Optical diffraction by three-dimensional quasi-repetitive structures : some aspects of thick holograms

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Temporal Characteristics of Boreal Forest Radar Measurements

Radar observations of forests are sensitive to seasonal changes, meteorological variables and variations in soil and tree water content. These phenomena cause temporal variations in radar measurements, limiting the accuracy of tree height and biomass estimates using radar data. The temporal characteristics of radar measurements of forests, especially boreal forests, are not well understood. To fill this knowledge gap, a tower-based radar experiment was established for studying temporal variations in radar measurements of a boreal forest site in southern Sweden. The work in this thesis involves the design and implementation of the experiment and the analysis of data acquired. The instrument allowed radar signatures from the forest to be monitored over timescales ranging from less than a second to years. A purpose-built, 50 m high tower was equipped with 30 antennas for tomographic imaging at microwave frequencies of P-band (420-450 MHz), L-band (1240-1375 MHz) and C-band (5250-5570 MHz) for multiple polarisation combinations. Parallel measurements using a 20-port vector network analyser resulted in significantly shorter measurement times and better tomographic image quality than previous tower-based radars. A new method was developed for suppressing mutual antenna coupling without affecting the range resolution. Algorithms were developed for compensating for phase errors using an array radar and for correcting for pixel-variant impulse responses in tomographic images. Time series results showed large freeze/thaw backscatter variations due to freezing moisture in trees. P-band canopy backscatter variations of up to 10 dB occurred near instantaneously as the air temperature crossed 0⁰C, with ground backscatter responding over longer timescales. During nonfrozen conditions, the canopy backscatter was very stable with time. Evidence of backscatter variations due to tree water content were observed during hot summer periods only. A high vapour pressure deficit and strong winds increased the rate of transpiration fast enough to reduce the tree water content, which was visible as 0.5-2 dB backscatter drops during the day. Ground backscatter for cross-polarised observations increased during strong winds due to bending tree stems. Significant temporal decorrelation was only seen at P-band during freezing, thawing and strong winds. Suitable conditions for repeat-pass L-band interferometry were only seen during the summer. C-band temporal coherence was high over timescales of seconds and occasionally for several hours for night-time observations during the summer. Decorrelation coinciding with high transpiration rates was observed at L- and C-band, suggesting sensitivity to tree water dynamics.The observations from this experiment are important for understanding, modelling and mitigating temporal variations in radar observables in forest parameter estimation algorithms. The results also are also useful in the design of spaceborne synthetic aperture radar missions with interferometric and tomographic capabilities. The results motivate the implementation of single-pass interferometric synthetic aperture radars for forest applications at P-, L- and C-band

Numerical and experimental methods applied to human exposure to electromagnetic fields

The research focuses on a hybrid experimental-numerical technique, based on Boundary Element Method (BEM), to reconstruct the electromagnetic eld distribution in the space surrounding unknown sources, in both low and high frequency range. The same procedure also allows to evaluate the induced electric eld non-invasively when human body presents near such sources. By applying BEM (including Green function) to a discretized surface that enclosing the sources, the electromagnetic elds outside the surface (source free region) can be received from an integration of same quantities over this surface. At low frequency range (up to 100 kHz), the induced electric eld inside human body can be also calculated as an inverse process, i.e. applying again BEM over a discrete body surface on which the magnetic elds are provided through the above procedure, to compute the elds at any point inside this surface. The only approximation during this procedure is assuming that on each discrete element, the eld values are uniform. Measurement can be performed on a grid with regular step over any known surfaces and both the magnitude and phase are required for each component of the electric and magnetic elds. The experimental validation at low frequency range has been carried out around a Helmholtz coil system enclosed by a wooden frame, which is used to position the 3D magnetic eld probe. Numerous eld distributions can be generated through this system by separately imposing the currents which supply the two coils, and three of them are applied in the validation procedure. The three voltage signals detected by the eld meter (corresponding to the three components of the magnetic elds) are sampled synchronously with the fourth one, which is picked up from the supply circuit and acts like a trigger, in order to compute the phases of the other three signals. The measured data is tted by an interpolation/extrapolation technique before adopted as input for BEM reconstruction in free space. Reconstruction quality through proposed BEM procedure has been investigated through dierent approaches, as well as the accuracy of the induced electric eld evaluation inside the human body. Measurement uncertainty propagation has been estimated through Monte Carlo method coupled with a discrete numerical technique. At last, the prediction of the radiation emission generated by a radio frequency model has been also presented, as an example of application for the proposed eld reconstruction in high frequency (300 MHz). A satisfactory accuracy is obtained through the comparison with another numerical metho

