New Light Detector Could Allow Broadband in Space

Researchers at the Massachusetts Institute of Technology have developed a tiny light detector that could one day boost interplanetary communications to broadband speeds.

The work could permit the transmission of color video between astronauts and satellites and scientists on Earth across interplanetary distances, something that is not practical with current technologies.

The new light detector improves detection efficiency to 57 percent at a wavelength of 1,550 nanometers--the same wavelength used by optical fibers on Earth to carry broadband signals to homes and offices. Currently, light detectors only absorb about 20 percent of the light they receive.

"It can take hours with the existing wireless radio frequency technology to get useful scientific information back from Mars to Earth," said study team member Karl Berggren from the Massachusetts Institute of Technology. "But an optical link can do that thousands of times faster."

Currently, many spacecrafts still use radio signals to send data back to Earth. Two-way laser communication in space would enable data transmission rates that are 10 to 1,000 times higher, scientists predict.

The Mars Telecommunications Orbiter spacecraft, set to launch in 2010, but cancelled last summer due to budget problems, would have used lasers to transmit data between 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. 

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

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

The new detector is so sensitive it can detect single photons from light or laser signals in the infrared part of the optical spectrum. Photons are the smallest and most basic unit of light.

To boost the sensitivity of the new detector, the researchers added an anti-reflective coating that helps prevent light from bouncing off the device surface and escaping.

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

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

Other researchers have developed single-photon detectors before, but they were not both speedy at efficient at detecting light. Aside from interplanetary communication, Berggren thinks the detector could also find uses in quantum cryptography and biomedical imaging.

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

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