Bragg Grating Pressure Sensor Using Single Mode Fiber
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Solution Overview
Problem
Uni-axial pressure sensors using fiber Bragg gratings face challenges with polarization state maintenance and wavelength shifts due to built-in bi-refringence in polarization maintaining fibers, leading to unwanted shifts even without applied pressure, and difficulty in maintaining stability against vibrations and temperature changes.
Innovation Solution
A system utilizing single mode fiber with a polarization controller to create and maintain orthogonal polarization states, allowing for the detection of maximum wavelength shift between these states, which is only present when uni-axial pressure is applied, using techniques such as rapid alternation of polarization states and dual light sources with frequency modulation for stable measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If polarization maintaining fiber is used to maintain polarization state, then polarization state stability is improved, but built-in bi-refringence causes unwanted wavelength shifts even without applied pressure
Solution Approach 1:
The patent extracts and eliminates the built-in bi-refringence stress from the fiber structure by using standard single mode fiber instead of polarization maintaining fiber, removing the source of unwanted wavelength shifts while maintaining polarization control through external means (polarization controller)
Solution Approach 2:
The patent employs dynamic polarization control by rapidly alternating the input polarization state between two orthogonal states, allowing the system to adapt to environmental changes and vibrations while measuring pressure through wavelength shift detection
2Ease of manufacture
If standard single mode fiber is used instead of polarization maintaining fiber, then cost and ease of manufacture are improved, but polarization state cannot be maintained over distance
Solution Approach 1:
The patent introduces a polarization controller as an intermediary device that actively manages and maintains the desired polarization state in standard single mode fiber, enabling the use of cheaper, easier-to-install fiber while preserving polarization control functionality
Solution Approach 2:
The patent changes the operating parameters by using rapid polarization state alternation and detecting wavelength shifts between orthogonal polarization states, allowing accurate pressure measurement in standard fiber without requiring polarization maintaining properties
3Measurement precision
If polarization controller is introduced to maintain orthogonal polarization states, then measurement accuracy is improved, but device complexity increases
Solution Approach 1:
The patent implements self-service by using the fiber optic cable's existing polarization mode dispersion characteristics to naturally create and maintain orthogonal polarization states, eliminating the need for external polarization controllers or alignment mechanisms
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 accurate and cost-effective uni-axial pressure sensing over long distances without requiring rotational alignment, maintaining stability against environmental influences by identifying and maximizing the mutual shift in reflection spectra of orthogonal polarization states.
Implementation Method 1
Fiber Bragg gratings are used for pressure sensing where the strain in the material is translated to a wavelength shift of a Bragg reflection
Implementation Method 2
A single fiber can incorporate multiple sensors realized with Bragg gratings that reflect at different wavelengths and optionally multiple Bragg gratings reflecting at the same wavelength are incorporated in the fiber and differentiated by the 'time of flight' of the reflections induced by these gratings when illuminated by an optical light pulse at the input to the fiber
Implementation Method 3
Uni-axial pressure sensors rely on the bi-refringence of glass optical fibers under uni-axial pressure. This means that the wavelength shift of Bragg reflectors incorporated in such fibers due to pressure applied differs for polarizations aligned with the pressure axis or orthogonal to the pressure axis
Data Source
AI summary
A pressure sensor includes a single-mode optical fiber and a fiber Bragg grating coupled to the single model fiber. The sensor detects a wavelength shift of a reflection spectrum of the Bragg grating for input light signals to the single mode fiber at different polarization states. A pressure measurement is obtained by determining at least a pair of mutually orthogonal polarization states for the input light at which the wavelength shift of a reflection spectrum of the Bragg grating is a maximum.