Dual layer grating coupler

1. A system comprising:

a grating coupled laser comprising a first waveguide and a transmit grating coupler optically coupled to the first waveguide; and

a photonic integrated circuit (PIC) comprising a second waveguide and a receive grating coupler optically coupled to the second waveguide,

wherein:

the receive grating coupler is in optical alignment with the transmit grating coupler;

the receive grating coupler comprises a first grating and a second grating both in optical alignment together with the transmit grating coupler and both optically coupled to the second wavequide, the second grating being spaced apart from and above the first grating within the PIC; and

the first and second gratings are each formed in respective first and second layers of the PIC.

2. The system of claim 1 , wherein an optical mode size of the receive grating coupler is between 20 to 40 micrometers.

3. The system of claim 2 , wherein a coupling efficiency of the receive grating coupler is 0.5 to 3 decibels (dB) for incident light from the grating coupled laser that has a Gaussian mode with an optical mode size between 20 to 40 mirometers.

4. The system of claim 1 , wherein:

the first grating of the receive grating coupler comprises a silicon grating; and

the second grating of the receive grating coupler comprises a silicon nitride grating.

5. The system of claim 1 , wherein the second grating is spaced apart from the first grating by an intermediate layer of oxide disposed between the first and second layers.

6. The system of claim 1 , wherein at least one of:

the receive grating coupler comprises a positive angle grating coupler;

one or both of the first and second gratings of the receive grating coupler comprises grating lines that are straight and are spaced across the respective first and second layer; and

the second waveguide of the PIC is arranged at a perpendicular angle to the grating lines of one or both of the first and second gratings.

7. The system of claim 1 , wherein at least one of:

the receive grating coupler comprises a negative angle grating coupler;

one or both of the first and second gratings of the receive grating coupler comprises grating lines that are curved and are spaced across the respective first and second layer; and

the second waveguide of the PIC is arranged at a non-perpendicular angle to the grating lines of one or both of the first and second gratings.

8. The system of claim 1 , wherein the system is devoid of an optical isolator between the grating coupled laser and the PIC.

9. The system of claim 1 , wherein the PIC comprises:

a silicon substrate;

a silicon dioxide box layer formed above the silicon substrate;

a silicon layer formed above the silicon dioxide box layer and being the first layer in which the first grating is formed, the first grating comprising a silicon grating;

a gate oxide layer formed above the silicon layer and separating the first layer from the second layer;

a silicon nitride layer formed above the gate oxide layer and being the second layer in which the second grating is formed, the second grating comprising a silicon nitride grating, wherein the second waveguide is formed in the silicon nitride layer.

10. The system of claim 1 , wherein:

the transmit grating coupler comprises grating teeth formed on the first waveguide, the grating teeth extending upward from a top of a core of the first waveguide, the grating teeth each having a total height and a height above the top of the core of the first waveguide; and

a core index of refraction of the core of the first waveguide is greater than a first threshold value so that an effective index of the transmit grating coupler is sufficiently higher than a bottom cladding index of refraction of a bottom cladding of the transmit grating coupler to avoid leakage of a diffracted optical mode into a substrate of the transmit grating coupler.

11. The system of claim 1 , wherein:

the grating coupled laser further comprises an active section optically coupled to the first waveguide, a passive section that includes the first waveguide and the transmit grating coupler, and a ridge structure formed in the active section and the passive section; and

the ridge structure comprises a shallow ridge that extends downward to a first depth in the active section that is above a depth of a multiple quantum well layer of the active section and a deep ridge that extends downward to a second depth in the passive section that is below the depth of the multiple quantum well layer.

12. A system comprising:

a grating coupled laser comprising an active section and a passive section, the active section including a laser cavity, the passive section including a first waveguide and a transmit grating coupler, the first waveguide optically coupled end to end with the laser cavity; and

a photonic integrated circuit (PIC) comprising a second waveguide and a receive grating coupler optically coupled to the second waveguide, wherein:

the receive grating coupler is in optical alignment with the transmit grating coupler; and

the receive grating coupler comprises a first grating formed in a first layer of the PIC, a second grating formed in a second layer of the PIC, and a gate oxide layer formed between the first layer and the second layer of the PIC,

the first and second gratings both in optical alignment together with the transmit grating coupler and both optically coupled to the second waveguide.

13. The system of claim 12 , wherein:

an optical mode size of the receive grating coupler is between 20 to 40 micrometers; and at least one of the first grating or the second grating of the receive grating coupler is apodized such that an optical mode of the receive grating coupler overlaps an optical mode of light received from the grating coupled laser by at least 50%.

14. The system of claim 12 , wherein:

the first grating of the receive grating coupler comprises a silicon grating; and

the second grating of the receive grating coupler comprises a silicon nitride grating.

15. The system of claim 12 , wherein at least one of:

the receive grating coupler comprises a positive angle grating coupler;

one or both of the first and second gratings of the receive grating coupler comprises grating lines that are straight; and

the second waveguide of the PIC is arranged at a perpendicular angle to the grating lines of one or both of the first and second gratings.

16. The system of claim 12 , wherein at least one of:

the receive grating coupler comprises a negative angle grating coupler;

one or both of the first and second gratings of the receive grating coupler comprises grating lines that are curved; and

the second waveguide of the PIC is arranged at a non-perpendicular angle to the grating lines of one or both of the first and second gratings.

17. The system of claim 12 , wherein the system is devoid of an optical isolator between the grating coupled laser and the PIC.

18. The system of claim 12 , wherein the PIC comprises: a silicon substrate;

a silicon dioxide box layer formed above the silicon substrate;

the first layer comprising a silicon layer formed above the silicon dioxide box layer and in which the first grating is formed, the first grating comprising a silicon grating;

the gate oxide layer formed above the silicon layer; and

the second layer comprising a silicon nitride layer formed above the gate oxide layer and in which the second grating is formed, the second grating comprising a silicon nitride grating, wherein the second waveguide is formed in the silicon nitride layer.

19. The system of claim 12 , wherein:

the transmit grating coupler comprises grating teeth formed on the first waveguide, the grating teeth extending upward from a top of a core of the first waveguide, the grating teeth each having a total height and a height above the top of the core of the first waveguide; and

a core index of refraction of the core of the first waveguide is greater than a first threshold value so that an effective index of the transmit grating coupler is sufficiently higher than a bottom cladding index of refraction of a bottom cladding of the transmit grating coupler to avoid leakage of a diffracted optical mode into a substrate of the transmit grating coupler.

20. A system comprising:

a grating coupled laser comprising an active section and a passive section, the active section including a laser cavity, the passive section including a first waveguide and a transmit grating coupler, the first waveguide optically coupled end to end with the laser cavity; and

a photonic integrated circuit (PIC) comprising:

a silicon substrate;

a silicon dioxide box layer formed above the silicon substrate;

a silicon layer formed above the silicon dioxide box layer and in which a silicon grating is formed;

a gate oxide layer formed above the silicon layer; and

a silicon nitride layer formed above the gate oxide layer and in which a silicon nitride grating is formed and in which a second waveguide is formed that is optically coupled to the silicon grating and the silicon nitride grating,

wherein the silicon grating and the silicon nitride grating form a receive grating coupler, the silicon grating and the silicon nitride grating both being in optical alignment together with the transmit grating coupler and both being, optically coupled to the second waveguide, the receive grating coupler positioned to couple light received from the transmit grating coupler into the second waveguide.