Modelling of pressure-driven gas flow in a nodeless Anti-Resonant Hollow Core Fiber for laser absorption spectroscopy

One of the most unique features of the hollow-core fibers (HCF) is the possibility to fill the fiber core with desirable gas, which can be useful, especially in laser absorption spectroscopy (LAS) of gases. Sensitivity of LAS-based gas sensors can be easily and efficiently enhanced by increasing the gas molecules-laser light interaction path within the sensor’s setup. In commonly used multi-pass cells-based sensors, the increase in optical path length comes at the cost of the increased sensor’s complexity, leading to the reduced detection capability. HCFs, that are long, flexible, and devoid of additional optics, allows for constructing gas absorption cells that give the possibility to design less-complex sensor's. One of the main problems in developing HCFs-based sensors is the filling time of the fiber core with the target gas. The core diameter of an Anti-Resonant Hollow Core Fiber (ARHCF) is typically on the order of tens of micrometers, leading to a low volumetric flow rate. It was reported in [1] that the time required to fill a 1 meter-long ARHCF with a core diameter of ~84 μm with the target gas using relatively low overpressure (800 Torr) is about 20 seconds. Additionally, the nodeless type of ARHCF is made of separated capillaries, forming its cladding ( Fig. 1a ). The velocity of gas flow in the gaps between the cladding capillaries is far lower than velocity in the core. We believe that this has a significant impact on the time of pressure-driven gas-filling of such fibers, hence it is necessary to perform simulations on more complex geometric models than for the gap-less ARHCFs, where the core can be approximated by a simple 2D channel.