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The ability to measure both optical efficiency and dynamic response to changes in optical signal is crucial to the development of Transition Edge Sensors (TESs) for far-infrared astronomical instruments. We have devised and implemented a cryogenic test facility for ultra-low-noise far-infrared TESs, designed for the SAFARI grating spectrometer on the cooled-aperture space telescope SPICA. Whilst our experimental arrangement is suitable for the whole of the SAFARI wavelength range, 34-230 μm, we focus here on representative optical measurements at 60-110 μm. Detectors are illuminated with a few-mode beam having modal characteristics identical to those of an ideal imaging telescope. In addition, a fast thermal infrared source allows direct measurement of the TES response to tiny changes in incident optical power. We describe the measured functional forms of TES transient responses both to fast optical pulses and to modulation of the power dissipated in the bilayer, in the presence of background optical loading through to TES saturation.

We present an on-chip superconducting filter bank spectrometer based on transition-edge sensors (TES) as a technology for realising a microwave atmospheric sounding instrument with several hundred channels and sky- noise limited performance. Each device consists of a wideband feed coupled to a transmission line filter bank, with a TES behind each filter. In the first instance we have targeted atmospheric temperature sounding using the oxygen (O2) absorption line at 60 GHz, however the device is being scaled to 180 GHz for humidity sounding. The technology developed is also generally applicable to astronomical instrumentation. We have fabricated a set of test devices to demonstrate key device technologies, such as channel placement, spectral resolution and sensitivity. We will describe device design, test configuration and results.

The mid/far infrared hosts a wealth of spectral information that allows direct determination of the physical state of matter in a large variety of astronomical objects, unhindered by foreground obscuration. Accessing this domain is essential for astronomers to much better grasp the fundamental physical processes underlying the evolution of many types of celestial objects, ranging from protoplanetary systems in our own milky way to 10-12 billion year old galaxies at the high noon of galaxy formation in our universe. The joint ESA/JAXA SPICA mission will give such access for the astronomical community at large, by providing an observatory with unprecedented mid- to far-infrared imaging, polarimetric and spectroscopic capabilities.

The optical modelling of far-infrared partially-coherent grating spectrometers has long been considered difficult, due to the multi-mode diffractive nature of the grating optics. However, for the next generation of far-infrared space missions the need for understanding the complex behaviour of these grating spectrometers has intensified. Conventional modelling techniques are difficult to apply because i) the field is partially coherent; ii) diffraction and focusing effects are crucially important; iii) diffraction integrals need to be sampled finely over large optical surfaces. We describe an effective approach based on propagating the correlation functions of the radiation field using the natural modes of the optical system. First, the transformation matrix of the system, T, is determined, which captures the natural modes of the optics. Next, the correlations functions are propagated through the optics using T. The result is a modal optics technique that captures all performance information, in terms of the spectral, spatial and coherence details, within a single framework. In the paper, we explain the foundations of the method and demonstrate its applicability based on a number of standard far-infrared optical systems. Our scheme is numerically powerful, and provides insights into the trade-offs needed to optimise performance. The analysis we will extended to partially coherent far-infrared grating spectrometers as a function of the incident spectral field compositions, scattering at the grating optics, and detector geometry to improve our understanding of such systems.

We give an overview of the baseline detector system for SAFARI, the prime focal-plane instrument on board the proposed space infrared observatory, SPICA. SAFARI's detectors are based on superconducting Transition Edge Sensors (TES) to provide the extreme sensitivity (dark NEP$\le2\times10^{-19}\rm\ W/\sqrt Hz$) needed to take advantage of SPICA's cold (, 7 pages, 3 figures, Proc. SPIE 10708, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy IX, 107080K (9 July 2018); (fixed typo in abstract)

Ultra-low-noise Transition Edge Sensors (TESs) have been selected for the far-infrared Fourier transform spectrometer SAFARI on the space telescope SPICA, now under study as an M5 mission, operating in three wavelength bands: S-band from 34-60 μm, M-band from 60-110 μm and L-band from 110-210 μm. We report the fabrication and optical characterisation of a linear TES array for the SAFARI M-band, integrated with micromachined reflective backshorts and profiled pyramidal optical feedhorns. The design and construction of the cryogenic optical test facility used to illuminate the devices under test are described, featuring a variable temperature blackbody load, band-defining filters and an optical aperture. We observe effective numbers of optical modes, Nef f = 0.41 ± 0.03, and near-unity optical efficiencies in TES-backshort assemblies, with some loss of efficiency in the presence of horns. Stray light control measures are discussed in the context of a significant reduction achieved in long wavelength stray light detected by these devices.

