Study On Photoacoustic Spectroscopy Based On Tunable Single-frequency 2?m Laser

Trace gas detection technology is capable of monitoring dangerous gases in industrial production to ensure personal safety.Also,this technique can not only monitor air pollution and be helpful for building up a n environmental evaluation system,but also detect gases exhaling from human body which couldimprove the convenience and accuracy of disease diagnosis.Photoacoustic spectroscopy(PAS),one of the trace gas detection techniques,has the strengths of high sensitivity and strong selectivity,which can be used to retrieve the gas concentration according to the detected acoustic signal.The amplitude of the acoustic signal is proportional to the intensity of the absorption line and the optical power,and it can be improved by selecting the strong absorption line and increasing the power of laser.The optical power of the mid-infrared solid-state laser can be up to 100 m W,and the absorption line intensity is larger in mid-infrared region.Therefore,the study of mid-infrared solid-state laser source to improve the detection performance of PAS is of great value.To begin with,the principle of single-frequency output of etalon was introduced,and the narrowing effect of etalon on the gain linewidth was calculated theoretically.Afterwards,the influence of the reflectivity and thickness of an etalon on the single-frequency selection effect has been worked out.The theoretical work of single frequency selection of the composite etalon was carried out by choosing Tm:YAP as the gain medium,and the thickness and reflectivity of the etalon were determined in the subsequent experiments.On the basis of single frequency output,the theoretical research of fine frequency modulation technology has been developed.Meanwhile,the technology of joint control concerning TEC temperature and tilt angle of the etalon would be applied in the fine frequency modulation research.Then,a Tm:YAP solid-state laser was built,and the continuous output performance of the laser was studied and optimized.The Tm:YAP single frequency output was realized by using the method of double etalons,with the features being studied.To realize high sensitivity monitoring of a variety of gases,on one hand,the tuning range of Tm:YAP single frequency output wavelength has been worked out to cover the absorption lines of gases in the subsequent PAS system;on the other hand,the single frequency output power has been adjusted to the optimal value.Besides,fine frequency modulation of the laser was studied by controlling the TEC temperature and tilt angle of the etalon by Labview.Finally,the H₂O-PAS technology and NH₃-PAS technology were studied by using self-developed Tm:YAP single-frequency laser which was used as the light source.During the study of PAS technology,the performance of the PAS system was optimized mainly from two aspects,including increasing the power of laser and enhancing the intensity of gas absorption line.For H₂O and NH₃ under test,the absorption line corresponding to mid-infrared wavelength was selected for detection,and the power of light source in the experiment was in the order of 100 m W.And a one-dimensional resonant photoacoustic cell is used to amplify the signal.The final experiment proves that the PAS system with laser serving as the light source has excellent detection performance.