Embeddable Polarimetric Fiber Optic Sensor for Structural Stress Monitoring
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Solution Overview
Problem
Current fiber optic stress sensing technologies, such as FBGs and FPEs, require pre-designation of potential stress areas and removal of protective coatings, limiting their robustness and applicability in distributed sensing and hostile environments.
Innovation Solution
A distributed fiber optic stress sensing system using white-light polarimetric interferometry with polarization-maintaining optical fibers that can detect strain along the entire length without the need for discrete sensor sites or protective coating removal, employing a broadband optical source, optical interferometer, and signal processing unit to calculate stress concentrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Fiber Bragg Gratings (FBGs) are used for stress sensing, then localized strain detection is achieved, but the sensor cannot detect strain outside the grating area and requires pre-determined sensor locations
Solution Approach 1:
The optical fiber is segmented into multiple discrete sensing regions along its length, with each segment capable of independent strain measurement. This is achieved through periodic modulation of the fiber properties at specific locations, allowing the fiber to function as both a transmission medium and a distributed array of sensors.
Solution Approach 2:
The sensing capability is extended from a single localized point to multiple points along the fiber length by adding the spatial dimension. The fiber optic cable transforms from a simple point sensor into a distributed sensing network that can monitor strain at multiple locations simultaneously along its entire length.
2Measurement precision
If protective coating is removed from optical fiber to write FBGs, then sensing sensitivity is improved, but robustness and environmental tolerance are reduced
Solution Approach 1:
The fiber structure is modified locally at specific sensing positions rather than along the entire length. The protective coating is removed only at discrete locations where sensing regions are created, allowing the majority of the fiber to retain its protective coating and maintain robustness while still achieving sensitive measurement at required points.
Solution Approach 2:
The sensing regions are pre-defined and pre-prepared during fiber manufacturing before deployment. The periodic modulation and selective coating removal are performed in advance at factory conditions, eliminating the need for field processing that would compromise fiber protection and reliability.
3Adaptability or versatility
If multiple FBGs are multiplexed along a single fiber, then distributed sensing is achieved, but device complexity and instrumentation cost increase
Solution Approach 1:
The fiber optic cable serves multiple functions simultaneously: it acts as both the transmission medium for optical signals and as the sensing element itself. The periodic modulation creates multiple sensing regions that can be interrogated using simplified equipment, reducing overall system complexity compared to traditional multiplexed FBG systems.
4Measurement precision
If FBG locations are predetermined and matched to structure sites, then measurement precision at specific points is achieved, but adaptability to unknown stress areas is lost
Solution Approach 1:
The sensing system transitions from a static, pre-configured arrangement to a dynamic, flexible monitoring network. The distributed sensing regions along the fiber allow the system to detect and respond to stress concentrations at any location along the fiber path, enabling adaptation to unexpected damage areas without requiring reconfiguration.
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 robust, cost-effective, and widespread monitoring of structural stress without pre-identification of stress areas, using commercially available instrumentation and increasing the sensitive area for damage detection, thus reducing maintenance and replacement costs.
Implementation Method 1
A broadband optical source launches a polarized short coherence length optical pulse into one end of the optical fiber sensor. Localized stresses within the structure delay the transmission of that pulse as it travels down the fiber.
Implementation Method 2
At the other end of the fiber, a polarizer projects the polarized output light onto a common polarization axis. The light then enters the interferometer where light from one leg of the interferometer is superimposed onto light from the other leg at varying amounts of delay.
Implementation Method 3
A broadband optical source launches a polarized short coherence length optical pulse into one end of the optical fiber sensor. At the other end of the fiber, a polarizer projects the polarized output light onto a common polarization axis.
Data Source
AI summary
The present invention provides the capability of ascertaining, through a quick and simple measurement, locations on a structure that may have experienced damage that could result in reduced structure lifetime, strength, or reliability. The sensing element is a connectorized section of polarization maintaining (“PM”) optical fiber, where a length of PM fiber represents a fully distributed sensor array. Stress-induced changes to the sensor are measured through white-light Polarimetric interferometry. The output of the measurement is a data array representing the stress concentration magnitude at an array of locations along the length of the sensor. In an application, the knowledge of the optical fiber position on the structure, coupled with the measurement of stress locations along the fiber length, allows the user to determine locations on the structure with large stress concentrations. These locations may signify structural damage. This knowledge would allow the user to employ a more sophisticated system, albeit a larger and slower one, to fully characterize and evaluate that area of potential damage and take appropriate action.