Multimode Fiber Modal Characteristic Measurement via OFDR
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Solution Overview
Problem
Existing methods struggle to accurately determine modal characteristics of multimode optical fibers due to weak Rayleigh scatter measurements and conventional assumptions that cross correlations of different modes in multimode fibers do not provide useful results.
Innovation Solution
The use of Optical Frequency Domain Reflectometry (OFDR) to measure and correlate Rayleigh scatter reflections from multimode fibers, allowing for the determination of modal characteristics such as refractive indices and group indices by aligning and scaling Rayleigh scatter data to emphasize cross-correlation peaks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Rayleigh scatter measurements are performed on multimode fiber, then modal characteristics can be measured, but the measurement signal strength is weak
Solution Approach 1:
The patent applies preliminary action by performing multiple scans of the Rayleigh scatter signal and accumulating the results before analysis. The system performs N scans and accumulates the Rayleigh scatter signals, then performs FFT on the accumulated data. This pre-processing step enhances the signal strength before the actual measurement analysis, allowing weak modal characteristics to be detected despite the inherently weak Rayleigh scatter from multimode fiber.
2Measurement precision
If cross-correlation of Rayleigh scatter patterns is performed on multimode fiber, then modal characteristics can be determined, but conventional assumptions indicate cross-correlations would not provide useful results
Solution Approach 1:
The patent applies segmentation by separating the Rayleigh scatter signal into different modal components through FFT analysis. By transforming the time-domain Rayleigh scatter signal into the frequency domain, the system can identify and analyze specific modal characteristics (LP01, LP11, LP21 modes) independently. This segmentation allows cross-correlation to be performed on specific modal components rather than the mixed total signal, making the cross-correlation technique useful for determining modal characteristics in multimode fiber.
3Productivity
If short segment Rayleigh scatter data is used for correlation, then measurement speed is improved, but correlation peaks become low level and difficult to analyze
Solution Approach 1:
The patent applies preliminary action by performing multiple scans and accumulating the Rayleigh scatter signals before performing FFT and correlation analysis. The system performs N scans, accumulates the signals, and then processes the accumulated data. This pre-accumulation step enhances the correlation peak levels, making them sufficiently high for accurate analysis while maintaining the ability to use relatively short fiber segments for the measurement.
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 non-destructive, simple measurement of modal characteristics in multimode fibers, distinguishing between step-index and graded-index fibers, and overcoming the challenge of weak signal levels by using cross-correlation techniques.
Implementation Method 1
Rayleigh scatter reflections associated with a segment of the waveguide are detected
Data Source
AI summary
One or more modal characteristics are determined for a waveguide that supports more than two modes. In an example implementation, optical frequency domain reflectometry (OFDR) is used to couple light into the waveguide and detect Rayleigh scatter reflections associated with a segment of the waveguide. An original set of Rayleigh scatter data associated with the detected Rayleigh scatter reflections is generated. In addition, a scaled set of Rayleigh scatter data associated with the detected Rayleigh scatter reflections is generated. The original set of Rayleigh scatter data is correlated with the scaled set of Rayleigh scatter data. One or more modal characteristics of the waveguide are determined based on the correlation.