A high performance edge filter for wavelength-division multi-demultiplexer units

The theory for realizing edge interference filters, exhibiting minimum polarization at the wavelengths to be separated, despite oblique beam incidence, is extended to versions for any angle. An edge filter (19 alternating layers of SiO ₂ and TiO ₂) for a beam incident at 20° from glass realized through tradeoff between acceptable insertion loss and desired crosstalk attenuation is presented. Due to its steep edge, it is capable of separating the emissions of two led's at close wavelengths. Whereas the reflectance for the wavelength to be passed is almost zero, that for the wavelength to be reflected is almost complete.