Patent ID: 8254028

Claim:
A diffraction grating structure comprising: a substrate having a first surface and a second surface opposite to the first surface, wherein a first grating is defined on the first surface and a second grating is defined on the second surface, the first grating includes a plurality of first grooves and has a first grating period d 1 , and the second grating includes a plurality of second grooves and has a second grating period d 2 , and wherein the profile of the first grooves of the first grating are described by the following function: f 1 ⁡ ( x 1 ) = d 1 ⁢ tan ⁢ ⁢ θ π ⁡ [ sin ⁢ 2 ⁢ πx 1 d 1 - 1 2 ⁢ sin ⁢ 4 ⁢ πx 1 d 1 + 1 3 ⁢ sin ⁢ 6 ⁢ πx 1 d 1 ] , and the profile of the second grooves of the second grating are described by the following function: f 2 ⁡ ( x 2 ) = d 2 ⁢ tan ⁢ ⁢ θ π ⁡ [ sin ⁢ 2 ⁢ πx 2 d 2 - 1 2 ⁢ sin ⁢ 4 ⁢ πx 2 d 2 + 1 3 ⁢ sin ⁢ 6 ⁢ πx 2 d 2 ] , where θ is a half of angle of two coherent beams radiating on the diffraction grating structure, tan θ represents the tangent of the angle θ, x1 is a longitudinal coordinate which has an origin at the centre of the first grating, and x2 is another longitudinal coordinate which has another origin at the centre of the second grating.