Two-dimensional grating coupler and methods of making same
Disclosed are apparatus and methods for optical coupling. In one example, a described apparatus includes: a planar layer; a grating region comprising an array of scattering elements arranged in the planar layer to form a two-dimensional grating; a first taper structure formed in the planar layer connecting a first side of the grating region to a first waveguide, wherein a shape of the first taper structure is a first triangle that is asymmetric about any line perpendicular to the first side of the grating region in the planar layer; and a second taper structure formed in the planar layer connecting a second side of the grating region to a second waveguide, wherein a shape of the second taper structure is a second triangle that is asymmetric about any line perpendicular to the second side of the grating region in the planar layer, wherein the first side and the second side are substantially perpendicular to each other.
1. An apparatus for optical coupling, comprising:
a planar layer;
a grating region comprising an array of scattering elements arranged in the planar layer to form a two-dimensional (2D) grating;
a first taper structure formed in the planar layer connecting a first side of the grating region to a first waveguide, wherein a shape of the first taper structure is a first triangle that is asymmetric about any line perpendicular to the first side of the grating region in the planar layer; and
a second taper structure formed in the planar layer connecting a second side of the grating region to a second waveguide, wherein a shape of the second taper structure is a second triangle that is asymmetric about any line perpendicular to the second side of the grating region in the planar layer, wherein the first side and the second side are substantially perpendicular to each other, wherein:
the array of scattering elements are evenly distributed in the planar layer such that there is a same distance between centers of every two adjacent scattering elements along either a first direction perpendicular to the first side of the grating region or a second direction perpendicular to the second side of the grating region,
the first triangle has a first base side residing on the first side of the grating region, has a first vertex opposite to the first base side, and has a first length along the first direction,
the first vertex has a first distance to a perpendicular bisector of the first base side in the planar layer,
the second triangle has a second base side residing on the second side of the grating region, has a second vertex opposite to the second base side, and has a second length along the second direction,
the second vertex has a second distance to a perpendicular bisector of the second base side in the planar layer; and
the first distance and the second distance are equal to each other.
2. The apparatus of claim 1 , wherein a shape of the grating region is a square in the planar layer.
3. The apparatus of claim 2 , wherein:
the array of scattering elements are arranged in the planar layer at a plurality of intersections of a first set of straight lines crossing with a second set of straight lines;
each of the first set of straight lines is parallel to the first side of the grating region; and
each of the second set of straight lines is parallel to the second side of the grating region.
4. The apparatus of claim 1 , wherein each scattering element in the array of scattering elements has a same square shape with a same size in the planar layer.
5. The apparatus of claim 1 , wherein each scattering element in the array of scattering elements has a same circular shape with a same size in the planar layer.
6. The apparatus of claim 1 , wherein the first triangle and the second triangle are congruent.
7. The apparatus of claim 1 , wherein the first triangle and the second triangle are symmetric to each other about a diagonal line crossing the grating region.
8. The apparatus of claim 1 , wherein:
the 2D grating is configured for receiving an incident light from an optical fiber with an incident angle that is non-zero;
the incident angle is measured in plane of incidence between an axis of the optical fiber and a direction perpendicular to the planar layer; and
the first length, the second length, the first distance and the second distance are determined based on the incident angle and a position of the optical fiber relative to the 2D grating.
9. The apparatus of claim 8 , wherein:
the first taper structure is configured for transmitting a first portion of the incident light to the first waveguide to achieve a minimum insertion loss; and
the second taper structure is configured for transmitting a second portion of the incident light to the second waveguide to achieve a minimum insertion loss, wherein
the first portion of the incident light is substantially a parallel polarization component of the incident light,
the second portion of the incident light is substantially an orthogonal polarization component of the incident light,
each of the parallel polarization component and the orthogonal polarization component comprises a polarized light, and
the polarized light has a transverse magnetic (TM) polarization mode or a transverse magnetic (TE) polarization mode split from the incident light.
10. The apparatus of claim 1 , wherein:
each of the first length and the second length is between 20 and 500 micrometers; and
each of the first distance and the second distance is between 0 and 20 micrometers.
