JP7626398B2 - Optical fiber vibration sensing device and optical fiber vibration sensing method - Google Patents

Description

This disclosure relates to optical fiber vibration sensing technology using an optical fiber interferometer.

As a fiber optic vibration sensing technology using an optical fiber interferometer, a method has been proposed and commercialized to detect anomalies such as intrusions by configuring multiple optical fiber ring interferometers and detecting vibrations on an optical fiber ring interferometer system basis (see, for example, Non-Patent Document 1).

As a method for identifying the vibration generation position using an optical fiber interferometer, a method has been proposed in which a sub-loop is added to an optical fiber ring interferometer. By passing through the sub-loop multiple times, multiple optical paths are seemingly created within the same optical fiber ring, and the position can be identified by comparing the interference light output from the light that has passed through each path (see, for example, Non-Patent Document 2).

Fujikura Engineering Co., Ltd., "Optical Fiber Intrusion Detection System", https://www.fen.fujikura.jp/products/pro01/tel-co-pdf02.pdf (Accessed 2021.1.7) Tatsuya Omori, Hiroaki Ogihara, Kenya Hashimoto, Masatsune Yamaguchi, "Vibration Position Identification Method in Optical Fiber Ring Interferometer Vibration Sensor", IEICE Transactions on Electronics, Information and Communication Engineers, Vol. J89-B, No. 4, pp. 594-601, 2006 Nobutomo Hanzawa, Kuimasa Saitoh, Taiji Sakamoto, Takashi Matsui, Kyozo Tsujikawa, Masanori Koshiba, and Fumihiko Yamamoto, "Two-mode PLC-based mode multi/demultiplexer for mode and wavelength division multiplexed transmission", 21 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE. 21.025752 ｜ OPTICS EXPRESS 25752

However, the device put into practical use in Non-Patent Document 1 requires the installation of a return device called a branch coupler for each detection section. In order to detect vibrations in a distributed manner, the required number of branch couplers must be installed, which poses a major challenge in terms of installation.

In addition, in Non-Patent Document 2, it is possible to identify the vibration position. However, since it is based on the premise that vibration is applied to one point on the optical fiber ring interferometer, if the entire optical fiber ring interferometer is implemented in a single optical fiber cable that is not in a ring form, vibration will occur in two places, which makes it difficult to apply.

This disclosure was made in light of the above circumstances, and aims to realize optical fiber vibration sensing with a simple configuration that can identify the position where vibration is applied.

For an optical transmission medium with multiple optical transmission paths, the interference signal is measured from opposite directions using a Mach-Zehnder interferometer, and the location where the vibration was added is identified by detecting the arrival time difference according to the location where the vibration occurred.

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
an optical transmission medium having a plurality of optical transmission paths;
a forward Mach-Zehnder interferometer which, at one end of the optical transmission medium, leaves a portion of the light in fundamental mode as it is and converts the remainder to a higher-order mode, combines the light left in fundamental mode with the converted light in higher-order mode, and inputs the combined light into an optical transmission line, and at the other end of the optical transmission medium, leaves the light in fundamental mode of the light that has propagated through the optical transmission line in fundamental mode and converts the light in higher-order mode to fundamental mode, and combines and receives the light left in fundamental mode with the converted light in fundamental mode;
a reverse Mach-Zehnder interferometer which, at the other end of the optical transmission medium, leaves a part of the light in fundamental mode as it is and converts the remainder to a higher-order mode, combines the light left in fundamental mode with the converted light in higher-order mode, and inputs the combined light to another optical transmission path, and at one end of the optical transmission medium, leaves the light in fundamental mode of the light propagating through the other optical transmission path in fundamental mode and converts the light in higher-order mode to fundamental mode, and combines and receives the light left in fundamental mode with the converted light in fundamental mode;
Equipped with.

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
the optical transmission medium includes a first optical transmission path and a second optical transmission path;
The forward Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a forward measurement light source that emits light;
a first optical mode coupler that leaves a portion of the fundamental mode light from the forward direction measurement light source as it is in the fundamental mode and converts the remainder into a higher order mode, combines the light that remains in the fundamental mode with the converted light in the higher order mode, and inputs the combined light into the first optical transmission line;
At the other end of the optical transmission medium,
a second optical mode coupler that, among the light propagating through the first optical transmission line, leaves light in a fundamental mode as it is and converts light in a higher order mode to the fundamental mode, and couples the light that remains in the fundamental mode with the converted light in the fundamental mode;
a forward direction measurement optical receiver that receives light from the second optical mode coupler;
The reverse Mach-Zehnder interferometer comprises:
At the other end of the optical transmission medium,
a light source for backward measurement that emits light;
a third optical mode coupler that leaves a part of the fundamental mode light from the backward direction measurement light source as the fundamental mode and converts the remainder into a higher order mode, combines the light that remains in the fundamental mode with the converted light in the higher order mode, and inputs the combined light into the second optical transmission line;
At one end of the optical transmission medium,
a fourth optical mode coupler that, among the light propagating through the second optical transmission line, leaves light in a fundamental mode as it is and converts light in a higher order mode to the fundamental mode, and couples the light that remains in the fundamental mode with the converted light in the fundamental mode;
a backward direction measurement optical receiver that receives light from the fourth optical mode coupler;
The present invention is characterized by comprising:

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
the optical transmission medium includes a first optical transmission path, a second optical transmission path, a third optical transmission path, and a fourth optical transmission path;
The forward Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a forward measurement light source that emits light;
a first optical mode coupler that leaves a part of the fundamental mode light from the forward direction measurement light source as it is in the fundamental mode and converts the remainder into a higher order mode, combines the light that remains in the fundamental mode with the converted light in the higher order mode, and inputs the combined light into the first optical transmission line;
a forward direction measurement optical receiver that receives light from the third optical transmission path,
At the other end of the optical transmission medium,
a second optical mode coupler which, among the light propagating through the first optical transmission line, leaves light in a fundamental mode as it is and converts light in a higher-order mode to a fundamental mode, combines the light which remains in the fundamental mode with the converted light in the fundamental mode, and outputs the combined light to the third optical transmission line;
The reverse Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a light source for backward measurement that transmits light to the fourth optical transmission line;
a fourth optical mode coupler that, among the light propagating through the second optical transmission line, leaves light in a fundamental mode as it is and converts light in a higher order mode to the fundamental mode, and couples the light that remains in the fundamental mode with the converted light in the fundamental mode;
a backward direction measurement optical receiver that receives light from the fourth optical mode coupler,
At the other end of the optical transmission medium,
The present invention is characterized in that it further comprises a third optical mode coupler which leaves a portion of the fundamental mode light from the fourth optical transmission path in the fundamental mode and converts the remainder to a higher mode, combines the light left in the fundamental mode with the converted higher mode light, and inputs the combined light to the second optical transmission path.

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
The first optical mode coupler or the third optical mode coupler is an off-axis connection part in which the centers of the cores of two optical fibers are connected with an offset.

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
The optical fiber on the forward direction measurement light source side of the off-axis connection part as the first optical mode coupler or on the reverse direction measurement light source side of the off-axis connection part as the third optical mode coupler propagates only fundamental mode light.

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
The second optical mode coupler or the fourth optical mode coupler is an off-axis connection section in which the centers of the cores of two optical fibers are connected with an offset, and the optical fiber on the side of the forward direction measurement receiver of the off-axis connection section as the second optical mode coupler or on the side of the reverse direction measurement receiver of the off-axis connection section as the fourth optical mode coupler propagates only light in the fundamental mode.

