Asymmetric Directional Coupler for Compact InP Waveguides
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Solution Overview
Problem
Existing directional couplers face challenges in transitioning optical power between InP-based high-mesa optical waveguides efficiently, requiring large device sizes and being vulnerable to manufacturing errors due to narrow acceptable device length tolerances.
Innovation Solution
A directional coupler design featuring first and second optical waveguides with high-mesa structures, asymmetric widths, and a gap core layer with a lower equivalent refractive index than the core layers, optimized to reduce optical power transition distance and enhance robustness against manufacturing errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If the optical waveguides are brought close to each other to reduce transition distance, then the device size is reduced, but the manufacturing precision requirement becomes extremely narrow (around 1 micrometer admissible error range)
Solution Approach 1:
The patent applies asymmetry by setting different widths for the first and second optical waveguides (W1 ≠ W2). This asymmetric waveguide width configuration modifies the optical coupling characteristics, enabling sufficient power transition over a shorter interaction length while providing a broader tolerance range against manufacturing errors. The asymmetric structure creates a more robust coupling condition that is less sensitive to dimensional variations.
Solution Approach 2:
The patent introduces a sub-wavelength grating structure that adds periodic modulation in the transverse dimension (width direction) of the optical waveguides. This periodic structure with period smaller than the optical wavelength creates effective index modulation that enhances coupling efficiency over a shorter longitudinal distance, thereby reducing overall device length while maintaining coupling performance and tolerance.
2Length of stationary object
If the optical waveguides are brought close to each other to achieve sufficient power transition, then the transition distance is reduced, but the device becomes more vulnerable to manufacturing errors
Solution Approach 1:
The asymmetric waveguide widths (W1 ≠ W2) create a coupling regime that is inherently more tolerant to manufacturing variations. The differential width configuration establishes a coupling condition where the power transition is less sensitive to small changes in waveguide separation distance, thereby improving reliability while achieving compact dimensions.
Solution Approach 2:
The patent modifies the coupling parameters by introducing different waveguide widths and implementing sub-wavelength grating modulation. These parameter changes transform the coupling characteristics to achieve a optimal balance between compact transition distance and robustness against manufacturing tolerances, making the device more reliable.
3Ease of manufacture
If conventional directional coupler design is used with InP-based high-mesa optical waveguides, then the device can be manufactured with standard processes, but the device size becomes large due to required long transition distance
Solution Approach 1:
The patent applies sub-wavelength grating structures that introduce periodic modulation in the transverse dimension of the waveguides. This grating structure with period smaller than the optical wavelength creates effective index modulation that dramatically enhances coupling efficiency, reducing the longitudinal interaction length required for power transition while maintaining compatibility with InP-based high-mesa waveguide fabrication processes.
Solution Approach 2:
The asymmetric waveguide width configuration (W1 ≠ W2) combined with the sub-wavelength grating structure creates a compact coupling regime that is well-suited for InP-based high-mesa waveguides. This asymmetric design enables sufficient power transition over a short distance while being manufacturable using standard semiconductor processing techniques.
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 directional coupler achieves a compact device size with improved resistance to manufacturing errors, enabling efficient optical power transition and maintaining a desired power ratio with reduced length requirements.
Implementation Method 1
A directional coupler optically couples light propagation modes of two independent optical waveguides by bringing the optical waveguides close to each other to a distance equal to or shorter than a wavelength of propagated light
Implementation Method 2
An optical waveguide traps light into a local region by making a refractive index higher than a periphery and causes light to propagate in a desired direction by forming the region into a linear shape
Data Source
AI summary
An interval between the first and second optical waveguides (2c,3c) of the optical power transition unit is equal to or less than a wavelength of the light. Each of the first and second optical waveguides (2c,3c) is a high-mesa structure which includes a lower cladding layer (5), a core layer (6a,6b), and an upper cladding layer (7a,7b) which are sequentially stacked on the semiconductor substrate (1). The first optical waveguide and the second optical waveguide have different widths. A gap core layer (6c) is formed on the lower cladding layer between the core layers of the first and second optical waveguides of the optical power transition unit. An equivalent refractive index of the gap core layer when leakage of the light in a height direction is taken into consideration is lower than an equivalent refractive index of the core layers of the first and second optical waveguides.