All-Optical-Fiber Orbital Angular Momentum Mode Generator With a Helical Phase Disk Inserted Between Fibers

An all-optical-fiber orbital angular momentum (OAM) mode generator is designed based on the effective medium theory. The generator is formed by many coaxial vortex structures with the same height and gradually decreasing width following the azimuth angle, which causes the linear decrement of the effective refractive index with the azimuth angle. The different effective refractive index will cause the different optical path and the corresponding phase delay along the azimuthal angle. The OAM generation process is simulated using a finite-difference time-domain (FDTD) software package.