Simulation of gas diffusion in hollow-core photonic crystal fiber

Hollow-Core Photonic Crystal Fibers (HC-PCFs) have aroused great interest in the field of fiber-based gas sensing, where high sensitivity, fast response, and distribution sensing are required. One of the most important factors for gas sensing is the response time that the gas fills the hollow core of the fiber. In order to achieve rapid response for gas sensing, various techniques have been investigated in the literature, such as lateral drilled side holes. Though the gas concentration along the fiber at a certain time can be expressed, it do not give a satisfied explanation to the mechanism of gas flow in HC-PCFs drilled with side holes, which is essential to determine the optimum design parameters of the fiber sensor, such as the size and spacing of drilled side holes. This study aims to analyze the gas flow behavior and determine the response time of HC-PCFs with drilled side holes by developing and applying a numerical model based on gas diffusion in a microchannel. The diffusion takes place at the atmospheric temperature and pressure. Both ends of the HC-PCF are spliced to single mode fibers, the side holes whose diameter is 10 μm are open to the air. The open side holes are embraced with gas species, the transient diffusion process between the air and the gas species. The model is validated against the results of two different experimental studies. The model is then applied to determine the response time as a function of the length, the number and spacing of side holes. It is found that an inverse relationship exists between the effects of number and spacing of side holes on the response time, suggesting that an optimum design point exists. The results obtained can be utilized for the fiber sensor design.