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Researchers: Broadband Possible in Space

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CAMBRIDGE, Mass., March 22, 2006 -- MIT researchers have developed a tiny light detector that may allow for superfast broadband communication over interplanetary distances. Currently, even still images from other planets are difficult to retrieve, they said.

"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.

MIT electrical engineering and computer science assistant professor Karl Berggren works on nanowires in the lab. (Photo: Donna Coveney)
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 (MSPU). The new detector improves the detection efficiency to 57 percent at a wavelength of 1550 nm, the same wavelength used by optical fibers that currently carry broadband signals to offices and homes. That's nearly three times the current detector efficiency of 20 percent, Berggren said.

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, he said.

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 antireflective coating to keep light from bouncing off its surface.

Berggren said 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, he said.

Berggren and his colleagues published their discovery in the Jan. 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 MSPU. The work was partially funded by the US Air Force.

The researchers are now working to make the detector even more efficient, Berggren said. For more information, visit:
Mar 2006
Indicating a capability to deal with a relatively wide spectral bandwidth.
1. A device designed to convert the energy of incident radiation into another form for the determination of the presence of the radiation. The device may function by electrical, photographic or visual means. 2. A device that provides an electric output that is a useful measure of the radiation that is incident on the device.
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
BerggrenbroadbandCommunicationsdetectorinterplanetarylightLincoln LaboratoryMITMoscowMSPUnanowireNews & Featuresphotonphoton trapRLESensors & Detectorsultrafast

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