Mode Coupling in Multi-Mode Optical Fiber Transmission
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
In multi-mode optical fiber transmission systems, generating sufficient mode coupling is challenging due to increasing propagation constant differences between modes, leading to performance degradation and increased digital signal processing load, especially with mode-dependent loss and group delay spread issues.
Innovation Solution
The installation of multiple mode converters along the optical fiber transmission line, utilizing an optical fiber with a grating pitch corresponding to the propagation constant difference between modes, ensures efficient mode coupling by increasing the coupling amount beyond a threshold, thereby reducing group delay spread and mode-dependent loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If mode coupling is generated by making the propagation constant difference Δβ as small as possible, then mode coupling is easy to generate with slight disturbance, but Δβ tends to increase in higher-order modes, making it difficult to generate sufficient mode coupling
Solution Approach 1:
The patent divides the optical fiber transmission line into multiple sections and installs mode converters at multiple discrete locations rather than relying on a single coupling mechanism. This segmentation allows each converter to contribute to the total coupling amount, enabling sufficient mode coupling even when individual Δβ values are large in higher-order modes.
Solution Approach 2:
The patent combines the coupling effects of multiple mode converters installed at different locations along the transmission line. By merging the coupling amounts from multiple converters, the system achieves sufficient total mode coupling that would be unattainable with a single converter, particularly for higher-order modes with larger Δβ.
2Device complexity
If a single mode converter is installed in the transmission line, then the structure is simple, but the mode coupling amount is determined by input power and is insufficient due to power changes between degenerate modes
Solution Approach 1:
Instead of using a single mode converter whose performance depends on input power stability, the patent segments the coupling function across multiple converters. This distribution makes the total coupling amount more stable because power fluctuations between degenerate modes at one location are compensated by the cumulative effect of other converters.
Solution Approach 2:
The patent changes the parameter of coupling amount from being determined by a single converter's input power to being determined by the sum of coupling amounts from multiple converters. This parameter change stabilizes the total coupling effect against power fluctuations that occur in single-converter systems.
3Loss of information
If MIMO equalizer is used at receiving end, then inter-mode crosstalk is compensated, but performance degradation occurs due to mode dependent loss
Solution Approach 1:
The patent applies preliminary action by generating mode coupling before the signals experience significant mode-dependent loss. The mode converters are installed along the transmission line to continuously mix modes, ensuring that when MIMO equalization is applied at the receiver, the coupled modes have already undergone sufficient mixing to mitigate the impact of MDL on individual mode signals.
4Productivity
If group delay spread is large at receiving end, then the transmission capacity is affected, but the digital signal processing load increases
Solution Approach 1:
The patent segments the group delay spread management by installing mode converters at multiple locations rather than relying on a single compensation point. This distributed approach reduces the accumulated group delay spread along the transmission line, thereby reducing the complexity and energy consumption of DSP required at the receiver while maintaining transmission capacity.
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 enhances mode coupling efficiency, reduces digital signal processing load, and stabilizes power fluctuations, achieving effective long-distance and high-capacity optical fiber transmission.
Implementation Method 1
utilizing an optical fiber with a grating pitch corresponding to a propagation constant difference between the propagation modes in a longitudinal direction
Implementation Method 2
propagation constant difference between the propagation modes
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
An optical transmission system according to the present disclosure is a mode multiplexed optical transmission system using a multi-mode optical fiber in which a plurality of propagation modes propagate as a transmission line, the optical transmission system including an optical fiber transmission line (83) that includes an optical fiber with two or more propagation modes; and a plurality of mode converters (91) that are configured to generate mode coupling between at least one pair of the propagation modes, in which a variation in an installation interval of the plurality of mode converters (91) is equal to or less than a threshold value determined by the transmission line length (Lt) of the optical fiber transmission line (83).