AlN Electro-Optic Modulator With Doped Polysilicon Grating Electrodes
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Solution Overview
Problem
Conventional electro-optic modulators using lithium niobate (LN) suffer from low modulation efficiency and integration challenges, while aluminum nitride (AlN) modulators have very low modulation efficiency due to their small Pockels coefficient and require electrodes to be placed far from the waveguide to avoid optical loss, leading to inefficient performance.
Innovation Solution
The use of doped polysilicon sub-wavelength gratings as electrodes for AlN modulators, which are embedded in silicon oxide layers, allows for closer placement to the waveguide, enhancing modulation efficiency by reducing the equivalent separation between electrodes and maintaining low optical loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If AlN modulators use conventional electrode placement far from the waveguide, then optical loss is reduced, but modulation efficiency becomes very low (about 200 V·cm)
Solution Approach 1:
The electrode structure is segmented into sub-wavelength grating fingers rather than a continuous electrode. This segmentation allows the electrode to be placed closer to the waveguide while maintaining low optical loss by reducing the effective interaction area with the optical mode, thus resolving the contradiction between proximity (for efficiency) and distance (for low loss).
Solution Approach 2:
The electrode geometry is changed from a continuous structure to a sub-wavelength grating structure with specific finger dimensions and spacing. This parameter change enables the electrode to achieve both close proximity to the waveguide for high field overlap and sufficient separation through the grating periodicity to minimize optical loss, achieving Vπ·Lπ of approximately 17.2 V·cm.
2Ease of manufacture
If conventional LN modulators are fabricated using proton exchange or titanium diffusion, then modulator functionality is achieved, but index contrast is low leading to poor optical confinement and low modulation efficiency
Solution Approach 1:
The waveguide structure uses a composite material system with AlN core layer deposited on silicon substrate with silicon oxide cladding layers. This composite structure achieves high index contrast for poor optical confinement without compromising fabrication ease, as AlN is CMOS compatible and can be deposited using standard semiconductor processes.
3Productivity
If LNOI modulators are used to improve performance, then modulation efficiency and optical confinement are significantly improved, but cost increases and monolithic integration on silicon platform becomes difficult
Solution Approach 1:
The patent replaces expensive LNOI material with cheaper AlN material that is CMOS compatible. Although AlN has lower intrinsic Pockels coefficient, the sub-wavelength grating electrode structure compensates for this by enhancing field overlap, achieving comparable performance at lower cost with full silicon platform integration capability.
4Productivity
If doped polysilicon sub-wavelength gratings are used as electrodes, then equivalent separation between electrodes is reduced enhancing modulation efficiency, but optical loss may increase due to closer placement
Solution Approach 1:
The sub-wavelength grating parameters (finger width, spacing, period, depth) are optimized to achieve the right balance. The grating period is kept below the optical wavelength to prevent diffraction, while the fill factor and finger dimensions are tuned to maximize electrical field overlap with the optical mode while minimizing optical absorption and scattering losses in the doped polysilicon.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed modulator achieves performance comparable to commercial LN modulators with improved integration capabilities on a silicon platform, reducing Vπ·Lπ to approximately 17.2 V·cm and enabling high-speed operation up to 1-40 GHz.
Implementation Method 1
The electro-optic modulator may be configured to modulate the optical light upon application of a potential difference between the first electrically conductive sub-wavelength grating and the second electrically conductive sub-wavelength grating based on Pockels effect
Data Source
AI summary
Various embodiments may relate to an electro-optic modulator. The electro-optic modulator may include a waveguide configured to carry optical light along a longitudinal length of the waveguide. The electro-optic modulator may also include a first electrically conductive sub-wavelength grating on a first side of the waveguide, the first electrically conductive sub-wavelength grating including a plurality of fingers extending substantially perpendicular to the longitudinal length of the waveguide. The electro-optic modulator may further include a second electrically conductive sub-wavelength grating on a second side of the waveguide opposite the first side, the second electrically conductive sub-wavelength grating including a plurality of fingers extending substantially perpendicular to the longitudinal length of the waveguide. The electro-optic modulator may be configured to modulate the optical light upon application of a potential difference between the first electrically conductive sub-wavelength grating and the second electrically conductive sub-wavelength grating based on Pockels effect.