US-12619109-B2 - Differential drive modulator structures for a linear electro-optic platform

Abstract

Disclosed are differential-drive electro-optic modulator structures for linear electro-optic (Pockels) platforms such as thin-film LiNbO3 (TFLN), directly compatible with traditional differential-output analog drivers, such as a linear electro-optic modulator, including: a first signal trace; a second signal trace, wherein the first signal trace and the second signal trace are driven by a differential signal; a first optical arm; and a second optical arm, wherein a geometry of the first signal trace and the second signal trace causes the differential signal to modulate optical signals in the first optical arm and the second optical arm, such that when the optical signals are combined to create a resultant optical signal, the geometry mitigates chirp in the resultant optical signal. Other embodiments are disclosed.

Inventors

• Maxime Jacques
• Gregory Brookes
• Michael Vitic
• Alexandre Simard
• Douglas McGhan
• Ying Gao

Assignees

• CIENA CORPORATION

Dates

Publication Date

20260505

Application Date

20230406

Claims (20)

1. 1 . A linear electro-optic modulator, comprising: a first signal trace; a second signal trace, wherein the first signal trace and the second signal trace are driven by a differential signal; a first optical arm; and a second optical arm, wherein a geometry of the first signal trace and the second signal trace causes the differential signal to modulate optical signals in the first optical arm and the second optical arm, such that when the optical signals are combined to create a resultant optical signal, the geometry mitigates chirp in the resultant optical signal, wherein the geometry comprises an electrical crossover of the first signal trace and the second signal trace, and wherein a first ratio of a segment length before and after the electrical crossover is less than one and is the inverse of a second ratio of an average attenuation before and after the electrical crossover.
2. 2 . The linear electro-optic modulator of claim 1 , wherein the electrical crossover is optimized to minimize radio frequency crosstalk.
3. 3 . The linear electro-optic modulator of claim 1 , wherein the geometry comprises a fold of the first signal trace and the second signal trace and at least one crossover of the first optical arm or the second optical arm and the first signal trace or the second signal trace.
4. 4 . The linear electro-optic modulator of claim 3 , wherein the geometry comprises a first crossover of the first optical arm and the first signal trace, a second and a third crossover of the first optical arm and the second signal trace, and a fourth crossover of the second optical arm and the second signal trace.
5. 5 . The linear electro-optic modulator of claim 3 , wherein the geometry comprises a first crossover of the first optical arm and the first signal trace and a second crossover of the second optical arm and the second signal trace.
6. 6 . The linear electro-optic modulator of claim 1 , wherein the geometry comprises a first plurality of T-rail segments of the first signal trace and a second plurality of T-rail segments of the second signal trace, wherein the second plurality of T-rail segments are inter-digitated with the first plurality of T-rail segments.
7. 7 . The linear electro-optic modulator of claim 6 , wherein a distance between ends of consecutive T-rail segments is smaller than a length of a top side of a T-rail segment.
8. 8 . The linear electro-optic modulator of claim 6 , wherein the first optical arm passes in a second gap between the first signal trace and a top side of each T-rail segment of the second plurality of T-rail segments, and wherein the second optical arm passes in a third gap between a bottom side of each T-rail segment of the first plurality of T-rail segments and a bottom side of each T-rail segment of the second plurality of T-rail segments.
9. 9 . The linear electro-optic modulator of claim 8 , wherein the second gap is larger than the third gap.
10. 10 . The linear electro-optic modulator of claim 6 , wherein the geometry comprises a fold having no crossovers in the fold of the first signal trace, the second signal trace, the first optical arm and the second optical arm.
11. 11 . The linear electro-optic modulator of claim 6 , wherein the geometry comprises a fold having crossovers in the fold of the first signal trace, the second signal trace, the first optical arm and the second optical arm, and wherein a length of the first optical arm and the second optical arm in the fold is increased to achieve a velocity matching condition.
12. 12 . The linear electro-optic modulator of claim 6 , wherein the geometry comprises two electrical crossovers of each T-rail segment in the first plurality of T-rail segments with a respective T-rail segment of the second plurality of T-rail segments.
13. 13 . The linear electro-optic modulator of claim 1 , wherein the first optical arm and the second optical arm comprise a thin-film lithium niobate.
14. 14 . The linear electro-optic modulator of claim 13 , comprising a substrate of silicon or quartz.
15. 15 . A method of modulating an optical signal, comprising: splitting the optical signal into a first optical signal and a second optical signal; providing the first optical signal to a first optical arm of a linear electro-optic modulator; providing the second optical signal to a second optical arm of the linear electro-optic modulator; applying a differential electrical signal to a first signal trace and a second signal trace of the linear electro-optic modulator to modulate the first optical signal and the second optical signal, resulting in a modulated first optical signal and a modulated second optical signal; and combining the modulated first optical signal and the modulated second optical signal, wherein a geometry of the first signal trace and the second signal trace of the linear electro-optic modulator causes the differential electrical signal to modulate first optical signal in the first optical arm and the second optical signal in the second optical arm, such that when the optical signals are combined to create a resultant optical signal, the geometry mitigates chirp in the resultant optical signal, wherein the geometry comprises an electrical crossover of the first signal trace and the second signal trace, and wherein a first ratio of a segment length before and after the electrical crossover is less than one and is the inverse of a second ratio of an average attenuation before and after the electrical crossover.
16. 16 . The method of claim 15 , wherein the electrical crossover is optimized to minimize radio frequency crosstalk.
17. 17 . The method of claim 15 , wherein the geometry comprises a first plurality of T-rail segments of the first signal trace and a second plurality of T-rail segments of the second signal trace, wherein the second plurality of T-rail segments are inter-digitated with the first plurality of T-rail segments.
18. 18 . A method for manufacturing a linear electro-optic modulator, comprising: forming a first optical arm; forming a second optical arm; and forming a first signal trace and a second signal trace, wherein a geometry of the first signal trace and the second signal trace causes a differential signal applied to the first signal trace and the second signal trace to modulate a first optical signal in the first optical arm and a second optical signal in the second optical arm, such that a combination of the first optical signal and the second optical signal creates a resultant optical signal, wherein the geometry mitigates chirp in the resultant optical signal, wherein the geometry comprises an electrical crossover of the first signal trace and the second signal trace, and wherein a first ratio of a segment length before and after the electrical crossover is less than one and is the inverse of a second ratio of an average attenuation before and after the electrical crossover.
19. 19 . The method of claim 18 , wherein the second ratio is based on an average attenuation level over each segment length before and after the electrical crossover.
20. 20 . The method of claim 19 , wherein the average attenuation level is over frequency.

