Impact of internal pressure and gas composition on the long-term optical performance of hollow core optical fibres

Abstract

Hollow core optical fibres (HCFs) are a specialty optical fibre, where light can be transmitted with low loss within an air-filled core. This is in contrast to conventional optical fibres, where light is usually transmitted in glass (most often silica). The advantages of HCFs over conventional fibres include ultra-low loss, low latency and low non-linearity, and rapid progress in recent years is now leading to their deployment in real-world applications [1]. Hence the long-term reliability and consistency of their optical performance is becoming one of the current essential research topics. In a HCF, light is confined with low loss within the air core by a carefully designed microstructured cladding, that, depending on fibre design, can consist of up to several hundred air-filled holes, defined by thin silica membranes. It is reasonable to consider that these air holes, which extend longitudinally along the full length of a HCF, add extra complexity to their ageing mechanism(s) compared to conventional solid fibres. The extensive surface areas due to the hollow structures within the fibres increase the potential for optical degradation through reactions with gas species within the hollow structure. This study describes progress in understanding and identification of processes, and their origins, which affect the long-term fibre optical performance, and suitable treatments, during or post fabrication, are considered as a means to improve the fibre’s durability. This investigation is performed considering two key silica surface areas with an HCF: the inner surfaces created by the thin silica membranes which extend throughout the fibre length and also the fibre end-faces, which are created when an HCF is cleaved. Although these separate works have their own specific focuses, they are closely interlinked with each other and their study is relevant to a wide range of applications and to understand the long term optical performance of a HCF in various scenarios. Within this work, the high potential for atmospheric water vapour to degrade the inside of a HCF was revealed. Water vapour not only absorbs light at specific wavelengths (absorption resonances) but also accumulates on the inner surfaces within a HCF via chemical and physical reactions with the silica membranes. Most of the water vapour involved in the reaction originates from the fibre’s surrounding atmosphere, initially being drawn into the open-ended fibre due to sub-atmospheric absolute pressure inside a HCF immediately after fabrication. It was found that the formation of the surface water groups on the inner silica surface increased the loss of hollow core photonic bandgap fibres when the fibre was open to standard atmosphere (HC-PBGFs, a specific type of HCF), but the increase in loss due to the atmospheric water vapour was shown to be negligible within state-of-art, low loss hollow core anti-resonant fibres (HC-ARF). This can be attributed to the substantially lower overlap between the guided light in and the microstructured cladding within a HC-ARF as compared to a HC-PBGF. One of the simplest methods to extend the lifespan of HCFs, which is suggested here, will be to isolate the inside of the hollow structure from the humidity containing atmosphere as much as possible after fibre draw; this is practical in many applications, such as telecommunications, where the fibre can be hermetically spliced to other optical components. Degradation of the end-faces of a HCF due to crystalline contamination has been previously identified and is a known concern for reliability of a HCF when the fibre is used in a free-space optical set-up. However, prior to this work, several fundamental pieces of information about this phenomena had been left unsolved. Here, the contaminant appearing on a cleaved end-face surface of a HCF was identified as the ammonium chloride crystal and it was confirmed that this contamination occurs specifically on microstructured fibres which contain longitudinal air holes. The source of the chlorine required to form this crystal contaminant is the raw material used for fabrication of HCFs (high purity silica glass containing a small amount of chlorine). Whilst the origin of the ammonium is still unclear, our findings through different experiments suggest that an intermediate molecule is produced under the high temperature environment required during fibre fabrication and is present on the inner silica surfaces within a HCF. With this understanding, several methods to mitigate/stop the degradation are demonstrated and proposed: use of a lower-chlorine containing material, removal of the hydrogen chloride gas and treatment of a cleaved end-face.

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ORCID for Natalie Wheeler: orcid.org/0000-0002-1265-9510

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