Astrophysical measurements at far-infrared (FIR) wavelengths have many applications, including probing the fuel for cosmic star formation and understanding the role of dust in the interstellar medium. We are currently developing an on-chip spectrometer operable in the 100-200µm [50-100 cm−1 ] band with sub-micron spectral resolution coupled to a low-noise kinetic inductance detector (KID) array with planned noise equivalent power (NEP) < 10−19 WHz1/2 . Ultimately we will need to evaluate the spectral response of these chips at very low photon backgrounds. While the spectral performance can be simulated, it is crucial to make measurements of the as-built properties of the chip with a calibrator. To this end, we have designed and fabricated a cryogenic Fourier Transform Spectrometer (FTS). Since self-emission from the FTS at room temperature would dominate a cold black body source, the FTS optics are cooled to a temperature of 5K, which greatly reduces the photon background and simulates space-like conditions. The FTS is based on the Michelson design, with 6 fixed Aucoated Al mirrors, back-to-back moving mirrors that increase the effective optical path length by a factor of 2, and a kapton film beamsplitter. Light is coupled into the FTS using HDPE collimating lenses protected by bandpass filters, and the output beam is coupled directly into the device under test through a shared vacuum space. In this paper, we report on the FTS design and discuss its fabrication and testing plan.
QUBIC (Q and U bolometric interferometer for cosmology) is an international ground-based experiment dedicated to the measurement of the polarized fluctuations of the cosmic microwave background (CMB). It is based on bolometric interferometry, an original detection technique which combines the immunity to systematic effects of an interferometer with the sensitivity of low temperature incoherent detectors. QUBIC will be deployed in Argentina, at the Alto Chorrillos mountain site near San Antonio de los Cobres, in the Salta province. The QUBIC detection chain consists of 2048 NbSi transition edge sensors (TESs) cooled to 320 mK. The voltage-biased TESs are read out with time domain multiplexing based on superconducting quantum interference devices (SQUIDs) at 1 K and a novel SiGe application-specific integrated circuit (ASIC) at 60 K allowing an unprecedented multiplexing (MUX) factor equal to 128 to be reached. The current QUBIC version is based on a reduced number of detectors (1/4) in order to validate the detection technique. The QUBIC experiment is currently being validated in the lab in Salta (Argentina) before going to the site for observations. This paper presents the main results of the characterization phase with a focus on the detectors and readout system.
LiteBIRD, the Lite (Light) satellite for the study of B-mode polarization and Inflation from cosmic background Radiation Detection, is a space mission for primordial cosmology and fundamental physics. JAXA selected LiteBIRD in May 2019 as a strategic large-class (L-class) mission, with its expected launch in the late 2020s using JAXA's H3 rocket. LiteBIRD plans to map the cosmic microwave background (CMB) polarization over the full sky with unprecedented precision. Its main scientific objective is to carry out a definitive search for the signal from cosmic inflation, either making a discovery or ruling out well-motivated inflationary models. The measurements of LiteBIRD will also provide us with an insight into the quantum nature of gravity and other new physics beyond the standard models of particle physics and cosmology. To this end, LiteBIRD will perform full-sky surveys for three years at the Sun-Earth Lagrangian point L2 for 15 frequency bands between 34 and 448 GHz with three telescopes, to achieve a total sensitivity of 2.16 μK-arcmin with a typical angular resolution of 0.5° at 100 GHz. We provide an overview of the LiteBIRD project, including scientific objectives, mission requirements, top-level system requirements, operation concept, and expected scientific outcomes.
