Study Of High Sensitivity Optical Feedback Cavity-enhanced Absorption Spectroscopy Based On High-quality V-shaped Optical Cavity

The light absorption spectroscopy is an important technique to understand the structure and property of gas molecules,and it has played an important role in the research of physics and chemistry.The qualitative and quantitative analysis of gas molecules can be achieved by direct absorption spectroscopy to determine their transition frequency,line intensity and line profile.Since the 1960s,the application of laser sources to the absorption spectroscopy make the measurement system more sensitive,and a higher spectral resolution can be obtained.Among the laser absorption spectroscopy techniques,the cavity-enhanced absorption spectroscopy(CEAS)has the advantage of simple structure,real-time measurement and high sensitivity,and is becoming more and more popular.Based on the detailed theoretical analysis and simulation of optical feedback effect,two optical feedback cavity-enhanced absorption spectroscopy(OF-CEAS)measurement system are established,a minimum detectable absorption coefficient of?10^-9cm^-1 is achieved in a single scan with 5 seconds,corresponding to a noise equivalent absorption coefficient of?10^-10cm^-1Hz^-1/2.A large amount of experimental studies are operated with these two setups.The primary works and achievements are summarized as follows:(1)Based on the Beer-Lambert law,the 4 parameters to achieve the qualitative and quantitative analysis of gas molecules are determined as:The transition frequency,line intensity,line profile and the effective absorption path length.Applying a distributed feedback(DFB)laser diode as the light source of experimental system,the frequency locking,linewidth narrowing and noise reduction of optical feedback effect is analyzed theoretically.The influence of optical feedback to laser power coupling,spectral resolution and frequency tuning rate are also simulated.In short,the controlling of feedback ratio,phase of feedback light and the frequency tuning rate of OF-CEAS system are summarized.(2)The spectrum parameters of C₂H₂,NH₃ and HCN,such as the line position,line intensity and the full width of half maximum(FWHM)are discussed.The characteristic absorption line are determined according to the principle of selection.The effect of temperature and pressure on the line position,line intensity and the line profile of absorption lines are analyzed.(3)Two V-shaped OF-CEAS measurement setups are designed and established based on the theoretical analysis of optical feedback effect,and the quantitative analysis method of V-shaped OF-CEAS is derived based on the normalized absorption coefficient.The DFB laser diode and its driver,V-shaped cavity,the piezpelectric transducer,the near-infrared photodetector,the fast optical switch and the controlling system of gas are discussed,simultaneously,their performances are tested.Based on the Lab VIEW platform,a software modules including date acquisition,cavity length modulation,feedback phase control,date analysis and cavity ring-down measurement are constructed.(4)The detection characteristics of C₂H₂,NH₃,HCN are tested with two OF-CEAS measurement system.The methods to improve the sensitivity and detection accuracy are introduced,including the Allan-variance analysis,pressure control and applying of mirrors with higher reflectivity.In the measurement of NH₃ in exhaled human breath,the main interference of gases such as H₂O?CO₂ are approximately eliminated at the same time,leading to a improved detection accuracy.High spectral resolution absorption spectra of NH₃ are demonstrated with both optical feedback cavity-enhanced absorption spectroscopy(OF-CEAS)and optical feedback cavity ring-down spectroscopy(OF-CRDS)using the same device.Furthermore,the measurement of different spetral regions are obtained simultaneously with this fibered OF-CEAS system.