Description

CROSS REFERENCE TO RELATED APPLICATIONS The subject disclosure claims the benefit of U.S. Provisional Patent Application No. 63/481,502 filed Jan. 25, 2023, which is incorporated by reference herein. FIELD OF THE DISCLOSURE The subject disclosure relates to differential drive modulator structures for a linear electro-optic (Pockels) platforms, such as thin-film lithium niobate (TFLN). BACKGROUND Differential-output drivers are generally preferred over single-end output, among others because they allow to 1) cancel or strongly suppress common-mode noise and other common impairments (even-order harmonics) present at the DAC/driver channel output, 2) increase the available swing at the modulator (by using both complements of a differential driver as opposed to one), and 3) for a fixed total driver swing and fixed modulator termination resistance, up to halve the dynamic power consumption at the travelling-wave (TW) Mach-Zehnder modulator (MZM) termination. This is because for a prescribed optical phase shift at the TW-MZM, differential-driving allows to halve the swing of each signal (within the pair) compared to single-ended driving. This allows for a theoretical 2× reduction in dynamic driver power consumption as set forth in the following equation: 2⁢x⁢(Vr⁢m⁢s/2)2Rterm=12⁢Vrms2Rterm Furthermore, differential-output drivers can also reduce static power consumption by reducing the supply voltage since the signal swing is shared between two complements of a differential pair. The driving signals reduce the supply voltage by ½ the swing, while still maintaining the same headroom as single-ended drive. Finally, differential driving also allows for unterminated topologies such as Emitter Follower, Push Pull and Open-Collector (OC) topologies, whereas single-ended driven MZMs would be essentially restricted to Traveling-Wave Amplifiers (TWA) or a terminated single-ended driver. In the context of TW-MZMs, drivers with differential channel outputs are usually employed to drive coplanar strip (CPS) or dual coplanar waveguide (CPW) RF electrode layouts of the type: S-S(CPS, ‘series push-pull’ MZM architecture for SiPhot),G-S-S-G (dual-CPS),G-S-G-S-G (dual-CPW, fully shielded), orslightly different variations thereof. These TW electrode layouts are fairly common in integrated modulator technologies such as InP and SiPhot, which respectively rely on the quantum-confined Stark effect (QCSE) and plasma dispersion (free-carrier refraction). However, for linear-phase electro-optic (E-O) crystals such as bulk LiNbO3 (LN), or the more recent integrated platform thin-film LiNbO3 (TFLN), the fixed crystal orientation (or a direction of the highest Pockels coefficient) across a chip, the low linear capacitance (absence of pn junction loading in the optical waveguides) and the field-effect nature of the modulators seem to point toward optimal RF electrode configurations of the type G-S-G (ground-signal-ground, i.e., single-ended), and S-S-S or G-S-S-S-G (differential). The G-S-G configuration is by far the most prevalent for E-O (Pockels) modulators. Generally, this modulator category also includes EO Polymer-based devices. BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: FIGS. 1A-1F are block diagrams illustrating various prior art modulator RF electrode configurations. FIG. 2 is a block diagram illustrating a conventional single-ended TFLN modulator structure having an equivalent cross section illustrated in FIG. 1A, scheme 1. FIG. 3 is a block diagram illustrating an example, non-limiting embodiment of TFLN modulator in accordance with various aspects described herein. FIG. 4 is a block diagram illustrating an example, non-limiting embodiment of enhancing the field and optimizing the RF crossing location of TFLN modulator of FIG. 3 in accordance with various aspects described herein. FIG. 5 is a block diagram illustrating an example, non-limiting embodiment of a GSSG MZM configuration for Pockels-based modulators with a fold and a swap of the interferometer arm that is modulated before/after the fold in accordance with various aspects described herein. FIG. 6 is a block diagram illustrating an example, non-limiting embodiment of a GSSG MZM configuration for Pockels-based modulators with a fold and both interferometer arms being modulated before/after the fold in accordance with various aspects described herein. FIG. 7 is a block diagram illustrating an example, non-limiting embodiment of a GSSG MZM configuration for Pockels-based modulators with or without a fold and having inter-digitated T-rail capacitive segments in accordance with various aspects described herein. FIG. 8 is a block diagram illustrating an example, non-limiting embodiment of a GSSG MZM configuration for Pockels-based modulators having inter-digitated T-rail capacitive segments and a symmetrical RF design for SS in accordance with various aspects described herein