Low-noise Fourier Transform Spectroscopy Enabled by Superconducting On-Chip Filterbank Spectrometers

TL;DR

The paper tackles the bottleneck of achieving high-mapping-speed mm/sub-mm spectroscopy for large fields of view by addressing detector-count and photon-noise limitations in traditional Fourier transform spectrometers. It introduces a filterbank-dispersed Fourier transform spectrometer (FBDFTS), which uses a medium-resolution FTS with a low-resolution on-chip filterbank spectrometer as a post-dispersion stage to preserve imaging performance while substantially reducing photon noise. The authors quantify mapping speeds and detector requirements, showing that an FBDFTS could enable near-term R ~ 1000 LIM measurements with signal-to-noise ratios of 10–100 over tens to hundreds of thousands of spectrometer-hours on platforms like JCMT, with potential improvements of roughly an order of magnitude over conventional FTS approaches. This architecture promises practical, high-sensitivity mm/sub-mm LIM surveys, provided careful calibration and systematics control for the FTS component are implemented.

Abstract

Historically employed spectroscopic architectures used for large field of view mapping spectroscopy in millimetere and sub-millimetre astronomy suffer from significant drawbacks. On-chip filterbank spectrometers are a promising technology in this respect; however, they must overcome an orders-of-magnitude increase in detector counts, efficiency loss due to dielectric properties, and stringent fabrication tolerances that currently limit scaling to resolutions of order 1000 over a large array. We propose coupling a medium-resolution Fourier transform spectrometer to a low-resolution filterbank spectrometer focal plane, which serves as a post-dispersion element. In this arrangement, medium resolution imaging spectroscopy is provided by the Fourier transform spectrometer, while the low resolution filterbank spectrometer serves to decrease the photon noise inherent in typical broadband Fourier transform spectrometer measurements by over an order of magnitude. This is achieved while maintaining the excellent imaging advantages of both architectures. We present predicted mapping speeds for a filterbank-dispersed Fourier transform spectrometer from a ground-based site and a balloon-borne platform. We also demonstrate the potential that an instrument of this type has for an R~1000 line intensity mapping experiment using the James Clerk Maxwell Telescope as an example platform. We demonstrate that a filterbank-dispersed Fourier transform spectrometer would be capable of R~1000 measurements of CO power spectra with a signal-to-noise ratio of 10--100 with surveys of $10^5$--$10^6$ spectrometer hours.

Figures (6)

• Figure 1: The total number of MKIDs employed at the focal plane of various instruments for astronomy. The shaded green area shows a linear fit to the data with a preferential weighting scheme in which more recent/future focal planes are assigned weights that are as much as twice that of early focal planes.
• Figure 2: Efficiency loss in a single half-wave filter as a consequence of dielectric loss. Decreasing the bandpass of the filter (increasing its quality factor/$R$) results in greater efficiency loss.
• Figure 3: The reduction in the photon noise of an FTS provided by a low-resolution post-dispersion spectrometer under a fixed optical load. The optical load estimate has been set as the mean expected optical load from a 15 m telescope located at Maunakea coupled to a room-temperature FTS.
• Figure 4: The projected mapping speed of an FBDFTS from the CCAT-prime/FYST site and from a balloon platform compared to other prominent mapping spectrometers in similar spectral bands. The instantaneous bandwidth of heterodyne spectrometers is denoted by vertical bars subdividing their respective curves. Mapping speeds have been determined using published sensitivity values and are discussed more fully in Section \ref{['sec:FBDFTS']}.
• Figure 5: Simulated detector signals from continuum measurements with three spectral channels of an FBDFTS. The low-frequency channel (with a spectral band centred at $\nu=125$ GHz, marked by the heavily weighted orange line) emphasises the decaying cosinusoidal modulation that would be measured by a single detector in an FBDFTS. The signal frequency in the temporal domain is set by the scanning speed of the FTS mirror ($2\,$cm/s) in the FBDFTS and the central frequency of the filter channel.