Research On Frequency Locking Technology In Laser Cavity Ring-down Spectroscopy

Cavity ring-down spectroscopy technology（CRDS）is a high-quality enhanced absorption spectrum technology,with its high detection sensitivity,high resolution,high stability and technical characteristics that are not affected by light source fluctuations,wildly used in the fields of atmospheric,biomedical,and isotopes detection.Frequency locking technology provides a method for cavity ring-down spectroscopy technology,which further improves the detection sensitivity of CRDS technology,and promotes the development of cavity ring-down spectroscopy technology in the field of spectral absorption.Based on the basic principles of classic cavity ring-down spectroscopy technology and the basic principles of locking technology,the matching point feedback adjustment mechanism is established by using matching points,and the 1654nm methane trace gas are measured.This thesis mainly includes the following contents:（1）The technical principles of cavity decaying spectrum and the theoretical technology of locking theoretical are analyzed.Starting from the"light bullet model"of the pulse cavity,the classic cavity spectral theory was introduced,and the formula of the gas concentration detection and the dying time was obtained.And then,the characteristics of the Gaussian beam in the ring-down cavity and the pattern matching principle are studied,which provides the theoretical basis for the choice of matching mirrors during the ring-down cavity design.Finally,based on the"multi-light beam interference theory",from the perspective of the frequency domain,the relationship between frequency locking in the cavity and the intensity of transmission light is obtained.（2）Through the research on the impact of the measurement accuracy and sampling frequency of the threshold,not only introduces the matching point-based frequency lock method based on the matching point,but establishes a matching point feedback adjustment algorithm model.Firstly,simulation and experimental verification of the device sampling accuracy at different thresholds are implemented.The results show that the threshold is positively correlated with the sampling accuracy,but related to the negative of the sampling frequency.And then,the dynamic adjustment current is achieved according to the matching point.Based on the principle of current modulation,dynamic frequency locking is finally realized.In this part of study,the process of matching point data was introduced,and a matching point feedback adjustment model was established too.（3）The 1654nm methane gas cavity decaying device was built,and the cavity calibration measurement and measurement of methane marks were performed.First of all,a schematic diagram of the 1654nm methane gas cavity decay detection device was designed,which introduces the optical devices and main experimental parameter measurement during the construction process.And then,the air cavity calibration measurement is performed on the device.The result shows that the decay time of the device during the table experiment and after the installation separately is 39.026us and 38.944us,and the decay time standard difference separately is 0.0875us and 0.2176us.Based on these values,the baseline drifting value of this device can be calculated at 3.5214×10^-8m^-1,the minimum detection limit also be calculated at 3.1×10^-10m^-1.After that,the device is measured by 1ppmv methane trace gas.The result shows that the measurement of methane trace gas concentration is N=1.0081ppmv,and its measurement accuracy is 8ppbv.