Synthesize materials of optical fibre sensor for chemical liquid detection

Abstract

This study aims to improve the detection sensitivity and to reduce temperature effect of the optical fibre chemical sensor (OFCS) system. This system was built for chemical liquid leakage detection using optical time domain reflectometer (OTDR) as an analyzer and unclad multimode optical fibre (MMF) as a sensing part (MMF sensor). A chemical etching technique was used to unclad 1 cm { 3 cm MMF fibre. The OTDR is used as analyzer to record return loss from the OTDR trace. The scale of return loss is determined by the refractive index of chemical sample which is in contact with the MMF sensor. To improve the sensitivity of the OFCS system, the MMF sensor is coated by a thin layer of gold nanoparticles (AuNPs) using sputtering method. The performance of the system is further analyzed to characterize the return loss in relation to the sensor length and the temperature variation. The results show that the sensitivity of the OFCS system using the unclad 3 cm MMF sensor tested with Cargille solutions (refractive indexes 1.440 { 1.474) was found to be 1.825 dB=RI. The best sensitivity of 18.293 dB=RI was achieved when the unclad 3 cm MMF sensor was coated with 14.3 nm AuNPs. The MMF sensor coated with silicon carbide (SiC) was conducted in this study to reduce the influence of the temperature on the sensor response. Analyses on the results reveal that the temperature sensitivity of the OFCS system using MMF sensors coated with SiC and AuNPs was 0.025 dB=oC and 0.042 dB=oC respectively when both sensors were tested in toluene. This means the OFCS system coated with SiC is less affected by temperature variation compared to the one coated with AuNPs. However, the sensitivity of OFCS sensor coated with AuNPs to detect the change of the refractive index is much better than the one coated with SiC. Therefore, the optimum performance where sensitivity of the OFCS system to detect the change the refractive index is good and the sensitivity to temperature is minimized was achieved when the MMF sensor is coated with a bilayer coating (AuNPs and SiC). The results reveal that the sensitivity of the OFCS system to temperature with bilayer coating was 0.029 dB=oC. This figure is a little higher than the temperature sensitivity achieved using MMF sensor coated with SiC (0.025 dB=oC). However, it is far better than the MMF sensor coated with AuNPs and yet still shows a good sensitivity to the change in the refractive index.