WO2025225633A1

WO2025225633A1 - Sensing system and sensing method

Info

Publication number: WO2025225633A1
Application number: PCT/JP2025/015668
Authority: WO
Inventors: Murtuza PETLADWALA; Toshiro Yamamoto; Takeshi Kojima; Ryuji Wakakusa
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2024-04-24
Filing date: 2025-04-23
Publication date: 2025-10-30
Anticipated expiration: 2026-10-24

Abstract

An object is to monitor a plurality of optical fibers in a distributed acoustic sensing system with a simple system configuration. A plurality of optical fibers for distributed acoustic sensing are disposed in areas where a target is detected. A measurement unit outputs probe light and receives backscattered light to sensing for each optical fiber. A light distribution unit switches an optical path between the measurement unit and the plurality of the optical fibers in such a manner that the probe light is input to one optical fiber in the plurality of the optical fibers and the measurement unit receives the backscattered light from the one optical fiber. A control unit controls the light distribution unit to connect each optical fiber to the measurement unit in a sequential manner.

Description

SENSING SYSTEM AND SENSING METHOD

　　The present disclosure relates to a sensing system and a sensing method.

　　When a sound wave is applied externally to an optical fiber, light passing through the optical fiber is modulated. Then, by detecting reflected light, backscattered light, or transmitted light, sound waves can be monitored at a remote location. In general, this sensor system has recently been referred to as Distributed Acoustic Sensing (DAS). Fiber optic sensors do not need to be powered and do not require electrical wiring because the detected signals are transmitted by light. Therefore, fiber optic sensors are not affected by lightning strikes and are less susceptible to induction noise.

　　A distributed acoustic sensing system for detecting a target has been widely developed. In this system, an optical fiber-based vibration sensor, i.e., a Distributed Acoustic Sensing (DAS) interrogator, sends a light pulse into an optical fiber and calculates perturbations throughout the fiber from a backscattered light.

　　Patent Literature 1 discloses a method and system for distributed fiber optic sensing across multiple fiber ports with multiple corresponding fiber paths. In this configuration, at least one optical signal transmitter transmits a train of optical signals through a fiber optic network including a plurality of optical fibers distributed over a geographic area. An optical switching arrangement sequentially distributes optical signals in the train through the plurality of optical fibers via respective optical fiber ports. A plurality of optical receivers receive backscattered optical signals from the plurality of optical fibers. A processing unit demodulates data from the backscattered optical signals to perform DAS sensing.

[Patent Literature 1] Japanese Unexamined Patent Application Publication No. 2022-507455

　　As described above, a general DAS system, such as a system of Patent Literature 1, requires a plurality of optical receivers to monitor a plurality of optical fibers. In addition, depending on the configuration of the DAS system, a plurality of optical transmitters may also be required. As a result, it is difficult to miniaturize and reduce the cost of the general DAS system.

　　An aspect of the present disclosure is a sensing system including: a plurality of optical fibers for distributed acoustic sensing arranged in areas where a target is detected; a measurement unit configured to output probe light and receive backscattered light for sensing for each optical fiber and to determine whether the target affects the optical fiber according to the backscattered light; a light distribution unit configured to switch an optical path between the measurement unit and the plurality of the optical fibers in such a manner that the probe light is input to one optical fiber in the plurality of the optical fibers and the measurement unit receives the backscattered light from the one optical fiber; and a control unit configured to control the light distribution unit to connect each optical fiber to the measurement unit in a sequential manner.

　　An aspect of the present disclosure is a sensing method including: outputting probe light and receiving backscattered light for sensing to each of a plurality of optical fibers for distributed acoustic sensing arranged in areas where a target is detected, and determining whether the target affects the optical fiber according to the backscattered light by a measurement unit; switching an optical path between the measurement unit and the plurality of the optical fibers in such a manner that the probe light is input to one optical fiber in the plurality of the optical fibers and the measurement unit receives the backscattered light from the one optical fiber; and controlling the switching of the optical path to connect each optical fiber to the measurement unit in a sequential manner.

