Asymmetrical semiconductor-based optical modulator
An optical modulator includes a semiconductor substrate and an optical waveguide portion disposed on the semiconductor substrate. A signal contact that extends alongside the optical waveguide portion is disposed on the semiconductor substrate. A first ground line is disposed on the semiconductor substrate spaced away from the signal contact by a first spacing. A second ground line is disposed on the semiconductor substrate spaced away from the signal contact by a second spacing opposite the first ground line. The first spacing is different from the second spacing.
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 signal contact disposed on the semiconductor substrate, the signal contact extending alongside the optical path;
a first ground line disposed on the semiconductor substrate and spaced a first distance away from the signal contact, the optical waveguide portion disposed between the signal contact and the first ground line, the first distance selected based on a desired field strength across the optical waveguide portion; and
a second ground line disposed on the semiconductor substrate spaced a second distance away from the signal contact, the first distance different from the second distance, the signal contact disposed between the optical waveguide portion and the second ground line, the second distance selected to be longer than the first distance based on a target impedance for a travelling electrical-domain signal.
2 . The optical modulator of claim 1 , wherein an electrical-domain refractive index associated with travel along the signal contact is 3.2 or greater.
3 . The optical modulator of claim 1 , wherein the optical waveguide portion is configured to undergo an optical domain refractive index shift responsive to a travelling electrical domain signal on the signal contact.
4 . The optical modulator of claim 1 , wherein the second ground line is configured to cause a match of wave velocities between a travelling electrical-domain signal on the signal contact and the light that is to be modulated on the optical path.
5 . The optical modulator of claim 1 , wherein a width of a transverse-to-signal-travel cross-section of the signal contact provides the target impedance for the travelling electrical-domain signal in combination with the second distance.
6 . The optical modulator of claim 5 , wherein the width of the transverse-to-signal-travel cross-section of the signal contact is between 5 microns and 15 microns.
7 . The optical modulator of claim 1 , wherein:
the optical waveguide portion is disposed within a first layer of the semiconductor substrate; and
the signal contact is disposed on the semiconductor substrate within an intermediate layer deposited on top of the first layer.
8 . The optical modulator of claim 7 , wherein the first ground line and second ground line are disposed on the semiconductor substrate within the second layer.
9 . The optical modulator of claim 8 , wherein the signal contact and the first ground line are electrically coupled to the optical waveguide portion using one or more vias.
10 . The optical modulator of claim 1 , wherein:
the first ground line and second ground line are asymmetrically positioned alongside the signal contact within a first arm of a Mach-Zehnder modulator; and
the Mach-Zehnder modulator includes a second arm including another signal contact asymmetrically positioned between two second-arm ground lines.
11 . The optical modulator of claim 10 , wherein:
two second-arm ground lines include the first ground line as a shared ground line.
12 . The optical modulator of claim 1 , wherein:
the second ground line is disposed on the semiconductor substrate opposite the first ground line, the second distance selected to be longer than the first distance to match respective wave velocities of an electrical-domain signal provided to the signal contact and the light that is to be modulated on the optical path.
13 . The optical modulator of claim 12 , wherein the second distance is positioned such that the signal contact has a target impedance for a travelling electrical-domain signal based at least in part on the second distance.
14 . The optical modulator of claim 12 , wherein the second ground line is configured to cause a match of a phase velocity of the electrical-domain signal with a group velocity of the light that is to be modulated based at least in part on the second distance.
15 . The optical modulator of claim 12 , wherein the signal contact extends parallel to the optical path such that the light to be modulated travels parallel to a direction of travel of the electrical-domain signal.