New Light Detector Could Allow Broadband in Space

Researchersat the Massachusetts Institute of Technology have developed a tiny lightdetector that could one day boost interplanetary communications to broadbandspeeds.

The work couldpermit the transmission of color video between astronauts and satellites andscientists on Earth across interplanetary distances, something that is notpractical with current technologies.

The newlight detector improves detection efficiency to 57 percent at a wavelength of1,550 nanometers--the same wavelength used by optical fibers on Earth to carrybroadband signals to homes and offices. Currently, light detectors only absorbabout 20 percent of the light they receive.

"It cantake hours with the existing wireless radio frequency technology to get usefulscientific information back from Mars to Earth," said study team member KarlBerggren from the Massachusetts Institute of Technology. "But an optical linkcan do that thousands of times faster."

Currently,many spacecrafts still use radio signals to send data back to Earth. Two-way lasercommunication in space would enable data transmission rates that are 10 to1,000 times higher, scientists predict.

The MarsTelecommunications Orbiter spacecraft, set to launch in 2010, but cancelledlast summer due to budget problems, would have used lasers to transmit databetween Earth and Mars at a rate of between 1 to 30 million bits per second,depending on how close the two planets are to each other. 

Whilelasers and radio transmissions both travel at light speed, lasers can pack moredata. Currently, the maximum data rate between Earth and Mars is about 128,000bits per second.

Because ofthe large distances involved, current optical systems require large lasers anda lot of power to beam data at high rates between planets. This is usually notpossible on power-starved satellites and spacecrafts. The new detector wouldget around these requirements because it can receive weaker signals fromsmaller lasers that do not use much power, Berggren explained.

The newdetector is so sensitive it can detect single photons from light or lasersignals in the infrared part of the optical spectrum. Photons are the smallestand most basic unit of light.

To boostthe sensitivity of the new detector, the researchers added an anti-reflectivecoating that helps prevent light from bouncing off the device surface andescaping.

They alsoadded a "photon trap" to the detector. The trap is made from tightly coilednanowires that are super-cooled to just above absolute zero. This increases theability of the detector to absorb photons; the more photons that are absorbed,the greater the efficiency of the detector.

If thephoton is not absorbed the first time it touches the wire, it bounces back andforth between the wire and a mirror which is also included in the photon trap,thus increasing the chance that it will eventually be captured.

Otherresearchers have developed single-photon detectors before, but they were notboth speedy at efficient at detecting light. Aside from interplanetary communication,Berggren thinks the detector could also find uses in quantum cryptography andbiomedical imaging.

The work isdetailed in the Jan. 23 issue of the journal Optics Express.

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