Nonquarterwave multilayer filters: optical monitoring with a minicomputer allowing correction of thickness errors

Many spectral filtering problems require assemblies of layers having thicknesses that bear no obvious relationship to one another. After a brief review of the optical methods used to monitor deposition of multilayers containing nonintegral thicknesses, we show that the performance of monitoring systems can be improved further by including the real-time calculation of any necessary layer thickness changes that may be required to compensate any errors that might still occur. The apparatus described consists of a minicomputer coupled to a rapid-scanning spectrometer. Such a procedure working in real time avoids the cumulative effects of successive errors. The technique is demonstrated in the production of a beam splitter.