Filter-bank Spectrometer Research Articles

A Vacuum Waveguide Filter Bank Spectrometer for Far-Infrared Astrophysics

Traditional technologies for far-infrared (FIR) spectroscopy generally involve bulky dispersive optics. Integrated filter bank spectrometers promise more compact designs, but implementations using superconducting transmission line networks become lossy at terahertz frequencies. We describe a novel on-chip spectrometer architecture designed to extend this range. A filter bank spectrometer is implemented using vacuum waveguide etched into a silicon wafer stack. A single trunk line feeds an array of resonant cavities, each coupled to a kinetic inductance detector fabricated on an adjacent wafer. We discuss the design and fabrication of a prototype implementation, initial test results at ambient temperature, and prospects for future development.

Fabrication Development for SPT-SLIM, a Superconducting Spectrometer for Line Intensity Mapping

Line Intensity Mapping (LIM) is a new observational technique that uses low-resolution observations of line emission to efficiently trace the large-scale structure of the Universe out to high redshift. Common mm/sub-mm emission lines are accessible from ground-based observatories, and the requirements on the detectors for LIM at mm-wavelengths are well matched to the capabilities of large-format arrays of superconducting sensors. We describe the development of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R$</tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda / \Delta \lambda = 300$</tex-math></inline-formula> on-chip superconducting filter-bank spectrometer covering the 120–180 GHz band for future mm-LIM experiments, focusing on SPT-SLIM, a pathfinder LIM instrument for the South Pole Telescope. Radiation is coupled from the telescope optical system to the spectrometer chip via an array of feedhorn-coupled orthomode transducers. Superconducting microstrip transmission lines then carry the signal to an array of channelizing half-wavelength resonators, and the output of each spectral channel is sensed by a lumped element kinetic inductance detector (leKID). Key areas of development include incorporating new low-loss dielectrics to improve both the achievable spectral resolution and optical efficiency and development of a robust fabrication process to create a galvanic connection between ultra-pure superconducting thin-films to realize multi-material (hybrid) leKIDs. We provide an overview of the spectrometer design, fabrication process, and prototype devices.

A horn-coupled millimetre-wave on-chip spectrometer based on lumped-element kinetic inductance detectors

Context. Millimetre-wave astronomy is an important tool for both general astrophysics studies and cosmology. A large number of unidentified sources are being detected by the large field-of-view continuum instruments operating on large telescopes. Aims. New smart focal planes are needed to bridge the gap between the large bandwidth continuum instruments operating on single-dish telescopes and high spectral and angular resolution interferometers (e.g. ALMA in Chile and NOEMA in France). The aim is to perform low to medium spectral resolution observations and select a lower number of potentially interesting sources (i.e. high-redshift galaxies) for further follow-up. Methods. We have designed, fabricated, and tested an innovative on-chip spectrometer sensitive in the 85–110 GHz range. It contains 16 channels, each of which covers a frequency band of about 0.2 GHz. A conical horn antenna coupled to a slot in the ground plane collects the radiation and guides it to a millimetre-wave microstrip transmission line placed on the other side of the mono-crystalline substrate. The millimetre-wave line is coupled to a filter-bank spectrometer. Each filter is capacitively coupled to a lumped-element kinetic inductance detector (LEKID). The microstrip configuration provides the benefit of low loss, due to the mono-crystalline substrate, and protects the LEKIDs from illumination by stray un-filtered light. Results. The prototype spectrometer exhibits a spectral resolution R = λ/Δλ ≈ 300. The optical noise equivalent power is in the low 10−16 W Hz−1/2 range for an incoming power of about 0.2 pW per channel. The device is polarisation-sensitive, with a cross-polarisation lower than 1% for the best channels.

Biosensing with Silicon Nitride Microring Resonators Integrated with an On-Chip Filter Bank Spectrometer.

