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Far-infrared spectroscopic observations of the universe provide a unique means of addressing some of the leading questions in modern astrophysics from the formation of stars and planets in our own galaxy to the evolution of galaxies over cosmic time. The technique of Fourier transform spectroscopy, with its ability to cover a broad spectral range at varying spectral resolution, has played a central role in such observations. The history of Fourier transform spectroscopy is reviewed and the evolving role of this technique in the era of photon counting infrared detectors discussed.

Recent advances in far-infrared detector technology have led to increases in raw sensitivity of more than an order of magnitude over previous state-of-the-art detectors. With such sensitivity, photon noise becomes the dominant noise component, even when using cryogenically cooled optics, unless a method of restricting the spectral bandpass is employed. One method is to use a low-resolution diffraction grating spectrometer to post-disperse the signal from a high-resolution instrument, such as a Fourier transform spectrometer (FTS). This concept has been adopted for the SAFARI instrument on the SPICA mission. This paper discusses the development of a prototype cryogenic grating spectrometer that has been used to evaluate the concept of a post-dispersed polarizing FTS over the range from 285-500 μm.

The sensitivity of state-of-the-art superconducting far-infrared (FIR) detectors is such that wideband spectroscopic observations, particularly those employing Fourier transform spectrometers (FTS), will require techniques to reduce the spectral bandwidth of a detector to limit the photon noise from an astronomical source. The proposed SPICA SAFARI instrument employs grating spectrometers to post-disperse the light that has been modulated by a polarizing FTS onto a detector array, thereby reducing the photon noise on each detector. While the principles of this method are understood, to date an integrated system has not been realized in the laboratory. We present the development of a FIR post-dispersed polarizing FTS (PDPFTS) consisting of a warm FTS and a 4 K grating spectrometer as a first step to a fully cryogenic PDPFTS demonstrator. Realistic astronomical spectra are generated by combining line emission from a tunable THz photomixer source with continuum emission from a variable blackbody source.

The sensitivity of state-of-the-art superconducting far-infrared detectors is such that astronomical observations at these wavelengths are limited by photon noise from the astronomical source unless a method of restricting the spectral bandpass is employed. One such method is to use a high resolution Fabry-Perot interferometer (FPI) in conjunction with a lower resolution, post-dispersing system, such as a grating spectrometer. The resonant wavelength of an FPI is typically tuned by changing the spacing or medium between the parallel reflecting plates of the etalon. We previously reported on a novel design in which the wavelength is tuned by scanning the angle of incidence, which simplifies the cryo-mechanical design, actuation and metrology. Here we present first light results from the realized instrument.

The sensitivity of state-of-the-art superconducting far-infrared detectors is such that astronomical observations at these wavelengths are limited by photon noise from the astronomical source unless a method of restricting the spectral bandpass is employed. One such method is to use a high resolution Fabry-Pérot interferometer (FPI) in conjunction with a lower resolution, post dispersing system, such as a grating spectrometer. The resonant wavelength of an FPI is typically tuned by changing the spacing or medium between the parallel reflecting plates of the etalon. We present a novel design in which the wavelength is tuned by scanning the angle of incidence, which simplifies the cryo-mechanical design, actuation and metrology. The effects on the spectral response as a function of incident angle have been simulated and shown to be manageable.

Imaging applications at terahertz (THz) frequencies are limited to relatively low spatial resolution due to the effects of diffraction. A subwavelength aperture can be used to improve the resolution at the cost of low transmission. Plasmonic lenses in the form of bullseye structures, consisting of a single subwavelength circular aperture surrounded by concentric periodic corrugations, have shown enhanced transmission and beam confinement. In this paper, we discuss the design and performance of plasmonic lenses optimized for transmission at 325 GHz., 2015 40th International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), August 23-28,2015, Hong Kong, China

We report on the design and fabrication of an 8 K superconducting transition edge sensor (TES) bolometer with active heater feedback, for applications at far-infrared wavelengths. The bolometer is made using standard micro-fabrication techniques. The expected NEP is better than 1 pW/√ Hz.

