Precision laser micromachining of hollow core negative curvature fibres

Abstract

The principal aim of this work was to develop a novel micromachining strategy for a new class of hollow core silica optical fibre, the Negative Curvature Fibre (NCF). Processing techniques were investigated to increase the physical access to the hollow core (along the length of the fibre) in order to enhance the interaction of chemical species with the light and hence enable practical sensing devices. Because of the unique internal structure of these NCFs, consisting of a fine (sub-micron) silica webbing, a highly precise and controllable machining process was required. Due to the well-known advantages of femtosecond laser machining such as the ability for inscription in any material, small volume removal and the non-thermal nature of the process, resulting in machined structures with an almost negligible heat-affected zone, a new femtosecond laser micromachining process was developed. A methodology was successfully demonstrated which gives the capability to precisely machine away the solid outer cladding fibre and then controllably remove the silica webbing and expose the hollow core of the fibre. This single step process provides a more direct way of machining a fibre (compared to previously reported hybrid techniques such as laser machining plus chemical etching). Parameter optimisation allowed control of the removal depth, minimisation of re-deposited material and avoidance of damage to the remaining silica web which is very important for NCF due to its guidance mechanism. An NCF, which was machined through to the hollow core, exhibited no significant disruption to the guidance preserving the confinement of light to the hollow core. Hence the laser micromachining strategy presented in this work paves the way to develop new optical sensing devices exploiting the unique properties of these novel fibre geometries.