We describe an interferometric technique capable of fully characterizing the optical response of few-mode and multi-mode detectors using only power measurements, and its implementation at 1550 nm wavelength. EnergyAbsorption Interferometry (EAI) is an experimental procedure where the system under test is excited with two coherent, phase-locked sources. As the relative phase between the sources is varied, a fringe is observed in the detector output. Iterating over source positions, the fringes’ complex visibilities allow the two-point detector response function to be retrieved: this correlation function corresponds to the state of coherence to which the detector is maximally sensitive. This detector response function can then be decomposed into a set of natural modes, in which the detector is incoherently sensitive to power. EAI therefore allows the reconstruction of the individual degrees of freedom through which the detector can absorb energy, including their relative sensitivities and full spatial forms. Coupling mechanisms into absorbing structures and their underlying solidstate phenomena can thus be studied, with direct applications in improving current infrared detector technology. EAI has previously been demonstrated for millimeter wavelength. Here, we outline the theoretical basis of EAI, and present a room-temperature 1550 nm wavelength infrared experiment we have constructed. Finally, we discuss how this experimental system will allow us to study optical coupling into fiber-based systems and near-infrared detectors.

Here we report on the optical design and on the spectral-spatial characterisation of a small 16 pixel camera. The prototype uses TES detectors with NEPs ~10-16 W/Hz0.5 which have been fabricated with near identical optical coupling structures to mimic their much lower NEP counterparts (~10-19 W/Hz0.5). This modification, which is achieved through changing only the pixel thermal conductance, G, has allowed us to perform spectral/spatial cryogenic testing using a 100mK ADR to view room temperature thermal sources. The measurements show a flat spectral response across the waveband and minimal side lobe structure in the antenna patterns down to 30dB.

We describe the geometry, architecture, dark- and optical performance of ultra-low-noise transition edge sensors as THz detectors for the SAFARI instrument. The TESs are fabricated from superconducting Mo/Au bilayers coupled to impedance-matched superconducting ß-phase Ta thin-film absorbers. The detectors have phonon-limited dark noise equivalent powers of order 0.5 - 1.0 aW/$\sqrt{Hz}$ and saturation powers of order 20 - 40 fW. The low temperature test configuration incorporating micro-machined backshorts is also described, and construction and typical performance characteristics for the optical load are shown. We report preliminary measurements of the optical performance of these TESs for two SAFARI bands; L-band at 110 - 210 µm and S-band 34 - 60 µm .

We have characterized the optical response of prototype detectors for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI's three bolometer arrays will image a 2’×2’ field of view with spectral information over the wavelength range 34—210 μm. SAFARI requires extremely sensitive detectors (goal NEP ~ 0.2 aW/√Hz), with correspondingly low saturation powers (~5 fW), to take advantage of SPICA's cooled optics. We have constructed an ultra-low background optical test facility containing an internal cold black-body illuminator and have recently added an internal hot black-body source and a light-pipe for external illumination. We illustrate the performance of the test facility with results including spectral-response measurements. Based on an improved understanding of the optical throughput of the test facility we find an optical efficiency of 60% for prototype SAFARI detectors. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Ultra-sensitive superconducting tunnel junction heterodyne receivers used for astronomy research require relatively low levels of local oscillator (LO) power. When configured as an imaging array, however, the LO power required substantially increases and the provision and distribution of a harmonically generated LO signal to multiple pixel elements becomes a technically challenging task. Furthermore, the difficulty of generating LO power is compounded as the operational frequency is increased into the supra-THz region (

