Phase Based Sensing Using Frequency Shifted Return Signals
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Solution Overview
Problem
Distributed acoustic sensing (DAS) systems face limitations in dynamic range, especially when sensing long fibers, and existing techniques struggle to effectively apply derivative sensing methods to continuous return signals.
Innovation Solution
The method involves frequency shifting the first and second return signals in a distributed acoustic sensing system, allowing for the application of derivative sensing techniques to continuous signals, using an output interferometer with frequency modulators to separate and analyze the signals, and adjusting pulse width and separation to optimize channel resolution without modifying the fiber.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If heterodyne interferometry is used to improve measurement precision, then sensitivity is improved, but dynamic range is limited especially when sensing very long fibres
Solution Approach 1:
The continuous return signal from DAS is segmented into discrete temporal channels by dividing the fiber into multiple sections and assigning each to a specific time window. This segmentation allows derivative sensing to be applied to each channel independently, effectively extending the dynamic range while maintaining sensitivity across the entire fiber length.
Solution Approach 2:
The system uses periodic pulse injection into the fiber, where pulses are sent at regular intervals and the return signals are sampled in corresponding time windows. This periodic action creates discrete measurement opportunities that enable derivative sensing calculations while maintaining continuous monitoring capability across the full dynamic range.
2Adaptability or versatility
If derivative sensing technique is applied to improve dynamic range, then dynamic range is improved, but the technique struggles with continuous return signals from DAS
Solution Approach 1:
The continuous return signal is divided into discrete temporal segments corresponding to different fiber sections. By assigning specific time windows to specific fiber segments, the system creates discrete measurement channels that can process derivative information independently, reducing the complexity of processing the entire continuous signal at once.
Solution Approach 2:
The system pre-organizes the continuous return signal into discrete temporal channels before derivative sensing calculation. By establishing the temporal segmentation and channel assignment in advance, the system prepares the data structure needed for derivative processing, simplifying the subsequent calculation steps.
3Ease of manufacture
If standard fibre is used without modification to reduce manufacturing complexity, then ease of manufacture is improved, but sensing resolution and accuracy may be limited
Solution Approach 1:
The system changes the interrogation parameters (pulse width, pulse separation, duty cycle) to optimize the resolution and accuracy of sensing using standard fiber. By adjusting these parameters, the system can achieve high sensing resolution without requiring modified fiber, thereby maintaining ease of deployment while improving measurement precision.
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 enables the use of unmodified standard fibers for DAS, providing improved sensitivity and flexibility in resolving acoustic signals along long fiber lengths, with enhanced dynamic range and spatial resolution, and allows for simultaneous measurement of phase and phase rate changes.
Implementation Method 1
In distributed acoustic sensing, Rayleigh backscattering is normally used. Due to random inhomogeneities in standard optic fibres, a small amount of light from a pulse injected into a fibre is reflected back from every location along the length of the fibre
Implementation Method 2
An alternative approach to DAS is based on heterodyne interferometry. In this approach light which has passed through a given section of fibre is interfered with light that has not. Any disturbance to this section of fibre causes a phase change between the two portions of light that interfere
Implementation Method 3
the first return signal is frequency shifted relative to said second return signal
Data Source
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AI summary
A method of distributed acoustic sensing (DAS) whereby the derivative or rate of change of a signal backscatted from a fibre is measured. The change, or derivative of the phase measured in this way has a much smaller amplitude than the signal itself if the difference between the two times at which the signal is measured is much less than the period of the signal being measured, resulting in lower sensitivity. Frequency shifts can be applied to temporally displaced return signals to compare the rate of change, for example by employing an output interferometer arranged to modulate the signal in each arm by a different frequency shift.