Effects of Invention

　　According to the present disclosure, it is possible to monitor a plurality of optical fibers in a distributed acoustic sensing system with a simple system configuration.

Fig. 1 is a diagram schematically illustrating a configuration of a distributed acoustic sensing (DAS) system according to an example embodiment. Fig. 2 is a diagram illustrating a configuration example of a DAS system according to an example embodiment. Fig. 3 is a flow chart illustrating a sensing operation of a DAS system according to an example embodiment. Fig. 4 is a diagram illustrating an example of a DAS system including two optical fibers. Fig. 5 is a table illustrating a sensing operation when a DAS system includes two optical fibers. Fig. 6 is a table illustrating a sensing operation of a DAS system according to an example embodiment. Fig. 7 is a table illustrating a sensing operation of a comparison example. Fig. 8 is a table illustrating a sensing operation of a DAS system according to an example embodiment.

　　Hereinafter, example embodiments of the present disclosure will be described with reference to the drawings. In the drawings, the same element is denoted by the same reference numeral, and redundant description is omitted as necessary.

　　Hereinafter, when referring to "an example embodiment", "An example embodiment" can be applied to any one of example embodiments described below or applied to a combination of two or more example embodiments. Application of "an example embodiment" is not limited to any particular example embodiment.

　　A distributed acoustic sensing (DAS) system according to a first example embodiment will be described. Fig. 1 is a diagram schematically illustrating a configuration of a DAS system according to an example embodiment. A DAS system 100 includes a measurement unit 1, a light distribution unit 2, a control unit 3, and optical fibers F1 to FN. Note that N is an integer equal to or greater than two.

　　The optical fibers F1 to FN are sensing elements of DAS sensing and are disposed in an area where effects of targets, such as humans, animals, or the like, on sensing parts therein are detected. One end of each of the optical fibers F1 to FN is connected to respective ports PT1 to PTN of the light distribution unit 2. In the present embodiment, the optical fibers F1 to FN have the same distance from the respective ports of the light distribution unit 2 to the sensing part therein.

　　The measurement unit 1 is connected to the light distribution unit 2. The light distribution 2 connects the measurement unit 1 to one optical fiber to be sensed which is an optical fiber selected among from the optical fibers F1 to FN. Hereinafter, one optical fiber to be sensed by the measurement unit 1 is referred to as a target optical fiber FT.

　　The measurement unit 1 and the light distribution unit 2 will be described in more detail. Fig. 2 is a diagram illustrating an example configuration of a DAS system according to an example embodiment.

　　The measurement unit 1 includes a light source unit 11, an optical reception unit 12, and a processing unit 13. The light source unit 11 outputs the probe light P through the light distribution unit 2 to the target optical fiber FT according to a control signal CON1 provided by the control unit 3. The optical reception unit 12 receives the backscattered light B from the target optical fiber FT through the light distribution unit 2. Then, the optical reception unit 12 converts the received backscattered light B, which is an optical signal into a reception signal R, which is an electrical signal. The processing unit 13 performs predetermined processing for DAS sensing on the reception signal R to detect whether the target affects the target optical fiber FT.

　　The light distribution unit 2 includes an optical circulator 21 and an optical switch 22. The optical switch 22 is interposed between the light source unit 11, the optical reception unit 12, and the optical switch 22, respectively.

　　The optical circulator 21 outputs the probe light P input from the light source unit 11 to the optical switch 22. The optical circulator 21 outputs the backscattered light B input from the optical switch 22 to the optical reception unit 12.

　　The optical switch 22 is configured as a single-pole and N-throw switch. The optical switch 22 connects the port PT0 and one of the ports PT1 to PTN according to a control signal CON2 provided by the control unit 3. Thus, the measurement unit 1 and the target optical fiber FT are connected. Accordingly, the light source unit 11 can output the probe light P to the target optical fiber FT through the light distribution unit 2. The reception unit 12 can receive the backscattered light B from the target optical fiber FT through the light distribution unit 2.

