Research On Ultrasonic Sensing And Imaging Technology Based On In-fiber Mode Interferometer

Ultrasound has a wide range of applications in non-destructive testing industrial,medical imaging diagnosis,marine fishery,and other fields,because of its good directivity,concentrated energy,and strong penetrating power.For ultrasonic detection,traditional electrical ultrasonic sensors can no longer meet the needs of industrial development due to their large acoustic impedance,narrow response bandwidth,and poor reusability.Compared with electrical ultrasonic sensors,fiber-optic ultrasonic sensors have many advantages such as compact structure,high sensitivity,and anti-electromagnetic interference,so they have become the research focus in the field of ultrasonic applications.At present,fiber-optic ultrasonic sensors mainly include fiber grating-type and interference-type.Fiber grating-type ultrasonic sensors have relatively fixed structures and are easily affected by temperature.Ultrasonic sensors based on fiber Fabry-Perot interferometers have complex manufacturing processes and material compatibility and other issues.So this paper proposes an in-fiber ultrasonic sensor based on mode interference.The main research contents are as follows:(1)An in-fiber ultrasonic sensor based on the interference of fiber non-circular symmetry high-order mode and fundamental mode is proposed.The mode interference theory and ultrasonic sensing theory on the basis of the mode characteristic equation are established,point out that the introduction of perturbation points in the optical fiber can effectively excite non-circular symmetry high-order modes,and clarify that the fiber mode interferometer ultrasonic sensing principle is based on the ultrasonic pair mode the dynamic modulation of the length of the interferometer sensing optical fiber.in view of the weak ultrasonic detection signal and the large bandwidth,quotes the intensity demodulation method and discusses and analyzes the demodulation principle and working point selection of the intensity demodulation method.(2)A two-mode fiber mode interferometer based on misalignment offset fusion splicing is proposed.Displacement welding is used to introduce perturbation points to realize the interference of LP₀₁ and LP₁₁ modes in two-mode fibers,and by optimizing the dislocation offset distance,discharge power,overlap distance,and other parameters,an in-fiber mode interferometer with low insertion loss and high contrast is prepared.Using a sensor with an interference length of 3cm,the focus is on the ultrasonic sensing characteristics of the in-fiber mode interferometer.The experimental results show that the in-fiber ultrasonic sensor can accurately detect 40k Hz-2MHz ultrasonic waves.In addition,the ultrasonic sensitivity is improved by adopting a reflective sensing structure and increasing the interference length.(3)The application research of ultrasonic imaging in the fiber mode interferometer is initially carried out.Based on ultrasonic testing,an ultrasonic imaging device was established to perform ultrasonic scanning on a plastic rod with a diameter of 4 mm and a hollow pen core with an outer diameter of 6 mm,and we record the axial sound pressure distribution point by point.Combined with the filtered back-projection algorithm,we finally realized ultrasound tomography with millimeter resolution.