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
the optical transmission medium comprises an optical transmission path,
the forward Mach-Zehnder interferometer and the backward Mach-Zehnder interferometer share a fifth optical mode coupler that combines and branches light at one end of the optical transmission medium, and share a sixth optical mode coupler that combines and branches light at the other end of the optical transmission medium;
The forward Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
A forward measurement light source is provided to emit light,
the fifth optical mode coupler keeps a part of the fundamental mode light from the forward direction measurement light source as the fundamental mode and converts the remainder into a higher order mode, combines the light kept in the fundamental mode with the converted higher order mode light, and inputs the combined light into the optical transmission line;
At the other end of the optical transmission medium,
a forward direction measurement receiver for receiving light;
the sixth optical mode coupler, among the light propagating through the optical transmission line, leaves the light in a fundamental mode as it is, converts the light in a higher order mode to the fundamental mode, combines the light that remains in the fundamental mode with the converted light in the fundamental mode, and outputs the combined light to the forward direction measurement optical receiver;
The reverse Mach-Zehnder interferometer comprises:
At the other end of the optical transmission medium,
a backward measurement light source that emits light of a wavelength different from that of the forward measurement light source;
the sixth optical mode coupler leaves a part of the fundamental mode light from the backward measurement light source as the fundamental mode and converts the remainder into a higher-order mode, combines the light that remains in the fundamental mode with the converted higher-order mode light, and inputs the combined light into the optical transmission line;
At one end of the optical transmission medium,
a backward direction measurement optical receiver that receives light from the fifth optical mode coupler,
The fifth optical mode coupler is characterized in that, of the light propagating through the optical transmission line, the light in fundamental mode is left as it is in the fundamental mode and the light in higher modes is converted to the fundamental mode, the light that is left as it is in the fundamental mode and the converted light in the fundamental mode are combined, and output to the reverse direction measurement optical receiver.

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
the optical transmission medium includes a first optical transmission path, a third optical transmission path, and a fourth optical transmission path;
the forward Mach-Zehnder interferometer and the backward Mach-Zehnder interferometer share a fifth optical mode coupler that combines and branches light at one end of the optical transmission medium, and share a sixth optical mode coupler that combines and branches light at the other end of the optical transmission medium;
The forward Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a forward measurement light source that emits light;
a forward direction measurement optical receiver that receives light from the third optical transmission path,
the fifth optical mode coupler keeps a part of the fundamental mode light from the forward direction measurement light source as the fundamental mode and converts the remainder into a higher order mode, combines the light kept in the fundamental mode with the converted higher order mode light, and inputs the combined light into the first optical transmission path;
At the other end of the optical transmission medium,
the sixth optical mode coupler, among the light propagating through the first optical transmission line, leaves the light in fundamental mode as it is, converts the light in higher-order modes to fundamental mode, combines the light that remains in the fundamental mode with the converted light in fundamental mode, and outputs the combined light to the third optical transmission line;
The reverse Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a backward direction measurement light source that transmits light of a wavelength different from that of the forward direction measurement light source to the fourth optical transmission line;
a backward direction measurement optical receiver that receives light from the fifth optical mode coupler,
the fifth optical mode coupler, among the light propagating through the first optical transmission line, leaves the light in a fundamental mode as it is, converts the light in a higher order mode to a fundamental mode, combines the light that remains in the fundamental mode with the converted light in the fundamental mode, and outputs the combined light to the backward direction measurement light source;
At the other end of the optical transmission medium,
The sixth optical mode coupler leaves a portion of the fundamental mode light from the fourth optical transmission line as the fundamental mode and converts the remainder into a higher-order mode, combines the light that remains in the fundamental mode with the converted higher-order mode light, and inputs the combined light into the first optical transmission line.

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
a first AD converter that performs AD conversion on an electrical signal from the forward Mach-Zehnder interferometer;
a second AD converter that AD converts an electrical signal from the reverse Mach-Zehnder interferometer;
an analysis device that identifies a location where vibration occurs using a signal from the first AD converter and a signal from the second AD converter;
The present invention is characterized by further comprising:

Specifically, the optical fiber vibration sensing device of the present disclosure comprises:
a first AD converter that performs AD conversion on an electrical signal from the forward direction measurement light receiver;
a second AD converter that performs AD conversion on the electrical signal from the reverse direction measurement light receiver;
an analysis device that identifies a location where vibration occurs using a signal from the first AD converter and a signal from the second AD converter;
The present invention is characterized by further comprising:

Specifically, the optical fiber vibration sensing method of the present disclosure includes:
a fixed-time data acquisition step of acquiring interference wave signals w1(t) and w2(t) from a forward Mach-Zehnder interferometer and a backward Mach-Zehnder interferometer in which the light travels in the opposite direction to that of the forward Mach-Zehnder interferometer, respectively;
a cross-correlation calculation step of changing τ in formula (8) and determining τ when the correlation value (w1 * w2) (τ) is maximized as ΔT;
(w1*w2)(τ) = Σ w1(t)×w2(τ-t) (8)
a vibration generation position calculation step of calculating a distance Lv from the other end of the optical transmission line to a point where vibration is applied using equation (4);
Lv = (L-c2×ΔT)c1/(c1+c2) (4)
Here, L is the total length of the optical transmission medium having a plurality of optical transmission paths, c1 is the speed of light in the optical transmission path of the forward Mach-Zehnder interferometer, and c2 is the speed of light in the optical transmission path of the backward Mach-Zehnder interferometer.

The optical fiber vibration sensing device and optical fiber vibration sensing method disclosed herein can realize optical fiber vibration sensing that can identify the position where vibration is applied with a simple configuration.

1 is a diagram showing an example of the configuration of an optical fiber vibration sensing device according to the present disclosure. 1 is a diagram showing an example of the configuration of an optical fiber vibration sensing device according to the present disclosure. 1 is a diagram showing an example of the configuration of an optical fiber vibration sensing device according to the present disclosure. 1 is a diagram showing an example of the configuration of an optical fiber vibration sensing device according to the present disclosure. 1 is a diagram showing an example of the configuration of an optical fiber vibration sensing device according to the present disclosure. FIG. 2 is a diagram showing an example of processing of the optical fiber vibration sensing method of the present disclosure. 1 is a diagram illustrating an example of a configuration of an optical mode coupler according to the present disclosure. 1 is a diagram illustrating an example of a configuration of an optical mode coupler according to the present disclosure. 1 is a diagram illustrating an example of a configuration of an optical mode coupler according to the present disclosure. 1 is a diagram illustrating an example of a configuration of an optical mode coupler according to the present disclosure. 1 is a diagram illustrating an example of a configuration of an optical mode coupler according to the present disclosure. 1 is a diagram illustrating an example of a configuration of an optical mode coupler according to the present disclosure.

The following describes in detail the embodiments of the present disclosure with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings are mutually identical.

(Embodiment of Basic Configuration)
The configuration of the optical fiber vibration sensing device of this embodiment is shown in Fig. 1. In Fig. 1, reference numeral 1 denotes an optical transmission medium having a plurality of optical transmission paths, 10 denotes a forward Mach-Zehnder interferometer, and 11 denotes a backward Mach-Zehnder interferometer.

Examples include a multi-core optical fiber cable or ribbon-type optical fiber cable having multiple optical fibers as the optical transmission medium, with multiple optical fibers as the multiple optical transmission paths, and a multi-core optical fiber or multi-core optical fiber cable having multiple cores as the multiple optical transmission paths, with the cores of a multi-core optical fiber as the optical transmission medium.

In the forward Mach-Zehnder interferometer 10, at one end of the optical transmission medium 1 having multiple optical transmission paths, a portion of the light in the fundamental mode is left in the fundamental mode and the remainder is converted to a higher-order mode, the light left in the fundamental mode and the converted light in the higher-order mode are combined and input into the optical transmission path, and at the other end of the optical transmission medium 1 having multiple optical transmission paths, the light in the fundamental mode that has propagated through the optical transmission path is left in the fundamental mode and the light in the higher-order mode is converted to the fundamental mode, and the light left in the fundamental mode and the converted light in the fundamental mode are combined and received. In the reverse Mach-Zehnder interferometer 11, at the other end of the optical transmission medium 1 having multiple optical transmission paths, a portion of the fundamental mode light is kept in the fundamental mode and the remainder is converted to a higher mode, the light kept in the fundamental mode and the converted light in the higher mode are combined and input to another optical transmission path, and at one end of the optical transmission medium 1 having multiple optical transmission paths, the fundamental mode light of the light propagating through the other optical transmission path is kept in the fundamental mode and the higher mode light is converted to the fundamental mode, and the light kept in the fundamental mode and the converted light in the fundamental mode are combined and received. The forward Mach-Zehnder interferometer 10 and the reverse Mach-Zehnder interferometer 11 have opposite directions of light travel.

