This disclosure relates generally to coherent distributed acoustic sensing (DAS) systems, methods and structures using Rayleigh backscattering, which exhibit random and independent Rayleigh fading and low signal levels—or even no signal—at some fiber locations at any given time.
As will be understood by those skilled in the art, DAS systems exploit a Rayleigh scattering effect in optical fiber to detect changes in the fiber strain. The obtained dynamic strain signal is used to determine the vibration and acoustic signal along the entire length of the fiber optic cable under interrogation and the fiber location of such vibration. Coherent DAS extracts the phase from the complex signal to detect the strain. However, phase calculation is sensitive to signal strength under the same noise level. In DAS system relying on the detection of Rayleigh scattered signal, there is a possibility that at certain locations the signal strength will fade and be inundated by noise. This causes instability in the phase measurement.
An advance in the art is made according to aspects of the present disclosure that advantageously employs a moving average using polarization combining output—to reduce any Rayleigh fading before phase determination—which advantageously improves output signal quality. To ensure proper alignment, the present disclosure describes a method that aligns multiple fiber locations involved in the averaging.
In sharp contrast to the prior art, systems, methods, and structures according to aspects of the present disclosure employ a novel moving average method among locations along an optical fiber, using a user configured number of taps. As we shall show and describe, there are two operations that advantageously strengthen the signal(s) and reduce the possibility of fading.
The first operation involves location grouping and group internal phase alignment wherein 1) locations along the fiber are divided into non-overlapping fixed size groups, with the number of locations in each group equal to the spatial averaging taps; 2) within each group, the location having the maximum averaged power (“elected location”) is identified—for each DAS frame cycle, the averaged power is updated along with the elected location; 3) each location within the group has a phase rotation value, which is used to rotate the complex signal of the corresponding location to align with other locations within the group. This rotation value is updated in every DAS frame cycle; and 4) the averaged direction of the rotated complex signal in the elected location (“reference direction”) is identified. For each location our method calculates the averaged difference with this reference direction and updates its rotation.
The second operation according to aspects of the present disclosure is inter-group phase alignment and combining and involves: First, calculate the averaged phase difference between the rotated complex value of the elected locations in each adjacent groups, as inter-group phase offset; Second for averaging that spans the groups, rotate the locations in one group to compensate the inter-group offset and then combine; Third, our method uses two steps of rotation to achieve the alignment: intra-group rotation, and then inter-group rotation, to align the phase of all the participating locations of an average wherein 1) Intra-group alignment uses the location of maximum power as reference and rotates the other locations to align their phase. This is achieved by comparing the phase difference between each other location and the elected location and rotating the other locations by the phase difference. This enables the tracking of phase change caused by both polarization switching and fiber stress, covering the full band of the sampled signal; and Inter-group alignment involves rotating one of the groups using the phase difference between the two groups. This method saves the phase difference between every two groups, rather than each location participating in an averaging, to reduce the calculation complexity and required buffer size from O(N) to O(1) where N is the number of averaging taps. Our method uses the rotated value of the elected location, to calculate the phase difference, for both intra- and inter-group alignments. This enables phase continuity when changing the elected location and tracks the direction of the maximum power location, so any information in the signal (including ultra-low frequency) will be preserved
A more complete understanding of the present disclosure may be realized by reference to the accompanying drawing in which:
The illustrative embodiments are described more fully by the Figures and detailed description. Embodiments according to this disclosure may, however, be embodied in various forms and are not limited to specific or illustrative embodiments described in the drawing and detailed description.
The following merely illustrates the principles of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope.
Furthermore, all examples and conditional language recited herein are intended to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions.
Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.
Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure.
Unless otherwise explicitly specified herein, the FIGs comprising the drawing are not drawn to scale.
By way of some additional background—we again note that in recent years, distributed fiber optic sensing (DFOS) systems including distributed vibration sensing (DVS) and distributed acoustic sensing (DAS) have found widespread acceptance in numerous applications including—but not limited to—infrastructure monitoring, intrusion detection, and earthquake detection. For DAS and DVS, backward Rayleigh scattering effects are used to detect changes in the fiber strain, while the fiber itself acts as the transmission medium for conveying the optical sensing signal back to an interrogator for subsequent analysis.
As previously noted, coherent DAS uses differential beating for every two selected locations along an optical fiber to detect fiber stress at location(s) in between the two selected locations. Coherent optical detection has X and Y polarization diversities, which changes randomly due to fiber movement or other factors. For this reason, the beating may use X-X, X-Y, Y-X, and Y-Y to fully utilize all the power, which results in 4 polarization diversities ζxx, ζxy, ζyx, and ζyy. Subsequent processing is required to combine the 4 diversity terms into a single term.
In a particular embodiment, the coherent receiver may employ multiple LO frequencies that are offset from interrogating frequencies by different amounts, to detect Rayleigh reflected signals. As part of this methodology, a Tx/Rx framing scheme may be employed which advantageously provide a frequency offset between the interrogation and coherent detection.
Operationally, DAS received signal samples are received in sequence of location-by-location within each frame, while the polarization diversity combining process requires a frame-by-frame processing for each location. The sequence conversion requires large amount of memory and bandwidth. Doubling the diversity terms from beating process further doubles the memory and bandwidth needed.
Systems, methods, and structures according to aspects of the present disclosure generally operate within or in conjunction with the receiver, and advantageously reduces the memory and bandwidth required by reducing beating diversity terms.
