Air flowing induced thermo-optic effect for thermal sensitivity reduction in anti-resonant hollow core fibers

The signal propagation delay through an optical fiber changes with environmental temperature, imposing a fundamental limit on performances in many fiber-optic applications. It has been shown that the thermal coefficient of delay (TCD) in hollow core fibers (HCFs) can be 20 times lower than in standard single-mode fibers (SSMFs). To further reduce TCD over a broad wavelength range at room temperature, so that to enrich fiber-optic applications in time- synchronization scenarios, the thermal expansion effect of silica glass must be compensated for. Exploiting the thermo-optic effect of air inside an anti-resonant hollow core fiber (ARF) can be a feasible solution. Nevertheless, an accurate description of the air flow in the course of temperature variation is highly needed to predict the influence of this effect. This work develops an analytical model for quantitatively calculating this temperature-induced air-flowing effect. Across a range of parameters of core diameter, fiber length, and temperature change rate, the experimentally measured propagation delay changes agree well with our model. The resultant low thermal sensitivity is also validated in non-steady conditions and in a practically usable SSMF-ARF-SSMF chain. Our model indicates that a >40-fold TCD reduction relative to SSMFs can be realized in a 60-m-long, 50-µm-diameter ARF, and further TCD reduction should be possible by properly engineering the gas type and the ambient pressure.