April 26, 2002

Scientists complete laser link between Lab, Mount Diablo

Scientists have successfully completed a 28-kilometer high-capacity laser communication link between the Laboratory and Mount Diablo.

"This represents one of the longest terrestrial high-capacity air-optics links currently in existence," said Tony Ruggiero, principal investigator on the project to develop an optical wireless testbed for evaluating new laser communication technologies.

The experiments are being conducted as part of the Secure Air-Optic Transport and Routing Network, or SATRN program. SATRN is an NAI LDRD Strategic Initiative to develop advanced technologies for long-range laser communications.

Proliferation detection, counterproliferation, arms control, counterterrorism and warfighting all require the timely and secure communication of information in situations where fiber-optic cable is physically or economically impractical and data requirements exceed radiofrequency (rf) or microwave wireless capacity.
The initial Lab-Mount Diablo link transmitted data at a 2.5-Gbit/s single-channel data rate — equivalent to the transmission of 1600 conventional T1 data lines, 400 TV channels or 40,000 simultaneous phone calls.
"This preliminary experiment puts the SATRN team in a class with less than a handful of research groups worldwide that have successfully implemented long-range air-optic links," Ruggiero said.

Laser communication consists of an optical system in which information is encoded on a laser beam and transmitted to a receiver telescope. Functionally similar to radiofrequency or microwave communications, lasers use the optical part of the electromagnetic spectrum. The laser beams used for communication are not visible or harmful in any way.

Systems for transmitting data using lasers over short distances of 100 to 500 meters — between buildings, for example — are well established. The challenge of the SATRN project is to extend that range to tens of kilometers while maintaining a high availability, or percentage of time the link is accessible at an acceptably low bit-error rate. The bit-error rate quantifies the number of errors generated per number of bits sent. For example, one error in every million bits transmitted would correspond to a bit-error rate of 10-6.

"The bit-error rates for this first baseline attempt were quite reasonable for an unoptimized system without forward error correction," Ruggiero said. "We learned a lot from this first attempt that will allow us to improve the system for subsequent experiments."

Variable mountain weather conditions during the course of the experiment also allowed researchers to gain experience with operation in freezing temperatures, winds up to 40 mph, low-visibility conditions and light fog.

The next step is a 24-hour full-duplex data collect to get a statistical sampling of the fade statistics, bit-error rates, and effects of turbulence and weather conditions on link performance, Ruggiero said.

This will allow the team to establish a solid baseline for the single-channel long-range link performance of the system. Scientists will then begin data-capacity scaling via wavelength division multiplexing to match the existing SATRN 20-Gbit/s building-to-building link.

Future long-range experiments will involve implementation and testing of new enabling laser communication technologies designed to increase link performance, validate atmospheric beam propagation models, evaluate and characterize the effects of varying weather conditions, and assess higher-level communications protocols such as Gigabit Ethernet.

"Ultimately we plan to demonstrate a 28-km air-link with an aggregate bit rate of 100 Gbit/s, 50 times the capacity of the first experiment," Ruggiero said.

Members of the SATRN team involved in fielding this new technology and the successful implementation of this first long-range link experiment were John Cornish, Jeff Cooke, Gary W. Johnson, Steve Mostek, Alex Pertica, Dean Rippee, Jim Thournir, Jeff Wilburn and Rick Young.