Patent ID: 7702200

Claim:
A planar router, comprising: an input coupler; an output coupler; and a grating having a plurality of waveguides forming arms of the grating, wherein: successive arms of the grating have a path length difference ΔL which has a substantially constant value from each arm to a corresponding next arm; the grating includes an input array of radial waveguides, a first curved array of waveguides, a central array of essentially straight waveguides, a second curved array of waveguides, and an output array of radial waveguides; each waveguide of the first curved array is characterized by a curvature radius R that varies as a function of an angle of rotation θ of the waveguide and variation of R is minimized by choosing parameters of the first curved array so that a derivative of said variation of R is essentially zero for a particular principal angle of rotation θ=θ 0 inside an aperture of the first curved array, so that a radius of a principal waveguide corresponding to said principal angle of rotation θ 0 is a smallest curvature radius in said first curved array; each waveguide of the second curved array is characterized by a curvature radius R′ that varies as a function of an angle of rotation θ′ of the waveguide and variation of R′ is minimized by choosing parameters of the second curved array so that a derivative of said variation of R′ is essentially zero for a particular principal angle of rotation θ′=θ′ 0 inside an aperture of the second curved array, so that a radius of a principal waveguide corresponding to said principal angle of rotation θ′ 0 is a smallest curvature radius in said second curved array; the first and second curved arrays have opposite curvatures and said principal angles of rotations θ 0 and θ′ 0 are chosen so as to produce nonzero diffraction orders.