Study on the Phenomenon of Double Dominant Modes in Vortex Fiber with Ring-Core Structure

The vortex fiber is a fiber that can generate and transmit orbital angular momentum (OAM) modes. The vector modes in the same mode group in the vortex fiber with ring-core structure are no longer degenerate. We find that when the refractive index of core much lower than the ring, the dominant mode will appear in both the ring (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$HE_{11}$</tex>) and the core (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$HE_{12}$</tex>). After making this kind of fiber into a Bragg grating, it is found that the dominant modes of the outer and inner sides will both produce strong reflection peaks. In addition, in the process of grating measurement, the reflection peak intensity of the inner dominant mode can be reduced by means of dislocation until it disappears. The phenomenon found in this paper has important application value in the field of mode filter and mode separation.