A compact beamformer of diversity multiplexing of multiple modes by utilizing few-mode long-period fiber gratings

Abstract

In this paper, a few-mode true delay beamforming network with two mode combinations is proposed to obtain continuously adjustable time delay. The excitation of LP modes is realized by using few-mode long-period fiber gratings (LPFGs), and the time delay differences between LP modes are controlled by two 2 × 2 optical switches. The system integrates three channels into one, and then demultiplexes LP modes through a mode selective photonic lantern. At first, the relationships of the distance between few-mode LPFGs are derived so that the beamformer can be rationally designed by them. Then, the amplitude errors of the antenna elements, radio-frequency gain and noise figure of each channel of the system affected by the conversion rates of few-mode LPFGs and the LP mode coupling of few-mode fibers and mode selective photonic lantern are also given. Finally, the experimental measurements show that variable optical attenuators can effectively eliminate the above effects, which further proves the feasibility of the beamformer.