Spatial averaging methods for coherent distributed sensing

Abstract

Aspects of the present disclosure describe systems, methods, and structures for coherent distributed sensing employing a moving average method between locations along the sensing fiber employing a user-configured number of taps, with location grouping and intra-group phase adjustment followed by inter-group phase adjustment and combination. To achieve this adjustment, the method uses two stages of rotation, i.e., intra-group rotation and inter-group rotation, to adjust the phase of all participating sensing fiber locations.

Description

[Technical field]

[0001] The present disclosure generally relates to coherent distributed acoustic sensing (DAS) systems, methods, and structures using Rayleigh backscattering that exhibit random and independent Rayleigh fading and low signal levels, or even no signal, at several fiber locations at any given time. [Background technology]

[0002] As one skilled in the art can appreciate, DAS systems utilize the Rayleigh scattering effect in optical fibers to detect changes in strain in the fiber. The resulting dynamic strain signal is used to determine vibration and acoustic signals along the entire length of the fiber optic cable under interrogation, as well as the fiber location of such vibrations. Coherent DAS extracts the phase from the complex signal to detect strain. However, phase calculations are sensitive to signal strength under the same noise level. DAS systems that rely on the detection of Rayleigh scattering signals may experience attenuation in signal strength and overflow with noise at certain locations, which results in unstable phase measurements. Summary of the Invention

[0003] An advancement in the art is made by aspects of the present disclosure that advantageously employ a moving average with the polarization combined output to reduce any Rayleigh fading prior to phase determination, advantageously improving output signal quality. To ensure proper alignment, the present disclosure describes methods to align multiple fiber positions involved in the averaging.

[0004] In a significant departure from the prior art, systems, methods, and structures according to aspects of the present disclosure employ a novel moving average method between positions along the optical fiber using a user-configured number of taps. As will now be shown and explained, there are two actions that advantageously strengthen the signal and reduce the likelihood of fading.

[0005] The first operation involves grouping of locations and intra-group phase adjustment, where 1) the locations along the fiber are divided into non-overlapping fixed size groups, with the number of locations in each group equal to the spatial average taps, 2) within each group, the location with the highest average power is identified (the "selected location") and the average power is updated with the selected location every DAS frame cycle, 3) each location in the group has a phase rotation value that is used to rotate the complex signal at the corresponding location to align with the other locations in the group. This rotation value is updated every DAS frame cycle, and 4) an average direction of the rotated complex signal at the selected location (the "reference direction") is identified. For each location, the average difference from this reference direction is calculated and its rotation is updated.

[0006] The second operation according to the embodiment of the present disclosure is inter-group phase adjustment and synthesis, which includes: first, calculating the average phase difference between the rotated complex values ​​of the selected positions in each adjacent group as the inter-group phase offset; second, for averaging across groups, rotating the positions in one group to correct the offset between the groups, and then synthesizing; third, the method adjusts the phase of all participating positions of the average using two stages of rotation, such as intra-group rotation and then inter-group rotation, to achieve the adjustment, where 1) the intra-group adjustment uses the position of maximum power as a reference and rotates other positions to adjust their phases. This is achieved by comparing the phase difference between each other position and the selected position and rotating the other positions by the phase difference. This allows tracking the phase changes caused by both polarization switching and fiber stress and covering the full band of the sampled signal, and the inter-group adjustment rotates one of the groups using the phase difference between the two groups. This method stores the phase difference for each of the two groups instead of for each position involved in the averaging, reducing the computational complexity and required buffer size from O(N) to O(1), where N is the number of averaged taps. The method uses the rotation values ​​of the selection positions to calculate the phase difference for both intra-group and inter-group adjustments. This allows the phase to be continuous as the selection position is changed, and to track in the direction of the maximum power position, preserving all information in the signal (including very low frequencies). [Brief description of the drawings]

[0007] A more complete understanding of the present disclosure may be realized by reference to the accompanying drawings.

[0008] [Figure 1(A)] FIG. 1 is a schematic diagram of an exemplary prior art DAS configuration.

