Fibre-Optic Interferometer Parasitic Reflection Elimination
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Solution Overview
Problem
Fibre-optic interferometers using single-frequency sources are plagued by parasitic reflections, which generate significant noise due to the long coherence length of these sources, interfering with the main signal and causing disturbances in measurements.
Innovation Solution
A fibre-optic interferometric system employing a single-frequency laser source that modulates the source beam at specific frequencies and wavelengths to eliminate parasitic reflections, using techniques such as power modulation and wavelength tuning, combined with phase modulation and electronic filtering to isolate the main signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single-frequency laser source is used in a fibre-optic interferometer, then the coherence length is extended and measurement sensitivity is improved, but parasitic reflections generate significant noise that disturbs measurements
Solution Approach 1:
The patent applies periodic action by modulating the laser source at a specific frequency (the natural frequency of the optical fibre coil) and using periodic sampling of the interferometric signal. This periodic modulation creates a time-varying interference pattern that allows the main signal to be distinguished from parasitic reflections through synchronous detection and demodulation processes, thereby eliminating the harmful noise while preserving measurement sensitivity.
Solution Approach 2:
The patent employs feedback mechanisms through the modulation of the laser source at the natural frequency of the coil, which creates a feedback loop where the modulated signal interacts with the interferometric output. This feedback approach enables the system to reject parasitic reflections through phase-sensitive detection, as the reflected signals do not satisfy the same frequency and phase conditions as the main interferometric signal.
2Object-affected harmful factors
If a broadband source is used in a fibre-optic interferometer, then parasitic reflections and Rayleigh retro-reflection are limited, but the coherence length is reduced and measurement sensitivity decreases
Solution Approach 1:
The patent resolves this contradiction by implementing periodic modulation of a single-frequency source at the natural frequency of the optical fibre coil. This periodic action creates a time-varying interference pattern that enables sophisticated signal processing techniques (modulation and demodulation) to distinguish the main signal from parasitic reflections, thereby achieving both high coherence length for sensitivity and effective rejection of harmful reflections through frequency-selective detection.
3Object-affected harmful factors
If the laser source is modulated at the natural frequency of the optical fibre coil, then parasitic reflections are eliminated, but the device complexity increases due to modulation and demodulation systems
Solution Approach 1:
The patent applies periodic action through modulation of the laser source at the natural frequency of the optical fibre coil. This modulation approach, combined with synchronous demodulation of the interferometric signal, enables the system to reject parasitic reflections effectively. While this introduces modulation and demodulation circuitry, the use of a fixed natural frequency allows for simplified implementation compared to more complex adaptive systems.
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 system effectively eliminates parasitic reflection effects, reducing noise and improving measurement accuracy and stability in fibre-optic interferometers.
Implementation Method 1
a laser source (8) configured to emit a source beam (10) comprising at most 4 longitudinal modes at each time instant t
Implementation Method 2
the optical coupler-splitter (3) being able to inject each secondary beam (11, 12) at one end of the optical fibre coil (4) in such a way that the two secondary beams (11, 12) propagate in mutually opposed directions in the optical fibre coil (4)
Implementation Method 3
the optical coupler-splitter (3) being able to recombine the two secondary beams (11, 12) at the exit of the optical fibre coil (4) to form an interference beam (15)
Implementation Method 4
the source splitter (2) being able to transmit the interference beam (15) towards the photodetector (5)
Data Source
AI summary
This relates to an interferometer including a light generator, a fibre-optic coil, an optical coupler/splitter, a photodetector and an electronic signal-processing system. The light generator includes a laser source able to emit a source beam split into two secondary beams passing through the coil with a travel time τ defining a natural frequencyfp=12τ.According to the invention, the laser source is a monofrequency laser source and the light generator includes modulation means designed to modulate the source beam at a modulation frequency equal to2fp2n+1where n is an integer greater than or equal to 1, and the photodetector and the electronic signal-processing system are configured to acquire and process a signal representative of the interference beam at a demodulation frequency equal tofp2n+1.