1,721,631 research outputs found
A review of metal mesh filters
No full textThe Astronomical Instrumentation Group at Cardiff University has been developing metal mesh optical filters for more than 30 years, which are currently in use in many ground-, balloon- and space-based instruments. Here we review the current state of the art with respect to these quasi-optical components (low-pass, high-pass and band-pass filters, dichroics and beam-dividers) as developed for the FIR and sub-millimetre wavelength region. We compare performance data with various modelling tools (HFSS, transmission line theory or Floquet mode analysis). These models assist with our understanding of the behaviour of these filters when used at non-normal incidence or in the diffraction region of the grid structures. Interesting artefacts, such as the Wood anomalies and behaviour with S and P polarisations, which dictate the usage of these components in polarisation sensitive instruments, will be discussed
Polarisation effects investigations in quasi-optical metal grid filters
No full textMany theories have been developed to describe metal grid filters that are commonly used for wavelength selection in far-infrared and millimetre wavelengths experiments. Application of these filters has shifted from simple photometric measurements to the more demanding requirements needed to accurately measure the polarisation. It has therefore become necessary to investigate residual polarisation effects inherent in these kinds of filter structures. In this work we present an alternative modelling approach, based on finite-element analysis software (HFSS) available commercially. To validate the model we have made Fourier transform spectrometer, FTS, measurements of a real capacitive grid component as a function of incidence angle and grid orientation for both s and p polarisations. The excellent agreement between model and data enable accurate predictions of performance of these structures when used in polarimetric experiments as well as determining the cause of parasitic effects. © 2005 Elsevier B.V. All rights reserved
Effects of quasi-optical components on feed-horn co- And cross-polarisation radiation patterns
No full textAstrophysical experiments dedicated to the study of the Cosmic Microwave Background are in needs of very well defined beam shape in order to get an accurate re-construction of the anisotropies power spectrum. These beams have to be carefully designed, but also properly characterised. Some of these instruments will be located in a cryostat for which filters and windows are necessary, and the effects of these additional optical components on the beam shape need to be taken into account. We present here, measurements of such effects on the co- and cross-polarisation radiation patterns of corrugated horns using a Vector Network Analyser
Achromatic half-wave plate for submillimeter instruments in cosmic microwave background astronomy: Modeling and simulation
No full textWe adopted an existing formalism and modified it to simulate, with high precision, the transmission, reflection, and absorption of multiple-plate birefringent devices as a function of frequency. To validate the model, we use it to compare the measured properties of an achromatic five-plate device with a broadband antireflection coating to expectations derived from the material optical constants and its geometric configuration. The half-wave plate presented here is observed to perform well with a phase shift variation of <2° from the ideal 180° over a bandwidth of Δv/v ∼ 1 at millimeter wavelengths. This formalism represents a powerful design tool for birefringent polarization modulators and enables its optical properties to be specified with high accuracy. © 2006 Optical Society of America
Metal-mesh achromatic half-wave plate for use at submillimeter wavelengths
No full textA metal-mesh achromatic half-wave plate (HWP) has been designed, manufactured, and tested for potential use in millimeter and submillimeter astronomical instruments. The prototype device presented here is based on a 12-grid Shatrow [IEEE Trans. Antennas Propag. 43,109 (1995)] recipe to operate over the frequency range of 120-180 GHz. Transmission line modeling and finite-element analysis [Ansoft HFSS website: http://www.ansoft.com/hfss/1 were used to optimize the design geometrical parameters in terms of the device transmission, reflection, absorption, phase-shift, and cross-polarization as a function of frequency. The resulting prototype device was constructed and characterized using incoherent radiation from a polarizing Fourier transform spectrometer to explore its frequency and polarization behavior. These measurements are shown to be in excellent agreement with the models. Lists of the achieved HWP performance characteristics are reported. © 2008 Optical Society of America
Achromatic half-wave plate for submillimeter instruments in cosmic microwave background astronomy: Experimental characterization
No full textAn achromatic half-wave plate (HWP) to be used in millimeter cosmic microwave background (CMB) polarization experiments has been designed, manufactured, and tested. The design is based on the 5-plates Pancharatnam recipe and it works in the frequency range 85-185 GHz. A model has been used to predict the transmission, reflection, absorption, and phase shift as a function of frequency. The HWP has been tested by using coherent radiation from a back-wave oscillator to investigate its modulation efficiency and with incoherent radiation from a polarizing Fourier transform spectrometer (FTS) to explore its frequency behavior. The FTS measurements have been fitted with an optical performance model which is in excellent agreement with the data. A detailed analysis of the data also allows a precise determination of the HWP fast and slow axes in the frequency band of operation. A list of the HWP performance characteristics is reported including estimates of its cross polarization. © 2006 Optical Society of America
Modelling and measurement of frequency selective surfaces
No full textFrequency Selective Surfaces, FSS's, are used to make filters in the far infrared and submillimetre region, which can have low-pass, high-pass and band selection properties. For basic characterisation of grid optical properties we use transmission line theory. In addition it has proven necessary to use High Frequency Structure Simulator software, HFFS, to investigate the Woods-type anomalies and the absorption observed in these devices
Metal mesh based metamaterials for millimetre wave and THz astronomy applications
No full textMetal mesh technology has been employed for decades to build high performance filters working at millimetre and sub-millimetre wavelengths. They have found a wide range of applications, mainly targeted to astronomical instrumentation. Mesh filters, dichroics, beam dividers and polarizers have been used in a multitude of ground based, balloon-borne and satellite projects. The technology is well proven and space qualified. The same technology can be used to develop more general quasi-optical devices able to manipulate and transform the electromagnetic field amplitude and phase across their surface. We summarize recent developments in this field discussing devices such as mesh Half Wave Plates, flat mesh lenses, polarization splitting devices and mesh prisms
The SCUBA-2 polarimeter
No full textA polarimeter is built to be used with the SCUBA-2 camera of the James Clerk Maxwell Telescope to study polarized sub-millimeter radiations. We simulated the effect of the polarimeter on image quality and polarization measurements. © 2006 Optical Society of America
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