| Along with the development of modernization and industrialization, a large amount of harmful gases (such as sulfur oxides, nitrogen oxides, organic compounds, halides, carbon compounds, etc.) generated from industrial production and daily life has been emissioned into the atmosphere, which seriously pollutes the atmosphere around us. Air pollution not only brings huge economic losses for mankind, but also endangers the human health and the survival and development of biological, and also leads to corrosion of acid rain, ozone depletion, global warming. Therefore, in order to reduce the damage of air pollution as small as possible, detection of the pollution gas and master the gas emission and distribution have a special significance for environmental protection.Continuous wave cavity ringdown spectroscopy (CW-CRDS) is a kind of highly sensitive laser absorption spectroscopy, in which the absorption of gas species is deduced from the decay of laser light confined in the cavity. Compared with other techniques, CW-CRDS has several advantages:(1) Since the absorption is obtained from the temporal behavior of signal, it is independent of the fluctuations of laser intensity; (2) Owing to the near unit reflectivity of cavity mirrors, a very long effective optical path length could be reached in a high finesse optical cavity; (3) CW-CRDS is kind of calibration-free technique; (4) It is very simple since few instruments can construct a CRDS setup. In this thesis, we focus on the research of CW-CRDS. We firstly constructed an experimental setup based on CW-CRDS, and then investigated and optimized the influence factors during experiments. In addition, the investigations of trace gas species detection were performed based on the constructed system. The key parts contain following aspects:1. Based on the fundamental characteristic of optical cavity, we established the relationship between the lifetime of photon confined in the cavity and the intracavity loss, and demonstrated the principle of CW-CRDS. Furtherly, the response characteristic of the ringdown cavity, the propagation of Gauss beam, the theory of cavity mode and the mode matching between laser and the cavity were analyzed, which laid a solid foundation for the investigation of CW-CRDS.2. Experimental setup used for trace gas detection was designed and constructed. The ringdown cavity was consisted of two mirrors with reflectivity of about 99.35%, which corresponds to the ringdown time constant of about 2.02/us, and cavity finesse of 4832. Then the performances of the diode laser, threshold circuit, and acousto-optic modulator were tested. In order to realize the mode matching between the laser beam and the cavity geometry, a proper match lens was selected based on the measurement of the Gauss beam and the calculation of the Gauss mode inside the cavity. In addition, we investigated the influence factors during experiments and optimized them.3. Experimental investigations on trace gas detection were performed based on the established setup. Firstly, we measured the absorption line of C2H2 at different wavenumber based on a 1531 nm fiber laser and a 1528nm diode laser, and the detection sensitivity of 7.5 ppb and 9.9 ppb were obtained with the sample pressure of 78.2 Torr and 92 Torr, respectively. Then the absorption line of CO2 at the wavenumber of 6330.821 cm-1 was measured, and the long term monitoring of the indoor concentration of CO2 was performed when the laser wavelength was stabilized to the center of this absorption line. After threshold denosing, many times averaging and Kalman filtering, the data could stand for the variation of the concentration of CO2 real time and truly.4. We presented a kind of spatial effect of ringdown time detection in continuous wave cavity ringdown spectroscopy (CW-CRDS) experiments. This effect demonstrated that the measured ringdown time was related to the spatial detection position of photodetector (PD) and the spot size of light. The specific response measurements of PD illustrated that the highest detection efficiency and response speed of PD could be guaranteed only in its effective detection area (EDA). Thus a simple model based on the convolution of the decay time of cavity and the response time of PD was proposed, which can account for the spatial effect observed in our experiments. In addition, according to the absorption measurements of C2H2 gas, we found that neglecting this effect will lead to underestimation of intracavity loss. Therefore, for reliable and stable measurements in CW-CRDS, one should make sure that the PD was aligned perfectly and the laser beam was focused properly to match the size of EDA of PD. The research of this effect will help to precisely evaluate the reflectivity of super mirrors and trace gas concentration.5. The theoretical and experimental investigations of the dynamic response of the ringdown cavity were shown. The results indicated that the intensity of the cavity reflection was more stable than the cavity transmission;: whenever the modulation frequency of cavity length or the intracavity absorption loss was changed. This characteristic makes the cavity reflection more advantageous in the signal processing. In addition, we found that through simulating the dynamic response of cavity reflection or transmission can obtain the stretching velocity of PZT, which has special significance in the correction of the nonlinear response of PZT.6. A new type of continuous-wave cavity ringdown spectrometer based on the control of cavity reflection for trace gas detection was designed and evaluated with a fiber laser. We suggested the reflected light could be used to produce the trigger signal to optical switch and then the ringdown event was fully provided by the cavity transmission. This scheme avoided the employment of threshold circuit and reduced the electronic noise in the signal processing. Moreover, according to the dynamic response of the optical cavity, we know that the intensity of the cavity reflection is more stable than the cavity transmission when the modulation frequency of cavity length or the intracavity absorption loss is changed. Thus the slow changes of the dip depth of reflection can increase the dynamic range of gas concentration measurement, which makes the application of cavity reflection to CRDS even superior.The creative works are as follows:1. In the experimental process, the ringdown time was measured under different conditions, such as different fitting points, different resonant modes, different threshold values, different photodetector, and scanning the laser frequency. According to the experimental results, we analyzed their influences on CRDS measurements, and optimized the experimental system to ensure precisely measurements.2. We observed a kind of spatial effect of ringdown time detection during the experiments. This effect demonstrates that the measured ringdown time is related to the spatial detection position of PD and the spot size of light. Experimental investigations showed that the effect was induced by the inhomogeneous response of PD. A simple model based on the convolution of the cavity decay and the response of PD was proposed to account for the spatial effect observed in our experiments. For reliable and stable measurements in CW-CRDS, one should make sure that the PD was aligned perfectly and the laser beam was focused properly to match the size of the EDA of PD.3. The theoretical and experimental investigations of the dynamic response of the ringdown cavity were shown. The results indicated that the intensity of the cavity reflection was more stable than the cavity transmission whenever the modulation frequency of cavity length or the intracavity absorption loss was changed. This characteristic makes the cavity reflection more advantageous in the signal processing. In addition, we found that through simulating the dynamic response of cavity reflection or transmission can obtain the stretching velocity of PZT, which has special significance in the correction of the nonlinear response of PZT.4. We proposed and developed a new kind of CW-CRDS based on the control of cavity reflection. The keypoint of this method was that the reflected light was used to produce the trigger signal to optical switch for laser beam interruption. Under this scheme, then the ringdown event was fully provided by the cavity transmission and the threshold circuit was replaced. Moreover, according to the dynamic response of the optical cavity, we know that the intensity of the cavity reflection was more stable than the cavity transmission when the modulation frequency of cavity length or the intracavity absorption loss is changed. Thus the slow changes of the dip depth of reflection can increase the dynamic range of gas concentration measurement, which makes the application of cavity reflection to CRDS even superior. |
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