Rotational frequency shift in cylindrical vector beam due to skew rays in few-mode optical fibers

Abstract

We report here the demonstration of rotational Doppler Effect (RDE) and measurement of rotational frequency shifts (RFS) in single-charge helical-phased cylindrical vector beams directly generated using a two-mode optical fiber. The vector-vortex beam with a shifted vortex core, generated by propagating the Gaussian laser beam as an offset-skew ray selectively excites both the fundamental and first low-order waveguide modes simultaneously in the two-mode optical fiber. Rotation frequency of the output beam around a shifted axis of the beam is measured as a function the analyzer rotation for changing handedness of the input circular polarization to demonstrate RDE in the directly excited cylindrical vector-vortex beams. Even small variations in the input launch conditions were found to dramatically alter the stability of the vortex beams and hence the demonstration of RDE.