　　The control unit 3 controls which optical fiber in the optical fibers F1 to FN the light distribution unit 2 connects to the measurement unit 1. Specifically, the control unit 3 outputs the control signal CON2 to the light distribution unit 2 to specify the target optical fiber FT. In response to the control signal CON2, the light distribution unit 2 configures a light path therein to connect the specified target optical fiber FT to the measurement unit 1.

　　According to the configuration of the DAS system 100, the measurement unit 1 outputs probe light P to the target optical fiber FT through the light distribution unit 2. Then, the measurement unit 1 receives backscattered light B from the target optical fiber FT. Thus, the measurement unit 1 can determine whether the target affects the target optical fiber FT based on the backscattered light B by using DAS sensing technology. Since the DAS sensing technology is well known in the sensing field, descriptions of the principle and operation thereof are omitted.

　　In the present configuration, the light distribution unit 2 switches the target optical fiber FT in the optical fibers F1 to FN so that each of the optical fibers F1 to FN is sensed. Hereinafter, for simplicity, it is assumed that the light distribution unit 2 switches the target optical fiber FT from the optical fiber F1 to the optical fiber FN in ascending order. However, an order of selecting the target optical fiber FT is not limited to this. As long as each of the optical fibers F1 to FN is sensed, the target optical fiber FT is selected in any order such as descending order and random order.

　　Next, a sensing operation of the DAS system 100 will be described. Fig. 3 is a flowchart illustrating a sensing operation of a DAS system according to an example embodiment.

Step S0
　　The control unit 3 sets a parameter i for specifying one optical fiber, which is the target optical fiber FT among from the optical fibers F1 to FN to "1", which is an initial value.

Step S1
　　The control unit 3 instructs the light distribution unit 2 to specify the optical fiber Fi as the target optical fiber FT by using the control signal CON2. The light distribution unit 2 connects the specified optical fiber Fi to the measurement unit 1 as the target optical fiber FT in response to the control signal CON2.

Step S2
　　The control unit 3 instructs the light source unit 11 to output the probe light P by using the control signal CON1. Further, the control unit 3 starts to measure a sensing time t from an output timing of the probe light P. In response to the control signal CON1, the light source unit 11 outputs the probe light P to the target optical fiber FT.

Step S3
　　The optical reception unit 12 detects the backscattered light B from the target optical fiber FT. The optical reception unit 12 converts the received backscattered light B into the reception signal R. The processing unit 13 performs predetermined processing for DAS sensing on the reception signal R to detect whether the target affects the target optical fiber FT.

Step S4
　　The control unit 3 continuously monitors whether the sensing time t reaches a sampling rate R.

Step S5
　　When the monitoring time t reaches the sampling rate R, the control unit 3 determines whether i is equal to N. If i is equal to N, the process returns to Step S0.

Step S6
　　If i is not equal to N, the control unit 3 increments the parameter i by one. The process then returns to Step S1.

　　According to the operation described above, the DAS system 100 can repeatedly perform the sensing cycle including sensing operations for the optical fibers F1 to FN.

　　In the present embodiment, the sampling rate R is determined in such a manner that the sensing operations of the optical fibers F1 to FN do not overlap. Here, a distance to the sensing part in each of the optical fibers F1 to FN from the corresponding port of the light distribution unit 2 is represented as d. n_g is a refractive index of the optical fibers F1 to FN. c is the speed of light in a vacuum. A light traveling time T in which the light travels back and forth between the corresponding port of the light distribution unit 2 and the sensing part in the target optical fiber FT is expressed by the following expression.
Expression 1

　　Thus, by determining the sampling rate R as a value greater than the light traveling time T, the sensing operations of the optical fibers F1 to FN can be performed in a sequential manner without overlapping.

　　Next, an example where two optical fibers are disposed will be described. Fig. 4 is a diagram illustrating an example of a DAS system including two optical fibers. In this example, a first optical fiber is denoted by F1 and a second optical fiber is denoted by F2.

