Patent ID: 7400448

Claim:
A birefringent spectral filter comprising: a first birefringent stage adapted to generate an optical path difference (OPD) between two polarizations of incident light propagating along each of two orthogonal axes, respectively, the first birefringent stage comprising: a first layer of a first birefringent material having a first depth and a first extraordinary axis aligned with a first of the two orthogonal axes; a second layer of the first birefringent material having the first depth and a second extraordinary axis, the second extraordinary axis being orthogonal to the first extraordinary axis; and a first half-wave plate disposed between the first and second layers of birefringent material; a third layer of a second birefringent material optically coupled to the first birefringent stage, the third layer having a second depth and a third extraordinary axis aligned with one of the two orthogonal axes; and a fourth layer of the second birefringent material optically coupled to the third layer of birefringent material, the fourth layer having the second depth and a fourth extraordinary axis, the fourth extraordinary axis being orthogonal to the third extraordinary axis, wherein the second birefringent material is different from the first birefringent material, the third and fourth layers are adapted to generate a further OPD between the two polarizations of incident light, and the second depth is different from the first depth and is adapted to enlarge a field of view by controlling an amount of contribution to the total OPD from second and higher orders of the sine of a cone angle, θ, of light incident upon the filter; and a polarizing beam splitter optically coupled to the first birefringent stage.