Multi-Carrier Coherent Coded DAS for Fading Mitigation
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Solution Overview
Problem
Coherent fading limits the distance over which distributed acoustic/vibration sensing (DAS/DVS) systems can effectively detect fiber events in optical fibers, due to coherent fading effects that result in weak backscatter signals and unreliable phase estimates, especially in long fiber sections used in telecom transmissions.
Innovation Solution
The implementation of a purely-digital spectral-diversity coherent differential-phase optical time-domain reflectometry (OTDR) system with polarization diversity and multi-carrier coded probing sequences, which introduces frequency diversity to overcome coherent fading by using binary-phase shift keying (BPSK) or quadrature-phase shift keying (QPSK) codes for X and Y polarizations, enabling more reliable detection of fiber events along the optical fiber.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If coherent detection is used to improve sensitivity and linearity in DAS/DVS systems, then measurement precision is improved, but coherent fading occurs causing signal strength to deteriorate
Solution Approach 1:
The patent applies parameter changes by modulating the laser source with multiple frequency chirps instead of a single frequency, transforming the optical signal parameters to achieve spectral diversity. This allows the system to overcome coherent fading by varying the frequency parameters across multiple sub-bands, ensuring reliable phase detection even when individual frequencies experience fading conditions
2Length of stationary object
If multi-frequency-chirps source is used to achieve spectral diversity, then sensing distance is improved, but device complexity increases due to optical elements
Solution Approach 1:
The patent replaces complex optical elements (EOM and acousto-optic modulators) with a purely digital signal processing approach. By using digital signal processing to generate and process multi-frequency chirps, the system achieves the same spectral diversity and extended sensing distance without the mechanical and optical complexity of traditional modulators
Solution Approach 2:
The patent changes the approach from optical parameter modulation to digital signal processing parameters. By implementing multi-frequency chirps through digital processing rather than optical modulation, the system maintains extended sensing capability while significantly reducing device complexity
3Device complexity
If single polarization operation is used, then device complexity is reduced, but signal reliability deteriorates due to polarization-induced fading
Solution Approach 1:
The patent implements polarization diversity by processing both X and Y polarization components simultaneously through the same digital signal processing pipeline. This multi-functional approach allows the system to handle multiple polarization states without requiring separate processing paths, maintaining signal reliability while avoiding the complexity of dedicated polarization management for each channel
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 extends the distance over which reliable fiber sensing can be performed by managing coherent fading, enhancing the performance and flexibility of DAS/DVS systems, particularly in telecom fiber applications, by increasing the number of detected fiber events and reducing false negatives.
Implementation Method 1
modulate a source signal based on one or more digital code sequences for each of multiple sub-bands to inject, via an optical circulator, a multi-carrier modulated optical signal into an optical fiber
Implementation Method 2
receive a multi-carrier reflected optical signal corresponding to reflections of the multi-carrier modulated optical signal from the optical reflectors
Implementation Method 3
process the multi-carrier reflected optical signal using the at least one digital coding sequence to detect fiber events along the optical fiber
Implementation Method 4
Distributed acoustic/vibration sensing (DAS/DVS) allows for detection of mechanical and/or chemical perturbation at various locations along an optical fiber using Rayleigh backscattering
Data Source
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AI summary
An optical sensing system (100) has (a) an optical transmitter (110) that modulates a laser signal (115b) based on one or more digital code sequences (129) for each of multiple sub-bands to inject a multi-carrier modulated optical signal (119) into an optical fiber (160) having reflectors (Sm) distributed along the fiber and (b) an optical receiver (120) that (i) receives a multi-carrier reflected optical signal (161) corresponding to reflections of the multi-carrier modulated signal from the reflectors and (ii) processes the multi-carrier reflected signal using the at least one digital coding sequence to detect fiber events along the fiber. In an embodiment, X and Y polarization axes of the multiple sub-bands are encoded using BPSK or QPSK codes corresponding to different rows of X- and Y-axis OFDM codes. The fiber events are detected using differential phase data generated from sub-band Jones matrices derived from sub-band data recovered from the reflected signal.