Photon detector is precursor to broadband in space

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Photon detector is precursor to broadband in space

* 18:45 21 March 2006
* NewScientist.com news service
* Kimm Groshong

US researchers have nearly trebled the efficiency of a miniscule detector capable of capturing single photons of light – the technology could one day be used to receive information through a laser stream of data sent from Mars to Earth. The finding could lead to speedier, reliable relays of huge amounts of data across interplanetary distances, setting up a form of broadband communication in space.

"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," says Karl Berggren, at MIT.

His team has boosted the photon-capturing abilities of the detector by using an extremely thin nanowire detector. They combined it with an anti-reflection coating to stop light bouncing away and a "photon trap" that helps channel incoming photons to be absorbed and not lost.

The trap – a cavity between a sheet of glass and a gold mirror at a set distance – reflects photons that would normally transmit straight through the detector back onto the coiled nanowire, where they can be absorbed.
Superconductor

The special add-ons increased the detector's absorption efficiency from 20% – the previous best for previous single-photon detectors – to 57% at the wavelength used for broadband signal transmission.

And it does so quickly. "The speed comes from the fact that it's a superconducting nanowire," Berggren says. The nanowire is wrapped tightly into a configuration with dimensions many times smaller than a human hair. It is cooled to just above absolute zero, where it becomes a superconductor sensitive to absorbed photons.

The speed and efficiency of detection would be crucial in interplanetary optical communications. Unlike traditional radio links, clouds become a problem in the transmission of photons. And a laser beam would spread out many times by the time it reached the Earth, making the received signal very weak. So the efficiency of detecting light particles is vital in developing any kind of optical communication.

"They can work with very weak signals and very few photons," Andrew Kerman, a member of the technical staff at Lincoln Laboratory in Lexington, Massachusetts, US, told New Scientist. "In communicating with Mars, for example, you're going to be stuck not being able to transmit very many photons."
Quantum cryptography

When NASA cancelled its Mars Telecommunications Orbiter in July 2005, a team of researchers had been developing an optical communication laser for just such a purpose. The laser was designed to beam back between 1 million and 30 million bits per second, depending on the distance between Mars and Earth at any given time.

The currently orbiting Mars Odyssey spacecraft relays about 128,000 bits per second using radio waves. The leap in capacity offered by lasers is due to the different wavelengths of light carrying the data. The Mars Telecommunications Orbiter was to use infrared light with a wavelength of 1.06 microns – thousands of times shorter than radio waves. Light and radio travel at the same speed through space, but shorter wavelengths carry more information in the same transmission time.

But interplanetary communication is not the only application for such an efficient single-photon detector. Berggren also foresees applications in quantum cryptography where the state of individual photons encodes information that needs to be secure.

Journal Reference: Optics Express (vol 14, p 527)
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Mars laser will beam super-fast data

* 12:47 16 September 2004
* NewScientist.com news service
* Maggie McKee

A laser that can beam data from Mars to Earth at 10 times the rate of current radio links will be sent to the Red Planet in 2009, say NASA scientists. The laser will be the first test of such technology in deep space and may usher in a new era of space communication.

"It is the next big thing," says Stephen Townes, deputy manager of the Mars Laser Communication Demonstration at NASA's Jet Propulsion Laboratory in Pasadena, California. "There is the promise we will be able to get high data returns with lower power and lower mass than the typical systems out there."

NASA's Mars Odyssey spacecraft currently boasts the highest data transmission rate at 128,000 bits per second. The new laser will beam back between one million and 30 million bits per second, depending on the distance between Mars and Earth.

That leap in capacity is due to the different wavelengths of light carrying the data. The laser will use infrared light with a wavelength of 1.06 microns, which is thousands of times shorter than radio waves. Since all light travels at the same speed through space, shorter wavelengths carry more information in the same time.

That is crucial for the increasing number of ambitious space missions, says Joss Bland-Hawthorn, head of instrument science at the Anglo-Australian Observatory in Sydney.

"Astronomers are losing vast amounts of data from recent satellite missions to Mars," he told New Scientist. "We collect a hundred times more than we can transmit back."
Cloudy skies

But so-called optical communication has certain drawbacks compared with time-tested methods. Unlike radio waves, clouds can block the laser's photons. And laser beams are narrower than the distended radio wave cones that wash over the entire Earth, making precise pointing of the laser important.

The new Mars laser project will use two different sites to detect the laser on Earth - the 5-metre Hale Telescope in southern California and an array of four 0.8-metre telescopes whose location has yet to be determined. If the weather is overcast at one location, astronomers can try the next. Future projects are likely to have a dozen telescopes spaced around the world.

The laser will be transmitted from a 0.3-metre telescope on a spacecraft orbiting Mars, but the beam will spread to a width of a few hundred kilometres by the time it reaches Earth. That makes picking up the laser's relatively weak signal "very difficult," says Townes, adding that the team is developing photon detectors that are orders of magnitude more sensitive than those now used.
Live footage

Despite the signal's weak strength, "optical astronomers are quite alarmed about the prospect of this being generalised", says Tomas Gergely, electromagnetic spectrum manager at the National Science Foundation in Arlington, Virginia, US.

A few satellites orbiting Earth are already testing the technology to talk to each other or to send data to ground-based telescopes. A glut of such nearby lasers could bounce off the foil shrouds covering satellites and create light pollution, explains Gergely.

Bland-Hawthorn agrees that space-based lasers can cause unwanted light but adds that astronomers on Earth already cause similar problems by using laser dots for telescope calibration. He says that one day space agencies could send live video footage - which requires data rates of billions of bits per second - from Jupiter to Earth.

The new Mars laser, allocated $270 million from NASA, will undergo a design review in early 2005 and will fly on NASA's Mars Telecommunications Orbiter in 2009. That spacecraft, which will also have traditional radio links, will be the first mission designed mainly for communication between other space missions.

[url=\"http://www.newscientistspace.com\"]source: New Scientist: Space Section[/url]