Thin-Film Bandpass / Notch Filter Designer

A Narrow Window of Transmission

An all-dielectric bandpass filter is a Fabry–Pérot cavity built entirely from quarter-wave layers: two TiO₂/SiO₂ mirror stacks surround a half-wave SiO₂ spacer. Only a narrow band near the design wavelength passes; everything else inside the mirror stopband is reflected.

Why the Passband Narrows

The two quarter-wave stacks act as high reflectors, so the cavity between them has very high finesse. Adding more pairs per mirror raises that reflectance, which narrows the passband and deepens the out-of-band blocking within the mirror stopband. A complementary notch filter rejects, rather than passes, the cavity wavelength.

H and L are quarter-wave TiO₂ and SiO₂; (2L) is a half-wave cavity. More pairs p → higher mirror reflectance → narrower FWHM and stronger blocking. Wide rejection beyond the mirror stopband needs additional blocker stacks.

Interactive design tool

This page includes a live transfer-matrix calculator for this design. Adjust the parameters to recompute spectra, layer stack, and performance figures in the browser, then open the design in the full calculator.