Method and Apparatus for Optical Waveguide-to-Semiconductor Coupling for Integrated Photonic Circuits

Abstract

A grating coupler couples a waveguide to a beam and is formed of patterned shapes in a first and second layer of planar material, the shapes embedded in background material, the layers separated by less than one wavelength. The shapes are organized as a plurality of adjacent unit cells arranged along a direction of propagation of light with each unit cell including a shape of the first material and a shape of the second material, each unit cell having design parameters including a period, a width wb of the shape of first planar material, a width wt of the shape of second planar material, and an offset between the shapes. The coupler has a directivity ratio D is at least 10 dB between “up” and “down” radiation; and unit cells differ in at least one parameter selected from period, wb, wt, and offset to provide a predetermined beam shape.

What is claimed is: 
     
         1 . A grating coupler configured in a configuration selected from the group consisting of being configured to receive light of a wavelength from a waveguide and emit a beam, and being configured to receive light from a beam and emit light of the wavelength through the waveguide, the grating coupler comprising:
 patterned shapes in each of a first and second layer of planar material and the shapes of at least the first or the second layer being embedded in background material, a plane defined by the shapes of the first planar material separated from a plane defined by the shapes of the second material by less than one wavelength in a direction perpendicular to the plane defined by the shapes of the first planar material;   the first and second layer of planar material having similar refractive index;   a plurality of adjacent unit cells arranged with a direction of propagation of the light each comprising a shape of the first material and a shape of the second material, each unit cell having design parameters comprising a cell width A equal to a distance between a first edge of a shape of the first planar material and a first edge of a next shape of the first planar material, a width wb of the shape of the first planar material, a width wt of the shape of the second planar material, and an offset between the first edge of a shape of the first planar material and the first edge of the shape of the second material;   wherein a directivity ratio D of a first and a second unit cell is at least 10 dB; and   where the first unit cell differs from the second unit cell of the plurality of unit cells in at least one parameter selected from Λ, wb, wt, and offset to provide a predetermined beam shape the beam having a direction not coplanar with the plane defined by shapes of the first material layer.   
     
     
         2 . The grating coupler of  claim 1  where at least two of the parameters of the first cell differ from corresponding parameters of the second cell. 
     
     
         3 . The grating coupler of  claim 2  where a wb/wt ratio of the first cell is not equal to a wb/wt ratio of the second cell. 
     
     
         4 . The grating coupler of  claim 3  where the offset of the first cell is not equal to the offset of the second cell. 
     
     
         5 . The grating coupler of  claim 1  wherein the first unit cell lies near a peak power emissions point of the grating. 
     
     
         6 . The grating coupler of  claim 1 , where the second unit cell is configured to have a radiation strength α that differs from that of the first unit cell by at least 20%, and the second unit cell also has a directionality of at least 10 dB. 
     
     
         7 . The grating coupler of  claim 1 , wherein the second unit cell has a cell width that, when simulated in a uniform periodic grating, produces an output radiation at an angle that differs from that of the first cell by at least one degree, the combination of first and second unit cells adapted to produce a converging beam. 
     
     
         8 . The coupler of  claim 1  wherein the first unit cell is closer to the input waveguide than a third unit cell, the third unit cell being closer to the input waveguide than the second unit cell, and the first unit cell is configured to have an output angle θ 1  that leans forward away from the input waveguide and the second unit cell is configured to have an output angle θ 2  that leans backward, toward the input waveguide. 
     
     
         9 . A grating coupler configured in a configuration with a waveguide port and a beam port the grating coupler comprising:
 patterned shapes in a first layer of a first planar material and patterned shapes in a second layer of a second planar material, the shapes of the first material and the shapes of the second material being embedded in background material;   a plurality of adjacent unit cells arranged with a direction of propagation of the light each comprising a shape of the first material and a shape of the second material, each unit cell having design parameters comprising a cell width A equal to a distance between a first edge of a shape of the first planar material and a first edge of a next shape of the first planar material, a width wb of the shape of the first planar material, and a width wt of the shape of the second planar material; and   where a first unit cell of the unit cells lie closer to the waveguide than a second unit cell of the unit cells and width wt of the shape in the second planar material is greater in the second unit cell than in the first unit cell.   
     
     
         10 . The grating coupler of  claim 9 , where the unit cells have a monotonically increasing width wt of the shapes in the second planar material from the waveguide to an end of the coupler distant from the waveguide. 
     
     
         11 . The grating coupler of  claim 9  wherein the waveguide comprises a shape formed in the first planar material layer. 
     
     
         12 . The grating coupler of  claim 9 , where the unit cells have a monotonically decreasing width wt of the shapes in the second planar material from the waveguide to a distal terminus of the coupler. 
     
     
         13 . The grating coupler of  claim 9  wherein the waveguide comprises a shape formed in the second planar material layer. 
     
     
         14 . A method of designing a grating coupler for an output angle of interest and a desired beam power distribution comprising:
 selecting a center wavelength of operation, a mean output angle of beam, and a material stack providing a first and a second patternable planar device layers where the second patternable device layers lies less than the center wavelength above the first patternable device layer;   determining layer thicknesses and refractive indices of the patternable device layers, and at least one refractive index of a surrounding material;   using a Bloch Mode Solver with a periodic Bloch boundary condition along horizontal axis and radiation absorbing boundaries and a model of a unit cell of the structure for a plurality of sets of particular choices of parameters, the parameters comprising a cell width, a width of a scattering element formed in each of the first and second patternable layers, and an offset between the scattering element in the first patternable layer and the scattering element in the second patternable layer at the center wavelength, to provide Bloch field distribution in the unit cell and the complex propagation constant, extracting from the Bloch field distribution for each set of particular choices of parameters an angle θ of emitted radiation, a directivity D, and an emissions strength α and placing θ, D, and a as entries in a four dimensional table;   selecting entries in the four dimensional table according to the output angle of interest;   determining an approximate desired a for each of a plurality of unit cells in the grating coupler;   finding entries in the four dimensional table corresponding to maximal directivity D and desired a for each of the plurality of unit cells.   
     
     
         15 . The grating coupler of  claim 6  wherein the first and second unit cell lie within a mode field diameter of each other.