Probing the Early Universe with the PLANCK Surveyor

Abstract included in text

Gaussian beam mode analysis of standing waves between two coupled corrugated horns

Abstract—In this paper we present the theoretical analysis of the effects of standing waves between coupled horn antennas that can occur in terahertz quasi-optical systems. In particular we illustrate the approach for the case of two coupled horn antennas as the distance between them is varied. The full mode matching scattering matrix approach is based on combining a standard waveguide mode description of the horn antenna and a quasi-optical Gaussian beam description of the free space propagation. Track is kept of both the backward and forward going components of the propagating fields.We compare theoretical predictions with actual experimental test results for a quasi-optical system operating at a frequency of 0.480 THz

Gaussian beam mode analysis of standing waves between two coupled corrugated horns

Abstract—In this paper we present the theoretical analysis of the effects of standing waves between coupled horn antennas that can occur in terahertz quasi-optical systems. In particular we illustrate the approach for the case of two coupled horn antennas as the distance between them is varied. The full mode matching scattering matrix approach is based on combining a standard waveguide mode description of the horn antenna and a quasi-optical Gaussian beam description of the free space propagation. Track is kept of both the backward and forward going components of the propagating fields.We compare theoretical predictions with actual experimental test results for a quasi-optical system operating at a frequency of 0.480 THz

Simulation and performance prediction for the radiation patterns of few-moded and multi-moded corrugated horns

Corrugated horn assemblies are the most common method of feeding bolometer cavities for power detection on CMB experiments. Traditionally these have been single moded, whether polarised or unpolarised, but if polarisation is not required greater throughput can be obtained from few-moded or multi-moded horns. The simulation of these assemblies is far more computationally intensive than for single moded systems, and the effort is often compounded by the design of the assemblies. Here we show that multi-moded horn assemblies can be modelled reliably and do perform as required, but performance prediction requires attention to manufacturing tolerances as well as the system design

Perturbation analysis of Gaussian-beam-mode scattering at off-axis ellipsoidal mirrors

We present an approximate technique, based on the principles of multimode Gaussian optics and perturbation theory, for calculating the distortions that occur when an off-axis ellipsoidal mirror is placed in the path of a weakly diffracting beam. More specifically, we calculate the Gaussian-beam-mode scattering matrices of off-axis ellipsoidal mirrors. The technique can be applied when the phase errors across the surface of the mirror are small. In this case, power is only scattered into a few neighbouring modes, and simple closed-form expressions can be derived for the elements of the scattering matrix

Optical Modelling of the HFI Instrument on Board the Planck Surveyor

The PLANCK SURVEYOR is a European Space Agency satellite mission to image the very faint anisotropies in the temperature of the Cosmic Microwave Background (CMB) radiation. Maynooth is actively participating in an international collaboration of scientists involved with the optical design of the High Frequency Instrument (HFI). This paper outlines research which has been undertaken in Maynooth concerned with numerical modelling of the optical characteristics of the multi-frequency array of detectors making up the HFI instrument. In the study the commercial software package ZEMAX was used to model the coupling of the focal plane HFI detectors to the PLANCK telescope. This package is particularly useful in the optical design of PLANCK because of the powerful optimisation features of the software. It is thus possible to readily determine the optimum positioning of the detectors in the focal plane of the telescope. Although the package is based on a numerical ray tracing approach, diffraction effects can be included making it possible to model the beam patterns for the HFI array on the sky. As a method of validating this approach the results of the ZEMAX model of the system were compared with that of a more rigorous physical optics simulation. It was concluded from the study that the level of agreement between the two approaches support the proposed use of ZEMAX. Once the detector positions were optimised using ZEMAX the physical optics approach was used to determine a more accurate set of beam patterns for the array

The Gaussian beam mode analysis of off-axis aberrations in long wavelength optical systems

An issue of major concern in the design of long wavelength systems is optical aberration or distortion, which can be particularly severe in off-axis systems. Aberrations occur in both lenses and mirrors and in this paper we present a novel method capable of modelling off-axis mirror configurations. Aberrations degrade fundamental receiver coupling coefficients such as aperture efficiency while increasing spillover power losses. For single pixel instruments this will lead to much longer integration times and the possibility of stray light. For imaging arrays the aberrations cause a departure from perfect point imaging by increasing coupling to array detectors located at angles further off the bore sight of the telescope. This paper verifies a matrix-based scheme using Gaussian beam mode analysis (GBMA) for predicting aberrations from off-axis mirrors. The applied technique was originally described in (S. Withington, A. Murphy, G. Isaak, Representation of mirrors in beam waveguides as inclined phase transforming surfaces, Infrared Phys. Tech. 36(3) (1995) 723–734. [1]) and in this paper we exploit the theory and validate the approach with a series of examples using off-axis conic sections. We present the predictions for both a fundamental Gaussian and a scalar horn field illuminating various off-axis mirror configurations including different angles of incidence. A commercially available physical optics (PO) software package, GRASP8™, is used to validate the accuracy of these scalar GBMA predictions

The quasi-optical analysis of Bessel beams in the far infrared

We discuss the Gaussian beam mode analysis of Bessel beams, eigen-solutions of the wave-equation in cylindrical polar coordinates which neither change form nor spread out as they propagate. Approximate, limited diffraction finite aperture, pseudo-Bessel beams having intense on-axis spots with large depths of field can be produced experimentally in the far infrared by using plastic conical lenses, known as axicons. We illustrate the physical insight provided by Gaussian beam mode analysis of such systems. Such pseudo-Bessel beams can be usefully approximated by high-order Gaussian–Laguerre modes, which have similar propagation characteristics. The size of the on-axis spot produced by an axicon, and its depth of focus, can be estimated from a single best-fit high-order Gaussian–Laguerre mode, and a more detailed description of behaviour can be achieved by adding a few additional modes of neighbouring orders. The strength of Gaussian beam mode analysis is that it is straightforward to model the propagation of Bessel beams through complex systems of long wavelength optical components, such as apertures, mirrors, and lenses. We report the experimental generation and measurement of a 0.1 THz Bessel beam, and show that useful performance is possible for an axicon having a scale size just one order of magnitude greater than the wavelength. This work confirms the technical feasibility of designing and building long-wavelength optical systems based on Bessel 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

Quasi-optical multiplexing using reflection phase gratings

Heterodyne array receiver systems for both ground based and satellite telescope facilities are now becoming feasible for imaging in the submillimetre/terahertz regions of the EM spectrum. Phase gratings can be usefully employed as high efficiency passive multiplexing devices in the local oscillator (LO) injection chain of such receivers, ensuring that each element of the array is adequately biased and that the reflected LO power level at the array is minimised. For the wavelengths of interest both transmission and reflection gratings can be manufactured by milling an appropriate pattern of slots into the surface(s) of a suitable material. Thus, the required phase modulation is produced by the resulting pattern of varying optical path lengths suffered by the incident wave-front. We report on work we are undertaking to develop all reflection quasi-optical multiplexing systems so as to reduce reflection losses at the grating and minimise the number of surfaces that can contribute to standing wave effects in the optical system. As part of this endeavour we have also developed a quasi-optical technique for analysing the inevitable degradation due to multiple reflections on transmission grating design. This analysis is based on the Gaussian beam mode technique, and a further application of this technique allows one to assess tolerance limitations on the grating