Wearable, mobile, and point-of-care (POC) sensors comprise a rapidly expanding field of devices aimed at improving human health by relaying real-time biometric data such as heart rate and glucose levels. The current scope of what these devices can offer healthcare is limited by their inability to measure biomarkers associated with inflammation, well-being, and disease. Photonic biosensors that integrate sensing elements directly with spectrometers, lasers, and detectors are an attractive approach to enabling POC sensors, with distinct advantages in terms of size, weight, power consumption, and cost. Here, we have demonstrated for the first time the integration of photonic microring resonator biosensors with an on-chip microring filter bank spectrometer for the controlled detection of inflammatory biomarker C-reactive protein (CRP) in serum. We demonstrate that sensor and spectrometer performance is tolerant of temperature variation, as temperature dependence moves in parallel. Finally, we assess the impact of manufacturing variability on the 300 mm wafer scale on the performance of the spectrometer. Taken together, these results suggest that integration of on-chip ring filter bank spectrometers with ring resonator-based biosensors constitutes an attractive approach toward cost-effective integrated sensor development.

Design of the SPT-SLIM Focal Plane: A Spectroscopic Imaging Array for the South Pole Telescope

The Summertime Line Intensity Mapper (SLIM) is a mm-wave line-intensity mapping (mm-LIM) experiment for the South Pole Telescope (SPT). The goal of SPT-SLIM is to serve as a technical and scientific pathfinder for the demonstration of the suitability and in-field performance of multi-pixel superconducting filterbank spectrometers for future mm-LIM experiments. Scheduled to deploy in the 2023-24 austral summer, the SPT-SLIM focal plane will include 18 dual-polarisation pixels, each coupled to an R = lambda / Delta lambda = 300 thin-film microstrip filterbank spectrometer that spans the 2 mm atmospheric window (120–180 GHz). Each individual spectral channel feeds a microstrip-coupled lumped-element kinetic inductance detector, which provides the highly multiplexed readout for the 10k detectors needed for SPT-SLIM. Here, we present an overview of the preliminary design of key aspects of the SPT-SLIM focal plane array, a description of the detector architecture and predicted performance, and initial test results that will be used to inform the final design of the SPT-SLIM spectrometer array.

The Simulation and Design of an On-Chip Superconducting Millimetre Filter-Bank Spectrometer

Superconducting on-chip filter banks provide a scalable, space saving solution to create imaging spectrometers at millimetre and submillimetre wavelengths. We present an easy to realise, lithographed superconducting filter design with a high tolerance to fabrication error. Using a capacitively coupled lambda /2 microstrip resonator to define a narrow (lambda /Delta lambda = 300) spectral pass band, the filtered output of a given spectrometer channel directly connects to a lumped-element kinetic inductance detector. We show the tolerance analysis of our design, demonstrating <11% change in filter quality factor to any one realistic fabrication error and a full filter-bank efficiency forecast to be 50% after accounting for fabrication errors and dielectric loss tangent.

SPT-SLIM: A Line Intensity Mapping Pathfinder for the South Pole Telescope

The South Pole Telescope Summertime Line Intensity Mapper (SPT-SLIM) is a pathfinder experiment that will demonstrate the use of on-chip filter-bank spectrometers for mm-wave line intensity mapping (LIM). The SPT-SLIM focal plane consists of 18 dual-polarization R=300 filter-bank spectrometers covering 120-180 GHz, coupled to aluminum kinetic inductance detectors. A compact cryostat holds the detectors at 100 mK and performs observations without removing the SPT-3G receiver. SPT-SLIM will be deployed to the 10-m South Pole Telescope for observations during the 2023-24 austral summer. We discuss the overall instrument design, expected detector performance and sensitivity to the LIM signal from CO at 0.5 < z < 2. The technology and observational techniques demonstrated by SPT-SLIM will enable next-generation LIM experiments that constrain cosmology beyond the redshift reach of galaxy surveys.

First characterization of a superconducting filter-bank spectrometer for hyper-spectral microwave atmospheric sounding with transition edge sensor readout

We describe the design, fabrication, integration, and characterization of a prototype superconducting filter bank with transition edge sensor readout designed to explore millimeter-wave detection at frequencies in the range of 40–65GHz. Results indicate highly uniform filter channel placement in frequency and high overall detection efficiency. The route to a full atmospheric sounding instrument in this frequency range is discussed.

