Detection Of Vibration Signals Based On High Accuracy Fiber-optic Sensing System

Large-scale infrastructure,such as bridges and dams,bring many benefits to our lives.However,due to construction design,environmental pollution caused by rapid development,materials and long-term service,some structures have certain safety hazards,which need to be taken some measures accurately assess the health of their structure to prevent it from happening.Monitoring the vibration signals generated during structural service is an important tool for analyzing many studies related to structural damage.The three main parameters describing vibration signals are amplitude,frequency and phase.One of the methods of analyzing the vibration signals' frequency is commonly used in structural health monitoring.At present,electromagnetic and piezoelectric vibration monitoring sensors have been relatively mature,but the common shortcomings are complex structure,low anti-interference ability and high cost.Therefore,the optical fiber that can overcome these shortcomings has begun to receive attention in the field of sensing,and has the advantages of high accuracy,strong anti-electromagnetic interference capability,and long-distance online monitoring.At present,fiber Bragg Grating(FBG)sensors are mainly used for vibration analysis,which belong to the wavelength coding with high cost of the direct wavelength demodulation,but the demodulation system are complicated and not easy to integrate.For such sensors,the development of a low-cost,easy-to-integrate all-fiber detection system is still a hot topic at home and abroad.Based on the above background,two all-fiber detection systems are designed to convert the wavelength shift of fiber Bragg grating into intensity change in vibration environment to make the detection system with the advantages of high precision,fast speed and low cost.The main contents include the following work:(1)The research background and dynamics of optic fiber vibration sensors at home and abroad are analyzed and summarized.The research status and significance of vibration sensors based on fiber structure are introduced.The research content and direction of this topic are obtained.A method based on Er-doped optic fiber(EDF)for detecting mechanical vibration by two-wave mixing with formation of dynamic population gratings is proposed.In the erbium-doped fiber,two mutually coherent waves propagate in opposite directions form a standing wave field,in this way dynamic population gratings is written along the longitudinal direction of the fiber.The gratings is dynamically written and erased along with the establishment and disappearance of the interference field.At the same time,the mechanism of two-wave mixing for writing gratings in rare earth doped fiber is theoretically deduced and experimentally verified.The experimental results show that the linear two-wave mixing all-optical system based on EDF can be used to monitor the vibration signal generated by piezoelectric transducers with a frequency response range of 50 Hz to 10 kHz with the minimal detecting amplitude of 0.19 ?m,which with a good "time domain" and "spectrum" response.(2)The working mechanism of fiber grating sensors are analyzed briefly.The domestic and international status of fiber gratings vibration sensors demodulation technology are briefly described.At present,most vibration sensing systems of fiber gratings based more likely use FBG sensors.The traditional wavelength demodulation technologies are relatively expensive and much more complicated of these wavelength-encoding sensors.Therefore,a two-wave mixing based on saturated erbium-doped fiber by formation of dynamic population gratings is proposed to detect the dynamic strain signal on the FBG sensor.In the sensing part,we chose a fiber ring laser sensor based on a semiconductor optical amplifier.The FBG acts as a reflection head for the ring laser and at the same time is a sensor that is bonded to the PZT with optical glue.Experimental data showed that the system can respond to dynamic signals in the frequency range of 2 kHz to 10 kHz with the strain of 5 ??.To further investigate the system's detectable range,we attached FBG to an aluminum plate to detect ultrasonic waves with a response frequency of 33.75 kHz and with the strain of 3 ??.(3)Based on the mechanism of dynamic population grating recording,the output two-wave mixing signal of the system can accurately and quickly detect the vibration signal generated by PZT and the dynamic strain signal come from the FBG.However,the frequency range that the demodulation system can detect is limited.For overcoming the shortcoming,we have detected the dynamic signals come from multiplexed semiconductor optical amplifier-based ring laser sensor based on the principle of matching filter demodulation.Two-channel simultaneous signal detection is set as an example.For detection,the system can respond 5 ?? dynamic strain signals with the highest detecting frequency up to 200 kHz.In addition,two cascaded sensing gratings were respectively immersed in two ultrasonic cleaner achieved synchronous ultrasonic detection,and the detected frequency was 33 kHz with the strain of 10 ??.