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 fibers. When the fiber 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 fiber recovers to that expected from the morphological structure of the fiber. Our measurements indicate an initial chlorine concentration of 0.45 µmol/cm 3 in the hollow core, equivalent to 1.1 mol% Cl 2 at atmospheric pressure.