Semiconductor-based optical modulator
An optical modulator includes, a semiconductor substrate, an optical waveguide portion disposed on the semiconductor substrate, a first P-N junction disposed on the semiconductor substrate, and a second P-N disposed on the semiconductor substrate. The optical waveguide portion provides an optical path for light that is to be modulated. The first P-N junction is disposed on the semiconductor substrate along the optical path and defines a border between an N-doped portion disposed on the semiconductor substrate and a P-doped portion disposed on the semiconductor substrate. The second P-N junction is disposed on a portion of the semiconductor substrate alongside the optical path and spaced apart from the first P-N junction.
1. An optical modulator, including:
a semiconductor substrate;
an optical waveguide portion disposed on the semiconductor substrate, the optical waveguide portion configured to provide an optical path for light that is to be modulated;
a first P-N junction disposed on the semiconductor substrate along the optical path, the first P-N junction defining a border between an N-doped portion disposed on the semiconductor substrate and a P-doped portion disposed on the semiconductor substrate, the P-doped portion having a uniform doping level; and
a second P-N junction disposed on a portion of the semiconductor substrate alongside the optical path and spaced apart from the first P-N junction by the P-doped portion, the P-doped portion extending from the first P-N junction to the second P-N junction; and
a reference point disposed on the semiconductor substrate alongside the first P-N junction, the reference point electrically coupled in series with the first P-N junction and the second P-N junction, the reference point configured to be held at, via electrical coupling, a DC reference potential.
2. The optical modulator of claim 1 , wherein the first P-N junction and the second P-N junction are electrically coupled in series along an electrical path transverse to the optical path.
3. The optical modulator of claim 1 , wherein the N-doped portion and the P-doped portion are configured to undergo a shift in refractive index relative to one another while an electrical potential is applied across the first P-N junction.
4. The optical modulator of claim 1 , wherein:
the optical modulator is structured as a ring modulator including a resonator portion forming a ring; and
the optical waveguide portion, N-doped portion, and P-doped portion include at least parts of concentric annular regions of the resonator portion.
5. The optical modulator of claim 1 , wherein:
the optical waveguide portion, the first P-N junction, and the second P-N junction are components of a Mach-Zehnder modulator; and
the optical waveguide forms an arm of the Mach-Zehnder modulator.
6. The optical modulator of claim 1 , wherein the second P-N junction defines a border between the P-doped portion and an opposingly-doped portion disposed on the semiconductor substrate.
7. The optical modulator of claim 6 , wherein the N-doped portion and/or the opposingly-doped portion include sub-portions with differing dopant levels.
8. The optical modulator of claim 7 , wherein, separately in each of the N-doped and the opposingly-doped portions, any sub-portions are ordered such that sub-portion dopant level increases with increasing distance from the optical path.
9. The optical modulator of claim 6 , wherein:
the opposingly-doped portion includes a second N-doped portion; and
the second P-N junction defines a border between the P-doped portion and the second N-doped portion.
10. The optical modulator of claim 1 , wherein a distance between the first and second P-N junctions is selected based on a size of an optical mode the optical waveguide portion is configured to guide.
11. The optical modulator of claim 10 , wherein the distance is further selected based on a ratio between an expected optical loss from interference with the optical mode by the second P-N junction and expected electrical loss due to resistivity of a portion of the semiconductor substrate between the first and second P-N junctions.
12. The optical modulator of claim 1 , further including:
a first metal contact disposed on the semiconductor substrate alongside the first P-N junction, the first metal contact configured to receive, via electrical coupling, a modulation signal;
wherein the reference point is electrically coupled to the first metal contact in series through the first and second P-N junctions.
13. The optical modulator of claim 12 , wherein the reference point includes a second metal contact disposed on the semiconductor substrate alongside the first P-N junction.
14. The optical modulator of claim 1 , further including:
a second optical waveguide portion disposed on the semiconductor substrate, the second optical waveguide portion configured to provide a second optical path for light that is to be modulated;
a third P-N junction disposed on the semiconductor substrate along the second optical path; and
a fourth P-N junction disposed on a portion of the semiconductor substrate alongside the second optical path and spaced apart from the third P-N junction; and
wherein the reference point is disposed on the semiconductor substrate between the two optical waveguide portions.
