The microwave properties of tissue and other lossy dielectrics

This thesis describes work on the theoretical modelling and experimental measurement of the complex permittivity of dielectrics. The main focus of research has been into the characterisation of permittivity of planar and layered samples within the millimetre wave band. The measurement method is based on the free-space measurement of the transmission and reflection coefficients of samples. A novel analytical method of determining the transmission and reflection coefficients as functions of frequency arising from a generalised structure of planar dielectric layers is also described and validated. The analytical method is based on signal flow techniques. The measurement and analytical techniques have been applied in two main areas: firstly, the acquisition of new data on human skin in the band 57 to 100GHz and secondly, the detection and location of defects in composite materials for which a band of 90 to 100GHz was used. Measurements have been made on the complex permittivity of a single sample of excised human skin fixed in formaldehyde. The experimental results have been corrected to account for the fixing process in formaldehyde and are projected to body temperature. This data is, to the best of the author’s knowledge, the first of its kind to be published. Predicted skin permittivity based on various relaxation models varies widely and only partially fits the measured data. The experimental results have been used to determine the parameters of a Cole-Cole function which gives the best fit to the measured data. The measured skin data has also been used to calculate power deposition in skin exposed to millimetre wave radiation. This work concludes that a skin surface temperature rise of only 0.20C results from a thirty second exposure to signals of 100W/m2. Experimental work with fibreglass composite samples has shown that defects such as delaminations, voids, matrix cracks and improper cure result in resolvable differences in the dielectric properties of the samples at 90 – 100GHz. The measurement technique is particularly sensitive to the detection of cracks and its spatial resolution is 20mm or better. Whilst confirming the general conclusions of previously published work, the specific findings of this study are novel.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Concealed Explosives Detection using Swept Millimetre Waves

The aim of this project is to develop a system for the stand-o detection (typically ten metres) of concealed body-worn explosives. The system must be capable of detecting a layer of explosive material hidden under clothing and distinguishing explosives from everyday objects. Millimetre wave radar is suitable for this application. Millimetre Waves are suitable because they are not signi cantly attenuated by atmospheric con- ditions and clothing textiles are practically transparent to this radiation. Detection of explosive layers from a few mm in thickness to a few cm thickness is required. A quasi optical focussing element is required to provide su cient antenna directivity to form a narrow, highly directional beam of millimetre waves, which can be directed and scanned over the person being observed. A system of antennae and focussing optics has been modelled and built using designs from nite element analysis (FEA) software. Using the developed system, represen- tative data sets have been acquired using a Vector Network Analyser (VNA) to act as transmitter and receiver, with the data saved for processing at a later time. A novel data analysis algorithm using Matlab has been developed to carry out Fourier Transforms of the data and then perform pattern matching techniques using arti cial neural networks (ANN's). New ways of aligning and sorting data have been found using cross-correlation to order the data by similar data slices and then sorting the data by amplitude to take the strongest 50% of data sets. The signi cant contribution to knowledge of this project will be a system which can be eld tested and which will detect a layer of dielectric at a stando distance, typically of ten metres, and signal processing algorithms which can recognise the di erence 17 between the response of threat and non-threat objects. This has partially been achieved by the development of focussing optics to acquire data sets which have then been aligned by cross-correlation, sorted and then used to train a pattern matching technique using neural networks. This technique has shown good results in di erentiating between a person wearing simulated explosives and a person not carrying simulated explosives. Further work for this project includes acquiring more data sets of everyday objects and training the neural network to distinguish between threat objects and non-threat objects. The operational range also needs increasing using either a larger aperture optical element or a similarly sized Cassegrain antenna. The system needs adapting for real time use with the data processing techniques developed in Matlab. The VNA is operated over a band of 14 to 40 GHz, future work includes moving to a stand-alone transmitter and receiver operating at w-band (75 to 110 GHz)

A Millimeter-Wave Radar Microfabrication Technique and Its Application in Detection of Concealed Objects.

Millimeter-wave (MMW) radars are envisioned for a number of safety and security applications such as collision-avoidance, navigation and standoff target detection in all weather conditions. This work focuses on two MMW radar applications: (1) phenomenology of radar backscatter from the human body for the purpose of identification and detection of concealed objects on the body (2) microfabrication of advanced MMW radar to achieve compact and low-cost systems for autonomous navigation. In MMW band, the wavelength (1 mm ~ 1 cm) is long enough to allow signal penetration through cluttered atmosphere and clothing with little attenuation and short enough to allow for fabrication of small-size radar systems. Hence, this frequency band is well suited for the design of small sensors capable of obstacle detection and navigation in heavily cluttered environment and detecting hidden objects carried by individuals. For this purpose, a novel non-imaging approach is developed for distinction of walking human body and concealed carried object using polarimetric backscatter Doppler spectrum. This approach does not need radiometric calibration of the radar and preparation of the subject for radar interrogation. It is shown that a coherent polarimetric radar at W-band (95 GHz) or higher frequencies can be used for standoff detection of concealed carried objects. Motivated by these results, the thesis also includes an investigation on developing a technology for compact MMW radar systems. A micromachined, high-resolution, compact and low-power imaging MMW radar operating at 240 GHz intended for obstacle detection in complex environment is introduced. A frequency scanning antenna array micromachined from three layers of stacked silicon wafers is designed to provide 20 beamwidth in azimuth and 80 in elevation with azimuthal beam scanning range of ± 250. The frequency beam scanning is enabled by a meander rectangular waveguide with a slot array on its broad wall to feed linear microstrip patch antennas microfabricated on a suspended Parylene membrane. This technique offers high fabrication precision; provide easy fabrication and integration with active devices. The performances of the passive components of the radar system are verified using a WR-3 S-parameter and a near-field measurement systems.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91484/1/mvahid_1.pd

Remote Detection of Concealed Guns and Explosives

A reliable method of remotely detecting concealed guns and explosives attached to the human body is of great interest to governments and security forces throughout the world. This thesis describes the development and trials of a new remote non-imaging concealed threat detection method using active millimetre wave radar using the microwave and mmwave frequencies bands 14 – 40 and 75 – 110 GHz (Ku, K, Ka and W). The method is capable of not only screening for concealed objects, like the current generation of concealed object detectors, but also of differentiating between mundane and threat objects. The areas focused upon during this investigation were: identifying the impact of different commonly worn fabrics as barriers to detection; consulting with end users about their requirements and operational needs; a comparison of different frequency bands for the detection of guns and explosives; exploring the effects of polarisation on object detection; a performance comparison of different detection schemes using Artificial Neural Networks; improving existing data acquisition systems and prototyping of a real-time capture system

Microwave Imaging for Diagnostic Application

Imaging of the human body makes a significant contribution to the diagnosis and succeeding treatment of diseases. Among the numerous medical imaging methods, microwave imaging (MWI) is an attractive approach for medical applications due to its high potential to produce images of the human body safely with cost-efficiency. A wide range of studies and research has been done with the aim of using the microwave approach for medical applications. The focus of this research is developing MWI algorithms, which is the Huygens Principle (HP) based and to validate the capability of the proposed MWI algorithm to detect skin cancer and bone lesion through phantom measurements. The probability of the HP procedure for skin cancer detection has been investigated through design, and fabrication of a heterogeneous phantom simulating the human forearm having an inclusion mimicking a skin cancer. Ultrawideband (UWB) MWI methods are then applied to the phantom. The S21 parameter measurements are collected in an anechoic chamber environment and processed via HP technique. The tumour is successfully detected after applying appropriate artefact removal procedure. The ability to successfully apply HP to detect and locate a skin cancer type inclusion in a multilayer cylindrical phantom has been verified. The feasibility study of HP-based MWI procedure for bone lesion detection has also been investigated using a dedicated phantom. Validation has been completed through measurements inside the anechoic chamber in the frequency range of 1–3 GHz using one receiving and one transmitting antennas in free space. The identification of the lesion’s presence in different bone layers has been performed on images. The quantification of the obtained images has been performed by introducing parameters such as the resolution and signal-to-clutter ratio (S/C). The impact of different frequencies and bandwidths (in the 1–3 GHz range) in lesion detection has been investigated. The findings showed that the frequency range of 1.5–2.5 GHz offered the best resolution (1.1 cm) and S/C (2.22 on a linear scale). Subtraction between S21 obtained using two slightly displaced transmitting positions has been employed to remove the artefacts; the best artefact removal has been obtained when the spatial displacement was approximately of the same magnitude as the dimension of the lesion. Subsequently, a phantom validation of a low complexity MWI device (based on HP) operating in free space in the 1-6.5 GHz frequency band using two antennas in free space has been applied. Detection has been achieved in both bone fracture lesion and bone marrow lesion scenarios using superimposition of five doublet transmitting positions after applying the rotation subtraction method to remove artefact. A resolution of 5 mm and the S/C (3.35 in linear scale) are achieved which is clearly confirming the advantage of employing multiple transmitting positions on increased detection capability. The finding of this research verifies the dedicated MWI device as a simple, safe and without any X-ray radiation, portable, and low complexity method, which is capable of been successfully used for bone lesion detection. The outcomes of this thesis may pave the way for the construction of a dedicated bone imaging system that in future could be used as a safe diagnostic device even in emergency sites

