Disk Resonator Optical Modulator Using Composite Silicon Structure
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Solution Overview
Problem
Existing silicon-based optical modulators face limitations such as high optical waveguide loss, low modulation speed, and high power consumption due to indirect band-gap absorption, minority-carrier diffusion, and increased capacitance, which restrict their performance in high-speed applications.
Innovation Solution
An integrated circuit with a composite silicon structure incorporating a bus optical waveguide and a disk resonator, featuring a metal-oxide-semiconductor (MOS) capacitor with a polysilicon layer on an oxide layer, and silicon layer, optimized for high-speed and low-power operation by minimizing capacitance and series resistance, and using optical tapers to reduce loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If carrier injection-based silicon optical modulators are used, then modulation efficiency is improved and optical loss is reduced, but modulation speed deteriorates due to minority-carrier diffusion
Solution Approach 1:
The patent replaces the carrier injection mechanism with a plasma dispersion effect mechanism. Instead of injecting minority carriers that diffuse slowly, the invention uses free-carrier depletion in a reverse-biased pn junction to change the refractive index, eliminating the speed limitation imposed by minority-carrier diffusion while maintaining low optical loss.
Solution Approach 2:
The patent changes the operating parameters by using reverse-biased pn junctions instead of forward-biased pin diodes. This parameter change enables the use of free-carrier depletion rather than injection, achieving both high modulation speed and low optical loss by controlling the concentration of free carriers through voltage modulation.
2Speed
If carrier depletion-based silicon optical modulators are used, then modulation speed is improved, but modulation efficiency deteriorates and insertion loss increases
Solution Approach 1:
The patent applies local quality by creating a highly localized depletion region within the pn junction that precisely overlaps with the optical mode. This localized charge distribution maximizes the modulation efficiency while minimizing the required doping concentration, thereby reducing insertion loss. The optical waveguide is designed so that the optical field is concentrated in the region where the depletion effect occurs.
3Power
If MOS-capacitor structures are used, then phase shift efficiency is improved with small bias voltage, but capacitance increases leading to RC-limited bandwidth
Solution Approach 1:
The patent extracts the capacitance problem by using a pn junction structure instead of a MOS capacitor. The pn junction depletion region provides the necessary phase modulation without requiring the high capacitance associated with MOS structures. By taking out the oxide layer and using direct semiconductor junctions, the RC time constant is significantly reduced, enabling higher bandwidth operation.
4Speed
If shallow-etched optical waveguides are used, then bandwidth is improved, but optical mode overlap with electrical charges is reduced lowering modulation efficiency
Solution Approach 1:
The patent solves the overlap problem by transitioning from a planar geometry to a three-dimensional structured waveguide. The etched waveguide creates vertical confinement that concentrates the optical mode in the horizontal plane where the pn junction charges are located. This dimensional change ensures strong overlap between the optical field and the charge distribution, maintaining high modulation efficiency while supporting shallow etching for broadband operation.
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 solution enables high-speed modulation up to 25 Gb/s with ultra-low power consumption and reduced optical loss, achieving high modulation efficiency and broadband operation while simplifying design and fabrication.
Implementation Method 1
optical modulation in silicon can also be implemented by exploiting the free-carrier dispersion effect of silicon, in which the concentration of free charges in silicon changes the real and imaginary parts of the index of refraction
Implementation Method 2
optical tapers to reduce loss
Implementation Method 3
disk resonator
Data Source
AI summary
An optical modulator is described. This optical modulator may be implemented using silicon-on-insulator (SOI) technology. In particular, the optical modulator may include a carrier-accumulation-type micro-disk resonator fabricated using optical waveguides having a composite structure. Moreover, the composite structure may embed a metal-oxide semiconductor capacitor in the disk resonator. For example, the composite structure may include polysilicon disposed on an oxide layer, which is disposed on a silicon layer in an SOI platform. The optical modulator may have high modulation efficiency and high-speed operation. In addition, the optical modulator may have a compact footprint with low power consumption.