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
It is possible to use wave-front reconstruction for imaging at millimetre wavelengths employing off-axis holography (a frequently used technique at visible wavelengths). We report on how the technique can also be used for imaging the phase centre of non-standard feed antennas at millimetre wavelengths such as planar lens antennas for example. Holography provides a method for recording a lens-less image of an object reducing loss of spatial frequency information important for maximum resolution. An experimental arrangement at 100 GHz based on a simple form of near-field off-axis holography was developed, with the object and reference beams derived from two radiating horn antennas fed by a single coherent source via a 3dB cross-guide coupler. The reference beam derived from a well understood and characterised horn was collimated using a large off-axis mirror, while the object beam was derived directly from the horn antenna whose pattern is to be measured. The hologram (or intensity pattern) resulting from the interference of the two beams was recorded over an area of 150 × 150 mm with a spatial resolution of 1 mm by a scanning detector and the object wave-front recovered by simulating the reconstruction through near-field diffraction of the reference beam. It is possible to model the propagation of the recovered object beam back towards the horn and recover the object horn fields in the vicinity of the waist (the effective phase centre of the horn). This is a useful inexpensive experimental method for recovering the phase centre position of non-standard feeds.
In this paper, we report on our investigations of novel imaging techniques such as holography, the generation of limited diffraction beams with large depths of focus and the use of binary optics for millimeter wave systems. Holography, widely used at visible wavelengths is simulated and tested in a simple optical sep-up at 100 GHz using an off-axis lensless configuration. Such a technique can be used to measure absorption characteristics of materials, and can also help classify radiating horns and lens antennas. Gaussian Beam Mode Analysis is used as an efficient computational technique to investigate the propagation of non-diffracting beams, and in particular, Bessel beams, at millimeter wavelengths. Because of the limited throughput of millimeter-wave systems, due to the long wavelength and the need for compact optics for practical applications, modal analysis is a very computationally efficient means for computing propagation characteristics. Typically, the axicon, or conical lens, is the most common optical component used for the generation of such zeroth order Bessel beams, but we show that holographic simulation can be used to design binary holograms for the generation of higher order non-diffracting beams. Furthermore, we describe a practical design for such a simple alternative to the axicon through the manufacture of a binary analogue of this component, which successfully produces diffraction invariant beams.
Because of their special properties profiled corrugated waveguide-horn structures are popular as both single-moded and multi-moded bolometer feeds in CMB experiments (e.g. PLANCK, Archeops, QUaD). Although optimised at a spot frequency the horns are usually employed over a relatively wide bandwidth and for single-moded horns the waveguide itself acts as the high pass filter. The horns can be profiled to reduce the horn length and sometimes also a front flared section is added to provide minimum edge taper and spill-over levels (e.g. on PLANCK). In this paper we report on our detailed analysis of the bandwidth properties of such corrugated horns. In the case of multi-moded horns an important issue is how the number of modes varies across the band with the resulting impact on the beam patterns. The so-called "phase centre" location 3 is also an issue. For polarisation selective systems we probe the polarisation purity of the relayed signal across the band and also investigate waveguide details that determine the exact location of band edges. Furthermore any leakage below the expected cut-off will lead to non-idealised cross-polar effects. We have also undertaken a series of laboratory measurements of bandwidth effects in corrugated waveguides to verify the models used.
QUaD is a ground-based high-resolution (up to l ≈ 2500) instrument designed to map the polarisation of the Cosmic Microwave Background and to measure its E-mode and B-mode polarisation power spectra. QUaD comprises a bolometric array receiver (100 and 150 GHz) and re-imaging optics on a 2.6-m Cassegrain telescope 2. It will operate for two years and begin observations in 2005. CMB polarisation measurements will require not only a significant increase in sensitivity over earlier experiments but also a better understanding and control of systematic effects particularly those that contribute to the polarised signal. To this end we have undertaken a comprehensive quasi-optical analysis of the QUaD telescope. In particular we have modelled the effects of diffraction on beam propagation through the system. The corrugated feeds that couple radiation from the telescope to phase-sensitive bolometers need to have good beam symmetry and low sidelobe levels over the required bandwidth. It is especially important that the feed horns preserve the polarisation orientation of the incoming fields. We have used an accurate mode-matching model to design such feed horns. In this paper we present the diffraction analysis of the QUaD front-end optics as well as the electromagnetic design and testing of the QUaD corrugated feeds.
In this paper, we report on extending a theoretical framework based on Gaussian Beam Mode Analysis for modelling standing waves in receiver systems coupled to submillimetre wave telescopes. This analytical technique includes a full electromagnetic description of corrugated detector horns, used as a standard feed horn in the THz frequency range. In previous papers we reported on the underlining theory and described some important examples including reflections between a feed horn and telescope secondary mirror and also reflections between a horn and a plano-convex lens. As the theory uses a full multi-moded scattering matrix description within the horn, which can then be transformed to equivalent free space modes, mulitple reflections between the detector, located at the back of the horn, and any arbitrary surface in the optical path can be accurately analysed. We present an experimental validation of the model, comparing predicted standing wave patterns occuring between two corrugated horns to laboratory measurements, owrking in a frequency range around 0.1THz.