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.