Unguided optical signaling or free-space optics has been in use for centuries.
Paul Revere used binary optical signaling from the Old North Church just prior to
his famous ride. A more modern application is to connect the LANs in two buildings
via lasers mounted on their rooftops. Optical signaling using lasers is
inherently unidirectional, so each end needs its own laser and its own photodetector.
This scheme offers very high bandwidth at very low cost and is relatively
secure because it is difficult to tap a narrow laser beam. It is also relatively easy
to install and, unlike microwave transmission, does not require an FCC license.
The laser’s strength, a very narrow beam, is also its weakness here. Aiming a laser beam 1 mm wide at a target the size of a pin head 500 meters away requires the marksmanship of a latter-day Annie Oakley. Usually, lenses are put into the system to defocus the beam slightly. To add to the difficulty, wind and temperature changes can distort the beam and laser beams also cannot penetrate rain or thick fog, although they normally work well on sunny days. However, many of these factors are not an issue when the use is to connect two spacecraft.
One of the authors (AST) once attended a conference at a modern hotel in Europe at which the conference organizers thoughtfully provided a room full of terminals to allow the attendees to read their email during boring presentations. Since the local PTT was unwilling to install a large number of telephone lines for just 3 days, the organizers put a laser on the roof and aimed it at their university’s computer science building a few kilometers away. They tested it the night before the conference and it worked perfectly. At 9 A.M. on a bright, sunny day, the link failed completely and stayed down all day. The pattern repeated itself the next two days. It was not until after the conference that the organizers discovered the problem: heat from the sun during the daytime caused convection currents to rise up from the roof of the building, as shown in figure. This turbulent air diverted the beam and made it dance around the detector, much like a shimmering road on a hot day. The lesson here is that to work well in difficult conditions as well as good conditions, unguided optical links need to be engineered with a sufficient margin of error.
Unguided optical communication may seem like an exotic networking technology
today, but it might soon become much more prevalent. We are surrounded by cameras (that sense light) and displays (that emit light using LEDs and other
technology). Data communication can be layered on top of these displays by encoding
information in the pattern at which LEDs turn on and off that is below the
threshold of human perception. Communicating with visible light in this way is
inherently safe and creates a low-speed network in the immediate vicinity of the
display. This could enable all sorts of fanciful ubiquitous computing scenarios.
The flashing lights on emergency vehicles might alert nearby traffic lights and
vehicles to help clear a path. Informational signs might broadcast maps. Even festive
lights might broadcast songs that are synchronized with their display.
The laser’s strength, a very narrow beam, is also its weakness here. Aiming a laser beam 1 mm wide at a target the size of a pin head 500 meters away requires the marksmanship of a latter-day Annie Oakley. Usually, lenses are put into the system to defocus the beam slightly. To add to the difficulty, wind and temperature changes can distort the beam and laser beams also cannot penetrate rain or thick fog, although they normally work well on sunny days. However, many of these factors are not an issue when the use is to connect two spacecraft.
One of the authors (AST) once attended a conference at a modern hotel in Europe at which the conference organizers thoughtfully provided a room full of terminals to allow the attendees to read their email during boring presentations. Since the local PTT was unwilling to install a large number of telephone lines for just 3 days, the organizers put a laser on the roof and aimed it at their university’s computer science building a few kilometers away. They tested it the night before the conference and it worked perfectly. At 9 A.M. on a bright, sunny day, the link failed completely and stayed down all day. The pattern repeated itself the next two days. It was not until after the conference that the organizers discovered the problem: heat from the sun during the daytime caused convection currents to rise up from the roof of the building, as shown in figure. This turbulent air diverted the beam and made it dance around the detector, much like a shimmering road on a hot day. The lesson here is that to work well in difficult conditions as well as good conditions, unguided optical links need to be engineered with a sufficient margin of error.
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