Characterisation of hollow core fibres

Abstract

For conventional optical fibres such as single mode fibres (SMFs), the development of characterisation techniques has played a crucial role in their success. This is because characterisation contributes in various aspects such as performance improvement and quality assurance. However, characterization for the emerging low loss hollow core fibres (HCFs) has regrettably lagged behind the rapid advancements in HCFs over the past three decades. This becomes particularly urgent when their loss (0.174 dB/km) is reaching the level of SMFs. In response to this disparity, this Thesis endeavours to explore and develop novel characterisation techniques for low loss HCFs, aiming to facilitate a deeper
understanding and enhance the optimization of their performance. The thesis unfolds in two main topics. The first delves into the impact of coating on the overall thermal sensitivity of HCFs. In both experimental observations and simulations, we demonstrate that the coating significantly influences thermal sensitivity. Remarkably, the coating is observed to introduce relaxation effects in optical phase stability (phase change goes back partly once temperature stays unchanged), which has not been previously discussed in the HCFs literature, to the best of our knowledge. The analysis on this effect is conducted via simulations, suggesting that this effect is caused by the viscoelastic properties of the coating. Finally, based on the studies of coating, a novel strategy is proposed to reduce the thermal sensitivity of a HCF when spooled, of interest to applications such as ultra-stable laser locking. The second topic focuses on the distributed HCF characterisation using the technique of Optical Time Domain Reflectometer (OTDR). We firstly build a high dynamic range OTDR system (>45 dB) with high spatial resolution (<2 m). The system enables measurement of the backscattering in HCFs, allowing the distributed characterisation of HCFs not only when air-filled, but also when being evacuated. This enables real time distributed monitoring of HCFs evacuation, serving as a tool for studying the gas flow in HCFs. Furthermore, the backscattering of the HCFs varies with the air pressure within the core and the core size along the HCF length. This variation poses challenges for direct distributed loss measurement, as the backscattering signal depends on both the backscattering coefficient and the loss. The strategy is demonstrated on how to measure and process OTDR traces to enable distributed loss measurement. Finally, this technique also enables monitoring of the backscattering coefficient variation along the HCF length.

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