Study On Generation Mechanism And Correction Techniques Of Spectral Distortion For TDLAS

To achieve fast and accurate on-line monitoring and early warning for toxic and harmful gases can be effective in preventing poisoning, explosions and other disasters. In order to suppress the harmful gas emissions and protect the people’s life and property security, gas sensors have emerged. As one of the most important components of the artificial intelligence system, research and development of gas sensor have drawn increasing attention. The gas sensor based on infrared absorption spectrum method is a sensor family’s rising star. However, by virtue of its unique advantages, it has become a hot discussion in the current sensing field. Until now, the infrared gas sensor has achieved rapid development. Infrared gas sensing technology has rendered many branches. Among them, tunable diode laser absorption spectroscopy (TDLAS) is applied widely in situ. This technology not only possess high accuracy, fast response and the ability of measuring a variety of gases simultaneously, but also is running low cost, easy to install, easy to upgrade.Domestic and foreign researchers have done a lot of research on technological progress of TDLAS. However, most works are committed to improving the detection sensitivity of TDLAS system to achieve a high detection limit, which has very important significance for scientific exploration. Admittedly, the system sensitivity is a very important indicator of TDLAS sensor; however, driving R&D to market makes other important indicators equally important, such as accuracy and stability. As we all know, the difference process and harmonic extraction are usually employed in TDLAS system to obtain gas absorption spectrum. The intensity of the absorption peak is used to calculate the gas concentration, which requires an absorption line without any spectral distortion can faithfully reflect the relationship between the absorption peak and gas concentration. But when the surrounding environment parameters (such as temperature, pressure, fiber bending, etc.) and some system parameters (such as wavelength modulation frequency in the wavelength modulation spectroscopy, etc.) changes, spectral distortion will occur, the detection error tends to be several orders of magnitude higher than the minimum detection limit without calibrating. So only discuss detection sensitivity without correcting the spectral distortion is pointless. In addition, the reliability will also determine the long-term stability of the sensor, getting rid of frequent manual calibration is an important sign which can make sensor actual utility. A censusanticipation to the current domestic market suggests the more mature and stable TDLAS gas sensors are basically from abroad. There are little competing products in our country. The technology monopoly of foreign companies leads to the high cost of the instrument. Therefore, development of domestic TDLAS gas sensor with core competence is imperative; the first technical problem to be solved is the prevention and correction of spectral distortion in TDLAS.In our research, a detailed theoretical and experimental study on the mechanism and correction of spectral distortion for TDLAS system is launched. The main contribution and innovation of the thesis are as follows:1. After analyzing the theory of near-infrared selective absorption of gas, the academic base of the measurement of the gas is described by absorption theory. Physical model of the absorption line strength and the absorption line shape function is set out in detail, as well as the variation relation of water vapor absorbance per unit length with temperature and pressure.2. Detection principle and detection systems of the two TDLAS technical branches (direct absorption spectroscopy and wavelength modulation spectroscopy) are discussed in different chapters. The physical models and circuit schematic diagrams of the three demodulation methods (subtraction circuit, division circuit, balanced ratiometric detector circuit) commonly used in direct absorption spectroscopy technique are described. Basic principles and testing procedures of lock-in amplifier applied in wavelength modulation spectroscopy technology are represented. System structure of wavelength modulation spectroscopy and the origin and extraction methods of the second harmonic are introduced.3. The model selections and main indicators of various optical devices in TDLAS experimental systems built-in this study are listed, including laser, photodetector and gas chamber. Schematic diagrams of current drive circuit and temperature control circuit of the laser are given, we also describe their workflow. Methods and effects for enhancing SNR are described.4. Spectral distortion mechanism for TDLAS system (primarily direct absorption spectroscopy) caused by the non-absorption loss is analyzed. Various Sources of non-absorption loss are introduced, including the scattering effect, fiber bending loss, collimation mismatch of the gas chamber, splitting ratio changes of the fiber coupler. The leading role of various scattering effects (Rayleigh scattering, Mie scattering, non-selective scattering) in diverse situations is distinguished, and the magnitude of scattering loss in the lab environment