The Research On The Design,fabrication,and Performance Optimization Of Fiber Bragg Grating Ultrasonic Sensor

Fiber-optic ultrasonic sensors have many advantages,such as strong electromagnetic interference resistance,corrosion resistance,small volume,lightweight,high-pressure resistance,high sensitivity,and fast response,and so on,which have become a focus in the field of fiber-optic sensing research in recent years.Common types of fiber-optic ultrasonic sensors mainly include interference type and fiber grating type.Interferometric optical fiber ultrasonic sensors have various forms,wide detection frequency bandwidth,and other advantages.However,it's easily damaged,hard multiplexed,and it's also sensitive to parameters,such as temperature,strain.In the process of ultrasonic detecting,it's easily influenced by low-frequency signal interference,causing a big noise signal.In comparison,the fiber Bragg grating ultrasonic sensor has important study value because of its advantages of the stable system,strong anti-interference capacity,good multiplexing,and so forth.At present,the fiber Bragg grating-based ultrasonic sensor needs further research in sensitivity improvement,structure optimization,multi-parameter detection,and other aspects.In view of these problems,according to the relevant theoretical analysis and experimental research,three kinds of fiber Bragg grating ultrasonic sensors are designed and manufactured,and their ultrasonic sensing characteristics are tested,evaluated,and analyzed to meet the requirements of ultrasonic detection in different applications scenarios.Its main work is as follows.(1)An ultrasonic sensor based on wavelength mismatching fiber Bragg gratings is proposed and demonstrated to solve the problem of the low sensitivity of ultrasonic sensors based on a single fiber Bragg grating structure.The innovation point is that two fiber Bragg gratings with matching wavelengths are stretched and squeezed under the action of ultrasound because of the specially designed sensitization bracket,causing wavelength mismatching,and then the response sensitivity of the sensor to ultrasound is improved.The experimental results indicate that the ultrasonic signals detecting with different frequencies of 300 KHz,1 MHz,and 5 MHz by the proposed sensor have a big signal-noise ratio of56.9 d B,53 d B,and 31.8 d B,which is higher than the reported ultrasonic sensor based on interference.(2)In order to avoid the use of the sensitizer bracket,miniaturized sensor structure,to achieve the sensor layered detection and imaging of underwater objects,a micro-cavity modulated fiber Bragg grating ultrasonic sensor is designed and manufactured.The deformation of the micro-cavity and the response of the fiber grating to ultrasonic waves are the important reasons for improving the sensitivity of the sensor.The layered detection imaging results show that the detection depth of the sensor to the underwater object is about 60.9mm.(3)Aiming at the problem that the fiber Bragg grating ultrasonic sensor cannot obtain the magnetic information of geological structure in geomagnetic exploration,a fiber Bragg grating dual-parameter(ultrasonic and magnetic field)sensor is designed and manufactured.Compared with the previous two sensors,the coupling mode of the sensor in ultrasonic detection is changed from liquid coupling(water)to air coupling,which can better meet the needs of an actual survey.In addition,the sensor also realizes the stability detection of the magnetic field,that is,it has the characteristics of multi-function.(4)The advantages and disadvantages of the three designed sensors have been summarized.The deficiency of the previous work has been analyzed,and the next work plan is given according to the problem in this paper.