Grating-coupler assembly with small mode-field diameter for photonic-integrated-circuit systems

1. A grating-coupler assembly for optically coupling light into or out of a photonic integrated circuit (PIC), comprising:

a silicon-on-insulator (SOI) substrate;

at least one silicon waveguide supported by the SOI substrate, wherein the silicon waveguide is optically coupled to one or more PIC components on the PIC;

at least one grating coupler supported by the SOI substrate and optically coupled to the at least one silicon waveguide;

an optical fiber having an end and at least one core having a mode-field diameter MFD 60 in the range 5 μm≦MFD 60 ≦6 μm, wherein the optical-fiber end is disposed adjacent the at least one grating coupler; and

wherein the at least one core of the optical fiber and the at least one grating coupler define a coupling efficiency CE≧0.7.

2. The grating-coupler assembly according to claim 1 , further comprising an intervening medium of refractive index n m >1 between the optical-fiber end and the grating coupler.

3. The grating-coupler assembly according to claim 2 , wherein the intervening medium comprises either an epoxy or an index-matching gel.

4. The grating-coupler assembly according to claim 1 , wherein the optical fiber includes a tapered end portion that includes the optical-fiber end.

5. The grating-coupler assembly according to claim 1 , wherein the optical fiber comprises multiple cores and wherein the at least one grating coupler comprises corresponding multiple grating couplers.

6. The grating-coupler assembly according to claim 1 , wherein the at least one grating coupler has a coupler footprint that is reduced by at least 15% as compared to a grating-coupler footprint as defined by a single-mode fiber having a mode-field diameter MFD C of nominally 10 μm.

7. The grating-coupler assembly according to claim 1 , wherein the mode-field diameter MFD 60 is nominally 5.4 μm.

8. The grating-coupler assembly according to claim 1 , wherein an area reduction ratio R ranges from 0.25 to about 0.35 compared to a grating-coupler footprint defined by a single-mode fiber having a mode field diameter MFD C of about 10 μm.

9. A grating-coupler assembly for optically coupling light into or out of a photonic integrated circuit (PIC), comprising:

a silicon waveguide supported by a silicon-on-insulator (SOI) substrate, wherein the silicon waveguide is optically coupled to one or more PIC components on the PIC;

a grating coupler supported by the SOI substrate and optically coupled to the silicon waveguide, the grating coupler having a coupler footprint;

an optical fiber having an end and at least one core having a mode-field diameter MFD 60 in the range 5 μm≦MFD 60 ≦6 μm, wherein the optical-fiber end is disposed adjacent the grating coupler, with an intervening medium of refractive index n m >1 between the optical-fiber end and the grating coupler;

wherein the at least one core of the optical fiber and the grating coupler define a coupling efficiency CE≧0.7; and

wherein the coupler footprint is reduced by at least 10% from a coupler footprint defined by a single-mode optical fiber having a mode-field diameter of nominally 10 μm.

10. The grating-coupler assembly according to claim 9 , wherein the optical fiber includes a tapered end portion that includes the optical-fiber end.

11. A method of optically coupling light into a photonic integrated circuit (PIC) that includes at least one grating coupler optically coupled to at least one silicon waveguide, and one or more PIC components on the PIC optically coupled to the silicon waveguide, the method comprising:

transmitting light through at least one core of an optical fiber having an end, the at least one core defining a mode-field diameter MFD 60 for the transmitted light, wherein the mode-field diameter MFD 60 is in the range 5 μm≦MFD 60 ≦6 μm;

emitting the transmitted light from the optical-fiber end to the at least one grating coupler; and

directing the emitted light from the at least one grating coupler to the at least one silicon waveguide such that the optical coupling of light from the at least one core to the at least one silicon waveguide is accomplished with a coupling efficiency CE≧0.7, and the emitted light is received by the one or more PIC components from the at least one silicon waveguide.

12. The method according to claim 11 , further comprising transmitting the emitted light through an intervening medium of refractive index n m >1 that resides between the optical-fiber end and the at least one grating coupler.

13. The method according to claim 12 , wherein the optical fiber comprises multiple cores, the at least one grating coupler comprises multiple grating couplers and wherein each core is optically coupled to one of the grating couplers.

14. The method according to claim 12 , wherein the at least one grating coupler has a coupler footprint that is reduced by at least 15% as compared to a grating-coupler footprint as defined by a single-mode fiber having a mode-field diameter of nominally 10 μm.

15. A method of optically coupling light from a photonic integrated circuit (PIC) that includes at least one grating coupler optically coupled to at least one silicon waveguide, and one or more PIC components on the PIC optically coupled to the silicon waveguide, the method comprising:

transmitting light from the at least one silicon waveguide to the at least one grating coupler;

directing the transmitted light from the at least one grating coupler to at least one core of an optical fiber having an end arranged adjacent the at least one grating coupler, the at least one core defining a mode-field diameter MFD 60 for the transmitted light, wherein the mode-field diameter MFD 60 is in the range 5 μm≦MFD 60 ≦6 μm; and

wherein optical coupling of light from the at least one silicon waveguide to the at least one core is accomplished with a coupling efficiency CE≧0.7, and the transmitted light is received by the one or more PIC components from the at least one silicon waveguide.

16. The method according to claim 15 , further comprising transmitting the light through an intervening medium of refractive index n m >1 that resides between the optical-fiber end and the at least one grating coupler.

17. The method according to claim 15 , wherein the optical fiber comprises multiple cores, the at least one grating coupler comprises multiple grating couplers and wherein each core is optically coupled to one of the grating couplers.

18. The method according to claim 15 , wherein the at least one grating coupler has a coupler footprint that is reduced by an amount by at least 15% as compared to a grating-coupler footprint as defined by a single-mode fiber having a mode-field diameter of nominally 10 μm.