Photonic input/output coupler alignment

1. A photonic chip comprising:

a substrate;

a photonic integrated circuit (PIC) comprising a plurality of photonic devices formed in the substrate, the plurality of photonic devices comprising a first plurality of input/output couplers arranged in an array having a constant pitch; and

at least one loopback alignment feature formed in the substrate optically unconnected to the PIC, the at least one loopback alignment feature comprising a closed loop formed by at least two couplers and at least two waveguides connecting the at least two couplers, a distance between the at least two couplers being equal to an integer multiple of the constant pitch of the array of input/output couplers of the PIC.

2. The photonic chip of claim 1 , wherein the at least one loopback alignment feature comprises a first loopback alignment feature comprising the closed loop and a second loopback alignment feature, optically unconnected to the first loopback alignment feature, comprising two additional couplers connected to each other by at least one waveguide, a distance between the two additional couplers being equal to an integer multiple of the constant pitch of the array of input/output couplers of the PIC.

3. The photonic chip of claim 1 , wherein the closed loop is formed by at least three couplers and at least three optical waveguides connecting the at least three couplers, and wherein the at least one loopback alignment feature comprises two loopback alignment features comprising two respective pairs of couplers among the at least three couplers.

4. The photonic chip of claim 1 , wherein the at least three couplers are uniformly spaced in a linear arrangement.

5. The photonic chip of claim 1 , wherein the at least two couplers of the at least one loopback alignment feature comprise grating couplers.

6. The photonic chip of claim 1 , wherein the at least two couplers of the at least one loopback alignment feature comprise turning mirrors.

7. A method for aligning an optical connector with input/output couplers of a photonic integrated circuit (PIC) formed in a photonic chip, the method comprising:

actively aligning the optical connector with a loopback alignment feature formed in the photonic chip optically unconnected to the PIC by coupling light from a light source external to the photonic chip via a first channel of the optical connector into a first coupler of the loopback alignment feature and measuring the light at a second coupler of the loopback alignment feature via a second channel of the optical connector with a detector external to the photonic chip, the light transmitted from the first coupler to the second coupler via two separate optical connections each comprising a waveguide;

after active alignment of the optical connector with the loopback alignment feature, moving the optical connector, based at least in part on a known positions of the loopback alignment feature relative to the input/output couplers of the PIC, to a position aligned with the input/output couplers of the PIC; and

locking the optical connector in place in the position aligned with the input/output couplers of the PIC.

8. The method of claim 7 , further comprising actively aligning the optical connector with an additional loopback alignment feature formed in the photonic chip optically unconnected to the PIC, and calibrating a coordinate system in which the PIC and the loopback alignment features are defined relative to a movement of the optical connector based on a known relative position between the loopback alignment features.

9. The method of claim 8 , wherein the loopback alignment feature and the additional loopback alignment feature form a single closed loop comprising the first coupler, the second coupler, and a third coupler in a linear arrangement.

10. The method of claim 8 , wherein the loopback alignment feature and the additional loopback alignment feature form separate, optically unconnected structures.

11. The method of claim 7 , wherein the optical connector is aligned with the input/output couplers of the PIC without supplying electrical power to the PIC.

12. The method of claim 7 , wherein the optical connector is aligned with the input/output couplers of the PIC without establishing electrical connections between an alignment system and the PIC.

13. The method of claim 7 , wherein the optical connector is moved into the position aligned with the input/output couplers of the PIC using a robot with high-accuracy translational and rotational stages.

14. The method of claim 7 , wherein the optical connector is aligned to the input/output couplers of the PIC at a temperature selected based on an intended range of operating temperatures of the PIC.

15. A method of manufacturing a photonic chip comprising at least one loopback alignment feature, the method comprising:

photolithographically patterning a substrate to simultaneously define, with a single photomask, a plurality of input/output couplers of a photonic integrated circuit (PIC) and the at least one loopback alignment feature, the plurality of input/output couplers being arranged in an array having a constant pitch, and the at least one loopback alignment feature comprising a closed loop formed by at least two couplers and at least two waveguides connecting the at least two couplers, a distance between the at least two couplers being equal to an integer multiple of the constant pitch of the array of input/output couplers of the PIC; and

processing the patterned substrate to create the photonic integrated circuit and the at least one loopback alignment feature in the substrate.

16. The method of claim 15 , wherein the at least two couplers comprise grating couplers.

17. The method of claim 16 , wherein the grating couplers comprise at least one of silicon-nitride-based or indium-phosphide-based grating couplers.

18. The method of claim 15 , wherein the at least one loopback alignment feature comprises two loopback alignment features forming a single structure.

19. The method of claim 18 , wherein the two loopback alignment features comprise at least three couplers uniformly spaced in a linear arrangement.

20. The method of claim 15 , wherein the at least one loopback alignment feature comprises two loopback alignment features forming separate, optically unconnected structures.