February, the first laser communication link between Lawrence Livermore
and the top of the 915-meter-high Mount Diablo, 28 kilometers away,
transmitted data at 2.5 gigabits per second on a single laser channel,
a rate comparable to 1,600 conventional T1 (local area network)
data lines, 400 channels of television, or 40,000 simultaneous phone
calls. That event was one of the longest terrestrial high-capacity
airoptics links ever, says Tony Ruggiero, principal
investigator for the project.
though theres the ever-present beeping of cell phones, buzzing
of pagers, and notices popping up that youve got mail, users
still demand better, faster communications. The demand is especially
high from the military, whose highly sensitive, remote, sensor-based
intelligence, surveillance, and reconnaissance (ISR) systems collect
massive quantities of data.
improvements have been made on data collection capabilities. Now
the challenge is delivering data for timely evaluation and action.
Advanced sensors can collect data at rates of up to a gigabit
per second, notes Ruggiero. But the fastest that the
data can be transmitted is currently 270 megabits per second using
state-of-the-art radio frequency links. For most ISR applications,
data from several types of sensors must be aggregated to be useful,
driving the total data collection rate into the tens of gigabits
per second and creating a massive bottleneck.
the February test, Ruggieros project, the Secure AirOptic
Transport and Routing Network (SATRN), has closed the link between
the Laboratory and Mount Diablo at 10 gigabits per second using
four 2.5-gigabit-per-second channels running at slightly different
wavelengths. The team collected extensive performance data under
a variety of atmospheric and weather conditions. Soon, Ruggiero
expects to be delivering data via a laser beam at the rate of 100
gigabits per second.
success of the SATRN project may finally give the U.S. military
the means to eliminate the bottlenecks that have hindered information
transfers to date. Data will also be able to move quickly and securely
among various kinds of platformsbetween a moving plane and
ship, for example, or from a plane to a ground-based facility.
|Airoptic laser communications
can connect an assortment of platforms, including planes, ground-based
facilities, and ships, and can allow large transfers of data
in real time. Data transfer rates will finally be as fast as
data collection rates.
Reducing the Response Timeline
The bad guys feed off latencythe delay
between gathering intelligence and being able to use it, says
Ken Israel, former director of the Defense Airborne Reconnaissance
Latency is a challenge when,
for example, ISR sensors onboard an unmanned aerial vehicle (UAV)
detect enemy activity. As shown in the figure below, the chain of
events that follows the detection is to reorient sensors to gain
additional information and then use high-resolution imagery to verify
the activity, target it, and finally destroy it. Reducing this sensor-to-shooter
timeline is a primary goal of the SATRN project.
Today, about 30 minutes
of image data from the UAV would take 83 days to transmit over a
56-kilobit ISDN (digital phone) line, 3 days over a T1 line, or
15 minutes over the best transfer technology available. With a 1-gigabit-per-second
laser communication line, data transfer would occur in real time.
Verification, targeting, and destruction would follow almost immediately.
With data transfers
at 40 to 100 gigabits per second, multiple sensors could be combined
in a single platform, says Ruggiero. A UAV could carry
synthetic aperture radar, signal intelligence, and video, and all
of them could be transmitting information at once to the decision
makers in command.
|Ready, aim, fire! The sensor-to-shooter
timeline includes detecting the initial threat, reorienting
sensors to collect any additional information, and using high-resolution
imagery to fully recognize the target, aim the weapon, and destroy
Technologies Make It Work
Laser communication is already
in use but only to transmit information very short distances, typically
from 100 to 500 meters and usually between buildings. Extending
laser communication over longer distances and between mobile platforms
has been hampered by the effects of the atmosphere on the laser
The atmosphere is composed
of random pockets of slightly varying temperature that destroy the
spatial properties of an electromagnetic beam and cause the beams
intensity to fluctuate at the receiver. This is a much greater problem
for the shorter wavelengths used in laser links than it is for radio
and microwaves. Atmospheric attenuationthe interaction of
the laser beam with gases and particulate matter in the airis
another problem that causes an overall reduction in the detected
power level of the beam. At the same time, atmospheric turbulence
causes the beam to break up, spread, and wander, so that its power
fluctuates. Livermores SATRN team is developing several innovative
technologies to cut through the atmosphere, minimize beam fading,
and amplify the power of the beam.
Cutting through the atmosphere
to maximize transmitter and receiver beam coupling can be done most
efficiently with adaptive optics. Livermore-developed adaptive optics
systems have already proved their mettle in astronomical observatories,
where they mitigate the atmospheric disturbances that prevent astronomers
from having a clear view of stars. (See S&TR, July/August
1999, A New View
of the Universe, and June
Optics Sharpen the View from Earth.) For SATRN, the team is
producing two versions of adaptive optics to enhance laser communications.
One is based on micro-electrical-mechanical systems and builds on
adaptive optics technology that Livermore has been working on for
almost a decade. The other is an entirely new methodology based
on nonlinear optics in fibers and semiconductor systems. Still in
the research and development phase, these approaches show promise
of exceeding the performance of current adaptive optics receiver
Minimizing beam fading caused
by beam wander and obscurations such as birds in the laser path
requires a process known as forward error correction. Conventional
error correction methods do not work for laser communications because
of the high data rate and relatively long duration of the atmospheric
fades. The SATRN team is collaborating with industry to develop
new error correction techniques specifically for airoptic
Lastly, to overcome path
losses due to poor air quality and fog, new high-power fiber amplifier
technologies are based on photonic crystal fiber technologies. For
this work, Livermore is collaborating with researchers at the University
of Bath in the United Kingdom, where photonic crystal fibers were
invented. The new technology may provide 10 times the power of current
commercial amplifiers that are designed for use in wavelength-division-multiplexed
Crucial to all of this work
is modeling the laser beam both as it propagates normally through
the atmosphere and as it propagates with various new technologies.
Modeling is helping the team to optimize the design of the optical
system and predicting the performance of open-air links under specified
atmospheric conditions and ranges. We will soon integrate
the codes to provide an unprecedented capability at Livermore for
simulating terrestrial laser communications, notes Ruggiero.
|Adaptive optics systems in
use today in astronomical observatories have deformable mirrors
that move extremely quickly to compensate for atmospheric disturbances.
Nonlinear adaptive optics, a revolutionary new technology, uses
fiber optics and semiconductor chip technology to make real-time
corrections to the spatial profile of a laser beam.
Up to the Future
next major experiment for SATRN will be to create a link to airplanes
and UAVs in collaboration with the U.S. Navys Third Fleet
and the Naval Postgraduate School in Monterey, California. That
effort will take place in 2003.
to date on SATRN has been internally funded by Laboratory Directed
Research and Development. Beginning next year, the Department of
Defense and other government sponsors will fund further development
and experimental deployments. SATRN technologies will be integrated
into the Tera-Hertz Operational Reachback (THOR) program of the
Defense Advanced Research Projects Agency, the primary research
and development organization for the Department of Defense. The
goal of THOR is to develop high-bandwidth air-to-air, air-to-ground,
ground-to-air, and air-to-sea optical links to the tactical warfighter.
SATRN will fit right in.
Key Words: laser
communications link, Secure AirOptic Transport and Routing
For further information contact Tony Ruggiero (925) 423-1020 (firstname.lastname@example.org).