Changes in the Multilayer Dielectric Coating Reflection Coefficient under Variation in the Medium Humidity

Abstract

It is found that the reflection coefficient of multilayer dielectric mirrors strongly depends on the medium (gas) humidity. This effect can result in both an increase and decrease in the reflection coefficient, which is determined by a change in the refractive indices of the dielectric layers (when filling with water vapor). The mirror reflection coefficient can increase up to 0.9% in a gas with the humidity close to the dew point. Changes in the reflection coefficient of a mirror in gaseous media which contain different water vapor isotopes (H₂¹⁶O, H₂¹⁸O, and D₂O) are studied. Mirrors of a CRDS spectrometer with the reflection coefficient R = 0.9999 are studied and the upper bound of the variation in the reflection coefficient versus air humidity is estimated.