[0009] [Figure 1(B)] FIG. 1 is a schematic diagram of an exemplary coherent DAS configuration according to an aspect of the present disclosure.

[0010] [Figure 1(C)] FIG. 2 is a schematic diagram illustrating an example process for a coherent DAS configuration according to an embodiment of the present disclosure.

[0011] [Diagram 2] 1 illustrates spatial averaging according to an aspect of the present disclosure.

[0012] [Diagram 3] FIG. 11 is a flow diagram illustrating the calculation of intra-group phase differences and rotation angles according to aspects of the present disclosure, separated into per-group processing, inter-group phase difference processing, and adjustment and combination processing operations.

[0013] [Figure 4] FIG. 1 is a schematic diagram illustrating grouping of locations according to an aspect of the present disclosure.

[0014] [Figure 5(A)] 1 is a pair of plots illustrating intra-group adjustments according to an aspect of the present disclosure. [Figure 5(B)] 1 is a pair of plots illustrating intra-group adjustments according to an aspect of the present disclosure.

[0015] [Figure 6(A)] 13 illustrates intra-group phase difference calculation and rotation value update according to an aspect of the present disclosure. [Figure 6(B)] 13 illustrates intra-group phase difference calculation and rotation value update according to an aspect of the present disclosure.

[0016] [Figure 7] FIG. 13 is a flow diagram illustrating intra-group rotation and adjustment according to an aspect of the present disclosure.

[0017] [Figure 8] FIG. 1 is a schematic diagram illustrating inter-group reconciliation and groups requiring further reconciliation according to aspects of the present disclosure.

[0018] [Figure 9] FIG. 1 is a schematic diagram illustrating inter-group coordination according to aspects of the present disclosure.

[0019] [Figure 10] FIG. 13 is a schematic diagram illustrating inter-group coordination in an implementation according to an aspect of the present disclosure.

[0020] Exemplary embodiments are more fully described in the drawings and detailed description, however, embodiments in accordance with the present disclosure may be embodied in various forms and are not limited to the specific or exemplary embodiments set forth in the drawings and detailed description. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The following merely illustrates the principles of the present disclosure, and 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 present disclosure and are included within its spirit and scope.

[0022] Furthermore, all examples and conditional terms described herein are intended to be for educational purposes only to aid the reader in understanding the concepts contributed by the inventors to further the principles and techniques of the present disclosure, and should not be construed as being limited to such specifically recited examples and conditions.

[0023] Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, such equivalents are intended to include both currently known equivalents as well as equivalents developed in the future, i.e., elements developed that perform the same function, regardless of structure.

[0024] 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.

[0025] Unless otherwise specified herein, the figures making up the drawings are not drawn to scale.

[0026] As some additional background, it is worth noting that in recent years, distributed fiber optic sensing (DFOS) systems, including distributed vibration sensing (DVS) and distributed acoustic sensing (DAS), have gained widespread acceptance in many applications, including but not limited to infrastructure monitoring, intrusion detection, and earthquake detection. DAS and DVS use the backscattering Rayleigh effect to detect changes in strain in the fiber, with the fiber itself acting as the transmission medium to carry the optical sensing signal back to the interrogator for subsequent analysis.

[0027] Figure 1(A) shows a simplified schematic diagram of a prior art DFOS / DVS / DAS system that uses an interrogator / coherent receiver / detection / analysis system. In operation, such a system generates and applies an optical Tx signal to an optical sensing fiber, which results in a reflected / scattered optical signal returned to the receiver / analysis system, which receives / detects / analyzes the reflected / scattered and subsequently received signal. The signal is analyzed and an output is generated that is indicative of the environmental conditions encountered along the length of the fiber.

[0028] FIG. 1(B) is a schematic block diagram of an exemplary coherent DAS system according to an embodiment of the present disclosure. Coherent DAS systems generally use an interrogator including a transmitter (Tx) that periodically generates an optical signal (pulse or code) to detect acoustic signals along a sensing fiber using Rayleigh scattering. The optical signal is directed into a distributed sensing fiber. Each location along the fiber reflects a small portion of the optical signal back to the interrogator. The backscattered signal is processed in the Rx section to recover the acoustic signal or detect vibrations.

