Fig. 1. Experimental device. (a) Diffraction device of LG beam; (b) interference device of spherical wave and diffracted beam

Fig. 2. Diagram of incident vortex beam and diffraction plate. (a) Intensity distribution of incident vortex beam; (b) Fermat spiral distribution with number of Fermat spirals m=6 and counterclockwise rotation; (c) Fermat spiral distribution with number of Fermat spirals m=6 and clockwise rotation; (d) Fermat spiral distribution with number of Fermat spirals m=8 and counterclockwise rotation; (e) Fermat spiral distribution with number of Fermat spirals m=8 and clockwise rotation

Fig. 3. Change process of diffracted beam after passing through diffraction plate with m=6 behind focal plane. (a1) Intensity distribution of whole beam calculated by simulation; (a2) intensity distribution of whole beam measured by experiment; (b1)--(b5) (e1)--(e5) intensity distribution calculated by simulation after LG beam passing through diffraction plate with counterclockwise rotation and clockwise rotation; (c1)--(c5) (f1)--(f5) phase distribution of calculated by simulation after LG beam passing through diffraction plate with counterclockwise rotation and clockwise rotation; (d1)--(d5) (g1)--(g5) intensity distribution measured by experiment after LG beam passing through diffraction plate with counterclockwise rotation and clockwise rotation

Fig. 4. Change process of diffracted beam after passing through diffraction plate with m=8 behind focal plane. (a) Intensity distribution of beam passing through counterclockwise diffraction plate; (b) intensity distribution of beam passing through clockwise diffraction plate

Fig. 5. Phase distribution of diffracted beam and its phase distribution and intensity distribution after interference with spherical wave. (a) Simulated phase distribution of diffracted beam passing through mirror 1; (b) simulated phase distribution of diffracted beam after interference with spherical wave; (c) experimental intensity distribution of diffracted beam after interference with spherical wave; (d) simulation results of OAM spectrum of diffracted beam