Fibre Optic Distributed Acoustic Sensing Frequency Measurement
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Solution Overview
Problem
Fibre optic distributed acoustic sensors face challenges in providing quantitative information about environmental stimuli due to variable intensity changes across channels and difficulty in detecting low-frequency disturbances, mainly because of the variability in channel gain and non-linear responses, which complicates accurate measurement of stimuli acting on the sensing fibre.
Innovation Solution
The method involves repeatedly interrogating the sensing fibre with coherent optical radiation pulses and detecting the frequency of variation in backscatter intensity, utilizing the phase change caused by optical path length changes to determine the amplitude of disturbances, allowing for quantitative analysis of stimuli by relating the frequency of intensity variation to the rate of stimulus, effectively addressing the challenges of channel variability and low-frequency detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If intensity-based DAS sensing is used, then the sensor can detect acoustic stimuli, but quantitative measurement is difficult due to variable channel gain
Solution Approach 1:
The patent changes the measurement parameter from intensity to frequency. By detecting the frequency of intensity variations rather than absolute intensity levels, the system eliminates sensitivity to channel gain variations. The frequency of oscillation in the backscatter intensity directly corresponds to the acoustic stimulus frequency, providing a reliable quantitative measure independent of channel-specific gain characteristics.
2Manufacturing precision
If short duration pulses are used to achieve high spatial resolution, then spatial resolution is improved, but less light is injected into the fibre reducing detection sensitivity
Solution Approach 1:
The patent changes the detection parameter from intensity magnitude to intensity variation frequency. This allows the use of short pulses for high spatial resolution while maintaining detection sensitivity, because the frequency measurement depends on the temporal pattern of intensity variations rather than the absolute intensity level, which would be reduced by shorter pulses.
3Reliability
If repeated interrogations are performed to detect low frequency disturbances, then detection capability is improved, but channel gain variability complicates quantitative analysis
Solution Approach 1:
The patent measures the frequency of intensity variations across repeated interrogations rather than intensity changes alone. This frequency-based approach provides a stable quantitative metric for low frequency disturbances that is independent of channel gain variability, enabling both reliable detection and precise quantitative 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 accurate and quantitative measurement of environmental changes, including strain and temperature variations, by correlating the frequency of intensity variation with the stimulus rate, enhancing the sensitivity and reliability of fibre optic distributed acoustic sensing systems.
Implementation Method 1
Various types of DAS sensor have been demonstrated including sensors based on Rayleigh scattering of light from the sensing fibre. Light transmitted into an optical fibre will be scattered from the various inherent scattering sites within an optical fibre. A mechanical vibration of the fibre, such as caused by an incident acoustic wave, will alter the distribution of scattering sites resulting in a detectable change in the properties of the Raleigh backscattered light.
Implementation Method 2
As the backscatter from the various scattering sites within the sensing portion of fibre will interfere to produce the resulting intensity, a change in optical path length will vary the degree of interference and thus result in a change in backscatter intensity.
Data Source
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AI summary
This application describes method and apparatus for fibre optic distributed acoustic sensing (DAS) that allow for quantitative estimation of relatively large and continuous stimuli acting on the sensing fibre. An optical fibre (101) is interrogated with optical pulse and the Rayleigh backscatter detected to provide a DAS sensor. The method involves identifying a first stimulus acting on at least one sensing portion of the optical fibre, which results in an effective optical path length change within said sensing portion of at least the wavelength of the optical radiation. Such a path length change will result in signal wrapping leading to an observed variation (401) in backscatter intensity. The frequency of variation is detected and can be used to estimate the rate of change of path length. The method can be used to estimate strain rate and/or rate of change of temperature.