Few-Mode Fiber Index Ring Reduces Mode Coupling
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Solution Overview
Problem
Current few-mode fiber designs face challenges in increasing transmission capacity while maintaining low attenuation losses and high effective area, as mode coupling increases with the number of linear polarization modes, leading to limitations in fiber transmission reach and capacity.
Innovation Solution
The design incorporates an index ring in the cladding to spatially separate higher-order modes from the modes used for transmission, minimizing spatial overlapping and reducing mode coupling, which allows for guiding more LP modes than used for transmission, thereby enhancing transmission capacity and maintaining low attenuation losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the number of LP modes used for transmission is increased to enhance transmission capacity, then mode coupling increases, but transmission reach is limited
Solution Approach 1:
The patent segments the cladding region into multiple concentric zones with different refractive indices: an inner cladding region, an index ring region, and an outer cladding region. This segmentation allows different regions to serve different functions - the index ring specifically targets and confines higher-order modes while leaving lower-order modes undisturbed, thereby reducing mode coupling and extending transmission reach while maintaining high transmission capacity through multiple LP modes
Solution Approach 2:
The patent applies local quality by creating a localized index ring structure with higher refractive index than the outer cladding, positioned at a specific radius from the fiber axis. This local modification of refractive index properties selectively affects only the higher-order modes that extend into the cladding region, while preserving the propagation characteristics of lower-order modes confined to the core, thus reducing mode coupling without compromising transmission capacity
2Reliability
If core refractive index is increased to reduce mode coupling, then attenuation losses increase and effective area decreases
Solution Approach 1:
The patent extracts the mode coupling reduction function from the core region and relocates it to the cladding region through the index ring structure. By placing the refractive index modification in the cladding rather than increasing the core index, the patent achieves mode coupling reduction through spatial separation of mode fields, avoiding the penalties of increased attenuation and reduced effective area that would result from core index increases
Solution Approach 2:
The patent transitions from modifying the core region (one-dimensional approach) to modifying the cladding region (extending to another spatial dimension). The index ring in the cladding provides an additional spatial dimension for controlling mode coupling, allowing the system to achieve mode separation without the adverse effects associated with core index modifications
3Reliability
If core refractive index is increased to reduce mode coupling, then effective area decreases, but intra-mode non-linearity must be limited
Solution Approach 1:
The patent extracts the mode coupling control mechanism from the core and implements it in the cladding through the index ring. This extraction preserves the core's large effective area while achieving mode coupling reduction through the cladding's index structure, thereby maintaining high effective area for reduced intra-mode non-linearity while still achieving the desired mode coupling reduction
Solution Approach 2:
The index ring acts as an intermediary structure between the core and outer cladding, mediating the interaction between different modes. This intermediary structure provides the necessary mode coupling reduction through its specific refractive index and radial positioning, while allowing the core to maintain its large effective area for minimizing intra-mode non-linear effects
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 approach enables increased transmission capacity with low attenuation losses and high effective area, facilitating the use of simple MIMO techniques and reducing non-linear effects in the fiber.
Implementation Method 1
the refractive index of the core, n co , is greater than the one of the cladding, n cl ... the refractive index profile that associates the refractive index (n) with the radius (r) of the optical fiber
Data Source
Figure 1~2
Figure 3~5
Figure 4A~4B
AI summary
A few mode optical fiber comprises an optical core and an optical cladding surrounding the optical core. The FM F has a step-index profile. The optical core has a core outer radius R 1 > 7.5μm and a core refractive index difference Δη 1 such that 14.5x 10-3 < Δη 1 < 24x 10-3. The optical cladding comprises: - an index ring with: o a ring inner radius Rr 1 between 12μm and 19μm; o a ring refractive index difference Δη 1 such that Δη 1 / Δη r between 2 and 4; o a ring volume V ring =πΔη r (Rr 2 2— Rr 1 2) between 1.8μm 2 and 4.1μm 2 where Rr 2 is the ring outer radius; an inner cladding between the optical core and the index ring, with an inner cladding inner radius R i 1 and an inner cladding outer radius R i 2, the inner cladding having an inner cladding refractive index difference Δη clad 1 between—1.0x10-3 and 1.0x10-3.