Waveguide modulators structures

1. A Mach-Zehnder waveguide modulator comprising:

a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide;

wherein each of the left and right optical waveguides comprises a PIN junction region and a plurality of electrodes for providing a bias across the PIN junction to enable control of the phase of light travelling through the respective PIN junction region via dispersion;

wherein each of the PIN junctions is formed from:

a first semiconductor region corresponding to either a p-doped region or an n-doped region;

a second semiconductor region corresponding to the other of the p-doped or n-doped region; and

a central SiGe waveguide region;

wherein the first semiconductor region of the left optical waveguide is integral with the first semiconductor region of the right optical waveguide in a region between the left and right arms, forming a common doped region; and

the plurality of electrodes is configured to apply a forward bias across one or both of the PIN junctions.

2. A Mach-Zehnder waveguide modulator according to claim 1 , wherein:

at least one of the first semiconductor region or second semiconductor region includes a vertical doped portion which extends along the side of the waveguide.

3. A Mach-Zehnder waveguide modulator according to claim 2 , wherein the vertical doped portion only extends in the vertical direction along a portion of the side wall of the waveguide, such that the central SiGe waveguide region has a greater height than the vertical doped portion.

4. A Mach-Zehnder waveguide modulator according to claim 2 , wherein the vertical doped portion extends in the vertical direction along the entire side of the waveguide.

5. A Mach-Zehnder waveguide modulator according to claim 2 , wherein, for a given waveguide arm of the modulator, no more than one side of the waveguide comprises a vertical doped portion.

6. A Mach-Zehnder waveguide modulator according to claim 1 , wherein

the Mach-Zehnder waveguide modulator further comprises:

an intervening lightly p-doped semiconductor region located between the p-doped semiconductor region and the central waveguide region;

wherein the intervening lightly p-doped semiconductor region has a lower dopant concentration than the p-doped semiconductor region.

7. A Mach-Zehnder waveguide modulator according to claim 1 , wherein

the Mach-Zehnder waveguide modulator further comprises:

an intervening lightly n-doped semiconductor region located between the n-doped semiconductor region and the central waveguide region;

wherein the intervening lightly n-doped semiconductor region has a lower dopant concentration than the n-doped semiconductor region.

8. A Mach-Zehnder waveguide modulator according to claim 7 , wherein:

the Mach-Zehnder waveguide modulator further comprises an intervening lightly p-doped semiconductor region located between the p-doped semiconductor region and the central waveguide region, wherein the intervening lightly p-doped semiconductor region has a lower dopant concentration than the p-doped semiconductor region; and

the central waveguide region, the n-doped semiconductor region, the p-doped semiconductor region, the lightly p-doped semiconductor region, and the lightly n-doped semiconductor region are all composed of SiGe.

9. A Mach-Zehnder waveguide modulator according to claim 1 , wherein:

the first semiconductor region; and the second semiconductor region are each composed of Si.

10. A Mach-Zehnder waveguide modulator according to claim 1 , wherein:

the central SiGe waveguide region, the first semiconductor region, and the second semiconductor region are all composed of SiGe.

11. A Mach-Zehnder waveguide modulator according to claim 1 , wherein the plurality of electrodes includes a common electrode located at the common doped region.

12. A Mach-Zehnder waveguide modulator according to claim 1 wherein, for each waveguide arm:

the first semiconductor region includes a first lateral portion extending laterally away from the waveguide wall on a first side of the waveguide;

the second semiconductor region includes a second lateral portion extending laterally away from the waveguide wall on a second side of the waveguide; and

the plurality of electrodes comprises:

a first electrode located directly on top of the first lateral portion; and

a second electrode located directly on top of the first lateral portion.

13. A Mach-Zehnder waveguide modulator according to claim 1 , wherein the plurality of electrodes includes a plurality of lumped electrodes.

14. A Mach-Zehnder waveguide modulator according to claim 1 , configured to operate in a push-pull mode.

15. A Mach-Zehnder waveguide modulator comprising:

a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide;

wherein each of the left and right optical waveguides comprises a PIN junction region and a plurality of electrodes for providing a bias across the PIN junction to enable control of the phase of light travelling through the respective PIN junction region via dispersion;

wherein each of the PIN junctions is formed from:

a first semiconductor region corresponding to either a p-doped region or an n-doped region;

a second semiconductor region corresponding to the other of the p-doped or n-doped region;

and a central SiGe waveguide region;

wherein the first semiconductor region of the left optical waveguide is electrically isolated from the first semiconductor region of the right optical waveguide by a central isolated region between the left and right arms; and

the plurality of electrodes is configured to apply a forward bias across one or both of the PIN junctions.

16. A Mach-Zehnder waveguide modulator according to claim 15 , wherein:

at least one of the first semiconductor region or second semiconductor region includes a vertical doped portion which extends along the side of the waveguide.

17. A Mach-Zehnder waveguide modulator according to claim 16 , wherein the vertical doped portion only extends in the vertical direction along a portion of the side wall of the waveguide, such that the central SiGe waveguide region has a greater height than the vertical doped portion.

18. A Mach-Zehnder waveguide modulator according to claim 16 , wherein the vertical doped portion extends in the vertical direction along the entire side of the waveguide.

19. A Mach-Zehnder waveguide modulator according to claim 15 , wherein the Mach-Zehnder waveguide modulator further comprises:

an intervening lightly p-doped semiconductor region located between the p-doped semiconductor region and the central waveguide region;

wherein the intervening lightly p-doped semiconductor region has a lower dopant concentration than the p-doped semiconductor region.

20. A Mach-Zehnder waveguide modulator according to claim 15 , wherein the Mach-Zehnder waveguide modulator further comprises:

an intervening lightly n-doped semiconductor region located between the n-doped semiconductor region and the central waveguide region;

wherein the intervening lightly n-doped semiconductor region has a lower dopant concentration than the n-doped semiconductor region.