LiteBIRD has been selected as JAXA’s strategic large mission in the 2020s, to observe the cosmic microwave background (CMB) B-mode polarization over the full sky at large angular scales. The challenges of LiteBIRD are the wide field-of-view (FoV) and broadband capabilities of millimeter-wave polarization measurements, which are derived from the system requirements. The possible paths of stray light increase with a wider FoV and the far sidelobe knowledge of -56 dB is a challenging optical requirement. A crossed-Dragone configuration was chosen for the low frequency telescope (LFT : 34–161 GHz), one of LiteBIRD’s onboard telescopes. It has a wide field-of-view (18° x 9°) with an aperture of 400 mm in diameter, corresponding to an angular resolution of about 30 arcminutes around 100 GHz. The focal ratio f/3.0 and the crossing angle of the optical axes of 90◦ are chosen after an extensive study of the stray light. The primary and secondary reflectors have rectangular shapes with serrations to reduce the diffraction pattern from the edges of the mirrors. The reflectors and structure are made of aluminum to proportionally contract from warm down to the operating temperature at 5 K. A 1/4 scaled model of the LFT has been developed to validate the wide field-of-view design and to demonstrate the reduced far sidelobes. A polarization modulation unit (PMU), realized with a half-wave plate (HWP) is placed in front of the aperture stop, the entrance pupil of this system. A large focal plane with approximately 1000 AlMn TES detectors and frequency multiplexing SQUID amplifiers is cooled to 100 mK. The lens and sinuous antennas have broadband capability. Performance specifications of the LFT and an outline of the proposed verification plan are presented.
LiteBIRD is a JAXA-led Strategic Large-Class mission designed to search for the existence of the primordial gravitational waves produced during the inflationary phase of the Universe, through the measurements of their imprint onto the polarization of the cosmic microwave background (CMB). These measurements, requiring unprecedented sensitivity, will be performed over the full sky, at large angular scales, and over 15 frequency bands from 34 GHz to 448 GHz. The LiteBIRD instruments consist of three telescopes, namely the Low-, Medium-and High-Frequency Telescope (respectively LFT, MFT and HFT). We present in this paper an overview of the design of the Medium-Frequency Telescope (89{224 GHz) and the High-Frequency Telescope (166{448 GHz), the so-called MHFT, under European responsibility, which are two cryogenic refractive telescopes cooled down to 5 K. They include a continuous rotating half-wave plate as the first optical element, two high-density polyethylene (HDPE) lenses and more than three thousand transition-edge sensor (TES) detectors cooled to 100 mK. We provide an overview of the concept design and the remaining specific challenges that we have to face in order to achieve the scientific goals of LiteBIRD.
QUBIC (a Q and U Bolometric Interferometer for Cosmology) is a next generation cosmology experiment designed to detect the B-mode polarisation of the Cosmic Microwave Background (CMB). A B-mode detection is hard evidence of Inflation in the ΛCDM model. QUBIC aims to accomplish this by combining novel technologies to achieve the sensitivity required to detect the faint B-mode signal. QUBIC uses technologies such as a rotating half-wave plate, cryogenics, interferometric horns with self-calibration switches and transition edge sensor bolometers. A Technical Demonstrator (TD) is currently being calibrated in APC in Paris before observations in Argentina in 2021. As part of the calibration campaign, the spectral response of the TD is measured to test and validate QUBIC's spectro-imaging capability. This poster gives an overview of the methods used to measure the spectral response and a comparison of the instrument data with theoretical predictions and optical simulations.
The Q and U Bolometric Interferometer for Cosmology (QUBIC) Technical Demonstrator (TD) aiming to shows the feasibility of the combination of interferometry and bolometric detection. The electronic readout system is based on an array of 128 NbSi Transition Edge Sensors cooled at 350mK readout with 128 SQUIDs at 1K controlled and amplified by an Application Specific Integrated Circuit at 40K. This readout design allows a 128:1 Time Domain Multiplexing. We report the design and the performance of the detection chain in this paper. The technological demonstrator unwent a campaign of test in the lab. Evaluation of the QUBIC bolometers and readout electronics includes the measurement of I-V curves, time constant and the Noise Equivalent Power. Currently the mean Noise Equivalent Power is ~ 2 x 10-16W= p √Hz
QUBIC, the Q & U Bolometric Interferometer for Cosmology, is a novel ground-based instrument that has been designed to measure the extremely faint B-mode polarisation anisotropy of the cosmic microwave background at intermediate angular scales (multipoles of 𝑙 = 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 systematic error control of an interferometer. It will directly observe the sky through an array of 400 back-to-back entry horns whose signals will be superimposed using a quasi-optical beam combiner. The resulting interference fringes will be imaged at 150 and 220 GHz on two focal planes, each tiled with NbSi Transition Edge Sensors, cooled to 320 mK and read out with time-domain multiplexing. A dichroic filter placed between the optical combiner and the focal planes will select the two frequency bands. A very large receiver cryostat will cool the optical and detector stages to 40 K, 4 K, 1 K and 320 mK using two pulse tube coolers, a novel 4He sorption cooler and a double-stage 3He/4He sorption cooler. Polarisation modulation and selection will be achieved using a cold stepped half-wave plate (HWP) and polariser, respectively, in front of the sky-facing horns. A key feature of QUBIC’s ability to control systematic effects is its ‘self-calibration’ mode where fringe patterns from individual equivalent baselines can be compared. When observing, however, all the horns will be open simultaneously and we will recover a synthetic image of the sky in the I, Q and U Stokes’ parameters. The synthesised beam pattern has a central peak of approximately 0.5 degrees in width, with secondary peaks further out that are damped by the 13-degree primary beam of the horns. This is Module 1 of QUBIC which will be installed in Argentina, near the city of San Antonio de los Cobres, at the Alto Chorrillos site (4869 m a.s.l.), Salta Province. Simulations have shown that this first module could constrain the tensor-to-scalar ratio down to σ(r) = 0.01 after a two-year survey. We aim to add further modules in the future to increase the angular sensitivity and resolution of the instrument. The QUBIC project is proceeding through a sequence of steps. After an initial successful characterisation of the detection chain, a technological demonstrator is being assembled to validate the full instrument design and to test it electrically, thermally and optically.
