Four-mirror Ring Cavity Ring-down Detection Technology Based On DFB Semiconductor Laser

As a high-sensitivity,high-resolution spectroscopic detection technology,cavity ring-down spectroscopy(CRDS)is developed from the special characteristics of high finesse optical resonators.CRDS is widely used in environmental monitoring,disease diagnosis,and safety assurance.In recent years,the technology of CRDS has seen its rapid development with the appearance of new light sources,new frequency matching modulation methods,and new cavity designs.In this dissertation,a narrow linewidth DFB laser is selected as the excitation light source to perform the theoretical and experimental studies in the CRDS technology.It mainly includes the following:1?Research on the principle of CRDS technology.Taking into consideration the process of laser injection into the resonating cavity and using the theory of multiple beam interference,a linear cavity is studied with the light radiation dynamic characteristics in the frequency domain analyzed.The relationship between the acceptable deviation of the central incident laser wavelength and the bandwidth is obtained.The spectral tunability of the DFB laser is utilized to achieve the proper matching between the incident laser frequency and the longitudinal mode of the resonant cavity.A set of coupling optics is designed to get a proper transverse mode matching between the incident laser and the resonating beam in a stable cavity.2?Analysis of the CRDS process.Starting from the theory of multiple beam interference,a mathematical model describing the temporal evolution of the intracavity radiation is established.Different stages of a CRDS process are theoretically studied,including the processes of radiation storage,incidence turn-off and the final ring-down processes.The influences of different bandwidths and central frequency deviations of the incident laser beam on the intracavity radiation charging process are numerically simulated.Combined with the transient relationship between the turn-off and the ring-down stages,the influence of the different turn-off time on the ring-down curve is also discussed.3?An open path four-mirror ring-cavity CRDS experiment.A self-built open path four-mirror ring-cavity CRDS apparatus is design and build.With this apparatus,the concentration of atmospheric carbon dioxide in the laboratory environment is measured.Using the HITRAN database,the discrete point parameters of CO₂ molecules is used to fit the absorption cross-sectional density at different temperatures and pressures.The static characteristics of the DFB laser at the peak absorption wavelength are analyzed and the beam profile measured.With the ABCD theory,the cavity stability of the four-mirror ring is analyzed and the intracavity beam profile on both the meridian and sagittal planes cavity calculated.Three algorithms are studied and compared in their ability in the fitting accuracy and time consumption in the dealing of ring-down traces,including Levenberg-Marquardt,Discrete Fourier Transform,and Linear Regression Sum algorithms.Finally the Levenberg-Marquardt algorithm is selected to get the ring-down times.With a proper deviation from the absorption peak,the cavity ring-down time of the open cavity under a non-absorption situation is obtained.At the spectral resonance between the gas absorption and the incident laser,the concentration of CO₂in the laboratory environment is measured to be 486 ppmv by this open four-mirror ring cavity CRDS.