Linear multimode interference fiber temperature sensor using the liquid in glass thermometer principle

A temperature sensor based on a multimode interference thermometer is designed and fabricated. The operation mechanism is based on the thermal expansion of a specific volume of ethylene glycol contained in a glass bulb that is connected to a capillary of the same material, with a no-core fiber (NCF) inserted and centered into the capillary tube. As the temperature is increased, the liquid is expanded, and the NCF is gradually covered by the liquid, resulting in a peak wavelength shift that is correlated to the temperature variations. A sensitivity of 0.4447 nm/°C and highly linear response with an R² of 0.99962 are obtained. The advantage of this configuration is that the sensing temperature range can be adjusted by changing either the inner diameter of the capillary tube or the bulb volume. We can also measure negative temperatures by simply modifying the freezing point of the liquid, which demonstrates the viability of the sensor for many applications.