W. Lanigan

Novel techniques for millimeter wave imaging systems operating at 100GHz

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.

Terahertz holographic image reconstruction and analysis

We report on the reconstruction of terahertz images from digitally recorded holograms. An off-axis lens-less configuration is explored using a test set-up at 0.1 THz. A backward propagation algorithm and Gaussian beam mode analysis are used to determine the transmission properties of transparent materials and scattering properties of rough surfaces.

Millimetre-wave and Terahertz Imaging Systems with Medical Applications

The properties of terahertz (THz) radiation 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 and non-biological samples at 0.1THz with a view to developing THz and millimetre-wave imaging schemes. In particular we have investigated the effects of standing waves on such systems and the sensitivity to water content of the sample as a means to extract medically useful information.

A mechanical mounting system for functional near-infrared spectroscopy brain imaging studies

In this work a mechanical optode mounting system for functional brain imaging with light is presented. The particular application here is a non-invasive optical brain computer interface (BCI) working in the near-infrared range. A BCI is a device that allows a user to interact with their environment through thought processes alone. Their most common use is as a communication aid for the severely disabled. We have recently pioneered the use of optical techniques for such BCI systems rather than the usual electrical modality. Our optical BCI detects characteristic changes in the cerebral haemodynamic responses that occur during motor imagery tasks. On detection of features of the optical response, resulting from localised haemodynamic changes, the BCI translates such responses and provides visual feedback to the user. While signal processing has a large part to play in terms of optimising performance we have found that it is the mechanical mounting of the optical sources and detectors (optodes) that has the greatest bearing on the performance of the system and indeed presents many interesting and novel challenges with regard to sensor placement, depth of penetration, signal intensity, artifact reduction and robustness of measurement. Here a solution is presented that accommodates the range of experimental parameters required for the application as well as meeting many of the challenges outlined above. This is the first time that a concerted study on optode mounting systems for optical BCIs has been attempted and it is hoped this paper may stimulate further research in this area.

The quasi-optical design of the QUaD Telescope

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.

Quasi-optical millimetre-wave imaging with bio-medical applications

We report on millimeter-wave imaging systems being developed for both near-field analysis and quasi-optical image processing. As well as simple near field transmission, we also consider Gaussian beam mode telescope based imaging, which utilizes spatial filtering techniques from Fourier optics, for the imaging of biological samples and other applications. We also report on near-field wave-front reconstruction techniques from holography and show how the techniques can also be used for imaging the phase centre of nonstandard feed antennas. Finally, we briefly summarise progress on the development of Fourier gratings as a method for producing multiple images of a coherent local oscillator source. Such schemes will be necessary for the development of inexpensive multiplexers for heterodyne detection schemes for sensitive array imagers and cameras.

Novel optical design for terahertz imaging applications

We report on our investigations of a number of issues for terahertz medical imaging including optimisation of spatial resolution, characterisation of bio-medical tissues, efficient quasi-optical design and novel delivery systems. In tandem with our experimental approach, we are developing effective computational models with which to analyse system performance.

Phase gratings for the sub-millimetre band

This presentation is concerned with the development of phase gratings for use at terahertz frequencies (sub-mm wavelengths). Design, fabrication and testing of two grating types (Dammann and Fourier) are discussed. Modelling using quasi-optical techniques is outlined and the implementation of a suitable algorithm (Gerchberg-Saxton) to find solutions for the phase-retrieval problem discussed.

Amplitude and phase recovery using holographic imaging of antenna feeds

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.

Quasi-optical analysis of binary optical components at 100 GHz

We report on the use of Gaussian beam mode analysis (GBMA) to study the focusing properties of binary counterparts of lenses, axicons and Gabor zone plates. Optical components at millimetre wavelengths with short focal ratios are inevitably rather thick. It is therefore advantageous to devise a method that can maximize space in a linear imaging system. This can be achieved by using a binary form of refractive and diffractive focusing elements to localize power from beams subject to a large degree of diffraction as is common in quasi-optics at these wavelengths. In this paper, we present numerical analysis of the aforementioned elements using GBMA as a computationally efficient alternative to Fresnel-Kirchoff diffraction integrals.