Coherent Fiber Sensor LO Noise Suppression via Phasor Correction
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Solution Overview
Problem
Existing fiber optical sensor systems suffer from noise induced by the local oscillator, which degrades the signal-to-noise ratio and limits the measurement range and spatial resolution, particularly in coherent optical time domain reflectometry (c-OTDR) systems.
Innovation Solution
A method and system for measuring and correcting fluctuations in the local oscillator phasor by generating an interrogation signal, mixing it with reflected light, and applying distributed back-reflection processing to reduce the impact of local oscillator noise on the receiver output signal, using techniques such as pulse compression filtering and noise suppression filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If coherent optical mixing with local oscillator is used to optimize signal-to-noise ratio, then receiver sensitivity is improved, but local oscillator instability introduces phase noise that degrades measurement precision
Solution Approach 1:
The patent implements feedback by continuously monitoring the local oscillator phase using an auxiliary interferometer and using this information to compensate for phase fluctuations in the main measurement signal. The system measures the local oscillator phase noise and applies corrective transformation to the received signal, creating a closed-loop feedback mechanism that actively suppresses the harmful phase noise while preserving the measurement precision
Solution Approach 2:
The patent introduces an auxiliary interferometer as an intermediary system that separately measures the local oscillator phase fluctuations. This intermediary measurement channel allows the system to characterize and compensate for the local oscillator instability without directly affecting the main sensing measurement, effectively isolating and correcting the harmful phase noise
2Productivity
If frequency swept signal is used to increase duty-cycle to 100%, then measurement efficiency is improved, but source induced noise becomes dominant particularly at short ranges
Solution Approach 1:
The system uses feedback by measuring the actual local oscillator phase fluctuations during the frequency swept interrogation and applying real-time compensation to the received signal. This allows the system to maintain 100% duty-cycle operation while actively suppressing the source induced noise that would otherwise dominate at short ranges
Solution Approach 2:
The patent converts the harmful source induced noise into a measurable parameter by using the auxiliary interferometer to characterize the local oscillator phase fluctuations. This measured noise characteristic is then used to create a correction that transforms the previously harmful effect into a compensatable parameter, improving the overall signal quality
3Measurement precision
If narrowband laser source is used to reduce source induced noise, then signal-to-noise ratio is improved, but frequency stability requirements become more stringent
Solution Approach 1:
The patent implements feedback monitoring of the local oscillator phase using a separate auxiliary interferometer. This feedback mechanism allows the system to detect and compensate for frequency instabilities in the narrowband laser, maintaining the signal-to-noise ratio benefits while reducing the stringent stability requirements through active correction
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
Improves the signal-to-noise ratio and spatial resolution by effectively suppressing local oscillator noise, enabling more accurate and extended measurements in fiber optical sensor systems.
Implementation Method 1
coherent optical mixing of the reflected signal by a local oscillator (LO) reference light
Implementation Method 2
naturally occurring and unavoidable density variations in the fiber gives rise to Rayleigh backscattering
Data Source
AI summary
A method for measuring a response from an optical fiber providing distributed back reflections using a system comprising an optical source comprising a laser, an optical receiver and a processing unit is disclosed. The method comprises generating an interrogation signal and an optical local oscillator using the optical source, the interrogation signal being represented by an interrogation phasor and the optical local oscillator being represented by a local oscillator phasor; transmitting the interrogation signal into the optical fiber; and mixing the optical local oscillator with reflected light from the optical fiber and detecting a mixing product with the optical receiver to achieve a receiver output signal. The method further comprises performing a measurement that characterizes fluctuations in the local oscillator phasor; processing the receiver output signal based on the measurement result to provide a corrected receiver output signal such that an effect of fluctuations in the local oscillator phasor on the corrected receiver output signal is reduced; and applying distributed back-reflection processing on the corrected receiver output signal. Finally, the method comprises extracting the response from the optical fiber from the distributed back-reflection processing output. A system for measuring a response from an optical fiber providing distributed back reflections is also disclosed.