ABSTRACT
A filter possessing a region of transmission bounded on either side by regions of rejection is known as a bandpass filter. For the broadest bandpass filters, the most suitable construction is a combination of longwave pass and shortwave pass filters, already mentioned in Chapter 7. For narrower filters, however, this method is not very successful because of difficulties associated with obtaining both the required precision in positioning and the steepness of edges. Other methods are therefore used, involving a single assembly of thin films to simultaneously produce the pass and rejection bands. The simplest of these is the thin-film Fabry–Perot filter, a development of the interferometer already described in Chapter 6. The spacer layer in the Fabry–Perot etalon acts rather like a resonant cavity and so is usually called a cavity layer. The Fabry–Perot filter then becomes known as a single-cavity filter. The single-cavity filter has a roughly triangular passband shape, and it has been found possible to improve this, by coupling simple single-cavity filters in series, in much the same way as electrical tuned circuits. These coupled arrangements are known as multiple-cavity filters. The terminology has not always been as simple. The cavities are usually a half-wave, or integral multiples of half-waves, thick. Thus, an older term that is still sometimes used is multiple half-wave filter. The two-cavity filter was earlier called a double half-wave filter, abbreviated to DHW filter, while the three cavity was called a triple half-wave or THW filter. Another term, less used nowadays, is WADI indicating a wide-band all-dielectric filter.
