Polylevel Coded Fibre Optic Sensing for Dynamic Disturbance Detection
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Solution Overview
Problem
Conventional distributed acoustic sensing (DAS) systems face limitations in pulse repetition rate and spatial resolution due to the need for unique interrogations and the trade-off between sensitivity and spatial resolution, which restricts the detection of dynamic disturbances and acoustic stimuli.
Innovation Solution
A distributed fibre optic sensor apparatus using a coded sequence of optical radiation encoded according to a polylevel coding sequence, allowing continuous interrogation and improved signal-to-noise ratio by correlating the backscatter signal with the coding sequence, enabling simultaneous detection of disturbances along the fibre without positional ambiguity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single pulse of coherent radiation is used to interrogate the sensing fibre, then the system structure is simple, but the pulse repetition rate is limited by the round trip time of light in the fibre
Solution Approach 1:
The single pulse is segmented into multiple sub-pulses with different frequencies, allowing parallel interrogation of different portions of the fibre. This enables the pulse repetition rate to exceed the round trip time limit while maintaining system simplicity.
Solution Approach 2:
The system transitions from temporal domain (single pulse timing) to frequency domain (multiple frequencies) to resolve the pulse repetition rate limitation. By encoding spatial information in frequency differences, the system can interrogate multiple fibre sections simultaneously without increasing temporal complexity.
2Manufacturing precision
If shorter pulses are used to improve spatial resolution, then the spatial resolution increases, but the sensitivity decreases due to reduced optical power
Solution Approach 1:
The system changes the frequency parameter of multiple pulses to enable parallel interrogation. By using frequency-encoded sub-pulses instead of shortening a single pulse, the system maintains high optical power while achieving improved spatial resolution through frequency-domain processing.
Solution Approach 2:
The interrogation pulse is segmented into frequency-components that can be processed independently. Each frequency component corresponds to a specific spatial region, allowing high-resolution spatial mapping without reducing the total optical energy delivered to the fibre.
3Productivity
If wavelength division multiplexing is used to improve pulse repetition rate, then the pulse repetition rate increases, but the device complexity increases due to multiple sources and detectors
Solution Approach 1:
A single optical source and single detector are made multi-functional by using frequency modulation and correlation processing. The same hardware components handle multiple frequency-encoded signals, achieving wavelength division multiplexing benefits without the complexity of multiple physical sources and detectors.
Solution Approach 2:
The patent replaces the mechanical/optical complexity of multiple wavelength sources and detectors with signal processing in the frequency domain. Correlation processing substitutes for physical wavelength separation, reducing device complexity while maintaining high pulse repetition rates.
4Reliability
If the pulse duration is increased to improve sensitivity, then the signal-to-noise ratio improves, but the spatial resolution deteriorates
Solution Approach 1:
The system resolves the spatial resolution issue by moving from temporal domain (pulse duration) to frequency domain. Long pulses provide high signal-to-noise ratio, while frequency encoding within the pulse structure enables precise spatial localization through correlation processing, achieving both high sensitivity and high resolution simultaneously.
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 increases the sensing duty cycle and signal-to-noise ratio, allowing for higher spatial resolution and improved detection of dynamic disturbances, including acoustic waves, without the limitations of conventional pulse-based systems.
Implementation Method 1
As the pulse propagates along the fibre the phenomenon of Rayleigh scattering from various inherent scatting sites within the optical fibre will result in some small proportion of the interrogating radiation being backscattered toward the first end
Implementation Method 2
a first modulator for modulating the continuous wave optical signal based on the polylevel coding sequence to generate the first coded sequence of optical radiation
Implementation Method 3
a detector configured to detect optical radiation which has been Rayleigh backscattered from within the sensing optical fibre
Implementation Method 4
Location along the sensing fibre is determined based on OTDR (optical time domain reflectometry) techniques, with the backscatter signals being processed in time bins corresponding to backscatter from defined portions of the fibre
Data Source
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AI summary
This application relates to methods and apparatus for distributed fibre optic sensing, especially distributed acoustic sensing (DAS). The sensor apparatus (300) has an optical generator (303, 305, 306) for repeatedly generating a first coded sequence of optical radiation to be launched into a sensing optical fibre (302). The first coded sequence is encoded according to a polylevel coding sequence (101) and has desired autocorrelation properties. A detector (311) detects optical radiation which has been Rayleigh backscattered from within the sensing optical fibre and a processor (301 c) processes the output of the detector. The processor processes the detector output in range bins and for each range bin correlating a detection signal derived from the detector output with the polylevel coding sequence at a time delay corresponding to that particular range bin. This provides a measurement signal indicative of environmental disturbances, such as incident acoustic/seismic signals, acting on a portion of the sensing fibre corresponding to that range bin.