Apparatus for the Measurement of Optical Absorptivity in Laser Mirrors

Abstract

The construction and operation of an apparatus for the measurement of optical absorptivity in mirrors at various laser wavelengths are described in detail. The sample mirror, in vacuum, is irradiated with a laser, and the reflected laser beam is absorbed in a blackened detector cone. The equilibrium temperature rise of the sample and detector cone, due to heating by the laser, is recorded. With the laser beam interrupted, these same temperatures are reproduced by the use of heaters attached to the sample and cone. Measurement of the power to the two heaters allows a very accurate determination of the mirror absorptivity. This apparatus is capable of measuring mirrors of 0.01 absorptivity with a precision of ≤1.4% and a systematic error of ≤1.6%. Illustrative data for mechanically polished copper mirrors yield an average value of 0.0076 for the absorptivity at a wavelength of 10.6 μm.