The technical demonstrator is a scaled-down version of Module 1 in terms of the number of detectors, input horns and pulse tubes and a reduction in the diameter of the combiner mirrors and filters, but is otherwise similar. The demonstrator will be upgraded to the full module in 2019. In this paper we give an overview of the QUBIC project and instrument.
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 𝑙 = 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 elsewhere1,2,3,4; 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 (the Q and U Bolometric Interferometer for Cosmology) is a ground-based experiment which seeks to improve the current constraints on the amplitude of primordial gravitational waves. It exploits the unique technique, among Cosmic Microwave Background experiments, of bolometric interferometry, combining together the sensitivity of bolometric detectors with the control of systematic effects typical of interferometers. QUBIC will perform sky observations in polarization, in two frequency bands centered at 150 and 220 GHz, with two kilo-pixel focal plane arrays of NbSi Transition-Edge Sensors (TES) cooled down to 350 mK. A subset of the QUBIC instrument, the so called QUBIC Technological Demonstrator (TD), with a reduced number of detectors with respect to the full instrument, will be deployed and commissioned before the end of 2018.
The voltage-biased TES are read out with Time Domain Multiplexing and an unprecedented multiplexing (MUX) factor equal to 128. This MUX factor is reached with two-stage multiplexing: a traditional one exploiting Superconducting QUantum Interference Devices (SQUIDs) at 1K and a novel SiGe Application-Specific Integrated Circuit (ASIC) at 60 K. The former provides a MUX factor of 32, while the latter provides a further 4. Each TES array is composed of 256 detectors and read out with four modules of 32 SQUIDs and two ASICs. A custom software synchronizes and manages the readout and detector operation, while the TES are sampled at 780 Hz (100kHz/128 MUX rate).
In this work we present the experimental characterization of the QUBIC TES arrays and their multiplexing readout chain, including time constant, critical temperature, and noise properties.
QUBIC, the QU Bolometric Interferometer for Cosmology, is a novel forthcoming instrument to measure the B-mode polarization anisotropy of the Cosmic Microwave Background. The detection of the B-mode signal will be extremely challenging; QUBIC has been designed to address this with a novel approach, namely bolometric interferometry. The receiver cryostat is exceptionally large and cools complex optical and detector stages to 40 K, 4 K, 1 K and 350 mK using two pulse tube coolers, a novel 4He sorption cooler and a double-stage 3He/4He sorption cooler. We discuss the thermal and mechanical design of the cryostat, modelling and thermal analysis, and laboratory cryogenic testing.
In this activity, we develop novel focal plane detector pixels for the next generation CMB B mode detection missions. Such future mission designs will require focal plane pixel technologies that optimizes the coupling from telescope optics to the large number of detectors required to reach the sensitivities required to measure the faint CMB polarization traces. As part of an ESA Technical Research Programme (TRP) programme we are tasked with developing, manufacturing and experimentally verifying a prototype multichroic pixel which would be suitable for the large focal plane arrays to reduce the focal plane size requirement. The concept of replacing traditional single channel pixels with multi frequency pixels will be a key driver in future mission design and the ability to couple radiation effectively over larger bandwidths (30 - 100%) is a real technical challenge. In the initial part of the programme we reviewed the science drivers and this determined the technical specifications of the mission. Various options for focal plane architectures were considered and then after a tradeoff study and review of resources available, a pixel demonstrator was selected for design manufacture and test. The chosen design consists of a novel planar mesh lens coupling to various planar antenna configurations with Resonant Cold Electron Bolometer (RCEB) for filtering and detection of the dual frequency signal. The final cryogenic tests are currently underway and a final performance will be verified for this pixel geometry.