Each of the above optical transmission paths has a propagation constant that allows high-order mode light to propagate at the wavelengths of light used by the forward Mach-Zehnder interferometer 10 and the reverse Mach-Zehnder interferometer 11.

In FIG. 1, when vibration is applied to the optical transmission medium 1 having multiple optical transmission paths, a corresponding vibration is also applied to the optical transmission paths constituting the forward Mach-Zehnder interferometer 10 and the reverse Mach-Zehnder interferometer 11. The vibration leaves the electric field distribution of the fundamental mode light unchanged, but changes the electric field distribution of the higher-order mode light. When the electric field distribution changes, the conversion efficiency changes when the higher-order mode light that has propagated through the optical transmission path is converted to the fundamental mode. The fundamental mode light remains in the fundamental mode, and the higher-order mode light is converted to the fundamental mode, and a beat is generated when the light that remained in the fundamental mode is combined with the converted fundamental mode light.

As a result, the forward Mach-Zehnder interferometer can observe an interference wave with a beat corresponding to the point Pv where the vibration is applied. Similarly, the reverse Mach-Zehnder interferometer can observe an interference wave with a beat corresponding to the point Pv where the vibration is applied.

The optical fiber vibration sensing device of this embodiment may further include a first AD converter that performs AD conversion on the electrical signal from the forward Mach-Zehnder interferometer, a second AD converter that performs AD conversion on the electrical signal from the backward Mach-Zehnder interferometer, and an analyzer that identifies the location of vibration using the signal from the first AD converter and the signal from the second AD converter. The beat interference waves observed by the forward Mach-Zehnder interferometer and the backward Mach-Zehnder interferometer are taken into the analyzer via the first AD converter and the second AD converter, respectively, and the analyzer identifies the location of vibration.

(Application embodiment 1)
The configuration of the optical fiber vibration sensing device of this embodiment is shown in Fig. 2. In Fig. 2, 1 is an optical transmission medium having a plurality of optical transmission paths, 20 is a first optical transmission path, 21 is a second optical transmission path, 100 is a forward direction measurement light source, 101 is a backward direction measurement light source, 200 is a forward direction measurement light receiver, 201 is a backward direction measurement light receiver, 300 is a first optical mode coupler, 310 is a second optical mode coupler, 301 is a third optical mode coupler, 311 is a fourth optical mode coupler, 400 is a first AD converter, 401 is a second AD converter, and 500 is an analysis device.

The optical fiber vibration sensing device is equipped with a forward Mach-Zehnder interferometer and a reverse Mach-Zehnder interferometer, with the light traveling in opposite directions.

As a forward Mach-Zehnder interferometer, a forward measurement light source 100 that transmits light is provided at one end of an optical transmission medium 1 having a plurality of optical transmission paths, and a first optical mode coupler 300 that leaves part of the fundamental mode light from the forward measurement light source 100 in the fundamental mode and converts the remainder to a higher mode, combines the light that remains in the fundamental mode with the converted light in the higher mode, and inputs it to a first optical transmission path 20. At the other end of the optical transmission medium 1 having a plurality of optical transmission paths, a second optical mode coupler 310 that leaves the fundamental mode light of the light propagating through the first optical transmission path 20 in the fundamental mode and converts the higher mode light to the fundamental mode, combines the light that remains in the fundamental mode with the converted light in the fundamental mode, and a forward measurement receiver 200 that receives light from the second optical mode coupler 310 is provided.

In the forward Mach-Zehnder interferometer, a forward measurement light source 100 at one end of an optical transmission medium 1 having multiple optical transmission paths sends out light, and a first optical mode coupler 300 leaves part of the fundamental mode light from the forward measurement light source 100 in the fundamental mode and converts the rest to a higher mode, combines the light that has been kept in the fundamental mode with the converted light in the higher mode, and inputs it to the first optical transmission path 20. A second optical mode coupler 310 at the other end of the optical transmission medium 1 having multiple optical transmission paths leaves the fundamental mode light of the light that has propagated through the first optical transmission path 20 in the fundamental mode and converts the higher mode light to the fundamental mode, combines the light that has been kept in the fundamental mode with the converted light in the fundamental mode, and a forward measurement receiver 200 receives the light from the second optical mode coupler 310.

The backward Mach-Zehnder interferometer is provided with a backward measurement light source 101 that transmits light at the other end of an optical transmission medium 1 having multiple optical transmission paths, a third optical mode coupler 301 that leaves part of the fundamental mode light from the backward measurement light source 101 in the fundamental mode and converts the remainder to a higher mode, combines the light that remains in the fundamental mode with the converted light in the higher mode, and inputs it to the second optical transmission path 21. The backward measurement optical receiver 201 that receives light from the fourth optical mode coupler 311 is provided at one end of the optical transmission medium 1 having multiple optical transmission paths, leaves the fundamental mode light in the fundamental mode and converts the higher mode light to the fundamental mode, and combines the light that remains in the fundamental mode with the converted light in the fundamental mode.

In the backward Mach-Zehnder interferometer, the backward measurement light source 101 at the other end of the optical transmission medium 1 having multiple optical transmission paths sends out light, and the third optical mode coupler 301 leaves part of the fundamental mode light from the backward measurement light source 101 in the fundamental mode and converts the rest to a higher mode, combines the light that has been kept in the fundamental mode with the converted light in the higher mode, and inputs it to the second optical transmission path 21. The fourth optical mode coupler 311 at one end of the optical transmission medium 1 having multiple optical transmission paths leaves the fundamental mode light of the light that has propagated through the second optical transmission path 21 in the fundamental mode and converts the higher mode light to the fundamental mode, combines the light that has been kept in the fundamental mode with the converted light in the fundamental mode, and the backward measurement optical receiver 201 receives the light from the fourth optical mode coupler 311.

An example of the configuration of the first optical mode coupler 300 is shown in FIG. 7. In FIG. 7, the first optical mode coupler 300 is composed of an optical branching/coupling coupler 300-1 and an optical mode conversion coupler 300-2. The optical branching/coupling coupler 300-1 branches the fundamental mode light input to one input terminal into two fundamental mode lights. The optical mode conversion coupler 300-2 leaves the fundamental mode light input to one input terminal as it is, converts the fundamental mode light input to the other input terminal into a higher mode light, and combines and outputs the light that remains in the fundamental mode and the converted higher mode light. Optical mode conversion couplers are described in detail in Non-Patent Document 3. A similar configuration can be exemplified for the third optical mode coupler 301. The same applies to the following application embodiments.

Another example of the configuration of the first optical mode coupler 300 is shown in FIG. 11. In FIG. 11, 300-3 is an axis-offset connection part. In the axis-offset connection part 300-3, two optical fibers are connected with the centers of the cores offset. The axis-offset connection part 300-3 may be fused with the centers of the cores of the two optical fibers offset, or may be made by connecting them with a connector. Due to the axial discontinuity of the cores of the two optical fibers, light propagating in the fundamental mode is coupled to the guided mode of the fundamental mode and the higher mode. With this configuration, an inexpensive optical mode coupler can be obtained. It is desirable that the optical fiber on the side of the forward measurement light source 100 of the axis-offset connection part 300-3 is an optical fiber that propagates only in the fundamental mode. It can be a high-precision optical fiber vibration sensing device. A similar configuration can be exemplified for the third optical mode coupler 301. The same applies to the following application embodiments.

An example of the configuration of the second optical mode coupler 310 is shown in FIG. 8. In FIG. 8, the second optical mode coupler 310 is composed of an optical mode conversion coupler 300-2 and an optical branching and coupling coupler 300-1. The optical mode conversion coupler 300-2 here is a reverse of the input/output relationship of the optical mode conversion coupler 300-2 in the first optical mode coupler 300. That is, the optical mode conversion coupler 300-2 outputs the fundamental mode light of the light input to one input terminal to one output terminal while keeping it in the fundamental mode, and converts the light of the higher order mode to the fundamental mode and outputs it to the other output terminal. The optical branching and coupling coupler 300-1 here is a reverse of the input/output relationship of the optical branching and coupling coupler 300-1 in the first optical mode coupler 300. That is, the optical branching and coupling coupler 300-1 combines the fundamental mode light input to one input terminal with the fundamental mode light input to the other input terminal, and outputs the fundamental mode light to the output terminal. The optical mode conversion coupler is described in detail in Non-Patent Document 3. A similar configuration can be exemplified for the fourth optical mode coupler 311. The same applies to the following application embodiments.