The Greenland Telescope project will deploy and operate a 12m sub-millimeter telescope at the highest point of the Greenland i e sheet. The Greenland Telescope project is a joint venture between the Smithsonian As- trophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA). In this paper we discuss the concepts, specifications, and science goals of the instruments being developed for single-dish observations with the Greenland Telescope, and the coupling optics required to couple both them and the mm-VLBI receivers to antenna. The project will outfit the ALMA North America prototype antenna for Arctic operations and deploy it to Summit Station,1 a NSF operated Arctic station at 3,100m above MSL on the Greenland I e Sheet. This site is exceptionally dry, and promises to be an excellent site for sub-millimeter astronomical observations. The main science goal of the Greenland Telescope is to carry out millimeter VLBI observations alongside other telescopes in Europe and the Americas, with the aim of resolving the event horizon of the super-massive black hole at the enter of M87. The Greenland Telescope will also be outfitted for single-dish observations from the millimeter-wave to Tera-hertz bands. In this paper we will discuss the proposed instruments that are currently in development for the Greenland Telescope - 350 GHz and 650 GHz heterodyne array receivers; 1.4 THz HEB array receivers and a W-band bolometric spectrometer. SAO is leading the development of two heterodyne array instruments for the Greenland Telescope, a 48- pixel, 325-375 GHz SIS array receiver, and a 4 pixel, 1.4 THz HEB array receiver. A key science goal for these instruments is the mapping of ortho and para H2D+ in old protostellar ores, as well as general mapping of CO and other transitions in molecular louds. An 8-pixel prototype module for the 350 GHz array is currently being built for laboratory and operational testing on the Greenland Telescope. Arizona State University are developing a 650 GHz 256 pixel SIS array receiver based on the KAPPa SIS mixer array technology and ASIAA are developing 1.4 THz HEB single pixel and array receivers. The University of Cambridge and SAO are collaborating on the development of the CAMbridge Emission Line Surveyor (CAMELS), a W-band `on- hip' spectrometer instrument with a spectral resolution of R ~ 3000. CAMELS will consist of two pairs of horn antennas, feeding super conducting niobium nitride filter banks read by tantalum based Kinetic Inductance Detectors.

We investigate a new concept, where a single superconductor-insulator-superconductor (SIS) based mixer chip is switched between two RF frequency bands. A single broadband antenna is used to couple the RF/LO signal to two SIS mixers via a power splitter and two superconducting on/off switches, forming a switching circuit. The planar on/off switches comprise a superconducting microstrip bridging two transmission lines used to alternate the RF/LO signal between the two branches of the power splitter circuit by switching the impedance of the microstrip from the superconducting to normal state, and vice versa. An important application of this dual-band design is to enable combination of adjacent observing astronomical windows into a single receiver cartridge, freeing valuable space in the receiver cabin.

The Far-Infrared Fourier transform spectrometer instrument SAFARI-SPICA which will operate with cooled optics in a low-background space environment requires ultra-sensitive detector arrays with high optical coupling efficiencies over extremely wide bandwidths. In earlier papers we described the design, fabrication and performance of ultra-low-noise Transition Edge Sensors (TESs) operated close to 100mk having dark Noise Equivalent Powers (NEPs) of order 4 × 10−19W/√Hz close to the phonon noise limit and an improvement of two orders of magnitude over TESs for ground-based applications. Here we describe the design, fabrication and testing of 388-element arrays of MoAu TESs integrated with far-infrared absorbers and optical coupling structures in a geometry appropriate for the SAFARI L-band (110 − 210 μm). The measured performance shows intrinsic response time τ ~ 11ms and saturation powers of order 10 fW, and a dark noise equivalent powers of order 7 × 10−19W/√Hz. The 100 × 100μm2 MoAu TESs have transition temperatures of order 110mK and are coupled to 320×320μm2 thin-film β-phase Ta absorbers to provide impedance matching to the incoming fields. We describe results of dark tests (i.e without optical power) to determine intrinsic pixel characteristics and their uniformity, and measurements of the optical performance of representative pixels operated with flat back-shorts coupled to pyramidal horn arrays. The measured and modeled optical efficiency is dominated by the 95Ω sheet resistance of the Ta absorbers, indicating a clear route to achieve the required performance in these ultra-sensitive detectors.

Understanding the thermal behaviour of low-dimensional dielectric support structures patterned in

SPICA is an infra-red (IR) telescope with a cryogenically cooled mirror (~5K) with three instruments on board, one of which is SAFARI that is an imaging Fourier Transform Spectrometer (FTS) with three bands covering the wavelength of 34-210 μm. We develop transition edge sensors (TES) array for short wavelength band (34-60 μm) of SAFARI. These are based on superconducting Ti/Au bilayer as TES bolometers with a Tc of about 105 mK and thin Ta film as IR absorbers on suspended silicon nitride (SiN) membranes. These membranes are supported by long and narrow SiN legs that act as weak thermal links between the TES and the bath. Previously an electrical noise equivalent power (NEP) of 4×10-19 W/√Hz was achieved for a single pixel of such detectors. As an intermediate step toward a full-size SAFARI array (43×43), we fabricated several 8×9 detector arrays. Here we describe the design and the outcome of the dark and optical tests of several of these devices. We achieved high yield (

