Usage and technologies

Free-space optical links can be implemented using infrared laser light, although low-data-rate communication over short distances is possible using LEDs. IrDA is a very simple form of free-space optical communications. Free Space Optics are additionally used for communications between spacecraft. Maximum range for terrestrial links is in the order of 2 to 3 km (1.2 to 1.9 mi), but the stability and quality of the link is highly dependent on atmospheric factors such as rain, fog, dust and heat. Amateur radio operators have achieved significantly farther distances (173 miles in at least one occasion) using incoherent sources of light from high-intensity LEDs. However, the low-grade equipment used limited bandwidths to about 4 kHz.

In outer space, the communication range of free-space optical communication is currently in the order of several thousand kilometers, but has the potential to bridge interplanetary distances of millions of kilometers, using optical telescopes as beam expanders. The present distance records for optical communications have involved detection and emission of laser light by space probes. The current two-way distance record for communication is held by the Mercury laser altimeter instrument aboard the MESSENGER spacecraft. This infrared diode neodymium laser, designed as a laser altimeter for a Mercury orbit mission, was able to communicate across a distance of 15 million miles (24 million km), as the craft neared Earth on a fly-by in May, 2005. The previous record had been set with a one-way detection of laser light from Earth, by the Galileo probe, as two ground-based lasers were seen from 6 million km by the out-bound probe, in 1992.

Secure free-space optical communications have been proposed using a laser N-slit interferometer where the laser signal takes the form of an interferometric pattern. Any attempt to intercept the signal causes the collapse of the interferometric pattern. This technique has been demonstrated to work over propagation distances of practical interest and, in principle, it could be applied over large distances in space.

Intense research into the feasibility of using a white LED-based space lighting system for indoor LAN communications is being undertaken. These systems present advantages over traditional UHF RF-based systems from improved isolation between systems, the size and cost of receivers/transmitters, RF licensing laws and by combining space lighting and communication into the same system. A low-cost white LED (GaN-phospor) which could be used for space lighting can typically be modulated up to 20 MHz. Data rates of over 100Mbps can be easily achieved using efficient modulation schemes and Siemens claim to have achieved over 500Mbps. Some research has been undertaken into using a similar system for futuristic traffic control of automated vehicles with LED traffic lights.

June 25, 2011

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