[0029] As mentioned above, coherent DAS uses differential beats at every two selected positions along the optical fiber to detect fiber stress at positions between the two selected positions. Coherent optical detection has A and Y polarization diversity, which varies randomly due to fiber movement or other factors. Therefore, the beats can use XX, XY, YX, and YY to fully utilize all the power, resulting in four polarization diversity.

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[0030] In certain embodiments, a coherent receiver may use multiple LO frequencies offset by different amounts from the interrogation frequency to detect Rayleigh reflected signals. As part of this method, a Tx / Rx framing scheme may be used that advantageously provides a frequency offset between interrogation and coherent detection.

[0031] In operation, DAS receive signal samples are received sequentially for each position within each frame, whereas polarization diversity combining processing requires frame-by-frame processing for each position. The sequence conversion requires a large amount of memory and bandwidth. Doubling the diversity term from the beat processing again doubles the memory and bandwidth required.

[0032] Systems, methods, and structures according to aspects of the present disclosure generally operate within or in conjunction with a receiver and advantageously reduce memory and bandwidth requirements by reducing beat diversity terms.

[0033] According to an embodiment of the present disclosure, because polarization switching is a slow process compared to the position sampling rate (i.e., the DAS pulse or frame repetition rate), the A and Y polarizations are integrated pre-beat. Operationally, the two polarizations are first aligned in the same direction by rotating one polarization (A or Y) to the other polarization (Y or A) before integration, and then rotated to maintain phase continuity.

[0034] The two polarizations are first aligned to the one with the higher average power (call it pol-P). The XY composite signal is then passed to a beat module for differential beating, followed by phase extraction or other additional processing.

[0035] Advantageously, systems, methods, and structures according to aspects of the present disclosure can combine the two polarizations into one output before the beat, such that there is only a single input to the beat module and only one output from the beat. This overall inventive operation advantageously reduces processing complexity and memory size.

[0036] As previously discussed, systems, methods, and structures according to embodiments of the present disclosure are for a coherent DAS that uses differential phasing to detect stress along a fiber, as shown in Figure 1(C). The signal inputs X(z,n) and Y(z,n) are of two polarizations X / Y, where z is the position along the fiber and n is the time series of that position. The differential beats produce:

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[0037] It should be noted that an embodiment of the present invention can be treated as a moving average function in the spatial domain, which can be expressed as follows:

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[0038] FIG. 2 illustrates spatial averaging according to an embodiment of the present disclosure. As previously described and further explained, systems, methods, and structures according to an embodiment of the present disclosure divide the positions along the fiber into non-overlapping groups. Each position has a phase rotation R(z,n) that is used to align with other positions in that group. In each group, the position with the highest average power (or amplitude) is selected as the position z. e The selection position is used as the "selection position" shown as

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[0039] For every two adjacent groups, an intergroup phase rotation R is used to align the phase of group (g+1) to group g. g There is a group variable g (g+1,n). This signal is the average of the phase difference between group (g+1) and group g,

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[0040] For the final synthesis, each position is first rotated by R(z,n)

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[0041] As mentioned above, aspects of the present disclosure provide a moving average with the polarization combined output, which is expected to reduce the possibility of Rayleigh fading before sending to phase calculation and improve the quality of the output signal. In polarization combined, each position may point to a different position and cannot be combined directly. The present disclosure provides a method to adjust the multiple positions involved in the averaging.

[0042] This disclosure describes dividing an input signal into groups with N positions within each group, as shown in Figure 4 (a schematic diagram showing position grouping according to an embodiment of the disclosure), where N is the averaging taps. For example, for 5 tap averaging, positions 0-4 belong to group 0, positions 5-9 belong to group 1, and so on.

