Mode coupling effects in vortex mode generation using a micro structure phase plate.

A micro structure spiral phase plate (SPP) which is used to generate vortex modes is proposed. The SPP which is designed to be established on a multimode fiber end facet is composed of several groups of fan-shaped glass and air. The effective refractive index changes along the angular direction by changing the ratio of glass to air, and different topological charges are realized by controlling the groove depth. The coupling and propagation processes of Gaussian beams transmitting through the SPP of different topological charges and subsequently into multimode fibers is studied by finite difference time domain (FDTD) simulations. The results show that the Gaussian beam is transformed into several vortex modes after passing through the phase plate, which follows the mode selection rule, that is, the polarization form remains unchanged, the topological charge is equal to that of the SSP, and the root number of the mode is limited to that of the cut-off mode of the fiber. Increase of the absolute value of topological charge leads to the increase of mode size which results in the decrease of the total transmittance. [ABSTRACT FROM AUTHOR]