Research On Tunable Laser Absorption Spectroscopy Gas Sensors Performance Improvement

With the development of science and technology and social progress,there are lots of needs for real-time monitoring industrial production and social life to ensure the safety.Source,intermediate and exhaust gas detection are parts of gas detection technology.Near infrared absorption spectroscopy detection technology is more and more widely used among various kinds of gas detection technologies.On the one hand,most target gases which are used to production have spectral absorption properties in the near infrared region.On the other hand,the near infrared spectral absorption gas detection technology has benefited a lot from the development of optical communications and semiconductor laser technologies.Optical fiber has advantages of resistance to electromagnetic interference,small transmission losses and light weight,which makes near infrared spectral absorption detection based optical fiber technology being widely used in all kinds of harsh environment.Among all kinds of near infrared spectral absorption detection technologies,tunable diode laser absorption spectroscopy(TDLAS)technology is the most widely used.And commonly used branches include direct absorption technology,wavelength modulation spectroscopy technology and photoacoustic spectroscopy technology.All kinds of sensors and smart instruments based on the technologies mentioned above are becoming the important direction of national strategic development.For practical application of TDLAS sensors,the detection environment and application range are determined by the sensors parameters.Stability,resolution,detection limit and other indicators decide whether the sensors could achieve high precision and reliability detection.And in order to facilitate the spectral information from the gas information inversion,waveform adjustment and recovery technology are also important parts.Therefore,research and improvement of these parameters are the main research theme of this paper.Taking water vapor detection as an example,the paper introduces selection and system set up,stability,detection limitation and resolution,waveform adjustment and examples of detection systems of TDLAS devices.The main research contents are listed as follows.1.The mechanism of molecular absorption of infrared and the theory of infrared absorption detection technology are introduced.The key parameters include absorption line strength,full width at half maximum and line shape functions are introduced.The temperature and pressure effects on line width,line shape function and measurement of detection results are analyzed as well.We also introduce and compare the technologies of direct absorption,wavelength modulation spectroscopy and photoacoustic spectroscopy.The commonly used devices and system structures of TDLAS sensors are introduced and compared as well.2.The stability improvement methods of TDLAS sensors are introduced.The effect brought by environment and background absorption can be suppressed by improving optical devices;the effect brought by low-frequency scanning baseline,background absorption inside optical devices and environment can be suppressed by the technologies combination of wavelength modulation spectroscopy and subtraction;the effect brought by non-absorption transmission losses can be suppressed by comparing artificial absorption and actual absorption during each scanning period;the effect brought by phase shift drifting of diode lasers can be suppressed by phase shift drifting suppression technology.3.The detection limitation improvement methods of TDLAS sensors are introduced.The detection limitation and resolution can be doubled by the technologies combination of Herriott gas cell and mirror;the effect brought by irregular background noise can be suppressed and signal to noise ratio can be improved by artificial absorption technology;signal to noise ratio can be improved and detection consumption can be reduced by wavelength modulation based frequency stabilization technology.4.The waveform adjustment methods of TDLAS sensors are introduced.The effect brought by intensity modulation of diode laser can be suppressed by technologies combination of subtraction and division and wavelength modulation;waveform can be adjusted by modulation index adjustment at each scanning point,and it can be used to improve sampling rate and reduce errors.5.The practical products are chosen as examples to introduce TDLAS technology application.In the field of food packing and substation monitoring,water vapor is used to ensure safety;in the field of environment monitoring and food safety inspection,carbon monoxide sensor is installed and applied.During the accomplishment of this paper,some innovations have been proved to be valid.The main innovations are listed as follows.1.The effects brought by optical devices are taken into consideration,and corresponding improvement methods are put forward.The optical path can be reduced by placing the parts inside the devices;the devices can be unaffected by environment temperature by applying temperature control system;the concentration of background gas can be reduced by packing under high purity nitrogen environment.2.The artificial absorption technology is firstly introduced.The effect brought by non-absorption losses can be suppressed by detecting artificial absorption and actual absorption during each scanning period;detection limitation can be improved by combining artificial absorption and actual absorption to exceed irregular background noise;at the same time,artificial absorption can be used to select reference point for direct absorption system.3.The phase shift drifting effect brought by long time working of diode laser is studied.And the solution is firstly introduced.The reference demodulation signal is generated by laser instead of signal generator.Then phase shift drifting can be suppressed and long term stability can be improved.4.Frequency stabilization technology are used to reduce bandwidth of lock-in amplifiers and improve signal to noise ratio.At the same time,rapid processing data makes it possible for rapid detection such as combustion and explosion detection.In addition,frequency stabilization method is introduced to suppress the wavelength drifting effect brought by long term working of diode lasers.5.The modulation index adjustment method for waveform adjustment is firstly introduced.The whole waveform can be adjusted by adjusting modulation index at each scanning point.According to pure frequency modulation signal,the actual signal can be adjusted to suppress intensity modulation of diode lasers.Sampling insufficient problem can be solved by improving turning points of harmonic signals.6.Improvement methods for common mode noise suppression with subtraction,intensity modulation effect suppression with division,detection limitation and resolution improvement with long optical path gas cell are introduced.At the same time,the actual sensors with these technologies applied are introduced as well.