An Ultrafine No-Core Fiber-Based Sensor for Simultaneous Liquid Level and Temperature Measurement

In this article, a Section of ultrafine no-core fiber (UNCF) is fused between two sections of single-mode fiber (SMF) to form a multimode interference (MMI)-based sensor, which enables dual-parameter measurement of liquid level and temperature. The effect of UNCF diameter on the liquid-level sensing performance is analyzed theoretically and experimentally, and the results show that the sensitivity can be improved by reducing the UNCF diameter. When the UNCF diameter is reduced to
$30 \mu \text{m}$
, the sensitivity of the sensor is 1008.2 pm/mm for water with a refractive index (RI) of 1.333 in the liquid-level range of 0–30 mm. Its temperature sensitivity is −221.02 pm/°C over a detection range of 5 °C–60 °C. Attractively, the sensor shows almost no response to pressure variation. When applied to the liquid-level measurement of a water cooler, the sensitivity is 1898 pm/mm. The sensor has the advantages of high sensitivity, good repeatability and stability, and satisfactory anti-interference ability. It has great potential for applications in the health management of lasers, chips, transformers, and other instruments.