N. Trappe

Novel techniques for millimeter wave imaging systems operating at 100GHz

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.

Ultra-low-noise transition edge sensors for far infrared wavelengths: optical design, measurement and stray light control

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.

Modeling of millimetre-wave and terahertz imaging systems

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.

Quasi-optical analysis of the HIFI instrument for the Herschel Space Observatory

In this paper we detail the optical design of HIFI, one of three instruments to be flown on the Herschel Space Observatory and also outline the general approach taken in analysing the quasi-optical performance of the instrument. Gaussian Beam Mode Analysis theory is the principle technique chosen for the quasi-optical analysis, supplemented by the commercially available optical analysis package GLAD. These quasi-optical techniques are used for validation of the optical design and predicting the performance of the HIFI system. The HIFI system was checked for issues such as truncation, mechanical misalignment and aberrational losses, along with aperture efficiency and LO coupling efficiency. The quasi-optical analytical methods described could be applied generally to long wavelength optical instruments.

The SAFARI detector system

We give an overview of the baseline detector system for SAFARI, the prime focal-plane instrument on board the proposed space infrared observatory, SPICA. SAFARIs detectors are based on superconducting Transition Edge Sensors (TES) to provide the extreme sensitivity (dark NEP≤2×10-19 W/√Hz) needed to take advantage of SPICAs cold (<8 K) telescope. In order to read out the total of ~3500 detectors we use frequency domain multiplexing (FDM) with baseband feedback. In each multiplexing channel, a two-stage SQUID preamplifier reads out 160 detectors. We describe the detector system and discuss some of the considerations that informed its design.

Next generation sub-millimetre wave focal plane array coupling concepts: an ESA TRP project to develop multichroic focal plane pixels for future CMB polarisation experiments

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.

Radiation patterns of multimode feed-horn-coupled bolometers for FAR-IR space applications

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.

Optical Characterisation of a Camera module Developed for Ultra-low NEP TES Detector Arrays at FIR Wavelengths

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.

Next generation sub-millimeter wave focal plane array coupling concepts: an ESA TRP project to develop multichroic focal plane pixels for future CMB polarization experiments polarization experiments

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.

Next Generation Sub-millimetre Wave Focal Plane Array Coupling Concepts - An ESA TRP project to develop multichroic focal plane pixels for future CMB polarisation experiments

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.