Fibre Optic Distributed Sensing Low-Frequency Phase Isolation
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Solution Overview
Problem
Fibre optic distributed sensing systems, such as DTS and DAS, require long time averages and have limited resolution, making them less effective for detecting rapid temperature changes and ignoring low-frequency phase changes which can indicate strain or temperature variations.
Innovation Solution
A method involving series of interrogations with pulse pairs in optical fibres, where pulses have a time interval, sampling Rayleigh backscattered radiation to isolate low-frequency phase modulation below a threshold frequency, allowing for detection of slow-acting changes like strain or temperature variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If long time averages are used in DTS systems to improve measurement precision, then temperature measurement accuracy is improved, but the system cannot detect rapid temperature changes
Solution Approach 1:
The patent employs periodic pulse pairs with different frequencies launched at regular intervals to interrogate the fibre. By using periodic interrogation with pulse pairs having frequency separation, the system achieves both adequate signal averaging for precision and sufficient update rate to capture rapid temperature changes, resolving the contradiction between measurement precision and response speed.
2Adaptability or versatility
If conventional DAS methods are used to detect acoustic signals, then high-frequency vibrations are detected, but low-frequency phase changes indicating strain or temperature variations are ignored
Solution Approach 1:
The patent extracts and processes only the low-frequency phase modulation components from the backscattered signal by filtering out high-frequency acoustic components. This extraction of the specific low-frequency phase change information allows the system to detect strain and temperature variations that would be obscured or ignored in conventional DAS methods, thereby preventing information loss.
3Adaptability or versatility
If pulse pairs with frequency separation are used to enable DAS functionality, then acoustic sensing capability is achieved, but the ability to detect low-frequency phase changes is compromised
Solution Approach 1:
The patent segments the signal processing into distinct frequency bands: high-frequency components are processed for acoustic sensing while low-frequency phase modulation components are separately extracted and processed for strain and temperature detection. This segmentation allows both acoustic sensing capability and low-frequency phase change detection to coexist without compromising either function.
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
Enables the detection of low-frequency phase shifts indicative of strain or temperature changes, improving the sensitivity and accuracy of fibre optic sensing systems for applications like leak detection in pipelines.
Implementation Method 1
sampling optical radiation which is Rayleigh backscattered from within said fibre
Data Source
AI summary
A method of distributed fibre optic sensing is described. In an example, a series of interrogations are launched into an optical fibre, each interrogation comprising interrogating radiation in at least one pulse pair, wherein the pulses of a pulse pair are introduced to the optical fibre with a time interval therebetween. Radiation backscattered therefrom is sampled, so as to obtain at least one sample from each interrogation. Phase modulation in the samples is determined and components of the phase modulation which are below a threshold frequency are isolated. Such a method of sensing could be used, for example, to monitor changes in temperature of the optical fibre.