Adaptive Demodulation System Based On Two-Wave Mixing In Photorefractive Crystal

Compared with the traditional piezoelectric sensors,fiber Bragg grating(FBG)sensors have a wide application in the area of structural health monitoring,aerospace and civil due to the characteristics of electromagnetic interference resistance,compact structure,light weight,corrosion resistance,high real-time,high sensitivity,easy to form large-scale quasi distributed FBG sensor network and integrating sensing and transmission.Since the occurrence time and location of impact events or acoustic emissions are transient and unpredictable,it is necessary to install multiple FBG sensors on the structure to achieve real-time measurement of the health status of the structure.Therefore,there are two technical challenges for FBG sensors in dynamic strain caused by impact and acoustic emission events.Firstly,it is necessary to develop a demodulator that can demodulate multiple FBG sensors at the same time.Secondly,high-frequency dynamic signals of general interest often coexist on a larger quasi-static signal background,and the FBG sensor itself is sensitive to quasistatic signals and dynamic strains.Therefore,it is very important to develop a FBG wavelength demodulator which can adapt to the external large and low frequency vibration.Based on the above background,an adaptive two-wave mixing wavelength demodulator based on photorefractive In P:Fe crystal is designed.The two-wave mixing in the photorefractive crystal can not only automatically eliminate the low-frequency FBG wavelength drift,but also multiplex.The main work of this paper is as follows:(1)The writing process of photorefractive dynamic grating has been theoretically analyzed using one center-two band nonlinear transport equations,which are linearized by perturbation method.Therefore,the steady-state space charge field under the small intensity modulation has been solved.The relationship between space charge field and gain coefficient has been established by coupled wave equations.The effects of temperature,pump intensity,external DC electric field and mixing angle on the gain coefficient are calculated numerically.The optimal pump intensity is 218 m W/cm~2 and the optimally mixing angle is 5°at the temperature of 298 K and an external electric field of 5k V/cm.The experimental results show reasonable agreement with the theoretical predictions,which verify the rationality of the one center-two band model.Finally,the adaptivity of the two-wave mixing process in photorefractive material was discussed based on the photorefractive response time.(2)The direct detection has the simplest optical structure,which can realize the linear shift of small phase conversion in drift mode,and the interferometer still has the best signal-to-noise ratio limited by shot noise.In the isotropic configuration,because there are two polarization states after passing through the crystal,the phase compensation plate can be used to realize the orthogonal condition.However,this leads to more complex optical equipment in the isotropic configuration and reduces the signal-to-noise ratio to a certain extent.Therefore,the direct detection scheme is adopted in this work,and the frequency response of the direct detection is finally calculated.The result shows that the photorefractive beam combiner is equivalent to a"smart"high-pass filter(cut-off frequency is 120Hz).(3)This paper presents an intelligent optical fiber dynamic strain sensing system based on semiconductor optical amplifier(SOA)fiber ring laser.When the reflection spectrum of FBG sensor changes due to the dynamic strain,the wavelength of laser output shifts accordingly,and then it is converted into corresponding phase shift and demodulated by the In P:Fe photorefractive crystal two-wave mixing interferometer.The relationship between demodulated signal amplitude and optical path difference is analyzed theoretically.It is found that the demodulation signal amplitude of FBG sensor with 0.2 nm bandwidth is the largest when the optical path difference is about 4 mm,and the larger the signal amplitude is affected by the applied DC electric field.The dynamic response results show that the sensing system can adapt to large low-frequency vibration,and the low-frequency cut-off frequency is about 50 Hz.It also shows that the system can be used for impact and ultrasonic detection,and finally proves that the sensing system can be multiplexed.TWM wavelength demodulator has the characteristics of real-time,adaptive and multiplexing,so it is expected to be used in FBG dynamic strain demodulation of structural health detection such as acoustic emission.