In-situ Measurement Of Dissolved Gases In Water Using Off-axis Integrated Cavity Spectroscopy

In this paper,the Off-Axis Integrated Cavity Output Spectroscopy(OA-ICOS)is studied and applied to high-sensitivity,in situ on-line measurement of dissolved trace gases in water.By introducing white noise into the laser actively,the cavity mode noise is effectively suppressed,and the signal-to-noise ratio of the system is improved,which provides an important reference for the further development of high-precision in-situ measurement technology.Based on the characteristics of OA-ICOS technology,a new integrated cavity structure with high stability was designed,and a temperature control box was developed to further ensure the stability of the optical test prototype.The influence of the uncoupled radiation signal on the absorption signal is studied.The 1550nm high-pass filter is selected to optimize the optical path of the multi-parameter gas measurement system according to the parameters of the photodetector and mirrors.The RF white noise perturbation is introduced into the laser drive current to suppress the cavity mode noise in the ICOS system.The effect of the injected RF noise power on the cavity mode noise and the absorption line is studied.The results show that the higher the injected RF noise power,the lower the cavity mode noise,the wider the linewidth of the absorption spectrum and the smaller the peak value of the second harmonic signal,but the integrated area of the absorption spectrum seems not significantly affected.The signal-to-noise ratio(SNR)analysis results show that the optimal noise perturbation conditions for both OA-ICOS and WM-OA-ICOS systems are-20 dBm RF white noise which improves the detection sensitivity of CH4 by a factor of about 3 times and 4 times,respectively.The relationship between the CH4 concentration(0.2?10.3 ppmv)and the absorption integrated area was studied.The results show that the concentration inversion formula needs to be corrected when the single-pass absorption loss is larger.Another concentration measurement test result shows that the RF white noise perturbation will not break the linear relationship between CH4 concentration(0.2?2.0 ppmv)and the peak of the 2f signal.Based on the off-axis integrated cavity output spectroscopy technology,a set of test prototype that can measure methane and carbon dioxide simultaneously.The measurement accuracy,stability,and noise source of the prototype are analyzed.The curve relationship between the relative integrated area and concentration of the two absorption signals was analyzed and fitted.The detection limit of the measured methane concentration was 7.7 ppbv,the detection limit of carbon dioxide is about 1.4 ppmv.A comparison test with the Picarro formula G2201-i gas analyzer shows the reliability of the test sample.The self-built water-gas separation system was connected with the prototype to analyze the dissolved gas components in the tap water.