This is a close-up view of the “spectrometer-on-a-chip” technology that could dramatically reduce the size of spectrometers in the future. Credit: NASA Goddard/Chris Gunn |
The
Composite Infrared Spectrometer (CIRS) is big. It’s powerful and it
discovered, among other things, that Saturn’s mysterious moon Enceladus
was one of the very few worlds in the solar system that radiated several
gigawatts of heat into space, primarily along prominent fractures
dubbed “tiger stripes.”
If
a team of technologists at NASA’s Goddard Space Flight Center in
Greenbelt, Md., succeeds, however, scientists in the future won’t
observe these far-flung worlds with instruments the size of dishwashers.
Rather,
they will make their discoveries with dramatically smaller, more
efficient models whose critical components fit onto a silicon wafer and
do not require moving parts to operate—unlike the breadbox-size
components found inside the Goddard-developed CIRS, which flew on the
flagship Cassini-Huygens mission to Saturn.
“The
Holy Grail is reducing the number of moving parts, which will allow us
to build lighter, smaller instruments,” said team member John Allen.
“The smaller the device, the better. That’s where the power of our
effort really begins to take off.”
Potentially revolutionary FTS
The
potentially revolutionary Miniaturized Waveguide Fourier Transform
Spectrometer (FTS), which, like CIRS, would be sensitive to the
mid-infrared bands, is a greatly scaled down version of the
Michelson-type FTS commonly used to study the spectra of planets and
stars and identify their chemical makeup and other physical properties.
With
traditional Michelson-type instruments, a beamsplitter takes the
incoming light gathered by a telescope and divides it into two optical
beams. One beam of light re?ects off a ?at mirror that is fixed in place
on one arm of the instrument; the other re?ects off another mirror,
which is attached to a mechanism that moves the mirror a very short
distance—typically a few millimeters (or tenth(s) of an inch)—away from
the beamsplitter. At the end of the optical path, the two beams
recombine.
Because
the path that one beam travels is ?xed in length and the other is
constantly changing as its mirror moves, the signal that exits the
device is the result of these two beams “interfering” with one another.
When analyzed with the Fourier transform, a mathematical theorem
invented by Joseph Fourier in 1811, the resulting interferogram produces
a spectrum revealing the various wavelength frequencies emitted by the
source.
Though
this type of spectrometer is ubiquitous in science investigations,
“obtaining an accurate spectrum with the instrument exacts a penalty in
the form of mass, electrical power, and moving parts that can fail,”
explained Shahid Asalm, the principal investigator leading the effort
funded by NASA’s Center Innovation Fund and Goddard’s Internal Research
and Development program. To find berths on future planetary missions,
instruments would have to be significantly smaller, lighter, and more
capable, he said.
Lightening the load
To
lighten the load and reduce complexity, the team wants to replace the
mirrors and associated hardware with a microscale photonic system
featuring 60 hollow waveguides, or tunnel-like circuits, 10 times
thinner than a human hair.
|
Etched
inside a silicon wafer, these tiny circuits would carry out the job of
more traditional Michelson-type spectrometers. The light would travel
down these tiny tubes, hit a Y-junction, split, and then continue its
journey down two separate channels. One beam would take a relatively
straight path; the other would take a path that loops slightly to create
a longer route. The two beams would then recombine to create one
intensity point on an interferogram from which to derive a spectrum.
“The
result is a spectrometer-on-a-chip that fits in the palm of a hand,
excludes moving parts, and samples the complete inteferogram
simultaneously,” Aslam said. “In addition, the device does not require
mechanical power to move the mirror, nor any bulky, high-precision
free-space optics as in classical Fourier transform spectrometers. The
significance of our research is that we’re transforming how we propagate
light. We’re replacing large, high-precision optics with microscale
light pipes.”
Although
researchers have reported creating a similar device tuned to the
visible and near-infrared wavelength bands, no one has attempted it for
instruments sensitive to mid- and long-infrared light. “When we do
simulations, we find the physics are true. No rules are violated. In
principal, we should be able to make a device like this,” Aslam said,
adding that he and his team, also including Co-Principal Investigator
Tilak Hewagama and engineer George Shaw, plan to demonstrate the device
by the end of the year.
Demonstrating
that the approach can produce an interferogram is one challenge. The
other will be in engineering an instrument that can take advantage of
the micro optics and gather useful science. “This is embryonic,” Allen
agreed. “We’ll need a few more years to advance this technology and I’m
sure we’ll encounter a lot of pitfalls. But we have to try to understand
how these photonic circuits work efficiently,” Asalm said, adding it
took a Goddard team nearly 10 years to develop CIRS.
Should
the group succeed, Aslam said the payoff will be huge, not only for
NASA’s science disciplines, but for biotechnology and other commercial
applications. Asalm envisions using the technology for detecting
dangerous pathogens and other life-threatening materials in trucks and
shipping containers because of its potential to reduce the size of
instruments for more mobile use. “Anywhere you’re trying to remotely
sense, I think this technology has a multitude of applications,” Allen
said.
Source: NASA Goddard Space Flight Center