Research On Interferometric Optical Fiber Ultrasonic Sensor And Demodulation Technology

Ultrasonic signal has a wide range of applications in ultrasonic testing,medical examination,imaging technology,industrial automation and other fields because of its strong penetration,good directivity,nondestructive testing and other advantages.Optical fiber ultrasonic sensors have the advantages of wide response frequency band,small size and light weight,which is expected to break through the limitations of existing conventional ultrasonic detection technology,such as low sensitivity,could not work in strong electromagnetic interference and harsh environments,so the research of optical fiber ultrasonic sensors has important significance.Based on this,several interferometric optical fiber ultrasonic sensors are proposed in this paper,the ultrasonic detection system is built experimentally,and the high sensitivity detection of underwater ultrasonic signals is realized.The main research content of this paper is as follows:（1）A kind of Mach-Zehnder interferometric optical fiber ultrasonic sensor based on fiber bitaper sandwitching in single mode-multimode-single mode（SMS）structure is studied,and the principle of curvature sensing and ultrasonic sensing of the sensor is discussed.The sound pressure of the ultrasonic signal can modulate the physical condition of the sensor and the bending curvature of the sensor at certain specific frequencies.Therefore,it is necessary to analyze the static curvature performance of the sensor before conducting ultrasonic experiments on the sensor.The static curvature measurement results show that the maximum sensitivity is-34.3935 nm/m^-1 in the curvature measurement range of 2.96-4.19 m^-1,the temperature sensitivity is 0.0106 nm/℃,and the temperature crosstalk can be ignored in practical application.The stability test shows that the measurement standard error is 0.0004 m^-1,confirming it has the ability of long-term operation.Meanwhile,an underwater ultrasonic detection system is built.The experimental results show that the sensor has a high response to pulsed ultrasonic signals,and the signal noise ratio is 21 d B at 110 k Hz.（2）A Michelson interferometric optical fiber ultrasonic sensor based on cantilever beam is studied,which is designed as single mode fiber-bitaper-multimode fiber-fiber sphere structure.The experimental results of static curvature measurement of the sensor show that the maximum sensitivity is 0.6888 nm/m^-1 in the curvature measurement range of 0-12.2894 m^-1,the temperature sensitivity is 0.0078 nm/℃,eliminating the influence of ambient temperature.Due to the cantilever design of the sensor head and the high strain-stress conversion efficiency,the response of spectral intensity of the sensor to curvature changes is significantly improved,so the sensor is easier to detect ultrasonic signals based on intensity demodulation technology.Ultrasonic response experiments show that the sensor has a high response to continuous ultrasonic signals and can successfully detect 110 k Hz continuous ultrasonic wave with a signal noise ratio of 23 d B.（3）In order to meet the measurement requirements of narrow space and simplify packaging technology,a Fabry-Perot interferometric optical fiber ultrasonic sensor based on filled with（dimethylsiloxane）is proposed.The filling technique is used to combine poly（dimethylsiloxane）material with low Young modulus and optical fiber to form an ultrasonic probe.The ultrasonic probe size is small and the ultrasonic response of the sensor is enhanced.The ultrasonic response experiment shows that the signal noise ratio of the sensor can reach 29 d B at 110 k Hz,and the sensor has a high response to a 1 MHz continuous ultrasonic signal.The sensor has obvious wavelength response to the ultrasonic signal,and the wavelength sensitivity is 0.0068 nm/V with a linearity of 0.9891.The sensor has virtues of robustness,integrated in fiber,high response sensitivity,low-temperature crosstalk,and can absorb residual ultrasonic field,making it has a wide range of application scenarios.（4）A demodulation system for optical fiber ultrasonic sensors based on sideband filtering technology is developed.The hardware part of the demodulation system is optimized,and the nonlinear and high DC bias of the common photoelectric conversion circuit are improved by the current mode detection method.The T-type negative feedback network is used to amplify the voltage in two stages.The experimental results show that the signal noise ratio of the demodulation system is improved by about 10 d B for the optical fiber ultrasonic sensor,thus improving the performance of the ultrasonic detection system.