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Physical Optics Analysis of the ALMA Band 5 Front End Optics

The Atacama Large Millimetre Array will be a ground based millimetre to submillimetre band interferometer. The instrument will be comprised of up to 50 high precision 12m Cassegrain antennas. Each antenna will cover a frequency range from 30 to 950 GHz, which will be split into 10 observing channels/bands. Each frequency channel will have its own specifically designed front end optics to couple radiation from the secondary reflector focal plane to the accompanying receiver. We present a full electromagnetic analysis of the band 5 front end optics system using physical optics, which covers a range from 163 to 211 GHz. This band is being developed by the Group for Advanced Receiver Development (GARD) at Chalmers University, Gothenburg, Sweden. Two software packages are utilised for this analysis; the industry standard reflector antenna software package GRASP9 developed by TICRA [1] and a new optical software package MODAL [2,3] (Maynooth Optical Design Analysis Laboratory) developed at NUI Maynooth, Ireland. Electromagnetic predictions of beam patterns are presented at the Cassegrain focal plane and at the subreflector vertex. The basis of the analysis is primarily to determine optical performance and efficiency and the effects of beam truncation by the off-axis reflectors of the front end optics. Three levels of beam truncation are modelled varying rim diameter

Physical Optics Analysis of the ALMA Band 5 Front End Optics

The Atacama Large Millimetre Array will be a ground based millimetre to submillimetre band interferometer. The instrument will be comprised of up to 50 high precision 12m Cassegrain antennas. Each antenna will cover a frequency range from 30 to 950 GHz, which will be split into 10 observing channels/bands. Each frequency channel will have its own specifically designed front end optics to couple radiation from the secondary reflector focal plane to the accompanying receiver. We present a full electromagnetic analysis of the band 5 front end optics system using physical optics, which covers a range from 163 to 211 GHz. This band is being developed by the Group for Advanced Receiver Development (GARD) at Chalmers University, Gothenburg, Sweden. Two software packages are utilised for this analysis; the industry standard reflector antenna software package GRASP9 developed by TICRA [1] and a new optical software package MODAL [2,3] (Maynooth Optical Design Analysis Laboratory) developed at NUI Maynooth, Ireland. Electromagnetic predictions of beam patterns are presented at the Cassegrain focal plane and at the subreflector vertex. The basis of the analysis is primarily to determine optical performance and efficiency and the effects of beam truncation by the off-axis reflectors of the front end optics. Three levels of beam truncation are modelled varying rim diameter

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

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

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