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 main objective of this activity is to develop new focal plane coupling array concepts and technologies that
optimise the coupling from reflector optics to the large number of detectors for next generation sub millimetre
wave telescopes particularly targeting measurement of the polarization of the cosmic microwave background
(CMB). In this 18 month TRP programme the consortium are tasked with developing, manufacturing and
experimentally verifying a prototype multichroic pixel which would be suitable for the large focal plane arrays
which will be demanded to reach the required sensitivity of future CMB polarization missions. One major
development was to have multichroic operation to potentially reduce the required focal plane size of a CMB
mission. After research in the optimum telescope design and definition of requirements based on a stringent
science case review, a number of compact focal plane architecture concepts were investigated before a pixel
demonstrator consisting of a planar mesh lens feeding a backend Resonant Cold Electron Bolometer RCEB for
filtering and detection of the dual frequency signal was planned for manufacture and test. In this demonstrator
the frequencies of the channels was chosen to be 75 and 105 GHz in the w band close to the peak CMB signal.
In the next year the prototype breadboards will be developed to test the beams produced by the manufactured
flat lenses fed by a variety of antenna configurations and the spectral response of the RCEBs will also be
verified.
In this paper we present the development and verification of feed horn simulation code based on the mode- matching technique to simulate the electromagnetic performance of waveguide based structures of rectangular cross-section. This code is required to model multi-mode pyramidal horns which may be required for future far infrared (far IR) space missions where wavelengths in the range of 30 to 200 µm will be analysed. Multi-mode pyramidal horns can be used effectively to couple radiation to sensitive superconducting devices like Kinetic Inductance Detectors (KIDs) or Transition Edge Sensor (TES) detectors. These detectors could be placed in integrating cavities (to further increase the efficiency) with an absorbing layer used to couple to the radiation. The developed code is capable of modelling each of these elements, and so will allow full optical characterisation of such pixels and allow an optical efficiency to be calculated effectively.
As the signals being measured at these short wavelengths are at an extremely low level, the throughput of the system must be maximised and so multi-mode systems are proposed. To this end, the focal planes of future far IR missions may consist of an array of multi-mode rectangular feed horns feeding an array of, for example, TES devices contained in individual integrating cavities. Such TES arrays have been fabricated by SRON Groningen and are currently undergoing comprehensive optical, electrical and thermal verification. In order to fully understand and validate the optical performance of the receiver system, it is necessary to develop comprehensive and robust optical models in parallel. We outline the development and verification of this optical modelling software by means of applying it to a representative multi-mode system operating at 150 GHz in order to obtain sufficiently short execution times so as to comprehensively test the code.
SAFARI (SPICA FAR infrared Instrument) is a far infrared imaging grating spectrometer, to be proposed as an ESA M5 mission. It is planned for this mission to be launched on board the proposed SPICA (SPace Infrared telescope for Cosmology and Astrophysics) mission, in collaboration with JAXA. SAFARI is planned to operate in the 1.5-10 THz band, focussing on the formation and evolution of galaxies, stars and planetary systems. The pixel that drove the development of the techniques presented in this paper is typical of one option that could be implemented in the SAFARI focal plane, and so the ability to accurately understand and characterise such pixels is critical in the design phase of the next generation of far IR telescopes.
Big Bang cosmologies predict that the cosmic microwave background (CMB) contains faint temperature and polarisation
anisotropies imprinted in the early universe. ESA's PLANCK satellite has already measured the temperature
anisotropies1 in exquisite detail; the next ambitious step is to map the primordial polarisation signatures which are
several orders of magnitude lower. Polarisation E-modes have been measured2 but the even-fainter primordial B-modes
have so far eluded detection. Their magnitude is unknown but it is clear that a sensitive telescope with exceptional
control over systematic errors will be required.
