Research On Interferometric Optical Fiber Water Level Sensing System And Demodulation Method

The tsunami is a serious natural disaster,and the coasts of Pacific and Indian Ocean are threatened by the tsunami all the year round,and also the coastal cities are frequently severely damaged because of this reason.Since the tsunami disaster is energy-intensive,wide-ranging and extremely lethal,it is difficult to escape form the tsunami and further leads to serious casualties and economic property losses.Therefore,it is quite significant to realize effective tsunami early-warning to make the guard against in advance.Amongst of the early-warning techniques,real-time monitoring of ocean water level is an important means to prevent the occurrence of tsunami.The research on the water level monitoring system has been carried out for a long time,however,since the marine environment is very complicated and the current water level systems suffer from electromagnetic interference,chemical corrosion,short transmission distance,etc.,and thus cannot work in the marine environment for a long time.While the fiber optic sensor is quite suitable for monitoring the water level in the marine environment beacaue of possessing many excellent properties such as small size,light weight,high sensitivity,anti-electromagnetic interference and corrosion resistance,it has been the preferred method for the water level sensing system.Based on the characteristics described above,in this dissertation,the system is mainly built based on optical fiber sensing technology to monitor the ocean water level in real time.The main purpose of this dissertation is to design an interferometric optical fiber water level sensing system and corresponded structure and demodulation method for marine water level monitoring in view of the ocean-water's characteristics of wide level changing range and rapid changing speed,and eventually realize the water level monitoring with the performance of large-range,high-precision and fast response.In this paper we firstly introduce the complex environment of the ocean and the huge disasters and losses caused by the tsunami,and then propose constructing an interferometric fiber-optic water level sensing system to realize ocean water level monitoring and tsunami early warning based on the advantages of fiber sensors.Secondly,we make a introduction for the basic knowledge of fiber,including the principle of light interference and various phase modulation mechanisms.Furthermore,introduce the commonly used fiber-optic interference sensing structure and phase demodulation methods.On this basis,in view of the application environment and background of the system,it is proposed to improve the existing optical fiber interferometric sensing structure to meet the needs of large-scale ocean water level monitoring;Meanwhile since the current phase demodulation method is difficult to satisfy the accuracy and response time requirements of the water level system,an effective demodulation method is also needed.Then design and implement the interferometric optical fiber water level sensor system.In this system,an improved Mach-Zehnder interferometer structure based on double collimator alignment is used as the sensitive probe to meet the needs of a large range.The internal structure of the sensitive probe is designed to convert the change of water level value into the change of the distance between two collimators,thus realizing a one-to-one relationship between the water level value and the distance between the two collimators.The laser,collimator,detector,acquisition card and other hardware devices are studied and selected to build a complete water level system.Subsequently,two demodulation methods are proposed,similarity demodulation method and linear-frequency modulation phase demodulation method.Similarity demodulation method constructs a series of analog interferometric signals and compares them with the signals to be measured.The demodulation accuracy can be optimized through increasing the number of subdivisions,but the more subdivisions,the greater the amount of computation,it is difficult to satisfy the requirements of fast response simutaneouly.Linear-frequency modulation phase demodulation method combines the characteristics of the linear-modulation light source of the system and applies differential and linear fitting to suppress the effects of the irrelevant parameters,to obtain the relationship between the real-time phase and the length difference of the interferometer arm and further obtaining the water level.This method greatly reduces the amount of calculation and the response time,meanwhile ensuring the accuracy at the same time.Finally,verify the performance of the water level system through experiment.According to the test,the measuring range of the system can reach 109.14 m,which meets the needs of large range of sea level changes.The linear frequency modulation phase demodulation method can realize the demodulation accuracy of 1 cm water level and 0.01 s response time,which meets the requirements of ocean water level for accuracy and response time.At the same time,the application of interference structure of double collimator alignment in the system can effectively avoid the influence of temperature changes.The successful development of the interferometric optical fiber water level sensor system is of great significance to the monitoring of ocean water level and tsunami early warning.