The sensitivities, saturation powers, and response times of Kinetic Inductance Detectors depend on the degree to which the quasiparticle and phonon temperatures differ when readout and signal power are applied. Previously, we proposed a model for saturation in KIDs, and found that hysteretic switching, which is seen experimentally, emerges from the model in a natural way. Here we extend our work, and show through a diagrammatic rep- resentation of operating-point trajectories that output pulse shapes can be calculated for non-hysteretic and hysteretic changes of state driven by readout-frequency, readout-power, and signal-power pulses. The work has several applications: (i) Understanding quasiparticle and phonon relaxation, and their dependence on film and substrate thickness. (ii) Recovering quasiparticle cooling functions from measurements. (iii) Understanding the relationship between optimum readout power and device and material parameters. (iv) Characterising the time-dependent behaviour of photon-counting KIDs, including nonlinearity and pile up.

The next generation of space missions targeting far-infrared wavelengths will require large-format arrays of extremely sensitive detectors. The development of Transition Edge Sensor (TES) array technology is being developed for future Far-Infrared (FIR) space applications such as the SAFARI instrument for SPICA where low-noise and high sensitivity is required to achieve ambitious science goals. In this paper we describe a modal analysis of multi-moded horn antennas feeding integrating cavities housing TES detectors with superconducting film absorbers. In high sensitivity TES detector technology the ability to control the electromagnetic and thermo-mechanical environment of the detector is critical. Simulating and understanding optical behaviour of such detectors at far IR wavelengths is difficult and requires development of existing analysis tools. The proposed modal approach offers a computationally efficient technique to describe the partial coherent response of the full pixel in terms of optical efficiency and power leakage between pixels. Initial wok carried out as part of an ESA technical research project on optical analysis is described and a prototype SAFARI pixel design is analyzed where the optical coupling between the incoming field and the pixel containing horn, cavity with an air gap, and thin absorber layer are all included in the model to allow a comprehensive optical characterization. The modal approach described is based on the mode matching technique where the horn and cavity are described in the traditional way while a technique to include the absorber was developed. Radiation leakage between pixels is also included making this a powerful analysis tool.

Future Far-IR space telescopes, such as the SAFARI instrument of the proposed JAXA/ESA SPICA mission, will use horn antennas to couple to cavity bolometers to achieve high levels of sensitivity for Mid-IR astronomy. In the case of the SAFARI instrument the bolometric detectors susceptibility to currents coupling into the detector system and dissipating power within the bolometers is a particular concern of the class of detector technology considered.1 The simulation of such structures can prove challenging. At THz frequencies ray tracing no longer proves completely accurate for these partially coherent large electrical structures, which also present significant computational difficulties for the more generic EM approaches applied at longer microwave wavelengths. The Finite Difference Time Domain method and other similar commercially viable approaches result in excessive computational requirements, especially when a large number of modes propagate. Work being carried out at NUI-Maynooth is utilising a mode matching approach to the simulation of such devices. This approach is based on the already proven waveguide mode scattering code "Scatter"2 developed at NUI-Maynooth, which is a piece of mode matching code that operates by cascading a Smatrice while conserving power at each waveguide junction. This paper outlines various approaches to simulating such Antenna Horns and Cavities at THz frequencies, focusing primarily on the waveguide modal Scatter approach. Recently the code has been adapted to incorporate a rectangular waveguide basis mode set instead of the already established circular basis set.

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.

The next generation of space missions targeting far-infrared bands will require large-format arrays of extremely lownoise detectors. The development of Transition Edge Sensors (TES) array technology seems to be a viable solution for future mm-wave to Far-Infrared (FIR) space applications where low noise and high sensitivity is required. In this paper we concentrate on a key element for a high sensitivity TES detector array, that of the optical coupling between the incoming electromagnetic field and the phonon system of the suspended membrane. An intermediate solution between free space coupling and a single moded horn is where over-moded light pipes are used to concentrate energy onto multimoded absorbers. We present a comparison of modeling techniques to analyze the optical efficiency of such light pipes and their interaction with the front end optics and detector cavity.

