Few-Mode Fiber Hybrid Distributed Acoustic Testing
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Solution Overview
Problem
Existing optical fiber distributed sensing systems face challenges in simultaneously measuring vibration and temperature due to fiber nonlinearity issues, particularly in single mode fibers, which affect the operation of distributed acoustic sensors (DAS) and distributed temperature sensors (DTS), and multimode fibers complicate the measurement of vibrations due to independent interference behavior among modes.
Innovation Solution
The use of few-mode fibers as a compromise between single and multimode fibers, employing wavelength division multiplexing and mode multiplexing/demultiplexing techniques to separate and analyze backscattered Rayleigh, Stokes Raman, and Anti-Stokes Raman signals, allowing for simultaneous vibration and temperature sensing by demultiplexing signals from few-mode fibers to single-mode fibers for precise detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If single mode fiber is used for distributed temperature sensing, then temperature measurement is achieved, but fiber nonlinearity (stimulated Raman scattering) inhibits proper operation
Solution Approach 1:
The patent introduces a few-mode fiber as an intermediary medium between the pump light source and the sensing region. This few-mode fiber has a higher nonlinearity threshold that acts as a buffer, allowing the pump power to be sufficiently high for good temperature sensing while preventing the stimulated Raman scattering that would occur in single-mode fiber, thus maintaining system operation stability
Solution Approach 2:
The patent changes the fiber mode parameter from single-mode to few-mode operation. By launching pump light into specific modes (LP01 and LP11) of the few-mode fiber, the system exploits the higher power threshold for nonlinear effects in these modes, enabling both accurate temperature measurement and stable operation
2Measurement precision
If multimode fiber is used for distributed acoustic sensing, then vibration sensing capability is improved, but independent interference behavior among modes complicates measurement
Solution Approach 1:
The patent segments the few-mode fiber operation into distinct mode groups - specifically utilizing LP01 and LP11 modes for acoustic sensing. By separating the sensing into these specific modes rather than using all possible modes, the system maintains the vibration sensing capability of multimode fibers while reducing the complexity of mode interference management
Solution Approach 2:
The few-mode fiber serves multiple functions simultaneously: it provides the high power threshold needed for distributed temperature sensing while also supporting distributed acoustic sensing through specific modes. This multi-functionality eliminates the need for separate single-mode and multimode fiber systems
3Length of stationary object
If few mode fiber is used for simultaneous sensing, then sensing range is extended and costs are reduced, but signal separation and detection complexity increases
Solution Approach 1:
The patent employs periodic pulse transmission at different wavelengths (e.g., 1550nm and 1625nm) through the few-mode fiber. By using time-domain multiplexing with periodic pulses, the system can separate Rayleigh backscatter signals (for acoustic sensing) from Raman backscatter signals (for temperature sensing) based on their different temporal and spectral characteristics, reducing signal processing complexity
Solution Approach 2:
The patent introduces wavelength division multiplexing filters and mode demultiplexers as intermediary devices to automatically separate the different signal types. These intermediaries perform the complex signal separation task, reducing the burden on subsequent detection and processing stages while enabling simultaneous vibration and temperature sensing over extended ranges
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 simultaneous distributed vibration and temperature sensing with a longer sensing range compared to single-mode fiber-based systems, reduces overall costs by integrating both sensing functions into a single platform, and mitigates dispersion issues in multimode fiber-based systems, ensuring reliable operation and continuous real-time monitoring.
Implementation Method 1
separating, by a wavelength division multiplexing filter and from a few mode optical fiber, a collection of backscattered Rayleigh signals
Implementation Method 2
consecutive Rayleigh backscattered traces are recorded in the time domain. Each Rayleigh trace has a speckle-like profile because of coherent interference of the signals reflected by scattering centers within the injected pulse duration
Implementation Method 3
The pulses are backscattered by thermal excitement of the FUT. At the FUT input port, backscattered Stokes Raman and anti-Stokes Raman signals are compared to determine the temperature of the FUT
Implementation Method 4
This fiber nonlinearity, such as stimulated Raman scattering, inhibits the proper operation of DTS systems
Data Source
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AI summary
The subject matter of this specification can be embodied in, among other things, a method that includes separating, from a few mode optical fiber, a collection of backscattered Rayleigh signals based on a vibration of the few mode optical fiber at a vibration frequency at a first location along the few mode optical fiber, separating, from the few mode optical fiber, a collection of backscattered Stokes Raman signals and Anti-Stokes Raman signals based on a temperature of the few mode optical fiber at a second location along the few mode optical fiber, detecting the separated Rayleigh signals and Raman signals, determining, based on detecting the collection of backscattered Rayleigh traces, at least one of the first location, the vibration frequency, and an amplitude of the vibration, and determining, based on the detecting the collection of backscattered Raman signals, the temperature at the second location.