Thermal Phase Sensitivity Reduction in Hollow-Core Photonic Bandgap Fibers Enabled by a Stress-Buffering Coating Architecture

Low thermal sensitivity of the optical propagation phase in optical fiber is critical for high-precision fiber systems, including applications such as ultra-low noise lasers, highly sensitive interferometric sensing, and time-frequency transfer. Although Hollow-Core Photonic Bandgap Fibers reduce the thermo-optic noise compared to conventional single-mode fibers (e.g., SMF-28), their ultimate performance is constrained by the polymeric coating. Thus, minimizing the coating’s mechanical influence on the fiber is crucial for enhancing thermal performance. This study investigates strain transfer between the bare fiber and outer coating layer, as well as the resulting impact on thermal sensitivity, using numerical simulations and delay interferometry experiments. We compare thermal sensitivity of fibers with a high-modulus coating to those featuring a buffered low-modulus architecture. Experimental results show that the optimized fiber achieves a sensitivity of 3.3 rad/m/°C, representing a twofold improvement over single-layer coatings with identical standard coating dimensions. This result shows good agreement with simulations. By balancing thermal stability with engineering-grade coating thickness, this work provides optical fibers with both low thermal phase sensitivity and mechanical robustness.