Multi-Span DAS with Negative-Dispersion Fiber and Staggered Pulses
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Solution Overview
Problem
Existing distributed acoustic sensing (DAS) systems are limited in range to less than 50 km to 150 km and can only sense one fiber span due to fiber nonlinearities, with a maximum peak power of approximately 23 dBm, restricting their sensing capabilities.
Innovation Solution
Implementing negative dispersion fiber (D- fiber) in DAS systems, using two circulators to route forward and backward Rayleigh backscattered signals, and employing staggered sensing pulses to mitigate modulation instability and four-wave mixing, allowing higher optical power launch and improved signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If higher optical power is launched to improve signal-to-noise ratio, then sensing sensitivity is improved, but fiber nonlinearities (modulation instability and four-wave mixing) increase and limit the sensing range
Solution Approach 1:
The patent changes the dispersion parameter of the optical fiber from positive to negative dispersion. This parameter change fundamentally alters the nonlinear optical processes in the fiber, suppressing modulation instability and four-wave mixing effects that normally limit sensing range. By operating in the negative dispersion regime, the system can tolerate up to 10 dB higher optical power without experiencing the same nonlinear penalties, thereby improving signal-to-noise ratio and sensing sensitivity while extending the sensing range beyond conventional limits.
2Length of stationary object
If the sensing range is extended beyond 50-150 km, then the monitoring coverage is improved, but the signal quality degrades due to accumulated dispersion and nonlinearities
Solution Approach 1:
The patent employs negative dispersion fiber to change the dispersion parameter, which compensates for accumulated dispersion over long distances and suppresses nonlinear effects. This allows the system to maintain signal quality over extended sensing ranges beyond the conventional 50-150 km limit.
Solution Approach 2:
The patent uses staggered sensing pulses with different time offsets for different fiber spans. This periodic action with varying time delays allows the system to interrogate multiple spans sequentially with optimized power levels, maintaining signal quality across the extended sensing range by managing the temporal distribution of optical power throughout the fiber network.
3Length of stationary object
If two-way sensing is implemented to double the sensing range, then the monitoring coverage is improved, but the system complexity increases with additional components
Solution Approach 1:
The patent implements a multi-span two-way DAS system where the same optical infrastructure and sensing mechanism are used to serve multiple fiber spans in both directions. The system achieves doubled sensing range by utilizing bidirectional sensing capabilities, where each span can be sensed from both ends, effectively multiplying the monitoring coverage without proportionally increasing system complexity.
Solution Approach 2:
The patent divides the extended sensing range into multiple manageable spans, with each span being independently managed through staggered pulse timing. This segmentation allows the system to handle long-distance sensing by breaking it into discrete segments that can be interrogated sequentially, reducing the complexity of simultaneous multi-span management while achieving extended total coverage.
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
Enhances DAS sensitivity by tolerating up to 10 dB more optical power, doubling the sensing range, and maintaining high signal quality through reduced nonlinearities and improved signal-to-noise ratio.
Implementation Method 1
the DAS system may be based on Rayleigh backscattering (otherwise referred to as a Rayleigh-backscattering-based DAS system)
Implementation Method 2
Optical phase shifts between pulses may be proportional to strain in the fiber, leading to the ability to detect vibrations and the like, as measured by the effect of such perturbations on the phase
Implementation Method 3
signal power depletion due to modulation instability (MI)
Implementation Method 4
signal power depletion due to modulation instability (MI) and Four Wave Mixing (FWM)
Data Source
Figure 1A~1B
Figure 2A~2B
Figure 3A
AI summary
A system may include a distributed acoustic sensing (DAS) station, comprising a DAS transmitter to launch an outbound DAS signal in a first direction, over at least one span of an optical communications link. The system may further include a DAS receiver to receive a backscattered Rayleigh signal, based upon the outbound DAS signal, wherein the DAS signal is transmitted at least in part over a D- fiber.