Research On Optical Feedback Linear Cavity Ringdown Spectroscopy And Its Application

Cavity ringdown spectroscopy is a highly sensitive gas detection technique,which is suitable for atmospheric trace gas detection.However,low coupling efficiency of laser to optical cavity caused by laser frequency noise greatly limits the detection sensitivity of cavity ringdown spectroscopy.Optical feedback technology is an economical and effective laser frequency stabilization technology,which can effectively suppress the laser frequency noise and improve the coupling efficiency of laser to optical resonator.Most cavity enhancement spectroscopy has a linear cavity composed of two high reflectivity mirrors,while cavity ringdown spectroscopy based on optical feedback is suspected that the direct reflected light of the incident mirrors will cause optical feedback competition,and uses a V-shaped resonator composed of three high reflectivity mirrors.However,the V-shaped resonator structure increases the optical loss,reduces the stability of the system structure,and causes the inevitable phenomenon of "odd and even mode".In recent years,the optical feedback linear cavity enhanced direct absorption spectroscopy technique has proved that the direct reflection of the incident cavity mirror does not compete with the optical feedback generated by the cavity leakage light.However,its sensitivity is limited by laser intensity noise.In order to combine the advantages of optical feedback effect,the stability of linear cavity and the advantages of cavity ringdown spectroscopy,an optical feedback linear cavity ringdown spectroscopy is developed in this dissertation.The main contents include:1.For linear cavity optical feedback system,the phase model of light wave is established,and the frequency response function of linear-cavity optical feedback is derived.The difference between the V-shaped cavity optical feedback system and the linear cavity optical feedback system is compared,and the influence of different parameters on the frequency response of the linear cavity optical feedback is analyzed.The results show that by controlling the distance between the laser and the resonator,the directly reflected light does not participate in the optical feedback competition.2.The key parts of optical feedback linear cavity ringdown system are designed and optimized,including laser-to-cavity pattern matching,optical feedback rate control,feedback phase control and cavity ringdown signal excitation.Aiming at the ringdown signal triggered by the current off mode of laser,the accurate ringdown signal was obtained by selective filtering of the output signal of the detector.3.The main factors affecting the measurement uncertainty of cavity ringdown time in optical feedback linear cavity ring-down spectral system are studied,including optical feedback rate,detector space position and trigger threshold of ringdown signal.Using low optical feedback rate,the amplification effect of optical feedback on cavity birefringence can be reduced,and the measurement uncertainty of cavity ringdown time is reduced from 0.12%to 0.04%.By focusing the transmitted spot to the center of the detector,the junction capacitance effect of the detector can be reduced,the fitting residual of the ringdown signal is optimized from 20 mV to 1 mV,and the measurement uncertainty of cavity ringdown time is reduced from 0.13%to 0.03%.When the trigger threshold of cavity ringdown signal is set at 60%～90%of cavity mode amplitude,a larger signal-to-noise ratio can be obtained,and the measurement uncertainty of cavity ringdown time is reduced from 0.11%to 0.02%.Based on the above analysis results,the detection sensitivity of the system is 1.29×10-10 cm-1 and the detection limit of CH4 at 1654 nm is 0.35 ppb using a linear ringdown cavity with 7800 fineness.4.The atmospheric methane concentration is detected,and the spectral signal-to-noise ratio is 440,which is better than the measurement result of optical feedback cavity enhancement technique.Aiming at the system drift problem of long-term measurement,the stability of long-term measurement results is improved 4.6 times by periodic correction of cavity ringdown time.In this dissertation,the optical feedback linear cavity ringdown spectroscopy is developed to supplement the theoretical model and experimental analysis results of linear cavity light feedback.The technique combines the stability of linear cavity,the narrowing effect of laser linewidth with optical feedback and the advantages of cavity ringdown.The detection sensitivity of this technique is better than that of conventional cavity ringdown spectroscopy and optical feedback cavity enhancement spectroscopy.It is suitable for the observation of the background and annual growth of atmospheric greenhouse gases.