The use of solid frustrated total internal reflection (FTIR) layers to control the polarization effects in optical coatings usually results in large substrate sizes and complicated designs. To overcome this problem, it is proposed to incorporate an air FTIR layer into the multilayer thin film coatings. The low refractive index of air not only helps to reduce the substrate sizes, but also simplifies coating designs or improves the performance. The principle and layer structures of the proposed multilayers are described. Examples of polarizing- and non-polarizing beam-splitters for the infrared spectral region are given. Some practical manufacturing issues are also discussed.
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