is also given. The mathematical formulas for calculating macrobending loss and microbending loss are introduced. We analyzed the physical models of the three kinds of optical mismatch (off-axis mismatch, angle mismatch, distance mismatch). The amount of variation with temperature of insertion loss and light source power is provided by experiments. We also describe the physical principles of optical fiber coupler splitting ratio functioning with temperature, and give a detailed experimental data.5. The degree of spectral distortion for TDLAS system based on three different demodulation circuits (subtraction circuit, division circuit, balanced ratiometric detector circuit) under the influence of non-absorption loss is compared by the theoretical simulation and experimental verification. The detection error of three demodulation methods in various environmental factors (fiber bending loss, changes in ambient temperature) is also compared. When the ambient temperature is increased by 1℃, the deviation of the division method is only 0.29%, which obviously outperforms the subtraction method (2.90%) and the balanced ratiometric detector (BRD) method (0.55%). Furthermore, a concept, "power fluctuation rejection ratio," is introduced to compare the suppression effects of the subtraction, division, and BRD methods on the laser light source power fluctuation. The study results demonstrate that the division method provides better reliability when the ambient temperature or bending loss is varied.6. A single-beam optical sensor system with automatic photoelectric conversion gain control is proposed for doing high reliability water vapor detection under relatively rough environmental conditions. Comparing to a dual-beam system, it can distinguish the finer photocurrent variations caused by the optical power drift and provide timely compensation by automatically adjusting the photoelectric conversion gain. Spectral distortion under the fluctuation of the optical power caused by the non-absorption loss is not obvious in this system. Experiments have shown that more detection accuracy can be afforded by the new system which is more apt for remote sensing application which is often subject to light transmission loss.7. A single-beam BRD TDLAS system is proposed for high accuracy gas detection, the design of which demodulation circuit is based on the Ebers-Moll model. The majority of photocurrent attenuation caused by the optical loss can be effectively compensated by automatically adjusting the splitting ratio of probe photocurrent. We present a theoretical analysis which can be suppressed the photocurrent attenuation caused by optical loss from 0.5552dB to 0.0004dB by using the all-electronic feedback stabilization. And the system with a significant advantage in terms of suppressing the non-absorption loss is proved through the fiber bending loss tests.8. The main source of background absorption spectrum is concluded through analysis of internal mechanical structure of different optoelectronic devices (lasers, photodetectors, optical fiber collimator). Then we take the direct absorption spectroscopy for example, demonstrate the fact that distortion of observed absorption line shapes occurs when the background absorption spectrum of moisture existing inside internal end-face gaps of optical components at atmospheric pressure is superposed on the desired absorption spectrum of moisture in gas cell where the pressure exceeds 1 atm. Errors that arise with the measurements of moisture concentration and pressure because of line shape distortion effects in infrared absorption spectroscopy are quantitatively analyzed. A correction algorithm is proposed for the dual-beam balanced ratiometric detector strategy to improve the measuring precision. This algorithm corrects the line shape distortion effects caused by moisture inside optical components and conducts a successful moisture measurement and control program for SF6 GIE. In our experiments, impacts of moisture inside optical components have been suppressed, and the mean absolute errors have been decreased by 86.0% and 76.1%, respectively.9. The physical mechanism of the generation of residual amplitude modulation (RAM) is analyzed, and the spectral line shapes of first harmonic and second harmonic signals with the presence or absence of RAM are compared. A new concept, asymmetry factor, is introduced to facilitate the evaluation of the degree of spectral distortion (asymmetry) caused by RAM effect. The influences of the phase shift between amplitude modulation and wavelength modulation, the phase shift between the reference signal and the measured signal, the modulation index on the distortion of the second harmonic signal is given. A continuously wavelength-tunable light source with constant-power output is presented. The unwanted optical power modulation companying with the wavelength modulation of the distributed feedback laser diode by the injection current is successfully offset by the external amplitude modulation of electro-optic modulator. The light sources are employed for gas detection systems based on direct absorption spectroscopy and wavelength modulation spectroscopy respectively. The results prove that the source is more effective for eliminating the RAM and amplitude modulation.