Detection of vortex beams based on mutual interference

The mutual interference method is introduced to detect vortex beams. For the coaxial interference between l₁-order and l₂-order vortex beams, the intensity distribution is spirally fan-shaped. The partition number N=|l2-l1|, and the spiral direction can distinguish the sign of topological charge with larger absolute value between |l1| and |l2|. Fork-shaped fringes appear in the center for the small-angle interference between vortex beams with incident angle β1 and β2. The forking number between two fringes agrees with N=|l2-l1|, and the forks face upward when l₁<l₂ & β₁<β₂, or l₁<l₂ & β₁<β2, and face downward when l1<l2 & β1<β2, or l1<l2 & β1<β2. Especially, when one of the beams is a Gaussian beam, such as l₁=0, the value and sign of the topological charge l₂ of the other vortex beam can be simply detected. The mutual interference method can conveniently detect the value and sign of vortex beams without borrowing redundant devices.