Investigation of statistical characteristics of partially coherent laser beams for application in optical communication systems

Electro-magnetic waves propagation in atmosphere is of great theoretical and practical interest because using laser radiation for information transmission through the earth's atmosphere has many obvious advantages over traditional communication methods. However, fluctuations of the refractive index in the atmosphere, caused by turbulence, lead to the optical field fragmentation in the detecting area and to the beam wondering in regards to the initial direction. These factors bring high error rate during signal decoding and are major obstacles for the high-speed long distance optical communications. Therefore, development of methods for suppression of laser beam intensity fluctuations (scintillation) in turbulent atmosphere is extremely important, because so far the problem of high-speed information transmission in the turbulent atmosphere is not solved. Fundamental study of speckle fields and optical vortex fields spatial dynamics in conditions of refractive index fluctuations depending on the statistical characteristics of random atmosphere states is necessary, and it is one of the general problems of today’s optics.

So, the present investigation analyzes reasons of the atmospheric optical communications instability, by results of the numerical experiments it suggests methods for the improvement of laser beam statistical characteristics for data transmission in the turbulent atmosphere at distances about tens of kilometers in conditions of weak and strong turbulence.

It is found that in order to suppress the scintillation index it is necessary to compensate beam wandering and to ensure statistical independence of signals. These requirements are implemented by using a set of partially coherent beams (PCBs) with definite initial directions of propagation relative to the z-axis. A novel method to generate the PCBs by combining two laser beams - Gaussian and vortex beams, with different frequencies, is proposed and, according to our numerical studies, this approach is able to efficiently suppress scintillation index in the turbulent atmosphere, and also eliminates the need in high-frequency phase modulator.

The results of present investigation give theoretical basis for achieving gigabit data-rates in long-distance laser communication through turbulent atmospheres over distances of tens of kilometers. The methods of signal protection and spectral coding of optical channel are proposed.