MIT
researchers have developed a tiny light detector
that may allow for super-fast broadband communications
over interplanetary distances. Currently, even still
images from other planets are difficult to retrieve.
"It can take hours with the existing wireless radio
frequency technology to get useful scientific information
back from Mars to Earth. But an optical link can
do that thousands of times faster," said Karl Berggren,
assistant professor in the Department of Electrical
Engineering and Computer Science (EECS).
Berggren, who is also affiliated with the Research
Laboratory of Electronics (RLE), developed the detector
with colleagues from the RLE, Lincoln Laboratory
and Moscow State Pedagogical University.
The new detector improves the detection efficiency
to 57 percent at a wavelength of 1,550 nanometers
(billionths of a meter), the same wavelength used
by optical fibers that carry broadband signals to
offices and homes today. That's nearly three times
the current detector efficiency of 20 percent.
The result will be real-time collection of large
amounts of data from space. The work may ultimately
permit the transmission of color video between astronauts
or equipment in outer space and scientists on Earth.
The detector, which uses nanowires and superconductor
technology, can sense extremely low light or laser
signals in the infrared part of the optical spectrum
-- down to a single photon, the smallest and most
basic unit of light. That has not been possible using
conventional optical systems.
The detector also could be applied to quantum cryptography
and biomedical imaging, but the biggest application
is interplanetary communication, Berggren said.
Because of the vast distances between planets, current
optical systems would require a large laser and a
lot of power to send data at a high rate. And this
would have to be done on spacecraft, which are typically
starved for power. So there is a need for devices
like the new detector that can operate quickly and,
because they are more sensitive, receive signals
from smaller lasers that do not use much power, Berggren
said.
Single-photon detectors have been made by MIT and
other researchers in the past, but they have not
been both speedy and efficient at detecting light.
The way Berggren and his colleagues improved the
efficiency was to add a "photon trap" to the detector
as well as an anti-reflection coating to keep light
from bouncing off its surface.
The photon trap is an optical cavity consisting
of the nanowire detector, a carefully measured gap
of glass and a mirror. The nanowire is coiled tightly
like the metal on the back of a refrigerator to broaden
its area of overlap with the laser light.
The wire is then cooled to just above absolute zero.
That temperature is the point at which it becomes
a superconductor and at which it can detect the absorbed
photons. If a photon is not absorbed the first time
it touches the wire, it bounces back and forth between
the coiled nanowire and the mirror so it has more
opportunities to be absorbed. The more photons that
are absorbed, the greater the efficiency of the detector.
Berggren and his colleagues published their discovery
in the January 23 issue of Optics Express. His co-authors
are MIT RLE post-doctoral researcher Kristine Rosfjord
and RLE/EECS graduate students Joel Yang, Vikas Anant
and Eric Dauler; Lincoln Laboratory staff member
Andrew Kerman; and Boris Voronov and Gregory Gol'tsman
of Moscow State Pedagogical University.
The researchers are now working to make the detector
even more efficient.
This work was funded in part by the U.S. Air Force.
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