Internal Gas Composition and Pressure in As-drawn Hollow Core Optical Fibers

We present a study into the gas pressure and composition within a hollow core optical fiber immediately after fabrication. Results from three different experimental techniques indicate that the initial absolute pressure inside of the hollow core is significantly lower than atmospheric pressure. By measuring the equilibrium height to which water ingresses into the hollow core, we estimate the absolute internal gas pressure to be 20 kPa < ${{\boldsymbol{P}}_{\boldsymbol{i}}}$ < 29 kPa for the fibers reported here that were fabricated using standard techniques. The initial gas composition within the hollow core was studied using Raman and absorption spectroscopy and the evolution of the gas composition provides indirect information about the condition of the silica surfaces inside the fiber. The measurements indicate that these internal surfaces become saturated with atmospheric water vapor as this is drawn into the open-ended fiber, initially due to a pressure gradient post-fabrication. Our findings are an essential foundation for the study of long-term optical and mechanical performance of hollow core fibers and important for accurate characterization of these specialty fibers. The first is becoming increasingly important as commercial applications of these fibers expand.