Controlling the optical performance of a simply end-capped 4-tube hollow core fiber using selective gas pressure-controlled phase matching

The low loss light transmission and enhanced gas-light interaction length possible in a hollow core optical fiber (HCF) has attracted significant attention in the field of gas sensing and spectroscopy. Furthermore, independent control of the gas composition and/ or pressure within the core and/ or cladding of the HCF presents new possibilities for modifying the optical performance of a HCF. Here, we demonstrate a new end-capping approach to enable low loss selective gas filling into the core of a HCF. We use this novel end-cap to explore the impact of gas-induced differential refractive index (GDRI) by increasing the core gas pressure relative to the cladding gas pressure in an in-house fabricated 4-tube HCF. Experimental investigations of a fabricated end-capped 4-tube HCF gas cell, as well as theoretical simulations, show that the loss of the fiber changes dramatically due to via selective gas pressure-controlled phase matching. These results show that this approach offers flexibility in changing gas pressure in real time thus tuning control the optical properties of HCF for specific applications and to enable substantially improved optical transmission in HCF geometries that would otherwise be prohibitively lossy for practical use.