Simulations and performance of the QUBIC optical beam combiner

Abstract

QUBIC, the Q & U Bolometric Interferometer for Cosmology, is a novel ground-based instrument that aims to measure the extremely faint B-mode polarisation anisotropy of the cosmic microwave background at intermediate angular scales (multipoles of = 30 − 200). Primordial B-modes are a key prediction of Inflation as they can only be produced by gravitational waves in the very early universe. To achieve this goal, QUBIC will use bolometric interferometry, a technique that combines the sensitivity of an imager with the immunity to systematic effects of an interferometer. It will directly observe the sky through an array of back-to-back entry horns whose beams will be superimposed using a cooled quasioptical beam combiner. Images of the resulting interference fringes will be formed on two focal planes, each tiled with transition-edge sensors, cooled down to 320 mK. A dichroic filter placed between the optical combiner and the focal planes will select two frequency bands (centred at 150 GHz and 220 GHz), one frequency per focal plane. Polarization modulation will be achieved using a cold stepped half-wave plate (HWP) and polariser in front of the sky-facing horns.The full QUBIC instrument is described elsewhere 1,2,3,4 ; in this paper we will concentrate in particular on simulations of the optical combiner (an off-axis Gregorian imager) and the feedhorn array. We model the optical performance of both the QUBIC full module and a scaled-down technological demonstrator which will be used to validate the full instrument design. Optical modelling is carried out using full vector physical optics with a combination of commercial and in-house software. In the high-frequency channel we must be careful to consider the higher-order modes that can be transmitted by the horn array. The instrument window function is used as a measure of performance and we investigate the effect of, for example, alignment and manufacturing tolerances, truncation by optical components and off-axis aberrations. We also report on laboratory tests carried on the QUBIC technological demonstrator in advance of deployment to the observing site in Argentina.

QUBIC VIII: Optical design and performance

“…We 2https://www.ticra.com/software/grasp/ Figure 8. CAD model of the beam combiner showing the 3D model of the beams (here coloured yellow pink and green) propagating between the two mirrors [24], [11].…”

“…The focal plane image is sampled by an array of cooled transition-edge sensor (TES) bolometers, hence the term bolometric interferometry. [11]).…”

QUBIC: the Q and U bolometric interferometer for cosmology

“…The requirement to sample the narrowest fringes (the longest baselines of interest) with two or more 3-mm wide detectors limits the focal length to 300 mm. This short focal length means that aberrations are an issue, but our simulations have shown that they nevertheless only decrease the sensitivity of the instrument by about 6% 19 . The use of reflectors means that the system can be accurately modelled with physical optics (PO) software such as GRASP 20 .…”

QUBIC: Observing the Polarized Microwave Sky over the Puna

“…14 detectors will be reported elsewhere, after the calibration of the technological demonstrator is completed. More details on the simulation of the optical beam combiner and horns array can be found in [50].…”