The collimation of the Bessel Gauss beams using a composite optical system consisting of multiple pairs of Cassegrain antennas

Collimation of Bessel Gauss beams has always been a problem and has hindered its application in free space optical communication. In this work, the Bessel Gauss beam can have a diffraction-free distance of 10,000 km with the aid of a new composite optical system. The optical system consists of several pairs of Cassegrain antennas, each pair of which allows the divergence angle of the beam to be compressed to one-tenth of its original size. Bessel Gauss beams with a topological charge of one to five can achieve an efficiency of 81.05%, 88.68%, 88.68%, 85.75%, 80.12% when the wavelength of beams is 860 nm, after 10,000 km propagation in the composite optical system. At this time, the corresponding divergence angles of the first to fifth order beams are 0.118 μrad, 0.141 μrad, 0.169 μrad, 0.184 μrad, 0.192 μrad, respectively. We give a relationship between the ability to compress the angle of divergence of the system and the number of primary and secondary mirror pairs contained in the antenna and the focal length ratio of primary and secondary mirrors, which is verified by calculations that guide the design of the new system. It is possible to design an antenna suitable for different application scenarios by changing the number of primary and secondary mirror pairs and the focal length ratio of the primary and secondary mirrors.