| When used at oblique angle, the effective thickness of each layer becomes thinner than that at normal angle. Thus, the reflectance and transmittance of thin films exhibit strong polarization effects, particularly for the films inside a glass cube, which results from the fact that the tangential components of the electric and magnetic fields are continuous across each layer interface. However, for many applications it is undesirable and should be reduced. Therefore, the concept of non-polarizing beam splitter(NPBS) is proposed.Up till now, however, most of the reports of NPBSs are suitable for visible light. Therefore, it is necessary to find out some methods to reduce the polarization effects for infrared applications. A design method of infrared NPBS in a cube is proposed, the theoretical analysis is given, the designs for different substrates are demonstrated and the simulations of their optical properties are presented in the present paper.To begin with the basic theory of thin-film optics, this paper introduces the Fresnel's equation and the ways of a single-layer film and multiple-period films used in a thin-film system. Based on the development of the NPBS, the ways of NPBS's technology are given, mainly including Gilo's work. With the use of characteristic matrices, the NPBSs in the visible band are calculated, which prepares for infrared NPBSs. According to Cauchy's dispersion formula, refractive indices of film materials are obtained. For different glass substrates, such as ZF-7 glass and K9 glass, NPBSs in infrared band are investigated, respectively.Most of NPBS designed in the paper are appropriate. NPNS utilizing theprinciple of frustrated total reflection and mental-dielectric NPBS reduce the polarization effects in wide band. However, reflection-induced retardances are undesired. Further, material dispersion affects the property of NPBSs slightlyfrom the computer simulation. Finally, the technical problems of fabricating NPBS are discussed.
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