Multi-frequency Coherent DAS with Single Tx/Rx Pair
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Solution Overview
Problem
Distributed acoustic sensing (DAS) systems face signal fading and noise instability due to Rayleigh scattering, particularly in environments with large vibrations, leading to phase measurement instabilities and 'blind spots', as existing technologies rely on fixed frequency spacing and high-cost multiple laser sources.
Innovation Solution
A multi-frequency DAS system employing a single transmitter/receiver pair with an optical IQ modulator and multiple local oscillators, using an arrayed waveguide grating or RF frequency sources to generate flexible frequency channels, which reduces the need for additional high-cost optical laser sources and eliminates the requirement for high-speed analog-to-digital conversion and digital signal processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple optical laser sources are used to generate multiple frequency channels, then frequency spacing can be increased to mitigate Rayleigh fading, but system cost increases significantly
Solution Approach 1:
The patent changes the frequency parameter by using a single optical laser source combined with an acousto-optic modulator (AOM) to generate multiple frequency channels. The AOM shifts the laser frequency by different amounts for different channels, achieving frequency diversity without requiring multiple laser sources. This resolves the contradiction by maintaining signal stability through frequency spacing while avoiding the high cost of multiple optical lasers.
Solution Approach 2:
Instead of using multiple physical laser sources, the patent creates copies of the same laser frequency at different offset frequencies using the AOM. The AOM generates multiple frequency-shifted versions of the single laser source, effectively copying the laser output at different frequencies to mitigate Rayleigh fading without the cost of multiple lasers.
2Ease of manufacture
If fixed frequency spacing is used in DAS systems, then system design is simplified, but signal fading occurs at specific locations creating blind spots
Solution Approach 1:
The patent implements dynamic frequency selection where the frequency spacing between channels is not fixed but can be adjusted. The system uses variable frequency offsets in the AOM to adaptively choose frequency combinations that avoid fading conditions at different fiber locations. This resolves the contradiction by maintaining ease of system design while improving signal coverage through adjustable frequency spacing.
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 system effectively mitigates signal fading by increasing frequency spacing between Rayleigh scattering characteristics, ensuring signal stability in dynamic fading scenarios and reducing costs through a true single transmitter and receiver design, while maintaining high spatial resolution and signal-to-noise ratio.
Implementation Method 1
an additional optical IQ modulator to generate the multiple frequency channels for DAS interrogation
Implementation Method 2
RF frequency sources can be generated either by an arrayed waveguide grating (AWG)
Implementation Method 3
contemporary distributed acoustic sensing (DAS) systems exploit Rayleigh scattering effects in an optical fiber to detect changes in the fiber strain
Implementation Method 4
separate coherent beating of each individual frequency channel
Data Source
AI summary
Aspects of the present disclosure describe multi-frequency coherent distributed acoustic sensing with a single transmitter/receiver pair using an offset Tx/Rx framing scheme and an additional optical IQ modulator to generate the multiple frequency channels for DAS interrogation.