Optical Alignment Circuit for Silicon Photonics Grating Couplers
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Solution Overview
Problem
Precise alignment and polarized wave/wavelength matching between an optical fiber and a grating coupler in silicon photonics (SiPh) optical circuits are hindered by manufacturing variance and background noise from scattering rays and dark current, leading to a low signal-to-noise ratio (S/N) during alignment processes.
Innovation Solution
An alignment optical circuit with a semiconductor optical waveguide, a grating coupler, and a photodiode having p-type and n-type regions is used, where multiplexed light is employed for rough alignment, improving the S/N ratio by isolating the alignment signal from noise through direct absorption of red light by the photodiode, reducing interference from scattering rays and dark current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional alignment methods using return light or photocurrent are used, then alignment can be performed, but the signal-to-noise ratio is low due to background noise from scattering rays and dark current
Solution Approach 1:
The patent segments the alignment process into two distinct stages: rough alignment using a first wavelength (e.g., 635 nm red light) that is strongly absorbed by the photodiode, and fine alignment using a second wavelength (e.g., 1550 nm infrared light) for optical coupling. This segmentation allows each stage to use optimized light wavelengths, improving signal-to-noise ratio during rough alignment while maintaining coupling efficiency during fine alignment.
Solution Approach 2:
The patent changes the wavelength parameter of the alignment light based on the alignment stage. During rough alignment, a first wavelength with high photodiode absorption is used to maximize signal strength and overcome background noise. During fine alignment, a second wavelength optimized for optical coupling is used. This parameter change resolves the contradiction by matching light properties to specific alignment requirements.
2Reliability
If precise alignment and polarized wave/wavelength matching are performed, then optical coupling efficiency is improved, but the alignment process becomes more complex and time-consuming
Solution Approach 1:
The alignment process is segmented into rough alignment (positioning) and fine alignment (optimization) stages. Each stage has simplified requirements: rough alignment only needs positional accuracy using high-absorption light, while fine alignment handles the complex polarized wave and wavelength matching. This segmentation reduces overall process complexity while maintaining high coupling efficiency.
Solution Approach 2:
Rough alignment using the first wavelength is performed as a preliminary action before fine alignment. This preliminary positioning ensures the optical fiber is approximately aligned with the grating coupler, creating a favorable starting condition for the subsequent fine alignment process. This preliminary action simplifies the overall complexity by separating coarse positioning from fine optimization.
3Adaptability or versatility
If rough alignment is performed over a wide range to achieve maximum coupling, then alignment coverage is improved, but the time required for alignment increases
Solution Approach 1:
The patent changes the light wavelength parameter during the alignment process. Rough alignment uses a first wavelength (e.g., 635 nm) that provides strong signal response across a wide positional range, enabling fast coverage. Fine alignment then switches to a second wavelength (e.g., 1550 nm) for precise optimization. This parameter change allows wide coverage during rough alignment without excessive time consumption.
Solution Approach 2:
The alignment search space is segmented into a wide rough alignment phase and a narrow fine alignment phase. The first wavelength enables efficient wide-range scanning with high signal-to-noise ratio, quickly identifying the approximate coupling region. The second wavelength then performs detailed optimization in the identified region. This segmentation reduces total alignment time while maintaining comprehensive coverage.
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 enhances the S/N ratio during optical alignment, allowing for accurate rough and fine alignment without being buried in noise, thereby improving the efficiency of optical coupling between the optical fiber and the grating coupler.
Implementation Method 1
direct absorption of red light by the photodiode
Implementation Method 2
grating coupler through which external light can be coupled to a SiPh optical waveguide
Data Source
AI summary
A photodiode including a p-type region and an n-type region formed in a core of a grating coupler is provided. The p-type region and the n-type region are each formed as a region having a rectangular shape extending in an array direction of a grating as seen in plan view and are arranged in a direction orthogonal to the array direction of the grating and parallel to a plane of a substrate. A plurality of the p-type regions and a plurality of the n-type regions are formed and alternately arranged.