Multimode Fiber Graded Index Profile for Bandwidth
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Solution Overview
Problem
Multimode optical fibers with graded index profiles face challenges in maintaining low intermodal dispersion when used with polychromatic sources, leading to reduced bandwidth due to chromatic dispersion, which is not effectively compensated by existing methods such as fiber concatenation or signal conditioning, resulting in increased system complexity and cost.
Innovation Solution
Modifying the graded index profile of multimode fibers by adjusting the parameter α to introduce a modal dispersion that compensates for the chromatic dispersion-induced modal dispersion, allowing for zero effective modal dispersion within the same fiber, even with polychromatic sources, without the need for signal conditioning or fiber concatenation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a graded index profile with parameter α is used to reduce intermodal dispersion, then bandwidth is improved for a particular wavelength, but chromatic dispersion from polychromatic sources causes modal dispersion that reduces effective bandwidth
Solution Approach 1:
The patent modifies the graded index profile parameter α from its conventional optimal value (1.8-2.2) to a specific range (0.8-1.5) to compensate for chromatic dispersion effects. This parameter change creates a deliberate modal dispersion that counteracts the chromatic dispersion-induced modal dispersion, achieving zero effective modal dispersion for polychromatic VCSEL sources and enabling effective bandwidth greater than 6000 MHz-km
2Reliability
If fiber concatenation is used to compensate for modal dispersion, then bandwidth is improved, but device complexity and system cost increase
Solution Approach 1:
The patent extracts the dispersion compensation function from external system components (fiber concatenations, signal conditioning devices) and integrates it directly into the fiber's refractive index profile. By embedding the compensation mechanism within the fiber itself through a modified graded index profile, the system achieves high bandwidth without additional complexity or cost
3Reliability
If signal conditioning is used to manage dispersion, then bandwidth is improved, but device complexity and cost increase
Solution Approach 1:
The patent removes the need for external signal conditioning devices by incorporating the dispersion management function directly into the fiber's refractive index profile. The modified graded index profile with parameter α in the range 0.8-1.5 provides inherent compensation for chromatic dispersion effects, eliminating the need for additional system components
4Manufacturing precision
If the parameter α is optimized for a particular wavelength, then intermodal dispersion is minimized for that wavelength, but effective bandwidth decreases when used with polychromatic sources having wide spectral width
Solution Approach 1:
The patent changes the parameter α from its conventional optimal value (1.8-2.2) to a specific range (0.8-1.5) to create a graded index profile that compensates for chromatic dispersion effects across the VCSEL spectral width. This parameter modification transforms the fiber's dispersion characteristics to achieve zero effective modal dispersion for polychromatic sources, enabling effective bandwidth greater than 6000 MHz-km
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 ensures a significant increase in effective bandwidth, achieving bandwidths greater than 6000 MHz-km and improved system performances in high-speed Ethernet transmission networks by effectively managing modal and chromatic dispersion interferences.
Implementation Method 1
A value of the parameter α comprised between 1.8 and 2.2 allows a satisfactory limitation of the modal dispersion
Implementation Method 2
the chromatic dispersion of the sources used... each transverse mode of the laser having its own wavelength corresponding to the various peaks of the emission spectrum
Implementation Method 3
A graded-index profile can be defined by a relationship between the refractive index value n at a point as function of the distance r from this point to the center of the fiber
Data Source
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AI summary
A multimode optical fiber has a refractive index profile such that a measurement of the dispersion modal delay (DMD), carried out on a DMD graph modified by adding a delay (Δttrace) to each trace: Δttrace=ΔλmaxxDxLrtraceaβ where rtrace is the center position of the injection of the light pulse corresponding to the trace, α is the core radius of the multimode fiber, L is the length of the fiber, D is the chromatic dispersion of the multimode fiber at the wavelength of the light pulse, β is the coefficient of the delay and Δλmax is the maximum spectral width of the source intended to be used with the fiber, produces an equivalent modal dispersion value of less than 0.11 ps/m for (Δλmax x D) > 0.07 ps/m. The modification of the DMD graph to take account of the chromatic dispersion makes it possible to guarantee a calculated effective bandwidth EBc greater than 6000 MHz-km for multimode fibers used with multimode transverse sources.