Research On The Characteristics Of Cascade Structure Fiber Optic Liquid Level Sensing

Recently,fiber-optic mode interferometers(FOMI)have been well applied into the field of liquid level sensing because of the advantages such as high sensitivity,simple structure,ease of fabrication and the resistance to corrosion and electromagnetic interference.In this paper,in order to reduce temperature crosstalk and extend the linear response range,the FOMIs based on the structures of core-offset and core-mismatch are fabricated by the arc-discharge technique.Furthermore,the liquid level sensors with temperature self-compensation and the cascade structures are comprehensively studied and tested.The main contents are as follows:(1)The principles of multimode interference(MMI)and Mach-Zehnder interference(MZI)are studied in theory.By beam propagation method,the optimal fiber lengths in MMI and MZI are analyzed to get the interference spectra with high fringe contrast.For the proposed structures,their sensing principle of liquid level in wavelength and intensity are investigated.Also the possible temperature crosstalk is given in the liquid level measurement.(2)The sensing characteristics of liquid level are studied for typical mode interferometers based on core-offset and core-mismatch(MSM)structures.And their linear response region and temperature characteristic are analyzed.In order to eliminate temperature crosstalk,a thin-core fiber based mode interferometer with core-offset splicing is proposed and fabricated.Through adjusting the core-offset value precisely,the high temperature consistency of fringes is achieved and the temperature noise is effectively eliminated by introducing difference compensation.The experimental results show that the liquid level sensitivity is 34.9 pm/mm with the linearity of 0.99.(3)A novel cascaded mode interferometer is proposed to realize simultaneous measurement of liquid level and temperature.The experimental results show that its sensitivity of liquid level reaches 259.23 pm/mm and the temperature sensitivity is 15.96 pm/?.The calculated temperature crosstalk is about 0.062 mm/?.Furthermore,the liquid level sensors base on multi-order MSM structure are developed to extend the linear response range.The experimental results show that the second-order MSM structure has a clear feature of intensity modulation with the sensitivity is 0.26122 d B/mm in the linear range of 40 mm.And for the forth-order MSM,the corresponding linear response range reaches 65 mm,which may effectively increases the practicability of liquid level sensor.