| Optical fiber sensing technology has the advantages of anti-electromagnetic noise interference,fast response speed and small size,so it can be widely used in the detection of physical parameters in various complex scenarios.In the current demand of distributed multi-point real-time monitoring of water temperature and pressure of deep borehole groundwater,when using multiplexing technology for FBG distribution,the low precision of peak searching of demodulation system will be caused by the low efficiency of FBG utilization,cross-talk of return light and interaction of multiple wave peaks.The FBG demodulation system based on WDM+TDM hybrid multiplexing technology is studied and implemented to complete the high precision monitoring of water temperature and water pressure in a large range and multiple points.The demodulation system uses a tunable laser as the input light source of mixed multiplexed FBG,and uses FPGA to implement the correlation algorithm of demodulation system and control the circuit.After analyzing the current status of demodulation system and the application trend of FBG multiplexing technology,the scheme of demodulation system is determined according to the monitoring needs of practical applications.In the optical path part,WDM+TDM mixed multiplexing technology is used to improve the loading capacity of FBG,and the extension and complement structure design of 24 FBG distribution in 4 channels(up to 2000 theoretical additions)is completed.The circuit includes the design of power supply circuit,laser drive circuit and TEC temperature control circuit,optical signal processing circuit and communication circuit.In the demodulation process,after the photoelectric conversion of the returned optical signal is completed,adaptive filtering is firstly carried out to filter the interference noise,and then the time-domain coding of the sampled data and the determination of the channel where the returned light is located are carried out.The laser is used for step wavelength scanning,and the sampling threshold voltage filtering is completed as well as the comparison before and after the sampled data.The central wavelength interval of multi-wave peaks is determined and the peaks are separated from each other.Then the central wavelength is calculated using Gauss-polynomial fitting method.Finally,the demodulation data is transmitted to the top computer through Ethernet communication module.After the completion of the demodulation platform design,in order to verify its demodulation performance,the test scheme of the modulation system was designed,and the output control of the laser light source,the peaking of the FBG returned light multiplexed,and the fitting of temperature and pressure with the central wavelength were tested.The final test results show that the performance of the demodulation system is good.The error size of the tuned current output is within 0.04 m A,and the wavelength error size of the optical signal output by the light source is within 1pm.The demodulation system has a very high degree of differentiation of the peaks of the multi-modal returned light,and the central wavelength of each wave peak can be solved.The average error of the obtained center wavelength is only 3pm higher,which indicates that the demodulation system of the adopted mixed multiplexing technology has higher precision and is less affected by crosstalk.The linear fitting degree of temperature and pressure to the central wavelength is higher than 0.99,the temperature error is within 0.1℃,the pressure error is within 0.015 MPa.Figure [78] Table [18] Reference [87]... |
PDF Full Text Request