Asymmetric Grating Coupler Back-Reflection Reduction
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Solution Overview
Problem
Current grating couplers in silicon-on-insulator (SOI) photonic integrated circuits suffer from high back-reflection issues, which are undesirable for high-frequency modulation performance and interferometer-based devices, as they induce intensity noise and instability, and are challenging to reduce due to Fresnel reflections and grating structure dependencies.
Innovation Solution
The proposed solution involves an asymmetric grating coupler design with curved elongate trenches, where the length of the trenches is determined by the equation R = Nλ / (n_eff - ecos(ψ)), and the effective index of the grating region is asymmetrically distributed, changing the angle of reflected light paths to minimize back-reflections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional grating coupler is used in SOI photonic integrated circuits, then coupling between optical fibre and chip is achieved, but high back-reflection occurs which induces intensity noise and instability
Solution Approach 1:
The patent applies asymmetry by introducing a tapered structure at the input waveguide interface with asymmetric profiling. The taper has different slopes on opposite sides, creating an asymmetric effective index distribution that redirects reflected light away from the waveguide core, thereby reducing back-reflection while maintaining coupling efficiency
Solution Approach 2:
The patent transitions from a two-dimensional grating structure to a three-dimensional tapered structure by adding vertical profiling to the waveguide input interface. This dimensional addition allows the reflected light to be redirected in the vertical dimension, preventing it from coupling back into the waveguide mode
2Reliability
If the grating structure is optimized for high coupling efficiency, then more light is coupled to the grating trench area, but Fresnel reflection increases which is difficult to reduce
Solution Approach 1:
The patent applies local quality by implementing the tapered structure only at the input waveguide interface region, while keeping the rest of the grating coupler structure conventional. This localized modification addresses the Fresnel reflection problem at the critical interface without altering the overall grating design that provides high coupling efficiency
3Object-generated harmful factors
If a tilted elliptical-grating coupler design is used to reduce back-reflection, then back-reflections are lowered to about -40dB, but the mode cannot be confined by waveguide side walls and coupling to grating trench area decreases
Solution Approach 1:
The patent segments the device into two functional regions: a tapered input interface region that handles reflection management, and a conventional grating coupler region that handles light coupling. This segmentation allows each region to be optimized for its specific function without compromising the other
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
This design significantly reduces back-reflections to as low as -41 dB within a 1 dB bandwidth, improving the performance of photonic integrated devices by minimizing Fresnel reflections and maintaining high coupling efficiency.
Implementation Method 1
The two-dimensional grating structure (4) is adapted for diffracting radiation received from the waveguide towards a direction out of said photonic substrate
Implementation Method 2
The asymmetric grating trenches change the angle of the reflected light path
Implementation Method 3
The Fresnel reflection can be described as the reflection occurring when light is moving between different refractive index media
Data Source
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Figure 5~7
AI summary
The present invention relates to a photonic integrated device (1) comprising - a waveguide (2) embedded in a photonic substrate and having a waveguide radiation exit surface (3), - a grating structure (4) connected with said waveguide, said grating structure having a plurality of elongate grating trenches (5) adapted for diffracting radiation from said waveguide, whereby at least two of said elongate grating trenches with a different distance to the centre of said waveguide radiation exit surface have a different angle of arc with respect to said centre.