Fig. 1. Diagrammatic sketch of experiment setup. Laser, fs laser at a central wavelength of 810 nm with pulse duration of ∼140fs and repetition rate of 80 MHz; PBS, polarizing beam splitter; SLM, spatial light modulator; L11, L12, L21 and L22, lenses with the same focal length of 100 mm; AL1 and AL2, aspheric lenses; SFS, spatial filtering system; SMF, single-mode fiber; FMF, few-mode fiber; CCD, charge coupled device. Insets (a) and (b) show the refractive-index difference profiles of the G652B and OFS-1550TMF fibers, respectively.

Fig. 2. Experimental results of right-handed circularly polarized vortex beams with OAM passing through the G652B fibers with different lengths. (a) From top to bottom, it depicts the holograms loaded on the SLM (from left to right depicts input OAM=−1,0,1), generated mode intensity profiles after passing the 4f system, intensity profiles output from 15, 150, and 1500 m optical fiber, respectively. Then, (b), (c), and (d) are projection measurements corresponding to the last three rows of (a), respectively.

Fig. 3. Experimental results of light with OAM passing through OFS-1550TMF optical fibers of 150 m length. The graph is divided into two parts: (a) corresponds to OAM=0, 1, 2, 3, 4, and 5, and (b) corresponds to OAM=0,−1,−2,−3,−4, and −5. Each part is divided into three rows, corresponding to grating, and the intensity distribution of incident light and emitted light, respectively.

Fig. 4. OAM spectra of the output beams from the OFS-1550TMF fiber and the encoding results. (a) shows the results of projection measurement for the input beams with OAM=0,±1,±2,±3,±4, and ±5. If OAM=0, 1, 2, 3, 4, 5 are used for encoding, the results are shown in (b), and if OAM=0,−1,−2,−3,−4,−5 are used for encoding, the results are shown in (c).