Research On Detecting Signal Disturbance By Devices In TDLAS

With the development of society and the advancement of science and technology,people's demand for gas detection has been continuously improved,and the application scenarios have become increasingly widespread.Therefore,as a rising star in the field of gas sensing,the field of infrared absorption spectroscopy has become a gas sensor hot areas.Due to its unique advantages which including the large dynamic range,high sensitivity,excellent selectivity,long life and non-contact.The tunable diode laser absorption spectroscopy(TDLAS)is the most widely used and most representative in the field of infrared absorption spectroscopy.Because of its high precision,rapid response and multi-component detection in real-time field detection.Thus it has been extensively studied and explored depply.A lot of work has been done on researchers at home and abroad for TDLAS technology.For the detection limit of TDLAS,application scenarios and so on have been more comprehensive research.However,when we apply TDLAS to actual gas detection,we can still find many problems.For TDLAS gas detection,the improvement of detection sensitivity is not only the optimization of the system optical path and circuit,but also the optimization of detection means.More importantly,it studies the performance of some related devices in the system and restricts the detection limit.In the TDLAS,we generally use a tunable laser to output the optical signal,and analyze and detect the absorption signal after the gas to be tested.However,there are some factors in the optical signal other than the wavelength modulation and gas absorption given by itself.Including the abnormality of the output wavelength of the laser itself,the fluctuation of the intensity caused by the transmission of light in the fiber or the coupler,and the abnormal attenuation or loss caused by the absorption of the reciprocating mirror of the air chamber,etc.,which are not highly valued in the general TDLAS.But the part that is crucial for the entire TDLAS inspection.Therefore,in this paper,I will do a theoretical discussion and experimental research on some related parameters and key components in TDLAS gas detection.The main contents of this paper are as follows:The theoretical basis of infrared spectrum detection is given by the absorption line theory of gas molecules and the Beer-Lambert law.Two important branches of TDLAS gas detection are discussed:direct absorption spectroscopy and wavelength modulation spectroscopy.Furthermore,the TDLAS gas detection system and some key devices built in this experimental study are described in detail.Experimental studies were carried out on some correlation parameters that affect their detection performance and detection limit thresholds in TDLAS gas detection.It mainly includes the wavelength response of the fiber coupler to the optical signal,the response of the reciprocating absorption chamber to the gas flow rate and the change of the half-width of the gas absorption line in the actual measurement.Finally we mentioned the effect of background absorption signals caused by common gases in open spaces on detection.Temperature fluctuation inside the distributed feedback laser diode(DFB-LD)is found and analyzed in detail by experiments in this study.In tunable diode laser absorption spectroscopy(TDLAS)based gas sensing systems,the temperature of laser source is usually fixed and current signal is used to drive the laser for wavelength scanning or modulation.Even the laser is controlled by a robust temperature controller,the inside temperature still fluctuate due to the delay effect of controller when changing driving current sharply.More phase shifts between wavelength and power of the laser output are involved due to the temperature fluctuation,which would result in waveform distortion of absorption signal.All of these are investigated in detail by experiments,a resonance frequency that effectively suppresses the temperature effects is introduced as well in this study.