Few-mode optical fiber graded index profile
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Solution Overview
Problem
Few-moded optical fibers with step index cores face issues such as large differential group delays, high microbending losses, and reduced numerical aperture, leading to bit error rate penalties and alignment sensitivity due to mode mixing and leaky higher order modes.
Innovation Solution
The development of optical fibers with a glass core and graded refractive index profile, supported by a glass cladding, featuring a specific radius and refractive index profile that allows for the propagation of multiple LP modes with reduced differential group delays and increased numerical aperture, minimizing microbending losses and coupling issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the core diameter is increased relative to single mode fibers to support additional modes, then the fiber can transmit multiple LP modes, but large delay differences occur between the fundamental mode and higher order modes
Solution Approach 1:
The patent applies a graded refractive index profile where the refractive index varies continuously from the center of the core to the core-cladding interface. This local variation in optical properties allows different modes to propagate with controlled velocity differences, reducing differential group delay while maintaining multi-mode capability. The alpha value between 1.8 and 2.2 optimizes this gradient to balance mode support and delay reduction.
Solution Approach 2:
The patent changes the refractive index parameter across the core radius using a graded profile characterized by an alpha value between 1.8 and 2.2. This parameter modification transforms the uniform step index into a controlled gradient, which fundamentally alters mode propagation characteristics to reduce differential group delay while maintaining the ability to support multiple LP modes.
2Adaptability or versatility
If the core diameter is increased to support additional modes, then multi-mode transmission is enabled, but high microbending losses occur
Solution Approach 1:
The graded refractive index profile creates local variations in optical density that confine light more effectively throughout the core. This local optimization of light confinement reduces sensitivity to microbending deformations, thereby lowering microbending losses while maintaining the increased core diameter needed for multi-mode operation.
Solution Approach 2:
By modifying the refractive index parameter from uniform to graded distribution, the patent reduces the effective numerical aperture at the core edges, which in turn reduces the fiber's sensitivity to bending losses. This parameter transformation allows the fiber to maintain larger core dimensions for multi-mode support without suffering excessive microbending losses.
3Area of stationary object
If the refractive index of the core is reduced to achieve large effective area, then the effective area increases, but the numerical aperture of the fiber decreases
Solution Approach 1:
The graded refractive index profile creates a non-uniform distribution of optical properties within the core. This local quality variation allows the fiber to maintain a larger effective area for low loss transmission while concentrating the numerical aperture enhancement at specific radial positions, thereby improving coupling efficiency without sacrificing effective area.
Solution Approach 2:
The patent transforms the refractive index parameter from a uniformly reduced value to a spatially varying gradient characterized by alpha between 1.8 and 2.2. This parameter transformation enables the fiber to simultaneously achieve large effective area (for low loss) and adequate numerical aperture (for low coupling loss and alignment sensitivity) by optimizing the radial distribution of refractive index values.
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 achieves low attenuation, reduced differential group delays, and improved numerical aperture, enhancing the performance of optical fibers for long-haul transmission by supporting multiple LP modes with minimal signal degradation and alignment sensitivity.
Implementation Method 1
The glass core may have a graded refractive index profile with an alpha value greater than or equal to about 1.8 and less than about 2.2 at a wavelength of 1550 nm
Data Source
AI summary
Few moded optical fibers with small delay differences between the propagating modes are disclosed. In one embodiment, an optical fiber includes a glass core and a glass cladding surrounding and in direct contact with the glass core. The glass core may include a radius R1 from about 8 μm to about 13 μm; a graded refractive index profile with an alpha value between about 1.9 and 2.1 at a wavelength of 1550 nm; and a maximum relative refractive index Δ1MAX from about 0.6% to about 0.95% relative to the glass cladding. The effective area of the LP01 mode at 1550 nm may be between 80 μm2 and 105 μm2 such that the core supports the propagation and transmission of an optical signal with X LP modes at a wavelength of 1550 nm, wherein X is an integer greater than 1 and less than 10. The glass cladding may include a maximum relative refractive index Δ4MAX such that Δ1MAX>Δ4MAX. The optical fiber has DGD of less than or equal to about 150 ps/km at a wavelength of 1550 nm.