Wavelength-division multiplexed thin-film filters used in tilted incident angles of light

The properties of wavelength-division multiplexed (WDM) narrowband filters used in tilted collimated light are described. The wavelength shift for s polarization in a low-index-spacer filter is larger than that for p polarization when the filter is tilted, but it is smaller in a high-index-spacer filter. Therefore the passbands of the tilted filter can be centered at the same wavelength for two polarization modes by use of high- and low-index materials, as appropriate, as spacers or by selection of a moderate-index material instead of a spacer. With such spacers, WDM filters used for incident angles of 20 degrees or even greater are constructed. Experimental results agree with the computed ones.