Dual Laser Interferometer Absolute Distance Measurement
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Solution Overview
Problem
Existing interferometric measurement devices struggle to accurately determine absolute distance in repeatable measurements due to limitations in incremental methods and previous technologies, such as Fabry-Perot interferometers, which do not allow for precise absolute distance determination.
Innovation Solution
A device and method using a first tunable light source and a second light source with different wavelengths, modulated at distinct frequencies, coupled into a Fabry-Perot interferometer cavity, with a demodulator unit generating signals for computation of absolute distance through frequency sweeps, and a second tunable light source for correcting nonlinearities and enhancing accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If incremental methods are used for position tracking, then high positional resolution is achieved, but absolute distance determination in repeatable measurements is not possible
Solution Approach 1:
The patent uses a tunable laser source that changes wavelength (parameter change) to sweep through multiple fringes of the Fabry-Perot cavity. By monitoring the phase change during this wavelength sweep, the system can determine the absolute distance. This resolves the contradiction by transforming the incremental tracking approach into an absolute measurement approach through parameter variation.
2Device complexity
If a single wavelength is used in Fabry-Perot interferometer, then device complexity is reduced, but accurate absolute distance measurement is not achieved
Solution Approach 1:
The patent employs periodic wavelength sweeping of a single tunable laser source through the Fabry-Perot cavity. This periodic action allows the system to accumulate phase information over multiple fringes, enabling accurate absolute distance measurement while maintaining device simplicity. The periodic sweeping replaces the need for multiple fixed-wavelength sources.
3Length of stationary object
If frequency sweeping is performed over large interval, then absolute distance range is increased, but measurement precision deteriorates due to cavity drifts and nonlinearities
Solution Approach 1:
The patent uses feedback mechanisms to monitor and compensate for cavity drifts during the wavelength sweeping process. By continuously tracking the cavity resonance conditions and adjusting the measurement accordingly, the system maintains high precision even when sweeping over large frequency intervals. This feedback approach resolves the contradiction between measurement range and precision.
4Measurement precision
If tunable light source is used for frequency sweeping, then absolute distance measurement capability is enabled, but device complexity and cost increase
Solution Approach 1:
The patent makes the single laser source perform multiple functions: it serves as both the probe light source and the tuning mechanism for absolute distance measurement. The same tunable laser that provides the measurement beam also, through its wavelength sweeping capability, enables the determination of absolute distance by counting fringes. This multi-functionality reduces the need for separate components.
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 repeatable absolute distance measurement without the need for reference lengths or interferometers, providing a simple, robust, and cost-effective solution by using frequency division multiplexing and tunable light sources to correct for phase nonlinearities and cavity drifts.
Implementation Method 1
an interferometer detector is coupled to the Fabry-Perot interferometer cavity and configured to provide an interference measurement signal based on detected interference patterns of interferometric first and second wavelength light generated by the Fabry-Perot interferometer cavity
Implementation Method 2
a first wavelength light emitted from the first tunable light source is modulated by a first modulating frequency, and a second wavelength light emitted from the second light source is modulated by a second modulating frequency
Data Source
Figure 1~2
Figure 3~4B
Figure 5A~6
AI summary
A device for absolute distance measurement comprises a first tunable light source for emitting a first wavelength light of a first tunable frequency modulated by a first modulating frequency and a second light source for emitting a second wavelength light of a second frequency modulated by a second modulating frequency. An optical coupler couples the first wavelength light and the second wavelength light into a Fabry-Pérot interferometer cavity. An interferometer detector provides an interference measurement signal based on a detected interference pattern. A demodulator unit generates a first demodulation signal based on the interference measurement signal by demodulation with the first modulating frequency and a second demodulation signal based on the interference measurement signal by demodulation with the second modulating frequency. A computation unit computes an absolute distance by evaluating the first demodulation signal acquired during a sweep of the first tunable frequency and the second demodulation signal.