Radial optical coupler

1. An optical device, comprising:

an optical grating coupler formed along a planar surface of a substrate, said grating coupler including a pattern of concentric rings of ridges located on the surface, with each adjacent pair of the ridges being separated by a groove; and

a plurality of optical waveguides having first ends that terminate at an outermost one of the ridges, the first ends being about uniformly spaced along said outermost one of the ridges and being oriented radially with respect to a center of said concentric rings; and

a plurality of optical transition portions, each transition portion partially filling a region between adjacent ones of the first ends of the waveguides, wherein each of said transition portions includes a first subportion having a first thickness and a second subportion having a second thickness that is less than said first thickness.

2. The optical device of claim 1 , wherein said pattern of ridges is radially chirped with respect to the center.

3. The optical device of claim 1 , wherein said plurality of radial waveguides is configured to deliver a corresponding plurality of optical signals to said outermost one of the ridges with a substantially equal phase.

4. The optical device of claim 1 , further comprising a star coupler having a number of output or input ports equal to a number of said plurality of radial waveguides, each of the output or input ports being connected to a corresponding one of said radial waveguides.

5. The optical device of claim 4 , further comprising a plurality of polarization beam splitters, each polarization beam splitter being coupled to a corresponding port of said star coupler.

6. The optical device of claim 5 , further comprising a plurality of optical signal sources, each of said polarization beam splitters being connected to two of said optical signal sources, one of said two being configured to produce TE mode light and the other of the two being configured to produce TM mode light.

7. The optical device of claim 1 , further comprising a ring-core multimode optical fiber having an end located over and facing said circular grating coupler.

8. The optical device of claim 7 , wherein said outermost ridge has a diameter about equal to an outer diameter of said ring-core.

9. An optical device, comprising:

an optical grating coupler formed along a planar surface of a substrate, said optical grating coupler including a pattern of concentric rings of ridges located on the surface, with each adjacent pair of ridges being separated by a groove;

a plurality of optical waveguides having first ends that terminate at an outermost one of the ridges, the first ends being about uniformly spaced along said outermost one of the ridges and being oriented radially with respect to a center of said concentric rings, and

wherein said plurality of optical waveguides is configured to deliver a corresponding plurality of optical signals to said outermost ridge, with a phase of each optical signal of said corresponding plurality of optical signals increasing incrementally along said outermost ridge thereby advancing by a nonzero integer multiple of 2π per round of said outermost ridge.

10. The optical device of claim 9 , further comprising a plurality of optical transition portions, each transition portion partially filling a region between adjacent ones of the first ends of the waveguides.

11. The optical device of claim 1 , wherein said pattern of ridges is radially chirped with respect to the center.