Partially Coherent Optical Pulses for Disturbance Detection
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Solution Overview
Problem
Existing methods for detecting disturbances in optical fibers, such as those using coherent light, struggle to determine the location and direction of external influences like temperature and pressure changes with sufficient resolution and sensitivity, due to limitations in pulse length and power, which affects the accuracy of refractive index measurements.
Innovation Solution
The method employs partially coherent optical pulses launched into polarisation maintaining fibers, detecting temporal speckle patterns to determine location and direction information of disturbances by comparing changes in refractive index along the fiber, using two polarisation eigenmodes to provide radial direction information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the pulse length is reduced to increase spatial resolution, then the spatial resolution is improved, but the optical power in the pulse is reduced
Solution Approach 1:
The patent changes the coherence parameter of the light source, using partially coherent light instead of fully coherent light. This allows using longer pulse durations (lower temporal coherence) while maintaining the ability to detect disturbances, thereby resolving the contradiction between pulse length and optical power for achieving spatial resolution.
2Measurement precision
If a very coherent light source is used to detect disturbances, then the sensitivity of disturbance detection is improved, but the amount of optical power that can be launched into the fibre is severely limited
Solution Approach 1:
The patent modifies the coherence parameter by using partially coherent light sources with broader spectral widths. This parameter change allows launching higher optical power into the fibre while still maintaining sufficient sensitivity for disturbance detection, overcoming the severe power limitation imposed by highly coherent light sources.
Solution Approach 2:
The patent converts the previously harmful effect of Brillouin scattering (which limits coherent light power) into a beneficial approach by using partially coherent light with broader spectral widths that avoid the narrow bandwidth Brillouin scattering threshold, thereby enabling higher power operation.
3Power
If the spectral width of the light source is increased to avoid Brillouin scattering, then the optical power that can be launched is increased, but the spatial resolution may be affected
Solution Approach 1:
The patent optimizes the spectral width parameter to a specific range that balances two competing requirements: broad enough to avoid Brillouin scattering and enable high power operation, but narrow enough to maintain sufficient spatial resolution for disturbance location. This parameter optimization resolves the contradiction between power and resolution.
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 enhances the spatial resolution and sensitivity of disturbance detection, allowing for precise location and direction determination of disturbances along the fiber, overcoming limitations of previous techniques by minimizing Brillouin scattering and maintaining sufficient optical power.
Implementation Method 1
Light that is backscattered or reflected within the fibre returns back up the fibre and is detected by a photodetector at the point of injection of the optical pulses
Implementation Method 2
The coherence allows components of the backscattered light to interfere and contribute an intensity variation at the photodetector
Implementation Method 3
External influences or disturbances such as temperature and pressure or the presence of acoustic waves can cause changes in the refractive index of the optical fibre. These changes in refractive index result in a change to the speed of the light pulse and backscattered light along the fibre
Implementation Method 4
launching optical pulses into at least one polarisation eigenmode of a polarisation maintaining fibre
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
A fibre optic sensing method and apparatus for determining location and direction information of disturbances occurring in the environment of a sensor optical fibre are provided. The method comprises launching optical pulses into at least one polarisation eigenmode of a polarisation maintaining fibre as the sensor optical fibre, detecting temporal speckle patterns of light backscattered from the at least one polarisation eigenmode of the fibre, comparing the temporal speckle patterns to determine the location and direction information of a disturbance in the environment of the sensor optical fibre. The location information may be a distance along the fibre, and the direction information may be a direction radially from the axis of the fibre. The apparatus or instrument may be used to detect disturbance over long distances such as pipes, pipelines, or wells. Other applications include detecting intruders entering a controlled area.