Theories And Applications Of Highly Sensitive Liquid Level Sensors Based On Optical Fiber

Liquid level sensing is important for modern industries in many specific fields, such asfuel storage and measuring systems. In recent years, highly sensitive liquid level sensingfinds more and more applications in other fields like geologic monitoring and environmentalmonitoring. Meanwhile, the measurement of several related parameters, such as refractiveindex and temperature, tend to be incorporated to the process of liquid level sensing, aimingto realize multi-parameter sensing. On the other hand, optical fiber sensors offer manyadvantages under rigorous conditions due to its intrinsic properties such as non-conducting,anti-erosion, compact size and immune to electromagnetic interference. Among those, fiberBragg grating sensors, fiber laser sensors and fiber interferential sensors attract considerableattention for the distinguished properties of high sensitivity, compact size, etc.In the thesis, owing that the growing demand for highly sensitive liquid level sensing inindustrial production and environmental monitoring, we carried out the investigations ontheories, experiments and applications of the highly sensitive liquid level sensor based onoptical fiber. The main contents are as follows:Firstly, a highly sensitive liquid-level sensor based on weak uniform fiber Bragg gratingwith narrow-bandwidth is proposed and experimentally demonstrated. Based on couplingmode theory and transmission matrix formula, we thoroughly analyzed the sensing propertiesof the proposed configuration and experimentally proved the sensor have the outstandingmerits of highly sensitive, large measurement, great linearity, etc.Sencondly, for the first time to our knowledge, a highly sensitive liquid-level sensorbased on dual-wavelength single-longitudinal-mode fiber laser is proposed andexperimentally demonstrated. The theories of fiber ring laser and beat frequencyinterrogation technology are investigated. The laser is formed by exploiting two parallelarranged phase-shift fiber Bragg gratings, acting as ultra-narrow bandwidth filters, into adouble-ring resonators. The experiment for the intrinsic properties and liquid level sensingproperties are carried out respectively. Moreover, based on the laser configuration, a highlysensitive temperature sensor is proposed and experimentally demonstrated. The experimentalresults show that an ultra-high liquid-level sensitivity of2.12×107MHz/m within thelinearity of0.986and resolution of0.05μm is achieved. The sensor presents multiple merits including ultra-high sensitivity, thermal insensitive, good reliability and stability.Thirdly, a simple fiber sensor capable of simultaneous measurement of liquid level andrefractive index is proposed and experimentally demonstrated. The sensing head is anall-fiber modal interferometer manufactured by splicing an uncoated single-mode fiber withtwo short sections of multimode fiber. Experimental results show that the liquid level and RIsensitivities of the two dips are245.7pm/mm,-38nm/RI unit (RIU), and223.7pm/mm,-62nm/RIU, respectively. The approach has distinctive advantages of easy fabrication, low cost,and high sensitivity for liquid level detection with the capability of distinguishing therefractive index variation simultaneously.Fourthly, a novel fiber-optic water tilt meter and a rain gauge are proposed andexperimentally demonstrated, respectively. As for the water tilt meter, we designed twodifferent configurations based on fiber Bragg grating and fiber ring laser, respectively. Theexperimental results show good reliability, great stability and high sensivtity. As for the raingauge, we proposed a siphon-type rain gauge based on a fiber Bragg grating sensor andcarried out the experiments for the key parameters, siphon compensation and error analysis.According to the research process of theories, experiments and applications, weachieved the innovative results in different aspects. The achievements are important for thearea of geologic monitoring, environmental monitoring, biomedical detection and aerospacemonitoring, etc. Furthermore, the related achievements can be applied to the industry of theinternet of things and fiber-optic communication.