Fibre Optic Sensing With Baseband Filtering for Phase Accuracy
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Solution Overview
Problem
Distributed fibre optic sensing systems face challenges with low carrier-to-noise ratios in signals received from distant portions of long sensing fibres, leading to errors in demodulation, particularly 2π phase errors.
Innovation Solution
The system employs multiple pulses of coherent optical radiation at different frequencies, mixed with a local oscillator to generate separate carrier signals, and applies time-domain low-pass filtering to improve signal quality before converting to phase signals, reducing errors and increasing the effective range of sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If distributed fibre optic sensing is used to monitor long sensing fibres, then the sensing range is extended, but the carrier-to-noise ratio deteriorates leading to demodulation errors
Solution Approach 1:
The sensing fibre is divided into multiple discrete sensing portions or gauge sections along its length. Each portion is independently interrogated and processed, allowing the system to maintain measurement precision across long distances by treating distant segments with appropriate signal processing tailored to their specific carrier-to-noise conditions
Solution Approach 2:
The system dynamically adjusts processing parameters such as filter cut-off frequencies, integration times, and spatial averaging widths based on the carrier-to-noise ratio at different distances along the fibre. This adaptive parameter adjustment maintains phase detection accuracy despite the deteriorating signal quality over long sensing ranges
2Reliability
If multiple pulses with different frequencies are used, then the dynamic range is improved, but the signal processing complexity increases
Solution Approach 1:
Multiple optical pulses are transmitted periodically with different frequencies in a systematic sequence. This periodic multi-frequency interrogation allows the system to sample the fibre at different carrier frequencies, improving dynamic range and reliability through diversity while maintaining a structured processing approach that manages complexity
Solution Approach 2:
The patent introduces intermediate processing stages including frequency-specific filtering, down-conversion to baseband, and staged demodulation. These intermediary processing steps break down the complex multi-frequency signal into manageable components that can be processed sequentially, reducing overall system complexity while maintaining the benefits of multi-frequency operation
3Measurement precision
If time-domain low-pass filtering is applied, then the carrier-to-noise ratio is improved, but the bandwidth is reduced
Solution Approach 1:
The filter cut-off frequency is dynamically adjusted based on the specific sensing requirements and distance along the fibre. For distant portions with low carrier-to-noise ratios, a lower cut-off frequency provides greater noise rejection. For closer portions or high-frequency events, the cut-off frequency is increased to preserve bandwidth, optimizing the trade-off between signal quality and response speed
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 enhances the dynamic range and reduces errors in phase detection, allowing reliable sensing over longer fibre lengths by improving the carrier-to-noise ratio and enabling accurate tracking of phase changes due to environmental stimuli.
Implementation Method 1
One class of distributed fibre optic sensing is based on interrogating the sensing fibre with coherent optical radiation and detecting any of the interrogating radiation which has been Rayleigh backscattered from within the sensing fibre
Implementation Method 2
The backscatter from the sensing fibre is mixed with a local oscillator at a frequency different to that of each of the interrogation pulses, so as to form a signal component at a respective carrier frequency for each of the interrogating pulses
Implementation Method 3
A photodetector is used to detect the mixed backscatter/local oscillator signal at a relatively high sample rate
Data Source
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AI summary
This application relates to methods and apparatus for distributed fibre optic sensing. The apparatus includes an optical arrangement (103, 104, 105) configured to generate a local oscillator signal (LO) and also to repeatedly interrogate a sensing optical fibre (102). Each interrogation comprises at least one pulse of coherent optical radiation at a launch frequency which differs from that of the local oscillator signal by a carrier frequency. A mixer 9107) mixes backscatter signal from the sensing fibre with the local oscillator signal; and a detector arrangement (108X, 108Y) provides at least one corresponding digital detector output signal (DX, DY) A processor (110) processes each digital detector output signal to down-convert (202) the digital detector output signal to a respective baseband signal based for each carrier frequency and convert (206) each baseband signal to a phase signal. The processor is operable to apply time-domain low pass filtering (301) to each baseband signal with a cut-off frequency which is less than half the interrogation rate.