Fabry-Pérot Cavity Displacement Tracking Beyond Phase Ambiguity
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Solution Overview
Problem
Existing optical fiber-based Fabry-Perot cavity (FPC) sensors face limitations in measuring large displacements with high precision due to phase ambiguity in phase tracking methods and increased complexity in spectral transforms, while period tracking methods offer lower precision and are susceptible to errors.
Innovation Solution
A hybrid predictor-corrector scheme that combines period tracking for coarse estimation with phase tracking to identify correct peaks, using a median filter to enhance precision and overcome range limitations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase tracking methods are used to achieve nanometer-scale precision, then measurement precision is improved, but the measurement range is limited to ±15 μm due to phase ambiguity
Solution Approach 1:
The patent merges period tracking and phase tracking methods into a hybrid approach. Period tracking provides coarse estimation of peak locations to resolve phase ambiguity and extend measurement range, while phase tracking provides fine precision for nanometer-scale measurements. The combination allows measurements up to 24 μm displacement while maintaining nanometer precision.
Solution Approach 2:
The patent applies preliminary period tracking to predict peak locations before performing phase tracking. This preliminary action establishes the correct peak identification context, preventing phase unwrapping errors and enabling accurate measurement over extended ranges while maintaining high precision.
2Measurement precision
If spectral transforms (FFT, cepstral analysis, wavelet transforms) are used to achieve nanometer-scale precision, then measurement precision is improved, but processing complexity increases
Solution Approach 1:
The patent extracts and utilizes only the essential spectral information (peak locations and spacings) from the interference spectrum, rather than applying complex full-spectrum transforms. This extraction approach achieves nanometer precision through phase tracking of identified peaks while avoiding the computational complexity of FFT, cepstral analysis, or wavelet transforms.
3Length of moving object
If period tracking methods are used to extend measurement range, then measurement range is improved, but measurement precision decreases
Solution Approach 1:
The patent segments the measurement process into two distinct stages: coarse period tracking for range extension and fine phase tracking for precision. This segmentation allows each method to operate in its optimal performance regime, with period tracking handling large displacements and phase tracking providing nanometer precision, ultimately combining both advantages.
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
The hybrid method achieves high precision (nanometer-scale) measurements of up to 24 μm displacements, significantly improving on conventional methods by maintaining accuracy and reducing errors, particularly through the use of a median time-domain filter.
Implementation Method 1
The interrogator apparatus interrogates the Fabry-Perot cavity with light, produces a periodic spectral interference pattern from the light that interrogated the Fabry-Perot cavity
Data Source
AI summary
A system and method for monitoring Fabry-Perot cavity (“FPC”) displacement implementing a predictor-corrector scheme. The system includes an optical interrogator apparatus and a data processing apparatus. The optical interrogator apparatus interrogates the FPC, obtains a spectral interference pattern and outputs a corresponding signal including data associated with a plurality of peaks. The data processing apparatus processes the output signal to produce a prediction for a peak location based on the data associated with the plurality of peaks, and then uses the prediction to identify as correct one of the plurality of peaks. The data processing apparatus then determines and outputs a plurality of FPC length variations. In one embodiment, the data processing system implements a period tracking algorithm to produce the prediction based on the data associated with the plurality of peaks, and uses a phase tracking algorithm to determine an FPC length variation using the identified peak.