Propagation time through standard optical fibres changes with temperature at a rate of 40 ps/km/K. This can pose significant challenges in many diverse application areas of optical fibres in physics and engineering. Primary examples lie in applications in which very precise timing signals need to be disseminated for synchronization purposes in large experimental infrastructures such as synchrotrons, linear particle accelerators, large telescope arrays, and in phase arrayed antennae. A value of 40 ps/km/K equates to a phase temperature sensitivity of about 48 rad/m/K. This can adversely affect many applications relying on fibre interferometers (e.g. fibre optic sensors, quantum-optics, interferometric measurement techniques, and so on), in which maintaining stable interference would require temperature stabilization below mK level. Similarly, a few key optical metrology applications require the dissemination of optical signals at a precise frequency, for example to compare distant ultra-precise clocks (e.g., national standard clocks) with a precision (fractional stability) at/below the 10-18 level. Such a level of precision is easily compromised by thermally-induced changes in optical path length (temperature drift) with time that unavoidably result in a Doppler frequency shift.
Here, we review our recent results in which we show why and how Hollow-Core Fibres (HCF) are significantly better than solid-core fibres in terms of their sensitivity of propagation time and accumulated phase change to temperature and thus are a better alternative to standard fibres in the above-mentioned fields.