Study Of Interferometric Optical Fiber Sensing Technology And Its Application Based On Rectangular-pulse Binary Phase Modulation

Interferometric fiber-optic sensors have the advantages of high sensitivity,high detection resolution and strong anti-electromanetic interference capability,etc.They have been widely used in various fields such as oil detection,underwater acoustic monitoring,and perimeter security.Currently,there still exist some problems in the study of interferometric fiber-optic sensors,such as sensor structure,phase detection,networking,and real-time signal processing.In the field of oil detection,as the depth of the exploration increases,the requirement for high sensitivity geophone and low frequency detection is getting higher.In practical application systems,with the increasing size of sensor arrays,the system requires for higher computation speed.In addition,the use of multiplexing technology in the networking process will cause related crosstalk noise,decreasing the system performance.This paper studies the above issues,and the main research content includes the following parts:Aiming at the problem of high computational complexity in current phase detection technologies,we present a phase detection method based on rectangular-pulse binary phase modulation.It adopts rectangular-pulse phase modulation on the laser source and utilizes the characteristics of the length difference between the two interference arms of the unbalanced interferometer to generate three-step phase-shifting light intensity signals.Through time calibration,three-step phase-shifting light intensity signals can be obtained and orthogonal signals can be further extracted for phase calculation.This method has the characteristics of low computational complexity,high precision and good real-time performance.It provides theoretical basis for practical large-scale sensing applications.In order to effectively reduce the influence of high frequency noise on the phase demodulation accuracy of this method,a light-intensity-filtering based rectangular-pulse binary phase modulation method is proposed,which effectively suppresses the influence of high frequency noise and improves the signal to noise ratio.Then,a high-sensitivity fiber-optic accelerometer based on double-ended fixed beam is developed.The performance is tested by the phase detection method based on the rectangular-pulse binary phase modulation.The sensitivity is up to 3300 rad/g,which has good linearity and lateral interference resistance.It can effectively measure low-frequency vibration signals below 100Hz,providing the basis for the construction of the sensing array system and high-sensitivity oil detection.To solve the problem of crosstalk in sensor arrays,a wavelength-cross-combination technique is proposed.Combined with the rectangular-pulse binary phase modulation method,the interchannel crosstalk in array systems based on time division(TDM)/wavelength division multiplexing(WDM)technology has been reduced effectively.Experiments show that the system crosstalk is as low as-60d B when the extinction ratio is lower than 28d B,which effectively improves the system performance.To solve the polarization-fading problem existing in the sensor arrays,three-state polarization diversity technique is adopted,overcoming the complete polarization fading effectively.In order to improve the real-time processing capability of lagre-scale optical fiber sensor array,real-time signal processing technology based on Field Programmable Gate Array(FPGA)is studied.The hardware architecture of FPGA-based real-time sensing data processing based on the rectangular-pulse binary phase modulation is constructed.The rectangular-pulse binary phase modulation method is simple for signal extraction and only three points are needed to restore a single phase point with low sampling rate.The method is suitable for FPGA-based pipeline structure with high real-time performance.In the data processing module,the region growing theory is adopted to complete the extraction of light intensity,phase calculation and unpacking within 9 time cycles.The laytency is 180ns and the throughput of phase data per second is up to 5×10~7,which proves the performance with high accuracy and good real-time.Meanwhile,the analysis of system resource budget shows that the middle-end ATER Stratix III FPGA chip can support the real-time signal processing of more than 1024 sensors.The application of the interferometric optical fiber sensing technology in seismic wave detection is studied.An array system containing 64 sensing units based on TDM/WDM is built.The performance of the system is tested by experiments.The 64 sensing units can effectively reflect the time and amplitude information of the vibration signal,and the minimum detectable acceleration signal is about 100ng/Hz@100Hz.Meanwhile,the application in perimeter security is studied.The Michelson structure is adopted to design a 4-zone system and the length of each zone is 50m.Different intrusion behaviors are warned using time-domain level crossing statistics.The system has been tested for more than one month.The response time of the system is 0.5s.There is no false negative and the false report rate is less than 1%.