Another example of the configuration of the second optical mode coupler 310 is shown in FIG. 12. In FIG. 12, 300-3 is an axially offset connection section, and 300-4 is a single mode optical fiber. In the axially offset connection section 300-3, two optical fibers are connected with the centers of the cores offset. The axially offset connection section 300-3 may be fused with the centers of the cores of the two optical fibers offset, or may be made by connecting them with a connector. The single mode optical fiber 300-4 is an optical fiber that propagates only the fundamental mode at the wavelength of the light emitted by the forward measurement light source 100. Due to the axial discontinuity of the cores of the two optical fibers, the light propagating in the fundamental mode couples with the guided modes of the fundamental mode and the higher mode, and the light propagating in the higher mode couples with the guided modes of the fundamental mode and the higher mode. Of these, the single mode optical fiber 300-4 selects and propagates only the fundamental mode. That is, the optical mode coupler of FIG. 12 leaves some of the fundamental mode light of the light input to the input terminal as it is, and converts some of the higher mode light to the fundamental mode and outputs it. With this configuration, it is possible to make an inexpensive optical mode coupler. A similar configuration can be exemplified for the fourth optical mode coupler 311. The same applies to the following application embodiments.

The first optical transmission path 20 has a propagation constant that allows high-order mode light to propagate at the wavelength of the light emitted by the forward measurement light source 100. The second optical transmission path 21 has a propagation constant that allows high-order mode light to propagate at the wavelength of the light emitted by the backward measurement light source 101.

The optical fiber vibration sensing device disclosed herein is simple in configuration and can achieve optical fiber vibration sensing that can identify the position where vibration is applied.

(Application embodiment 2)
The configuration of the optical fiber vibration sensing device of this embodiment is shown in Fig. 3. In Fig. 3, 1 is an optical transmission medium having a plurality of optical transmission paths, 20 is a first optical transmission path, 21 is a second optical transmission path, 40 is a third optical transmission path, 41 is a fourth optical transmission path, 100 is a forward direction measurement light source, 101 is a backward direction measurement light source, 200 is a forward direction measurement light receiver, 201 is a backward direction measurement light receiver, 300 is a first optical mode coupler, 310 is a second optical mode coupler, 301 is a third optical mode coupler, 311 is a fourth optical mode coupler, 400 is a first AD converter, 401 is a second AD converter, and 500 is an analysis device.

The optical fiber vibration sensing device is equipped with a forward Mach-Zehnder interferometer and a backward Mach-Zehnder interferometer, with the light travelling in the opposite directions. The basic operating principle is the same as in application embodiment 1, but differs from application embodiment 1 in that the forward measurement receiver 200 of the forward Mach-Zehnder interferometer and the backward measurement light source 101 of the backward Mach-Zehnder interferometer are arranged at one end of an optical transmission medium 1 having multiple optical transmission paths.

As a forward Mach-Zehnder interferometer, a forward measurement light source 100 that transmits light is provided at one end of an optical transmission medium 1 having a plurality of optical transmission paths, a first optical mode coupler 300 that keeps a portion of the fundamental mode light from the forward measurement light source 100 in the fundamental mode and converts the remainder to a higher mode, combines the light that remains in the fundamental mode with the converted light in the higher mode, and inputs it to the first optical transmission path 20, and a forward measurement optical receiver 200 that receives light from the third optical transmission path 40. At the other end of the optical transmission medium 1 having a plurality of optical transmission paths, a second optical mode coupler 310 is provided that keeps the fundamental mode light of the light propagating through the first optical transmission path 20 in the fundamental mode and converts the light in the higher mode to the fundamental mode, combines the light that remains in the fundamental mode with the converted light in the fundamental mode, and outputs it to the third optical transmission path 40.

In the forward Mach-Zehnder interferometer, a forward measurement light source 100 at one end of an optical transmission medium 1 having multiple optical transmission paths sends out light, and a first optical mode coupler 300 leaves part of the fundamental mode light from the forward measurement light source 100 in the fundamental mode and converts the rest to a higher mode, combines the light that has been kept in the fundamental mode with the converted light in the higher mode, and inputs it to the first optical transmission path 20. A second optical mode coupler 310 at the other end of the optical transmission medium 1 having multiple optical transmission paths leaves the fundamental mode light of the light that has propagated through the first optical transmission path 20 in the fundamental mode and converts the higher mode light to the fundamental mode, combines the light that has been kept in the fundamental mode with the converted light in the fundamental mode, and outputs it to the third optical transmission path 40. A forward measurement receiver 200 at one end of the optical transmission medium 1 having multiple optical transmission paths receives the light from the third optical transmission path 40.

As a backward Mach-Zehnder interferometer, at one end of the optical transmission medium 1 having a plurality of optical transmission paths, there is provided a backward measurement light source 101 that transmits light to the fourth optical transmission path 41, a fourth optical mode coupler 311 that, among the light propagating through the second optical transmission path 21, leaves the light in the fundamental mode as it is and converts the light in the higher-order mode to the fundamental mode, and combines the light that has been kept in the fundamental mode with the light that has been converted into the fundamental mode, and a backward measurement optical receiver 201 that receives the light from the fourth optical mode coupler 311. At the other end of the optical transmission medium 1 having a plurality of optical transmission paths, there is provided a third optical mode coupler 301 that leaves a part of the light in the fundamental mode from the fourth optical transmission path 41 as it is in the fundamental mode, converts the rest to the higher-order mode, combines the light that has been kept in the fundamental mode with the light that has been converted into the higher-order mode, and inputs it to the second optical transmission path 21.

In the backward Mach-Zehnder interferometer, the backward measurement light source 101 at one end of the optical transmission medium 1 having multiple optical transmission paths sends light to the fourth optical transmission path 41. The third optical mode coupler 301 at the other end of the optical transmission medium 1 having multiple optical transmission paths leaves part of the fundamental mode light propagating through the fourth optical transmission path 41 in the fundamental mode and converts the rest to a higher mode, combines the light that has been kept in the fundamental mode with the converted light in the higher mode, and inputs it to the second optical transmission path 21. The fourth optical mode coupler 311 at one end of the optical transmission medium 1 having multiple optical transmission paths leaves the fundamental mode light of the light that has propagated through the second optical transmission path 21 in the fundamental mode and converts the light in the higher mode to the fundamental mode, combines the light that has been kept in the fundamental mode with the converted light in the fundamental mode, and the backward measurement receiver 201 receives the light from the fourth optical mode coupler 311.

The first optical transmission path 20 has a propagation constant that allows the propagation of higher-order mode light at the wavelength of the light emitted by the forward measurement light source 100. The third optical transmission path 40 has a propagation constant that allows only fundamental mode light or higher-order mode light to propagate at the wavelength of the light emitted by the forward measurement light source 100. The second optical transmission path 21 has a propagation constant that allows higher-order mode light to propagate at the wavelength of the light emitted by the backward measurement light source 101. The fourth optical transmission path 41 has a propagation constant that allows only fundamental mode light or higher-order mode light to propagate at the wavelength of the light emitted by the backward measurement light source 101.

The optical fiber vibration sensing device disclosed herein is simple in configuration and can realize optical fiber vibration sensing capable of identifying the position where vibration is applied. In addition, the optical fiber vibration sensing device disclosed herein can arrange the forward direction measurement light source 100, the backward direction measurement light source 101, the forward direction measurement receiver 200, and the backward direction measurement receiver 201, which are active devices, on the same side (one end of the optical transmission medium 1 having multiple optical transmission paths).

(Application embodiment 3)
The configuration of the optical fiber vibration sensing device of this embodiment is shown in Fig. 4. In Fig. 4, 1 is an optical transmission medium having a plurality of optical transmission paths, 20 is a first optical transmission path, 21 is a second optical transmission path, 100 is a forward direction measurement light source, 101 is a backward direction measurement light source, 200 is a forward direction measurement light receiver, 201 is a backward direction measurement light receiver, 320 is a fifth optical mode coupler, 330 is a sixth optical mode coupler, 400 is a first AD converter, 401 is a second AD converter, and 500 is an analysis device. In this application embodiment, different optical transmission paths are secured for a single optical fiber in the forward direction Mach-Zehnder interferometer and the backward direction Mach-Zehnder interferometer by wavelength division multiplexing technology.

