High Precision Optical Fiber Liquid Level Sensor Technology And Research Based On Multimode Interference Effect

Liquid level is one of the most important measurement parameters in the process of industrial production.Its accurate measurement is of great significance in the fields of aerospace,petroleum exploration,food processing and environmental protection.The fiber-optic liquid level sensor based on multimode interference effect has become a research hotspot in the field of fiber-optic liquid level sensor because of its simple structure,easy manufacture,low cost,portability and high integration.This paper mainly studies the basic theory of optical fiber liquid level sensor technology based on multi-mode interference effect,focusing on the existing problems of this kind of liquid level sensor,such as small measurement range,low accuracy and not easy to contact with liquid,and designs the corresponding solutions,and carries out experimental exploration.The main work of this paper includes the following aspects:1.Theoretical and Experimental Research on fiber-optic liquid level sensor based on multimode interference effect: the basic theory of fiber-optic multimode interference and the mechanism of liquid level sensor are firstly introduced in detail.Then through the combination of software simulation and experimental verification,three kinds of common fiber structures based on multimode interference theory,namely,single-mode-coreless-single-mode(SNS),multimode-coreless-multimode(MNM)and multimode-single-mode-multimode(MSM)are studied The relationship between resonance wavelength and liquid level is also deduced,which lays a theoretical foundation for the design optimization and Application Research of the sensor.2.Research on multi-mode interference reflective optical fiber liquid level sensing technology: two different reflective optical fiber liquid level sensing structures are proposed.they are single-mode-coreless-gold film and single-mode-coreless-single-mode-multilayer dielectric film.Through experiments,the sensitivity of 5.8cm Sn gold film structure and 20 cm SNS multilayer dielectric film structure to the liquid level measurement of distilled water solution is 185.2pm/mm and 41.6pm/mm respectively.Compared with the traditional transmission scheme,the reflective structure is easy to contact with the liquid,and it can adjust the light signal twice,which makes the actual liquid level measurement phenomenon more obvious.3.Research on high-precision optical fiber liquid level sensing technology based on spectrum reconstruction: high-precision liquid level measurement is realized by combining the spectrum reconstruction method of wavelength sampling and the minimum variance calculation method.This method abandons the traditional spectrometer structure,and uses tunable laser,photodiode and data acquisition card instead,which not only saves the system cost,but also improves in a certain range The measurement accuracy is obtained.Finally,a high-precision optical fiber liquid level measurement system is built.The measurement sensitivity of75.6pm/mm and the measurement accuracy of 0.1mm are achieved for dilute sulfuric acid solution in the range of 13 cm.4.An integrated optical fiber liquid level measurement prototype is built through hardware system selection,software programming and sensor packaging,which realizes stable and reliable automatic liquid level and refractive index measurement.