　　Here, a sensing operation is considered below when the sampling rate R is 150 ns. Fig. 5 is a table illustrating a sensing operation when the DAS system includes two optical fibers. The probe lights output to the optical fibers F1 and F2 are denoted by P1 and P2, respectively. The backscattered lights from the optical fibers F1 and F2 are denoted by B1 and B2, respectively. As illustrated in Fig. 5, by determining the sampling rate R as 150 ns, which is greater than the light traveling time T of 100 ns, the sensing operations of the optical fibers F1 and F2 can be prevented from overlapping.

　　As described above, according to the DAS system 100, the optical fibers can be independently sensed as described above. Thus, the DAS sensing results of the optical fibers can be obtained independently. Therefore, the DAS system according to the present example embodiment can advantageously perform DAS sensing operations of a plurality of optical fibers with a simple configuration including a light source unit and one optical reception unit.

　　Second Example Embodiment
　　In the first example embodiment, the sampling rate R is determined as the time longer than the light traveling time T for the optical fibers having the same distance to the sensing part. However, depending on the use of the DAS system, distances to sensing parts of all or a part of the optical fibers F1 to FN may be different. Therefore, in the present example embodiment, the sampling rate R is determined for a plurality of optical fibers in which distances to sensing parts are different.

　　In this case, the sampling rate R is determined based on the longest distance of the distances to the sensing parts in the optical fibers. The determination of the sampling rate R according to the second example embodiment is specifically described. In the DAS system 100, the sampling rate R is determined to be longer than the light travel time T_L of the longest distance in the distances between the respective ports and the sensing parts in the optical fibers F1 to FN. The light traveling time T_L is expressed by the following expression.
Expression 2

d_max is the longest distance among the distances between the respective ports and the sensing parts in the optical fibers F1 to FN.

　　In this case, the sampling rate R is determined as R>T_L, the DAS system 100 can sense each of a plurality of optical fibers in a sequential manner by the DAS sensing technology. An example case is described here. In this example, it is assumed that the light traveling time T1 for the optical fiber F1 is 100 ns and the light traveling time T2 for the optical fiber F2 is 200 ns. Therefore, the longest light traveling time T_L is 200 ns. Accordingly, the sampling rate R is determined as a value greater than the longest light traveling time T_L = 200 ns. Here, for example, the sampling rate R is determined to be 250 ns.

　　Fig. 6 is a table illustrating a sensing operation of the DAS system according to an example embodiment. As illustrated in Fig. 6, the DAS system 100 can monitor each of the optical fibers F1 to FN independently, even if the distances to the sensing parts of the optical fibers F1 to FN are not the same, by appropriately determining the sampling rate R as a value greater than the longest light traveling time T_L.

　　To facilitate an advantage of the DAS system 100, a comparison example where T_L =200 ns and R=100 ns is considered. Fig. 7 is a table illustrating a sensing operation of the comparison example. In the comparison example, since the sampling rate R is 100 ns, which is less than the longest light traveling time T_L, the probe light P is output to the optical fiber F1 or F2 alternately every 100 ns. In this case, at the time of 100 + 200r ns, where r is an integer equal to or greater than one, the backscattered light B1 from the optical fiber F1 and the backscattered light B2 from the optical fiber F2 return to the light distribution unit 2 at the same time. Therefore, the measurement unit 1 cannot monitor each of the optical fibers F1 and F2 independently.

　　Accordingly, by appropriately defining the sampling rate to be greater than the longest light traveling time, the DAS system can independently monitor each of a plurality of the optical fibers, even if the distances to the sensing parts of a plurality of the optical fibers are not the same.

　　Third Example Embodiment
　　In the present example, a method of determining a sampling rate that is different from that of the second example embodiment will be described. In the present embodiment, the sampling rate R is determined as a value other than the common multiples of light traveling times in a plurality of optical fibers.

　　Hereinafter, for ease of descriptions, an example in which two optical fibers are disposed, as in the first and second example embodiments, will be described below. Fig. 8 is a table illustrating a sensing operation of the DAS system according to an example embodiment.