QUBIC3 is a ground-based European experiment that aims to exploit the novel concept of bolometric interferometry in
order to measure B-mode polarisation anisotropies in the CMB. Beams from an aperture array of corrugated horns will
be combined to form a synthesised image of the sky Stokes parameters on two focal planes: one at 150 GHz the other at
220 GHz. In this paper we describe recent optical modelling of the QUBIC beam combiner, concentrating on modelling
the instrument point-spread-function and its operation in the 220-GHz band. We show the effects of optical aberrations
and truncation as successive components are added to the beam path. In the case of QUBIC, the aberrations introduced
by off-axis mirrors are the dominant contributor. As the frequency of operation is increased, the aperture horns allow up to five hybrid modes to propagate and we illustrate how the beam pattern changes across the 25% bandwidth. Finally we
describe modifications to the QUBIC optical design to be used in a technical demonstrator, currently being manufactured
for testing in 2016.
In this paper we describe the optical modelling of astronomical telescopes that exploit bolometric detectors fed by multimoded horn antennas. In cases where the horn shape is profiled rather than being a simple cone, we determine the beam at the horn aperture using an electromagnetic mode-matching technique. Bolometers, usually placed in an integrating cavity, can excite many hybrid modes in a corrugated horn; we usually assume they excite all modes equally. If the waveguide section feeding the horn is oversized these modes can propagate independently, thereby increasing the throughput of the system. We use an SVD analysis on the matrix that describes the scattering between waveguide (TE/TM) modes to recover the independent orthogonal fields (hybrid modes) and then propagate these to the sky independently where they are added in quadrature. Beam patterns at many frequencies across the band are then added with a weighting appropriate to the source spectrum. Here we describe simulations carried out on the highest-frequency (857-GHz) channel of the Planck HFI instrument. We concentrate in particular on the use of multimode feedhorns and consider the effects of possible manufacturing tolerances on the beam on the sky. We also investigate the feasibility of modelling far-out sidelobes across a wide band for electrically large structures and bolometers fed by multi-mode feedhorns. Our optical simulations are carried out using the industry-standard GRASP software package.
Astronomical observations in the far-infrared are critical for investigation of cosmic microwave background (CMB) radiation and the formation and evolution of planets, stars and galaxies. In the case of space telescope receivers, a strong heritage exists for corrugated horn antenna feeds to couple the far-infrared signals to the detectors mounted in a waveguide or cavity structure. Such antenna feeds have been utilized, for example, in the Planck satellite in both single-mode channels for the observation of the CMB and the multi-mode channels optimized for the detection of foreground sources. Looking to the demands of the future space missions, it is clear that the development of new technology solutions for the optimization and simplification of horn antenna structures will be required for large arrays. Horn antennas will continue to offer excellent control of beam and polarization properties for CMB polarisation experiments satisfying stringent requirements on low sidelobe levels, symmetry, and low cross polarization in large arrays. Similarly for far infrared systems, multi-mode horn and waveguide cavity structures are proposed to enhance optical coupling of weak signals for cavity coupled bolometers. In this paper we present a computationally efficient approach for modelling and optimising horn character-istics. We investigate smooth-walled horns that have an equivalent optical performance to that of corrugated horns traditionally used for CMB measurements. We discuss the horn optimisation process and the algorithms available to maximise performance of a merit parameter such as low cross polarisation or high Gaussicity. A single moded horn resulting from this design process has been constructed and experimentally verified in the W band. The results of the measurement campaign are presented in this paper and compared to the simulated results, showing a high level of agreement in co and cross polarisation radiation patterns, with low levels of integrated cross polar power. For future Far IR receivers using waveguide bounded bolometers and absorbers, an optimisation of the waveg-uide structures and absorber location within the integrating cavity is critical to maximise coupling performance particularly for multimoded systems. We outline the benefit of using multi-moded horns in focal plane arrays and illustrate the increased optical sensitivity associated with a many-moded approach, which may be optimized for coupling to particular incident beams.
This paper presents the key findings of an ESA-funded programme of work to investigate refractive systems and their application to precision polarimetry experiments. We briefly summarize the derivation of requirements on the optical system for CMB polarimetry, and the design of a refractive telescope system which meets these stringent requirements. An extensive programme of experimental work was undertaken in order to better understand the optical, thermal and mechanical characteristics of the lens material, and of lenses made from this material. A repeatable and controllable antireflection coating procedure was developed and validated, and used to coat lenses used in this study. Optical measurements before and after coating have been used to validate a new module for an industry-standard antenna modelling software package.