Transition-Edge Sensors (TESs) are sensitive devices us ed in astronomical detectors. Recent projects in ground-based and space astronomy demand the Noise Equivalent Power (NEP) of the TES to be reduced to the limitsneeded for accurate measurements, for example, of the B -mode polarisation of the CMB. Thus, we have measuredthermal conductance of Si x N y bridges of various geometries, and present the results that give insight intothe phonon transport mechanism inside these low-dime nsional structures. We also present a new method formeasuring the NEP of TESs using an on-chip black body radiator.Keywords: Transition-Edge-Sensors, Thermal properties of Si x N y , Johnson noise thermometry, On-chip black-body source 1. INTRODUCTION Silicon rich Silicon-Nitride ( Si x N y ) is the material of choice in many cryogenic devices. It has mechanicaland thermal properties that make it an ideal candidate in the fabrication of TESs operating either as micro-calorimeters or bolometers. Si x

This paper describes the key design features and performance of HARP, an innovative heterodyne focal-plane array receiver designed and built to operate in the submillimetre on the James Clerk Maxwell Telescope (JCMT) in Hawaii. The 4x4 element array uses SIS detectors, and is the first sub-millimetre spectral imaging system on the JCMT. HARP provides 3-dimensional imaging capability with high sensitivity at 325-375 GHz and affords significantly improved productivity in terms of speed of mapping. HARP was designed and built as a collaborative project between the Cavendish Astrophysics Group in Cambridge UK, the UK-Astronomy Technology Centre in Edinburgh UK, the Herzberg Institute of Astrophysics in Canada and the Joint Astronomy Centre in Hawaii. SIS devices for the mixers were fabricated to a Cavendish Astrophysics Group design at the Delft University of Technology in the Netherlands. Working in conjunction with the new Auto Correlation Spectral Imaging System (ACSIS), first light with HARP was achieved in December 2005. HARP synthesizes a number of interesting features across all elements of the design; we present key performance characteristics and images of astronomical observations obtained during commissioning.

We describe the optimization of transition edge superconducting (TES) detectors for use in a far-infrared (FIR) Fourier transform spectrometer (FTS) mounted on a cryogenically cooled space-borne telescope (e.g. SPICA). The required noise equivalent power (NEP) of the detectors is approximately 10 �19 W/ √ Hz in order to be lower than the photon noise from astrophysical sources in octave wide bands in the FIR. The detector time constants must be less than 10 ms in order to allow fast scanning of the FTS mechanism. The detectors consist of superconducting thermometers suspended on thin legs of thermally isolating silicon nitride and operate at a temperature of approximately 100 mK. We present the design of the detectors, a proposed focal plane layout and optical coupling scheme and measurements of thermal conductance and time constant for low NEP prototype TES bolometers.

We describe a new type of FIR detector based on lumped element superconducting resonators (LEKIDs). Thesedevices can act as distributed FIR radiation absorbers without the need for an additional coupling structure.In addition, these devices can be integrated into a compact filled array geometry with high filling factor. Wedescribe the optimization of lumped element resonators for high coupling efficiency to incoming radiation in thewavelength region from 200μm - 450μm, measurements of electrical and optical properties of these devices andthe design of a prototype array using these detectors.

Current and future astronomical detectors based on Transition Edge Sensors (TESs) need to achieve theoretically predicted current noise performance determined by the sum of contributions from thermal noise in the link to the heat bath, Johnson noise in the sensor itself and noise in the electrical readout circuit. Present TES geometries can have noise levels significantly above this limit. Our Mo/Cu bilayer TESs are fabricated on long, narrow, thermally isolating silicon nitride structures and are designed for operation at 360 or 200 mK. We briefly review the likely sources of the additional noise sources in this geometry and show results of measurements and modelling of the noise sources as the TES geometry is modified for TESs operated at both temperatures.

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.

We describe a modal theory of interferometry, suitable for the modelling of multimode bolometric interferometers, and present the first simulations of such an interferometer. The motivation for the work is the wish to combine the low noise properties of bolometers with aperture synthesis techniques to design far-infrared and sub-mm interferometers, without the need for single-mode components that restrict the power throughput and therefore reduce the signal to noise ratio. The analysis of such interferometers has proved impossible in the past because optical systems at sub-mm wavelengths are partially coherent, and bolometers respond not only to the intensity but also to the correlations in the field. In order to assess the viability of bolometric interferometers, we therefore require a multimode theory of interferometry. We show that the appropriate modes for describing an interferometer are the Hilbert-Schmidt decompositions of the kernels of the integral operators describing the individual telescopes, and demonstrate that these modes provide both a clear conceptual understanding of the operation of an interferometer and an extremely fast method of computation. We present simulations of idealized Michelson and Fizeau interferometers, and show that the normal behaviour of such interferometers is recovered. We show simulated sources, and dirty maps obtained from uv data for a simulated Michelson interferometer. We discuss the application of the theory to real instrument design.

