Construction Of Cavity Ring-down Spectrometer And Its Application In Reflectivity Measurement Of High Reflective Mirrors

With the development of thin film coating and chip polishing technique, the mirror that reflectivity higher than99.95%is already in common use. At the same time, the traditional reflectivity measurement can not meet the demand of the high reflectivity measurement, thus accurate measurement of high reflectivity becomes more important. In the1980s, cavity ring-down (CRD) technology was invented to solve this problem. Due to it’s high measurement accuracy and good reliability, CRD technology is considered to be the most effective method of high reflectivity measurement.This paper reviews different methods for measuring reflectivity. The advantages and disadvantages of these methods are discussed. Moreover, the development and application of cavity ring-down spectroscopy (CRDS) technology are reviewed. And the experiment principle of CRD method for measurement of high reflectivity is described in detail. The influence factors for CRD measurement error are systematically analyzed. Based on the CRD technology, we designed and established an experimental setup for measuring the reflectivity of high reflective mirrors. In our setup, a cheap, stable and high intensity laser diode was used. Then, a pair of similar reflectivity mirrors was measured by the straight cavity, and the single optical mirror was measured using a folded cavity which is formed from the straight cavity to be folded at19.2o angle at the wavelength of635nm. The reflectivity of the pair of cavity mirrors was measured to be (99.959±0.001)%and the single optical mirror was (99.917±0.003)%. The results show that the repeatability of the measurements is good. We studied on the measuring accuracy, the analysis shows that the average relative error of reflectivity is10-5. This experiment shows that this simple and easy operating setup can precisely measure reflectivity of high reflective mirrors and the measuring repetition is good.