The expansion of the universe has red-shifted remnant radiation, called the Cosmic Microwave Background (CMB)
radiation, to the terahertz band, one of the last areas of the electromagnetic spectrum to be explored. The CMB has
imprinted upon it extremely faint temperature and polarisation features that were present in the early universe. The next
ambitious goal in CMB astronomy is to map the polarisation characteristics but their detection will require a telescope
with unprecedented levels of sensitivity and systematic error control. The QUBIC (Q&U Bolometric Interferometer for
Cosmology) instrument has been specifically designed for this task, combining the sensitivity of a large array of wideband
bolometers with the accuracy of interferometry. QUBIC will observe the sky through an array of horns whose
signals will be added using a quasi-optical beam combiner (an off-axis Gregorian dual reflector designed to have low
aberrations). Fringes will be formed on two focal planes separated by a polarising grid.
MODAL (our in house simulation package) has been used to great effect in achieving a detailed level of understanding
of the QUBIC combiner. Using a combination of scalar (GBM) and vector (PO) analysis, MODAL is capable of high
speed and accuracy in the simulation of quasi-optical systems. There are several technical challenges to overcome but the
development of MODAL and simulation techniques have gone a long way to solving these in the design and analysis
phase.
In this paper I outline the quasi-optical modelling of the QUBIC beam combiner and work envisaged for the future.
Astronomical observations in the far-infrared are critical for investigation of cosmic microwave background (CMB) radiation and the formation and evolution of planets, stars and galaxies. In the case of space telescope receivers a strong heritage exists for corrugated horn antenna feeds to couple the far-infrared signals to the detectors mounted in a waveguide or cavity structure. Such antenna feeds have been utilized, for example, in the Planck satellite in both single-mode channels for the observation of the CMB and the multi-mode channels optimized for the detection of foreground sources. Looking to the demands of the future space missions, it is clear that the development of new technology solutions for the optimization and simplification of horn antenna structures will be required for large arrays. Horn antennas will continue to offer excellent control of beam and polarization properties for CMB polarisation experiments satisfying stringent requirements on low sidelobe levels, symmetry and low cross polarization in large arrays. Similarly for mid infrared systems multi-mode waveguide structures will give high throughput to reach the required sensitivities. In this paper we present a computationally efficient approach for modelling and optimising horn characteristics. We investigate smooth-walled profiled horns that have a performance equivalent to that of the corrugated horns traditionally used for CMB measurements. We discuss the horn optimisation process and the algorithms available to maximise performance of a merit parameter such as low cross polarisation or high Gaussicity.
Efficient optical modelling in the far infrared is challenging because of the dominance of diffraction effects in typical
astronomical instruments. With the development of the next generation of array imagers and multi-moded feed systems
the necessity for computational efficiency has become critical to ensure an optimised design, comprehensive system and
telescope analysis and image deconvolution. A multi-technique capability is necessary to simulate both efficiently and
accurately the propagation of the signal collected by the telescope through the quasi-optical beam guide and feed
structures using an appropriate combination of modelling tools, seamlessly passing from one regime to the next from
detector to sky. Physical optics for example, although computationally intensive, is useful tool when detailed telescope
beam analysis is required, particularly for providing cross-polarisation information. Modal analysis is often appropriate
for modelling beam guide structures while analysing the detector feed coupling may rely on a more complete
electromagnetic analysis because of the small sizes involved and the use of waveguide and planar structures. Image
recovery ideally requires a deconvolution technique based on a modal approach and precise knowledge of the beams on
the sky. In this paper we report on our work in the continued development of such appropriate techniques with the
particular goal of prototyping powerful efficient computational tools for imaging arrays and partially coherent systems.
In the presentation, we will discuss these issues and present examples from real instrumentation.
Special approaches unique to the waveband are required for the modelling of terahertz optical systems. Ray tracing is
inadequate because in typical instruments the propagating beams are not very many wavelengths in diameter and a
"quasi-optical" approach is required in which Fresnel diffraction effects can be efficiently and accurately simulated.