We report on the fabrication and characterization of a voltage-biased superconducting bolometer (VSB) in a cryogen-free 4 K cryostat for applications at far-infrared wavelengths. The VSB is made using standard microfabrication techniques. An optical NEP of 0.83 pW/√Hz has been measured with a T c ∼7.12 K.

We report on the performance of a Nb voltage-biased superconducting bolometer cooled by a closed cycle pulse tube cooler. The VSB has a Tc ∼ 8.1 K and an operating impedance of R ∼ 800 Ω. A preliminary value for the system optical noise equivalent power (NEP) = 1.8 × 10–12 WHz-1/2 and τ = 0.6 ms.

A Fourier transform spectrometer (FTS), with its high throughput and free spectral range, represents the most efficient class of spectrometer and is ideally suited to applications in the energy starved THz region. For broad-band/continuum observations, the challenging dynamic range requirements of the interferogram signal near zero optical path difference can be reduced by port balancing, a technique in which a broadband spectral source is placed at the second, complementary FTS input port. Studies of the performance of the Herschel SPIRE FTS instrument have shown that beamsplitter emission may contribute significantly to the measured interferogram when port balancing is optimum. In this paper we describe an experiment that has been designed to explore this effect further, and compare the results with a theoretical model.

Fourier transform spectroscopy (FTS) is now well established as a powerful diagnostic tool in far-infrared and submillimetre spectroscopic applications. The high throughput of an FTS is of particular importance in this energy starved region of the electromagnetic spectrum. It has been recognized for many years that an FTS can be readily modified for imaging spectroscopic applications by simply placing a detector array at one of its outputs. The development of array detectors operating at optical wavelengths, which has been driven in part by the consumer market, has been impressive. Similar advances in the development of array detectors have occurred at longer wavelengths, with cost increasing monotonically with wavelength. In particular, recent advances in the production of large format, TES detector arrays (e.g. SCUBA-2) presents a new opportunity for imaging spectroscopic applications at submillimetre wavelengths. The underlying principles of imaging Fourier transform spectroscopy (iFTS) are reviewed, and the challenges facing this field discussed with respect to two iFTS systems (SPIRE and FTS-2) that have been developed for submillimetre astronomical applications.

The Infrared Radiometer for Millimetre Astronomy (IRMA) is a 20 mum (15 THz) water vapour monitor. Using a simple infrared detector to measure the emitted flux across a spectral band containing only water vapour rotational transitions it is possible to determine the precipitable water vapour column abundance (PWV) using an atmospheric model. Water vapour is one of the principal sources of opacity at infrared wavelengths, thus measuring diurnal and seasonal trends in PWV plays an important role in observatory site assessment. The Antarctic plateau, with its high altitude, atmospheric stability, and extremely low humidity, represents a unique location for conducting astronomical observations. The site testing role of the IRMA instrument has been refined through remote operation in both Chile and Hawaii, and has now been extended to include Dome C, Antarctica. One IRMA unit will be integrated into the instrument suite of the University of New South Wales' AASTINO site monitoring facility in September 2007. We present here an overview of the IRMA concept and features that enables their use in site testing applications in remote locations of the Chilean desert. In addition, we describe the modifications required for operation in the low temperature environment (-80degC) encountered at Dome C.

The 20 mum (15 THz) Infrared Radiometer for Millimetre Astronomy (IRMA) monitors a narrow spectral band containing only water vapour molecular transitions. When used in conjunction with an accurate atmospheric model (BTRAM), it is possible to determine absolute precipitable water vapour (PWV) in a column of atmosphere to high accuracy. Flux calibration of IRMA is accomplished by using a calibrated blackbody source. The resulting PWV measurements can be used to determine atmospheric opacity and thus the potential to conduct infrared astronomical observations at the site. Since January 2007, three calibrated IRMA units have been deployed in the Americas as part of a site selection effort for the Thirty Meter Telescope (TMT) project. The three units were operated in parallel while co-located and viewing the same atmosphere. We present the parallel observation data, model sensitivity studies, and error analysis.

