrence.

This thesis investigates the computing capabilities of three distinct platforms for radio direction finding (RDF) applications in electronic warfare (EW) systems: the Raspberry Pi 4 Model B, Intel NUC NUC7i5BNH, and NVIDIA Jetson AGX Orin 64GB. RDF plays a critical role in locating radio emitters, demanding real-time processing for precise signal data analysis. The study aims to determine the maximum sampling frequency that each platform can maintain while meeting real-time requirements and identifies the most suitable RDF algorithm for platform assessment. The best-suited algorithm was found to be Phase Interferometry. Results indicate that the Raspberry Pi 4 Model B achieves a sampling frequency of 13.08 MHz, the Intel NUC NUC7i5BNH maintains 12.68 MHz, and the NVIDIA Jetson AGX Orin 64GB performs at 399.45 MHz (60W), 229.82 MHz (50W), 83.88 MHz (30W), and 54.12 MHz (15W).

Viscous drag reduction plays a vital role in increasing the performance of vehicles. However, there are only so many measurement techniques that can quickly and accurately measure this when compared to pressure drag measurement techniques. The current study makes use of one of the direct and robust measurement techniques that exist, called the Oil Film Interferometry (OFI) to estimate skin friction on the NASA/Virginia Tech BeVERLI (Benchmark Validation Experiment for RANS and LES Investigations) hill. This project aims to develop a detailed database of non-equilibrium, separated turbulent boundary layer flows obtained through wind tunnel experiments for CFD validation. Skin friction measurements are obtained at specific critical locations on the hill and in its close proximity. The challenges involved in obtaining skin friction data from these locations are discussed in detail. Detailed discussions on the experimental setup and data processing methodology are presented. Qualitative and quantitative results from each measurement location are discussed along with uncertainties to explain certain key flow physics. Additionally, skin friction coefficients from selected overlapping measurement locations from another experimental flow measurement technique called Laser Doppler Velocimetry (LDV) are compared with OFI, and a cross-instrument study is performed. Finally, results from well-refined RANS CFD simulations are assessed with the experimental results, and critical improvement areas are identified. / Master of Science / Drag force is a parameter that significantly contributes to the performance efficiency of any vehicle moving in a fluid. This force is categorised into two types - pressure and viscous drag- both of which need to be minimised as much as possible to contribute towards higher vehicle performance. While there are numerous measurement techniques and documentation currently available to measure pressure drag, this is not the case with the measurement of viscous drag. Skin friction measurement directly relates to the estimation of viscous drag, but accurate and quick measurement of this quantity highly challenging with countable measurement techniques currently available. Through this project, BeVERLI (Benchmark Validation Experiment for RANS and LES Investigations), a detailed documentation is developed for accurate measurement of skin friction through Oil Film Interferometry (OFI). The results obtained through this measurement is explained with a detailed experimental procedure as well as using a data processing code. The accuracy of these results are then discussed with the results from another flow measurement technique called Laser Doppler Velocimetry (LDV) and from Computational Fluid Dynamics (CFD).

Interferometric optical fiber sensors have proved many orders of magnitude more sensitive than their electrical counterparts, but they suffer from limitations in signal demodulation caused by phase ambiguity and complex fringe counting when the output phase difference exceeds one fringe period. Various signal demodulation methods have been developed to overcome some of the these drawbacks with limited success. This thesis proposes a new measurement system for the extrinsic Fabry-Perot interferometer (EFPI) sensor. Using a wavelength modulated source and a novel extended-gap EFPI, some of the limitations of interferometric signal demodulation are overcome. By scanning the output wavelength of a multilongitudinal mode laser diode through current modulation, the EFPI sensor signal is scanned through multiple fringes. Gap movement is then unambiguously determined by monitoring the phase of the multiple fringe pattern. / Master of Science

No / An experimental study of coupling compensation for AOA estimation using compact low profile antenna arrays with element separations of a quarter wavelength has been conducted. Two circular arrays of low profile miniaturised logarithmic spiral antennas deployed on a circular metal plate were used for data acquisition. Using the MUSIC direction-finding algorithm, the AOA estimation errors in receiving mode were observed before and after compensation: the errors were significantly decreased by coupling compensation.