Microstrip-coupled Transition Edge Sensors (TESs) are important because they can be combined with waveguide-horn technology to produce sensitive bolometric detectors with well-defined, single-mode beam patterns and polarisation characteristics. They also allow superconducting RF filters to be included on the detector chips. Our own design of TES uses a finline taper to transform between waveguide and superconducting Nb microstrip. The microstrip transports the signal to a matched Au-Cu resistor, which is deposited on a thermally isolated SiN membrane. The dissipated RF power causes the resistance of a Mo-Cu TES bilayer to increase, and the resulting reduction in bias current is read out by a SQUID. We have fabricated TES bilayers with critical temperatures of 400 to 600mK, and deduced dark NEPs as low as 3x10-17W/√Hz at 150GHz. In this paper we describe a number of experiments that were carried out in order to investigate the electrothermal behaviour of microstrip-coupled TESs. We show that the electrothermal behaviour of microstrip-coupled TESs can be as good as that of free-space TESs, and therefore that they are suitable for high-performance astronomical applications.

We present a new theory for the description of detectors in terms of modes. Although the theory is very general, it is expected to be of particular use in the modelling of far-infrared and submillimeter instruments. Such a theory is needed because at far-infrared frequencies, both optical systems and detectors show partially coherent behaviour. That is to say, even when the instrument is illuminated by an incoherent source, the resultant field at the detector is partially coherent, and the detector itself is sensitive to the coherence properties of the field and not just the intensity. We have previously developed a modal description of optical systems at far-infrared wavelengths; here we describe a modal theory for the detectors themselves. The theories can be combined to provide a complete modal description of far-infrared instruments. The theory presented here applies equally well to pulsed or ergodic radiation, and incorporates polarisation effects. We also show how the statistics of the detector output can be determined from the theory. This is important for instruments such as bolometers, where the internal noise of the detector is very low, and either sky noise or radiation from the optical components dominate. We illustrate our work with a number of simulations, showing signal to noise ratios for different detector types, and we show how how the theory may be applied to the array packing density problem.

CLOVER is an experiment which aims to detect the signature of gravitational waves from inflation by measuring the B-mode polarization of the cosmic microwave background. CLOVER consists of three telescopes operating at 97, 150, and 220 GHz. The 97-GHz telescope has 160 horns in its focal plane while the 150 and 220-GHz telescopes have 256 horns each. The horns are arranged in a hexagonal array and feed a polarimeter which uses finline-coupled TES bolometers as detectors. To detect the two polarizations the 97-GHz telescope has 320 detectors while the 150 and 220-GHz telescopes have 512 detectors each. To achieve the required NEPs the detectors are cooled to 100 mK for the 97 and 150-GHz polarimeters and 230 mK for the 220-GHz polarimeter. Each detector is fabricated as a single chip to guarantee fully functioning focal planes. The detectors are contained in linear modules made of copper which form split-block waveguides. The detector modules contain 16 or 20 detectors each for compatibility with the hexagonal arrays of horns in the telescopes' focal planes. Each detector module contains a time-division SQUID multiplexer to read out the detectors. Further amplification of the multiplexed signals is provided by SQUID series arrays. The first prototype detectors for CLOVER operate with a bath temperature of 230 mK and are used to validate the detector design as well as the polarimeter technology. We describe the design of the CLOVER detectors, detector blocks, and readout, and give an update on the detector development., Comment: 9 pages, 7 figures, SPIE Orlando 24-31 May 2006, Conference 6275

In the presentation we report on novel applications of Gaussian beam mode (GBM) analysis, including in image deconvolution and Fourier grating design. GBMs are the natural modes with which to describe propagation of quasi-collimated long-wavelength beams, with only a small number of modes required to reach adequate accuracy for many practical applications. GBMs provide a more efficient and natural basis set with which to describe propagation than for example plane wave decomposition, especially because of the limited spatial frequency content (only a few degrees of freedom are necessary to describe such beams and the degrees of freedom can be associated with component GBMs). We discuss how GBM analysis provides a useful alternative scheme to FFT approaches for performing deconvolutions and image retrieval in long-wavelength quasi-collimated systems. The convolving beam is usually described very efficiently in terms of beam modes and an SVD approach can be used to extract the mode coefficients of the deconvolved image. We discuss in particular the novel application to mapping in astronomical telescope observations. Another useful area of application is in the design of Fourier phase gratings. Fourier gratings can be used for beam multiplexing of local oscillator power in array imaging systems. In this case phase retrieval is often driven by an iterative approach to the solution based on FFTs and thus by implication plane waves. A GBM approach leads to a more efficient and physically more meaningful approach, especially again because of the limited spatial frequencies possible in long wavelength systems.

