Fibre Optic Sensing With Multi-Frequency Pulse Demodulation
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Solution Overview
Problem
Distributed fibre optic sensing systems face challenges with low carrier-to-noise ratio in signals from distant portions of long sensing fibres, leading to errors in demodulation, particularly 2π phase errors.
Innovation Solution
The system employs two pulses of coherent optical radiation at different frequencies, with each pulse having a distinct carrier frequency, and processes the backscatter signals using a local oscillator for amplification and independent demodulation. The processor applies time-domain low-pass filtering to the baseband signals with a cut-off frequency lower than half the interrogation rate to improve the carrier-to-noise ratio.
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 for distant portions leading to demodulation errors
Solution Approach 1:
The system uses periodic pulse interrogation of the sensing fibre with multiple pulses per interrogation cycle. Each pulse is separated by a time delay and has a different optical frequency, allowing repeated sampling of the same fibre section at different time instances. This periodic action enables coherent integration of multiple backscatter signals, improving the carrier-to-noise ratio for distant fibre portions while maintaining extended sensing range.
Solution Approach 2:
The system applies preliminary frequency shifting to each interrogation pulse using different optical frequencies before the pulses enter the fibre. This preliminary action creates distinct carrier frequencies in the backscatter signals that can be independently demodulated, allowing coherent integration across multiple pulses and improving signal quality for distant sensing portions before demodulation occurs.
2Adaptability or versatility
If multiple pulses with different frequencies are used for interrogation, then the dynamic range is improved, but the device complexity increases due to independent demodulation requirements
Solution Approach 1:
The system segments the demodulation process by assigning different carrier frequencies to different pulses within each interrogation cycle. Each pulse's backscatter signal is demodulated independently at its specific carrier frequency, allowing parallel processing of multiple signals. This segmentation enables the system to handle large dynamic ranges by processing multiple frequency components separately while using efficient digital signal processing techniques.
Solution Approach 2:
The system changes the optical frequency parameter of each interrogation pulse to create distinct carrier frequencies. By varying this physical parameter (frequency) across multiple pulses, the system enables independent demodulation channels that can be processed in parallel, improving dynamic range while managing complexity through standardized frequency separation and demodulation algorithms.
3Power
If the backscatter signal is mixed with a local oscillator, then the signal amplification is achieved, but the polarisation fading issues arise
Solution Approach 1:
The system uses periodic pulse interrogation where multiple pulses are sent through the fibre at different time instances within each interrogation cycle. By coherently integrating the backscatter signals from multiple pulses, the system achieves signal amplification through constructive interference while averaging out polarisation fading effects that occur randomly between pulses, thereby improving both signal power and polarisation stability.
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 demodulation, allowing longer sensing ranges with improved signal quality and accuracy.
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
As the interrogating radiation is coherent, the backscatter from the different scattering sites will interfere to provide an overall interference signal, which depends on the distribution of the scattering sites within the fibre
Implementation Method 3
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 4
A photodetector is used to detect the mixed backscatter/local oscillator signal at a relatively high sample rate
Data Source
AI summary
This application relates to methods and apparatus for distributed fibre optic sensing. The apparatus includes an optical arrangement configured to generate a local oscillator signal (LO) and also to repeatedly interrogate a sensing optical fibre. 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 mixes backscatter signal from the sensing fibre with the local oscillator signal; and a detector arrangement provides at least one corresponding digital detector output signal (DX, DY). A processor processes each digital detector output signal to down-convert the digital detector output signal to a respective baseband signal based for each carrier frequency and convert each baseband signal to a phase signal. The processor is operable to apply time-domain low pass filtering to each baseband signal with a cut-off frequency, which is less than half the interrogation rate.