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
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.
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.
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.
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. , 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.