Remnant radiation from the early universe, known as the Cosmic Microwave Background (CMB), has been redshifted and cooled, and today has a blackbody spectrum peaking at millimetre wavelengths. The QUBIC (Q&U Bolometric Interferometer for Cosmology) instrument is designed to map the very faint polaristion structure in the CMB. QUBIC is based on the novel concept of bolometric interferometry in conjunction with synthetic imaging. It will have a large array of input feedhorns, which creates a large number of interferometric baselines.
The beam from each feedhorn is passed through an optical combiner, with an off-axis compensated Gregorian design, to allow the generation of the synthetic image. The optical-combiner will operate in two frequency bands (150 and 220 GHz with 25% and 18.2 % bandwidth respectively) while cryogenically cooled TES bolometers provide the sensitivity required at the image plane.
The QUBIC Technical Demonstrator (TD), a proof of technology instrument that contains 64 input feed-horns, is currently being built and will be installed in the Alto Chorrillos region of Argentina. The plan is then for the full QUBIC instrument (400 feed-horns) to be deployed in Argentina and obtain cosmologically significant results.
In this paper we will examine the output of the manufactered feed-horns in comparison to the nominal design. We will show the results of optical modelling that has been performed in anticipation of alignment and calibration of the TD in Paris, in particular testing the validity of real laboratory environments. We show the output of large calibrator sources (50 ° full width haf max Gaussian beams) and the importance of accurate mirror definitions when modelling large beams. Finally we describe the tolerance on errors of the position and orientation of mirrors in the optical combiner.
A multimode horn differs from a single mode horn in that it has a larger sized waveguide feeding it. Multimode horns can therefore be utilized as high efficiency feeds for bolometric detectors, providing increased throughput and sensitivity over single mode feeds, while also ensuring good control of the beam pattern characteristics. Although a cavity mounted bolometer can be modelled as a perfect black body radiator (using reciprocity in order to calculate beam patterns), nevertheless, this is an approximation. In this paper we present how this approach can be improved to actually include the cavity coupled bolometer, now modelled as a thin absorbing film. Generally, this is a big challenge for finite element software, in that the structures are typically electrically large. However, the radiation pattern of multimode horns can be more efficiently simulated using mode matching, typically with smooth-walled waveguide modes as the basis and computing an overall scattering matrix for the horn-waveguide-cavity system. Another issue on the optical efficiency of the detectors is the presence of any free space gaps, through which power can escape. This is best dealt with treating the system as an absorber. Appropriate reflection and transmission matrices can be determined for the cavity using the natural eigenfields of the bolometer cavity system. We discuss how the approach can be applied to proposed terahertz systems, and also present results on how the approach was applied to improve beam pattern predictions on the sky for the multi-mode HFI 857GHz channel on Planck.
Multimode horn antennas can be utilized as high efficiency feeds for bolometric detectors, providing increased
throughput and sensitivity over single mode feeds, while also ensuring good control of beam pattern characteristics.
Multimode horns were employed in the highest frequency channels of the European Space Agency Planck Telescope,
and have been proposed for future terahertz instrumentation, such as SAFARI for SPICA. The radiation pattern of a
multimode horn is affected by the details of the coupling of the higher order waveguide modes to the bolometer making
the modeling more complicated than in the case of a single mode system. A typical cavity coupled bolometer system can
be most efficiently simulated using mode matching, typically with smooth walled waveguide modes as the basis and
computing an overall scattering matrix for the horn-waveguide-cavity system that includes the power absorption by the
absorber. In this paper we present how to include a cavity coupled bolometer, modelled as a thin absorbing film with
particular interest in investigating the cavity configuration for optimizing power absorption. As an example, the possible
improvements from offsetting the axis of a cylindrically symmetric absorbing cavity from that of a circular waveguide
feeding it (thus trapping more power in the cavity) are discussed. Another issue is the effect on the optical efficiency of
the detectors of the presence of any gaps, through which power can escape. To model these effects required that existing
in-house mode matching software, which calculates the scattering matrices for axially symmetric waveguide structures,
be extended to be able to handle offset junctions and free space gaps. As part of this process the complete software code
'PySCATTER' was developed in Python. The approach can be applied to proposed terahertz systems, such as SPICASAFARI.
The work of this research is the design, analysis and verification of the optical performance of a 4mmreceiver channel
for the 20 m telescope at Onsala Space Observatory, Onsala, Sweden. The 4 mm (75 GHz) receiver is a newly proposed
channel designed to be installed parallel to the existing 3 mm (100 GHz) channel targeting new science at that longer
wavelength. Gaussian beam mode analysis is used to produce the fundamental optical design of the system. The design
is then analysed more accurately with the physical optics approximation. We report on the comparison of simulation
and measurement and verification of the system design.