Study On Several Key Techniques Of Tunable Diode Laser Absorption Spectroscopy

Tunable diode laser absorption spectroscopy is a detection technique for measuring the information and status of species by tuning the laser emission wavelength over the characteristic absorption line according to the tuning signal. It is widely used in gas detection of industrial and environmental areas.The research purpose of the thesis was to offer technical support for the performance improvement and the detecting range extension of TDLAS. Researches of critical technologies were carried out aiming at system optimization and signal processing in TDLAS, including optimization of system parameters, data processing flow, procedure of wavelength correspondence and calculation method in system application. The major work completed in the thesis was summarized as following:1. The effect mechanism of modulation parameters and lock-in amplifier parameters on detecting signals was researched based on the TDLAS modulation theory. The influence of the parameters to second harmonic signals was observed by analyzing the signal characteristics for various system functions and demands. The basis and method of parameters optimization in concentration and pressure detecting system were summarized by the experiment result. Furthermore, the joint influence of lock-in time constant and modulation parameters to detecting signals was analyzed to make a rule of parameter selection in different conditions.2. The function and effect of each procedure in TDLAS signal processing were discussed and verified by observing its output result from different input signals. After comparing the signals between temperature tuning and current tuning, the characteristic of temperature tuning data which did not suit for subsequent processing was improved by numerical calculation method and signal processing technique in order to approximate the form of current tuning data. A method of calculating the columnar difference between adjacent extreme value points of detecting signal was proposed to extract the absorption feature simply. The processing result showed its advantage in signal processing or data storing and loading, it also provided the possibility for combining the columnar difference method with other matching and retrieval technique in practical system. The numerical characteristics of each absorption line were obtained by fitting the detecting data with the sum of line shape second order derivatives from the signals with overlapped absorption lines. 3. The error sources in the procedure of wavelength correspondence were studied combined with laser static tuning characteristics after analyzing the importance of the wavelength correspondence in pressure detecting. A method of searching the sampling point wavelength from quadratic fitting curves of laser tuning characteristics data was used to eliminate the impact of tuning non-linear property to spectral line shape detection. Modulation index calibration and scanning frequency calibration based on the effect mechanism of modulation parameters to laser emission wavelength were applied to reduce value difference between wavelength sequences of various modulation parameters and improve the line shape detection accuracy.4. Based on the above work, a research on TDLAS system application was carried out including single component concentration detection, multi-component gas detection, pressure detection and long path atmospheric monitoring by the experiment using different tuning way and signal processing method for different detection requirements. A set of calculation method and optimization process was summarized by selecting appropriate analytical means and fitting models to calculate the system output values, which provided an instructive basis for further development of such system.