Experimental Research On Interferometric Optical Fiber Ultrasonic Sensor And Its Imaging

Ultrasonic imaging technology has the advantages of no ionizing radiation,relatively safe,imaging with fast speed becoming a remarkable new technology that has been widely used in many fields,such as medical diagnosis,geological exploration,materials,structural health monitoring and so on.For this imaging technology,the core device to obtain the layered information in the detected object is the ultrasonic detector.Currently,PZT is mostly used as an ultrasonic detector in engineering application.However there is the limitations of large volume,low sensitivity,susceptible to electromagnetic interference environment,poor reusability in the application requirements of high precision and special environment.From the perspective of application demands and technology development,ultrasonic imaging technology is still an expected research for new progress and breakthroughs in technology.Therefore,in this thesis,the variety of fiber optic ultrasonic sensors based on interferometer were studied,ultrasonic detection system was set up,the multiple homemade seismic physical models were detected and imaged with high sensitivity,the automation of imaging system and elimination of temperature cross-sensitivity were studied using programmable program to control scanning and temperature self calibration method respectively.These studies are of great significance to accelerate the further development of ultrasonic imaging technology.The following are the research contents of this thesis:(1)The development of optical fiber sensing technology and optical fiber ultrasonic sensing technology were reviewed.The basic principle of interferometric fiber optic sensor,the method of ultrasonic generation,the propagation law of ultrasound in solid,liquid and gas,the reflection,refraction and attenuation of ultrasound in the propagation process,and the optimization method of ultrasonic detection system were discussed.The common ultrasonic detection methods were compared and analyzed.(2)A compact Michelson interferometric ultrasonic sensor was designed and fabricated by attaching two outputs of a 1×2 coupler with different lengths(coated on the end-face of two arms with gold film to improve reflectivity)to a tilted polypropylene tube.The corrosion operation of the interference arm improved the sound pressure sensitivity resulting a further improved the ultrasonic detection sensitivity.Combined with side-band filtering technology,the sensor detected pulse ultrasound at the frequency of 300 kHz withSNR of 15.56 dB and continuous ultrasound.In addition,the sensor showed a further enhanced the mechanical strength and improved the mobility of the sensor detection due to the tilted package of the sensor.It was demonstrated that the proposed sensor can be used for the identification of ultrasonic sources from unknown directions.(3)To improve the detected frequency range of the sensor,the air micro-bubble ultrasonic sensor based on FP interferometer was fabricated by repeated discharge operation for HCF.The length and wall thickness of the air micro-bubbles were about 120?m and 13 ?m respectively.The sensor presented good stability,repeatability and high sensitivity to 1MHz pulse ultrasound with SNR of 24.08 dB.To improve the sensitivity of the sensor,the HCF was pretreated with hydrogen which decreases the thickness of the micro-bubble wall and increases the length of the micro-bubble,thus improves the ultrasonic sensitivity(SNR:49.28 dB).Through the method of signal demodulation,the data processing and inversion imaging of the detected signal finally obtained the intuitive diagrams of seismic physical models.(4)In order to further improve the detection sensitivity to ultrasound,an ultrasonic sensor based on FP interferometer was fabricated by combining PVC film with the end face of SMF.The compact sensor showed a large ultrasonic sensitivity with SNR of 68.56 dB due to small young's modulus of the PVC film.By using the time-of-flight method to rebuild the detected ultrasonic signals,the two-dimensional structure diagrams of the trapezoidal seismic physical models and the three-dimensional structure diagram of the spherical seismic physical model were obtained respectively.(5)The ultrasounds emitted by PZT were coupled to POF as ultrasonic source,and the micro-bubble structure was used to detect it in different environments.The combination of air micro-bubble structure based on FP interferometer and POF ultrasonic source constituted an all-fiber ultrasonic detection/imaging system at small detection space.The experimental results showed that the system can detect ultrasound emitted by POF source at short distance in air,aluminum plate and water respectively.