Chapter 2 discussed the basic form of TMDs for discrete-layer, homogeneous designs and inhomogeneous rugate TMDs. For rugate designs, envelope functions are used to define the refractive-index profile of the rugate design. Typically, rugate designs with these envelope functions have reduced or suppressed reflected-ripple structure in their passbands, hence apodization. Discrete-layered TMDs can be apodized in a similar fashion except that the ripple structure of TMD passbands is not suppressed like apodized rugate designs. However, the apodization of TMDs does change the spectral performance of TMDs. The purpose of this chapter is to briefly investigate the relative stopband spectral positions of TMDs with the application of various envelope functions.
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