The optical fiber vibration sensing device is equipped with a forward Mach-Zehnder interferometer and a backward Mach-Zehnder interferometer, with the light travelling in the opposite directions. The basic operating principle is the same as in application embodiment 1, but it differs from application embodiment 1 in that the forward measurement light source 100 of the forward Mach-Zehnder interferometer and the backward measurement light source 101 of the backward Mach-Zehnder interferometer use different wavelengths, allowing the forward Mach-Zehnder interferometer and the backward Mach-Zehnder interferometer to share the same optical fiber.

The forward Mach-Zehnder interferometer and the reverse Mach-Zehnder interferometer share a fifth optical mode coupler 320 that is connected to the first optical transmission path 20 and the second optical transmission path 21 at one end of the optical transmission medium 1 having multiple optical transmission paths and that couples and branches light, and share a sixth optical mode coupler 330 that is connected to the first optical transmission path 20 and the second optical transmission path 21 at the other end of the optical transmission medium 1 having multiple optical transmission paths and that couples and branches light.

As a forward Mach-Zehnder interferometer, a forward measurement light source 100 that transmits light is provided at one end of an optical transmission medium 1 having a plurality of optical transmission paths, and a fifth optical mode coupler 320 that keeps part of the fundamental mode light from the forward measurement light source 100 in the fundamental mode and converts the rest to a higher mode, combines the light that remains in the fundamental mode with the converted light in the higher mode, and inputs it to a first optical transmission path 20. At the other end of the optical transmission medium 1 having a plurality of optical transmission paths, a sixth optical mode coupler 330 that keeps the fundamental mode light of the light propagating through the first optical transmission path 20 in the fundamental mode and converts the higher mode light to the fundamental mode, combines the light that remains in the fundamental mode with the converted light in the fundamental mode, and outputs it to a forward measurement optical receiver 200, and a forward measurement optical receiver 200 that receives light from the sixth optical mode coupler 330.

In the forward Mach-Zehnder interferometer, the forward measurement light source 100 at one end of the optical transmission medium 1 having multiple optical transmission paths sends out light, and the fifth optical mode coupler 320 leaves part of the fundamental mode light from the forward measurement light source 100 in the fundamental mode and converts the rest to a higher mode, combines the light that has been kept in the fundamental mode with the converted light in the higher mode, and inputs it to the first optical transmission path 20. The sixth optical mode coupler 330 at the other end of the optical transmission medium 1 having multiple optical transmission paths leaves the fundamental mode light of the light that has propagated through the first optical transmission path 20 in the fundamental mode and converts the higher mode light to the fundamental mode, combines the light that has been kept in the fundamental mode with the converted light in the fundamental mode, and outputs it to the forward measurement optical receiver 200. The forward measurement optical receiver 200 receives the light from the sixth optical mode coupler 330.

The backward Mach-Zehnder interferometer is provided with a backward measurement light source 101 that transmits light of a different wavelength from the forward measurement light source 100 at the other end of the optical transmission medium 1 having a plurality of optical transmission paths, a sixth optical mode coupler 330 that keeps a part of the fundamental mode light from the backward measurement light source 101 in the fundamental mode and converts the rest to a higher mode, combines the light that remains in the fundamental mode with the converted light in the higher mode, and inputs it to the second optical transmission path 21. The backward measurement optical receiver 201 is provided with a fifth optical mode coupler 320 that keeps the fundamental mode light of the light propagating through the second optical transmission path 21 in the fundamental mode and converts the higher mode light to the fundamental mode, combines the light that remains in the fundamental mode with the converted light in the fundamental mode, and outputs it to the backward measurement optical receiver 201, and receives the light from the fifth optical mode coupler 320.

In the backward Mach-Zehnder interferometer, the backward measurement light source 101 at the other end of the optical transmission medium 1 having multiple optical transmission paths sends out light, and the sixth optical mode coupler 330 leaves part of the fundamental mode light from the backward measurement light source 101 in the fundamental mode and converts the rest to a higher mode, combines the light that has been kept in the fundamental mode with the converted light in the higher mode, and inputs it to the second optical transmission path 21. The fifth optical mode coupler 320 at one end of the optical transmission medium 1 having multiple optical transmission paths leaves the fundamental mode light of the light that has propagated through the second optical transmission path 21 in the fundamental mode and converts the higher mode light to the fundamental mode, combines the light that has been kept in the fundamental mode with the converted light in the fundamental mode, and outputs it to the backward measurement optical receiver 201. The backward measurement optical receiver 201 receives the light from the fifth optical mode coupler 320.

An example of the configuration of the fifth optical mode coupler 320 is shown in FIG. 9. In FIG. 9, the fifth optical mode coupler 320 is composed of an optical branching/coupling coupler 320-1 and an optical mode conversion coupler 320-2. The optical branching/coupling coupler 320-1 branches the fundamental mode light input to one input terminal on the left side of the drawing into two fundamental mode lights, branches the fundamental mode light input to the other input terminal on the left side of the drawing into two fundamental mode lights, and outputs the fundamental mode light input to one input terminal and the fundamental mode light input to the other input terminal at each output terminal on the right side of the drawing after multiplexing them. It performs the opposite operation for light from the right side of the drawing. That is, the optical branching/coupling coupler 320-1 branches the fundamental mode light inputted to one input terminal on the right side of the drawing into two fundamental mode lights, branches the fundamental mode light inputted to the other input terminal on the right side of the drawing into two fundamental mode lights, and outputs the fundamental mode light inputted to one input terminal and the fundamental mode light inputted to the other input terminal to each output terminal on the left side of the drawing after multiplexing. The optical mode conversion coupler 300-2 keeps the fundamental mode light inputted to one input terminal on the left side of the drawing as it is, converts the fundamental mode light inputted to the other input terminal on the left side of the drawing into light of a higher order mode, multiplexes the light kept in the fundamental mode and the converted light of the higher order mode, and outputs it to the output terminal on the right side of the drawing. The optical mode conversion coupler 300-2 operates in the opposite manner for light from the right side of the drawing. The optical mode conversion coupler 300-2 keeps the fundamental mode light inputted to one input terminal on the right side of the drawing as it is, and outputs it to one output terminal on the left side of the drawing. Higher-mode light input to one input terminal on the right side of the drawing is converted to fundamental-mode light and output to another output terminal on the left side of the drawing. Optical mode conversion couplers are described in detail in Non-Patent Document 3. The same applies to the following application embodiments.

An example of the configuration of the sixth optical mode coupler 330 is shown in FIG. 10. The sixth optical mode coupler shown in FIG. 10 operates in the same manner as the fifth optical mode coupler 320. It is just that the configuration is reversed on the left and right in the drawing. Optical mode conversion couplers are described in detail in Non-Patent Document 3. The same applies to the following application embodiments.

The first optical transmission path 20 has a propagation constant that allows high-order mode light to propagate at the wavelength of the light emitted by the forward measurement light source 100. The second optical transmission path 21 has a propagation constant that allows high-order mode light to propagate at the wavelength of the light emitted by the backward measurement light source 101.

The optical fiber vibration sensing device disclosed herein is capable of realizing optical fiber vibration sensing capable of identifying the position where vibration is applied with a simple configuration. In addition, the optical fiber vibration sensing device disclosed herein can share the same optical fiber by using different wavelengths for the forward Mach-Zehnder interferometer and the reverse Mach-Zehnder interferometer.

(Application embodiment 4)
The configuration of the optical fiber vibration sensing device of this embodiment is shown in Fig. 5. In Fig. 5, 1 is an optical transmission medium having a plurality of optical transmission paths, 20 is a first optical transmission path, 21 is a second optical transmission path, 40 is a third optical transmission path, 41 is a fourth optical transmission path, 100 is a forward direction measurement light source, 101 is a backward direction measurement light source, 200 is a forward direction measurement light receiver, 201 is a backward direction measurement light receiver, 320 is a fifth optical mode coupler, 330 is a sixth optical mode coupler, 400 is a first AD converter, 401 is a second AD converter, and 500 is an analysis device. In this application embodiment, different optical transmission paths are secured for a single optical fiber in the forward direction Mach-Zehnder interferometer and the backward direction Mach-Zehnder interferometer by wavelength division multiplexing technology.

