Gaussian Beam Mode Analysis can be applied as a powerful technique approach in the development of phase gratings
for use at terahertz wavelengths, providing a physically intuitive approach relating Fourier and Fresnel diffraction
patterns to the scattering of the illumination beam at the grating. Fourier gratings in particular offer the possibility of
generating sparse arrays image of a single input beam, useful, for example, in active heterodyne systems with an LO
power source. The feasibility of the application of such gratings in real systems was investigated both by simulation and
experimental measurements.
The properties of terahertz (THz) radiation potentially make it ideal for medical imaging but the difficulty of
producing laboratory sources and detectors has meant that it is the last unexplored part of the electromagnetic
spectrum. In this paper we report on near-field reflection and absorption measurements of biological samples at
0.1THz as a first step towards developing THz and millimetre-wave imaging schemes. Variation of the absorption
and reflection of THz in these samples is investigated as a means of determining information about the sample
structure. Operating at 100 GHz with standard detecting devices we illustrate preliminary results in imaging
(transmission and reflection) measurements of meat samples using various optical configurations and draw
conclusions on the scope of the techniques. Some encouraging provisional results are discussed as well as
limitations in "intensity only" measurements due, primarily, to standing waves and a lack of dynamic range. These
experiments were performed as part of a Masters thesis. A discussion on a variety of absorbing materials utilized
to reduce reflected radiation from surrounding optical components is also given. In addition we report on initial
trials in extracting information about an object's size by sparsely measuring points in the equivalent Fourier plane
in a simple optical setup, thus avoiding the need for time consuming raster scanning. This technique has many
potential applications in detecting and scanning systems. Here the background theory and preliminary results are
presented.
In the presentation we report on novel applications of Gaussian beam mode (GBM) analysis, including in image
deconvolution and Fourier grating design.
GBMs are the natural modes with which to describe propagation of quasi-collimated long-wavelength beams, with only
a small number of modes required to reach adequate accuracy for many practical applications. GBMs provide a more
efficient and natural basis set with which to describe propagation than for example plane wave decomposition,
especially because of the limited spatial frequency content (only a few degrees of freedom are necessary to describe
such beams and the degrees of freedom can be associated with component GBMs).
We discuss how GBM analysis provides a useful alternative scheme to FFT approaches for performing deconvolutions
and image retrieval in long-wavelength quasi-collimated systems. The convolving beam is usually described very
efficiently in terms of beam modes and an SVD approach can be used to extract the mode coefficients of the
deconvolved image. We discuss in particular the novel application to mapping in astronomical telescope observations.
Another useful area of application is in the design of Fourier phase gratings. Fourier gratings can be used for beam
multiplexing of local oscillator power in array imaging systems. In this case phase retrieval is often driven by an
iterative approach to the solution based on FFTs and thus by implication plane waves. A GBM approach leads to a more
efficient and physically more meaningful approach, especially again because of the limited spatial frequencies possible
in long wavelength systems.
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