Solid solution GaSe1−xSx single crystals for THz generation

A table top source of coherent Terahertz (30-1000 µm) radiation, which is high power, narrow bandwidth, and broadly tunable, is high desired for applications in imaging, non-destructive testing (NDT), quantum, security and biomedical technologies. In spite of intensive research over many decades such a device remains elusive. Sulpher doped Gallium Selenide (GaSex−1Sx) solid solution ε-polytype crystals are an outstanding candidate for the efficient generation of radiation and tunability throughout the majority of the Terahertz (THz) regime; thanks to the prodigious linear and nonlinear optical properties of the Gallium Selenide (GaSe) parent crystal. Close control of doping and the crystal growth process enable the manufacture of superior quality nonlinear crystals, where the optical properties may be engineered and the mechanical properties vastly improved. Thus overcoming many of the physical issues that, despite its exceptional optical properties, have frustrated the widespread adoption of GaSe for laser frequency down conversion to the THz regime. In order to fully exploit the potential of GaSex−1Sx crystals and successfully design efficient sources for THz generation the optical properties of these crystals must be accurately determined and their transformation with doping well understood. The work in this thesis aims to accurately determine the optical properties of GaSe, Gallium Sulphide (GaS) and GaSex−1Sx crystals in the Far-Infrared and THz regimes to enable this exploitation. In the first phase of investigation we determine the linear refractive index (n) and absorption (α) coefficient for both the o and e waves in the THz regime (0.14.5 THz) using Terahertz - Time Domain Spectroscopy (THz-TDS) for GaSe, and a dense set of GaSex−1Sx crystals (x = 0.05 0.11 0.22 0.29 0.44). Measurements of THz dispersion and absorption properties of GaS crystals are performed for the first time. The transformation of the optical properties of the crystals and their phonon structure is studied. We examine the sources of inaccuracy in the THzTDs measurements of high refractive index birefringent crystals and propose a set of criteria for the selection of adequate data. The nonlinear Figure of Merit (FOM) of available high quality GaSex−1Sx crystals is found to be an order of magnitude less than that predicted in the literature, with FOM = 19.8 for GaSe, FOM = 17 for GaSex−1Sx, on the other hand estimates for double doping with Sulphur and Aluminium predict significant enhanced of these FOM values, up to 5-10 times. In the second phase of investigation we examine the phonon band of the GaSe, GaS and GaSex−1Sx by FTIR and Raman spectroscopy. For the first time we determine the absorption coefficients of the main phonon peak in the set of GaSex−1Sx crystals. The transformation of the phonon band with doping is studied. In the third phase of investigation we attempt to determine the nonlinear optical properties deff and n2 of GaSe and GaSex−1Sx in the Far Infra-Red (FIR) and THz regimes using the Maker fringe and Z-scan methods on the FELIX free electron laser

A feasibility study on the application of polarimetric decomposition algorithms to the detection of concealed weapons

