Ronan J. Mahon

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.

Applications of holography in the millimeter-wave and terahertz region

In this paper we report on the improvements in holographic techniques developed for applications in the millimeter-wave and terahertz range of the electromagnetic spectrum. An experimental arrangement, adapted from off-axis near-field holography at visible wavelengths, was employed that utilizes a raster scanning detector to record the holograms digitally. The object and reference fields were based on the beams from a pair of radiating antennas fed by a single coherent source via a cross-guide coupler. Using phase retrieval methods, the recorded holographic interference pattern can be used to determine the effective phase centers of radiating feed antennas, including non standard radiators such as planar lens antennas. By numerically propagating the recovered object beam back to the source plane the object beam in the vicinity of the waist (the effective phase center) can be recovered. Among the issues investigated was improvement in the accuracy of the phase retrieval process by taking account of the non-perfect reference beam. The technique has also been applied to the investigation of increased co-polarisation levels in the scattering of radiation from surface features of dielectric materials on millimeter-wave radiation.

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.

Diffraction of an optical pulse as an expansion in ultrashort orthogonal Gaussian beam modes

The Laguerre-Gaussian (LG) beam expansion is described as a numerical and physical model of paraxial ultrashort pulse diffraction in the time domain. An overview of the dynamics of higher-order ultrashort planar LG modes is given through numerical simulations, and the finite width of these beams is shown to induce a dispersive-like axial broadening of the fields, which creates related variations in the on-axis amplitude of such pulses. The propagation of a pulsed plane wave scattered at an aperture is then illustrated as a finite weighted sum of individual planar LG pulses, which allows for intuitive illustration of the convergence of this expansion technique. By applying such an expansion to diffraction at a hard aperture, the planar pulsed LG beams are described as the paraxial analogs of the Bessel boundary waves typically observed in such situations, with both exhibiting superluminal group velocities along the optical axis. Numerical results of pulse diffraction at an aperture highlight the suitability of the LG expansion method for efficient and practical simulation of ultrashort fields in the paraxial regime.

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.

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.

Gaussian beam mode analysis of optical pulses

The application of the Laguerre-Gaussian (LG) and Hermite-Gaussian (HG) series expansions in the paraxial simulation of optical pulses is described and presented through examples of pulse modulation by diffractive Fresnel lenses and axicons. Using the FDTD technique we exhibit the physical propagation of a Hermite-Gaussian mode outside of the paraxial regime, with a width parameter of the same order of magnitude as the wavelength. In higher order modes this causes evanescence at the source of such modes, and we describe the loss of energy caused by this phenomenon. Using recently derived expressions for the non-paraxial propagation of Hermite-Gaussian modes, we discuss the use of modal techniques outside of the usual paraxial restriction, which allows for an efficient modal synthesis of Rayleigh-Sommerfeld diffraction effects in the far-field.

Modeling of millimetre-wave and terahertz imaging systems

In order to improve the design and analyse the performance of efficient terahertz optical systems, novel quasi-optical components along with dedicated software tools are required. At sub-millimetre wavelengths, diffraction dominates the propagation of radiation within quasi-optical systems and conventional geometrical optics techniques are not adequate to accurately guide the beams or assess optical efficiency. In fact, in general Optical design in the terahertz waveband suffers from a lack of dedicated commercial software packages for modelling the range of electromagnetic propagation regimes that are important in such systems. In this paper we describe the physical basis for efficient CAD software tools we are developing to specifically model long wavelength systems. The goal is the creation of a user-friendly package for optical engineers allowing potential systems to be quickly simulated as well as also providing an analytical tool for verification of existing optical systems. The basic approach to modelling such optical trains is the application of modal analysis e.g. [1][2], which we have extended to include scattering at common off-axis conic reflectors. Other analytical techniques are also ncluded within the CAD software framework such as plane wave decomposition and full physical optics. We also present preliminary analytical methods for characterising standing waves that can occur in terahertz systems and report on novel binary optical components for this wavelength range. Much of this development work has been applied to space instrumentation but is relevant for all Terahertz Imaging systems.

Simulated propagation of ultrashort pulses modulated by low-Fresnel-number lenses using truncated series expansions.

Numerical simulation of the paraxial propagation of pulses modulated by lenses is demonstrated using the Laguerre-Gaussian (LG) series expansion method. This technique allows for relatively swift evaluation of the structures of several individual monochromatic fields transformed by arbitrary amplitude and phase modulating pupil functions, which can be superimposed via the inverse Fourier transform to determine the structure of a modulated pulse. The transformation of ultrashort pulses by spherical, diffractive, and conical lenses is simulated using this method, which is particularly effective with the use of vector and matrix techniques available in many popular numerical software packages. A description of the convergence of the LG series to the results of the conventional integral techniques is presented for a conical lens under illumination by a continuous wave from which a simple but robust criterion for axial accuracy in problems of circular symmetry is suggested.

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.