Encoding and decoding by the states of vector modes for vortex beams propagating in air-core fiber

In order to enhance the channel capacity and spectrum efficiency, the technology of space division multiplexing (SDM) has become research hotspot in optical communications. In this paper, a new encoding/decoding concept is proposed by the states of vector modes (polarized direction, rotational direction of phase and topological charge) for vortex beam propagating. To support encoded vector modes propagating in fiber, an OAM fiber with air-core structure is designed for encoding/decoding. Meanwhile, a new mode recognition method of judging states of the received vector modes is presented by the technology of digital image processing and digital signal processing (DSP). To verify the feasibility of encoding/decoding, an experimental platform to verify that encoded 16-QAM signal (0010_1100_1110_1010) is established. The vector modes of OAM beams can be propagated in OAM fiber with the length of 80 cm, and the received signal can be decoded to 16-QAM by image processing successfully. In addition, we also evaluate and analyze the influence factor (OAM fiber length and bit rate) on the transmitting performance in terms of BER, crosstalk and constellation figures.