Research On High Temperature Strain Measurement And Demodulation Technology Of Fiber Bragg Grating

Fiber Bragg grating strain sensor has become the current research hotspot of strain sensor due to its small size,high temperature resistance,long-distance transmission,and anti-electromagnetic interference.It has broad application prospects in heavy industries such as metallurgical industry,electric power,and petrochemical industry.Fiber grating has high reflectivity and narrow reflection bandwidth,and its wavelength is sensitive to strain and temperature changes.The key problem of fiber Bragg grating strain monitoring in high temperature environment is how to eliminate the influence of temperature on strain.Around this problem,this paper designs and makes fiber Bragg grating high temperature strain sensor and its demodulation system,the specific content is as follows:(1)The sensing theory of fiber Bragg gratings is studied and analyzed,and a substrate-type double-grating high-temperature strain sensor is designed and manufactured to solve the problem of temperature-strain cross-sensitivity of fiber Bragg gratings.In the form of double gratings,a femtosecond laser is used to write temperature gratings and strain gratings point by point on a single imide-coated optical fiber.Solid Works is used to design the substrate-type packaging structure,and simulation software is used to simulate and optimize the packaging structure,so that the temperature grating of the packaged sensor is only sensitive to temperature,and the strain grating is sensitive to temperature and strain.The measurement results of the temperature grating are used to perform thermal compensation on the monitoring results of the strain grating,eliminating the temperature effect on sensor strain monitoring.(2)The spectrometer demodulation system is designed based on FPGA,using SLD as the system light source,the spectrometer module is used to complete the conversion of optical signal to analog signal,and the analog signal is converted to digital signal through AD sampling circuit,and the FPGA control circuit is used to perform Gaussian smoothing filtering and centroid peak finding on the signal after AD sampling,and finally the reflection spectrum and central wavelength of the sensor are sent to the host computer for display through the serial port,so as to realize the demodulation of the central wavelength of the signal with high signal-to-noise ratio and high power narrow linewidth.(3)An experimental platform is built,and the strain measurement experiment and temperature sensing experiment are carried out on the sensor.The experiment proved that the strain range of the packaged sensor is 3000 με,which is 2.4 times that of the unpackaged sensor,and the temperature sensitivities of the temperature grating and the strain grating is 12.94 pm/°C and 13.89 pm/°C.Using a self-made demodulator and packaged sensor for high-temperature strain measurement,the experiment shows that after temperature compensation,the center wavelength drift of the strain grating of the sensor decreases from 4.6 nm to 0.2 nm,and the strain sensitivity is 1.91 pm/με,and the central wavelength of the temperature grating remains at 1571.633 nm,which realizes the simultaneous measurement of temperature and strain,and effectively solves the problem of temperature-strain cross-sensitivity of the fiber grating.