Comparison and evaluation of laser beam shaping techniques

Abstract

Over the past several decades, free-space optical (FSO) systems have gained a specific place in the wireless technology area. The application of these systems is advantageous for high bandwidths, a license free band and quick installation. The main drawback of FSO systems is their dependence on the state of the atmosphere causing deterioration of the FSO systems availability. One of the atmospheric effects which has an essential impact on the performance of the FSO systems is atmospheric turbulence. Atmospheric turbulence leads to fluctuation of the optical intensity in the plane of the receiving aperture. It has been shown that to reduce the effect of atmospheric turbulence, uniform distribution of the optical intensity within the cross section of the beam in the plane of transmitting aperture (phenomenon of diffraction is neglected) and a suffciently large diameter of the circularly symmetric receiving aperture (to achieve aperture averaging effect) are needed. The main idea of our paper is the problem of beam shaping at the transmitter. In our contribution the technique of transformation of a Gaussian beam into a beam with uniform distribution of optical intensity is discussed. For the mentioned transformation we experimentally tested several shaping methods such as multi aperture beam integrators, diffractive diffusers, etc. Usage of laser sources with different degrees of coherence was considered. The purpose of these techniques is to create an optical beam with uniform distribution of optical intensity on the transmitter output. In order to compare and evaluate the particular shaping techniques, a new Trans- formation Complex Quality (TCQ) parameter was defined. The TCQ parameter indicates the optimal shaping technique and also evaluates the quality of the resulting transformed beam with respect to its resistance towards atmospheric turbulence.