Under-filled Launch Configuration for Multi-mode Fiber Acoustic Sensing
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Solution Overview
Problem
Existing distributed acoustic sensing systems are limited by the use of single-mode optical fibers, which are not feasible to replace in many existing well installations that employ multi-mode optical fibers, hindering the adoption of distributed acoustic sensing technology.
Innovation Solution
Employing an under-filled launch configuration with multi-mode optical fibers to excite only the lowest-order modes, enabling the use of multi-mode fibers for distributed acoustic sensing while minimizing coupling losses and achieving performance comparable to single-mode fibers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If single-mode optical fiber is used to achieve adequate sensing performance, then measurement precision and coherency preservation are improved, but device complexity and installation feasibility worsen due to the need to replace existing multi-mode fiber infrastructure
Solution Approach 1:
The patent changes the operational parameters of multi-mode optical fiber by using an under-filled launch configuration with a launch diameter smaller than the fiber core diameter. This parameter change allows multi-mode fiber to operate in a regime that preserves coherency and enables distributed acoustic sensing, effectively making it perform like single-mode fiber without requiring infrastructure replacement
2Adaptability or versatility
If multi-mode optical fiber is used to maintain compatibility with existing installations, then ease of operation and adaptability improve, but measurement precision and sensing performance worsen due to mode dispersion and coupling losses
Solution Approach 1:
The patent applies local quality by creating a specialized launch condition at the input end of the multi-mode fiber. By confining the optical beam to a small core region (under-filled launch), the system locally modifies the excitation conditions to preferentially excite only low-order modes, thereby preserving coherency and enabling precise distributed acoustic sensing throughout the fiber length
3Loss of energy
If under-filled launch configuration is used with multi-mode optical fiber, then coupling losses are reduced and sensing performance is improved, but device complexity increases due to precise alignment requirements
Solution Approach 1:
The patent introduces an intermediary optical element (such as a lens or mode field adapter) that transforms the optical beam profile before it enters the multi-mode fiber. This intermediary component creates the required under-filled launch condition without demanding extreme alignment precision at the fiber input, thereby reducing coupling losses while maintaining practical ease of installation
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
Enables effective detection of distributed acoustical and vibrational energies with reduced coupling losses, allowing for the adaptation of existing multi-mode fiber installations for distributed acoustic sensing, providing reliable and efficient monitoring of downhole processes.
Implementation Method 1
detection of distributed acoustical and vibrational energies (DAS/DVS)
Data Source
AI summary
An illustrative distributed acoustic sensing system includes a multi-mode optical fiber cable for distributed sensing and a distributed acoustic sensing interrogator coupled to the multi-mode optical fiber cable via a single mode optical fiber. The interrogator derives distributed acoustic measurements from Rayleigh backscattering light that is initiated with a substantially under-filled launch configuration that is designed to excite only the lowest-order modes of the multi-mode optical fiber. Mode conversion within the multi-mode optical fiber is anticipated to be negligible. For elastic scattering (i.e., Rayleigh scattering), it is further anticipated that the scattered light will be primarily returned in the incident propagation mode, thereby escaping the extraordinarily large coupling loss that would otherwise be expected from coupling a single-mode optical fiber to a multi-mode optical fiber for distributed sensing. Experiments with graded index multi-mode optical fiber have yielded positive results.