Accuracy evaluation of a hetero-core splice fiber optic sensor

A Hetero-Core Splice Fiber Optic Sensor has a very simple structure in which a short-length, different core-size fiber is inserted in a single-mode-fiber network line with a low transmission loss for the wavelength of 1.31μm. In this study, a displacement sensor has been newly developed using a hetero-core spliced fiber optic element in a form of simple module structure and has been evaluated in terms of the accuracy as an OTDR(Optical Time Domain Reflectometer)-based measurement, for the purpose of the full-scale environmental monitoring. The developed sensor module is designed to have a coverage sensitive to a relatively large displacement in the range 0-5mm, for which a simple displacement-macrobending conversion mechanism has been adopted. A nickel alloy Invar with an extremely low linear expansion coefficient is used for the main parts of the module. Displacement is given to the module with a highly accurate micrometer of a minimum resolution of 0.5μm. The sensitivity showed a tendency proportional to the insertion length of hetero-core portion in the range from 1 to 2mm. The experiment promisingly showed sufficient reproducibility in the sensing operation with the accuracy less than 0.1% to the full span displacement of 5mm. Additionally, a pressure gauge using a hetero-core spliced fiber optic element has been newly developed in a form of module structure by applying the developed displacement sensor. The pressure gauge is designed to measure the change in length of a bellows due to pressurized air. The experiment achieved the measurement accuracy 0.3%[%FS] with highly showing the capability of all-light wave-based liquid level monitoring as a first demonstration based on a single mode fiber transmission concept, which could be very useful for environmental preservation.