The Study And Application Of Integrated Cavity Based On 1392nm With High Sensitivity

Trace gas detection is more and more widely used in many fields,such as industry,environment,medical treatment and so on.The integrated cavity output spectroscopy has been used as an effective method for trace gas detection because of its high sensitivity,simple device,easy operation,high time and spatial resolution,the capacity of resisting disturbance and it has wide development prospects.In this paper,the theory of the integrated cavity output spectroscopy and its application were mainly studied.The main noise in the integrated cavity output spectroscopy was excess laser intensity fluctuation caused by laser phase change which attributed to the F-P cavity interference,and this noise was called cavity mode noise.The cavity mode noise can be suppressed effectively by the off-axis alignment.In the experiment,an off-axis integrated cavity output spectroscopy(OA-ICOS)instrument based on DFB laser was designed.The light emitting from the laser centered at 1392 nm was directly focused into the cavity formed with two high reflectivity mirrors,separated by a distance of 60 cm.The output signal of the cavity was focused into the photoelectric detector.The experiment proved that the off-axis alignment can arouse a large number of high order modes which made the density of cavity mode increased and had almost no right mode structure characterizing the cavity,so that the cavity mode noise was restrained.The mirror reflectivity was calibrated by that of standard methane at different pressures.The reflectivity of the cavity mirror was measured to be 0.99843 in 1.599 kPa.The H2 O absorption lines at 7189.344 cm-1 and 7185.597 cm-1 were respectively used for measuring trace moisture in the high purity nitrogen gas and standard methane gas.The optimum noise-equivalent absorption coefficients ?min of 1.65×10-8 cm-1 and 3.75×10-8 cm-1 in 200 s were achieved,respectively,which correspond to detection limits(1?)of about 2.28 ppmv and 4 ppmv.In order to further improve the sensitivity of the system,OA-ICOS in conjunction with wavelength modulation spectroscopy(WMS)technique was developed.The absorption of methane at 7185.87 cm-1 was selected to detect methane gases at different concentrations.The instrument parameters(pressure,modulation frequency,phase and amplitude)were optimized.The system stability time was 203 s obtained by Allan variance calculation.The detection limit was 0.87 ppmv using the measurement time of 100 s,and the corresponding minimum detectable absorption was 2.2×10-6 Hz-1/2.Compared with the measurement by the OA-ICOS device,the sensitivity of the wavelength modulation off-axis integrated cavity output spectroscopy(WM-OA-ICOS)was increased at about 22-fold.The second harmonic peak height(2f)and the ratio of the peak height of second harmonic to the medium of first harmonic(2f/1f)was used during the measurement,the results showed that the method of 2f/1f had better stability and higher degree of linearity correlation compared with the method of the 2f.