What is a Band-Pass filter?

A band pass filter lets a wavelentgh band to pass while not letting wavelenghts around it. It is normally made of a cavity/spacer layer sandwiched between bragg mirrors. Bragg mirrors are stack of alternating high and low refractive index dielectric layers, that as the name suggest provide excellent reflection (and less absorbtion in comparison to metallic mirrors). Bragg mirrors have the dielectric layers at quarter wave thickness (at a specific design wavelength).

Within this excellent reflectance spectral region, by introducing a spacer in betwen two bragg mirrors, a small wavelength range can be tranmitted. This happens as within this cavity (also known here as fabry-perot cavity), resonance happens.

Bragg Mirror to a Band Pass Filter

First, a bragg mirror with alternating stacks of TiO2 and SiO2 are modelled using Transfer Matrix Method. The layers are at quarter wave thickness at the deisgn wavelength of 1550 nm. The Reflectance spectra is shown below with the effect of number of layers. The result shows that as the number of layers increase, the reflectance improves,

Let us stick with N = 8 from the above figure and now introduce a spacer material of half wave thickness. Here SiO2 is used (although there is no reason for it to be lower refractive index or either of the two use materials). To highlight the effect on reflectance due to this cavity, for the followign figure N = 4 has been used.

As is visible in the figure above, the R dips at around the deisgn wavelength of 1550 nm and the T goes up.