Mach-Zehnder Fourier transform spectrometer for astronomical spectroscopy at submillimeter wavelengths

Abstract

Astronomical spectroscopy at submillimeter wavelengths holds much promise for fields as diverse as the study of planetary atmospheres, molecular clouds and extragalactic sources. Fourier transform spectrometers (FTS) represent an important class of spectrometers well suited to observations that require broad spectral coverage at intermediate spectral resolution. In this paper we present the design and performance of a novel FTS, which has been developed for use at the James Clerk Maxwell Telescope (JCMT). The design uses two broadband intensity beamsplitters in a Mach-Zehnder configuration, which provide access to all four interferometer ports while maintaining a high and uniform efficiency over a broad spectral range. Since the interferometer processes both polarizations it is twice as efficient as the Martin-Puplett interferometer (MPI). As with the MPI, the spatial separation of the two input ports allows a reference blackbody to be viewed at all times in one port, while continually viewing the astronomical source in the other. Furthermore, by minimizing the size of the optical beam at the beamsplitter, the design is well suited to imaging Fourier transform spectroscopy (IFTS) as evidenced by its selection for the SPIRE instrument on Herschel.

Astronomical spectroscopy using an aliased step-and-integrate Fourier transform spectrometer

“…These encouraging results provided the impetus for developing a polarizing Martin-Puplett interferometer 2 . The most recent FTS design is based on a Mach-Zehnder configuration 3 . This design provides access to all four interferometer ports while maintaining a high and uniform efficiency over a broad spectral range.…”

SHIFTS: simulator for the Herschel imaging Fourier transform spectrometer

“…But in contrast to the Michelson inter ferometer, the modified MZ design provides access to all the two input and two output ports of the interferometer. The use of novel intensity beamsplitters (explained in Section 3.4) doubles the theoretical optical efficiency of the modified MZ design over that of a classic Martin-Puplett interfer ometer because light of all polarizations is detected [46,66,[90][91][92][93]. In the modified MZ design, the arms of the classic MZ interferometer are folded, reducing the mass and volume of the interferome ter (critical parameters for any space mission) [60,90].…”

“…Light from the two input ports of the intereferometer is split at the first intensity beam splitter [5], As indicated in Section 3.1, the successful implementation of the SPIRE spectrometer design relies entirely on the high and uniform efficiency of these beamsplitters over a wide spectral range [90,91], Built by the AIG at Cardiff University, the beamsplitters (SBS1 and SBS2 in In the discussion of the ideal Michelson interferometer in Section 2.3.2, we assumed the beamsplitter has 50 % reflectance and transmittance; in reality, no beamsplitter has such character istics [49,55]. fleeted and transmitted electric fields is shifted by the beamsplitters.…”

“…These considerations are all balanced in the optical design to optimize image quality and maximize throughput [66,83,86]. The SPIRE spectrometer employs a modified Mach-Zehnder (MZ) design, first proposed in Ade, Hamilton & Naylor [90] and field tested in Naylor et al [91] and Swinyard et al [46]. Like the Michelson interferometer (see Section 2.3), the modified MZ design measures an interferogram to determine a spectrum (see Sections 2.4 and 2.5) [90].…”

An update on the SCUBA-2 project

“…19 The FTS will cover at least a quarter of the field of view of the JCMT and will be used primarily for imaging spectroscopy of extended Galactic sources but will also provide useful information on bright nearby galaxies and planetary atmospheres.…”