Reducing the effects of weak atmospheric marine turbulence on scintillation by using different incident fields and adaptive optics mitigation techniqes

In this study, reducing the effects of weak atmospheric marine turbulence on the optical communication systems and improving the performance of optical communication links are examined. The first examining technique is to use different incidence fields and the second one is to apply adaptive optics mitigation. In the first technique, formulation of the on-axis scintillation index of a flat-topped Gaussian beam is derived by using the Rytov method and the intensity is also taken to be log-normal distribution. That is, average bit error rate for the log-normal distribution is also evaluated. The scintillation index and average bit error rate with respect to changes in propagation distance, wavelength, beam size, and average signal to noise ratio are exhibited. Our results showed that small advantage can be achieved in weak atmospheric marine turbulence when focal length equals to propagation distance and the order of flatness has a small value. The on-axis scintillation index for focused and collimated Gaussian beams in weak marine turbulence is formulated by using the Rytov method. Moreover average bit error rate is appreciated by using this formulation and the log-normal distributed intensity as well. The scintillation index and versus propagation distance and source size are demonstrated for the collimated and focused Gaussian beams, exhibited for wavelength, focal length, and average signal-to-noise ratio . The focused beams are more advantageous than collimated beams in an atmospheric marine medium. In addition to that, the beam spreading using the Rytov method in this medium is formulated. The beam spreading versus propagation distance, scintillation index, and average bit error rate are denoted for wavelength, source size, and average signal-to-noise ratio . Our results show that the effect of beam spreading is very clear in propagation distance greater than 3 km in the atmospheric marine medium. In the second technique, the formulation of an on-axis scintillation index with adaptive optics compensation derived by using Rytov method is represented in a weak marine medium. Our model covers weak marine turbulence, log-normal distribution, and on-off keying modulation over horizontal propagation paths. The on-axis scintillation index and the average bit error rate with a variation in propagation distances, receive aperture diameter, wavelength, beam size, and average signal-to-noise ratio (SNR) are indicated. Results point out that the bit-error rate becomes better by a few orders of magnitude. The adaptive optics is used to compensate for the scintillation. Low-order compensation is advantageous to reduce the scintillation in a marine medium and to improve the quality of the optical communication systems, especially in optical links less than 2 km. Keywords: Atmospheric propagation; laser beam propagation; atmospheric turbulence; marine turbulence; scintillation