It turns out that the OLPFs in DSLRs are not really Gaussian, but something much simpler: beam splitters. A slice of Lithium Niobate crystal, cut at a specific angle, presents a different index of refraction depending on the polarization of the incident light. Let us assume that a beam of horizontally polarized light passes through straight, for the sake of the argument. Vertically polarized light, on the other hand, refracts (bends) as it enters the crystal, effectively taking a longer path through the crystal. As the vertically polarized light leaves the crystal, it refracts again to form a beam parallel to the horizontally polarized beam, but displaced sideways by a distance dependent on the thickness of the crystal.

Using a single layer of Lithium Niobate crystal, you can split a single beam into two parallel beams separated by a distance d, which is typically chosen to match the pixel pitch of the sensor. Since this happens for all beams, the image leaving the OLPF is the sum (average) of the incoming image and a shifted version of itself, translated by exactly one pixel pitch.

If you stack two of of these filters, with the second one rotated through 90 degrees, you effectively split a beam into four, forming a square with sides equal to the pixel pitch (but often slightly less than the pitch, to improve resolution). A circular polariser is usually inserted between the two Lithium Niobate layers to "reset" the polarisation of the light before it enters the second Niobate layer.

Check more at http://mtfmapper.blogspot.ca/2012/06/nikon-d40-and-d7000-aa-filter-mtf.html