State of the art security screening technology is not meeting all modern day requirements. There exists a gap in the market for the development of real time systems capable of detecting weapons at standoff ranges. Researchers at the Centre of Sensing and Imaging at Manchester Metropolitan University have developed a radar based screening technology. This technology will offer new security screening capabilities, making it feasible to have portable systems that can detect concealed weapons, with the added advantage of being capable of screening people in a crowd. The next step in the development of this radar system is to investigate the potential of using polarimetric scattering effects to detect concealed weapons, with the aim of improving the robustness and detection capabilities in comparison with the current state-of-the-art systems. This thesis provides a feasibility study in the application of polarimetric decomposition techniques to Concealed Weapon Detection (CWD) and an experimental radar is developed to provide the measurements required for this study. The major outcome of this work is that polarimetric decompositions including the Pauli, Krogager SDH and H-α decompositions have been demonstrated as a viable means of interpreting data for the detection of concealed weapons. This will allow the next generation of radar based weapon detectors to reduce some of the orientation dependency on detection rates as observed in the current state-of-the-art systems. The work presented in this thesis has resulted in a clear understanding of what is required to implement a fully polarimetric radar based weapon detector. The detection of weapons using the developed fully polarimetric radar with the aid of polarimetric decomposition algorithms combined with calibration and signal-processing algorithms has been demonstrated in this thesis

Advanced Photonic Sciences

The new emerging field of photonics has significantly attracted the interest of many societies, professionals and researchers around the world. The great importance of this field is due to its applicability and possible utilization in almost all scientific and industrial areas. This book presents some advanced research topics in photonics. It consists of 16 chapters organized into three sections: Integrated Photonics, Photonic Materials and Photonic Applications. It can be said that this book is a good contribution for paving the way for further innovations in photonic technology. The chapters have been written and reviewed by well-experienced researchers in their fields. In their contributions they demonstrated the most profound knowledge and expertise for interested individuals in this expanding field. The book will be a good reference for experienced professionals, academics and researchers as well as young researchers only starting their carrier in this field

Frequency selective surfaces for Terahertz applications

This thesis presents both theoretical and experimental investigations of the performance and capabilities of frequency selective surfaces (FSS) applied at THz frequencies. The aim is to explore and extend the use of FSS, traditionally limited to microwave frequencies, towards the THz regime of the spectrum, where interesting applications such as imaging, sensing and communications exist. The contribution of this work lies in three main areas within the scope of THz FSS, namely, performance, prototyping and applications. Unlike microwave FSS where extensive research has been performed to evaluate the performance of different FSS designs, particular problems arise at THz frequencies, significantly, the ohmic losses. While a few notable studies can be found on the issue of ohmic losses, part of this thesis investigates, for the first time, the power dissipation due to the presence of both ohmic and dielectric losses, in relation to the power stored in the vicinity of the FSS, the currents induced in the elements of the array and the array’s terminal impedance. By doing so, a better understanding of the performance of THz FSS has been given in terms of their quality factor, allowing for design guidelines previously unavailable. In order to demonstrate multiband operation experimentally, a novel fabrication process has been designed and developed to manufacture capacitive or dipole-based THz FSS on a dielectric layer. Dry deep-reactive ion etching has been employed in order to avoid the use of wet etching to provide better control of etch characteristics. Various FSS operating around 15THz have been demonstrated experimentally. In addition, THz FSS have been investigated theoretically in the realm of three different applications, namely, multiband operation, sensing capability and reconfigurability. Multiband characteristics using single-screen FSS have been achieved by perturbed dipole FSS exhibiting up to four resonances due to the excitation of even and odd current modes. After studying the near-fields in perturbed FSS, it has been found that this type of FSS represent a very attractive candidate for sensing applications due to the revealed near-field enhancement phenomena related to the excitation of the odd mode, where currents flow in opposite directions. Finally, a novel tunability approach to reach frequency reconfigurability by varying the near-field coupling between two closely spaced layers in a dual-layer configuration has been proposed. A MEMS movable four-arm membrane has been suggested to vary the distance between the two layers mechanically, leading to the frequency tuning effect. This approach has been shown to be particularly suitable for THz frequencies, and has been applied to demonstrate theoretically tunable FSS and other periodic structures, such as artificial magnetic conductors and dielectric gratings