Polarisation effects investigations in quasi-optical metal grid filters
Introduction
Quasi-optical metal grid interference filters have become the standard method for waveband selection for far-infrared and submillimetre photometric instruments [1], [2], [3], [4]. They have enabled precise edge or band definition, dependent solely on the geometric properties of the metallic grids used in the filter construction, with low absorption loss. A complete filter comprises several metal grid sheets in a plane parallel stack with either air or dielectric filled spacers. These components have found ready application in nearly all sub-millimetre continuum instruments for waveband selection, thermal heat rejection, intensity beam division, spectral beam division, stopband rejection and for Fabry–Perot plates.
Many astronomical instruments’ receivers use metal grid filters in systems designed to measure polarisation. Applications abound and range from polarisation of the cosmic microwave background (CMB) anisotropy, star formation regions, synchrotron sources and laboratory diagnostic instruments. It has therefore become important to investigate residual polarisation effects inherent in the filter structures.
Metal grid filters constitute part of the more general family of frequency selective surfaces (FSS). Many theories have been developed to describe these structures, from simple transmission line based models [1], [2] to more sophisticated Floquet modes analysis [3], [4]. Here we used an alternative approach based on commercial finite-element analysis software high frequency structure simulator (HFSS) [5] that has been proved to be successful in similar types of analysis [6], [7]. This software enables a three-dimensional finite-element solution of Maxwell’s equations for grid filter structures in both the diffraction and non-diffraction regions as well as in the presence of Wood’s anomalies [8] at non-normal incidence. This modelling together with spectral measurements of a real capacitive grid component has enabled us to make accurate predictions of performance as well as determining the cause of the parasitic effects.
In the following two sections we present our modelling approaches followed by a brief description of the experimental measurements. In Sections 4 On-axis measurements, 5 Off-axis measurements we compare the results of the simulations with the experimental data for the capacitive grid described in the next section.
Section snippets
The HFSS model
Numerous techniques have been developed to model FSS. One of the earliest presented by Ulrich [2] for a simple square geometry, with the results of Marcuvitz [1], used a simple transmission line circuit representation. This model gives a reasonably accurate representation of their behaviour for frequencies below the diffraction region and allows for a good physical insight into their properties. Air gaps and dielectric layers are accounted for by incorporating transmission line sections in the
Spectral measurements
Experimental measurements where made using a polarising Martin–Puplett Fourier transform spectrometer (FTS), shown schematically in Fig. 3. The FTS was configured in a rapid scan mode using a 1.5-K bolometric detector with a spectral cut-off near 40 cm−1. The optical path difference was chosen to be 10 cm, giving an apodised resolution of 0.1 cm−1, which was adequate to resolve the expected spectral features. An adjustable Jacquinot stop in the FTS allowed the f-number of the incident beam at the
On-axis measurements
In this set of measurements the plane of the metal grid is orthogonal to the optical axis. In such a configuration any incident polarisation status can be decomposed into the sum of two orthogonal components projected along the metal grid symmetry axes. Thus, even using linearly polarised light we expect the transmission to be independent of the mesh rotation angle, α. In Fig. 5 the experimental data and the corresponding simulations are compared. The transmission spectra for different mesh
Off-axis measurements
The experimental results with the metal grid tilted with respect to the FTS optical axis are very different from those discussed above. As an example the results for ϑ = 15° are shown in Fig. 7, Fig. 8, respectively, for s and p polarisation as a function of the rotation angle ϕ. There are broad features around 25 cm−1, which depend strongly on the rotation angle. We also note that there is complementary behaviour between the s and p polarisations. Clearly these features are substantial and
Conclusions
The model presented here is a powerful tool to investigate and understand the response to polarised radiation of single FSS structures. Since multi-grid interference filters are made by superposition of many metal grids of the kind investigated here the importance of this study to the design and use of these in polarimetric instruments is clear. It should be noted that the analysis of a more complex multilayer FSS does not differ from the single grid case presented here. However, to reduce
Acknowledgments
We are grateful to the UK Particle Physics and Astronomy Research Council for support of this work and to Ansoft for providing the HFSS software. Special thanks to Josie Budd for manufacturing the grid filters, Carole Tucker and Beverley Holman for the experimental measurements and Damian Audley for the useful suggestions on the HFSS modelling.
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