Optical time domain reflectometry for hollow core optical fibres

A method of assessing an optical system comprising a hollow core optical fibre comprises providing an optical time domain reflectometry system comprising: an optical source 24 configured to generate optical pulses with wavelength λ; an optical detector 29 configured to detect light at wavelength λ; and an input/output fibre 22 comprising a solid core optical fibre optically coupled at a proximal end to receive optical pulses from the optical source 24 and deliver light to the optical detector 29, and having at its distal end 22a an end facet with an applied treatment configured to suppress back-reflection of light at wavelength λ caused at an interface of glass forming the core of the solid core optical fibre and air at the end facet; aligning the distal end of the input/output fibre with a proximal end 30a of a hollow core optical fibre 30 having gas present in the hollow core, for optical transmission between the input/output fibre and the hollow core optical fibre; operating the optical source to generate optical pulses for propagation along the input/output fibre and into the hollow core fibre; receiving backscattered light produced by Rayleigh scattering of the optical pulses from the gas in the hollow core of the hollow core fibre and detecting the backscattered light with the optical detector to generate a detected signal; and processing (processor 25) the detected signal to create an optical time domain reflectometry profile 26 comprising a distribution of backscattered optical power along a length of hollow core optical fibre.