Fiber Bragg Grating Dithering for Strain Measurement
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Solution Overview
Problem
Existing optical fiber strain sensors with multiple fiber Bragg gratings face challenges in accurately measuring strain due to the occurrence of 'ringing', which limits the ability to determine strain accurately when the total reflectivity exceeds 15-20%.
Innovation Solution
The use of an optical fiber with a series of fiber Bragg gratings where each grating has a unique light reflection frequency, varying within a defined range to minimize resonance and reduce the likelihood of ringing, achieved by etching refractive index changes at different angles across the fiber.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If many fiber Bragg gratings are etched into one optical fiber to measure strains at hundreds or thousands of locations, then the measurement coverage and productivity are improved, but the total reflectivity increases to 15-20% causing ringing that degrades measurement precision
Solution Approach 1:
The patent applies parameter changes by varying the spacing between adjacent fiber Bragg gratings along the optical fiber. Instead of uniform spacing, the gratings are positioned with different intervals, which changes the optical path differences and prevents constructive interference that causes ringing. This non-uniform spacing parameter modification allows maintaining high measurement coverage while avoiding the reflectivity threshold that triggers ringing phenomena.
Solution Approach 2:
The patent segments the optical fiber into multiple measurement sections, each with its own fiber Bragg grating at a unique position and spacing. By dividing the fiber into distinct measurement zones with different grating configurations, the system can measure strains at multiple locations independently without the cumulative reflectivity effects that would occur in a uniformly spaced array, thus preventing ringing while maintaining comprehensive coverage.
2Ease of manufacture
If fiber Bragg gratings are etched with uniform spacing to simplify manufacturing, then the ease of manufacture is improved, but ringing occurs when total reflectivity reaches 15-20% making accurate strain measurement difficult
Solution Approach 1:
The patent modifies the spacing parameter between gratings from a uniform value to a non-uniform distribution. This parameter change is implemented during the fabrication process using phase masks or other etching techniques, where the spacing between consecutive gratings is deliberately varied according to a predetermined pattern. This approach maintains manufacturing feasibility while eliminating the ringing condition that would otherwise occur with uniform spacing at high grating densities.
Solution Approach 2:
The patent introduces asymmetry in the grating spacing arrangement along the optical fiber. Instead of symmetric uniform intervals, the gratings are positioned with asymmetric spacing patterns where adjacent gratings have different intervals from their neighbors. This asymmetric configuration disrupts the regular interference patterns that cause ringing, thereby improving measurement reliability without significantly complicating the manufacturing process.
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 allows for accurate measurement of strain by maintaining low total reflectivity and preventing resonance, enabling precise determination of strain and other properties like temperature or pressure.
Implementation Method 1
Each fiber Bragg grating reflects light at a certain frequency depending on such factors as the magnitude of the refractive index changes in the optical fiber and the distance between the refractive index changes
Implementation Method 2
the magnitude of the refractive index changes in the optical fiber and the distance between the refractive index changes
Implementation Method 3
an optical fiber having a series of fiber Bragg gratings, each fiber Bragg grating in the series being characterized by a light reflection frequency
Implementation Method 4
When this point is reached, 'ringing' may occur. Ringing is an optical phenomenon wherein light undergoes multiple reflections within the optical fiber. That is the fiber Bragg gratings begin to act as an optical cavity to continuously reflect light between the gratings
Data Source
AI summary
An apparatus for determining a property, the apparatus including: an optical fiber having a series of fiber Bragg gratings, each fiber Bragg grating in the series being characterized by a light reflection frequency at which the fiber Bragg grating reflects light; wherein: the light reflection frequency for each fiber Bragg grating is different from the light reflection frequency of each adjacent fiber Bragg grating to minimize resonance of light between at least two of the fiber Bragg gratings in the series; at least two fiber Bragg gratings in the series have light reflection frequencies that overlap; and a change in the light reflection frequency of each fiber Bragg grating in the series is related to the property at the location of the each fiber Bragg grating.