Fresnel Reflection of Optical Fibres at Cryogenic Temperature for Absolute Radiometric Measurements

We have measured the temperature dependent Fresnel reflection loss and Rayleigh backscatter of SMF-28 fibre, polarisation maintaining (PM) single-mode fibre and solid core high index PM photonic crystal fibre (PCF). We show that the effective refractive index Neff of these fibres reduces by 0.11 %, 0.15 % and 0.30 % respectively from room temperature to 5 K. This relates to an increase in fibre output power of 0.02 %, 0.03 % and 0.06 %. The Rayleigh backscatter is shown to increase 15x for the standard single mode fibre, and 4x for the photonic crystal fibre at low temperature. We have quantified these changes in order to apply a correction to our fibre-coupled primary standard cryogenic radiometer for optical fibre power measurements. We use an in-situ beam-splitter measurement technique to measure the Fresnel reflection at 1310 nm and 1550 nm and we confirm the results at 1550 nm with an optical frequency domain reflectometer measurement.