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HIFI Instrument

The Heterodyne Instrument for the Far-Infrared (HIFI) is a very high resolution heterodyne spectrometer. It doesn't produce pictures of stars and galaxies, but rather extremely detailed spectra of their atoms and molecules.

rototype amplifier chain built at JPL for evaluation of the technologies involved. 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 observes the complete range of frequencies from 480-1250 GHz, and 1410-1910 GHz, divided into six bands. It is 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 has been designed and built by a consortium that includes NASA, and was led first by Thijs de Graauw and subsequently by Frank Helmich of Space Research Organization Netherlands (SRON), the Dutch space agency.

NASA contributed mixing elements for the two highest-frequency bands. Band 5 covers 1120-1250 GHz, and Band 6-High covers 1600-1910 GHz. NASA also provided 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 were 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.

Monolithic substrateless devices Frameless membrane devices Framed membrane based devices
Local oscillator technologies developed for HIFI at JPL. From left to right: monolithic substrateless device, frameless membrane device, framed membrane-based device.

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.
The SIS device with an expanded view of the mixer circuit.
The protoype band 5 mixer without its cover. Its size is 32x32x45mm.
The protoype band 5 mixer without its cover. Its size is 32x32x45mm.

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 does the job.

Hot Electron Bolometer
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.

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.

Local oscillator implementation

HIFI uses 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 detects 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 is used more selectively, targeting only certain ranges of wavelengths.

The Small Picture

HIFI's minute observations 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 has provided an inventory of chemicals in various regions of the local Universe, and added greatly to our understanding of how they evolve.

With its ultra-high frequency resolution, HIFI has produced very detailed spectra revealing the motions, temperatures, and other characteristics of the atoms and molecules it has observed. These in turn 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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