<title>New techniques for manufacturing optical-fiber-based fiber Fabry-Perot sensors</title>

Abstract

Optical fibre-based extrinsic Fabry-Perot interferometric (EFPI) sensors have been extensively deployed for sensing a number of measurands including temperature, strain, vibration and pressure. Their circular cross-section has made it relatively simple and attractive to embed them in advanced fibre reinforced composites (AFRCs) such as glass and carbon fibres. However, a typical construction of an EFPI consists of two optical fibres that are positioned and secured within a precision bore capillary. The relative outer diameters of the various key components are as follows: capillary = 300 µm; optical fibre = 125 µm; carbon and glass fibres = 8 and 14 µm respectively. This mismatch in relative diameters of the reinforcing and the sensor fibres can result in significant spatial distortion of the former. The location of the embedded sensing fibre in relation to the reinforcing fibre layers can also lead to the formation of resin-rich regions in the AFRC. These factors can have a detrimental effect on the compressive properties of the material. Therefore, there is significant attraction in reducing the overall diameter of the sensor.In this current paper, the feasibility of reducing the diameter of EFPI sensor design to that of the optical fibre is demonstrated via two techniques. The first technique involved the use of hydrofluoric acid to etch and create the FabryPerot cavity. In the second technique, the feasibility of using laser ablation to fabricate the Fabry-Perot cavity in silica and sapphire substrates is presented. The optical fibre-based Fabry-Perot cavity produced via acid etching was interrogated using white light interferometry.