All-Fiber Cylindrical Vector Beams Multiplexing Through a Mode-Selective Coupler

In order to improve channel capacity of a communication system, a novel concept of all-fiber cylindrical vector beams (CVB) multiplexing through a mode-selective coupler (MSC) is proposed. According to the proposed concept, a step-index few-mode fiber (FMF) is also designed and fabricated for supporting the first-order CVBs (TE01 and TM01) propagation. In addition, for generating and multiplexing the mutually orthogonal TE01 and TM01, a novel mode-selective coupler with two inputs and one output is also proposed and manufactured by the pre-tapering technology, which can be considered to be the superposition of two independent MSCs with one input and one output. The corresponding theory analysis and simulation with respect to the MSC are performed to prove the validity of the proposed concept in principle. To verify the feasibility of the proposed concept in practice, a novel experimental setup is also proposed and established. The measured result agrees well with the simulation on the whole. Bit error rate (BER) and constellation graph (CG) are employed to evaluate the system performances of the proposed concept. The measured results demonstrate that the BER performance of TM01 (TE01) decreases with the increase of FMF length and data rate, respectively. In addition, compared to the case of only TM01 (TE01) propagation, the measured system performances seriously decline when TM01 and TE01 are multiplexed. The received power distributions demonstrate that the crosstalk among first-order vector modes increases with the increase of FMF length when TM01 (TE01) is excited and propagated. Some advanced technologies [with respect to fibers, fabrication, (de)coding, etc.] are also discussed, which can be employed to improve the system performance in the future.