Two types of all-dielectric bandpass filters are possible: QWS filters that achieve a bandpass by optimization and Fabry-Perot filters.
7.1 Wide Bandpass Filters
Wideband filters can be designed by combining two reflectors, such as those used in Chapter 5 to make long- and short-pass filters. The 36-layer filter shown in Fig. 7.1 results from shifting the reflection zones of two 18-layer QWSs slightly to the right and left and optimizing to allow the green light to pass. Wideband filters typically will not have the smoothest shapes possible without using huge amounts of layers. Other design techniques are required to remove the wrinkles. A patented approach by Erdogan, Wang, and Clarke utilizes a large number of layers in a QWS that is optimized for a fast cutoff slope; the bandpass can be achieved by target selection (Fig. 7.2). The edges can be selected and improved; all of the layers are nonquarterwave. A long-pass filter can be deposited on the second surface to complete the filter.