Optoelectronic modulator, photonic integrated circuit, and method

1. A photonic integrated circuit, comprising:

an input waveguide configured to guide input light in a superposition of two polarisation modes of the input waveguide;

a polarisation splitter, connected to the input waveguide, and configured to provide, at a first output, light in a first polarisation mode of the two polarisation modes and, at a second output, light in a second polarisation mode of the two polarisation modes;

a first polarisation rotator, connected to the first output of the polarisation splitter, and configured to rotate light received therefrom from the first polarisation mode to the second polarisation mode;

an optoelectronic modulator, having a first modulation waveguide connected to the first polarisation rotator and a second modulation waveguide connected to the second output of the polarisation splitter, and configured, when the input light comprises both of the first and second polarisation modes, to modulate light in each waveguide according to the same modulation scheme, such that:

the power of light output from the first modulation waveguide is equal to the product of the power of light input to the first modulation waveguide and a modulation signal, and

the power of light output from the second modulation waveguide is equal to the product of the power of light input to the second modulation waveguide and the same modulation signal;

a first intermediate waveguide and a second intermediate waveguide, connected respectively to the first modulation waveguide and the second modulation waveguide; and

a polarisation combiner, connected to the first intermediate waveguide and the second intermediate waveguide, and configured to combine light received from each and provide the combined light to an output of the photonic integrated circuit as a modulated signal,

wherein one of the first intermediate waveguide and the second intermediate waveguide comprises a second polarisation rotator configured to rotate light received therein from one of the first and second polarisation modes to the other of the first and second polarisation modes.

2. The photonic integrated circuit of claim 1 , wherein the first intermediate waveguide comprises the second polarisation rotator, and the second polarisation rotator is configured to rotate light received therein from the second polarisation mode to the first polarisation mode.

3. The photonic integrated circuit of claim 1 , wherein the second intermediate waveguide comprises the second polarisation rotator, and the second polarisation rotator is configured to rotate light received therein from the first polarisation mode to the second polarisation mode.

4. The photonic integrated circuit of claim 1 , wherein the optoelectronic modulator comprises:

the first modulation waveguide, including a first modulation region;

the second modulation waveguide, including a second modulation region; and

a driver, operable to modulate light in both the first modulation waveguide and the second modulation waveguide,

wherein the driver is configured to send the same modulation signal to each of the first modulation region and the second modulation region.

5. The photonic integrated circuit of claim 1 , wherein either or both of the polarisation splitter and the polarisation combiner are provided as one or more multi-mode interference couplers.

6. The photonic integrated circuit of claim 1 , wherein either or both of the first polarisation rotator and the second polarisation rotator is provided as a rib waveguide, the rib waveguide including:

a slab portion; and

a ridge portion, which is disposed along a surface of the slab portion, and wherein:

the slab portion has a first slab region whose width, as measured in a direction perpendicular to a guiding direction of the waveguide, increases from a first slab width to a second slab width along a first length;

the ridge portion has a first ridge region whose width, as measured in the same direction as the slab widths, decreases from a first ridge width to a second ridge width along the same first length; and

such that the rotator is configured to rotate the polarisation of light during its transmission through the rib waveguide.

7. The photonic integrated circuit of claim 1 , wherein the polarisation splitter and first polarisation rotator, and/or the second polarisation rotator and polarisation combiner, are each provided as a single polarisation diverse grating coupler.

8. The photonic integrated circuit of claim 1 , wherein the circuit is a passive polarisation diverse modulator circuit.

9. The photonic integrated circuit of claim 1 , wherein the optoelectronic modulator comprises:

the first modulation waveguide, including a first modulation region;

the second modulation waveguide, including a second modulation region; and

a driver, operable to modulate light in both the first modulation waveguide and the second modulation waveguide,

wherein the driver is configured to send the same modulation signal to each of the first modulation region and the second modulation region.

10. The photonic integrated circuit of claim 9 , wherein the first modulation region and the second modulation region each comprises a first doped region and a second doped region.

11. The photonic integrated circuit of claim 10 , wherein the second doped region of each modulation region is contiguous with the other, and wherein a shared electrode is connected to the second doped regions of the modulation regions.

12. The photonic integrated circuit of claim 10 , wherein the first doped region of the first modulation region is connected to a first electrode and the first doped region of the second modulation region is connected to a second electrode.

