High-order optical vortex generation in a few-mode fiber via cascaded acoustically driven vector mode conversion.

We propose a method to generate the high-order optical vortex in a few-mode fiber via cascaded acoustically driven vector mode conversion. Theoretical analysis showed that the vector mode conversion induced by the acoustically induced fiber grating (AIFG) could occur between two HE modes with adjacent azimuthal numbers. In the experiment conducted at 532 nm, two AIFGs were simultaneously induced in the same segment of the fiber by a radio frequency source containing two different frequency components. One AIFG was used to convert the left- and right-handed circular polarization fundamental modes to the ±1-order vortex modes, which were then further converted to the ±2-order vortex modes by the other AIFG. The topological charges of the vortex modes were verified using both coaxial and off-axial interference methods, showing typical signature patterns of spiral forms and forklike fringes, respectively.