Fig. 1. (a) Mode group (MG) demultiplexing scheme based on diffraction optical network (DON). The input beam is regulated by several phase plates and cascade diffraction, and the outputs of the same MG are coupled to the same multimode fiber (MMF) channel. (b) Flowchart for optimizing the MG demultiplexing based on DON. (c) MG demultiplexing performance based on deterministic mode conversion (marked as fixed) and non-deterministic mode conversion (the marked six bases and 10 bases represent setting the number of mode bases of the MMF to 6 and 10, respectively) using different numbers of phase plates. IL_ave, average insertion loss; XT_ave, average inter-MG crosstalk.

Fig. 2. (P1)–(P5) Optimized five phase plates for the MG demultiplexing. (a1)–(a7) Superimposed intensity distribution in the demultiplexing process with input OAM channels l=−5  to  +5. (b1)–(b7) and (c1)–(c7) Complex field representations of the light in propagation process with input OAM channels l=−5 and l=+5, respectively. (d) MG transfer matrix at 1550 nm wavelength. (e) Broadband MG demultiplexing performance based on deterministic mode conversion and non-deterministic mode conversion [the meaning of the legend is consistent with Fig. 1(c)].

Fig. 3. Prepared phase plates and schematic diagram of experimental characterization. Above the picture are the local details of the prepared phase plates. These input OAM modes are stimulated by a spatial light modulator (SLM) and collimated from a 2 m few-mode fiber (FMF). The OAM modes of different MGs are demultiplexed to different locations after passing through the folded reflective cavity. At the receiving end, Setup 1 and Setup 2 are used for free space and fiber coupled detection, respectively. MLA, microlens array.

Fig. 4. (a) Collimated light field of different OAM modes after 2 m FMF. (b) Outputs corresponding to the input OAM modes. (c) MG demultiplexer with coupled MMF. (d) Detected transmission matrix. The relative power is the ratio of the energy of the output light of MMF to the energy of the input light from FMF.

Fig. 5. (a) MG demultiplexing loss under different numbers of mode groups and phase plates, with the number of phase plates ranging from 3 to 10 and the number of mode groups ranging from 3 to 12. (b1)–(b7) Superimposed intensity distribution in the demultiplexing process of 12 OAM MGs (|l|=0–11).