Typically, it is also necessary to be able to simulate the coupling of quasi-optical beams to feed antenna structures to
predict optical performance. In many systems the beams can be considered to be coherent and their propagation through
a beam guide consisting of re-focussing elements can be efficiently modelled using modal analysis, especially useful for
quick design purposes, beam control and optimisation. This modal approach has been extended to allow for aberrations
and truncation particularly relevant in compact mirror based systems. At the same time physical optics, although
computationally intensive, is also a useful tool when detailed analysis is required, particularly for providing crosspolarisation
information and high accuracy far-field beam patterns from large reflecting antennas, for example. However,
modal analysis in general is a very powerful tool, which enables one also to understand issues associated with throughput
when partially coherent systems are being considered. This is important for the efficient optical modelling of large arrays
systems now being developed for next generation astronomical instrumentation. In the presentation, we will discuss
these issues and present examples from real instrumentation. We also summarise our continuing work on the
development of computationally efficient modelling tools for fast quasi-optical design and analysis.
The modeling of millimeter and sub-millimeter-wave optical systems requires special approaches. In many systems the
beams can be considered to be coherent and their propagation can be efficiently modeled using modal analysis,
especially useful for quick design purposes. Physical optics is also a useful tool when detailed analysis is required.
Modal analysis in general, however, is a very powerful technique, which enables one also to understand issues associated
with throughput when partially coherent systems and arrays are being considered. In the paper we discuss these issues
and present some examples from millimeter and submillimeter wave astronomical instrumentation.
The Millimeter-Wave Bolometric Interferometer (MBI) is a ground-based instrument designed to measure the
polarization anisotropies of the Cosmic Microwave Background (CMB) and contains a number of quasi-optical
components, including a complex back-to-back system of corrugated feed-horn antennas. In this paper we use MBI as
an example to demonstrate the existing modeling techniques and as a focus to develop extended modeling capabilities.
The software we use to model this system targets the millimeter and sub-millimeter region of the electromagnetic
spectrum and has been extended to efficiently model the performance of back-to-back corrugated horns embedded in
larger optical systems. This allows the calculation of the coupling of radiation from the sky to the detector array through
a back-to-back horn feed system.
Gaussian Beam Mode Analysis can be applied as a powerful technique approach in the development of phase gratings
for use at terahertz wavelengths, providing a physically intuitive approach relating Fourier and Fresnel diffraction
patterns to the scattering of the illumination beam at the grating. Fourier gratings in particular offer the possibility of
generating sparse arrays image of a single input beam, useful, for example, in active heterodyne systems with an LO
power source. The feasibility of the application of such gratings in real systems was investigated both by simulation and
experimental measurements.
Optical design in the terahertz (THz) waveband can be challenging, especially for high-precision applications. In this
paper we summarise our experience with the quasi-optical design and subsequent performance of astronomical
telescopes designed to measure the faint temperature and polarisation properties of the Cosmic Microwave Background
Radiation, in particular QUaD1, the PLANCK Surveyor2 and MBI3. These telescopes contain a range of quasi-optical
components including corrugated feed horns, on- and off-axis conic mirrors and lenses. Knowledge of their optical
performance and beam patterns is critical for understanding systematic effects in the reliable extraction of feeble
polarisation signals.
Although Physical Optics can be used to characterise electromagnetic systems to high accuracy, it is computationally
intensive at these frequencies and often not suitable for the initial design or preliminary analysis of large multi-element
optical systems. In general there is a lack of dedicated software tools for modelling the range of components and
propagation conditions encountered in typical systems and we have employed a variety of commercial and in-house
software packages for this task. We describe the techniques used, their predictions and the performance of the
telescopes that have been measured to-date.
The properties of terahertz (THz) radiation potentially make it ideal for medical imaging but the difficulty of
producing laboratory sources and detectors has meant that it is the last unexplored part of the electromagnetic
spectrum. In this paper we report on near-field reflection and absorption measurements of biological samples at
0.1THz as a first step towards developing THz and millimetre-wave imaging schemes. Variation of the absorption
and reflection of THz in these samples is investigated as a means of determining information about the sample
structure. Operating at 100 GHz with standard detecting devices we illustrate preliminary results in imaging
(transmission and reflection) measurements of meat samples using various optical configurations and draw
conclusions on the scope of the techniques. Some encouraging provisional results are discussed as well as
limitations in "intensity only" measurements due, primarily, to standing waves and a lack of dynamic range. These
experiments were performed as part of a Masters thesis. A discussion on a variety of absorbing materials utilized
to reduce reflected radiation from surrounding optical components is also given. In addition we report on initial
trials in extracting information about an object's size by sparsely measuring points in the equivalent Fourier plane
in a simple optical setup, thus avoiding the need for time consuming raster scanning. This technique has many
potential applications in detecting and scanning systems. Here the background theory and preliminary results are
presented.
