Multimode Fiber Acoustic Sensing via Distributed Mode Coupling
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Solution Overview
Problem
Conventional distributed acoustic sensing (DAS) methods in optical fibers face limitations in detection speed, distance, and sensitivity due to reliance on Rayleigh scattering, which restricts the ability to detect acoustic vibrations effectively over long distances and with high sensitivity, particularly in applications like oil and gas well monitoring.
Innovation Solution
The system employs distributed mode coupling and delay in multimode optical fibers, allowing multiple laser pulses to propagate simultaneously, enhancing detection speed and sensitivity by utilizing the stronger signal-to-noise ratio of mode coupling over Rayleigh scattering, thereby increasing the detectable frequency and distance of acoustic vibrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional DAS methods using Rayleigh scattering are used, then acoustic vibrations can be detected along the optical fiber, but the detection speed is limited due to the round trip time of flight of a single laser pulse
Solution Approach 1:
The patent segments the single optical fiber into multiple spatial modes, allowing multiple laser pulses to propagate simultaneously through different modes. This segmentation enables parallel detection at multiple spatial positions, fundamentally increasing detection speed by eliminating the sequential limitation of conventional single-pulse DAS.
Solution Approach 2:
The patent transitions from single-dimensional temporal sequencing (one pulse at a time) to multi-dimensional parallel propagation by utilizing multiple spatial modes. This dimensional expansion allows simultaneous pulse injection and detection, resolving the time-of-flight bottleneck through spatial multiplexing.
2Length of stationary object
If Rayleigh scattering is used for DAS, then acoustic vibrations can be sensed, but the detectable distance is limited due to signal attenuation and weak scattering signal
Solution Approach 1:
The patent merges the signals from multiple spatial modes at the detection end, combining the backscattered light from different modes to achieve signal enhancement. This merging process compensates for attenuation losses by aggregating energy from multiple propagation paths, extending the detectable distance.
Solution Approach 2:
By continuously injecting multiple laser pulses into different spatial modes simultaneously, the system maintains continuous useful action along the entire optical fiber length. This continuous multi-pulse approach ensures that detection capability is maintained over extended distances without the signal dropping below the noise floor.
3Measurement precision
If Rayleigh scattering is used for DAS, then acoustic vibrations can be detected, but the sensitivity is limited due to low signal-to-noise ratio
Solution Approach 1:
The patent combines backscattered signals from multiple spatial modes to enhance the signal-to-noise ratio. By merging coherent signals from different modes while averaging out uncorrelated noise, the system achieves improved measurement precision and sensitivity for detecting weak acoustic vibrations.
Solution Approach 2:
The system creates multiple copies of the detection process by injecting identical or similar laser pulses into different spatial modes simultaneously. Each mode provides an independent copy of the acoustic sensing function, and combining these copies enhances the overall signal strength and sensitivity through coherent addition.
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 detection speed, distance, and sensitivity of acoustic vibrations, enabling more effective monitoring of oil and gas wells by overcoming the limitations of conventional DAS methods, particularly in detecting micro-seismic activity.
Implementation Method 1
DAS exploits Rayleigh scattering to sense acoustic vibrations
Implementation Method 2
Via the photo-elastic effect, if an acoustic vibration makes physical contact with the optical fiber at some point(s) along its length, the phase of the Rayleigh backscattering will change proportionally
Implementation Method 3
The system employs distributed mode coupling and delay in multimode optical fibers, allowing multiple laser pulses to propagate simultaneously
Implementation Method 4
The system employs distributed mode coupling and delay in multimode optical fibers, allowing multiple laser pulses to propagate simultaneously
Data Source
AI summary
A system and method are provided for distributed acoustic sensing in a multimode optical fiber. The system includes a transmitter for simultaneously propagating a sequence of M light pulses through the multimode optical fiber using a spatial mode selected from a set of N spatial modes provided by a spatial mode selector for the transmitter that is coupled to an input to the multimode optical fiber, with M and N being respective integers greater than one. The system further includes a receiver for receiving the sequence of M light pulses at an output of the multimode optical fiber and detecting an environmental perturbation in the multimode optical fiber based on an evaluation of a propagation of the sequence of M light pulses through the multimode optical fiber.