Analysis Of Detection Signals For Infrared Absorption Spectroscopy Technology Of Gases

To achieve further integration of industrial automation and information technology, we need online monitoring in many areas for ensuring product quality and production safety. For lots of monitorings, detections of material gas, intermediate gas and the waste gas are very important in industry monitoring. The gas detection systems or instruments based on infrared absorption spectroscopy have got a lot of public attentions. On one hand, majority of industrial gases have characteristics of infrared emission and absorption spectrum. On the other hand, the development of semiconductor laser technology, optical fiber sensing technology, and computer application technology facilitated the development of intelligent instruments. Currently, infrared absorption spectroscopy gas detection technology has been widely used in bad electronic conditions and combustible environments. For infrared absorption spectroscopy technology in gas detections, there are a few branches, among them tunable diode laser absorption spectroscopy (TDLAS) technology is the most porpular and widely used one for its years of researches, and photoacoustic spectroscopy (PAS) technique takes an important role in trace gas detection because of its no background absorption feature. As TDLAS techology and QEPAS technology have the similar theoretical basis and demodulation methods, we can discuss them together.For equipments in applications, a high resolution is important, and the accuracy and stability of the system in the application are of more importance. We should findout and deeply analyze how the signal works, what the noise sources are, and what changes the signal forms, and then we could know how to improve the system performance. Therefore, the detection signal analysis of a system is very important.In this dissertation, we discussed the system noise sources, residual amplitude modualtion, laser nonlinear effect, signal improvement, and detection system transplantation based on infrared absorption spectroscopy. The main work and innovations of the dissertation are as follows:1. We introduced the mechanism of molecular absorption of infrared and the theoretical basis of infrared absorption detection technology. We also detailed the key parameters of the theory, including the absorption line strength and the line shape functions, and discussed how the pressure and temperature affect the number density of gas molecules, the line strength, the full width at half maximum, the line shape functions, and the unit concentration absorption coefficient. We also introduced the mechanism of the photoacoustic signal generation, and gave out the theoretical derivation of photoacoustic signal in quartz-enhanced photoacoustic spectroscopy detection systems. The signal conversion process according to the actual application conditions was analyzed. We expanded the Lorentz absorption line function in Fourier series expansion, and explained the theoretical basis for harmonic detection technology. The analytical expressions for each harmonic coefficient were given. We also analyzed the sources and parameters related to the residual amplitude modulation.2. Presentations of system structure for tunable diode laser absorption spectroscopy (TDLAS) technology and quartz-enhanced photoacoustic spectroscopy (QEPAS) technology were given. We demonstrated three signal demodulation structures of TDLAS system:differential method, harmonic detection, and a method of harmonic detection after difference. Technologies of wavelength modulation and lock-in detection were introduced.3. An analysis of non-electronic noise sources of TDLAS system was done, roughly including three areas:power fluctuation, background absorption and etalon interference noise. We detailedly analyzed the fluctuation causes for the detection signal light power:fiber macrobending (jitter), the optical devices operating status changes caused by ambient temperature variation. As water vapor is a major constituent in atmospheric, air could be packaged in the device inevitably in packaging process, the device would cause absorption when laser passing through the devices, this was so called background absorption. By analyzing the internal structures of the system used devices in optical path, we found out the locations of air gaps and estimated the influence of such absorption. As there existed a few optical planes in the light path, light beams reflected among the planes and caused etalon interference noise. We gave out the theoretical analysis of the effect on the system detection limit by etalon noise.4. We discussed the effect on detection signals of phase difference between output modulated laser frequency and power. We studied the effect of phase difference on the second harmonic signal in single phase lock-in detection and dual phase lock-in detection systems. For single phase lock-in detection system we proposed a phase compensation method. We also studied how the scanning phase difference influenced the second harmonic signal. We proposed a phase difference measurement method based on the fact that the same absorption line appeared asymmetrically when laser was driven by a symmetrical signal. We offered the theoretical basis of such measurement method, and measured the phase difference with drive frequencies of 50 Hz?50 kHz. Finally, we discussed the versatility and performance of the method.5. Based on experiments and simulations we studied the influence of laser nonlinearity on harmonic signals in low concentration detection cases. We found that when there is no gas absorption, the background signals of one and second harmonics were related to the laser scanning current. Take the second harmonic for example such laser nonlinearity effect badly deformed the signal when the concentration was below 5 ppm with a 5-m gas cell. To suppress the signal distortion, we suggested the double light path structure.6. We discussed the signal enhancment methods of QEPAS system. We studied the best locations for the acoustic resonance tubes, and measured the magnification around the best incidence points. At the same time, we compared the resonance effect of quartz material and stainless steel material with the same physical dimensions. We proposed a QEPAS gas detection system with right-angle prism. Based on the fact that the right-angle prism could parallelly reflected light beam and the two optical axes did not overlap, we developed a double-pass structure. And we used two pairs of resonators to enhance the acoustic signal. Compared with the single beam structure, the double-pass system improved the acoustic enhancement factor from 16 to 22.4, and decreased the noise level frome 14.3?V to 11.6?V. The system signal to noise ratio was improved.7. We discussed the effect of diluent gas on detection signals. When using the methane detection system calibrated with nitrogen to test the methane levels in oxygen and ethane, signals were different at the same methane concentration within the two diluents, so the detection results deviated the standard values. Based on the experiments and simulations we found that changing diluents could cause absorption spectral line widths change. Finally the detected peak value of absorption line and the signal amplitude of second harmonic were changed. We tried to use mechanics of collision to explain how the dilution gases affect the spectral line widths.