15. The optical modulator of claim 1 , wherein the first P-N junction is disposed on the semiconductor substrate to bisect the optical waveguide portion.
16. The optical modulator of claim 1 , where the first P-N junction and the second P-N junction are laterally spaced apart.
17. The optical modulator of claim 1 , where the first P-N junction and the second P-N junction are spatially oriented in a same direction with respect to the semiconductor substrate.
18. A method of optical modulation, including:
optically coupling light to be modulated onto an optical path provided by an optical waveguide portion disposed on a semiconductor substrate; and
electrically coupling a modulation signal along an electrical path by:
electrically coupling across a first P-N junction disposed on the semiconductor substrate along the optical path and that defines a border between i) a first N-doped portion disposed on the semiconductor substrate and ii) a P-doped portion disposed on the semiconductor substrate, the P-doped portion having a uniform doping level; and
electrically coupling across a second P-N junction disposed on the semiconductor substrate alongside the optical path and spaced apart from the first P-N junction, the second P-N junction defining a border between i) a second N-doped portion disposed on the semiconductor substrate and ii) the P-doped portion;
holding a reference point at a DC reference potential via electrical coupling, the reference point i) disposed on the semiconductor substrate alongside the first P-N junction and ii) electrically coupled in series with the first P-N junction and the second P-N junction.
19. The method of optical modulation of claim 18 , wherein electrically coupling the modulation signal along the electrical path includes:
electrically coupling the modulation signal from a first metal contact to a second metal contact of the reference point in series through the first and second P-N junctions; and
wherein holding the reference point at the DC reference potential comprises holding the second metal contact at the DC reference potential.
20. The method of optical modulation of claim 18 , further including modulating the light by causing the N-doped and P-doped portions to shift in refractive index relative to one another responsive to an electrical potential of the modulation signal.
21. The method of optical modulation of claim 18 , wherein electrically coupling across the second P-N junction includes electrically coupling the modulation signal across a border between the P-doped portion and the second N-doped portion.
22. A method of manufacturing an optical modulator, including:
providing a semiconductor substrate;
fabricating an optical waveguide portion disposed on the semiconductor substrate, the optical waveguide portion configured to provide an optical path for light that is to be modulated;
fabricating a first P-N junction disposed on the semiconductor substrate along the optical path, the first P-N junction defining a border between i) a first N-doped portion disposed on the semiconductor substrate and ii) a P-doped portion disposed on the semiconductor substrate, the P-doped portion having a uniform doping level; and
fabricating a second P-N junction disposed on a portion of the semiconductor substrate alongside the optical path and spaced apart from the first P-N junction, the second P-N junction defining a border between i) a second N-doped portion disposed on the semiconductor substrate and ii) the P-doped portion; and
fabricating a reference point disposed on the semiconductor substrate alongside the first P-N junction, including fabricating the reference point to be electrically coupled in series with the first P-N junction and the second P-N junction, the reference point to be held at, via electrical coupling, a DC reference potential.
23. The method of manufacturing the optical modulator of claim 22 , wherein fabricating the first P-N junction and fabricating the second P-N junction are part of forming an electrical path having i) a first portion within the first N-doped region from a signal contact to the first P-N junction, and ii) a second portion within the P-doped region from the first P-N junction to the second P-N junction, the second portion of the electrical path having a second length shorter than a first length of the first portion of the electrical path.
24. The method of optical modulation of claim 18 , wherein electrically coupling the modulation signal along the electrical path further includes:
electrically coupling via a first portion of the electrical path within the first N-doped region from a signal contact to the first P-N junction; and
electrically coupling via a second portion of the electrical path within the P-doped region from the first P-N junction to the second P-N junction, the second portion of the electrical path having a second length shorter than a first length of the first portion of the electrical path.
25. The optical modulator of claim 1 , wherein the second P-N junction is electrically coupled in series along an electrical path having i) a first portion within the N-doped region from a signal contact to the first P-N junction, and ii) a second portion within the P-doped region from the first P-N junction to the second P-N junction, the second portion of the electrical path having a second length shorter than a first length of the first portion of the electrical path.