J. Anthony Murphy

The Millimeter-Wave Bolometric Interferometer

We report on the design and tests of a prototype of the Millimeter-wave Bolometric Interferometer (MBI). MBI is designed to make sensitive measurements of the polarization of the cosmic microwave background (CMB). It combines the differencing capabilities of an interferometer with the high sensitivity of bolometers at millimeter wavelengths. The prototype, which we call MBI-4, views the sky directly through four corrugated horn antennas. MBI ultimately will have ~ 1000 antennas. These antennas have low sidelobes and nearly symmetric beam patterns, so spurious instrumental polarization from reflective optics is avoided. The MBI-4 optical band is defined by filters with a central frequency of 90 GHz. The set of baselines, determined by placement of the four antennas, results in sensitivity to CMB polarization fluctuations over the multipole range ℓ = 150 - 270. The signals are combined with a Fizeau beam combiner and interference fringes are detected by an array of spider-web bolometers. In order to separate the visibility signals from the total power detected by each bolometer, the phase of the signal from each antenna is modulated by a ferrite-based waveguide phase shifter. Initial tests and observations have been made at Pine Bluff Observatory (PBO) outside Madison, WI.

A dual-polarization InSb receiver for 461/492 GHz

A dual-polarization InSb hot-electron bolometer-mixer receiver has been built for the James Clerk Maxwell Telescope, for operation at 461 and 492 GHz (the frequencies of theJ=4→3 rotational transition of CO and of the3P1→3P0 transition of neutral carbon). Receiver noise temperatures of 500K have been obtained at 461 GHz, in observing bandwidths of 3 MHz. The receiver was designed as a “common-user” or facility instrument. Here we describe those aspects of the design and construction which enabled this goal to be realized.

Far-Infrared Optics Design & Verification

Compact quasi-optics are difficult to design with any confidence using techniques developed for visible wavelengths. In this paper we investigate the performance of existing software design tools (ASAP, CODE V, GLAD) as well as a Gaussian beam mode analysis technique not yet available as commercial software. We have devised a set of test cases and used these to study the underlying methodologies and physics of these packages and we probe their suitability for the analysis of submillimetre-wave systems and components. We have used the physical optics package GRASP as our benchmark software.

Modal analysis of the quasi-optical performance of phase gratings

This paper is concerned with the analysis of phase gratings as passive quasi-optical multiplexing devices. One important application of such components is in the local oscillator injection chain of heterodyne array receivers. Gaussian beam mode analysis can be applied as a powerful tool when modelling the optical performance of phase gratings in a real submillimeter system of finite throughput and bandwidth. In our experimental investigations we have concentrated on the Dammann Grating (DG) which is a binary optical component and thus straightforward to manufacture. A number of quartz gratings were fabricated and carefully tested to evaluate the practical limitations of such quasi-optical components. Because of its convenient refractive index quartz can be used to produce gratings with very low reflection losses. The results presented confirm DGs to be particularly suitable multiplexers for sparse arrays of finite bandwidth.

Gaussian-beam mode analysis of reflection and transmission in multilayer dielectrics

The analysis of reflections from thin films or dielectric materials can be approached by a matrix method that treats any thin-layer device as a cascade of sequential, zero-thickness reflecting thin-layer surfaces [J. Opt. Soc. Am. A2, 1363 (1985)]. Our paper presents an alternative method for predicting the reflection/transmission characteristics of such dielectric films in a Fabry-Perot interferometer configuration based on a Gaussian-beam modal analysis within a scattering-matrix framework [in Proceedings of IEE 7th International Conference on Antennas and Propagation (IEE, 1991), Issue 15, p. 201.] We present and validate a scalar Gaussian-beam modal scattering-matrix approach using long-wavelength examples, where diffraction effects are important to model total transmission and reflection characteristics that also include a waveguide modal description of a corrugated horn. For optical beams the same technique is equally applicable, but diffraction is less severe within this framework. This approach is flexible and has many applications within laser optics and in far-infrared or submillimeter-instrumentation optical analysis, where it is possible to incorporate reflections in both waveguide and free space within the description of a whole system. To conclude and verify the accuracy of the technique, experimental measurements taken at 94 GHz are compared with theoretical predictions for a dielectric cavity of polyethylene sheets between corrugated source and detector antennas.

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.

Efficient modeling of detectors for far-IR astronomy

Far-IR receivers typically employ bolometers, a type of highly sensitive detector that absorbs the power in the incoming astronomical signal. Typical sources in the far-IR waveband include galaxies in the early universe and the cosmic microwave background. To prevent unwanted radiation from reaching the bolometers, they are often embedded in an electrically conducting structure that efficiently collects the incoming astronomical signal beam from the telescope and guides it to the detector. The system is usually housed inside a vacuum chamber known as a cryostat, which maintains the bolometers at a very low temperature that is close to absolute zero. It is important to be able to reliably model the full telescopereceiver system for optimal performance in obtaining sharp images, while also having the sensitivity to distinguish very faint features. Because of the relatively long wavelength of the far-IR signal compared to visible light, it is too simplistic to regard the beam as traveling purely in the form of rays. Instead, diffraction effects (or the bending that occurs when a wave is confined) as well as the electromagnetic nature of the signal beam become important when modeling transmission from the telescope through the receiver system to the bolometer. In developing these models, we need to simulate two distinct sections of the beam path: first, inside the conducting metal waveguide structure and, second, over the path from the telescope where the beam travels essentially as a free-space (i.e., vacuum) wave. The free-space propagation requires a technique known as quasi-optical analysis, which takes into account the effects of diffraction, especially if there are lenses or focusing mirrors in the path of the beam from the telescope. The propagation inside the electrically conducting waveguide structure, on the other hand, requires an extension of microwave techniques and a novel theoretical approach. Figure 1. (a) A section through a far-IR detector pixel showing multimode horn, detector cavity, and bolometer (thin absorbing disk). (b) Increase in beam width (and throughput) for a multimode horn compared with a single-mode horn for the same aperture size (both beams normalized to 0dB on axis).