Multi-band DAS System Inter-band Interference Reduction
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Solution Overview
Problem
Distributed Acoustic Sensing (DAS) systems face limitations in signal-to-noise ratio (SNR), sampling rate, and inter-location interference, particularly in long fiber lengths, due to Rayleigh scattering and harmonic frequency interference, which hinder effective detection of acoustic events along the fiber optic sensing fiber.
Innovation Solution
A multi-band DAS system is developed with frequency bands divided into groups to minimize inter-band interference, using an I-Q modulator and Acousto-Optic Modulator to create multi-frequency interrogation pulses, and applying a window function to reduce spectrum leakage, while interleaving bands for uniform sampling and increasing overall signal power.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multi-frequency pulses are used to increase repetition rate, then sensing frequency is improved, but inter-band interference increases
Solution Approach 1:
The frequency spectrum is segmented into multiple non-overlapping bands (e.g., 1-10 MHz, 11-20 MHz, 21-30 MHz) with guard bands between them. Each band is independently modulated and transmitted, preventing harmonic interference while maintaining high overall sensing frequency through parallel processing of multiple bands.
Solution Approach 2:
The system transitions from single-frequency time-domain multiplexing to multi-frequency spectrum-domain multiplexing. By distributing sensing across multiple frequency dimensions with orthogonal modulation schemes, the system achieves high repetition rates without temporal overlap of pulses from different locations.
2Productivity
If frequency bands are closely spaced to increase sampling rate, then sampling rate is improved, but spectrum leakage increases
Solution Approach 1:
Window functions (e.g., Hanning, Hamming) are applied to each frequency band signal before transmission to pre-suppress spectral leakage. This preliminary spectral shaping prevents energy from one band from leaking into adjacent bands, enabling closer frequency spacing while maintaining signal integrity.
Solution Approach 2:
Guard bands are introduced as intermediary frequency regions between adjacent signal bands. These guard bands act as spectral buffers that absorb any residual leakage energy, allowing the system to achieve high sampling rates through dense frequency packing while preventing inter-band interference.
3Productivity
If pulse interval is reduced to increase repetition rate, then sensing frequency is improved, but Rayleigh fading interference increases
Solution Approach 1:
The system changes from single-frequency pulse transmission to multi-frequency continuous wave modulation. By using frequency diversity across multiple bands and applying coherent integration techniques, the system overcomes Rayleigh fading at any single frequency/location, maintaining high repetition rates without suffering from fading-induced interference.
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
The solution enhances signal-to-noise ratio (SNR) and sampling rate, while minimizing inter-location interference, enabling more effective detection of acoustic events over long fiber lengths by reducing harmonic frequency interference and Rayleigh fading.
Implementation Method 1
using an I-Q modulator and Acousto-Optic Modulator to create multi-frequency interrogation pulses
Implementation Method 2
DAS systems use Rayleigh backscattering in an optical sensing fiber to detect strain changes in the optical sensing fiber
Implementation Method 3
applying a window function to reduce spectrum leakage, while interleaving bands for uniform sampling and increasing overall signal power
Data Source
AI summary
A distributed acoustic sensing (DAS) system that collects many frequency bands within the system (in particular, analog to digital converter, or ADC) bandwidth. The bands are divided into multiple groups, and within each group, the inter-band interference from band i to band j is at least l1 dB lower than the power of band j. For different groups, the inter-band interference is at least l2 dB lower than the power of band j, that l2≥l1+Cl, where Cl is a value related to round-trip fiber loss. The bands belonging to the same group are cascaded one after another, with little guard time in between, or packed back-to-back. This arrangement advantageously increases the overall signal power, resulting in an improved SNR.