The phase-length interference condition of Fabry-Perot cavity can effectively penetrate the light in a small range on both sides of the central wavelength and the central wavelength, while the cancellation interference can prevent the light transmission outside the passband. However, the cut-off bandwidth on either side of the central wavelength is very small. In order to increase the cut-off bandwidth of the filter, the broadband cut-off material can be used as the interlayer or substrate, or the broadband cut-off material can be plated on the interlayer or substrate. Although these materials can effectively cut off the transmission of incident light outside the passband, they will also reduce the transmission within the passband of the filter.