Remote sensing is the process of collecting information about an object or area without physically being in contact with it. Synthetic Aperture Radar (SAR) is a remote sensing technique that uses radar signals to capture images of the Earth’s surface. SAR is an active sensor that generates images by emitting electromagnetic pulses and receiving signals reflected from the target. Imaging radars can employ different wave polarizations in transmission and reception, and the selected combination significantly influences the backscattered signal, enabling us to better understand the properties of the targets in the scene. This technique is known as polarimetry. In addition, images acquired from different positions can be combined coherently, in another technique called interferometry. The motivation of this thesis is to contribute to the processing of multi-temporal polarimetric and interferometric SAR data and to evaluate the information content and applicability of such multidimensional data in different domains. The following specific objectives have been identified, each accompanied by clearly defined research questions. 1) Design, implement and evaluate a tensor-based filter that can fully exploit multi-temporal polarimetric and interferometric information for data processing. The proposed filter aims to maximize the utilization of information from multi-temporal polarimetric SAR data from different satellite and optimize the suppression of speckle while maintaining structural information. 2) Understand the physical reason of the contribution of polarimetric information to polarimetric persistent scatterer interferometry (PolPSI), with focus on dual-pol data from Sentinel-1. Compared to traditional experiments based on the single-polarization VV channel, the addition of the VH channel expands the dimensionality of the available data allowing for the analysis of how the VH channel helps increase the PS density and evaluate whether the PolPSI results are truly better than single-pol PSI results or not. 3) Analyse the contribution of polarimetric information to polarimetric distributed scatterers interferometry (PolDSI) based on Sentinel-1. The phase optimization of distributed scatterers is based on the polarimetric covariance matrix formed by the VV and VH channels. Compared to using the VV channel alone, the addition of VH channel introduces amplitude information and polarization coherence between the two channels. The optimized distributed scatterers then become synthesized persistent scatterers, and finally, decide whether it is worth to use always the VH channel, in addition to VV, or is it better to work with the VV channel alone. 4) Explore the potential of the polarimetric co-polar complex coherence in crop monitoring by investigating its sensitivity to crop growth. This experiment focuses on airborne data in the L-band with available ground measured data. The research carried out in this thesis has resulted in three publications in indexed journals and several presentations in international conferences and symposia. Imaging radars can employ different wave polarizations in transmission and reception, and the selected combination significantly influences the backscattered signal, enabling us to better understand the properties of the targets in the scene. This technique is known as polarimetry. In addition, images acquired from different positions can be combined coherently, in another technique called interferometry. / This thesis received support from the Spanish Ministry of Science and Innovation (State Agency of Research, AEI) under Project PID2020-117303GB-C22/AEI/10.13039/501100011033, European Space Agency under Contract 4000133590/20/NL/AS/hh, the ESA-MOST China DRAGON-5 project (ref. 59339), as well as the European Union’s Horizon Europe research and innovation programme under grant agreement No. 101129646. During my exchange at ETH, I received sponsorship from BANCO SANTANDER-UA for international doctoral mobility (2023-24)

The existence of other habitable worlds and the possible development of life elsewhere in the Universe have been among mankinds fundamental questions for thousands of years. These interrogations about our origins and place in the Universe are today at the dawn of being answered in scientific terms. The key year was 1995 with the discovery of the first extrasolar planet orbiting around a solar-type star. About 400 extrasolar planets are known today and the possibility to identify habitable worlds and even life among them largely contributes to the growing interest about their nature and properties. However, characterizing planetary systems is a very difficult task due to both the huge contrast and the small angular separation between the host stars and their environment. New techniques have emerged during the past decades with the purpose of tackling these fantastic observational challenges. In that context, infrared interferometry is a very promising technique, since it provides the required angular resolution to separate the emission of the star from that of its environment. This dissertation is devoted to the characterization of extrasolar planetary systems using the high angular resolution and dynamic range capabilities of infrared interferometric techniques. The first part of the present work is devoted to the detection with current interferometric facilities of warm dust within the first few astronomical units of massive debris discs around nearby stars. In order to extend the imaging of planetary systems to fainter discs and to extrasolar planets, we investigate in a second step the performance of future space-based nulling interferometers and make a comparison with ground-based projects. Finally, the third part of this work is dedicated to the impact of exozodiacal discs on the performance of future life-searching space missions, the goal being to characterize extrasolar planets with sizes down to that of the Earth.