Coherent Detection for Brillouin Scattering Strain and Temperature Separation
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Solution Overview
Problem
Conventional optical fiber strain and temperature measurement systems using Brillouin scattered light face challenges in achieving sufficient signal-to-noise ratio and separating strain and temperature changes due to the superimposition of phase differences and intensity changes in self-delayed heterodyne BOTDR systems.
Innovation Solution
An optical fiber strain and temperature measurement apparatus and method that utilize coherent detection to measure frequency changes as phase differences, incorporating a light source, splitting unit, optical frequency shifter, delay unit, multiplexer, coherent detection unit, and signal processing unit to separate strain and temperature changes by solving simultaneous equations using frequency shift and intensity information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If heterodyne detection is used to increase light receiving sensitivity, then the minimum light receiving sensitivity is improved, but the measurement system becomes more complex and requires separate reference light preparation
Solution Approach 1:
The patent applies self-service by using the Brillouin scattered light itself as the reference light for heterodyne detection, eliminating the need for separate reference light preparation. The scattered light is split into measurement light and reference light paths, where the reference light path processes the scattered light to serve as the heterodyne reference, making the system self-sufficient and reducing external component requirements.
Solution Approach 2:
The patent implements multi-functionality by making the Brillouin scattered light serve dual purposes: both as the measurement signal carrying strain/temperature information and as the reference light for heterodyne detection. This universal use of the scattered light reduces the need for separate reference light sources and simplifies the overall system architecture.
2Productivity
If self-delayed heterodyne BOTDR is used to measure frequency changes as phase differences, then measurement speed is improved, but strain and temperature changes cannot be separated due to superimposition of phase differences and intensity changes
Solution Approach 1:
The patent applies segmentation by separating the measurement into distinct phases: first measuring the phase difference to obtain frequency shift information, then measuring the intensity ratio to obtain Brillouin scattering coefficient changes. This segmentation allows the subsequent separation of strain and temperature effects by solving simultaneous equations with the two independent measurement sets, resolving the superimposition problem while maintaining fast measurement speed.
Solution Approach 2:
The patent introduces an intermediary measurement approach by using the intensity ratio of scattered light to Brillouin scattering coefficient changes as a mediator variable. This intermediary measurement, combined with phase difference measurements, enables the decoupling of strain and temperature effects through mathematical relationships, allowing separate determination of both parameters without direct interference.
3Measurement precision
If conventional BOTDR measures Brillouin frequency shift to determine strain and temperature, then measurement capability is achieved, but the intensity of scattered light is too low by two through three orders of magnitude compared to Rayleigh scattering
Solution Approach 1:
The patent applies self-service by using the weak Brillouin scattered light itself as the reference for heterodyne detection, maximizing the utilization of the available signal. By processing the scattered light through optical filters and using it in both measurement and reference paths, the system extracts maximum information from the limited signal without requiring external amplification or stronger light sources.
Solution Approach 2:
The patent replaces direct intensity measurement with heterodyne detection, substituting a mechanical/intensity-based measurement approach with an interference-based optical measurement. This substitution amplifies the weak scattered light signal through constructive interference with the reference light, converting the measurement from direct intensity detection to phase-sensitive heterodyne detection, thereby overcoming the low intensity limitation.
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 the separate measurement of strain and temperature changes, reducing measurement time and improving the accuracy of optical fiber strain and temperature monitoring by decoupling the effects of Brillouin scattering coefficient changes.
Implementation Method 1
An optical frequency shifter unit gives a frequency shift of beat frequency to light propagating through any one of the two branches
Implementation Method 2
The coherent detection unit performs heterodyne detection on the multiplexed light to output a difference frequency as a first electrical signal
Implementation Method 3
Brillouin backscattered light, which arise in the optical fiber under test owing to the probe light
Implementation Method 4
A delay unit is provided in any one of the first light path and the second light path and gives a delay between light propagating through the first light path and the second light path
Data Source
AI summary
The light source unit generates probe light. The splitting unit splits Brillouin backscattered light, which arise in the optical fiber under test owing to the probe light, into two branches of a first light path and a second light path. The delay unit gives a delay between light propagating through the first light path and the second light path. The multiplexer unit multiplexes light propagating through the first light path and the second light path to generate multiplexed light. The coherent detection unit performs heterodyne detection on the multiplexed light to output a difference frequency as a first electrical signal. The frequency shift amount obtaining unit performs homodyne detection on one of the two branches split from the first electrical signal and the second electrical signal having the same frequency as the frequency of the first electrical signal to obtain a frequency shift amount. The signal intensity obtaining unit generates intensity information of the first electrical signal as an intensity signal. The signal processing unit obtains strain δε and a temperature change δT separately from the frequency shift amount and the intensity.