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HIFI Instrument
The Heterodyne Instrument for the Far-Infrared (HIFI) is a very
high resolution heterodyne spectrometer. It won't produce pictures
of stars and galaxies, but rather extremely detailed spectra of
their atoms and molecules.
Prototype amplifier chain built at JPL for evaluation of the technologies
involved.
"Heterodyne" refers to a technique that mixes the frequency of each
incoming photon with one generated by the instrument, itself. The result
is a lower-frequency signal (microwave instead of the original far-infrared
or submillimeter) that is easier to amplify, copy, and resolve in extremely
fine detail.
HIFI will observe the complete range of frequencies from 480-1250 GHz, and
1410-1910 GHz, divided into six bands. It will be the first heterodyne device
to cover the far-infrared and submillimeter spectrum so comprehensively, and
the only instrument capable of making continuous-frequency high-resolution
spectral surveys in the sub-millimeter region.
HIFI is being designed and built by a consortium that includes NASA, and is
led by Thijs de Graauw of Space Research Organization Netherlands (SRON),
the Dutch space agency.
NASA is contributing mixing elements for the two highest-frequency bands.
Band 5 covers 1120-1250 GHz, and Band 6-High covers 1600-1910 GHz. NASA is
also providing the frequency sources (local oscillators) for Band 5 and all
of Band 6 (1410-1910 GHz), as well as components for the remaining frequency
sources. They are being developed by the Jet Propulsion Laboratory in
Pasadena, California.
No-Man's Land
Building heterodyne instruments to detect far-infrared and submillimeter
radiation is tricky. Conventional transistors can easily detect microwave
and longer wavelengths, but their performance rapidly degrades as the
wavelength gets shorter. Quantum electronics are well-suited to mid-infrared
and shorter wavelengths, but it's difficult to make quantum structures
large enough for the far-infrared. The far-infrared/submillimeter range
falls in-between the quantum and classical electronic regimes, in an
electromagnetic no-man's land. HIFI's receivers represent hybrids between
conventional and quantum electronics that push the limits of technology.
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Local oscillator technologies developed for HIFI at JPL. From left
to right: monolithic substrateless device, frameless membrane device,
framed membrane-based device.
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Each of HIFI's six bands is covered by a receiver consisting of a mixer and a
local oscillator. The mixer combines each incoming astronomical photon with a
signal generated by the local oscillator to down-convert its frequency, and
then sends the resulting "intermediate frequency" signal to the spectrometers
for analysis.
The SIS device with an expanded view of the mixer circuit.
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The protoype band 5 mixer without its cover. Its size is 32x32x45mm.
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For 480-1250 GHz observations, HIFI will use SIS (superconductor-insulator-superconductor)
mixers. Each uses tunnel junctions, which employ a quantum effect in which an electron
"tunnels" through an insulator. This kind of device is able to detect very weak signals
with very little noise. Receivers built with SIS mixers are the most sensitive kind of
receiver that current technology can build, and are nearly at the theoretical performance
limits.
Unfortunately for Band 6, current superconductor technology can't produce a SIS mixer
at the 1410-1910 GHz frequency. So for HIFI's highest band, an HEB (Hot Electron Bolometer)
mixer will do the job.
The Hot Electron Bolometer (HEB) for band 6 with an expansion from
a scanning electron microscope. The darker strip on the image
on the right is NbTiN. The rest of the material is gold.
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The HEB is a very small, very fast non-linear resistive mixer, which measures
changes in resistance across the device caused by the modulation of the signal.
Though not a tunnel junction, it too employs superconductor technology.
Both kinds of mixers have to be horrendously small to do their work. The SIS
junction is less than one-half square micron by 0.55 micron. The HEB measures
one-hundredth of a cubic micron. For comparison, a typical human hair is around
100 microns thick.
All of these innovations work together to provide record-setting performance.
HIFI will use two kinds of spectrometers to analyze the light the mixers feed
to it. One has greater bandwidth, the other features higher resolution.
The acousto-optical spectrometer will detect the full 4 GHz band that HIFI's
mixers supply at any given time, seeing the full spectrum of the light it
receives, but at low resolution. The auto-correlator spectrometer is capable
of much higher resolution, but will be used more selectively, targeting only
certain ranges of wavelengths.
The Small Picture
HIFI's minute observations will address some big questions about stars, galaxies,
the Solar System, and the chemicals of which we are all made.
Its capability for highly-detailed chemical identification of individual atoms
and molecules make it the instrument of choice for studying chemistry in the
interstellar medium and in the regions around embryonic and dying stars. HIFI
will provide an inventory of chemicals in various regions of the local Universe,
and add greatly to our understanding of how they evolve.
With its ultra-high frequency resolution, HIFI will produce very detailed
spectrographs revealing the motions, temperatures, and other characteristics
of the atoms and molecules it will study. This in turn will help scientists
understand the processes that govern comets, planetary atmospheres, star
formation, and the development of distant and nearby galaxies.
For more on HIFI, visit the
Herschel Space Observatory website at IPAC.
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