| During the course of establishing the modern civilization, human beings have changed their lives in the unprecedented speed and have simultaneously made natural environment destruction. Presently, many human diseases stem from rapidly deteriorated environment. In particular, after the 1960s, with the industry developing and civilian coal amount increasing, the quality of the atmosphere in our country started to drop, bringing the attention of the environmental protection organization and the government. In our country, 70% energy sources come from the coal. The daily average concentration of the total suspended particle (TSP), NO2, CO exceeds the allowed level in winter, which displays the typical pollution of burning coal. In recent years, as the motor vehicles increase, the type of air pollution is being transformed into the hybria of burning coal and motor waste gas. The particles that come from the vehicle exhaust, are very easy to adsorb the pollutant and then enter respiratory tract to do a bigger harm to the health. After the unleaded gasoline is promoted, the amount for unleaded reduces in atmosphere. However, volatile organic compound in the vehicle exhaust become more. Thus the air pollutant becomes more complex. Moreover, the greenhouse effect caused by carbon oxide and methane, and the industrial waste gases further accelerate to worsen our environment. In "Outline of the Eleventh Five-Year Plan", the government plans to invest a great amount of capital for environment control. Therefore, it is very important to develop all kinds of sensors to monitor the atmospheric pollutant, in particular, to develop portable sensors for gas detection based on high sensitive techniques.The traditional techniques for gas detection are to employ chemic methods. After the sampling is implemented in the field, the analysis is performed in laboratory. This may cause the error of measurement due to changed conditions and/or long delay, so that the accidents, if they are, cannot be dealt with on time. Because of the high-resolution, real-time merits of spectrum technology, it has been widespread applied to gas detection. The gas sensors concerning absorption spectroscopy generally employ the direct absorption technique and second-harmonic detection technique. The former is simpler, but its sensitivity is lower. In the latter, as the modulation frequency increases, the noise decreases, but electrical circuitry becomes more complicated and expensive.Although the cavity-enhanced techniques that are developed in recent several years are simpler and have high sensitivity, they mainly are used in laboratory to investigate the weaker molecular or atomic absorption lines, due to some limited factors. The motivation of the thesis is theoretically and experimentally to research on high sensitive gas detection by use of cavity-enhanced absorption spectroscopy (CEAS) and to make CEAS into practical application.To develop the gas sensors concerning CEAS, we firstly research on the theory of CEAS and make it fit for the condition of field measurement. Secondly, the spectrum features that are used to obtain the concentration information, including linestrength, pressure induced broadening coefficient, are measured. In laser-locked spectroscopy (LLS) or fixed-wavelength measurements, it is necessary to use the frequency-locked techniques. So they are also investigated. Finally, the gas sensors based on CEAS are experimentally researched in detail, combining the theory and spectrum data above. The whole thesis is divided into six chapters for above contents.The Chapter 1 is the introduction. At first, it reviews the conventional spectrum techniques in today laboratory. Then the source of polluted gases and its harm to people are described in order to emphasize the significance of polluted gas detection. At last, the research content of this thesis is educed.In Chapter 2, the theory of the quantitative absorption spectroscopy is described in detail, according to Beer-Lambert law. The linestrength as a function of the temperature and its conversion between several units is deduced. We also exhibit all kinds of broadening mechanism, including natural broadening, Doppler broadening, collision broadening and the important Voigt profile, giving their lineshapes. The calculation methods of Voigt function and spectrum fitting are discussed. The relationship between the concentration and spectrum data of the absorbing species is given. Finally, the minimum detectable absorbance (MDA) as a function of the temperature and the pressure are analyzed and the optimum conditions are determined.The research on the theory of CEAS is carried out in Chapter 3. At the beginning, the cavity modes are discussed by use of Fourier transform. Then the theoretical analyses of the laser-locked spectroscopy (LLS) and cavity-mode-peak spectroscopy (CMPS) employing a confocal Fary-Perot cavity (CFPC) are presented. The signal-to-noise ratio and the minimum detectable absorbance, which are also limited by either the shot noise or the amplitude noise due to the loose laser lock loop, are also discussed in detail. The comparison is implemented between the conventional nonconfocal and confocal Fary-Perot cavity configurations. At last, the theory of the incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) is introduced. The optimum reflectivity and maximal signal-to-noise enhancement factor are found when other significant time-independent background losses exist, apart from the absorption loss of interest.In Chapter 4, nitrogen dioxide and methane that are used as the investigated objects in our CEAS experiments are described from molecular structure to character. The electronic spectroscopy of nitrogen dioxide in visible region is shown. For methane, the vibrational modes and partition function are introduced. The rough spectrums of methane 2v3 and v2+2v3 band are obtained in virtue of a Tungsten lamp and monochrometer. The high-resolution absorption lines of R6, R7, R8 and R9 manifold are measured by use of an external cavity diode laser. Moreover, the parameters of the individual absorption line in R9 manifold are studied according to the theory of Chapter 2. In the end, the harmonic detection is