MODAL is an optical design and analysis package targeting the millimetre and sub-millimetre region of the
electromagnetic spectrum. It is being developed at NUI Maynooth with the aim of integrating advanced modelling
techniques and access to High Performance Computing into a user-friendly and yet very powerful tool for an
(quasi-)optical designer. MODAL has been recently extended to allow integrated simulation of custom corrugated
horns and dielectric lenses. This made it possible to model an existing instrument (QUaD), with the goal of
optimising its performance.
Here we present new results from analysis of the predicted performance of the QUaD telescope, with particular
emphasis on polarisation information. They were obtained by using MODAL to model the whole telescope, with
the distortion of the primary accounted for, for a range of component tilts and separations.
In order to improve the design and analyse the performance of efficient terahertz optical systems, novel quasi-optical components along with dedicated software tools are required. At sub-millimetre wavelengths, diffraction dominates the propagation of radiation within quasi-optical systems and conventional geometrical optics techniques are not adequate to accurately guide the beams or assess optical efficiency. In fact, in general Optical design in the terahertz waveband suffers from a lack of dedicated commercial software packages for modelling the range of electromagnetic propagation regimes that are important in such systems.
In this paper we describe the physical basis for efficient CAD software tools we are developing to specifically model long wavelength systems. The goal is the creation of a user-friendly package for optical engineers allowing potential systems to be quickly simulated as well as also providing an analytical tool for verification of existing optical systems. The basic approach to modelling such optical trains is the application of modal analysis e.g. [1][2], which we have extended to include scattering at common
off-axis conic reflectors. Other analytical techniques are also ncluded within the CAD software framework such as plane wave decomposition and full physical optics. We also present preliminary analytical methods for characterising standing waves that can occur in terahertz systems and report on novel binary optical components for this wavelength range. Much of this development work has been applied to space instrumentation but is relevant for all Terahertz Imaging systems.
Optical design in the terahertz (THz) waveband suffers from a lack of dedicated software tools for modelling the range of electromagnetic and quasi-optical propagation conditions encountered in typical systems. Optical engineers are forced to use packages written for very different wavelength systems and there is often a lack of confidence in the results because of possible inappropriate underlying physical models. In this paper we describe the analytical techniques and dedicated CAD software tools (MODAL) that we are developing for long-wavelength design and analysis in the THz waveband. Our basic approach to modelling long-wavelength propagation is the application of modal analysis appropriate to the problem under investigation. We have extended this to include the efficient description of common off-axis (tilted) components such as simple curved reflectors. In earlier research we have investigated the conditions under which approximate methods (ray tracing, paraxial modes) can provide extremely efficient and accurate solutions and situations where a more rigorous approach is required. As a rigorous model of electromagnetic wave propagation, physical optics can be used to characterize complete systems to high accuracy. However, the straightforward approach is computationally intensive and, therefore, not suitable for the initial design or preliminary analysis of large multi-element optical systems. In order to improve the computational efficiency of the usual PO approach we have developed fast physical optics software, initially for the analysis of the ESA PLANCK system. The MODAL code is modular and multi-platform, and different propagation models can be used within the same framework. Distributed parallel computing enables significant reduction of the time needed to perform the calculations. We present the new software and analyses of the QuaD and Herschel (HIFI) telescope systems.
The coherence analysis technique has been used to determine the state of polarizatitn of 228.8 nm radiation emitted by cadmium atoms excited to 51P1 state by electron impact. Data have been obtained for incident electron energy of 100 eV for scattering angles in the range from 10° to 40°. Reduced Stokes parameters and electron impact coherence parameters (EICP), characterizing the state of the excited atoms immediately after the collision, have been extracted from the measurements and compared with results of relativistic distorted-wave approximation calculations. Theoretical predictions are in good qualitative agreement with experimental values.
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