　　Here, as in the second example embodiment, it is assumed that the light traveling time T1 for the optical fiber F1 is 100 ns and the light traveling time T2 for the optical fiber F2 is 200 ns. With respect to the optical fiber F1, the common multiples of the light traveling time T1 is 100 (1+k) ns where k is an integer equal to or greater than one. With respect to the optical fiber F2, the common multiple of the light traveling time T2 is 200 (1+j) ns where j is an integer equal to or greater than one. In this case, the sampling rate R can be determined as a value other than the common multiples of 100 ns. Here, for example, the sampling rate R is determined to be 75 ns.

　　At t = 150(p-1), where p is an integer equal to or greater than one, the light distribution unit 2 connects the optical fiber F1 to the measurement unit 1. Then, the measurement unit 1 outputs the probe light P1 to the optical fiber F1.

　　At t = 150(p-1) + 75 that is a timing 75 ns after outputting the probe light P1, i.e., after a time is elapsed by the sampling rate R from the output of the probe light P1, the light distribution unit 2 connects the optical fiber F2 to the measurement unit 1. Then, the measurement unit 1 outputs the probe light P2 to the optical fiber F2.

　　At t = 150(p-1) + 100 that is a timing 100 ns after outputting the probe light P1, the light distribution unit 2 connects the optical fiber F1 to the measurement unit 1. Then, the measurement unit 1 detects the backscattered light B1 that corresponds to the probe light P1 that has been output at t = 150(p-1).

　　At t = 150(p-1) + 275 that is a timing 200 ns after outputting the probe light P2, the light distribution unit 2 connects the optical fiber F2 to the measurement unit 1. Then, the measurement unit 1 detects the backscattered light B2 that corresponds to the probe light P2 that has been output at t = 150(p-1) + 75.

　　In the present example, an order of the received backscattered lights is not sequential. For example, in the example illustrated in Fig. 8, the measurement unit 1 receives a first backscattered light B1 at 275 ns after receiving a first backscattered light B1 at 100 ns and a second backscattered light B1 at 250 ns. Therefore, the measurement unit 1 cannot receive the backscattered light B1 and B2 in a sequential manner. Therefore, the measurement unit 1 must perform an alignment of the backscattered lights.

　　The alignment of the backscattered light in the present example embodiment will be described. The processing unit 13 can determine whether the backscattered light returns from the optical fiber F1 or F2 according to a reception timing of the backscattered light. As described above, the reception timing of the backscattered light B1 is t = 150(p-1) + 100. The reception timing of the backscattered light B2 is t = 150(p-1) + 275. Thus, the processing unit 13 can determine whether the received backscattered light is the backscattered light B1 or B2 by referring to the reception timing. Accordingly, the processing unit 13 can easily perform the alignment of the backscattered lights.

　　Further, the processing unit 13 can perform the alignment of the backscattered light according to a time width of the backscattered light B. The received backscattered light B has the time width determined by the distance to the sensing part in the target optical fiber. Therefore, the processing unit 13 can determine whether the backscattered light B returns from the optical fiber F1 or F2 according to the time width of the backscattered light B by appropriately processing the reception signal R.

　　Accordingly, by appropriately defining the sampling rate as described above, the DAS system can independently monitor each of a plurality of the optical fibers, even if the distances to the sensing parts of a plurality of the optical fibers are not the same.

　　Other example embodiments
　　It should be noted that the present invention is not limited to the above example embodiments and can be suitably modified to the extent that it does not deviate from the purpose. For example, in the above-described example embodiments, although the example of two optical fibers has been described, and however, it is merely an example. It should be appreciated that the operation described for the case of two optical fibers may be applied to a case of three or more optical fibers.

　　The configurations of the measurement unit 1 and the light distribution unit 2 described above are merely an example. As long as the same operations as the measurement unit 1 and the light distribution unit 2 can be possible, other configurations may be adopted.

　　For example, the whole or part of the example embodiments disclosed above can be described as, but not limited to, the following supplementary notes.

　　The first to third embodiments can be combined as desirable by one of ordinary skill in the art.

　　While the disclosure has been particularly shown and described with reference to embodiments thereof, the disclosure is not limited to these embodiments.

　　It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the claims.

