Study On Distributed All Optical Fiber Laser-ultrasonic Transducer

In the fields of architecture,national defense and industry,it is necessary to carry out health monitoring on structures so as to avoid the occurrence of safety accidents and ensure the safety of workers' lives and property.Because of the large size of the structure(such as large buildings,aircraft,ships,etc.),the number of key target sites,so only a single point of monitoring is not enough.In this paper,the problem of multi-point excitation at the same time is solved by the ultrasonic generation mechanism and the control point.The main work includes:Aiming at the status quo of "single point" excitation excited by fiber-optic ultrasonic transducers.In this paper,energy-coupled multi-point TFBG laser ultrasonic transducers are proposed.The key device is TFBG with different coupling depth ghost mode.By changing the coupling depth of the ghost mode,the intensity control of single-point laser ultrasound excitation can be realized,and the multi-point excitation can be realized at the same time.In this paper,three different coupling depth ghost mode TFBG units are prepared.In order to realize the laser light source energy equally.The results show that three ultrasonic sequences with equivalent intensity are obtained,which verifies the feasibility of the design.Aiming at the TFBG for the production of difficult issues.In this paper,three kinds of micro-structure laser ultrasonic transducers are proposed based on M-Z interferometer.Among them,the fiber core misalignment of the laser ultrasonic transducer is by changing the amount of fiber core dislocation,the coupling to the fiber cladding of energy for precise control and distribution,and ultimately to achieve multi-point excitation."Bowknot-type taper" shaped laser ultrasonic transducer is to adjust the shape of the taper area to control the energy coupled to the fiber cladding and distribution,and ultimately to achieve multi-point excitation.The fiber bending ultrasonic transducers control and distribute the energy coupled to the optical fiber cladding by adjusting the bending radius of the fiber,and finally realize the multi-point excitation.In this paper,the fiber core misalignment type ultrasonic excitation transducer was designed.The experimental results show that the five ultrasonic sequences with the same intensity are obtained,which verifies the feasibility of the design.Furthermore,the verification experiments of "Bowknot-type taper" and the fiber bending scheme are carried out in this paper,and three ultrasonic sequences with equivalent intensity can be obtained in the experimental results,which proves that the design scheme is feasible.Aiming at the problem of mass production of micro-structure laser ultrasonic transducer,a laser ultrasonic transducer based on the splitter type is proposed in this paper.The intensity of single-point laser ultrasound excitation can be controlled by its splitting ratio.Only need to end in the fiber pigtail end directly coated with photoacoustic material to complete the preparation of excitation unit,which greatly reduces the difficulty of mass production.In this paper,four 50:50 splitters were used to produce four ultrasonic excitation units to achieve a four-division laser energy source.The experimental results show that four ultrasonic sequences with equivalent intensity are obtained,which verifies the feasibility of the design scheme.This scheme is the simplest method to fabricate the distributed al-fiber laser ultrasonic transducer.