Dual-slab-layer low-loss silicon optical modulator
A silicon optical modulator is fabricated to have a multi-slab structure between the contacts and the waveguide, imparting desirable performance attributes. A first slab comprises dopant of a first level. A second slab adjacent to (e.g., on top of) the first slab, comprises a doped region proximate to a contact, and an intrinsic region proximate to the waveguide. The parallel resistance properties and low overlap between the highly doped silicon and optical mode pigtail afforded by the multi-slab configuration, allow the modulator to operate with reduced optical losses and at a high speed. Embodiments may be implemented in a Mach-Zehnder interferometer or in micro-ring resonator modulator configuration.
1. An optical modulator comprising:
a waveguide having a first waveguide portion of a first conductivity type and a second waveguide portion of a second conductivity type opposite to the first conductivity type, the waveguide configured to communicate an optical signal along a first axis defined within a silicon substrate;
a first contact of the first conductivity type proximate to the first waveguide portion;
a second contact of the second conductivity type proximate to the second waveguide portion;
a first multi-slab structure offering parallel conductive pathways of different resistance between the first contact and the first portion; and
a second multi-slab structure offering parallel conductive pathways of different resistance between the second contact and the second portion, wherein:
a first slab of the first multi-slab structure proximate to the silicon substrate, is adjacent to a second slab of the first multi-slab structure distal from the silicon substrate, along a second axis orthogonal to the first axis.
2. An optical modulator as in claim 1 wherein the first slab exhibits a resistance lower than the second slab.
3. An optical modulator as in claim 2 wherein the first slab has a higher doping of the first conductivity type than the second slab.
4. An optical modulator as in claim 3 wherein the second slab comprises:
an intrinsic portion proximate to the first waveguide portion, and
a doped portion proximate to the first contact.
5. An optical modulator as in claim 1 comprising a Mach-Zehnder interferometer.
6. An optical modulator as in claim 1 comprising a micro-ring resonator.
7. An optical modulator as in claim 1 wherein the silicon substrate comprises a silicon-on-insulator (SOI) substrate.
8. An optical modulator as in claim 1 wherein the second slab exhibits a resistance lower than the first slab.
9. A method comprising:
creating a first conductive pathway of a first resistance between a first contact of a first conductivity type and a first waveguide portion of the first conductivity type;
creating a second conductive pathway of a second resistance lower than the first resistance, between the first contact and the first waveguide portion, the second conductive pathway parallel to the first conductive pathway;
creating a third conductive pathway of a third first resistance between a second contact of a second conductivity type opposite to the first conductivity type, and a second waveguide portion of the second conductivity type;
creating a fourth conductive pathway of a fourth resistance lower than the third resistance, between the second contact and the second waveguide portion, the fourth conductive pathway parallel to the third conductive pathway; and
applying a potential difference between the first contact and the second contact to reverse-bias a junction formed at an interface of the first waveguide portion and the second waveguide portion.
10. A method as in claim 9 further comprising communicating an optical signal through the first waveguide portion and the second waveguide portion along a first axis defined in a silicon substrate,
wherein the second conductive pathway and the fourth conductive pathway are proximate to the silicon substrate along a second axis orthogonal to the first axis, and
the first conductive pathway and the third conductive pathway are distal from the silicon substrate along the second axis.
11. A method as in claim 10 wherein:
the first conductive pathway comprises a first intrinsic portion proximate to the first waveguide portion; and
the third conductive pathway comprises a second intrinsic portion proximate to the second waveguide portion.
12. A method as in claim 9 wherein:
the first conductive pathway comprises a first intrinsic portion proximate to the first waveguide portion; and
the third conductive pathway comprises a second intrinsic portion proximate to the second waveguide portion.
13. A method as in claim 9 wherein the first waveguide portion and the second waveguide portion are part of a Mach-Zehnder interferometer.
14. A method as in claim 9 wherein the first waveguide portion and the second waveguide portion are part of a micro-ring resonator.
15. An optical modulator integrated with a silicon photonics system comprising:
a first phase-shifter on a silicon substrate;
a second phase-shifter on the silicon substrate;
a first 2×2 splitter having a first exit port coupled to an input port of the first phase-shifter and a second exit port coupled to an input port of the second phase-shifter; and
a second 2×2 splitter having a first entry port coupled to an output port of the first phase-shifter and a second entry port coupled to an output port of the second phase-shifter,
wherein the first phase-shifter comprises,
a first contact of the first conductivity type proximate to the first waveguide portion;
a second contact of the second conductivity type proximate to the second waveguide portion;
a first multi-slab structure offering parallel conductive pathways of different resistance between the first contact and the first portion; and
a second multi-slab structure offering parallel conductive pathways of different resistance between the second contact and the second portion, wherein:
a first slab of the first multi-slab structure proximate to the silicon substrate, is adjacent to a second slab of the first multi-slab structure distal from the silicon substrate, along a second axis orthogonal to the first axis.
16. An optical modulator as in claim 15 wherein the first slab exhibits a resistance lower than the second slab.
17. An optical modulator as in claim 16 wherein the first slab has a higher doping of the first conductivity type than the second slab.
18. An optical modulator as in claim 17 wherein the second slab comprises:
an intrinsic portion proximate to the first waveguide portion, and
a doped portion proximate to the first contact.
19. An optical modulator as in claim 15 comprising a Mach-Zehnder interferometer.
20. An optical modulator as in claim 15 comprising a micro-ring resonator.