Study on the phenomenon of double dominant modes in a ring-core structure fiber

The fiber with ring-core structure (or called vortex fiber) is mainly used for transmitting orbital angular momentum (OAM) modes. When the refractive index of the core is 10^-2 lower than that of the ring, a phenomenon that we named double dominant modes will appear. The double dominant modes means that one common dominant mode HE₁₁ exists in the ring area, and the other one dominant mode HE₁₂ exists in the core area (can be regard as the quasi-fundamental mode here) of the fiber. The mode effective index difference between HE₁₁ and HE₁₂ is very large and can reach near 1 × 10^-2. Through measuring the fiber Bragg grating (FBG), it can be found that the two dominant modes produce strong self-coupling reflection peaks without cross coupling reflection. These performances are different from common two-mode fibers.