A quasi-real-time inertialess microwave holographic imaging system

This thesis records the theoretical analysis and hardware development of a laboratory microwave imaging system which uses holographic principles. The application of an aperture synthesis technique and the electronic commutation of all antennae has resulted in a compact and economic assembly - which requires no moving parts and which, consequently, has a high field mapping speed potential. The relationship of this microwave holographic system to other established techniques is examined theoretically and the performance of the imaging system is demonstrated using conventional optically- and numerically-based reconstruction of the measured holograms. The high mapping speed potential of this system has allowed the exploitation of an imaging mode not usually associated with microwave holography. In particular, a certain antenna array specification leads to a versatile imaging system which corresponds closely in the laboratory scale to the widely used synthetic aperture radar principle. It is envisaged that the microwave holographic implementation of this latter principle be used as laboratory instrumentation in the elucidation of the interaction of hydrodynamic and electromagnetic waves. Some simple demonstrations of this application have been presented, and the concluding chapter also describes a suitable hardware specification. This thesis has also emphasised the hardware details of the imaging system since the development of the microwave and other electronic components represented a substantial part of this research and because the potential applications of the imaging principle have been found to be intimately linked to the tolerances of the various microwave components. Bibliography: pages 122-132

Microwave-induced thermoacoustic tomography: applications and corrections for the effects of acoustic heterogeneities

This research is primarily focused on developing potential applications for microwaveinduced thermoacoustic tomography and correcting for image degradations caused by acoustic heterogeneities. Microwave-induced thermoacoustic tomography was first used to verify the feasibility of noninvasively detecting the coagulated damage based on different dielectric properties between normal tissue and lesion treated with high intensity focused ultrasound. Good image contrasts were obtained for the lesions. A comparison of the size of the lesion measured with microwave-induced thermoacoustic tomography and the size measured by a gross pathologic photograph was presented to verify the effectiveness the proposed method. Clinical data for breast tumors were also collected to verify the feasibility of using microwave-induced thermoacoustic tomography in breast cancer imaging. Next, the effects of acoustic heterogeneities on microwave-induced thermoacoustic tomography in weakly refractive medium were investigated. A correction method based on ultrasonic transmission tomography was proposed to correct for the image distortion. Numerical simulations and phantom experiments verify the effectiveness of this correction method. The compensation is important for obtaining higher resolution images of small tumors in acoustically heterogeneous tissues. Finally, the effects of the highly refractive skull on transcranial brain imaging were studied. A numerical method, which considered wave reflection and refraction at the skull surfaces, was proposed to compensate for the image degradation. The results obtained with the proposed model were compared with the results without considering the skull-induced distortion to evaluate the skull-induced effects on the image reconstruction. It was demonstrated by numerical simulations and phantom experiments that the image quality could be improved by incorporating the skull shape and acoustic properties into image reconstruction. This compensation method is important when the thickness of skull cannot be neglected in transcranial brain imaging

Imaging and engineering optical localized modes at the nanoscale

In this thesis we experimentally developed high-resolution groundbreaking imaging techniques and novel methods suitable for nanophotonics materials. The experimental results are carefully supported by theory and numerical calculations. We engineered the propagation of light by exploiting devices that strongly localize electromagnetic fields at the nanoscale. The proposed techniques have a large field of application. We deeply investigated ordered and disordered based single and coupled nano-resonators, called photonicmolecules, and develop a laser-assisted local oxidation of the dielectric environment. These results put the basis for an unprecedented investigation of light behaviour in optical nano-resonators. Therefore, they would pave the way for novel devices that exploit the strong coupling regime between single light emitters and localized optical modes

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance