Continuous wave cavity ring-down spectroscopy(CW CRDS) is an ultra-sensitive spectroscopic technique being developed for the absorption measurement of gas-phase samples and the detection of trace species in an absolute fashion. In this dissertation, the theoretical modeling and simulation for the ring-down signals produced in CW CRDS are presented and a new simple design of the CW CRDS apparatus is proposed and tested experimentally. Numerical calculations have been performed for the coupling efficiency of a finite bandwidth laser into a scanning optical cavity. The theoretical results obtained for the laser coupling efficiency has also opened a new application field for the CW CRDS in the measurement of laser linewidth and mirror reflectivity.
A new design of CW CRDS has been proposed and implemented to a computer-interfaced spectrometer capable of fully automated data acquisition and in-situ analysis of spectroscopic data. The spectrometer produces a ring-down signal by blocking the incident CW laser through the fast scanning of a cavity toward off-resonance by using PZT. The detection sensitivity limit of the spectrometer has been assessed by theoretically estimating the minimum detectable absorption and investigating the experimental noise sources.
The principle of the linewidth measurement technique simply exploits the fact that the coupling efficiency is inversely proportional to the cavity scanning velocity with the proportionality depending on both the laser linewidth and the reflectivity of the cavity mirrors. The simple experimental setup required and the reliable results obtained in the linewidth measurement for a commercial CW dye laser and the reflectivity measurements for several high-finesse mirrors, have demonstrated the practical feasibility of the proposed technique.
The spectrometer in this study has demonstrated the minimum detectable absorption coefficient of $9.79×10^{-10} cm^{-1}$ which is about 3 times higher than the shot noise limit of $3.08×10^{-10} cm^{-1}$. The detection limits have been theoretically estimated by the quantitative error analysis of the uncertainties arising from specific statistical noise sources. The detection sensitivity of the spectrometer attained in this study is quite high already to explore the unknown weak absorption features of numerous samples.
Line broadening effect should be taken into account in the quantitative analysis of the spectral lines experimentally resolved. In case of CRDS, the sufficiently low pressure of gas samples allows Doppler broadening to predominate the others, which then requires the treatment of the lineshapes in Voigt profile. Nonlinear curve fit of the spectral lines to multiple Voigt profiles has been shown to give the reasonable measure of the linewidth for each spectral line even though the lines are appreciably overlapped.