11. A two-dimensional (2D) grating coupler, comprising:
a planar layer;
a grating region comprising an array of scattering elements arranged in the planar layer to form a 2D grating;
a first taper structure in the planar layer connecting a first side of the grating region to a first waveguide, wherein a shape of the first taper structure is a first triangle that is asymmetric about any line perpendicular to the first side of the grating region in the planar layer; and
a second taper structure in the planar layer connecting a second side of the grating region to a second waveguide, wherein a shape of the second taper structure is a second triangle that is asymmetric about any line perpendicular to the second side of the grating region in the planar layer, wherein the first side and the second side are substantially perpendicular to each other such that each of the first taper structure and the second taper structure has a shape and dimensions to reduce light power loss due to light transmission and reflection with respect to the 2D grating, wherein:
the array of scattering elements are evenly distributed in the planar layer such that there is a same distance between centers of every two adjacent scattering elements along either a first direction perpendicular to the first side of the grating region or a second direction perpendicular to the second side of the grating region,
the first triangle has a first base side residing on the first side of the grating region, has a first vertex opposite to the first base side, and has a first length along the first direction, and
the first vertex has a first distance to a perpendicular bisector of the first base side in the planar layer,
the second triangle has a second base side residing on the second side of the grating region, has a second vertex opposite to the second base side, and has a second length along the second direction,
the second vertex has a second distance to a perpendicular bisector of the second base side in the planar layer,
the first distance and the second distance are equal to each other,
the first taper structure is configured for transmitting a first portion of the incident light to the first waveguide to achieve a minimum insertion loss, and
the second taper structure is configured for transmitting a second portion of the incident light to the second waveguide to achieve a minimum insertion loss, wherein the first portion of the incident light is substantially a parallel polarization component of the incident light, and the second portion of the incident light is substantially an orthogonal polarization component of the incident light.
12. The 2D grating coupler of claim 11 , wherein:
a shape of the first taper structure is a first triangle that is asymmetric about any line perpendicular to the first side of the grating region in the planar layer;
a shape of the second taper structure is a second triangle that is asymmetric about any line perpendicular to the second side of the grating region in the planar layer; and
the first side and the second side are substantially perpendicular to each other.
13. The 2D grating coupler of claim 12 , wherein:
the first triangle has a first length from a first base side residing on the first side to a first vertex opposite the first base side, and has a first shift from the first vertex to a perpendicular bisector of the first base side; and
the second triangle has a second length from a second base side residing on the second side to a second vertex opposite the second base side, and has a second shift from the second vertex to a perpendicular bisector of the second base side.
14. The 2D grating coupler of claim 13 , wherein:
the first length, the second length, the first shift and the second shift have values to maximize light power in the first waveguide and the second waveguide.
15. The 2D grating coupler of claim 14 , wherein:
the values of the first length and the second length are between 20 and 500 micrometers; and
the values of the first shift and the second shift are between 0 and 20 micrometers.
16. An apparatus for optical coupling, comprising:
a planar layer;
a grating region comprising an array of scattering elements arranged in the planar layer to form a two-dimensional (2D) grating;
a first taper structure formed in the planar layer connecting a first side of the grating region to a first waveguide, wherein a shape of the first taper structure is a first triangle that is asymmetric about any line perpendicular to the first side of the grating region in the planar layer; and
a second taper structure formed in the planar layer connecting a second side of the grating region to a second waveguide, wherein a shape of the second taper structure is a second triangle that is asymmetric about any line perpendicular to the second side of the grating region in the planar layer, wherein the first side and the second side are substantially perpendicular to each other, wherein:
the array of scattering elements are evenly distributed in the planar layer such that there is a same distance between centers of every two adjacent scattering elements along either a first direction perpendicular to the first side of the grating region or a second direction perpendicular to the second side of the grating region,
the first triangle has a first base side residing on the first side of the grating region, has a first vertex opposite to the first base side, and has a first length along the first direction, and
the first vertex has a first distance to a perpendicular bisector of the first base side in the planar layer,
the second triangle has a second base side residing on the second side of the grating region, has a second vertex opposite to the second base side, and has a second length along the second direction,
the second vertex has a second distance to a perpendicular bisector of the second base side in the planar layer,
the first distance and the second distance are equal to each other,
the 2D grating is configured for receiving an incident light from an optical fiber with an incident angle that is non-zero,
the incident angle is measured in plane of incidence between an axis of the optical fiber and a direction perpendicular to the planar layer, and
the first length, the second length, the first distance and the second distance are determined based on the incident angle and a position of the optical fiber relative to the 2D grating.
17. The apparatus of claim 16 , wherein:
the first taper structure is configured for transmitting a first portion of the incident light to the first waveguide to achieve a minimum insertion loss; and
the second taper structure is configured for transmitting a second portion of the incident light to the second waveguide to achieve a minimum insertion loss, wherein the first portion of the incident light is substantially a parallel polarization component of the incident light, and the second portion of the incident light is substantially an orthogonal polarization component of the incident light.
18. The apparatus of claim 17 , wherein each of the parallel polarization component and the orthogonal polarization component comprises a polarized light.
19. The apparatus of claim 18 , wherein the polarized light has a transverse magnetic (TM) polarization mode or a transverse magnetic (TE) polarization mode split from the incident light.
20. The apparatus of claim 16 , wherein:
each of the first length and the second length is between 20 and 500 micrometers; and
each of the first distance and the second distance is between 0 and 20 micrometers.