Optical absorption spectrum reveals gaseous chlorine in anti-resonant hollow core fibres

We have observed unexpected spectral attenuation of ultraviolet light in freshly drawn hollow core optical fibres. When the fibre ends are left open to atmosphere, this loss feature dissipates over time. The loss matches the absorption spectrum of gaseous (molecular) chlorine and, given enough time, the transmission spectrum of the fibre recovers to that expected from the morphological structure of the fibre. Our measurements indicate an initial chlorine concentration of 0.45 $\mu$ mol/cm${}^3$ in the hollow core, equivalent to 1.1 mol% Cl${}_2$ at atmospheric pressure.