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One important challenge to implementation of efficient free- space optical (FSO) systems is optical signal scintillation and fade caused by atmospheric turbulence and optical aberration in output beam shaping devices and windows. A new method for mitigation of these harmful effects to delivery of optical radiation to remote subscriber terminals, such as aberration and refraction index non-uniformity in a free- space path, has been developed and tested in field experiments. A known approach to damping optical signal scintillation caused by turbulence in a free-space path was based on forming several substantially parallel optical beams modulated by the same transmit signal and overlapping such beams on a receive optical aperture. The beams transmitted through different free-space paths with uncorrelated optical inhomogenity have different, uncorrelated, transverse distribution of light intensity. Their overlapping provides for averaging out the light intensity non-uniformity and efficient suppression of the signal scintillation. The existing approach to mitigation of optical aberration in atmosphere requires targeting several beam shaping telescopes at a subscriber. This is not always practical. For example, in point-multipoint FSO systems servicing multiple subscribers it is advisable to allocate one telescope per subscriber to achieve highest compactness and cost effectiveness of a system. Also the existing method has limitations in solving a problem of window glass optical inhomogenity and aberration in the telescope itself. A new method for optical aberration mitigation is based on using an extended light source with sufficiently large emitting surface and properly selected width of output radiation angular spectrum coupled to the telescope targeted at a subscriber terminal. The method has been implemented in a point-multipoint base terminal having multiple output beams that could be independently targeted at different subscriber terminals. Results of the trial are presented in this paper. The extended source with given light emitting surface diameter d and angular spectrum width (Theta) may be implemented with an optical fiber having core diameter d and numerical aperture NA equals sin((Theta) /2) installed in optical path between a light source with compact light emitting surface, such as a semiconductor laser, and the telescope. Exit end of such fiber coupled to the telescope acts as an extended light source with angular size (alpha) determined by the fiber core diameter and a focal length of the telescope via a formula (alpha) equals d/f. It has been proven in our field experiments, that by using the source with properly selected angular size and angular spectrum width the following results may be achieved with single telescope targeted at a subscriber terminal: (a) damping of optical signal scintillation at a remote photo-detector (the signal standard deviation has been decreased by several times for wide range of scintillation indexes); and (b) elimination of the signal fade caused by aberration in the telescope and output window (in our experiments the extended source provided 5 to 30 times increase in average signal power at the photodetector for a variety of window glass samples used in residential construction).
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