We study how to control the orbital angular momentum, the intensity distribution, and the coherence length of a laser beam based on fiber array coherent combining. The features of the formation of nonzero orbital angular momentum of the beams when the controls the piston phase shift of the fiber array were theoretically and experimentally studied. A new method for controlling the distribution of the intensity of a light field synthesized on the basis of fiber array coherent combining were proposed. The possibilities of controlling the degree of spatial coherence of the synthesized laser beam were studied theoretically and experimentally.
The intermediate results of the experiment on vortex laser beam synthesis based on coherent combining laser subbeams formed by a fiber collimators array are presented. The wave field results from interference of individual subbeams in this case and is determined by the phase relationships between them. A functional diagram of the experimental setup is given. The experimental results of synthesis of a laser beam with an orbital angular momentum are described. It is suggested to use a spiral phase plate to determine the sign of the topological charge of an optical vortex; the efficiency of this approach is estimated.
The generation of optical beams with a possibility of quick variations in the orbital angular momentum (OAM) and the degree of spatial coherence is shown in laboratory experiments. The methods for OAM and coherence control are based on the phase control in the fiber array optical channels. The approach suggested allows one to change the OAM (the topological charge of a vortex beam) with a high speed determined by the phase shifter operation speed. The generation of a vortex beam is shown for six coherent Gauss-like beams arranged in a circle and having a constant phase shift between neighboring beams, providing the total phase shift equal to 2π around the circle. It is shown that the far field is characterized by an annular intensity distribution and a spiral-like distribution of the Poynting vector. In addition, the features of the OAM and the topological charge of a fiber-array-based vortex beam in a homogeneous medium are investigated in numerical experiments. The method for controlling the length of spatial coherence of the beam synthesized is based on introducing pseudo-random phase fluctuations in a fiber array. The value of the coherence length which exceeds the subbeam size is set by the correlation function of pseudo-random phase fluctuations in neighboring subbeams. The value of the coherence length smaller than beamlet size is set by the divergence of the subbeams with delta correlated phase fluctuations. The effect of the number of pseudo-random realizations of the fiber array phase on the average intensity distribution is studied in the laboratory experiments. The influence of the spatial coherence of the laser beam on the bit error rate of FSO communication systems in a turbulent atmosphere is studied theoretically.