In this paper, we report on a theoretical framework based on Gaussian Beam Mode Analysis for modelling standing waves in submillimetre optical systems. Standing waves or multiple reflections have been traditionally difficult to model but this analytical method proves to be very versatile in first order predictions. In previous papers we reported on the underlining theory and described some important examples including reflections between a feed horn and telescope secondary mirror and also reflections between two coupled corrugated horns. This technique can in addition be applied to reflections between components such as lenses and apertures. As our method uses a full multi-moded scattering matrix description of the feed horn (typically a corrugated horn), which is then transformed to equivalent free space Gaussian modes, multiple reflections between the source/detector device, located at the back of the horn, and any arbitrary surface in the optical path can be accurately analysed. An in-depth overview of the technique is presented including analysis of the eigenmodes or most natural mode set that describes the standing wave itself that can exist within a quasioptical system, which we hope will give new insights into optical cavity phenomena. We investigate mechanisms to reduce standing wave ripples often present in submillimeter optics and try to understand more deeply the form and structure of the reflected power component.

In this paper, we report on our investigations of novel imaging techniques such as holography, the generation of limited diffraction beams with large depths of focus and the use of binary optics for millimeter wave systems. Holography, widely used at visible wavelengths is simulated and tested in a simple optical sep-up at 100 GHz using an off-axis lensless configuration. Such a technique can be used to measure absorption characteristics of materials, and can also help classify radiating horns and lens antennas. Gaussian Beam Mode Analysis is used as an efficient computational technique to investigate the propagation of non-diffracting beams, and in particular, Bessel beams, at millimeter wavelengths. Because of the limited throughput of millimeter-wave systems, due to the long wavelength and the need for compact optics for practical applications, modal analysis is a very computationally efficient means for computing propagation characteristics. Typically, the axicon, or conical lens, is the most common optical component used for the generation of such zeroth order Bessel beams, but we show that holographic simulation can be used to design binary holograms for the generation of higher order non-diffracting beams. Furthermore, we describe a practical design for such a simple alternative to the axicon through the manufacture of a binary analogue of this component, which successfully produces diffraction invariant beams.

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.

We present a method for simulating the performance of millimetre-wave and submillimetre-wave STJ direct detectors when combined with commercially available op-amp based readout circuits. We employ full nonlinear modelling, together with frequency-domain analysis, to determine the responsivity, and then we use this responsivity, in conjunction with a detailed noise model, to calculate the NEP. By modelling the saturation of these devices, we are also able to calculate the dynamic range. Our method is capable of simulating a wide range of devices and takes into account the RF matching circuits. Using this approach, we have explored the effect of cooling STJs to different temperatures, and the effect of changing the frequency of operation. To achieve the best noise performance, the energy gap should be tailored to suit the operating frequency, and the device should be biased at a low voltage. We have performed detailed simulations to show that by using TaAl devices, and suitably chosen op-amp feedback components, an NEP of 6.0x10 -18 W/√Hz and dynamic range of 80 dB should be possible at 150 GHz: these conclusions draw on results already known from optical photon-counting experiments.

We describe a procedure for modelling the behaviour of multi-mode astronomical interferometers. The procedure is based on the concept of eigenfields. The input and output eigenfields are those field distributions on the sky and at the detector to which the individual telescopes of an interferometer can couple. The eigenfields of different telescopes are orthogonal, and therefore provide, when combined, a suitable basis set for propagating the second-order statistical properties of the field from a source through the telescopes, through the beam combiners, and onto the detectors. The scheme can be used at any wavelength, with any configuration of optical components (Michelson, Fizeau, etc.), with a source in any state of coherence and polarisation, and with any kind of detector.

