Usage and Technologies
Free-space point-to-point optical links can be implemented using infrared laser light, although low-data-rate communication over short distances is possible using LEDs. Infrared Data Association (IrDA) technology 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 using incoherent sources of light from high-intensity LEDs. One reported 173 miles (278 km) in 2007. However, physical limitations of the equipment used limited bandwidths to about 4 kHz. The high sensitivities required of the detector to cover such distances made the internal capacitance of the photodiode used a dominant factor in the high-impedance amplifier which followed it, thus naturally forming a low-pass filter with a cut-off frequency in the 4 kHz range.
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 distance records for optical communications involved detection and emission of laser light by space probes. A two-way distance record for communication was set 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.
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