Based on wavelength modulation spectroscopy and algebraic reconstruction technique, two-dimensional tomography of gas temperature in the range of 400K-1500K and H<sub>2</sub>O concentration in the range of 0.05-0.20 was realized. From the numerical simulation, the transmitted signals of two lines using wavelength modulation spectroscopy method are simulated, then the integrated areas of the absorption lines are obtained. Using the integrated areas of two lines, the two-dimensional tomographic reconstruction are derived. It is shown that the reconstructed gas temperature and H<sub>2</sub>O concentration are consistent with the predicted value and the most relative errors are 2.21% and 5.17%.
Integrated cavity output spectroscopy (ICOS) is a technique used for high sensitive measurement of absorption spectra. In this paper，we introduced the principle of ICOS measurement technology. By analyzing the theory of ICOS technology to effectively absorb the increase of optical path length, get the relationship of the specular reflectance and the selection for the length of cavity to the stability of cavity and detection sensitivity in the designing of experimental facility. We introduced the application of ICOS technology in atmospheric monitoring, medical testing, and industrial process and so on, summed up the role of wavelength modulation and off-axis incidence combined with ICOS on noise suppression. We summed up the advantages and disadvantages of ICOS technology in trace gas detection. At the end of the paper, the trend of ICOS technology was analyzed.