The optical fiber vibration sensing device includes a forward Mach-Zehnder interferometer and a backward Mach-Zehnder interferometer, with the light travelling in the opposite directions. The basic operating principle is the same as in the application embodiment 1, but differs from the application embodiment 1 in that the forward measurement receiver 200 of the forward Mach-Zehnder interferometer and the backward measurement light source 101 of the backward Mach-Zehnder interferometer are disposed at one end of the optical transmission medium 1 having multiple optical transmission paths. Also, it differs from the application embodiment 1 in that the forward measurement light source 100 of the forward Mach-Zehnder interferometer and the backward measurement light source 101 of the backward Mach-Zehnder interferometer use different wavelengths, allowing the forward Mach-Zehnder interferometer and the backward Mach-Zehnder interferometer to share the same optical fiber.

The forward Mach-Zehnder interferometer and the reverse Mach-Zehnder interferometer share a fifth optical mode coupler 320 that is connected to the first optical transmission path 20 and the second optical transmission path 21 at one end of the optical transmission medium 1 having multiple optical transmission paths and that couples and branches light, and share a sixth optical mode coupler 330 that is connected to the first optical transmission path 20 and the second optical transmission path 21 at the other end of the optical transmission medium 1 having multiple optical transmission paths and that couples and branches light.

As a forward Mach-Zehnder interferometer, a forward measurement light source 100 that transmits light is provided at one end of an optical transmission medium 1 having a plurality of optical transmission paths, a fifth optical mode coupler 320 that keeps a portion of the fundamental mode light from the forward measurement light source 100 in the fundamental mode and converts the remainder to a higher mode, combines the light that remains in the fundamental mode with the converted light in the higher mode, and inputs it to the first optical transmission path 20, and a forward measurement optical receiver 200 that receives light from the third optical transmission path 40. At the other end of the optical transmission medium 1 having a plurality of optical transmission paths, a sixth optical mode coupler 330 is provided that keeps the fundamental mode light of the light propagating through the first optical transmission path 20 in the fundamental mode and converts the light in the higher mode to the fundamental mode, combines the light that remains in the fundamental mode with the converted light in the fundamental mode, and outputs it to the third optical transmission path 40.

In the forward Mach-Zehnder interferometer, the forward measurement light source 100 at one end of the optical transmission medium 1 having multiple optical transmission paths sends out light, and the fifth optical mode coupler 320 leaves part of the fundamental mode light from the forward measurement light source 100 in the fundamental mode and converts the rest to a higher mode, combines the light that has been kept in the fundamental mode with the converted light in the higher mode, and inputs it to the first optical transmission path 20. The sixth optical mode coupler 330 at the other end of the optical transmission medium 1 having multiple optical transmission paths leaves the fundamental mode light of the light that has propagated through the first optical transmission path 20 in the fundamental mode and converts the light in the higher mode to the fundamental mode, combines the light that has been kept in the fundamental mode with the converted light in the fundamental mode, and outputs it to the third optical transmission path 40. The forward measurement receiver 200 at one end of the optical transmission medium 1 having multiple optical transmission paths receives the light from the third optical transmission path 40.

As a backward Mach-Zehnder interferometer, at one end of the optical transmission medium 1 having a plurality of optical transmission paths, there is provided a backward measurement light source 101 that transmits light of a wavelength different from that of the forward measurement light source 100 to the fourth optical transmission path 41, a fifth optical mode coupler 320 that, among the light propagating through the second optical transmission path 21, leaves the light in the fundamental mode as it is and converts the light in the higher order mode to the fundamental mode, combines the light in the fundamental mode with the converted light in the fundamental mode, and outputs it to the backward measurement optical receiver 201, and a backward measurement optical receiver 201 that receives the light from the fifth optical mode coupler 320. At the other end of the optical transmission medium 1 having a plurality of optical transmission paths, there is provided a sixth optical mode coupler 330 that leaves a part of the light in the fundamental mode from the fourth optical transmission path 41 as it is in the fundamental mode and converts the rest to the higher order mode, combines the light in the fundamental mode with the converted light in the higher order mode, and inputs it to the second optical transmission path 21.

In the backward Mach-Zehnder interferometer, the backward measurement light source 101 at one end of the optical transmission medium 1 having multiple optical transmission paths sends light to the fourth optical transmission path 41. The sixth optical mode coupler 330 at the other end of the optical transmission medium 1 having multiple optical transmission paths keeps a part of the fundamental mode light propagating through the fourth optical transmission path 41 as the fundamental mode and converts the rest to a higher mode, combines the light that has been kept in the fundamental mode with the converted light in the higher mode, and inputs it to the second optical transmission path 21. The fifth optical mode coupler 320 at one end of the optical transmission medium 1 having multiple optical transmission paths keeps the fundamental mode light of the light that has propagated through the second optical transmission path 21 as the fundamental mode and converts the light in the higher mode to the fundamental mode, combines the light that has been kept in the fundamental mode with the converted light in the fundamental mode, and outputs it to the backward measurement receiver 201. The backward measurement receiver 201 receives the light from the fifth optical mode coupler 320.

The first optical transmission path 20 has a propagation constant that allows the propagation of higher-order mode light at the wavelength of the light emitted by the forward measurement light source 100. The third optical transmission path 40 has a propagation constant that allows only fundamental mode light or higher-order mode light to propagate at the wavelength of the light emitted by the forward measurement light source 100. The second optical transmission path 21 has a propagation constant that allows higher-order mode light to propagate at the wavelength of the light emitted by the backward measurement light source 101. The fourth optical transmission path 41 has a propagation constant that allows only fundamental mode light or higher-order mode light to propagate at the wavelength of the light emitted by the backward measurement light source 101.

The optical fiber vibration sensing device disclosed herein can realize optical fiber vibration sensing capable of identifying the position where vibration is applied with a simple configuration. In addition, the optical fiber vibration sensing device disclosed herein can arrange the forward direction measurement light source 100, the backward direction measurement light source 101, the forward direction measurement receiver 200, and the backward direction measurement receiver 201, which are active devices, on the same side (one end of the optical transmission medium 1 having multiple optical transmission paths). Furthermore, the optical fiber vibration sensing device disclosed herein can share the same optical fiber by using different wavelengths for the forward direction Mach-Zehnder interferometer and the backward direction Mach-Zehnder interferometer.

(Application embodiment 5)
In the basic configuration embodiment and application embodiments 1 to 4, the optical fiber vibration sensing device may further include a first AD converter 400 that performs AD conversion of an electrical signal from a forward Mach-Zehnder interferometer (in application embodiments 1 to 4, the forward measurement receiver 200), a second AD converter 401 that performs AD conversion of an electrical signal from a reverse Mach-Zehnder interferometer (in application embodiments 1 to 4, the reverse measurement receiver 201), and an analysis device 500 that identifies the location of occurrence of vibration using the signal from the first AD converter 400 and the signal from the second AD converter 401.

In FIG. 1, if the total length of the optical transmission medium 1 having a plurality of optical transmission paths is L, and the distance from the other end of the optical transmission path to the point Pv where the vibration is added is Lv, the time T1 required for an interference wave with a beat caused by the vibration added at the point Pv to reach the other end of the optical transmission path of the forward Mach-Zehnder interferometer is given by
T1 = Lv/c1 (1)
Here, c1 is the speed of light in the optical transmission line of the forward Mach-Zehnder interferometer.

The time T2 required for the beat-containing interference wave caused by the vibration added at point Pv to reach one end of the optical transmission line of the reverse Mach-Zehnder interferometer is given by
T2 = (L-Lv)/c2 (2)
Here, c2 is the speed of light in the optical transmission line of the reverse Mach-Zehnder interferometer.