　　(Supplementary Note 1) A sensing system including:
　　a plurality of optical fibers for distributed acoustic sensing arranged in areas where a target is detected; a measurement unit configured to output probe light and receive backscattered light for sensing for each optical fiber, and to determine whether the target affects the optical fiber according to the backscattered light; a light distribution unit configured to switch an optical path between the measurement unit and the plurality of the optical fibers in such a manner that the probe light is input to one optical fiber in the plurality of the optical fibers and the measurement unit receives the backscattered light from the one optical fiber; and a control unit configured to control the light distribution unit to connect each optical fiber to the measurement unit in a sequential manner.

　　(Supplementary Note 2) The sensing system according to Supplementary Note 1, in which each of the plurality of the optical fibers includes a sensing part for distributed acoustic sensing, distances between the sensing parts of the plurality of the optical fibers and the light distribution unit are the same, and a sampling rate between a timing that outputs one probe light and a timing that outputs the next probe light is greater than a time in which light travels back and forth through the distance.

　　(Supplementary Note 3) The sensing system according to Supplementary Note 1, in which each of the plurality of the optical fibers includes a sensing part for distributed acoustic sensing, distances between the sensing parts of the plurality of the optical fibers and the light distribution unit are not the same, and a sampling rate between a timing outputting one probe light and a timing outputting the next probe light is greater than a time in which light travels back and forth through the longest distance.

　　(Supplementary Note 4) The sensing system according to Supplementary Note 1, in which each of the plurality of the optical fibers includes a sensing part for distributed acoustic sensing, distances between the sensing parts of the plurality of the optical fibers and the light distribution unit are not the same, and a sampling rate between a timing outputting one probe light and a timing outputting the next probe light is other than a common multiple of a time in which light travels back and forth through the distance of each optical fiber.

　　(Supplementary Note 5) The sensing system according to Supplementary Note 4, in which the measurement unit specifies the received backscattered light as corresponding to the probe light that was output earlier by the sampling rate.

　　(Supplementary Note 6) The sensing system according to Supplementary Note 1 or 2, in which the measurement unit includes: a light source unit configured to output the probe light; an optical reception unit configured to receive the backscattered light and convert the received backscattered light into a reception signal; 　　a processing unit configured to process the reception signal and detect the target according to a processing result.

　　(Supplementary Note 7) The sensing system according to Supplementary Note 6, in which each of the plurality of the optical fibers includes a sensing part for distributed acoustic sensing, distances between the sensing parts of the plurality of the optical fibers and the light distribution unit are not the same, a sampling rate between a timing outputting one probe light and a timing outputting the next probe light is other than a common multiple of a time in which light travels back and forth through the distance of each optical fiber, and the processing unit specifies the received backscattered light as corresponding to the probe light that was output earlier by the sampling rate.

　　(Supplementary Note 8) The sensing system according to Supplementary Note 6, in which the light distribution unit includes an optical switch inserted between the light source unit, the optical reception unit, and the plurality of optical fibers and configured to connect the light source unit and the optical reception unit to the one optical fiber in the plurality of optical fibers according to control by the control unit.

　　(Supplementary Note 9) The sensing system according to Supplementary Note 8, in which the light distribution unit includes an optical circulator inserted between the light source unit, the optical reception unit, and the optical switch and configured to output the probe light input from the light source unit to the optical switch and output the backscattered light input from the optical switch to the optical reception unit.

　　(Supplementary Note 10) A sensing method including: outputting probe light and receiving backscattered light to sensing for each of a plurality of optical fibers for distributed acoustic sensing arranged in areas where a target is detected, and determining whether the target affects the optical fiber according to the backscattered light by a measurement unit; switching an optical path between the measurement unit and the plurality of the optical fibers in such a manner that the probe light is input to one optical fiber in the plurality of the optical fibers and the measurement unit receives the backscattered light from the one optical fiber; and controlling the switching of the optical path to connect each optical fiber to the measurement unit in a sequential manner.

　　While the present invention has been particularly shown and described with reference to example embodiments thereof, the present invention is not limited to these example embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention.

　　This application is based upon and claims the benefit of priority from Japanese patent application No. 2024-70221, filed on April 24, 2024, the disclosure of which is incorporated herein in its entirety by reference.

