Hollow core optical fibres have many unique properties, especially compared to traditional glass-core optical fibres [1]. Firstly, the light path is accessible and light can thus interact with the gas inside over long lengths, making them interesting for applications in gas sensing or for nonlinear processes in gasses. Hollow core fibres can also operate at wavelengths, where silica glass has poor transmission and their chromatic dispersion is not compromised by the chromatic dispersion of bulk glass. Yet another unique feature is weak interaction of light with the guiding medium (typically air), significantly increasing the damage threshold and thus making them a good candidate for high-power (average or peak power) light delivery. Another group of unique features is related to how their properties (little) change with temperature.
In the presentation, we will firstly show where the common fibre optics wisdom (gained from work with standard optical fibres) tends to fail. In the second part, we will discuss how differently hollow core fibre change with temperature as compared to standard optical fibres and how it can be used for various applications, including fibre interferometry and time-stable signal transmission.
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.
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