Fiber Laser Liquid Level Sensor Based On Mode Interference

High-precision liquid level measurement is great significance in the fields of chemical industry,petroleum storage,food safety,and public water supply.Due to the advantages of low cost,light weight,anti-electromagnetic interference,ease of fabrication and corrosion resistance,optical fiber interferometer has been the important role in liquid level measurement.Recently,modal interferometer based schemes have received more attention in order to improve sensitivity.Nevertheless,linear range and crosstalk/noise caused by the difference of multi-excited high-order cladding modes become the open issues.In this paper,the thin core fiber based offset structure and fiber ring laser were comprehensively investigated,and the main contents are as follows:(1)By beam propagation method,the energy distribution of modes in thin-core fiber based offset structure was analyzed,and the optimal offset value is obtained for the state of quasi single-mode Interference.Moreover,the sensing principle of liquid level test was presented,and the possible crosstalk from temperature and refractive index in liquid level test were studied.(2)The sensing characteristics of liquid level in core-offset and core mismatch structures are analyzed.The main reason of low linearity in liquid level test is revealed through spatial frequency spectrum.Based on quasi single-mode interference state,the thin-core fiber offset structure was fabricated and the linear testing range is increased to500 mm,with the sensitivity of 41.8 pm/cm.Moreover,the linearity were always higher than 0.99 in the range of 20?500 mm.(3)A single longitudinal mode fiber ring cavity laser sensor is then built by combining the thin-core fiber offset structure,served as a passive filter.Owing to high extinction ratio(> 35 d B)and narrow line-width(< 0.1 nm),the actual detection-limit is improved 16 times at least.Furthermore,in order to eliminate the fluctuation of temperature and refractive index,the structure of down-taper cascaded fiber Bragg grating is introduced and a stable dual-wavelength laser operation is achieved.Through the wavelength and intensity joint-demodulation method,the dual-wavelength laser sensor completed high discriminative simultaneous measurement in terms of liquid level,refractive index and temperature.