Abstract

We give graphic descriptions of level monitoring of the thickness of optical layers and factors affecting the sensitivity of various schemes in detecting thickness errors. It is shown that the optical monitoring of a typical stack of quarterwave optical thickness layers has the greatest sensitivity when monitored at a wavelength near either edge of the stopband. Level monitoring can have cases where errors in layer thickness and index tend to contribute to a condition of runaway instability in the layer control. This was discussed by Zhao¹ along with a means of real time error correction. We elaborate here in graphic form on these problems and their correction. The effects of error compensation are described.

© 1989 Optical Society of America