Research On High Sensitivity Fiber Bragg Grating Sensing System Based On Composite Cavity Laser

The emergence of optical fiber makes optical fiber technology a very popular in the field of optics.At present,optical fiber sensing is one of the most important application fields of optical fiber technology.The fiber-optic Michelson interferometer plays an important role in the field of fiber-optic sensing by virtue of its special advantages.For example,flexible structure,good stability and multiplexing.The structure of the fiber Michelson interferometer is applied to the fiber laser system to form a composite cavity.The characteristics of the frequency modulation and mode selection of the composite cavity have been well applied to the fiber laser.Unlike passive fiber grating sensors,fiber laser sensors can usually achieve high signal-to-noise ratio(SNR),high sensitivity,long-distance sensing and multi-parameter sensing.Therefore,fiber laser sensors have been widely used in many fields such as bridge monitoring,military aviation,and industrial manufacturing.The application of interrogation technology is particularly important in fiber laser sensing systems.Among the current interrogation technologies,optical domain demodulation has high cost and low accuracy,while multi-longitudinal-mode laser beat frequency interrogation technology converts optical signals into Electrical signal processing,simple structure,good stability,low cost,is an important means of electrical domain interrogation.This thesis mainly focuses on the research of the composite cavity laser based on the fiber Michelson interferometer,and proposes corresponding schemes for the structural design and interrogation method of the sensing system.The specific research content of this thesis is as follows:(1)The principle of laser laser generation and the generation mechanism of laser multi-longitudinalmode beat frequency signals are described.Using the coupling transmission matrix method of the fiber Michelson interferometer,the transmission characteristics of the light wave in the fiber Michelson interferometer and the principle of interference in the photodetector are analyzed.The influence of the polarization state of the interference arm of the fiber Michelson interferometer and the defects of the demodulation technology are improved.The structure of the Michelson interferometer is applied to the fiber laser.The theoretical model of the composite cavity laser based on the Michelson interferometer is established,and the relationship between the longitudinal mode of the subcavity and the composite cavity mode and the generation of the beat signal are analyzed.(2)Through theoretical characteristics research,a composite cavity laser system based on Michelson interferometer is actually built.The fiber loop mirror and the Michelson interferometer are used as the two cavity mirrors of the laser cavity.The longitudinal mode generated by the laser samples the interference spectrum of the fiber Michelson interferometer.That is,the Michelson interferometer performs envelope modulation on the longitudinal mode of the laser.Stable laser output and beat frequency signal generation.The beat frequency signal interval of the envelope spectrum beat signal is determined by the arm length difference of the Michelson interferometer.(3)A high sensitivity fiber Bragg grating sensing interrogation system based on a composite cavity laser is proposed,and temperature sensing test is performed on it.The fiber Bragg grating(FBG)is used as the sensor head of the laser sensor system,and the chirped fiber Bragg grating(CFBG)is used as the reference grating.The large dispersion characteristic of the CFBG is used to convert the wavelength change into the cavity length change.The multi-longitudinal laser beat frequency interrogation technology is used to demodulate the wavelength change in the electrical domain,and the temperature change is detected by tracking the peak frequency change of the beat signal envelope.The temperature sensitivity is 8.03MHz/?.This thesis proposes a high-sensitivity fiber grating sensor interrogation system based on a composite cavity fiber laser.The system is not only simple in structure and low in cost,the laser locks the mode effectively reduces the influence of the polarization state,and the sensor system has the ability to sense different parameters by selecting different arm length differences.The sensor field has broad application prospects.