As a result, the time difference ΔT between when the beat-containing interference wave caused by the vibration applied at point Pv reaches the other end of the optical transmission path of the forward Mach-Zehnder interferometer and when it reaches one end of the optical transmission path of the backward Mach-Zehnder interferometer is given by
ΔT = T2 - T1
= (L-Lv)/c2-Lv/c1 (3)
From this, the distance Lv from the other end of the optical transmission line to the point Pv where the vibration is applied is obtained by the formula (4).
Lv = (L-c2×ΔT)c1/(c1+c2) (4)

If the speed of light in the optical transmission path of both the forward and reverse Mach-Zehnder interferometers is c, then equation (4) can be rewritten as Lv = (L - c × ΔT) / 2 (5)
For example, when the point Pv where the vibration is applied is at the midpoint of the optical transmission medium 1 having a plurality of optical transmission paths,
ΔT = 0 (6)
Equation (5) is
Lv = L/2 (7)
From formula (7), the point Pv where vibration is applied is a halfway position of the optical transmission medium 1 having a plurality of optical transmission paths, and therefore the validity of formula (4) is shown.

In this way, by measuring the time difference ΔT between when the beat interference wave caused by the vibration added at point Pv reaches the other end of the optical transmission path of the forward Mach-Zehnder interferometer and when it reaches one end of the optical transmission path of the reverse Mach-Zehnder interferometer, the distance Lv from the other end of the optical transmission path of the forward Mach-Zehnder interferometer to the point Pv where the vibration is added can be obtained. Note that the beat interference wave is only generated when the two lights are combined, but for convenience, it is explained here as if the beat interference wave propagates from the point where the vibration is added.

Two interference wave signals observed at the same time by the forward direction measurement receiver 200 and the reverse direction measurement receiver 201 are denoted as w1(t) and w2(t). ΔT is equal to the shift amount when the cross-correlation value of w1(t) and w2(t) is at its maximum. In other words, it can be obtained by changing τ in the following equation and setting τ at the time when the correlation value (w1*w2)(τ) is at its maximum as ΔT.
(w1*w2)(τ) = Σ w1(t)×w2(τ-t) (8)

The analysis device 500 captures the interference wave signal through the first AD converter 400 and the second AD converter 401, calculates the cross-correlation at regular intervals, and obtains ΔT, making it possible to identify the distance Lv to the vibration generation position Pv. Therefore, the point where vibration is applied can be obtained sequentially.

The analysis device 500 identifies the location of the vibration by taking into consideration the delay time of the optical signal from the other end of the optical transmission path to the forward measurement receiver 200, the delay time of the electrical signal from the forward measurement receiver 200 via the first AD converter 400 to the analysis device 500, the delay time of the optical signal from one end of the optical transmission path to the reverse measurement receiver 201, and the delay time of the electrical signal from the reverse measurement receiver 201 via the second AD converter 401 to the analysis device 500.

Figure 6 shows a flowchart of the optical fiber position sensing method executed by the analysis device 500. In the fixed-time data acquisition process 600, interference wave signals w1(t) and w2(t) are acquired from the forward Mach-Zehnder interferometer (forward measurement receiver 200 in application embodiments 1 to 4) and the reverse Mach-Zehnder interferometer (reverse measurement receiver 201 in application embodiments 1 to 4) in which the light travels in the opposite direction to that of the forward Mach-Zehnder interferometer, respectively.

In the cross-correlation calculation step 601, τ in the formula (8) is changed, and τ when the correlation value (w1*w2)(τ) is maximized is obtained as ΔT.
(w1*w2)(τ) = Σ w1(t)×w2(τ-t) (8)

In the vibration generation position calculation step 603, the distance Lv from the other end of the optical transmission line to the point where the vibration is applied is calculated using equation (4).
Lv = (L-c2×ΔT)c1/(c1+c2) (4)
Here, L is the total length of the optical transmission medium having a plurality of optical transmission paths, c1 is the speed of light in the optical transmission path of the forward Mach-Zehnder interferometer, and c2 is the speed of light in the optical transmission path of the backward Mach-Zehnder interferometer.

An effective time difference calculation step 602 may be provided between the cross-correlation calculation step 601 and the vibration occurrence position calculation step 603. The interference signals received by the forward direction measurement receiver 200 and the reverse direction measurement receiver 201 may have a poor signal-to-noise ratio (SNR) depending on the interference state, and may not be an effective time difference ΔT. Therefore, in the effective time difference calculation step 602, the maximum time difference ΔTmax expected from the longest distance L of the optical transmission medium 1 having multiple optical transmission paths is compared with the absolute value of ΔT,
|ΔT| ≦ ΔTmax (9)
If not, the process returns to the data acquisition step 600 for a certain period of time, and data acquisition is started again. Here, ΔTmax is obtained by the following formula.
ΔTmax = max{L/c1, L/c2} (10)
If the formula (9) is satisfied, the process proceeds to vibration occurrence position calculation step 603 .

As described above, the optical fiber vibration sensing device and optical fiber vibration sensing method disclosed herein can realize optical fiber vibration sensing that can identify the position where vibration is applied with a simple configuration.

The analysis device of the present invention can also be realized by a computer and a program. The program can be recorded on a recording medium or provided via a communication network.

This disclosure can be applied to the information and communications industry.

1: Optical transmission medium having a plurality of optical transmission paths 10: Forward direction Mach-Zehnder interferometer 11: Backward direction Mach-Zehnder interferometer 20: First optical transmission path 21: Second optical transmission path 40: Third optical transmission path 41: Fourth optical transmission path 100: Forward direction measurement light source 101: Backward direction measurement light source 200: Forward direction measurement receiver 201: Backward direction measurement receiver 300: First optical mode coupler 300-1: Optical branching and coupling coupler 300 -2: Optical mode conversion coupler 300-3: Off-axis connection section 300-4: Single mode optical fiber 310: Second optical mode coupler 301: Third optical mode coupler 311: Fourth optical mode coupler 320: Fifth optical mode coupler 320-1: Optical branching/coupling coupler 320-2: Optical mode conversion coupler 330: Sixth optical mode coupler 400: First AD converter 401: Second AD converter 500: Analysis device

Claims (11)

