ABSTRACT
When a hollow core fiber is drawn, the core and cladding holes within the internal cane geometry are pressurized with an inert gas to enable precise control over the internal microstructure of the fiber and counteract surface tension forces. Primarily by considering the temperature drop as the fiber passes through the furnace and the geometrical transformation of the internal microstructure from preform-to-fiber, we recently established that the gas pressure within the final 'as-drawn' fiber is substantially below atmospheric pressure. We have also established that slight changes in the gas refractive index within the core and surrounding cladding holes induced by changes in gas pressure are sufficient to significantly affect both the modality and loss of the fiber. Here we demonstrate, through both simulations and experimental measurements, that the combination of these effects leads to transient changes in the fiber's attenuation when the fibers are opened to atmosphere post-fabrication. It is important to account for this phenomenon for accurate fiber characterization, particularly when long lengths of fiber are drawn where it could take many weeks for every part of the internal microstructure to reach atmospheric pressure.
ABSTRACT
We demonstrate recent progress in the development of a Raman gas sensor using a single cladding ring anti-resonant hollow core micro-structured optical fiber (HC-ARF) and a low power pump source. The HC-ARF was designed specifically for low attenuation and wide bandwidth in the visible spectral region and provided low loss at both the pump wavelength (532â nm) and Stokes wavelengths up to a Raman shift of 5000â cm-1. A novel selective core pressurization scheme was also implemented to further reduce the confinement loss, improving the Raman signal enhancement by a factor of 1.9 compared to a standard fiber filling scheme. By exploiting longer lengths of fiber, direct detection of both methane and hydrogen at concentrations of 5 and 10â ppm respectively is demonstrated and a noise equivalent limit-of-detection of 0.15â ppm is calculated for methane.