Free-space laser communication using a partially coherent source beam

Free-space laser communication offers an attractive alternative for transferring high-bandwidth data when fiber optic cable is neither practical nor feasible. However, random fluctuations in the atmosphere's refractive index can severely degrade the signal-carrying laser beam, causing intensity fading at the receiver and increased system bit error rates. There is growing interest in developing techniques to overcome the turbulence-induced intensity fades that cause these bit error rates. It is shown here that using a partially coherent laser beam in a free-space laser communication system reduces system bit error rates by decreasing irradiance scintillations in the receiver focal plane.; A partially coherent laser beam can be easily created by the placement of a phase diffuser in front of the transmitting aperture. The properties of a partially coherent laser beam are studied using a derived analytic expression for the cross-spectral density function of a partially coherent, quasi-monochromatic Gaussian laser beam propagating through atmospheric turbulence. This expression allows for the focusing or diverging characteristics of the laser beam. The beam size, average intensity phase front radius of curvature, and coherence properties of the partially coherent signal-carrying laser beam at the receiver are derived from the cross-spectral density function.; The effect of receiver aperture averaging in reducing intensity scintillations for Gaussian beams of any degree of coherence is studied. An analytic expression is derived for the spatial covariance of irradiance fluctuations for a partially coherent Gaussian beam, subject to the restriction that the atmospheric turbulence-corrupted laser beam can be described as a Gaussian stochastic process. In the weak fluctuation regime this analytic expression is found to compare reasonably well with published data.; A model is described for a free-space laser communication system comprised of a laser transmitter with a phase diffuser, a lognormal atmospheric channel due to the presence of atmospheric turbulence, and a maximum likelihood receiver. This model offers a method for calculating optimal system performance with full consideration of the lognormal turbulent channel and source beam characteristics in the weak fluctuation regime. The impact of atmospheric turbulence strength and degree of source coherence on the average bit error rate are examined.