an optical transmission medium having a plurality of optical transmission paths;
a forward Mach-Zehnder interferometer which, at one end of the optical transmission medium, leaves a portion of the light in fundamental mode as it is and converts the remainder to a higher-order mode, combines the light left in fundamental mode with the converted light in higher-order mode, and inputs the combined light into an optical transmission line, and at the other end of the optical transmission medium, leaves the light in fundamental mode of the light that has propagated through the optical transmission line in fundamental mode and converts the light in higher-order mode to fundamental mode, and combines and receives the light left in fundamental mode with the converted light in fundamental mode;
a reverse Mach-Zehnder interferometer which, at the other end of the optical transmission medium, leaves a part of the light in fundamental mode as it is and converts the remainder to a higher-order mode, combines the light left in fundamental mode with the converted light in higher-order mode, and inputs the combined light to another optical transmission path, and at one end of the optical transmission medium, leaves the light in fundamental mode of the light propagating through the other optical transmission path in fundamental mode and converts the light in higher-order mode to fundamental mode, and combines and receives the light left in fundamental mode with the converted light in fundamental mode;
An optical fiber vibration sensing device comprising: the optical transmission medium includes a first optical transmission path and a second optical transmission path;
The forward Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a forward measurement light source that emits light;
a first optical mode coupler that leaves a portion of the fundamental mode light from the forward direction measurement light source as it is in the fundamental mode and converts the remainder into a higher order mode, combines the light that remains in the fundamental mode with the converted light in the higher order mode, and inputs the combined light into the first optical transmission line;
At the other end of the optical transmission medium,
a second optical mode coupler which, among the light propagating through the first optical transmission line, leaves the light in a fundamental mode as it is and converts the light in a higher order mode to the fundamental mode, and couples the light that remains in the fundamental mode with the converted light in the fundamental mode;
a forward direction measurement optical receiver that receives light from the second optical mode coupler;
The reverse Mach-Zehnder interferometer comprises:
At the other end of the optical transmission medium,
a light source for backward measurement that emits light;
a third optical mode coupler that leaves a part of the fundamental mode light from the backward direction measurement light source as the fundamental mode and converts the remainder into a higher order mode, combines the light that remains in the fundamental mode with the converted light in the higher order mode, and inputs the combined light into the second optical transmission line;
At one end of the optical transmission medium,
a fourth optical mode coupler that, among the light propagating through the second optical transmission line, leaves light in a fundamental mode as it is and converts light in a higher order mode to the fundamental mode, and couples the light that remains in the fundamental mode with the converted light in the fundamental mode;
2. The optical fiber vibration sensing device according to claim 1, further comprising: a backward measurement light receiver for receiving light from the fourth optical mode coupler. the optical transmission medium includes a first optical transmission path, a second optical transmission path, a third optical transmission path, and a fourth optical transmission path;
The forward Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a forward measurement light source that emits light;
a first optical mode coupler that leaves a part of the fundamental mode light from the forward direction measurement light source as it is in the fundamental mode and converts the remainder into a higher order mode, combines the light that remains in the fundamental mode with the converted light in the higher order mode, and inputs the combined light into the first optical transmission line;
a forward direction measurement optical receiver that receives light from the third optical transmission path,
At the other end of the optical transmission medium,
a second optical mode coupler which, among the light propagating through the first optical transmission line, leaves light in a fundamental mode as it is and converts light in a higher-order mode to a fundamental mode, combines the light which remains in the fundamental mode with the converted light in the fundamental mode, and outputs the combined light to the third optical transmission line;
The reverse Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a light source for backward measurement that transmits light to the fourth optical transmission line;
a fourth optical mode coupler that, among the light propagating through the second optical transmission line, leaves light in a fundamental mode as it is and converts light in a higher order mode to the fundamental mode, and couples the light that remains in the fundamental mode with the converted light in the fundamental mode;
a backward direction measurement optical receiver that receives light from the fourth optical mode coupler,
At the other end of the optical transmission medium,
The optical fiber vibration sensing device described in claim 1, characterized in that it is provided with a third optical mode coupler that leaves a portion of the fundamental mode light from the fourth optical transmission path in the fundamental mode and converts the remainder to a higher mode, combines the light that remains in the fundamental mode with the converted higher mode light, and inputs it to the second optical transmission path. The optical fiber vibration sensing device according to claim 2 or 3, characterized in that the first optical mode coupler or the third optical mode coupler is an off-axis connection portion in which the centers of the cores of two optical fibers are connected with an offset. The optical fiber vibration sensing device according to claim 4, characterized in that the optical fiber on the side of the forward measurement light source of the axially offset connection portion as the first optical mode coupler or the side of the reverse measurement light source of the axially offset connection portion as the third optical mode coupler propagates only light in the fundamental mode. An optical fiber vibration sensing device as described in any one of claims 2 to 5, characterized in that the second optical mode coupler or the fourth optical mode coupler is an off-axis connection section in which the centers of the cores of two optical fibers are connected with an offset, and the optical fiber on the side of the forward direction measurement receiver of the off-axis connection section as the second optical mode coupler or the side of the reverse direction measurement receiver of the off-axis connection section as the fourth optical mode coupler propagates only light in the fundamental mode. the optical transmission medium comprises an optical transmission path,
the forward Mach-Zehnder interferometer and the backward Mach-Zehnder interferometer share a fifth optical mode coupler that combines and branches light at one end of the optical transmission medium, and share a sixth optical mode coupler that combines and branches light at the other end of the optical transmission medium;
The forward Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
A forward measurement light source is provided to emit light,
the fifth optical mode coupler keeps a part of the fundamental mode light from the forward direction measurement light source as the fundamental mode and converts the remainder into a higher order mode, combines the light kept in the fundamental mode with the converted higher order mode light, and inputs the combined light into the optical transmission line;
At the other end of the optical transmission medium,
a forward direction measurement receiver for receiving light;
the sixth optical mode coupler, among the light propagating through the optical transmission line, leaves the light in a fundamental mode as it is, converts the light in a higher order mode to the fundamental mode, combines the light that remains in the fundamental mode with the converted light in the fundamental mode, and outputs the combined light to the forward direction measurement optical receiver;
The reverse Mach-Zehnder interferometer comprises:
At the other end of the optical transmission medium,
a backward measurement light source that emits light of a wavelength different from that of the forward measurement light source;
the sixth optical mode coupler leaves a part of the fundamental mode light from the backward measurement light source as the fundamental mode and converts the remainder into a higher-order mode, combines the light that remains in the fundamental mode with the converted higher-order mode light, and inputs the combined light into the optical transmission line;
At one end of the optical transmission medium,
a backward direction measurement optical receiver that receives light from the fifth optical mode coupler,
The optical fiber vibration sensing device described in claim 1, characterized in that, of the light propagating through the optical transmission path by the fifth optical mode coupler, the light in fundamental mode is left as it is in fundamental mode, and the light in higher modes is converted to fundamental mode, and the light that is left as it is in fundamental mode and the converted light in fundamental mode are combined and output to the reverse direction measurement receiver. the optical transmission medium includes a first optical transmission path, a third optical transmission path, and a fourth optical transmission path;
the forward Mach-Zehnder interferometer and the backward Mach-Zehnder interferometer share a fifth optical mode coupler that combines and branches light at one end of the optical transmission medium, and share a sixth optical mode coupler that combines and branches light at the other end of the optical transmission medium;
The forward Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a forward measurement light source that emits light;
a forward direction measurement optical receiver that receives light from the third optical transmission path,
the fifth optical mode coupler keeps a part of the fundamental mode light from the forward direction measurement light source as the fundamental mode and converts the remainder into a higher order mode, combines the light kept in the fundamental mode with the converted higher order mode light, and inputs the combined light into the first optical transmission path;
At the other end of the optical transmission medium,
the sixth optical mode coupler, among the light propagating through the first optical transmission line, leaves the light in fundamental mode as it is, converts the light in higher modes to fundamental mode, combines the light that remains in fundamental mode with the converted light in fundamental mode, and outputs the combined light to the third optical transmission line;
The reverse Mach-Zehnder interferometer comprises:
At one end of the optical transmission medium,
a backward direction measurement light source that transmits light of a wavelength different from that of the forward direction measurement light source to the fourth optical transmission line;
a backward direction measurement optical receiver that receives light from the fifth optical mode coupler,
the fifth optical mode coupler, among the light propagating through the first optical transmission line, leaves the light in a fundamental mode as it is, converts the light in a higher order mode to a fundamental mode, combines the light that remains in the fundamental mode with the converted light in the fundamental mode, and outputs the combined light to the backward direction measurement light source;
At the other end of the optical transmission medium,
The optical fiber vibration sensing device described in claim 1, characterized in that the sixth optical mode coupler leaves a portion of the fundamental mode light from the fourth optical transmission path in the fundamental mode and converts the remainder to a higher mode, and combines the light remaining in the fundamental mode with the converted higher mode light and inputs it to the first optical transmission path. a first AD converter that performs AD conversion on an electrical signal from the forward Mach-Zehnder interferometer;
a second AD converter that AD converts an electrical signal from the reverse Mach-Zehnder interferometer;
an analysis device that identifies a location where vibration occurs using a signal from the first AD converter and a signal from the second AD converter;
2. The optical fiber vibration sensing device according to claim 1, further comprising: a first AD converter that performs AD conversion on an electrical signal from the forward direction measurement light receiver;
a second AD converter that performs AD conversion on the electrical signal from the reverse direction measurement light receiver;
an analysis device that identifies a location where vibration occurs using a signal from the first AD converter and a signal from the second AD converter;
9. The optical fiber vibration sensing device according to claim 2, further comprising: a fixed-time data acquisition step of acquiring interference wave signals w1(t) and w2(t) from a forward Mach-Zehnder interferometer and a backward Mach-Zehnder interferometer in which the light travels in the opposite direction to that of the forward Mach-Zehnder interferometer, respectively;
a cross-correlation calculation step of changing τ in formula (8) and determining τ when the correlation value (w1 * w2) (τ) is maximized as ΔT;
(w1*w2)(τ) = Σ w1(t)×w2(τ-t) (8)
a vibration generation position calculation step of calculating a distance Lv from the other end of the optical transmission line to a point where vibration is applied using equation (4);
Lv = (L-c2×ΔT)c1/(c1+c2) (4)
Here, L is the total length of an optical transmission medium having a plurality of optical transmission paths, c1 is the speed of light in the optical transmission path of the forward Mach-Zehnder interferometer, and c2 is the speed of light in the optical transmission path of the reverse Mach-Zehnder interferometer.

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