Research And Application Of New-type Laser Absorption Spectroscopy Based On High-quality Optical Cavity

As an ancient and young science, spectroscopy is in the process of continual development all the time. In the1960's, the invention of laser technique made the spectral resolution and detective sensitivity of traditional spectroscopy gain a great breakthrough. Henceforth, spectroscopy stepped into the age of laser spectroscopy. In the recent 20 years, new types of laser absorption spectroscopy, which are mainly based on high-quality passive optical cavity, have advanced the spectroscopy science again. The new types of spectroscopy techniques characterize with more simple structure and better sensitivity, and now, they have become the typical techniques in modern spectroscopy and been widely applied in many fields such as analytical chemistry and physical chemistry. Besides of cavity ring-down spectroscopy (CRDS, including pulsed CRDS and continuous-wave CRDS), the new types of laser absorption spectroscopy also include cavity enhance absorption spectroscopy (CEAS), phase-shift cavity ring-down spectroscopy (PS-CRDS), integrated cavity output spectroscopy (ICOS), and recently developed, off-axis integrated cavity output spectroscopy (Off-axis ICOS). Here, according to the present situation of domestic research, we choose the CW-CRDS as the main object of the dissertation to research in detail. And then, Off-axis ICOS is primarily studied based on those works of CW-CRDS. In this dissertation, many important problems related to CW-CRDS are presented and researched thoroughly. And several sets of spectroscopy systems are established, and their noise (RMS) equivalent absorption coefficients are obtained as the level of ~10-9 cm-1. Various experimental studies are then operated on with these systems. The main research contents of this dissertation can be generalized as the following parts:(1) According to the energy conservation principle and the definition of passive cavity's Q value, the measuring principle of CW-CRDS is first deduced from a new point of view in this dissertation. And then some important problems, such as the possible influences of incident light's shut-off time and passive cavity's filled-light insufficiency on the measurement results of CW-CRDS, are analyzed and simulated based on these deduced results. Subsequently, some important theoretical problems related to CW-CRDS's sensitivity and precision are also discussed specially, including the mode coupling and frequency matching between incident light and passive cavity, and detector's response characteristics, etc. Additionally, to explain some strange phenomena in experiments, the spectral characteristics of the multilayer high-reflectivity coatings are analyzed and simulated, and their influences on the passive cavity's loss are also discussed.(2) Based on the theoretical analyses, the structural design of CW-CRDS system and the realization of its hardware and software are introduced in detail. Using the DFB laser as the light source and taking quartz glass and glass-ceramic with ultra-low expansion coefficient as the cavity material, several sets of high sensitivity CW-CRDS systems are built, and they have different structures and cavity lengths. In these CW-CRDS systems, the characteristic of DFB laser's current modulation is used to act as fast optical switch device of conventional CW-CRDS system, and the mode matching lens is also saved by the structure design of passive cavity, parameter selection of fiber collimator and application of signal peak detector. Additionally the spectral scanning system is not also needed because DFB laser has the properties of temperature and current tuning. Tests of these CW-CRDS systems'parts show that the equivalent shut-off time of incident light can be less than 80 ns and the laser frequency stability is ~0.002 cm-1, and the response times of PDA400 in different gain settings are obtained too. The mode beating phenomenon and the etaloning effect found in CW-CRDS system's early experiments are studied, and they are explained by combining with theoretical analysis, then technical solutions of eliminating these phenomena are proposed and verified by experiments later.(3) Respective characteristics and applications of CW-CRDS systems with different cavity structures and different schemes are researched, including straight, fold-type and ring cavity structures as well as wavelength-tuning and cavity-adjusting schemes. Among them, the noise (RMS) equivalent absorption coefficient of 4.1×10-9 cm-1 is obtained in the quartz-glass cavity system with the length of ~360 mm, and the noise (RMS) equivalent absorption coefficient of 3.2×10-9 cm-1 is reached in the glass-ceramic cavity system with the length of ~249 mm. Making use of the glass-ceramic cavity CW-CRDS system, the spectra of N2O, H2O and CH4 are measured. And the measured spectral line parameters are compared with HITRAN2004 database, 48 new spectral lines of CH4 molecule unlisted in HITRAN2004 database are observed in the range of 6586.5~6595.5 cm-1. Some other experiments are also operated on with these CW-CRDS apparatus, for example, the systems are applied to detecting gas concentration and to calibrating the wavelength-current relation of DFB laser. The experimental results show that the measurement sensitivity of these systems can reach the level of ppm and ppb as to the concentration detections of N2O and C2H2 gases respectively, and the wavelength precision calibrated with this dissertation's method can be 2~3 times better than the conventional methods.(4) The Z-shaped fold-type CW-CRDS system with the total cavity length of ~750 mm (cavity dimension ~250mm) is experimentally studied, and its noise (RMS) equivalent absorption coefficient is measured as ~1.5×10-7 cm-1. The ripple phenomenon of cavity loss spectrum is theoretically analysed and experimentally studied And then the etaloning effect is confirmed as the origin of the phenomenon. Furthermore, it is observed that the inner field of folding mirror coating fluctuates periodly with the scanning of laser frequency. This is an important discovery. This discovery will be valuable for separating some special coating losses of a high-reflective mirror from its total loss, and it can provide an important reference for the absorption measurement of a high-reflective mirror in high-power laser. Additionally, it can also be helpful to the progress of optical coating. Following that, the 8-shaped ring cavity system with the total cavity length of ~1 m (cavity dimention ~250mm) is tested and the above discovery is confirmed. And its noise (RMS) equivalent absorption coefficients of wavelength-tuning scheme and of cavity-adjusting scheme are obtained as ~1.02×10-8 cm-1 and ~ 6.6×10-9 cm-1 respectively. The measured results of straight and folded cavity systems are compared, and their differences confirm theoretical analysis of cavity structure's influence on the measuring accuracy of system.(5) Off-axis integrated cavity output spectroscopy (Off-axis ICOS) and wavelength modulation Off-axis ICOS (WM Off-axis ICOS) are primarily studied. Their theories and signal expressions are analyzed and deduced based on the character of passive optical cavity and the theory of signal and system. Especially, the choices of some parameters in WM-Off-axis-ICOS are discussed and simulated in detail such as the wavelength modulation amplitude and frequency. A set of low-cost and compact Off-axis ICOS system is built, and its noise (RMS) equivalent sensitivity can reach ~2.0×10-9 cm-1. The spectrum of water vapor molecule in the spectral region of 6587~6595.5 cm-1 is then measured with the Off-axis ICOS system, and the measured results are compared with those results of CW-CRDS and HITRAN2004 database. Additionally, the system based on WM-Off-axis-ICOS and its application are also primarily studied, and some problems related to theoretical analysis are experimentally confirmed.