Performance Simulation of Hollow-Core Photonic Crystal Fibre as a Gas Sensor for Industry Applications

Gas sensing systems form a crucial component of monitoring systems in many industrial and environmental applications. In the context of underground mining, effective monitoring of toxic and explosive gases such as methane, nitrogen dioxide, carbon monoxide and hydrogen sulphide is essential. Current monitoring systems in underground mines are subject to a number of limitations such as cross-sensitivity, limited lifetime, high maintenance cost, regular calibration, long response time and installation complexity. Conversely, fibre-optic based gas sensing systems have some advantages over conventional gas sensors as they can be used for long-distance and distributed gas measurements. Both are important in underground mining applications. Using their specific design characteristics, Hollow‑Core Photonic Crystal Fibres (HC‑PCFs) have the potential to add significant benefits to the mining industry and to be considered as the future fibre-optic based gas sensors.Short response time is typically favourable for gas sensors, where the response time of HC‑PCFs depends on the time of gas filling and evacuation into and from the fibre core. In order to allow the gas to enter the core, various techniques such as lateral drilled side-holes and pressure-driven gas flow have been investigated in the literature. Firstly, it is essential to understand the mechanisms of gas flow in HC-PCFs with drilled side-holes in order to determine the optimum design parameters, such as the size and spacing of drilled side-holes. Secondly, the characteristics of gas flow in long HC-PCFs is critical where it is essential for sensing low-concentration gases. Very few studies have focused on the modelling of the gas flow in HC‑PCFs, and these studies do not cover the main aspects of the gas filling process such as gas flow through drilled side-holes and pressure-driven gas flow for low concentration gases. In addition, no comparison has been made between the effect of design parameters (such as core diameter) on the optical and gas flow properties. This study aims to analyse the gas flow behaviour and determine the response time of non‑drilled and drilled side‑hole HC-PCFs by developing and applying numerical models.A diffusion-physisorption phenomenon based mathematical model has been developed to understand and predict the gas flow dynamics in a HC-PCF. A numerical model has also been developed to analyse the drilled HC‑PCF, based on gas diffusion equations and validated against experimental studies from the literature. Another gas flow model was then developed to analyse pressure-driven gas flow in the longer length HC-PCF, based on the Navier-Stokes equations with gas diffusion equations. This model has been validated against experiments of continuous-wave modulated photothermal spectroscopy. Finally, the light propagation characteristics have been numerically analysed to determine the effective index, the confinement loss, the effective mode area and the relative sensitivity for different HC-PCF structures. The optical properties and response time have been compared to optimize the design parameters of HC-PCF.  The results of this work show the change in gas concentration over time along the length of the core. For drilled HC-PCF, an inverse relationship between the effect of number and spacing of side-holes on the response time and the optical loss has been found, suggesting the existence of an optimum design point. For pressure-driven gas flow, a lower gas filling time has been achieved as the pressure difference between the inlet and outlet increased, the core diameter increased, and/or the core length decreased. The developed numerical model is well capable of providing some valuable insights about cross-sectional velocity profiles and gas flow rates that cannot be readily obtained from typical analytical models. The mode analysis of different core diameter HC-PCFs suggests that the small HC-PCF is better in terms of lower confinement loss and higher sensitivity.