demonstrated based on a weak absorption line of methane 2v3 band. The result shows that a better signal-to noise ratio can be achieved in the high-harmonic detection of tracegases. The contents of Chapter 5 are the frequency stabilization study of laser diode. We employ an absorption line of R9 manifold of methane 2v3 band, which is experimentally obtained in Chapter 4, to realize a frequency reference at 1.6μm. According to Tsuchida's theory, various limiting factors are analyzed and compared with experimental results, so that the way to improve the stability is given. Furthermore, a novel control system has been developed for avoiding the manual operation during traditional frequency locking. As long as derivative-like error signals of absorption as a function of wavelength are scanned out, the laser can be automatically locked to any expected absorption line. The system reduces the equipment, has no adjusting time and has better long-term stabilization.According to the theory of Chapter 2 and Chapter 3 and the spectrum data of Chapter 4, the optical gas sensor based on CEAS are experimentally investigated in Chapter 6. At first, we put forward to two common questions of CEAS: small dynamic range and auto calibration. They need be solved before CEAS is used in the field. Then, the methods of resolving the questions are found by analyzing the signal of the front cavity mirror, so that a high-sensitivity, large-dynamic range, auto-calibration methane optical sensor using a short CFPC is developed. In the experiment of IBBCEAS, a high-power light emitting diode (LED) is used instead of the Xenon lamp to implement the trace NO2 detection. The results verified its feasibility. This made it become possible to develop a portable device based on IBBCEAS for the field gas detection.The innovations of this paper:1. We have presented a complete theoretical description of LLS and CMPS employing a CFPC. The CFPC is freed from the necessity to mode matching due to the degeneracy of the transverse mode, whereas it can obtain the same efficient absorption path length as that of the conventional configuration which is popular to use and need cumbersome mode matching. The SNR with a CFPC introduces a maximum enhancement factor Q = 21/2(1-R)-1 comparing single pass absorption measurement. By constructing a model, we have evaluated the effect of the performance of the frequency lock loop on the MDA. The related results were published in Journal of Korean Physical society 2006 Vol.48, p56.2. We have presented a methane sensor based on a short. CFPC. The sensor used not only the transmitted signal from the back cavity mirror to obtain high sensitivity but also the reflected signal from the from cavity mirror to extend efficient dynamic range and to cancel emitter-amplitude variations. Moreover, a new type of signal processing providing further auto calibration has been employed to make the useful signal independent on the coupling coefficient of the cavity and the gain of the amplifier. Thus we achieved not only the minimum detection absorbance of 4.1×10-5, but also the larger dynamic range that span four orders of magnitude. Simultaneously, the auto-calibration was performed. The problem of auto calibration and small dynamic range were ingeniously solved. The prototype system has been demonstrated with an ultimate sensitivity of 2.9 ppm m and a large linear response ranging from 50 ppm to 5×105 ppm (0.005%-50%). The related results were published in Sensors and Actuators B: Chemical (2007), doi:10.1016/j.snb.2007.04.030 (in press).3. In IBBCEAS, the high-power LED has been employed as light source instead of Xenon lamp. This not only reduces the power consuming and the price, but also removes the elements of the cooling and of the high voltage. The substitution makes it become possible to develop a portable device based on IBBCEAS for the field gas detection. The related results are being prepared.4. The direct absorption spectra of R manifold of methane 2v3 band were measured by use of an ECDL. The Voigt function was employed to fit the spectra so that every individual linestrength of R9 manifold is obtained with the uncertainty of 3%. It provides support for methane sensor based on CEAS. These results were compared with those that have been done by J. S. Margolis with Fourier transform infrared spectrometer. Apart from line 3 and line 4, they are consistent. But compared with our portable apparatus, Margolis's that is bulky. At last, we employed the harmonic detection to investigate a weak absorption line of methane 2v3 band. The related results were published in Acta Optica Sinica 2004 Vol.24, p709; Laser & Infrared 2004 Vol.34, p109; Spectroscopy and Spectral Analysis 2005 Vol.25, p473.5. A frequency reference at 1.6μm has been obtained by stabilizing the frequency of an ECDL onto an absorption line of methane. The frequency fluctuation of the stabilized laser was held within 5.6 MHz. It was better than 160 MHz of the free running laser. The root Allan variance of error signals reached a minimum of 1.6×10-11 when averaging time is 64 s. And various limiting factors were analyzed. It finds that the stability was mostly limited by the noise of photodetector and frequency modulation in the experiment. Moreover, a novel system has been developed for avoiding the manual operation during traditional frequency locking. The system used a computer with a commercial data acquisition card and Labview language. This accomplished the whole operation of frequency locking, including generating ramp, searching locking point, engaging a proportional-integral-differential (PID) regulator on proper time and outputting PID compensation signal. And a new method has also been employed to make the control system accurately identify the locking points of all absorption lines within the scanning range, not the locking range, so that the laser frequency can be automatically firmly brought onto any selected absorption line center without any adjusting time. The related results were published in Acta Photonica Sinica 2005 Vol.34, p489; Journal of Optoelectronics·Laser 2005 Vol.16, p255; Measurement Science and Technology 2007 Vol.18, p1447.
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