Full-Array Noise Performance of Deployment-Grade SuperSpec mm-Wave On-Chip Spectrometers

SuperSpec is an on-chip filter-bank spectrometer designed for wideband moderate-resolution spectroscopy at millimeter wavelengths, employing TiN kinetic inductance detectors. SuperSpec technology will enable large-format spectroscopic integral field units suitable for high-redshift line intensity mapping and multi-object spectrographs. In previous results we have demonstrated noise performance in individual detectors suitable for photon noise limited ground-based observations at excellent mm-wave sites. In these proceedings we present the noise performance of a full $R\sim 275$ spectrometer measured using deployment-ready RF hardware and software. We report typical noise equivalent powers through the full device of $\sim 3 \times 10^{-16} \ \mathrm{W}/\sqrt{\mathrm{Hz}}$ at expected sky loadings, which are photon noise dominated. Based on these results, we plan to deploy a six-spectrometer demonstration instrument to the Large Millimeter Telescope in early 2020.

Superconducting Coplanar Waveguide Filters for Submillimeter Wave On-Chip Filterbank Spectrometers.

We show the first experimental results which prove that superconducting NbTiN coplanar–waveguide resonators can achieve a loaded Q factor in excess of 800 in the 350 GHz band. These resonators can be used as narrow band pass filters for on-chip filter bank spectrometers for astronomy. Moreover, the low-loss coplanar waveguide technology provides an interesting alternative to microstrip lines for constructing large scale submillimeter wave electronics in general.

CMB Science: Opportunities for a Cryogenic Filter-Bank Spectrometer

Cosmic microwave background (CMB) spectral science is experiencing a renewed interest after the impressive result of COBE–FIRAS in the early Nineties. In 2011, the PIXIE proposal contributed to reopen the prospect of measuring deviations from a perfect 2.725 K planckian spectrum. Both COBE–FIRAS and PIXIE are differential Fourier transform spectrometers (FTSes) capable to operate in the null condition across \(\sim \)2 frequency decades (in the case of PIXIE, the frequency span is 30 GHz–6 THz). We discuss a complementary strategy to observe CMB spectral distortions at frequencies lower than 250 GHz, down to the Rayleigh–Jeans tail of the spectrum. The throughput advantage that makes the FTS capable of achieving exquisite sensitivity via multimode operation becomes limited at lower frequencies. We demonstrate that an array of 100 cryogenic planar filter-bank spectrometers coupled to single mode antennas, on a purely statistical ground, can perform better than an FTS between tens of GHz and 200 GHz (a relevant frequency window for cosmology) in the hypothesis that (1) both instruments have the same frequency resolution and (2) both instruments are operated at the photon noise limit (with the FTS frequency band extending from \(\sim \)tens of GHz up to 1 THz). We discuss possible limitations of these hypotheses, and the constraints that have to be fulfilled (mainly in terms of efficiency) in order to operate a cryogenic filter-bank spectrometer close to its ultimate sensitivity limit.

On-chip filter bank spectroscopy at 600–700 GHz using NbTiN superconducting resonators

We experimentally demonstrate the principle of an on-chip submillimeter wave filter bank spectrometer, using superconducting microresonators as narrow band-separation filters. The filters are made of NbTiN/SiNx/NbTiN microstrip line resonators, which have a resonance frequency in the range of 614-685 GHz, two orders of magnitude higher in frequency than what is currently studied for use in circuit quantum electrodynamics and photodetectors. The frequency resolution of the filters decreases from 350 to 140 with increasing frequency, most likely limited by dissipation of the resonators.

