Continuous Wave Cavity Ring-down Spectroscopy Device And Trace Methane Detection

The Cavity Ring-Down Spectrometer?CRDS?utilizes the molecular?atomic?"fingerprint"absorption characteristics to obtain the information about the gas in the cavity by comparing the decay time of the light emitted from the cavity.In this paper,we focus on the CRDS?CW,Continuous Wave?technology.The optical axis structure characteristics in the cavity structure,dynamic response characteristics of intracavity optical field,characteristics of light transmitted from the cavity and system detection error sources are studied.And we designed the trace methane measuring device and a large number of experiments were carried out.The main contents of this paper and the conclusions obtained can be summarized as follows:First,the properties of triangular ring passive cavity are studied.In order to improve the utilization rate of light energy,the change of energy loss caused by misalignment and mismatching with the structure of optical axis is analyzed and verified.The expression of light energy transfer efficiency and the maximum tolerance of the system are given in the case of existing adjustment errors.In order to reduce the error caused by the dynamic response of the optical field,the analysis of the triangular ring-down cavity is carried out,and the dynamic response of the cavity optical field is simulated when the incident laser frequency changes and the cavity length changes.The difference between cavity length and laser frequency modulation is compared,and the dynamic response characteristics of the intracavitary cyclic optical field,reflection field and transmission field and PDH?Pound-Drever-Hall?signal are given.It is shown that the oscillation of the amplitude and phase of the intracavitary optical field in the cavity causes the oscillation of the detected signal,which provides a basis for optimizing the scanning speed utilizing the transmitted optical signal of the cavity to reduce the error.Finally,an experimental scheme for inducing the optical field oscillation in the cavity to determine the moving speed of the mirror or the changing speed of the incident light frequency is given.Second,the CW-CRDS technical solutions,including optical systems,electronics systems,hardware and software,are established.The structural parameters of the high-quality passive cavity composed of two plane mirrors and a spherical mirror are determined,which eliminates the reflected light and reduces the background noise.The measurement and locking of the laser frequency is done with a wavelength meter,which reduces the difficulty of tuning.The performance test of the main components in the system is completed.The turn-off time of the laser is within 70 ns,the PZT cavity length adjustment sensitivity is 0.3 nm,the adjustment frequency is 1 kHz,the detection accuracy of wavelength meter at 1653 nm is as high as±0.2 pm,and the stability of the laser is 2.6MHz.Third,for the problem of that,the cavity transmitted light signal is weak,the general direct detection method can not achieve high-precision detection,the optical heterodyne detection technology is applied to the detection of light emitted from passive resonant cavity.P-polarized light is used as the local oscillator light,and the cavity transmitted light is modulated by the eccentric wheel to generate Doppler shift as the signal light.Finally,the two beams are combined to form a coherent optical field with the same polarization direction for detection.This method can improve the conversion gain and reduce the signal-to-noise ratio loss,the minimum detectable power is up to 1.1×10^-5pW.Fourth,the experiment was set up,the attenuation data of the cavity?filled with pure nitrogen?was measured,and the reflectivity of the cavity mirror was calibrated with an accuracy of 10^-7.A trace amount of methane?CH₄?sample was detected by the device,and the methane absorption spectrum in the range of 6046.7 to 6047.2cm^-1 was measured,and the detection sensitivity was 54×10^-9.Finally,the error sources in CW-CRDS technology are analyzed and some experiments are carried out,including laser characteristics?including bandwidth and stability?,incident light off time,cavity length adjustment accuracy,different reflectivity of cavity mirror,cavity length scanning speed,detector noise,temperature and pressure within the ring-down cavity,and other gases present in the chamber.the effects of errors are reduced or eliminated,and the sensitivity of the CW-CRDS system are increased.