Research on a New Type of Ring-core Fiber and Mode Selective Coupler

【Objective】Mode-Selected Coupler (MSC) is a key component of Mode Division Multiplexing (MDM) for optical fiber communication system. In a MDM system, MSC can realize the transmission of different signals in different modes, thus increasing the transmission capacity. However, when fabricating a MSC with low insertion loss, the fabrication difficulty of the MSC is heightened due to the very close propagation constants between the modes. Therefore, there is an urgent need for a Few-Mode Fiber (FMF) with easily separable modes to serve as a key material for the fabrication of MSCs, thereby reducing the fabrication difficulty.【Methods】In this letter, a new type of ring-core FMF is proposed and fabricated. The FMF supports three linearly polarized modes LP₀₁, LP₁₁ and LP₂₁ in the C-band for fiber communication. Based on the theory of mode coupling, the article analyzes the impact of various parameters of the MSC on mode coupling and studies the coupling patterns between modes as influenced by fiber core spacing and coupling distance.【Results】The fused biconical tapering method is used to fabricate the MSCs of LP₀₁, LP₁₁ and LP₂₁. The experimental results show that the insertion losses of the MSCs of LP₀₁, LP₁₁ and LP₂₁ at 1 540, 1 530 and 1 550 nm are 0.10, 0.17 and 0.30 dB, respectively. The mode conversion efficiencies are 97.72%, 96.16% and 93.32%, respectively. The mode conversion bandwidths of LP₀₁, LP₁₁ and LP₂₁ are 125, 120 and 35 nm, respectively.【Conclusion】This research provides a new type of ring-core FMF, which can reduce the fabrication difficulty of MSCs. The low-loss MSCs have great potential application for MDM optical fiber communication system.