Fiber-Optic Acoustic Sensing With Pulse Labeling
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Solution Overview
Problem
Conventional distributed fiber optic acoustic sensing systems, particularly those using time division multiplexing (TDM) architecture, face limitations in bandwidth, especially for high-frequency acoustic detection, as they are restricted by the sensing length and suffer from cross-talk and heterodyne detection limitations, which hinder the detection of frequencies above 20-30 kHz.
Innovation Solution
The system employs direct measurement of baseband acoustic phase information and utilizes heterodyne detection to reject undesired contributions from delayed backscattered pulses, incorporating frequency or wavelength pulse labeling techniques within a segmented sensing fiber network with remote circulators and a controller module for precise identification of acoustic noise sources, enabling higher bandwidth and reduced cross-talk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If time division multiplexing (TDM) architecture is used for distributed sensing, then the system can multiplex a large number of sensing elements with high sensitivity and low cross-talk, but the bandwidth of detection is limited and depends on the sensing length
Solution Approach 1:
The sensing fiber is divided into multiple segments with each segment having its own circulator and up-lead fiber link. This segmentation allows independent interrogation of each segment, enabling higher bandwidth detection while maintaining the TDM architecture's low cross-talk performance. The segmented approach eliminates the bandwidth limitation imposed by total sensing length in conventional TDM systems.
Solution Approach 2:
The patent introduces frequency labeling as an additional dimension to the time-division multiplexing scheme. By assigning unique frequency labels to different time slots or segments, the system achieves both high bandwidth and low cross-talk simultaneously, resolving the contradiction between these two parameters.
2Reliability
If multiple interrogating pulse pairs with phase modulators are used to improve signal-to-noise ratio, then duty cycle and acoustic bandwidth can be increased, but heterodyne interrogation limits detection to frequencies up to 20-30 kHz only
Solution Approach 1:
The patent replaces the conventional heterodyne interrogation method with direct detection of baseband acoustic phase information. This substitution eliminates the frequency limitation inherent in heterodyne detection, enabling detection of acoustic frequencies above 30 kHz while maintaining good signal-to-noise ratio through the use of frequency pulse labeling.
Solution Approach 2:
The patent changes the detection parameter from heterodyne frequency mixing to direct baseband phase measurement. By using frequency-labeled pulses and measuring phase changes directly, the system achieves both high signal-to-noise ratio and extended acoustic frequency detection capability beyond the 20-30 kHz limit of conventional heterodyne methods.
3Speed
If the pulse interrogation rate is increased over the length of sensing fiber to detect higher frequencies, then acoustic bandwidth can be increased, but cross-talk between sensing elements increases
Solution Approach 1:
By segmenting the sensing fiber and assigning unique frequency labels to each segment or time slot, the system can increase the pulse interrogation rate for high-frequency detection while preventing cross-talk through frequency discrimination. Each segment's signal can be independently identified by its frequency label.
Solution Approach 2:
The patent adds frequency labeling as an additional dimension to the time-division multiplexing scheme. This allows the system to achieve high pulse interrogation rates for extended bandwidth while maintaining low cross-talk through frequency-based identification of signals from different segments or time slots.
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 allows for the detection of high-frequency acoustic signals up to 100 kHz with improved signal-to-noise ratio and spatial resolution, effectively identifying acoustic noise sources along infrastructure, such as leaking pipelines, while maintaining compatibility with existing fiber-optic installations.
Implementation Method 1
high resolution technique for distributed fiber optic acoustic sensing rely on Coherent Rayleigh (CR) effect which is applied for measurements of dynamic strain induced in the sensing fiber by propagating high pressure acoustic waves
Implementation Method 2
Interrogation of the backscattered pulses is done by using heterodyne demodulation, where an array of sensors is interrogated with two optical pulses that are frequency shifted relative to each other
Data Source
AI summary
The present invention relates to distributed acoustic sensing using fiber-optic system. More particularly, the present invention describes use of frequency pulse labeling techniques and wavelength pulse labeling techniques for providing high bandwidth acoustic sensing in applications such as infrastructure monitoring. In one embodiment, a segmented sensing fiber is used with corresponding circulators in an architecture that controls the interrogation of each segment of the fiber. In another embodiment, a single continuous length of sensing fiber is used, but a plurality of pulse sequences with different wavelengths are used to interrogate. In both configurations, heterodyne beat frequency components are rejected by a processing scheme, resulting in a simple direct measurement of baseband phase information.