According to aspects of the present disclosure, X and Y polarizations are merged before beating, since polarization switching is a slow process as compared to location sampling rate (i.e., DAS pulse or frame repetition rate). Operationally, the two polarizations are first aligned to the same direction before merging, by rotating one of the polarizations (X or Y) to the other (Y or X), then rotated to maintain phase continuity.
The two polarizations first align to the one having higher averaged power (say pol-P). The X-Y combined signal is then passed to the beating module for differential beating, followed by phase extraction or other additional processing.
Advantageously, systems, methods, and structures according to aspects of the present disclosure may combine the two polarizations into one output before beating, such that there is only a single input to a beating module and only one output from beating. This overall inventive operation advantageously reduces the processing complexity and memory size.
As noted, systems, methods, and structures according to aspects of the present disclosure are for coherent DAS, which uses differential phase to detect the stress along the fiber, as illustrated in
Note that aspects of the present invention can be treated as a moving average function in spatial domain, which may be represented by:
ζ′(z,n)=Σl=−N
as illustrated in
As we have noted and now describe further, systems, methods and structures according to aspects of the present disclosure divide the locations along the fiber into non-overlapping groups. Each location has a phase rotation R(z, n), which is used to align with the other locations inside its group. In each group, the location of the highest averaged power (or amplitude) is used as the “elected location”, denoted as location ze. The signal of the elected location ζ(ze, n) is first rotated to have ζr(g, n)=(ze, n)·R(ze, n), which is used as reference for this group. Then the input signal of each other location within the group calculates the phase difference with this reference by ζ(z, n)*·ζr(g, n), where ζ(z, n)* is the conjugate of the input signal ζ(z, n). This difference is used to update R (z, n) using a low pass filter. This procedure is shown in
Between every two neighbor groups, there is an inter-group phase rotation R9(g+1, n) (variable g for “group”) used to align the phase of group (g+1) with group g. This signal is the average of the phase difference between group (g+1) and g, which is avg(ζr(g, n)·ζr(g+1, n)*). The calculation of R9(g+1, n) is shown in
For final combining, each location first performs group internal alignment, by rotating R(z, n) to have ζr(z, n). For averaging that involves group g only, take sum(ζr(z, n)) that z ∈ group g. For those involving both group g and group (g+1), take the group rotation of Rg(g+1, n) for locations in group (g+1) and then combine. This is shown in
As described previously, aspects of the present disclosure provides for moving average using polarization combining output, to reduce the possibility of Rayleigh fading before sending to phase calculation, which is expected to improve the output signal's quality. Polarization combining may have each location pointing at different location, which makes it not possible to directly combine. The present disclosure provides a method to align the multiple locations that participate in an averaging.
The present disclosure describes operations that divides the input signals into groups with N locations in each group, where N is the averaging taps, as shown in
Each location has a rotation R(z, n), for the signal to shift from its pointing direction (considered as DC) to align with other members within the group.
For example, in
Each group chooses the location of the maximum averaged power as the elected location ze. The direction of the rotated signal ζr(ze, n) is considered as the group's reference direction. Each other locations in the group compares with this reference direction to update its R(z, n). The instant difference is calculated using ζdiff(z, n)=ζr(ze, n)·ζ(q, n)* as schematically shown in
In one embodiment, R(z, n) is updated by averaging (or low-pass filtering) the normalized ζdiff(z, n), which is avgn(ζdiff(z, n)/|ζdiff(z, n)|).
Once each group is internally aligned, the next step is inter-group alignment, for the combining of locations spanning two groups. As the example shown in
Each group (g+1) maintains the averaged phase difference Rg(g+1, n) with its previous one (group g). Inter-group alignment is by rotating all the aligned signals within group (g+1) to the direction of signals in group g using ζr(z, n)·Rg(g+1, n) where z ∈ group g. The multi-location combining will be Σz=gN+i(g+1)N−1ζr(z, n)+Σz=(g+1)N(g+1)N+i−1ζr(z, n)·Rg(g+1, n). This operation is illustrated in
Same as group internal alignment, the rotation in group (g+1) can be with normalized value which is ζr(z, n)·(Rg(g+1, n)/|Rg(g+1, n)|). In one embodiment, the multi-location combining can apply a weight to each location, based on its averaged power.
The group rotation Rg(g+1, n) is updated by the averaged difference between group g and (g+1), using the rotated signal of the elected locations in each group, which is avg(ζr(pe, n)·ζr(qe, n)*), where pe ∈ group g, and qe ∈ group g+1. In one embodiment, Rg(g+1, n) is generated from the normalized phase difference, ζr(pe, n)·ζr(qe, n)*/|ζr(pe, n)·ζr(qe, n)*|.
Giving location-by-location signal input, the inter-group alignment can be achieved by two parallel shift registers as shown in
At this point, while we have presented this disclosure using some specific examples, those skilled in the art will recognize that our teachings are not so limited. Accordingly, this disclosure should be only limited by the scope of the claims attached hereto.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 63/124,948 filed 14 Dec. 2020 the entire contents of which is incorporated by reference as if set forth at length herein.
Number | Name | Date | Kind |
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20190003879 | Bao | Jan 2019 | A1 |
20190186958 | Godfrey | Jun 2019 | A1 |
20190310304 | Yogeeswaran | Oct 2019 | A1 |
20200249076 | Ip | Aug 2020 | A1 |
20210172729 | Huang | Jun 2021 | A1 |
Number | Date | Country | |
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20220187121 A1 | Jun 2022 | US |
Number | Date | Country | |
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63124948 | Dec 2020 | US |