Soldiers may soon have a way to conveniently share large amounts of information, including video, thanks in part to APL’s developments in free-space optical communications, which sends data between two line-of-sight points through the atmosphere or space.

Free-space optical communications (FSOC), like fiber optics, relies on light waves to carry data — but without the fibers — and has several advantages over radio frequency (RF) and microwave links.

“The biggest advantage with free-space optical is its ability to handle very high data rates,” says D. Young, FSOC technical lead, in the Air and Missile Defense Department. “Through our field tests we’ve shown that data can be transmitted at aggregate speeds greater than 80 gigabits per second, which is more than 100 times faster than fielded RF systems.”

Military Applications
The military recognizes this communication method’s potential for handling uncompressed high-definition video streams or unprocessed synthetic aperture radar data that are too large for current RF links. It could also be used for the Global Information Grid that will put high-bandwidth data closer to the battlefield.

The directional nature and small beam size of free-space optical links make them nearly impossible to detect, intercept or jam, unlike RF or radar links. Compared with fiber optic links, they’re inexpensive and easy to set up.

Although the communications method has been around since the 1960s, a small team in AMDD’s Electro-Optical and Infrared Systems and Technologies (A2D) Group has only recently made strides in tackling some of the long-standing challenges that have hindered the technology’s potential as a feasible communications tool.

Early in an Internal Research and Development (IRAD) project that began in 2005, the A2D team realized that substantial improvements in current technology were required to produce high data-rate links. “We can significantly increase bandwidth by leveraging a fiber-optic communications technique called wavelength-division multiplexing that enables multiple streams of data to be simultaneously transmitted across one optical channel,” Young says. “Using this technique, FSOC links can transmit hundreds of gigabits of data per second, which is nearly as fast as fiber-optic links.”

Putting It to the Test
In one of three field tests, the group demonstrated an FSOC link between a moving ground vehicle and a tethered aerostat. It was the first known test of a free-space optical link to an airborne platform and at very high data rates, and proved one optical channel could be used to simultaneously transmit multiple data streams.

“We sent up to 80 gigabits of data per second over six individual channels,” says Young. “During the three-day test period, we transmitted more than three terabytes of data, which is about 10 times the size of APL’s Gibson Library if its printed materials were converted to digital form.”

The aerostat test, conducted in Elizabeth City, N.C., in May 2006, also highlighted additional challenges.

Atmospheric turbulence, such as dust, clouds, snow or anything that hinders sight lines, can disrupt FSOC links, as the team experienced during a 2006 field test in Hawaii, where they established a 93-mile (150-kilometer) link between the islands of Maui and Hawaii. Realizing that stabilizing the optical signal would provide a better link and improve data quality, the team decided to enhance their receiver technology, a critical element of the free-space optical communications pathway.

Under a project funded by the Air Force Research Laboratory, the team demonstrated a unique device, developed under an IRAD effort, known as an optical automatic gain control system, which reduces power fluctuations and ensures stable, uninterrupted power over multiple channels. They also developed an optical modem that enables two commercially available transmission devices to work together to send and receive signals over an FSOC link.

The devices were successfully tested in Campbell, Calif., in June 2007, and in Hawaii in August 2007, over a hybrid optical-RF link that, like Ethernet, has a capability to check for errors in real time and ensure data is successfully transmitted.

The team will perform additional science and technology work for the Air Force, focusing on advanced FSOC communications methods, and hopes to work this year with additional government sponsors. This growth area developed from having the “right group of people in the right place at the right time,” Young says, “and has flourished thanks to the support of AMDD’s new business development leaders.”