There is increasing interest in free space optical communications as an alternative to fibre optics and radio frequency communications, particularly in 'last mile' applications and applications with weight and power restrictions e.g. communications with unmanned aerial vehicles. The potential advantages of free space optical communications include: high bandwidth; no licensing issues; smaller, lighter payloads; low probability of intercept; and immunity from interference /jamming. However, propagation through the atmosphere is subject to atmospheric scintillation noise affecting the signal-to-noise ratio (SNR), effectively reducing the range and bandwidth of the communication link. This scintillation is experienced even over relatively short propagation paths and is caused by small temperature variations in the atmosphere, resulting in index of refraction changes. In this paper we present a technique to correct for atmospheric scintillation noise in free space optical communications and laser remote sensing. It uses common mode rejection to remove co-channel noise, where each channel is transmitted on separate, but closely spaced, wavelengths. The signal-to-noise ratio is significantly increased, thereby increasing the range and/or bandwidth of the link. To date, tests have been conducted with analogue audio and video transmissions. This has been successful, with improvements of up to 12dB in SNR having been demonstrated. This has been limited by the current implementation, which is only at prototype stage - the ultimate achievable improvement in SNR is anticipated to be significantly higher.
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