1 measurement unit
2 light distribution unit
3 control unit
11 light source unit
12 optical reception unit
13 processing unit
21 optical circulator
22 optical switch
100 DAS system
F1 to FN optical fibers

Claims

　　A sensing system comprising:
　　a plurality of optical fibers for distributed acoustic sensing arranged in areas where a target is detected;
　　a measurement unit configured to output probe light and receive backscattered light for sensing for each optical fiber, and to determine whether the target affects the optical fiber according to the backscattered light;
　　a light distribution unit configured to switch an optical path between the measurement unit and the plurality of the optical fibers in such a manner that the probe light is input to one optical fiber in the plurality of the optical fibers and the measurement unit receives the backscattered light from the one optical fiber; and
　　a control unit configured to control the light distribution unit to connect each optical fiber to the measurement unit in a sequential manner.
　　The sensing system according to claim 1, wherein
　　each of the plurality of the optical fibers includes a sensing part for distributed acoustic sensing,
　　distances between the sensing parts of the plurality of the optical fibers and the light distribution unit are the same, and
　　a sampling rate between a timing that outputs one probe light and a timing that outputs the next probe light is greater than a time in which light travels back and forth through the distance.
　　The sensing system according to claim 1, wherein
　　each of the plurality of the optical fibers includes a sensing part for distributed acoustic sensing,
　　distances between the sensing parts of the plurality of the optical fibers and the light distribution unit are not the same, and
　　a sampling rate between a timing outputting one probe light and a timing outputting the next probe light is greater than a time in which light travels back and forth through the longest distance.
　　The sensing system according to claim 1, wherein each of the plurality of the optical fibers includes a sensing part for distributed acoustic sensing,
　　distances between the sensing parts of the plurality of the optical fibers and the light distribution unit are not the same, and
　　a sampling rate between a timing outputting one probe light and a timing outputting the next probe light is other than a common multiple of a time in which light travels back and forth through the distance of each optical fiber.
　　The sensing system according to claim 4, wherein
　　the measurement unit specifies the received backscattered light as corresponding to the probe light that was output earlier by the sampling rate.
　　The sensing system according to claim 1 or 2, wherein
　　the measurement unit comprises:
　　　　a light source unit configured to output the probe light;
　　　　an optical reception unit configured to receive the backscattered light and convert the received backscattered light into a reception signal;
　　　　a processing unit configured to process the reception signal and detect the target according to a processing result.
　　The sensing system according to claim 6, wherein
　　each of the plurality of the optical fibers includes a sensing part for distributed acoustic sensing,
　　distances between the sensing parts of the plurality of the optical fibers and the light distribution unit are not the same, and
　　a sampling rate between a timing outputting one probe light and a timing outputting the next probe light is other than a common multiple of a time in which light travels back and forth through the distance of each optical fiber, and
　　the processing unit specifies the received backscattered light as corresponding to the probe light that was output earlier by the sampling rate.
　　The sensing system according to claim 6, wherein
　　the light distribution unit comprises an optical switch inserted between the light source unit, the optical reception unit, and the plurality of optical fibers and configured to connect the light source unit and the optical reception unit to the one optical fiber in the plurality of optical fibers according to control by the control unit.
　　The sensing system according to claim 8, wherein the light distribution unit comprises an optical circulator inserted between the light source unit, the optical reception unit, and the optical switch and configured to output the probe light input from the light source unit to the optical switch and output the backscattered light input from the optical switch to the optical reception unit.
　　A sensing method comprising:
　　outputting probe light and receiving backscattered light for sensing for each of a plurality of optical fibers for distributed acoustic sensing arranged in areas where a target is detected, and determining whether the target affects the optical fiber according to the backscattered light by a measurement unit;
　　switching an optical path between the measurement unit and the plurality of the optical fibers in such a manner that the probe light is input to one optical fiber in the plurality of the optical fibers and the measurement unit receives the backscattered light from the one optical fiber; and
　　controlling the switching of the optical path to connect each optical fiber to the measurement unit in a sequential manner.