Fig. 1. Optical path diagram for the measurement using topological charge difference method

Fig. 2. Conjugate interference modes of vortex beams with OAM of (a) l=6, (b) l=4 and (c) their superposition

Fig. 3. Intensity superposition distribution of two vortex beams with topological charge difference

Fig. 4. Intensity distribution of vortex beams with a difference of n=2, (a) l to be measured, and (b) l = 6

Fig. 5. Measurement process for off-axis vortex beam using the topological charge difference method

Fig. 6. Intensity distributions of the difference method at different off-axis rates

Fig. 7. Intensity distribution of partially received vortex beams using the topological charge difference method