Directional Fiber Optic Cable Using Evanescent Coupling
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Solution Overview
Problem
Fiber optic cables used in wellbores for downhole sensing lack directional sensitivity, making it difficult to determine the direction of external forces or environmental variations such as acoustic waves, pressure, strain-stress, or temperature changes.
Innovation Solution
A directional sensitive fiber optic cable system is developed, featuring a housing with acoustically isolated sections and evanescent electromagnetic coupling, where multiple sensing fibers carry distinct laser signals and are arranged around a signal fiber to isolate and detect sound waves from different directions, using acoustic reflective surfaces and a fiber optic gyro for orientation determination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If fiber optic cables are used for downhole sensing, then temperature and acoustic strain signals can be measured, but directional sensitivity is lost
Solution Approach 1:
The fiber optic cable is divided into multiple sensing elements (sensing fibers) arranged in different spatial orientations around a central signal fiber. Each sensing fiber is positioned to detect acoustic waves from specific directions, creating directional sensitivity through spatial segmentation of the sensing function.
Solution Approach 2:
Different sensing fibers are positioned at different locations and orientations within the cable housing to provide direction-specific sensing capabilities. The local spatial arrangement of each sensing fiber gives it a specific directional sensitivity characteristic, allowing the system to determine the direction of incoming acoustic waves.
2Measurement precision
If multiple sensing fibers are placed around a signal fiber, then directional sensitivity is achieved, but device complexity increases
Solution Approach 1:
Multiple sensing fibers carrying distinct laser signals are combined within a single housing structure around a central signal fiber. The system merges the functions of multiple fibers into one integrated assembly, allowing directional sensing to be achieved while maintaining a compact, unified device structure that can be deployed as a single unit in the wellbore.
3Measurement precision
If evanescent coupling is used to detect signals, then directional sensitivity is enabled, but signal isolation between fibers becomes challenging
Solution Approach 1:
The signal fiber acts as an intermediary that receives evanescent coupling signals from multiple sensing fibers. Each sensing fiber's laser signal couples evanescently to the signal fiber, allowing the signal fiber to mediate the detection process while maintaining signal distinction through frequency differentiation. This intermediary structure enables sensitive detection while preventing direct cross-contamination between sensing fibers.
Solution Approach 2:
Each sensing fiber carries a laser signal with a distinct frequency, analogous to different colors. This frequency differentiation allows the system to distinguish signals from different directional sources even when they couple to the same signal fiber, preventing cross-contamination through spectral separation.
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 directional sensitivity by isolating and amplifying sound waves from specific directions, allowing for accurate detection of environmental variations and fluid flow characteristics in wellbores, enhancing seismic applications and fluid differentiation.
Implementation Method 1
the signal fiber carries one or more evanescent coupling signals responsive to the laser signals in the plurality of sensing fibers
Data Source
AI summary
A fiber optic cable assembly includes an elongate housing, a signal fiber placed inside the housing and extending longitudinally, and a plurality of sensing fibers placed inside the housing and extending longitudinally. The plurality of sensing fibers is placed around the signal fiber. Each of the plurality of sensing fibers carries a respective laser signal of a distinct frequency. The signal fiber carries one or more evanescent coupling signals responsive to the laser signals in the plurality of sensing fibers.