Ultrahigh Transverse Mode Purity by Enhanced Modal Filtering in Double-Clad Single-Ring Hollow-Core Photonic Crystal Fiber

Maintaining high fundamental mode purity in low-loss single-ring hollow-core photonic crystal fiber remains a key challenge. Here a double-clad anti-resonant-reflecting design is reported that, through modal filtering, provides high suppression of higher-order modes while maintaining a low-loss fundamental mode. The structure consists of a ring of six thin-walled capillaries fused to the inner wall of a thicker-walled capillary that is in turn fused at a single point to the silica outer cladding. Greater than 40 dB m-1 suppression of higher order modes over a 6 nm wide window whose central wavelength can be tuned by adjusting the gas pressure is observed. The novel design paves the way to practical applications of such fibers in high-quality laser beam delivery and low-noise optical-signal transmission. In this work, a first-of-its-kind design of hollow-core antiresonant fiber with a double-clad structure is reported, providing unprecedented ability of high-order-mode suppression through enhanced mode filtering effects. The results represent a major step forward in the hollow-core fiber technology, providing new insight in fiber design and fabrication, laser-field fiber delivery, and optical fiber sensing. image