Coherent Rayleigh Backscatter Phase Detection for Linear Strain Measurement
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Solution Overview
Problem
Distributed fiber optic sensing systems face challenges in accurately classifying events and performing quantitative measurements due to the non-linear and unpredictable strain-optical signal transfer function, particularly in measuring vibration or strain using Rayleigh backscatter.
Innovation Solution
The implementation of coherent detection and phase-sensitive measurements in an optical time-domain reflectometry (OTDR) system, which records an intermediate-frequency waveform to measure both the phase and amplitude of the backscatter signal, allowing for the estimation of strain changes along the fiber by calculating phase differences between regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Difficulty of detecting and measuring
If amplitude of Rayleigh backscatter is measured to detect events, then event detection capability is improved, but measurement linearity and quantitative accuracy deteriorate due to non-linear strain-optical signal transfer function
Solution Approach 1:
The patent changes the measurement parameter from amplitude to phase of the backscatter signal. By measuring phase instead of amplitude, the system achieves a linear relationship between strain and optical signal, enabling accurate quantitative measurements while maintaining event detection capability
Solution Approach 2:
The patent replaces direct amplitude measurement with coherent phase detection using a local oscillator. This substitution transforms the non-linear amplitude-based detection into a linear phase-based measurement system through optical heterodyning or homodyning techniques
2Difficulty of detecting and measuring
If narrow band optical source is used to measure Rayleigh backscatter, then event detection capability is improved, but strain-optical signal transfer function becomes non-linear and unpredictable
Solution Approach 1:
The patent introduces a local oscillator as an intermediary reference signal that mixes with the backscatter signal. This intermediary enables coherent phase detection, providing a stable and predictable transfer function between strain and optical signal while maintaining the benefits of narrow band source for event detection
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 improves the linearity and accuracy of strain measurements, providing a quantitative and predictable strain-optical signal transfer function, enhancing the system's ability to detect and classify events along the sensing fiber.
Implementation Method 1
distributed fiber optic sensing systems typically measure the amplitude of Rayleigh backscatter returned from the fiber optic sensor when excited by a narrow band optical source
Implementation Method 2
coherent detection and phase-sensitive measurements in an optical time-domain reflectometry (OTDR) system, which records an intermediate-frequency waveform to measure both the phase and amplitude of the backscatter signal
Data Source
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AI summary
A fiber optic sensor system includes an optical source to output a first optical signal to launch into an optical fiber, and a coherent detector to mix a coherent Rayleigh backscatter signal generated by the optical fiber in response to the first optical signal with a second optical signal output by the optical source and to generate a mixed output signal. A phase detection and acquisition system determines a phase difference between first and second locations along the optical fiber based on phase information extracted from the mixed output signal and combines the phase information extracted from multiple acquisitions to detect strain on the optical fiber sensor.