Research On High Sensitivity Optical Fiber Sensor And Multipoint Fiber Laser Ultrasonic Transducers

Optical fiber devices have more performance advantages than ordinary electrical devices,which makes optical fiber devices more and more widely valued.In the detection of environmental health,optical fiber sensors can effectively detect environmental parameters changes.Fiber optic Fabry-Perot cavity sensor is used to realize sensing by two-beam interference.When the environment changes,the cavity length or refractive index of the sensor will change.This change will cause the interference spectrum to move.By demodulating this change,environmental parameters can be monitored.Fiber-optic Fabry-Perot cavity sensor not only has the advantages of simple structure,easy encapsulation and low cost,but also has high sensitivity and flexible fabrication method,which has attracted wide attention.In this paper,two novel sensitivity amplification schemes based on Vernier effect for optical fiber sensors are proposed.Aiming at the complexity of fabricating cascaded cavity Vernier sensor and the problem of constant magnification,this paper presents a dual-path discrete Vernier effect sensor system.In this scheme,two optical paths of the coupler are used to input the spectrum of the optical fiber sensor and the spectrum of the matching cavity respectively,so that the optical fiber sensor is separated from the preparation of the matching cavity.The Vernier effect of this scheme greatly improves the success rate of the sensor and the magnification of the sensor is replaceable.The key of fiber-optic Vernier effect sensor lies in the increase of magnification,which puts forward a high requirement for the preparation of the sensor.A single-cavity Vernier effect sensor is proposed in this paper.The sensor can be realized simply by an auxiliary matching device and a calculation module in the spectrometer.By recording the spectrum of the fiber optic sensor and matching it with an auxiliary matching device,the interference spectrum of the matching cavity can be recorded when the appropriate amplification spectrum is adjusted.Then the subtractive spectrum of the Vernier amplification effect can be realized by subtraction function of the spectrometer.Optical fiber laser ultrasonic transducer,as the excitation source in active ultrasonic monitoring,plays an important role in structural health monitoring of buildings.At present,the research on fiber laser ultrasonic transducer is focused on the structure of single-point ultrasonic transducer and the photoacoustic materials used,but the research on multi-point equalization ultrasonic transducer is seriously inadequate.The research point of multi-point equalization ultrasonic transducer is that the optical energy in the fiber core is coupled by the reasonable fiber coupling mechanism,so the multi-point ultrasonic transducer system has more engineering application value.In this paper,the transmission distribution of optical field in waist enlarged structure,peanut shape structure and single mode-coreless-single mode structure is analyzed by using beam propagation method.With software simulation,the result shows that the three kinds of fiber ultrasonic transducers can effectively couple the optical energy into the fiber cladding,and the coupling ratio of these ultrasonic transducers is related to the structural parameters of the fiber micro-fabricated structure.Then,the corresponding fiber-optic ultrasonic transducers are fabricated through experiments,and the experimental excitation of these ultrasonic transducers is studied.The results show that these ultrasonic transducers can achieve balanced multi-point ultrasonic excitation.