FTS-2, an imaging Fourier Transform Spectrometer (IFTS) for use with the SCUBA-2 sub-millimetre bolometer array camera on the James Clerk Maxwell Telescope (JCMT), is currently under development by the University of Lethbridge and will be operational in 2008. SCUBA-2 features two DC- coupled, monolithic TES filled arrays operating at 450 and 850 mum with a total of ~10,000 bolometers. With its larger format and increased sensitivity, SCUBA-2 promises a factor of 1000 increase in mapping speed compared to its predecessors. When coupled with SCUBA-2, FTS-2 will fill a niche between the dual band SCUBA-2 continuum images and the higher spectral resolution, but smaller images produced by the JCMT heterodyne facility instrument HARP-B. The preliminary design of the FTS-2 system has been presented elsewhere; here we review the final optical design of the instrument as the project enters the construction phase. Since the FTS-2 project began after the SCUBA-2 feed optics design was already well underway, the optical design of FTS-2 was tightly constrained and presented unique challenges. In order to maximize the use of the available space, FTS-2 incorporates a folded Mach-Zehnder design including corner cubes in the moving mirror assembly and powered optics with polynomial surfaces in the interferometer arms. A retractable pickoff mirror system allows SCUBA-2 to be used with or without the spectrometer. Both input ports are placed on the sky for atmospheric cancellation, and both output ports are imaged on separate quadrants of the SCUBA-2 arrays. INO (Institut National d'Optique), with guidance from the University of Lethbridge, optimized the optical model for FTS-2 using Zemax to maximize the instrument Field Of View (FOV) and efficiency over the desired resolution range. We present an overview of the optical design, and the projected performance of the IFTS in terms of achievable FOV, spot pattern and vignetting, at Zero Path Difference (ZPD) and for the 2 resolution modes (~0.1 and 0.006 cm-1).

In Fourier Transform Spectroscopy (FTS), observations of a broad continuum source produce an interferogram which has a large dynamic range around the position of zero optical path difference (ZPD). The process of port compensation involves a broadband spectral source placed at the second, complementary input port of an FTS, reducing the dynamic range requirements of the detector system. Port compensation is particularly advantageous in cases where blackbody emission from the focusing optics of an instrument produce a radiative background which dominates a much weaker source signal, as is often found in far-infrared and submillimetre astronomy. The Herschel/SPIRE imaging FTS uses a calibration source (SCAL) to compensate for the emission of the passively cooled telescope optics. In the case of Herschel, it will not be possible to determine the temperature and emissivity of the telescope accurately until after launch; therefore SCAL must have sufficient variability to accommodate this uncertainty. Although simple in theory, port compensation of the SPIRE FTS is non-trivial since it is not possible to match precisely the spectral signature of the Herschel optics over their possible temperature and emissivity parameter space. Typically only partial spectral cancelation can be expected which causes complications in the subsequent data processing and spectral analysis. We discuss the specific challenges to processing data from the SPIRE imaging FTS when both input ports are well balanced and present respective results from the ground- based test campaigns of the SPIRE imaging FTS flight model.

IRMA is a compact, low power, 20um IR water vapour monitor suitable for operation in the astronomical site testing role. IRMA units have been in operation at sites in Chile, Mexico and the USA for the TMT site testing campaign. An additional unit has been modified for operation in the extreme Antarctic environment at Dome C in 2007. We have developed a standalone solar panel/battery power supply and satellite phone based modem control system. This allows an IRMA unit to be powered and operated anywhere on the planet - for example, remote mountain tops in western China that might be suitable sites for telescope construction.