In this paper, we report on extending a theoretical framework based on Gaussian Beam Mode Analysis for modelling standing waves in receiver systems coupled to submillimetre wave telescopes. This analytical technique includes a full electromagnetic description of corrugated detector horns, used as a standard feed horn in the THz frequency range. In previous papers we reported on the underlining theory and described some important examples including reflections between a feed horn and telescope secondary mirror and also reflections between a horn and a plano-convex lens. As the theory uses a full multi-moded scattering matrix description within the horn, which can then be transformed to equivalent free space modes, mulitple reflections between the detector, located at the back of the horn, and any arbitrary surface in the optical path can be accurately analysed. We present an experimental validation of the model, comparing predicted standing wave patterns occuring between two corrugated horns to laboratory measurements, owrking in a frequency range around 0.1THz.

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 (MODAL1) 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 investigation2,3. We have extended this to include the efficient description of common off-axis (tilted) components such as simple curved reflectors4. 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 required5,6. 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 system7. 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 QuaD8 and Herschel (HIFI)9 telescope systems.

We report the successful operation of a 700 GHz SIS finline mixer employing a Nb tunnel junction and Nb transmission lines. In particular, we discuss the properties of a new mixer feed and the influence of tuning on the mixer performance. Experimental and simulation work shows that the performance of the mixer below the superconducting gap is strongly dependent on the electrical properties of the tuning stub, while at frequencies above the gap the mixer performance is dominated by both tuning and transmission line losses.

We describe a procedure for modelling the optical behaviour of planar, bolometric imaging arrays. Arrays of this kind axe being developed for the next generation of ground-based and space-borne, submillimetre-wave and far-infrared, astronomical telescopes. A unique feature of the scheme is that the partially coherent vector fields associated with the individual pixels are traced through the optical system simultaneously. Simultaneous tracing is achieved by propagating the second-order statistical properties of the total field. In the paper, we describe the theoretical basis of our method, and present the results of a number of illustrative simulations.

Astronomical Telescopes and Instrumentation, 2002, Waikoloa, Hawai'i, United States, Series: Proceedings of SPIE; no. 4855

We describe the preliminary design of the proposed Heterodyne Instrument for FIRST (HIFI). The instrument will have a continuous frequency coverage over the range from 480 to 1250 GHz in five bands, while a sixth band will provide coverage for 1410 - 1910 GHz and 2400 - 2700 GHz. The first five bands will use SIS mixers and varactor frequency multipliers while in the sixth band a laser photomixer local oscillator will pump HEB mixers. HIFI will have an instantaneous bandwidth of 4 GHz, analyzed in parallel by two types of spectrometers: a pair of wide-band spectrometers (WBS), and a pair of high- resolution spectrometer (HRS). The wide-band spectrometer will use acousto-optic technology with a frequency resolution of 1 MHz and a bandwidth of 4 GHz for each of the two polarizations. The HRS will provide two combinations of bandwidth and resolution: 1 GHz bandwidth at 200 kHz resolution, and at least 500 MHz at 100 kHz resolution. The HRS will be divided into 4 or 5 sub-bands, each of which can be placed anywhere within the full 4 GHz IF band. The instrument will be able to perform rapid and complete spectral line surveys with resolving powers from 103 up to 107 (300 - 0.03 km/s) and deep line observations.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper we discuss the extension of Gaussian beam analysis to multi-beam quasi-optical systems. The normal approach taken in analyzing the propagation of multiple beams is to ray trace the optical axes of the individual beams and then superimpose the beam evolution of the on-axis beam. This is a rather inelegant and cumbersome process. We show how an efficient paraxial description of off-axis beams using Gaussian beam modes can be developed. The computational feasibility of the beam mode description clearly depends on finding an on-axis mode set in terms of which any off-axis beam can be approximated by a modal sum of modest size. We show how the optimum choice is made, and illustrate the power of the approach with an example.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Quantum cascade lasers (QCLs) operating at 2.5 THz have been used for gas phase spectroscopy and as local oscillator in a heterodyne receiver. One QCL has a Fabry-Perot resonator while the other has a distributed feedback resonator. The linewidth and frequency tunability of both QCLs have been investigated by either mixing two modes of the QCL or by mixing the emission from the QCL with the emission from a 2.5 THz gas laser. The frequency tunability as well as the linewidth is sufficient for Doppler limited spectroscopy of methanol gas. The QCLs have been used successfully as local oscillators in a heterodyne receiver. Noise temperature measurements with a hot electron bolometer and a QCL yielded the same result as with a gas laser as local oscillator.