2012杭州薄膜会议第6个ppt：Limitations on wide passbands

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Limitations on wide passbands
There are differences in the behavior of wide passband edge filters with short wavelength passbands and those with long wavelength passbands. The bandwidth of the pass band is here defined by the longest wavelength in the band divided by the shortest wavelength. This is virtually unlimited in the case of a long wave pass filter. However, it is significantly limited in the case of the usual approach of using quarter wavelength layer thickness stacks for short wave pass filters. This limitation is encountered because the third harmonic of the blocking band appears at the short wavelength position where the quarter wave optical thicknesses of the layers for the blocking band stack of layers become three quarter waves at the wavelength of that harmonic. It appears that bandwidths of over 2 start to have increasingly higher reflection losses, and bandwidths of 2.5 become virtually impractical. When band-passes broader than about 2 are needed for edge filters with a short wave passband, recourse to rugate-like designs is needed. Such designs can be achieved with only two homogeneous materials by employing the concept of the Herpin approximation, although many layers may be required. The influence of the indices of refraction of the materials, number of layers, and design approach on the bandwidth, average reflectance in the passband, band edge steepness, blocking density, and "squareness" at the transition from the pass to blocking band are discussed.
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