Refractive index and temperature sensor based on large offset distance of coreless silica interferometer

Abstract

This thesis reports an original research on the development of refractive index (RI) sensor based on all-fiber Mach Zehnder Interferometer (MZI). The research development process involved design and analysis of sensor structure using BeamPROP software, in-house fabrication using pre-determined optical fiber splicing recipe and experimental work for verifying sensing performance. The fiber MZI sensor was realized from symmetrical offset of coreless silica fiber (CSF), where a section of CSF was spliced between two CSF sections in an offset manner. Thus, two distinct optical paths were created with large index difference, the first path through the connecting CSF sections and the second path at the outside of CSF through the surrounding media. RI sensing was established from direct interaction of light with surrounding media, hence high sensitivity can be achieved with a relatively compact sensor size. The use of CSF purposely to reduce the complexity of sensor fabrication as large diameter of CSF allows lower tolerance of getting the optimum offset distance. The offset distance was optimized using BeamPROP software for maximum fringe visibility at different sensor lengths. Fabrication recipe was meticulously refined and successfully employed in manufacturing the lateral offset structure. Three samples of sensor with different MZI arm length of 0.5 mm, 1.0 mm and 1.5 mm were experimented for (RI) sensing. The highest sensitivity of 1025 nm/RIU was recorded by the sensor of 1.0 mm arm length for RI range between 1.335 and 1.350. The flexibility of the sensor structure was further manifested in temperature sensing by filling the secondary path with high-thermo-optic material. Substantial temperature sensitivity enhancement from 28 pm/ºC to 3220 pm/ºC was achieved with regard to the original air filled secondary path structure. With the main attributes of high RI/temperature sensitivity and compact size, the proposed sensor would be an attractive sensing tool for many applications including include blood diagnosis, water quality control and food industries in near future.