Optical Heterodyne Cavity Ring-down Spectroscopy

Molecular spectroscopy is a primary method to study the interior structure and dynamics and physical and chemical properties of molecules. It is widely applied to the scientific investigations and material analysis. The spectroscopic technique is the experimental basic of molecular spectroscopy and can even promote the development of molecular spectroscopy, especially those of high sensitivity. Therefore, the study of high sensitive laser spectroscopy is an important component of molecular spectroscopy.Cavity ring-down spectroscopy (CRDS) is a high sensitive absorption spectroscopy detection technology, which has been developed not only to measure atoms, molecules and clusters but also to be used in the detection of trace absorption and quantitative analysis. However, conventional CRDS is implemented by directly measuring of the decay time. It demands the high performance of the data acquisition system and includes measuring a DC signal resulting from the loss of the cavity mirrors, which means that the further improvement of its sensitivity is restricted by the reflection of the cavity mirrors. Optical heterodyne (OH) detected CRDS can theoretically reach the quantum noise limit by avoiding the direct measurement of the decay time and eliminating the common mode DC signal [Ye J and John L H, 2000 Phys. Rev. A. 61, 061802]. A simplified OH-CRDS, which is relatively easier to implement and more suitable for molecular ro-vibrational spectrum study, is suggested in the thesis. Furthermore, its lineshape of the 1st deviation of Gaussian profile is theoretically analyzed, and the dependence of the spectral intensity and lineshape on some experimental parameters is also discussed, and thus, the optimal parameters are presented. Unfortunately, the experimental OH-CRDS spectrum was not obtained yet.