Distributed Acoustic Sensing with Frequency-Diverse Probe Pulses
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Solution Overview
Problem
Distributed optical sensors suffer from low sensitivity and cross-sensitivity issues due to weak scattering events and coherent fading, which limits their ability to accurately detect strain and other parameters, particularly in applications like seismic event detection where flexibility and high resolution are crucial.
Innovation Solution
The system employs a source generator to transmit a set of time-aligned optical probe pulses at distinct frequencies, which are backscattered and then mixed with frequency-shifted versions of these pulses to enhance sensitivity and reduce fading, using a combination of heterodyne and homodyne techniques to measure strain changes along the optical fibre.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If distributed optical sensors use weak scattering events for sensing, then the sensor can be repositioned and gauge length can be changed, but the sensitivity is significantly lower than discrete sensors
Solution Approach 1:
The system segments the optical signal into multiple frequency components by transmitting probe pulses at distinct frequencies. Each frequency component interacts with scattering events independently, and the receiver system processes each frequency separately to extract strain information, thereby improving sensitivity while maintaining distributed sensing capability
Solution Approach 2:
The system changes the frequency parameter of the probe pulses, transmitting a set of time-aligned optical probe pulses at distinct frequencies spaced by Δf. This frequency diversification allows the system to overcome coherent fading and improve sensitivity by averaging measurements across multiple frequencies
2Productivity
If distributed optical sensors rely on backscatter from random scatterer locations, then the system can provide distributed sensing along the fibre, but coherent fading occurs leading to signal distortion and non-linearity
Solution Approach 1:
The system transmits probe pulses at multiple distinct frequencies and processes the backscatter signals at each frequency separately. By diversifying the frequency parameter, the system reduces the probability that all frequency components will experience coherent fading simultaneously, thereby improving signal stability and reliability
Solution Approach 2:
The receiver system uses feedback mechanisms to monitor and compensate for fading effects. By continuously analyzing the backscatter signals and comparing them across multiple frequencies, the system can identify and correct for coherent fading distortions in real-time
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 significantly improves the sensitivity and dynamic range of distributed optical sensors by minimizing the probability of destructive interference and coherent fading, allowing for more accurate strain measurements and increased signal-to-noise ratio, even in the presence of birefringence and temperature variations.
Implementation Method 1
The main scattering mechanism of interest is Rayleigh scattering which results in backscatter at the same frequency as the propagating light due to elastic collisions with scattering sites
Implementation Method 2
the optical receiver system is configured to mix a delayed version of the backscattered light with at least one frequency-shifted version of the backscattered light
Implementation Method 3
using a combination of heterodyne and homodyne techniques to measure strain changes
Implementation Method 4
the scattered light from each scatterer adds coherently to form the backscatter pulse. Since the scatterer locations are random, this addition can be destructive and lead to no return signal
Implementation Method 5
Fading can also occur from birefringence effects
Data Source
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AI summary
Systems and methods for sensing changes in an optical sensing fibre, principally for detecting changes in strain. A plurality of optical probe pulses at different optical frequencies (f1, f2,... fn) spaced by Δf are transmitted into the sensing fibre. Light backscattered from the optical sensing fibre may be mixed with delayed backscatter frequency shifted by Δf+fm where fm is a heterodyne frequency. The backscattered or mixed light may be detected to determine changes in the sensing fibre. Δf may be chosen to optimise performance.