Highly sensitive OPD-demodulated fiber temperature sensor based on an ultra-long liquid-core with a syringe shape

An optical path difference (OPD)-demodulated fiber temperature sensor based on a Fabry-Perot interferometer with a liquid-core is proposed. The length of the liquid-core reaches more than 3 mm, and it shapes like a syringe. It is fabricated by filling cedar oil into a section of the hollow core fiber with two different inner diameters. The cedar oil whose refractive index is larger than that of silica can work as the liquid-core, and its length changes with the temperature variation sensitively owing to the high thermal expansion coefficient (TEC). Theoretical analysis proves that the temperature sensitivity based on the OPD demodulation is proportional to the length of the liquid-core, and the syringe shape can further amplify the sensitivity greatly. The filling length of the cedar oil can be improved greatly by the inside-instilling method through an ultra-fine silica needle. As a result, the OPD sensitivity is as high as 50.93 µm/°C, which is three orders of magnitude higher than that of most reported sensors. Besides, the sensor also offers excellent repeatability and stability owing to the absence of residual stress in the liquid material.