[0043] At each position there is a rotation R(z,n) that shifts the signal from its pointing direction (considered DC) in order to align with other members in the group. Figures 5(A) and 5(B) are a pair of plots illustrating intra-group alignment according to an embodiment of the present disclosure.

[0044] For example, in FIG. 5A, positions p and q both belong to group g, but the input signal

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[0045] Each group selects the position with the highest average power. e Select as. Rotation signal

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[0046] 6(A) and 6(B) show intra-group phase difference calculation and rotation value update according to an embodiment of the present disclosure. With reference to this figure, R(z,n) is filtered using a low-pass filter.

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[0047] In one embodiment, R(z,n) is normalized

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[0048] FIG. 7 is a flow diagram illustrating intra-group rotation and adjustment according to an embodiment of the present disclosure, summarizing the above steps.

[0049] Once each group is internally aligned, the next step is inter-group alignment to synthesize positions across two groups. The averaged result is shown in Figure 8 (a schematic diagram showing inter-group alignment and groups that require further alignment according to an embodiment of the present disclosure).

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[0050] Each group (g+1) is compared with the previous group (group g) by the average phase difference R g Maintain (g+1,n). Intergroup adjustment is

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[0051] Similar to the intra-group adjustment, the rotation within group (g+1) is calculated by the normalized value (

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[0052] Group Rotation R g (g+1,n) is the rotation signal (

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[0053] Given a signal input per position, inter-group adjustment can be realized by two parallel shift registers as shown in Figure 10. With reference to this figure, note that the "FF" used there refers to a "flip-flop" or a register. The left shift register has signals with no inter-group rotation, while the right shift register is rotated by the inter-group phase difference. A pulse is generated once per group and is used by the left register as a "clear" signal to clear registers 1 to (N-1) to zero, and by the right register as a "load" signal to load the value from the left register to the right register. When the pulse is inactive, the register loads the value from the previous register every cycle. This action ensures that the register at the right output always adjusts the signal appropriately.

[0054] At this point, the present disclosure has been presented using several specific examples, but those skilled in the art will recognize that the present teachings are not so limited. Accordingly, the present disclosure should be limited only by the scope of the claims appended hereto.

Claims

1. generating an interrogation light by an interrogator and transmitting the interrogation light to an optical sensing fiber; detecting, by a coherent detector, backscattered light resulting from the interrogation light transmitted into the optical sensing fiber; generating and outputting a signal indicative of the detected backscattered light by the coherent detector; 1. A spatial averaging method for a coherent distributed acoustic sensing (DAS) system, comprising: analyzing, by a signal processor, an output signal indicative of the detected backscattered light to determine a strain signal related to a position along the optical sensing fiber, the strain signal being indicative of a vibration and acoustic environment of the optical sensing fiber at said position; determining a phase change between all positions along the optical sensing fiber using differential beat and polarization synthesis of output signals of two positions along the optical sensing fiber; A moving average is applied to the differential beat and polarization combined output signal from multiple locations in the spatial domain; a method comprising grouping signals from individual locations along the optical sensing fiber into non-overlapping groups, wherein in each group, the location having the highest average power is used as a selected location, and wherein the signal at each other location in the group is adjusted in the same direction as the signal at the selected location, and inter-group adjustments are made to combine signals at locations across adjacent groups.

2. 2. The method of claim 1 further characterized in that each position in the group has a phase rotation for phase alignment with other positions in the group.

3. The method of claim 2 further characterized in that the rotation signal of the selected position is used as a reference for the group.

4. 4. The method of claim 3, further characterized by an intra-group adjustment procedure that determines a phase difference between the signal at each of the other positions in the group and the rotated signal at the selected position, and updates the phase rotation of each of the positions based on an average of the phase differences.

5. 5. The method of claim 4, further characterized by an inter-group adjustment procedure in which every group maintains a phase difference with its neighboring groups, and the phase difference is used to adjust the average position across the group and its neighboring groups, such that group G+1 maintains a phase difference with group G or group G maintains a phase difference with group G+1.

6. The method of claim 1 , wherein the number of locations in each group is equal to the spatial average taps.

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