Multi-Span DAS Fiber Layout With Dispersion Management
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Solution Overview
Problem
Existing distributed acoustic sensing (DAS) systems are limited to a range of <50 km to 150 km and can only sense one fiber span due to fiber nonlinearities, with a maximum peak power launch of approximately 23 dBm, restricting their sensing capabilities.
Innovation Solution
Incorporating negative dispersion fiber (D− fiber) into DAS systems and using external circulators to route outbound and backscattered Rayleigh signals, along with staggered sensing pulses to mitigate modulation instability and four-wave mixing, allowing for higher power tolerance and improved sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If conventional DAS systems use standard optical fiber, then the system can operate with simple fiber infrastructure, but the sensing range is limited to <50 km to 150 km and only one fiber span can be sensed due to fiber nonlinearities
Solution Approach 1:
The patent changes the dispersion parameter of the optical fiber by using D- fiber (negative dispersion fiber) instead of standard fiber. This parameter change allows the system to tolerate higher launch powers and extends the sensing range beyond conventional limits by mitigating the impact of fiber nonlinearities through the negative dispersion characteristic.
Solution Approach 2:
The patent employs a composite fiber approach by combining D- fiber (negative dispersion) with D+ fiber (positive dispersion) in a multi-span configuration. This composite structure allows different spans to have different dispersion characteristics, enabling extended sensing range while managing nonlinear effects through the interplay between negative and positive dispersion sections.
2Measurement precision
If DAS systems increase peak power launch to improve signal strength, then sensing sensitivity improves, but fiber nonlinearities increase and limit the sensing range
Solution Approach 1:
By changing the dispersion parameter to negative values using D- fiber, the system can launch at higher peak powers (up to 23 dBm and beyond) without experiencing the same level of nonlinear degradation. The negative dispersion parameter fundamentally alters how the fiber responds to high-power signals, allowing improved sensitivity while controlling nonlinear effects.
Solution Approach 2:
The patent converts the typically harmful effect of high launch power (which causes nonlinearities) into a benefit by using D- fiber. The negative dispersion transforms the nonlinear interaction, allowing the high power to be tolerated and even utilized for improved sensing sensitivity over extended ranges, effectively converting what would be a harmful condition into an advantageous operating regime.
3Length of stationary object
If DAS systems use external circulators to route signals over multiple spans, then the sensing capability extends to multiple fiber spans, but the device complexity increases
Solution Approach 1:
The patent introduces external circulators as intermediary devices to route the DAS signal through multiple fiber spans. These circulators act as mediators that direct the optical signal path, enabling the system to sense multiple spans by routing the outbound signal through different fiber sections and collecting backscattered signals from various spans, thereby extending sensing distance while managing system complexity through structured signal routing.
4Measurement precision
If DAS systems use staggered sensing pulses to mitigate nonlinearities, then the sensing range and sensitivity improve, but the system requires more complex pulse timing control
Solution Approach 1:
The patent employs periodic action by using staggered sensing pulses with specific timing intervals. Instead of continuous or uniformly spaced pulses, the system uses periodically staggered pulse sequences that are timed to exploit the dispersion characteristics of D- fiber. This periodic staggering allows the system to mitigate nonlinearities while maintaining improved sensitivity, as the timed intervals between pulses prevent harmful nonlinear interactions while still providing adequate sampling for accurate sensing.
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 use of D− fiber and circulators enhances the optical signal-to-noise ratio (OSNR) and extends the DAS range, enabling sensing over longer distances and multiple fiber spans with increased sensitivity and reduced nonlinearities.
Implementation Method 1
a DAS receiver to analyze the backscattered Rayleigh signal, based upon the outbound DAS signal
Implementation Method 2
the DAS signal is transmitted at least in part over a D− fiber
Data Source
AI summary
An undersea sensing system may include a first distributed acoustic sensing (DAS) station, comprising a first DAS transmitter and a first DAS receiver connected by an undersea optical communications cable. The first DAS transmitter is configured to launch a first outbound DAS signal in a first direction, over at least one sensing span of the optical communications cable. The DAS receiver is configured to receive a backscattered Rayleigh signal, based upon the first outbound DAS signal, wherein the first DAS signal is transmitted over a first D− fiber in a first half of the sensing span, and is transmitted over a first D+ fiber over a second half of the sensing span. The system further includes a second DAS station having a second DAS transmitter to launch a second outbound DAS signal in a second direction, opposite the first direction, over the at least one span. A second DAS receiver receives a second backscattered Rayleigh signal, based upon the second outbound DAS signal, wherein the second DAS signal is transmitted over a second D− fiber in the second half of the sensing span and is transmitted over a second D+ fiber over the first half of the sensing span.