Optical Fiber Laser Sensor For Liquid Level Measurement Based On Multimode Interference Effect

An optical fiber laser sensor for liquid level measurement based on multimode interference effect is proposed and demonstrated. The sensor head manufactured by splicing a section of no-core fiber (NCF) between two single-mode fibers. Due to the multimode interference effect, a transmission peak appears when light travelled through the structure. The surrounding liquid changes effective refractive index of related modes of the NCF and transmission peak is related to the length of NCF that immersed in the liquid, thus liquid level can be measured from wavelength shift of the transmission peak. By inserting the sensor head into the cavity of an erbium-doped fiber ring laser, enhanced measurement accuracy is achieved owing to the much narrower linewidth and higher signal-to-noise ratio of the laser. Excellent linear relationships between liquid level and lasing wavelength shift are achieved with various sensitivity of 163.9, 198.3 and 261.3 pm/mm for three kinds of liquids with different refractive index of 1.3618, 1.3807 and 1.4025, respectively.