Study On Cavity Ring-down Spectroscopy And Its Application

Spectroscopic technique is a primary method to differentiate the species and their individual components in the fields of chemistry and physics. The absorption Spectroscopy, well-known and simple in principle, is widely applied to from scientific investigations to industrial applications. At present, some well scaled absorption spectra, such as those of iodine, mercury, neon and sodium, are used as the absolute wavelength calibration in the experiment. The study of high sensitive laser spectroscopy is an important component of molecular spectroscopy.Cavity ring down spectroscopy (CRDS) not only has high sensitivity, but also can measure the absolute absorption of the sample. The decay times of the leakage of the light from the cavity at 17266.090 cm^-1 are recorded versus the different pressure of the oxygen gas filled inside, from the systematical vacuum limit to 1 atm. Therefore, the lifetime of the empty cavity is obtained to be 2.9174 ms by approaching method, and the absolute absorption cross section of ^RQ(5) ro-vibrational line of the (3, 0) band in the triple forbidden transition b¹Î£_g⁺ - X³Î£_g^- of O₂ is determined to be 1.4998(± 0.0967) × 10^-26 cm², which is in good agreement with previous estimation. Additionally, the reflectivity of the two identical cavity mirrors, obtained by the approached empty cavity lifetime, is determined to be 99.989 (± 0.001)%, which is more accurate than usual method, and the equivalent absorption length is prolonged by a factor of about 9090 in our experimental condition.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. With the optimal parameters obtained from theoretical investigation, this thesis focuses on the experimental study on OH-CRDS. The electronic circuits and the transverse mode matching of the laser beam to the confocal F-P cavity are implemented. The optical beat signal is obtained as well. Unfortunately, the experimental OH-CRDS spectrum has not been observed yet.