Optical waveguide modulator with an output MMI tap

1. A planar optical waveguide circuit, comprising:

an optical waveguide modulator comprising:

an input waveguide configured to receive input light, and

a modulator output waveguide configured to output modulated light; and

an in-line optical waveguide tap configured to tap off a fraction of the modulated light for monitoring, the in-line optical waveguide tap comprising:

a multi-mode interference (MMI) coupler comprising first and second input ports and first and second output ports, and

an unused waveguide that is directly coupled, at a first end, to the second input port of the MMI coupler and terminated, at a second end, in a first substrate to avoid picking up any stray portion of the input light or the modulated light propagating in the first substrate;

wherein the modulator output waveguide is optically coupled to the first input port of the MMI coupler to receive the modulated light from the optical waveguide modulator, and the MMI coupler is configured to couple non-equal fractions of the received modulated light into the first and second output ports.

2. The planar optical waveguide circuit of claim 1 , wherein the in-line optical waveguide tap is formed in the first substrate.

3. The planar optical waveguide circuit of claim 1 , wherein the second end of the unused waveguide is bent or routed away from the optical waveguide modulator.

4. The planar optical waveguide circuit of claim 1 , wherein the MMI coupler is configured to:

couple at least 80% of the modulated light received from the first input port into the first output port, and

couple at least 1% of the modulated light received from the first input port into the second output port.

5. The planar optical waveguide circuit of claim 1 , wherein the optical waveguide modulator is formed in or upon the first substrate integrally with the in-line optical waveguide tap.

6. The planar optical waveguide circuit of claim 1 , wherein the optical waveguide modulator is formed in or upon a second substrate.

7. The planar optical waveguide circuit of claim 6 , wherein the second substrate is butt-coupled to the first substrate.

8. The planar optical waveguide circuit of claim 6 , wherein

the first substrate comprises one of:

Silicon (Si),

Silica (SiO 2 ), or

Silica on Silicon, and

the second substrate comprises one of:

lithium niobate (LiNbO 3 ),

indium phosphide (InP),

silicon (Si), or

gallium arsenide (GaAs).

9. The planar optical waveguide circuit of claim 1 , wherein the first substrate comprises one of:

lithium niobate (LiNbO 3 ),

indium phosphide (InP),

gallium arsenide (GaAs),

Silicon (Si), or

Silica (SiO 2 ).

10. The planar optical waveguide circuit of claim 1 , wherein the MMI coupler comprises an MMI region of a rectangular shape.

11. The planar optical waveguide circuit of claim 1 , wherein the MMI coupler comprises an MMI region of a diamond-like shape.

12. The planar optical waveguide circuit of claim 1 , wherein the optical waveguide modulator comprises a Mach-Zehnder (MZ) modulator.

13. The planar optical waveguide circuit of claim 1 , wherein the optical waveguide modulator comprises an absorption modulator.

14. The planar optical waveguide circuit of claim 1 , wherein the second end of the unused waveguide is terminated at an optical termination.

15. The planar optical waveguide circuit of claim 14 , wherein the optical termination comprises metal loading.

16. The planar optical waveguide circuit of claim 14 , wherein the optical termination comprises a pit or groove etched across at least a portion of the unused waveguide.

17. An optical waveguide circuit, comprising:

a substrate;

an optical waveguide modulator formed in or upon the substrate and comprising an input waveguide for receiving input light and a modulator output waveguide for outputting modulated light; and

an in-line optical waveguide tap formed in or upon the substrate and configured to tap off a fraction of the modulated light for monitoring, the in-line optical waveguide tap comprising:

a multi-mode interference (MMI) coupler comprising first and second input ports and first and second output ports, and

an unused waveguide comprising a first end that directly connects to the second input port of the MMI coupler and a second end that is terminated, in the substrate, so as to avoid picking up any stray portion of the input or modulated light propagating in the substrate;

wherein the modulator output waveguide is coupled to the first input port of the MMI coupler for receiving the modulated light from the optical waveguide modulator, and the MMI coupler is configured to couple non-equal fractions of the received modulated light into the first and second output ports.

18. The optical waveguide circuit of claim 17 , wherein the optical waveguide modulator comprises an absorption modulator.

19. The optical waveguide circuit of claim 17 , wherein the optical waveguide modulator comprises a Mach-Zehnder (MZ) modulator.

20. A method comprising:

receiving, by an optical waveguide modulator, input light;

outputting, by the optical waveguide modulator, modulated light; and

tapping off, by an in-line optical waveguide tap, a fraction of the modulated light for monitoring,

wherein

the in-line optical waveguide tap comprises:

a multi-mode interference (MMI) coupler comprising first and second input ports and first and second output ports, and

an unused waveguide that is directly coupled, at a first end, to the second input port of the MMI coupler and terminated, at a second end, in a first substrate to avoid picking up any stray portion of the input light or the modulated light propagating in the first substrate,

the modulator output waveguide is optically coupled to the first input port of the MMI coupler to receive the modulated light from the optical waveguide modulator, and

the MMI coupler is configured to couple non-equal fractions of the received modulated light into the first and second output ports.