Temperature-Tunable Directional Coupler for Multiple OAM Modes Based on a Selectively Liquid-Filled Dual-Ring-Core Photonic Crystal Fiber

A temperature-tunable directional coupler is proposed and numerically investigated for power transfer of orbital angular momentum (OAM) modes between two ring cores. Selective infiltration of a thermosensitive liquid into the air holes of a dual-ring-core photonic crystal fiber provides thermal control of the coupling lengths of OAM states synthesised from the HE21, HE31, and EH11 mode families. At a fixed device length of 42 mm, the coupling efficiency and output mode purity remain above 94% and 90%, respectively, across all investigated states and wavelengths from 1520 to 1580 nm, with the temperature adjusted for each state and wavelength. The proposed design supports tunable, mode-preserving transfer of multiple OAM states at a fixed output plane, with potential applications in OAM-based fiber systems.