Optical Fiber Ring Laser Sensor System For Ultrasonic Detection

In daily life,aerospace vehicles,large offshore platforms,high-rise buildings and long-span bridges and other load-bearing structures are subject to different kinds of disasters that created by nature or human such as earthquakes and floods during service and long-term use(generally speaking,the life span of large civil structures can be decades or even hundreds of years),which will cause the change of mechanical parameters including strength and stiffness.Changes in these mechanical parameters can cause damage to the structure such as cracks,corrosion,and deformation,while undetected damage can cause structural damage,architecture failure and even more serious consequences.Materials or structures will generate ultrasonic signals in the process of deformation and damage,so how to detect such ultrasonic signals in time becomes very important in structural health monitoring.In order to eliminate hidden dangers or avoid catastrophic accidents to a certain extent,it is necessary to establish a health monitoring system for detecting structural damage.As a new type of sensor device,the fiber grating sensor has a series of advantages such as light weight,small size,high sensitivity,no electromagnetic interference,intrinsically explosion-proof,wide dynamic range,corrosion resistance,and easy to embed in materials to form intelligent materials and structures,which make it a research hotspot in various scientific fields in the world,and also make it well applicable to structural health monitoring in large structures.Based on the above problems,an ultrasonic detection system based on a fiber-optic ring laser sensing structure was designed.This system can convert the offset of the fiber Bragg grating reflection spectrum caused by external signals into intensity changes,so that the system has the characteristics of fast,high precision and low cost.The research content is divided into the following points:(1)The principle of intensity demodulation based on edge filtering in FBG sensing system is analyzed in this paper.A fiber ring laser sensing system combining fiber Bragg grating and non-tunable fiber Fabry-Pérot filter is proposed for the detection of ultrasonic dynamic strain signals.(2)A non-ring fiber grating sensing system based on an ASE broadband light source is described.The fiber Fabry-Pérot filter is used as a demodulation element to explore the response characteristics of dynamic strain signals,and the measured dynamic strain frequency reaches 30 kHz.(3)Two kinds of FBG sensing systems of fiber ring laser are established,which are fiber ring laser sensing system based on the demodulation of external Fabry-Pérot filter and fiber ring laser sensing system based on the demodulation of internal Fabry-Pérot filter respectively.The dynamic strain signal with demodulable ultrasonic frequency in these two systems is up to 500 kHz,and the dynamic strain signal of ultra-high frequency is up to 20 MHz.(4)The characteristics of two types of fiber ring laser sensing systems are studied and simulated.The optimal demodulation range and demodulation sensitivity are obtained.The dynamic strain signal of ultrasonic frequency and ultrasonic signal in water are detected.(5)The fiber sensing system based on a broadband source and ring laser sensing system are described and the sensing characteristics of these two types of systems are compared.It can be seen from the comparison results that the sensing performance of the fiber ring laser sensing system is better than that of the broadband source fiber grating sensing system,which has great advantages in the demodulation range,multiplexing and stability of the signal.