Phase Filtering for Distributed Acoustic Sensing
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Solution Overview
Problem
Existing signal processing techniques for distributed acoustic sensing (DAS) systems face limitations in accurately measuring acoustic fields, particularly for large strains and incorrect fiber scatter bias conditions, leading to distortion and generation of higher frequency components that do not truly represent the acoustic vibrations.
Innovation Solution
The proposed signal processing method involves filtering the phase of an input signal using an N-stage delay line, applying exponentiation to generate phase-modified signals, and cumulatively multiplying these signals to produce a filtered signal. This method acts directly on the phase information without crosstalk from amplitude, improving the detection of acoustic modulations along an optical path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional signal processing methods are used for DAS systems, then the system can operate with simpler processing, but the measurement precision deteriorates for large strains and incorrect fiber scatter bias conditions
Solution Approach 1:
The patent transforms the signal processing approach by changing the domain in which filtering is performed. Instead of filtering the complex signal directly (I-Q plane), the method converts to polar coordinates (amplitude-phase plane) and performs filtering on the phase component separately. This parameter transformation enables linear filtering of phase information without the nonlinear distortions that occur in traditional methods, thereby improving measurement precision for large strains while maintaining manageable processing complexity
Solution Approach 2:
The patent extracts the phase information from the complex signal by converting to polar coordinates, separating it from the amplitude component. This extraction allows the phase to be filtered independently using linear operations, avoiding the crosstalk between amplitude and phase that plagues traditional methods. The separated phase filtering approach maintains measurement precision even under incorrect fiber scatter bias conditions
2Measurement precision
If coherent detection is used to measure both phase and amplitude, then the dynamic range and correlation to acoustic field improve, but the operational range is limited
Solution Approach 1:
The patent changes the processing domain from Cartesian (I-Q) to polar coordinates (amplitude-phase), enabling separate filtering of the phase component. This transformation allows coherent detection to maintain its superior acoustic field correlation while extending operational range by preventing the signal distortion that normally limits distance. The linear phase filtering preserves signal integrity over longer fiber lengths
Solution Approach 2:
The patent applies preliminary filtering to the phase component before final signal reconstruction. By pre-filtering the phase information extracted from coherent detection, the system removes noise and artifacts that would otherwise accumulate over long distances, thereby extending the usable operational range while maintaining the high acoustic field correlation that coherent detection provides
3Productivity
If pulse repetition frequency is increased to improve sampling rate, then the acoustic frequency detection capability improves, but the fiber length limitation restricts the maximum achievable PRF
Solution Approach 1:
The patent transforms the signal to polar coordinates and filters the phase component separately, which enables effective noise reduction and signal enhancement. This improvement in signal quality allows for more robust detection at higher sampling rates, effectively enabling the system to achieve higher acoustic frequency detection capability even when constrained by fiber length limitations on PRF
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
The method enhances the signal-to-noise ratio and improves the detection of acoustic modulations by filtering phase information linearly without distorting it, even in the presence of large modulation depths, thus overcoming the limitations of traditional processing methods.
Implementation Method 1
These discontinuities lead to scattering of laser light passing through the optical fiber, particularly by Rayleigh scattering.
Implementation Method 2
Another way of measuring the amplitude and phase of the scattered light in a 'quantitative' system is to use a local oscillator reference signal and measure the phase of the scattered light in relation to this reference. This method is termed coherent detection.
Implementation Method 3
Other systems simultaneously measure the amplitude and phase of the scattered light, typically by comparing the phase of two sequential pulses or by comparing the phase of one pulse with a delayed copy of itself. In each case, said pulses are allowed to optically interfere and the resulting interference is measured.
Data Source
AI summary
The present disclosure relates to signal processing methods for an optical detection system, and systems for carrying out such processing. In particular, disclosed is a signal processing method for filtering the phase of an input signal, the method includes: receiving an input signal; applying an N-stage delay line to the input signal to generate a plurality of delayed signals, where N is an integer which may refer to the order of the filter, the value of which may be preselected to provide a desired balance between performance and processing power (as higher order filters may produce better results at the cost of increased processing power); applying a respective exponentiation to each delayed signal to generate a plurality of phase modified signals; and cumulatively multiplying each of the phase modified signals to generate a filtered signal.