Optical Fiber Strain Sign Determination via Frequency Swept Rayleigh Scattering
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Solution Overview
Problem
Current distributed fiber optic sensing techniques, such as DVS, struggle to accurately determine the correct sign of strain changes in structural monitoring applications, particularly in bridges, due to random scatter distribution and complex homodyne/heterodyne coherence detection methods, which are either ineffective or require costly and complex equipment.
Innovation Solution
A method involving a continuous wave laser with a monotonically changing supply current to generate coherent optical pulses, where the optical frequency changes strictly, allowing for the determination of strain changes with correct sign by analyzing power spectra backscattered from Rayleigh scatterers, using a device with a modulator and detection unit for spatially resolved measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If homodyne or heterodyne coherence detection methods are used to enable quantitative strain measurement, then measurement precision is improved, but device complexity increases due to narrow-linewidth lasers, additional modulators, GHz frequency sources, and polarization management requirements
Solution Approach 1:
The patent changes the detection parameter from phase-based coherence detection to frequency-based spectral analysis. By using a frequency-swept laser and analyzing the power spectrum of backscattered light at different frequencies, the system determines strain magnitude and sign without requiring complex interferometric setups. This parameter change from phase to frequency domain measurement resolves the contradiction by achieving quantitative strain measurement through a simpler spectral analysis approach.
2Device complexity
If single-pulse and single-wavelength direct detection approaches are used, then device complexity is reduced, but measurement precision deteriorates because only vibration detection is possible without quantitative strain measurement capability
Solution Approach 1:
The patent employs periodic frequency sweeping of the laser source, where the laser wavelength is modulated in a systematic sequence. By sweeping through multiple frequencies and measuring the backscattered power at each frequency point, the system builds a power spectrum that enables quantitative strain determination. This periodic frequency modulation transforms the simple direct detection approach into a capability for precise strain measurement while maintaining relatively simple device architecture.
3Measurement precision
If phase-resolved DVS techniques with dual-pulse approaches are used, then measurement precision is improved for quantitative strain changes, but device complexity increases due to additional interferometers, stabilization requirements, and polarization management
Solution Approach 1:
The patent extracts and utilizes only the power spectral information from the backscattered light, discarding the need for complex phase extraction and interferometric setups. By focusing solely on power measurements across a frequency sweep and analyzing spectral shifts, the system achieves quantitative strain measurement without requiring dual-pulse approaches, additional interferometers, or complex stabilization systems. This extraction of essential power spectral information eliminates unnecessary complexity.
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 determination of strain changes with correct sign, improving measurement precision and reducing complexity, suitable for structural monitoring and dynamic measurements with high spatial and temporal resolution.
Implementation Method 1
an optical fiber (60), which has Rayleigh scatterers in at least one section
Data Source
Figure 1~2
Figure 3A~5
Figure 6A~6B
AI summary
The invention relates to a method for determining, with the correct sign, a change in a physical parameter, which method comprises coupling a first pulse sequence into an optical fiber (60), which has Rayleigh scatterers at least in one section, the first pulse sequence having a plurality of optical pulses of identical pulse duration, which are each at least substantially coherent in the optical fiber, and the optical frequency changing monotonically in the first pulse sequence. In order to determine a first power spectrum, the power of a signal component backscattered by the Rayleigh scatterers is measured for each of the optical pulses of the first pulse sequence. A second pulse sequence, which corresponds at least substantially to the first pulse sequence, to a permutation of the first pulse sequence, to a part of the first pulse sequence or to a permutation of the part of the first pulse sequence and/or the optical frequency of which changes strictly monotonically in the second pulse sequence, is coupled into the optical fiber. In order to determine a second power spectrum, the power of a signal component backscattered by the Rayleigh scatterers is measured for each of the optical pulses of the second pulse sequence. In order to determine, with the correct sign, a change in a physical parameter of the optical fiber, an offset between the second power spectrum and the first power spectrum is determined. While each pulse sequence is coupled in, a feed current and/or a temperature of a continuous-wave laser (10) coupled to the optical fiber (60) is changed strictly monotonically.