Frequency Dithered DAS Suppressing Polarization Fading
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Solution Overview
Problem
Current Distributed Acoustic Sensing (DAS) technologies, particularly amplitude-based DAS, face challenges with linearity issues and measurement instability due to polarization fading and initial phase dependence, making them unreliable for accurate acoustic and vibration signal detection in optical fiber monitoring and troubleshooting.
Innovation Solution
The implementation of frequency dithered DAS (FD-DAS) using group data signal processing, where the coherent laser's optical frequency or phase is varied between data groups while remaining constant within each group, improves measurement reliability and suppresses polarization fading, allowing the use of low-cost single-frequency lasers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If amplitude-based DAS is used to simplify the instrument design, then device complexity is reduced, but measurement precision deteriorates due to lack of linearity and polarization fading
Solution Approach 1:
The patent changes the measurement parameter from direct amplitude detection to phase detection. By measuring the phase difference of coherent light before and after acoustic disturbance, the system achieves linear measurement relationship while maintaining simplified instrument design. The phase parameter is less sensitive to polarization fading than amplitude, resolving the contradiction between simplicity and precision.
Solution Approach 2:
The patent replaces the mechanical/optical amplitude detection mechanism with an interferometric phase detection mechanism. Instead of directly measuring light amplitude variations, the system uses interference patterns to encode acoustic information in phase differences, which are then decoded through signal processing. This substitution enables linear measurement while avoiding polarization fading issues.
2Reliability
If differential phase-based DAS is used to improve measurement reliability, then measurement precision is improved, but device complexity increases due to complicated instrument design
Solution Approach 1:
The patent extracts only the essential phase detection functionality from the complex differential phase-based DAS system. Instead of implementing the full differential phase measurement system with its complexity, the invention isolates and utilizes only the phase difference measurement between successive pulses, discarding the unnecessary complex components while retaining the core reliability benefit.
Solution Approach 2:
The patent segments the measurement process into distinct phases: coherent light transmission, phase encoding during acoustic disturbance, interferometric detection, and digital signal processing. By dividing the measurement into these manageable segments, the system achieves reliable phase-based measurement without requiring a monolithic complex instrument design.
3Ease of operation
If amplitude-based DAS measures intra-pulse interferences to detect disturbances, then ease of operation is improved, but measurement precision deteriorates due to unstable measurements from polarization fading
Solution Approach 1:
The patent introduces dynamic phase modulation through coherent light transmission. By continuously transmitting coherent light and dynamically measuring phase changes in response to acoustic disturbances, the system achieves stable measurements that are not affected by static polarization fading. The dynamic nature of phase measurement compensates for the simplicity of operation.
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 enhances the accuracy and reliability of acoustic and vibration signal measurements, reducing measurement errors and enabling cost-effective monitoring and troubleshooting in optical fiber telecommunication networks.
Implementation Method 1
Rayleigh scatter-based DAS uses a Coherent OTDR (C-OTDR) where a coherent laser pulse is sent along the optical fiber. As similar measurement technique in conventional OTDRs, for the C-OTDR the interfered intensities of any two or more reflected coherent lights are measured
Implementation Method 2
the interfered intensities of any two or more reflected coherent lights are measured as a function of time after transmission of the laser pulse
Implementation Method 3
frequency dithered distributed acoustic sensing (FD-DAS) using group data signal processing, where the continuous wave (CW) coherent laser's optical frequency or laser wavelength or optical phase for the FD-DAS interrogator may be set in evenly or randomly distributed manner for different data groups but they are kept as constant or with negligibly variation within each data group
Data Source
AI summary
There is provided a method and system of frequency dithered distributed acoustic sensing (FD-DAS) using group data signal processing, where the continuous wave (CW) coherent laser's optical frequency or optical phase for the FD-DAS interrogator may be set in evenly or randomly distributed manner for different data groups but they are kept as constant or with negligibly variation within each data group. Such FD-DAS may be used to improve acoustic or vibration signal measurement reliability, to allow to use a low-cost single-frequency laser such as a DFB laser, and/or to suppress polarization fading effect.