Fully differential traveling wave series push-pull mach-zehnder modulator
Disclosed herein are architectures for low power, low voltage traveling wave Mach-Zehnder optical modulators. By combining single-ended series push-pull modulator configurations with differential dual-drive modulator configurations, the advantages of each type may be utilized. In particular, the halved capacitance of single-ended series push-pull modulators may reduce microwave losses, thereby improving bandwidth performance within modulator configurations. Moreover, reduced required drive voltage of dual-drive modulators coupled with increased impedance may advantageously minimize the power consumption and maximize efficiency in the differential traveling wave series push-pull Mach-Zehnder modulator configurations disclosed herein.
1. An optical modulator comprising:
a differential driver connected to at least two transmission line conductors each connected to a respective first electrode configured to receive an RF+ signal from the differential driver and second electrode configured to receive an RF− signal from the differential driver;
an optical waveguide branching into top and bottom regions near an input port and recombining near an output port;
a central node;
a first sub-modulator encompassing the top region and connected to the first electrode, the first sub-modulator including a first node configured to receive a common mode voltage V CM via a first high impedance path;
a second sub-modulator encompassing the bottom region and connected to the second electrode, the second sub-modulator including a second node configured to receive the V CM via a second high impedance path, the first sub-modulator and the second sub-modulator being symmetrically arranged about the central node resulting in a central virtual ground; and
a plurality of diodes including a first diode between the first electrode and the first node such that an V CM +V RF voltage drop occurs across the first diode, a second diode between the first node and the central node such that an V CM −V RF voltage drop occurs across the second diode, a third diode between the second node and the central node such that an V CM +V RF voltage drop occurs across the third diode, and a fourth diode between the second electrode and the second node such that an V CM +−V RF voltage drop occurs across the fourth diode,
wherein the plurality of diodes is configured such that the optical modulator operates as a fully differential series push-pull modulator.
2. The optical modulator of claim 1 , wherein the first sub-modulator and the second sub-modulator each operate as a single-ended series push-pull modulator.
3. The optical modulator of claim 2 , wherein the first sub-modulator and the second sub-modulator each operate as the single-ended series push-pull modulator, based on a voltage drop on each diode of the plurality of diodes in the first sub-modulator and the second sub-modulator.
4. The optical modulator of claim 1 , wherein:
the first sub-modulator comprises a first sub-branch of the top region of the optical waveguide that splits and recombines near each distal end of the first sub-modulator, and
the second sub-modulator comprises a second sub-branch of the bottom region of the optical waveguide that splits and recombines near each distal end of the second sub-modulator.
5. A method comprising:
providing an optical modulator including:
a differential driver connected to at least two transmission line conductors each connected to a respective first electrode configured to receive an RF+ signal from the differential driver and second electrode configured to receive an RF− signal from the differential driver;
an optical waveguide branching into top and bottom regions near an input port and recombining near an output port;
a central node;
a first sub-modulator encompassing the top region and connected to the first electrode the first sub-modulator including a first node configured to receive a common mode voltage V CM via a first high impedance path;
a second sub-modulator encompassing the bottom region and connected to the second electrode, the second sub-modulator including a second node configured to receive the V CM via a second high impedance path, the first sub-modulator and the second sub-modulator being symmetrically arranged about the central node resulting in a central virtual ground; and
a plurality of diodes including a first diode between the first electrode and the first node such that an V CM +V RF voltage drop occurs across the first diode, a second diode between the first node and the central node such that an V CM −V RF voltage drop occurs across the second diode, a third diode between the second node and the central node such that an V CM +V RF voltage drop occurs across the third diode, and a fourth diode between the second electrode and the second node such that an V CM +−V RF voltage drop occurs across the fourth diode,
wherein the plurality of diodes is configured such that the optical modulator operates as a fully differential series push-pull modulator.
6. The method of claim 5 , wherein the first sub-modulator and the second sub-modulator each operate as a single-ended series push-pull modulator.
7. The method of claim 6 , wherein the first sub-modulator and the second sub-modulator each operate as the single-ended series push-pull modulator, based on a voltage drop on each diode of the plurality of diodes in the first sub-modulator and the second sub-modulator.
8. The method of claim 5 , wherein:
the first sub-modulator comprises a first sub-branch of the top region of the optical waveguide that splits and recombines near each distal end of the first sub-modulator, and
the second sub-modulator comprises a second sub-branch of the bottom region of the optical waveguide that splits and recombines near each distal end of the second sub-modulator.
9. A method of operating an optical modulator comprising:
producing an input signal via a differential driver connected to at least two transmission line conductors each connected to a respective first electrode receiving an RF+ signal from the differential driver and second electrode receiving an RF− signal from the differential driver;
providing an optical signal to an optical waveguide branching the optical signal into top and bottom regions near an input port and recombining near an output port;
modulating a first branch of the optical signal with a first sub-modulator encompassing the top region and connected to the first electrode; and
modulating a second branch of the optical signal with a second sub-modulator encompassing the bottom region and connected to the second electrode;
wherein the first sub-modulator and the second sub-modulator are symmetrically arranged about a central node resulting in a central virtual ground, and
wherein the optical modulator comprises a plurality of diodes including a first diode between the first electrode and the first node such that an V CM +V RF voltage drop occurs across the first diode, a second diode between the first node and the central node such that an V CM −V RF voltage drop occurs across the second diode, a third diode between the second node and the central node such that an V CM +V RF voltage drop occurs across the third diode, and a fourth diode between the second electrode and the second node such that an V CM +−V RF voltage drop occurs across the fourth diode, such that the optical modulator operates as a fully differential series push-pull modulator.
10. The method of claim 9 , wherein the first sub-modulator and the second sub-modulator each operate as the single-ended series push-pull modulator, based on a voltage drop on each diode in the first sub-modulator and the second sub-modulator.
11. The method of claim 9 , wherein the first sub-modulator and the second sub-modulator are arranged in a substantially symmetrical layout about the central virtual ground.
12. The method of claim 9 , wherein the first sub-modulator is driven by a +V RF signal from the differential driver and the second sub-modulator is driven by a −V RF signal from the differential driver.