Fibre Optic Distributed Sensing Baseband Noise
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Solution Overview
Problem
Fibre optic distributed acoustic sensors face challenges in distinguishing phase modulations due to acoustic stimuli from baseband noise, particularly due to random scattering site distribution and thermal drift, which degrades signal-to-noise ratio and requires high-frequency carrier signals and fast detection rates, leading to increased complexity and reduced sensitivity.
Innovation Solution
A distributed fibre optic sensor apparatus generates pulse pairs with controlled relative phase relationships, allowing for the extraction of phase modulations within the baseband noise by using a processing circuitry to determine phase values from backscattered radiation, eliminating the need for low-pass filtering and reducing the required detection bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high carrier frequencies are used to mitigate cross-over between measurement signal and baseband noise, then signal-to-noise ratio is improved, but detector bandwidth requirements increase and sensitivity is reduced
Solution Approach 1:
The patent changes the fundamental parameter of signal generation by using frequency-modulated pulse pairs instead of high carrier frequency continuous waves. The frequency difference between pulses creates a beat signal at baseband frequencies, eliminating the need for high carrier frequencies while maintaining signal-to-noise ratio through coherent detection of the modulated signal envelope.
Solution Approach 2:
The patent employs periodic pulse pair transmission with controlled frequency differences. By transmitting pairs of pulses at slightly different frequencies and detecting the resulting beat signal, the system creates a periodic modulation that can be detected at baseband frequencies, avoiding the sensitivity losses associated with high-frequency continuous wave detection.
2Measurement precision
If high carrier frequencies are used to distinguish measurement signal from baseband noise, then signal-to-noise ratio is improved, but detector sample rate requirements increase device complexity
Solution Approach 1:
The patent fundamentally changes the frequency parameter regime by using frequency-modulated pulse pairs that generate beat signals at baseband frequencies. This parameter change allows detection at much lower sample rates since the modulated signal envelope varies slowly compared to high carrier frequencies, directly reducing device complexity while maintaining measurement precision.
Solution Approach 2:
The patent replaces high-speed electronic detection mechanisms with a signal processing approach that operates at baseband frequencies. By converting the high-frequency optical modulation into a low-frequency electrical beat signal through coherent detection, the system substitutes fast electronic sampling with slower, more manageable signal processing operations.
3Adaptability or versatility
If pulse pairs with frequency difference related to pulse width are used, then acoustic signal detection is enabled, but baseband structure masks carrier signal and reduces signal-to-noise ratio
Solution Approach 1:
The patent applies preliminary frequency modulation to the pulse pairs before transmission. By pre-modulating the pulses with a known frequency difference, the system creates a coherent beat signal that remains distinguishable from baseband noise. This preliminary action ensures that the carrier information is embedded in a way that prevents masking by random baseband structures.
Solution Approach 2:
The patent employs feedback through coherent detection, where the detected beat signal is processed by comparing it with the expected modulation pattern. This feedback mechanism allows the system to distinguish the coherent acoustic signal from random baseband noise by analyzing the phase and frequency consistency of the modulated signal over time.
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
This approach enables the detection of phase modulations caused by acoustic stimuli with improved signal-to-noise ratio and sensitivity, allowing for accurate monitoring of acoustic, strain, and temperature changes without the need for high-frequency carrier signals and fast detection rates.
Implementation Method 1
measure changes to the radiation resulting from acoustic waves affecting the optical fibre
Implementation Method 2
responsive to stimuli inducing phase modulations within said fibre
Implementation Method 3
a detector configured to detect any radiation backscattered from said pulse pairs
Implementation Method 4
processing circuitry configured to determine a phase value for at least one given section of optical fibre based on the detected backscatter radiation
Data Source
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AI summary
The application describes methods and apparatus for distributed fibre sensing, especially distributed acoustic/strain sensing. The method involves launching at least first and second pulse pairs into an optical fibre, the first and second pulse pairs having the same frequency configuration as one another and being generated such that the phase relationship of the pulses of the first pulse pair has a predetermined relative phase difference to the phase relationship of the pulses of the second pulse pair. In one embodiment there is a frequency difference between the pulses in a pulse pair which is related to the launch rate of the pulse pairs. In another embodiment the phase difference between the pulses in a pair is varied between successive launches. In this way an analytic version of the backscatter interference signal can be generated within the baseband of the sensor.