Asymmetric Side-Hole Optical Waveguide for Distributed Pressure Sensing
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Solution Overview
Problem
Conventional optical fibers are impractical for pressure measurement in fluids due to their insensitivity to external pressure changes, and existing side-hole fibers with FBGs or rocking filters are limited by their restricted measurement locations, complexity, and high fabrication costs.
Innovation Solution
A single mode optical waveguide that deforms asymmetrically under fluid pressure, using polarized light and Polarization Optical Time Domain Reflectometry (POTDR) to detect birefringence changes along its entire length, allowing for precise pressure distribution measurement without the need for multiple sensors or gratings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional optical fibres are used for pressure measurement, then the device is simple and inexpensive, but the measurement precision is insufficient due to insensitivity to external pressure changes
Solution Approach 1:
The patent applies asymmetry by using a side-hole optical fibre with a non-circular cross-section containing air holes. This asymmetric structure causes the fibre core to experience differential strain when external pressure is applied, significantly enhancing the birefringence change and thus the sensitivity to pressure measurement compared to conventional circular symmetric fibres.
2Measurement precision
If side-hole fibres with FBGs or rocking filters are used, then the measurement precision is improved, but the device complexity and fabrication cost increase
Solution Approach 1:
The patent extracts and removes the complex FBG or rocking filter components from the side-hole fibre structure. Instead of using these additional elements, the invention relies solely on the inherent birefringence properties of the asymmetric side-hole fibre itself to enable pressure measurement, thereby simplifying the overall device structure while maintaining measurement precision.
3Measurement precision
If side-hole fibres with FBGs or rocking filters are used, then the measurement precision is improved, but the fabrication cost increases
Solution Approach 1:
The patent eliminates the need for expensive FBG writing or rocking filter fabrication processes. By using the natural birefringence of the side-hole fibre structure itself, the invention removes these complex and costly manufacturing steps, making the sensor more cost-effective while maintaining high measurement precision.
4Measurement precision
If side-hole fibres with FBGs or rocking filters are used, then the measurement precision is improved, but the measurement is restricted to specific locations along the fibre
Solution Approach 1:
The patent makes the entire length of the side-hole fibre functional for pressure measurement by utilizing its inherent birefringence properties along the whole fibre. This eliminates the need for specific FBG or rocking filter locations, allowing pressure measurement at any position along the fibre and enabling versatile applications such as distributed pressure sensing and tsunami detection.
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 measurement of pressure distribution along the entire length of the waveguide, improving sensitivity and reducing fabrication complexity, with the ability to detect both spatial and temporal pressure variations.
Implementation Method 1
When external pressure is applied to an optical fibre, the core of the fibre experiences strain which varies with the applied pressure. Due to a phenomenon known as the 'elasto-optic effect', the way in which light propagates in the fibre's core changes as the strain experienced by the core changes.
Implementation Method 2
The presence of the air holes means that the core of the fibre experiences less stress in a direction transverse to its length that extends between the air holes than in other directions transverse to its length when isotropic external pressure is applied to the fibre. The core therefore experiences greater strain in this direction than others. One consequence of the asymmetrical strain experienced by side-hole fibres is that the refractive index of the fibre's core changes more for light linearly polarised in the direction of greatest strain than for light linearly polarised in the orthogonal direction. In other words, the fibre's birefringence changes.
Data Source
AI summary
A pressure sensing apparatus has a light source for transmitting pulses of light along a monomode optical fiber. The polarization of light backscattered from the light pulses in the optical fiber is detected by a polarization processing unit (PPU) and a photo detector. The optical fiber is adapted to deform asymmetrically under the influence of applied external isotropic pressure, e.g. from a fluid. The deformation causes the birefringence of the optical fiber to change proportionally to the applied pressure. The change in birefringence can be determined from the detected polarization of the backscattered light, allowing detection of pressure distribution in the fluid. Importantly, the construction of the optical fiber is such that the birefringence beat length of the optical fiber at the wavelength of light propagated by the fiber remains more than twice the spatial length of the light pulses transmitted along the optical fiber. Applications of the invention include detection of fluid flow; location of an interface between two fluids of different density; and tsunami detection.