The construction of optical filters, which are required to have optical properties well defined over wide spectral regions, demands the use of multilayer designs in which there are no simple relationships between the thicknesses of the layers. There are considerable difficulties in the manufacture of such multilayers. First, we must be able to reproduce, sufficiently accurately, the indices of refraction that have been used in the theoretical design. Then we must be able to control the optical thicknesses of each layer, which necessitates measurement of the optical properties of the multilayer during deposition. The apparatus described consists of a minicomputer coupled to a rapid-scanning spectrometer that continuously measures the spectral profile during deposition of each layer. The precise measurement of the evolution of the optical properties during actual construction of a filter allows us to control layer thickness with very good accuracy. The technique is demonstrated in the monitoring of a beam splitter made of a few layers.
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