Air-hole-assisted ring-core fiber: a new platform for enhanced intermodal spatial overlap and nonlinearity

We introduces a novel application of a ring-core fiber (RCF) with a central air hole to significantly to enable highly efficient nonlinear parametric processes, with flexible signal and idler wavelength generation using standard pump lasers. In conventional step-index or small-core fibers, nonlinear processes such as FWM or parametric amplification are often constrained by poor spatial mode overlap, leading to low nonlinear efficiency. The proposed structure significantly enhances the efficiency of intermodal four-wave mixing (FWM) by improving the spatial mode overlap between the fundamental and higher-order modes, while simultaneously reducing the effective mode area of the modes. For a pump wavelength of 1550 nm, the proposed designs allow tunable idler and signal generation over wide spectral ranges of 992-1370 nm and 1783-3534 nm, respectively. This study introduces a new and efficient pathway to overcome the mode overlap limitations and achieving intermodal FWM high-efficiency through optimized fiber geometry, offering promising potential for entangled photon-pair sources and nonlinear photonic applications.