ABSTRACT
A filter which possesses a region of transmission bounded on either side by regions of rejection is known as a band-pass filter. For the broadest band-pass filters, the most suitable construction is a combination of longwave-pass and shortwave-pass filters, which we discussed in chapter 6. 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 produce simultaneously 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 5. The thin-film Fabry-Perot filter has a pass band shape which is triangular and it has been found possible to improve this by coupling simple filters in series in much the same way as tuned circuits. These coupled arrangements are known as multiple cavity filters or multiple half-wave filters. If two simple Fabry-Perot filters are combined, the resultant becomes a double cavity or double half-wave filter, abbreviated to DHW filter, while, if three Fabry-Perot filters are involved, we have a triple cavity filter, abbreviated normally to THW for triple half-wave. In the earlier part of this chapter, we consider single cavity filters. First of all, we examine combinations of edge filters.
