| Cavity ringdown spectroscopy (CRDS) is a highly sensitive optical spectroscopic technique. In contrast to the conventional optical absorption techniques, CRDS is based on the time measurement of the exponential decay of light intensity that is transmitted from the high reflective cavity mirror. Hence the CRDS method is insensitive to power fluctuations of the incident laser. This has been successfully applied in the precision measurement field.Electromagnetically induced transparency (EIT) is an important quantum coherence effect. In the EIT medium, almost all atoms are in a coherent superposition state induced by probe and coupling light field, and therefore absorption of the probe field at resonant frequency decreases, accompanied by a strong normal dispersion, which can be used to reduce group velocity. In this paper, we present a scheme which is based on the EIT effect to prolong the ringdown time and enhance the sensitivity of CRDS. The main results and innovation points are shown as the following:1. The expressions of the ringdown time and the sensitivity of CRDS with EIT are derived theoretically. And the influence factors of the ringdown time and the sensitivity are discussed. The light group velocity can be slowed down by EIT. It means that that lifetime of the intracavity photons is prolonged under EIT. We point out that CRDS could be improved through decreasing the residual absorption of the EIT medium and slowing down the light group velocity.2. In the experiment, we utilize the 87 rubidium to build the A-type three levels EIT for improving the ringdown time and the sensitivity of CRDS. The 87 rubidium is chosen as the sample and we measure the cavity transmission spectrum of CRDS with and without EIT under different sample lengths. The ringdown time and the sensitivity are calculated after some data processing. The results show that the ringdown time is prolonged, and the sensitivity is enhanced with an averaged factor about 1.5.3. In the experiment, we construct the V-type three levels EIT with the 87 rubidium to improve CRDS. The residual absorption of the EIT medium is reduced by the optical pump effect in the V-type EIT. In this way, the ringdown time is prolonged further and the average sensitivity enhancement factor is about 1.45. |
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