Research On High Precision Gas Detection Techniques Of Tunable Diode Laser Absorption Spectroscopy

Tunable diode laser absorption spectroscopy(TDLAS) technology, because of high sensitivity, high specificity, high spectral resolution, has become an effective method for trace gas detection field. During the course of development, a number of atmospheric sensing applications have been reported and successful at home and abroad, but there are still problems to be solved. In this context, the paper mainly studies background elimination and error reduction technology of TDLAS high precision gas detection, including system model, laser characteristics, the second harmonic background elimination technology and the first harmonic background elimination and baseline correction technology. The major content and innovation completed in the thesis is summarized as following:1. The simulation model on TDLAS system. The simulation model on TDLAS system is established, including the laser source, gas absorption cell and the signal detection module. The linear and nonlinear partial of light intensity impact on the second harmonic wave is analyzed. The reason for the second harmonic distortion and residual amplitude modulation(RAM) is found out. The paper gives change regulation on optical interference fringe originated from etalon and simulates the long and short etalon effects on harmonic. It provides a theoretical basis and guidance for background composition analysis and data processing of harmonic signal.2. The study on laser characteristics. It includes laser tuning characteristic, relative intensity noise(RIN), laser scanning step length and laser linewidth characteristics. Not only the tuning characteristic curve of output wavelength of laser, but also the first and secondary fitting results and expressions are given to improve the accuracy of wavelength correspondence. The RIN effect on the photodetector current is studied, and the curve between detector current noise spectral density and modulation frequency is given for different conditions, to provide the basis for the selection of system parameters. This paper analyzes the principle of relation between laser and gas absorption line, provides the principle of scanning signal resulting in scanning step length, and chooses the allowed number of ramp steps. The simulation results show that the curves is obtained between laser linewidth and the maximum amplitude of gas absorption line or linewidth error on the condition of different full width at half maximum(FWHM), and that the minimum allowable FWHM(FWHMmin) and applicable range of temperature and pressure is obtained with laser linewidth fixed. Study on laser characteristics can improve the accuracy of wavelength correspondence, also reduce RIN noise, scanning and linewidth error, then privide related theory basis for improving gas detection precision.3. The study on background elimination technology of the second harmonic. When background signal is to be eliminated, detecting gas can’t be replaced, and the device and system complexity are inconveniently increased for gas detection in the open and closed environment based on TDLAS. A background elimination method based on harmonic detection in none absorption spectral region(HDINASR) is proposed. The method changes the laser emission wavelength in the region without absorption lines by changing the laser operating temperature, and searches for the best background position using the standard background signal in order to eliminate background. The paper designs background separating technology, searching method and spectral line selection principle, then analyzes the stability of background signal. The curve correlation between the reference signal and measuring signal before and after background elimination, is used to describe the effect of HDINASR. With the advantage of avoiding replacing the gas, HDINASR provides an effective background elimination method for open and closed gas detection.4. Study on background elimination and baseline correction for the first harmonic. A new method of background elimination and baseline correction is proposed, in view that there are background signal and strong baseline signal in the first harmonic of TDLAS. It comprises of two steps. The first step is background elimination, which uses HDINASR to eliminate background. The second step is baseline correction. Because there will be a residual baseline after the first step, line fitting can be achieved by selecting a portion of the sampled data to correct baseline level. After eliminating background and correcting baseline, signal distortion is significantly improved and baseline is correctted. Then it is verified that the method in other operating temperature of the laser(26.7-27.2℃) is valid. And the improvement of detection precision of hydrogen fluoride(HF) gas concentration is quantitatively analyzed. It is convenient for the subsequent processing of the first harmonic signal.