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Chapter 7:
All-Dielectric Bandpass Filters
As mentioned in §1.6, a bandpass filter is highly transmissive in the spectral region of its passband. It rejects both the shorter and longer wavelengths contiguous to its passband, thus allowing it to isolate a limited region of the spectrum. The spectral width of its passband may be as small as a fraction of a nm for a filter used in fiber optical communications. A filter that isolates the FLIR band (7.5 μm to 13 μm) in the IR has a passband that is as wide as 4 μm. A bandpass may be constructed two ways: (1) with only nonabsorbing layers — termed an all-dielectric coating; or (2) with combinations of nonabsorbing and absorbing metal layers. Bandpasses containing absorbing materials (i.e., metal films) are reviewed in Chap. 8. Chapter 7 focuses on all-dielectric designs. Under what circumstances should each type of filter be used? Although there are inherent dangers in making sweeping generalizations, Table 7-1 lists the attributes of each filter. The advantages of the all-dielectric bandpass are its relatively high peak transmittance and, if required, a narrow spectral bandwidth. A desirable property of the bandpass containing metal layers is that it offers an immense amount of rejection — termed blocking in the trade—over a relatively wide spectral region. Blocking is illustrated in §1.6.5. An example is that the silver layers in the coating whose transmittance is shown in Fig. 8-29 attenuate quite effectively in the near IR.
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