Bend-insensitive grapefruit-type holey ring-core fiber for weakly-coupled OAM mode division multiplexing transmission

We propose a bend-insensitive grapefruit-type holey ring-core fiber to support orbital angular momentum (OAM) modes. The large air domain in the cladding results in a large refractive index contrast between the ring-core and the cladding, which makes this fiber bend-insensitive and makes the vector eigenmodes well separated. Therefore, the proposed fiber can be adopted for the OAM mode division multiplexing transmission without the use of any multiple-input multiple-output digital signal processing (MIMO-DSP). It is also proved that the thickness of the outer silica layer around the ring-core plays an important role in confining the light. If the OAM modes in the ring-core and the cladding modes in the outer silica layer are not phase-matched, the OAM modes in the ring-core will not be coupled into the outer layer of silica and can be well confined. We fabricate such fiber in two steps by using the “stack-and-draw” method. Two kinds of fibers with different thicknesses of the outer silica layer are fabricated under different pressures and temperatures. The testing results of the fabricated fiber demonstrate the mode purity and illustrate that the loss of the OAM $+1,1$ and OAM $+2,1$ modes are 0.095 dB/m and 0.25 dB/m, respectively.