CO2 Laser Polishing of Silica Optical Fibres

Abstract

Fibre side-polishing technology is experiencing a resurgence owing to the platform’s compatibility for enhanced light mater interactions with novel material systems, for example 2D materials. Conventional side-polished optical fibres are produced through the application of mechanical polishing. These approaches inevitably engender polished-surface defects and/or impurities. This study proposes an innovative side-polished optical fibre manufacturing technique with the use of a CO2 laser. Laser polishing of fused silica is a complex multi-physical process. A numerical COMSOL model and experimental analyses were employed to realise a comprehensive understanding of the surface formation during CO2 pulsed laser processing. The pulse parameters for the CO2 laser were chosen to selectively remove the cladding material from the side of the optical fibre while producing a flat surface for device fabrication. This approach has several benefits; specifically, it is non-contact, versatile, and scalable. As a consequence, the side-polished optical fibre not only possesses adiabatic polishing transitions, but also has a flat, uniform polished region. This polishing approach results in a clean polished surface with an RMS surface roughness below 2 nm. This method differs from traditional side-polishing systems that present multiple challenges such as the abrasion of hard tooling, the presence of surface flaws, and problems associated with residual abrasive particulates. This method is designed to establish a robust platform for innovative optical fibre devices that function in accordance with in-fibre light–matter interactions with exotic materials, including low-dimensional semi-conductors and topological insulators. In addition, this thesis proposes a one-step non-contact process to prepare substrates for use in Fabry-Pérot-based side-polished optical fibre sensors. The profile of the polished fibre surface is controlled by a CO2 laser cladding removal process, after which the polished substrate is micro-machined with two reflective cavities. The thermal response of these cavities are explored in this thesis. The device allows greater freedom in the design of optical fibre-based Fabry-Pérot sensors as is demonstrated by coating the substrate with a polydimethylsiloxane coating. The resulting sensor delivers a three orders-of-magnitude increase in sensitivity in comparison to the uncoated substrate. The surface is flat and polished, thereby facilitating the assimilation of novel sensing application materials, especially when it is critical to maintain polarization control for light-matter interaction, as is the case with low-dimensional materials, including graphene or MXenes. This technique is innately simple, mechanically stable, and scalable. Moreover, the CO2 laser polishing technique is also applicable to the fabrication of sidepolished, non-standard optical fibres, such as polarization-maintaining fibre and multi-core opti- cal fibre. A simple Fabry-Pérot-based, side-polished, 4-core multicore fibre is also demonstrated with this novel technique by combining the CO2 laser polishing with cladding micro-machining. Although the sensing properties of the device have not been examined, the ideal interference spectrum demonstrates the simplicity and high flexibility of this technique. It is expected that this technique will provide a reliable solution for the fabrication of atypical fibre-optic devices and a powerful platform for multi-parameter sensors with ultra-high coupling.