Optical Fiber Liquid Level Sensing Based On Brillouin Optical Time Domain Analysis

In recent years the fiber sensor technology develops rapidly,it has already been widely used in many kinds of areas.The fiber is used as not only the transmitting channels for signal,but also the sensing part of the sensors.So it has a wide range of sensing applications.It has many advantages over other methods,which are resistance to electromagnetic interference,strong corrosion resistance,high sensitivity and so on,so it has broader development space for practical engineering application.Brillouin optical time domain analysis(BOTDA)technology is a representative of the Brillouin distributed sensing that features high spatial resolution,long sensing distance and good signal to noise ratio have a series of advantages.This paper proposes a method that combined with differential pulse-width pair Brillouin optical time domain analysis(DPP-BOTDA)sensing system and the self-heated high attenuation fiber level sensor combination,details are as follows:Firstly,a DPP-BOTDA system which has a spatial resolution of 5cm is built to measure the temperature coefficient of the 22 m sensing fiber,the temperature coefficient of sensing optical fiber Brillouin frequency shift obtained from the experiment is 1.2MHz /TC(28)?,measurement accuracy is 0.275?,which provides the experimental basis for the sensor measurement of the liquid level.Secondly,the Brillouin frequency shift characteristics of the sensor fiber are studied under different heating conditions.In the experiment,the high attenuation optical fiber with attenuation coefficient of 10 d B/m and 20 d B/m with the sensing fiber are placed side by side respectively and stuck together.Through pouring the pump of different power into the high attenuation optical fiber make it heated,meanwhile by the physical heat conduction method heated the sensing optical fiber.The system obtains the Brillouin frequency shift distribution of the sensing fiber,and obtains the temperature distribution of the sensing fiber by the linear relationship between Brillouin frequency shift and temperature.Finally,the liquid level continuous measurement by using the DPP-BOTDAsystem is realized.Measuring principle is the difference of the heat capacity of the liquid and gas,there is a sudden temperature change at the gas-liquid interface,resulting in abrupt changes in the Brillouin frequency shift of the sensing fiber,the position of the liquid level can be transferred through the point to demodulate liquid level.In the experiment,a distributed liquid level measurement with a measuring range of 20 cm and a spatial resolution of 1cm was completed.