Problémes posés par la synthése et la réalisation de filtres spectraux multidiélectriques

Abstract

Many spectral filtering problems require the use of assemblies of layers having thicknesses which bear no obvious relationship to each other. This is often the case when precisely defined properties are required over a wide spectral range. Two major problems arise when such filters are to be realized. The first ofthem concerns the design of a stack best satisfying filtering requirements (synthesis). This design must be performed by successive iterations and since the number of unknown parameters is very large it is necessary to use fast procedures for the computation. We describe here a new procedure based on a Fourier transform method. With recent progress in this field, it is now possible to derive theoretical designs to satisfy almost any filtering requirement. The second problem to be solved regards the production of the multilayers so defined. For optical monitoring of non-quarter wave multilayer stacks it is necessary to use large bandwidth control systems. Such a monitoring method is described and its advantages over methods using one single wavelength are pointed out. Finally, we note that it may be interesting to compensate residual thickness errors by using a minicomputer working in real time during the deposition of successive layers. Such a procedure avoids the cumulative effects of successive errors.