U.S. Pat. No. 5,301,051 by Geller discloses a covert communication system that uses ultraviolet light as a medium for communication. Suitable wavelengths are chosen by examining atmospheric penetration, attenuation by clouds, presence of interfering sources, and ease of generation and detection.
It is well known that atmospheric gases such as ozone and oxygen strongly absorb light in the spectral range between 200 and 280 nm. It is called “solar blind” region of spectrum. It is beneficial to create a free-space communication link operating in this range since solar radiation will not interfere with the data transmission. Non-line of sight communication is based on the light scattering in atmosphere and detecting of at least some portion of the scattered light. Raleigh theory indicates a strong wavelength dependence of the scattering (˜λ−4) which means that blue light is scattered much more than red light. It is advantageous to use blue or UV light in NLOS communications since more light can be collected.
An optical communications transceiver of U.S. Pat. No. 6,137,609 comprises a transmitter that sends out the same information simultaneously in two channels with different wavelengths and a receiver for detecting and comparing the received data. Additional reliability of the communications is achieved by the transmission doubling.
Traditionally photomultipliers are used for UV light detection. Recently developed low noise high sensitive avalanche photodiodes are compatible with the photomultiplier in their characteristics while providing setup compactness. US patent application No. 20050098844, which addresses manufacturing of such detectors, is incorporated herein by reference.
There is still a need for improved communications system architecture to enhance detector sensitivity, information capacity and overall system reliability of non-line of sight UV optical communications.