Temperature sensitivity of a fibre optic timing link

Three single mode 1300 nm fibre optic links are used for a time and frequency transfer between two time laboratories equipped with cesium clocks. These laboratories determine the Czech National Time and Frequency Standard. Out of the total length of 10 km the main part (about 9 km) of the optic cable is laid in a communication cableduct whose depth varies between 2 and 3 m. A smaller part (about 800 m) is buried about 0.5 m underground. The results of routine daily phase time measurements carried out at both ends during nearly 2 years were used to determine the group delay variations of the timing link due to the temperature sensitivity of the fibre. It has been found that the group delay exhibits seasonal variations strongly correlated with the temperature changes in the cableduct effective depth. As a result, the timing links cause seasonal peak to peak group delay variations of 10 ns and corresponding long-term frequency offsets of the order of 1×10 ^-15. To correct for the degradation of the time and frequency transfer, the underground temperature variations induced by the mean daily outdoor temperature were modeled for different depths. Using the standard estimation method, the effective depth of the cable was found. The group delay instability corrected for this effect was reduced down to the background white noise PM