Uncertainty analysis of optical components absorption coefficient measurement using an intra-cavity device

In order to measure the absorption coefficient and performance degradation characteristics of optical components which used in high power laser system, an intra-cavity device was established based on a discharge-drived CW chemical laser. Two pieces of 45 degree reflecting mirrors were tested. Each mirror was tested for more than twenty times, and high power laser irradiation on the testing mirrors lasted 100 seconds continuously in each test. The dependence of absorption coefficients on irradiation times was acquired. The testing results of both reflecting mirrors showed that, the differences between the experimental absorption coefficients and their fitting curve were up to 30.7% and 21.6% respectively, and the differences were independent of irradiation condition, such as irradiation energy, irradiation power and beam cross-sectional area. The uncertainty of absorption coefficient was composed of two parts. For one thing, the uncertainty of the direct measurement results, such as the temperature rise of optical components, can cause the uncertainty of absorption coefficients. This part of uncertainty was about 11.3%. For another, the resonant cavity need to be adjusted again when other optical components were replaced, which lead to the change of the incident angle of the optical components to be measured. A typical film system of 24 layers (12 pairs) was calculated by Thin Film Design Software called TFCalc, which showed the absorption coefficients increased with the increase of incident angle. When the angle of incidence was 0.5 degree from the design value, there would be -60 similar to 71ppm difference of absorption coefficient from the original one, and the uncertainty was 14.5%. When there was a deviation of 1 degree, the difference of absorption coefficient and the uncertainty were -112 similar to 155ppm and 31.7% respectively. This results showed that, the deviation of incident angle was between 0.5 similar to 1 degree in the test. In order to reduce the testing uncertainty of absorption coefficients, the deviation between the incident angle of optical components and the design value should be reduced as much as possible. This provides guidance for measuring the absorption coefficients of optical components with an intra-cavity device.