13. The photonic integrated circuit of claim 10 , wherein the first doped region of each modulation region is connected to a second shared electrode.

14. The photonic integrated circuit of claim 9 , wherein each of the first modulation waveguide and the second modulation waveguide is a ridge waveguide.

15. The photonic integrated circuit of claim 9 , wherein the first and second modulation waveguides are formed from a III-V stack.

16. The photonic integrated circuit of claim 9 , comprising electrodes formed from gold.

17. The photonic integrated circuit of claim 9 , wherein the first modulation waveguide and the second modulation waveguide are counter-propagating waveguides.

18. The photonic integrated circuit of claim 9 , wherein the first modulation waveguide and the second modulation waveguide are co-propagating waveguides.

19. A method of modulating light in a photonic integrated circuit, comprising the steps of:

providing light from an input waveguide, said light comprising both of two polarisation modes of the input waveguide and being in a superposition of the two polarisation modes;

splitting the light into a first portion having a first polarisation mode of the two polarisation modes, and a second portion having a second polarisation mode of the two polarisation modes;

rotating the polarisation of the first portion, such that it has the second polarisation mode;

modulating the first portion and the second portion according to the same modulation scheme such that:

the power of the first portion after modulation is equal to the product of the power of the first portion before modulation and a modulation signal, and

the power of the second portion after modulation is equal to the product of the power of the second portion before modulation and the same modulation signal;

rotating the polarisation of one of the first portion and the second portion, so as to provide one portion having the first polarisation mode and one portion having the second polarisation mode; and

combining the first portion and the second portion.

20. The method of claim 19 , wherein the photonic integrated circuit comprises:

the input waveguide, configured to guide light in a superposition of the two polarisation modes of the input waveguide;

a polarisation splitter, connected to the input waveguide, and configured to provide, at a first output, light in the first polarisation mode and, at a second output, light in the second polarisation mode;

a first polarisation rotator, connected to the first output of the polarisation splitter, and configured to rotate light received therefrom from the first polarisation mode to the second polarisation mode;

an optoelectronic modulator, having a first modulation waveguide connected to the first polarisation rotator and a second modulation waveguide connected to the second output of the polarisation splitter, and configured to modulate light in each waveguide according to a modulation scheme;

a first intermediate waveguide and a second intermediate waveguide, connected respectively to the first modulation waveguide and the second modulation waveguide; and

a polarisation combiner, connected to the first intermediate waveguide and the second intermediate waveguide, and configured to combine light received from each and provide the combined light to an output of the photonic integrated circuit as a modulated signal,

wherein one of the first intermediate waveguide and the second intermediate waveguide comprises a second polarisation rotator configured to rotate light received therein from one of the first and second polarisation modes to the other of the first and second polarisation modes.

21. A method of modulating light in a photonic integrated circuit, the method comprising the steps of:

providing input light having a first power from an input waveguide, said input light being in a superposition of two polarisation modes of the input waveguide; and

modulating the input light according to a modulation function to have a second power equal to the product of the first power and the modulation function, the modulation function being independent of the polarisation of the input light,

wherein the photonic integrated circuit comprises:

the input waveguide;

a polarisation splitter, connected to the input waveguide, and configured to provide, at a first output, light in a first polarisation mode of the two polarisation modes and, at a second output, light in a second polarisation mode of the two polarisation modes;

a first polarisation rotator, connected to the first output of the polarisation splitter, and configured to rotate light received therefrom from the first polarisation mode to the second polarisation mode;

an optoelectronic modulator, having a first modulation waveguide connected to the first polarisation rotator and a second modulation waveguide connected to the second output of the polarisation splitter, and configured to modulate light in each waveguide according to a modulation scheme;

a first intermediate waveguide and a second intermediate waveguide, connected respectively to the first modulation waveguide and the second modulation waveguide; and

a polarisation combiner, connected to the first intermediate waveguide and the second intermediate waveguide, and configured to combine light received from each and provide the combined light to an output of the photonic integrated circuit as a modulated signal,

wherein one of the first intermediate waveguide and the second intermediate waveguide comprises a second polarisation rotator configured to rotate light received therein from one of the first and second polarisation modes to the other of the first and second polarisation modes.