Multilayer mirrors with high reflectance over an extended spectral region

The design of mirrors composed of multilayer stacks of dielectric films is considered. High reflectance over an extended spectral range is attained by positioning the stacks so that on a wavelength scale their high-reflectance bands are either contiguous or overlapping. Certain precautions must be taken in the choice of stacks to avoid deep-reflectance minima from developing within the extended high-reflectance region. Some of these are discussed and illustrated with both computional and experimental curves. The techniques of extending the high-reflectance regions are applicable not only to mirrors, but also to low- and high-pass cutoff filters and to multilayer polarizers.