Light-scattering characterization of transparent substrates for Laser-gyro application

摘要

To produce high-quality, low-scattering optical interference coatings for laser-gyro application, substrates with extremely smooth surfaces are required for deposition. For successful production of qualified mirrors, characterization of substrates before coating is essential. Light scattering has been used for decades to characterize the surface roughness of optical components. Its application in characterizing the surfaces of transparent substrates, commonly used for deposition, is difficult due to the low scattering of high-quality substrates and the scattering contribution of substrates' back surface and volume scattering. Light scattering characterization of transparent substrates for laser-gyro application has been studied in this paper. It has been found that collecting objective is more propitious to eliminate volume scattering and scattering from back surfaces of substrates than integrating sphere and more appropriate to characterize superpolished transparent surfaces of substrates for laser-gyro application. Collecting objective whose N.A. is 0.4 has been designed in ZEMAX. Careful analysis shows that scattering from back surfaces and volume scattering from points with axile distances to the front surface more than 1.5 millimeters can be eliminated completely. To solve the drift problem of PMT used for probing scattering light, specific structure of collecting system has been designed, so that scattering light and reflecting light can strike on the same PMT orderly. It has been found that the drift problem of PMT could be solved with this setting, so that the stability of the scattering measurement system over long time span could be improved greatly and its practicability in engineering has been ensured.