Information Capacity of an Optical Channel Through a Turbulent Atmosphere

Abstract

High temporal and spatial coherence property of laser radiation make it a very desirable source of communication providing high information capacity. This high degree of coherence is destroyed due to signal distortion produced by atmospheric turbulence. A laser beam passing through atmosphere suffers random beam scanning, random wavefront tilt, random phase, frequency amplitude and polarization fluctuations. Further attenuation, scattering, precipitation and the presence of some gases produce deleterious effect on channel capacity. A new parameter ‘Turbulence Factor’ is introduced to account all the atmospheric effects which defers the signal-to-noise ratio at detector end.

Considering the effect of random beam scanning and random phase front tilt only, an analytical expression for turbulence factor is derived. The channel capacity expression in the presence of atmospheric turbulence is also given.

Computed results of turbulence factor, for different degrees of turbulence and various signal-to-local oscillator beam spot diameters are reported. The modified channel capacity at frequencies ν=10¹⁴Hz, and ν=10¹⁵Hz and signal powers P_s=10⁻⁶ watts and P_s = 10⁻⁸ watts, is computed and plotted for various degrees of turbulence.