High sensitivity micro/nano singlemode-multimode-singlemode fibre sensors

Optical fibre interferometers have gained significant attention in the field of sensing, due to their advantages of high sensitivity, reliability, compact size, light weight, immunity to electromagnetic interference, environmental ruggedness and remote sensing capabilities. Among the interferometric sensors, singlemode – multimode – singlemode (SMS) fibre structures has advantages of easy and low cost fabrication and relative simplicity of integration with other optical devices. However, previous reported research in the SMS sensor has been limited to measurements of the traditional parameters such as temperature, strain, vibration and refractive index with limited sensitivity, which prevents further development of the sensing technique. In this thesis, temperature sensitivity enhancement and a new application in solid/liquid phase change detection have been firstly proposed and experimentally demonstrated based on the traditional SMS fibre structure; the traditional SMS fibre structure is then tapered and functionalised for magnetic field, humidity and biological sensing with ultrahigh sensitivity for human chorionic gonadotropin (hCG) detection. The key contributions of this thesis include: Modified SMS fibre for temperature sensing and phase transition monitoring of phase change materials Based on the modified SMS fibre structure, significant temperature sensitivity improvement (200 times over traditional SMS sensor) is experimentally demonstrated, by introducing a hollow core fibre filled with a polymer (refractive index 1.46), in the middle of the SMS structure, which constructed a new sensor of singlemode-multimode-polymer filled hollow core-multimode-singlemode fibre structure. Solid-liquid phase change materials (PCMs) have been widely used in latent heat thermal storage systems for heat pumps, solar engineering, and spacecraft thermal control. A platinum-coated singlemode-multimode (SM) structure has been proposed as an optical fibre sensor (OFS) to monitor the phase transition of a phase change material (PCM). The sensor has been experimentally demonstrated to be able to detect the phase change point of different PCMs, including paraffin wax and two salt hydrates (S32 and S46) with advantages of simple operation and low cost. Tapered optical fibre sensor for magnetic field sensing A tapered singlemode – no-core fibre – singlemode (SNCS) fibre sensor (taper waist diameter of 8 μm) is proposed and experimental demonstrated with a maximum sensitivity of 26,061 nm/RIU (refractive index unit), within the RI range of 1.4304 - 1.4320, which is over 10 times higher in sensitivity than that of a non-tapered SNCS structure. The tapered SNCS structure is then inserted into a glass tube (inner diameter of 1 mm) filled with magnetic fluid for measuring both magnetic field strength and direction. Experimentally a maximum sensitivity of 0.466 nm/mT has been achieved by using a tapered diameter of 10 μm and 1.3% ferro-magnetic particles in magnetic fluid. Functionalised optical fibre for bio-sensing A high sensitivity label free human chorionic gonadotropin (hCG) is proposed and experimentally demonstrated with a sensitivity of 0.188 nm spectral shift for 0.05 mIU/mL hCG, based on the tapered SNCS fibre structure by coating an anti-hCG-β antibody onto the fibre sensor to specifically capture the hCG hormone in the analyte such as urine sample of pregnant women. The sensitivity of the sensor can be improved by adding two different types of additional microspheres (1 and 0.3 μm diameter) functionalised with anti-hCG-α antibody into the analyte, which will be captured by the hCG (bind by anti-hCG-β antibody) on the fibre sensor. The sensitivity is enhanced to 0.273 nm spectral shift for 0.01 mIU/mL hCG, by adding anti-hCG-α antibody modified microspheres with 1 μm diameter in the hCG samples. Further sensitivity improvement has been achieved with sensitivity of 0.984 nm spectral shift for 0.01 mIU/mL hCG, by enriching the hCG sample using immuno-magnetic separation. The proposed sensor system has detection limit as low as 0.0001 mIU/mL provided the optical spectrum analyser has a resolution of 0.01 nm.