Mode Conversion for Multimode Fiber Modal Delay Reduction
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Solution Overview
Problem
The challenge in multimode fiber transmission systems is the significant modal delay between propagation modes, which limits the feasibility of long haul transmission and increases the required optical signal to noise ratio, reducing nonlinear tolerance and transmission distance, and complicates MIMO equalization at the receiver.
Innovation Solution
A method that involves converting the modes of transmission along the optical fiber to minimize modal group delay by using mode converters at specific positions along the fiber, ensuring that all signals experience approximately the same group delay, thereby reducing the accumulation of differential mode delay and allowing for efficient MIMO processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If mode division multiplexing is used to increase transmission capacity, then the number of channels increases, but modal delay between modes increases making MIMO equalization impractical
Solution Approach 1:
The transmission link is divided into multiple segments with mode converters placed at specific positions along the fiber. Each segment has controlled length and mode conversion characteristics, allowing the overall modal delay to be managed by proper segmentation of the transmission path.
Solution Approach 2:
The invention changes the propagation mode parameters by using mode converters that transform signals between different fiber modes at specific positions. This parameter transformation allows signals to experience different group delays in different segments, and through proper design, the cumulative modal delay can be minimized or equalized across all channels.
2Productivity
If denser constellation size is used to increase spectral efficiency, then capacity increases, but required optical signal to noise ratio increases and nonlinear tolerance decreases
Solution Approach 1:
Mode converters act as intermediary devices that transform the propagation characteristics of optical signals. By introducing these intermediary mode conversion points along the transmission link, the system can manage nonlinear effects and modal delay simultaneously, enabling the use of denser constellations over extended distances.
3Duration of action of stationary object
If multimode fiber is used to extend nonlinear tolerance, then transmission distance increases, but modal delay between modes increases making MIMO equalization difficult
Solution Approach 1:
The long transmission link is segmented into multiple shorter sections with mode converters positioned at specific points. This segmentation allows the total transmission distance to be extended while keeping the modal delay within each segment manageable, thereby simplifying the MIMO equalization requirements at the receiver.
Solution Approach 2:
Mode converters are placed periodically or at specifically designed intervals along the transmission link. This periodic mode conversion creates a structured pattern of modal delay accumulation that can be predicted and compensated, reducing the complexity of MIMO equalization compared to uncontrolled modal delay.
Data Source
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AI summary
A method of transmitting optical signals over an optical fiber in a mode division multiplexed system (100, 200), the optical fiber having a first and a second mode of transmission, the method comprising transmitting a first optical signal on the first mode of transmission (209) over a first portion (201) of the optical fiber, transmitting a second optical signal on the second mode of transmission (211) over the first portion (201) of the optical fiber, transmitting the first optical signal on the second mode of transmission (211) over a second portion (203) of the optical fiber, and transmitting the second optical signal on the first mode of transmission (209) over the second portion (203) of the optical fiber.