Research On Gas Detection System Based On Infrared Laser Absorption Spectroscopy Technique

This paper was supported by China national natural science foundation project “Research on new infrared gas and carbon monoxide detector” and the United States xx project “xxx”.This paper mainly studied gas detection systems based on infrared laser absorption spectroscopy.In this paper,two different laser absorption spectroscopy techniques were used to design four sets of gas detection systems,respectively,and the detection of methane,acetylene,water vapor and formaldehyde were achieved.The structure and principle of each detection system were introduced in detail,the parameters such as sensitivity,response time and stability of the detection system were characterized by experiments.First,the applications and hazards of methane,acetylene and formaldehyde were introduced in the introductory part.The monitoring of the concentration of these gases is important for the protection of life and the environment.Second,several common gas detection methods were listed and compared: electrochemical method,catalytic combustion method,gas chromatography and infrared absorption spectroscopy.Infrared absorption spectroscopy technique has fast response speed,high sensitivity,characteristic of non-contact measurement and long life,which has extensive application prospects in gas detection.The classification and development status of infrared gas detection technology were introduced in the first chapter.Recently,the common infrared gas detection techniques are direct absorption spectroscopy,photoacoustic spectroscopy,cavity ring-down spectroscopy,cavity enhanced absorption spectroscopy and wavelength-frequency modulation spectroscopy,etc.,which have different characteristics.The theoretical basis of gas detection using infrared laser absorption spectroscopy: molecular spectroscopy and Lambert-Beer law were introduced in the second chapter.The cause of the gas molecules' infrared absorption spectrum is the transition of vibrational energy levels and rotational energy levels.Different kinds of gas molecules have different spectral characteristics,which ensures the selectivity of infrared gas detection technology.According to Lambert-Beer's law,the absorption of a specific wavelength of laser light is related to the gas concentration,which is the basis of infrared gas detection.The third chapter mainly introduced the acetylene detection system based on near-infrared distributed feedback laser(DFB laser)and tunable diode laser absorption spectroscopy(TDLAS)technology.The structure of the system and detection performance were introduced in detail.The DFB laser driver and the lock-in amplifier in this system were all independently designed which can easily integrated.An integral discrete digital proportional-integral-differentiation algorithm was used in the laser temperature control module.The control process was rapid and stable with an accuracy of ±0.01 oC.The designed digital orthogonal lock-in amplifier is based on a digital processor chip which is compact and easy to be integrated.Two signal processing methods were compared through experiments and it is found that the system has a lower detection limit when using a divide circuit.The detection limit of the system is 3.97 ppm when the integration time is 1 s,the detection limit can be further reduced to 540 ppb when the integration time increased to 68 s.The methane detection system based on near infrared DFB laser and hollow core band gap photonic crystal fiber was introduced in chapter 4.Compared with single-mode fiber,hollow core bandgap photonic crystal fiber has many characteristics.The hollow core inside the fiber can not only conduct light,but also can be filled with gas as a gas chamber,which can greatly facilitate the detection system structure.According to Allan variance analysis,the detection limit of this system is 1.4 ppm at an integration time of 10 s.The cavity enhanced absorption spectroscopy based near-infrared water vapor detection system was introduced in chapter 5.A near-infrared cavity was designed in the water vapor detection system with a 15 cm cavity length which resulting in an equivalent absorption length of 49.7 m.The laser operation temperature was changed linearly at a low frequency to enable the laser emission wavelength sweep across the gas absorption line.An electro-optic modulator was used in the water vapor detection system to modulate the laser frequency and the repetitively mode-locked PDH technique was used to lock the laser frequency to the cavity.12 times of mode locking were generated in one wavelength scan period.The gas absorption spectrum can be obtained by measuring the intensity of transmitted light.The cavity enhanced absorption spectroscopy based mid-infrared formaldehyde detection system was introduced in chapter 6.Different from the water vapor detection system,radio frequency electrical modulation technique was used in this system to realize the laser frequency modulation which can simplify the system structure.A continuous PDH locking technique was used in this system to realize the locking of a interband cascaded laser and the microresonator with a cavity length of 2 cm.The detection of formaldehyde was achieved by detecting the intensity of the cavity transmitted light.The detailed structure of the system was introduced and the performance were experimentally tested.An effective absorption path up to several kilometers can be achieved by cavity enhanced absorption spectroscopy through a small volume of resonant cavity,so this technique has great application prospect in trace gas detection.The innovations of this article are shown as follows:1.The circuit part of the DFB laser and TDLAS based acetylene detection system has been independently designed and developed,which can achieve stable control of laser temperature,tuning of laser operating current,and detection of photoelectric signals.A division circuit was used in the system to eliminate the effect of residual amplitude modulation and improve the detection sensitivity.The structure of the optical fiber sensor probe used in the system is simple and stable,making the system have the potential of remote and distributed gas detection.2.An HC-PCF based infrared gas detection system was designed without a gas cell.A 0.8m-long HC–PCF was used as the gas cell and the optical path simultaneously,which simplified sensor structure and increases the design flexibility.A very long effective absorption path can be obtained with a small volume,without increasing the complexity of the system.3.The PDH-locked repetitively mode-locking technique was adopted in the cavity-enhanced absorption spectroscopy technology based water vapor detection system.Compared with the continuous locking technology,the wavelength scanning range of this technology is not limited by the length of the resonant cavity.Gas absorption spectra can be obtained over a wider wavelength range.4.In the formaldehyde detection system based on the interband cascade laser(ICL)and cavity-enhanced absorption spectroscopy technology,radio-frequency electrical modulation based Pound–Drever–Hall(PDH)frequency locking technique was used for stable mode locking between the laser mode and the cavity mode.Compared to electro-optical modulation,this method uses RF electronics rather than the expensive optical equipment which minimizes the sensor volume and cost.A compact high-finesse Fabry–Perot(F-P)cavity was designed in this system.A 20 m effective optical path was achieved with a 2 cm cavity length and the system detection limit reached ppb level.