A Novel Method Based On The Time-dependent Absorption Spectroscopy Analysis

Time-dependent absorption spectroscopy,a new type of absorption spectrum detection technology developed in recent 30 years,such as Cavity Ring-Down Spectroscopy(CRDS)and Phase-shift Spectroscopy(PSS),has always been the focus of researchers owing to its high detection sensitivity,fast response speed and unaffected by the fluctuation of light source intensity.Traditional absorption spectroscopy techniques including Direct Absorption Spectroscopy(DAS),Wavelength Modulated Spectroscopy(WMS)and Cavity Enhanced Absorption Spectroscopy(CEAS)are all based on lambert-beer law,which can be difficult to measure weak absorption under strong background light signal.And the instability of the light source also imposes certain limitations on detection.However,the time-dependent absorption spectroscopy technology can compensate for those defects of traditional absorption spectroscopy because it will not be affected by the fluctuation of light intensity.This paper first introduces the basic principles of Cavity Ring-down Spectroscopy and Phase-shift Spectroscopy,and some fundamental characteristics of resonant cavity are also analyzed.It can be found from the principle that the Cavity Ring-down Spectroscopy and Phase-shift Spectroscopy have the same components except the light source.So,in essence,this kind of spectral technology is dynamic response of resonant cavity to different light sources,different spectral technology corresponding to different dynamic response.In order to study the time-dependent absorption spectroscopy technology,the sensor theory is introduced.In this paper,the resonant cavity is regarded as a first-order system,the time-dependent absorption spectroscopy is analyzed by the first-order system theory,and the equation of time-dependent absorption spectroscopy in the first-order system is deduced.Then the equations derived from the first-order sensing theory are used to analyze the influence of laser pulse width in P-CRDS system and the influence of the relative depth of modulation in PSS system.In the P-CRDS system,the simulation results show that when the laser pulse width is not much smaller than the time constant of the resonator,there is an error between the fitting time constant and the real time constant.A series of corresponding fitting time constants are obtained by simulating a series of varying pulse width and time constants of resonant cavity.Through the analysis of these three sets of data,we get a relation expression.And through this expression,the fitting error is corrected so that the error of fitting time constant in a certain range is less than 1%,which satisfies the system measurement requirement.In PSS system,simulation results show that there will be fitting errors when the relative depth of modulation is small,while the error is less than 0.1%when the relative depth of modulation is greater than 0.23.At last,the corresponding measuring platform is built for P-CRDS system and PSS system.By measuring the stability of CRDS system,the optimal integration time of the system is 38s.In the atmospheric aerosol extinction coefficient measurement experiment,the comparison with the measurement results of the CAPS-EXT extinction instrument developed by Shoreline Science Research shows that the two are in good agreement.In PSS system,we measure the phase difference and the peak value at different frequencies.By the phase-frequency characteristic and amplitude-frequency characteristic deduced from the first-order transfer function,the time constant is fitted and the reflectivity of the cavity mirror is calculated.From the measurement results,the results of the two methods are basically the same.The result shows that the theory of the first-order system is fully applicable to the signal analysis of time-dependent spectroscopy.And the theory of first-order system also makes the theory of time-dependent spectroscopy unified.