| Liquid level refers to the information of liquid level in sealed or open containers.Liquid level is an important parameter in chemical production,oil storage and transportation,biomedicine,food and beverage,reservoir water level monitoring,water tank and other fields.High precision liquid level measurement technology not only brings more convenience to people’s life,but also plays an important role in the safe storage and transportation of liquid goods and the safe operation of automatic liquid level control system.The purpose of this paper is to explore the application of special optical fiber in liquid level measurement,and to improve the coupling efficiency and enhance the sensitivity of optical fiber structure to liquid level by designing and fabricating new type fiber Mach-Zehnder interferometer.In this paper,four kinds of fiber optic Mach-Zehnder interferometers are proposed,and the liquid level sensing characteristics of each interferometer are studied theoretically and experimentally.The main research contents and conclusions of this paper are as follows:（1）An optical fiber"peanut structure"is proposed and fabricated,which can be used as coupler in optical fiber interferometer.On this basis,a Mach-Zehnder interferometer based on single mode-peanut-single mode-peanut-single mode fiber structure is fabricated,and the response characteristics of liquid level and temperature are investigated experimentally.The experimental results show that the level sensitivity of the interferometer is-0.2595 nm/mm in the range of 0-36 mm.In the range of 35-90℃,the temperature sensitivity is 0.1603 nm/℃.Meanwhile,as a comparison,a Mach-Zehnder interferometer based on single mode-peanut-multimode-peanut-single mode fiber structure is designed and manufactured,and the liquid level response characteristics of the interferometer are studied.The results show that the interferometer is insensitive to the change of liquid level,and the level sensitivity is5.49×10-5nm/mm.（2）A Mach-Zehnder interferometer based on single mode-peanut-seven core-peanut-single mode fiber structure is proposed and fabricated,in which the single mode fiber and seven core fiber are connected by the fiber"peanut structure",which effectively improves the coupling efficiency.Three interferometers with lengths of 24 mm,28 mm and 32 mm are fabricated in the experiment,and the response characteristics of liquid level and temperature of the interferometers are investigated experimentally.The experimental results show that the level sensitivity of the three interferometers is-0.4069 nm/mm,-0.2739 nm/mm and-0.1653 nm/mm in the range of 0-24 mm,0-28 mm and 0-32 mm,respectively.In the temperature range of 35℃to 90℃,the temperature sensitivity of the three interferometers is 0.0885 nm/℃,0.0740 nm/℃and 0.0879 nm/℃.（3）A Mach-Zehnder interferometer based on single mode-coreless-seven core-coreless-single mode fiber structure is proposed and fabricated,and coreless fiber is used as fiber coupler.Three interferometers of different lengths are fabricated in the experiment.The experimental results show that the level sensitivity of the three interferometers is-0.2687 nm/mm,-0.2471nm/mm and-0.1734 nm/mm when the liquid level ranges are 0-18 mm,0-20 mm and 0-30 mm respectively.In the temperature range of 25℃to 95℃,the temperature sensitivity for the two interferometers is 0.0727 nm/℃and 0.0719 nm/℃,respectively.The wavelength fluctuation of the 18 mm interferometer at 8 mm and 16 mm is less than 0.03 nm and 0.08 nm respectively.In the stability test,the calculated standard measurement errors of the two interferometers is 0.11mm and 0.16 mm respectively.（4）A Mach-Zehnder interferometer based on single mode-multimode-seven core-multimode-single mode fiber is proposed and fabricated,in which the multimode fiber is the coupler between the single-mode fiber and the seven core fiber.The liquid level experiment results show that the sensitivity of the sensor is-0.1445 nm/mm and-0.5769 nm/mm at 0-18mm and 18-24 mm,respectively.The stability test shows that the wavelength fluctuation of the interference valley at the liquid level of 4 mm and 20 mm is less than 0.04 nm and 0.05 nm respectively.The above fiber Mach-Zehnder interferometers have the advantages of simple structure,low cost,easy manufacture,and could realize the highly sensitive liquid level sensing measurement,which has important research significance and application value. |
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