Efficient extension of conventional low frequency filter bank spectrometer by implementation of high resolution Chirp Z signal processing

The spectral analysis of any signal involves a signal-processing tool using Fourier processors. Chirp Z Transform (CZT) is a modern technique providing ideally any frequency resolution in the spectrum. The conventional filter bank type spectrometer provides low to moderate (few 100 kHz) frequency resolution and if its frequency resolution is to be increased further then the number of channels must be increased to a large extent, necessitating extra hardware and expense. To increase the frequency resolution of a conventional filter bank further, a novel approach using Chirp Z transform has been suggested at low frequency. It is a cost effective and easy to use solution capable of providing both Chirp Z signal processing and Discrete Fourier Transform (DFT) as a special case of CZT. It is an efficient way to perform offline signal processing in applications not requiring real time operation.

Measurements of broad-band millimeter-wave radiation from an IREB-plasma interaction system

High-power broad-band millimeter-wave radiation is emitted from a plasma in a strong Langmuir turbulence state driven by an intense relativistic electron beam. We measured directivity and spectrum of this radiation with a filter-bank spectrometer, a heterodyne spectrometer, and a filter-waveguide-combination spectrometer covering 18-140 GHz. The directivity measurement indicated that the radiation was relativistically beamed. The observed spectra were nearly flat up to about 40 GHz and declined steeply above 40 GHz. Discussion is given on the experimental results in connection with the collective Compton boosting model proposed by Benford and Weatherall (1992).

A low-cost filterbank spectrometer for submm observations in radio astronomy

We describe the design and construction of a filterbank of novel design which uses sections of coaxial transmission line as filter elements. This filterbank is a lower cost alternative to standard (solid-state) filterbank designs, and is appropriate in situations where large total bandwidths are required, as is the case for submillimeter observations of extragalactic objects.

An observed upper limit on stratospheric hydrogen peroxide

We have conducted a search for emission from the 70,7 → 61,5 rotational‐torsional transition of stratospheric hydrogen peroxide at 270.610 GHz, using a ground‐based millimeter‐wave heterodyne receiver which has been used successfully to measure stratospheric ClO, HO2, HCN and other trace gases. Observations were made at Mauna Kea, Hawaii (19.5°N) in late May and early June of 1983. No definitive signal was detected in the output of a 256‐channel filter bank spectrometer. From this result we deduce an upper limit on the H2O2 mixing ratio between approximately 30 and 50 km altitude which is approximately 2 times that predicted from a recent stratospheric model by Sze and Ko. Towards its middle altitude range, this limit overlaps a recent upper limit set by K. V. Chance and W. A. Traub (1984) between 22 and 38 km and is ∼2 × lower in the mixing ratio at 32 km than that implied by a tentative H2O2 detection by J. W. Waters et al. (1981).

Signal and noise response of a spectrum expanse

A spectrum expander samples an analog input signal periodically, stores these samples cyclically in a series of digital registers, and reads out these samples cyclically at a higher rate. An analysis of the signal and noise response of this device is given here. The above spectrum expander is precisely equivalent to an analog system consisting of 1) a filter-bank spectrometer, whose filter transfer functions are sine functions of frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(\sin c x\equiv \sin \pi x/ \pi x)</tex> spaced at the Nyquist interval; 2) a modulator bank that separates the filter bank outputs, spacing them equally in frequency; and 3) An adder, that sums the modulator bank outputs. Such a spectrum expander may be followed by an appropriate physical, fixed channel-bank spectrometer, and used to determine approximately the power spectrum of noise inputs with a variety of bandwidths. The input bandwidth can be accommodated to the fixed channel-bank spectrometer by adjusting the expansion ratio. Stating this another way, such a system is equivalent to a channel-bank spectrometer with variable resolution. Moreover, such a spectrum expander may be used for real-time signal processing, since it offers a digital means of dissecting a signal in the frequency domain. Each of the above applications suffers from impairments. The present analytical results permit us to evaluate the errors in various cases of interest. These include the effects of a finite number of digits in the storage registers (equivalent to quantizing the analog input) for Gaussian noise inputs. By using a weight function on the output samples, the transfer functions of the equivalent spectrometer filters described in item 1 above can be changed from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sin c</tex> functions of frequency to other desired characteristics. This too is amenable to treatment by extension of the present results.