Vibration sensing device and vibration sensing method
The vibration sensing device addresses the challenge of accessing both ends of optical fibers by using a simple configuration to detect vibrations in existing optical fiber networks, achieving efficient and cost-effective vibration sensing.
Patent Information
- Application Number
- JP2023179494
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2023-10-18
- Publication Date
- 2025-05-01
AI Technical Summary
Existing optical fiber networks pose challenges for vibration sensing due to the difficulty in accessing both ends for device installation, making it costly and complex to detect vibrations effectively.
A vibration sensing device that utilizes optical fibers with one end open, where test light with a wavelength shorter than the cutoff wavelength is input to excite multiple propagation modes, and the interference component is used to detect vibrations with a simple configuration.
Enables the detection of vibrations with a simple configuration, overcoming the challenges of accessing both ends of optical fibers in existing networks, and providing an efficient and cost-effective solution for vibration sensing.
Smart Images
Figure 2025069653000001_ABST
Abstract
Description
[Technical field]
[0001] The present disclosure relates to vibration sensing technology using optical fibers. [Background technology]
[0002] There is growing expectation for vibration sensing technology that can easily detect vibrations using existing communication optical fiber networks. In particular, inexpensive and easy-to-use vibration sensing technology is required for applications such as traffic monitoring and earthquake detection using communication optical fiber networks.
[0003] Toward these applications, a vibration sensing technology has been disclosed that uses test light with a wavelength shorter than the cutoff wavelength of single-mode optical fibers, which are used for communication, to excite multiple propagation modes in the optical fiber and utilize the interference between these modes (see, for example, Non-Patent Document 1).
[0004] Non-Patent Document 1 discloses a vibration sensing method that uses test light with a wavelength shorter than the cutoff wavelength of a single mode of a communication optical fiber to excite multiple propagation modes in the optical fiber and utilizes interference between the modes. [Prior art documents] [Non-patent literature]
[0005] [Non-Patent Document 1] A. Nakamura, T. Manabe, and Y. Koshikiya, “Vibration sensing based on inter-modal interference using two-mode region of conventional single-mode fibers,” Proc. 27th International Conference on Optical Fiber Sensors (OFS-27), Th4.28, 2022. Summary of the Invention [Problem to be solved by the invention]
[0006] However, in existing optical fiber networks, both ends are located at places distant from each other, and therefore it is often not easy from an operational and cost standpoint to access both ends and install devices there.
[0007] The present disclosure has been made in light of the above circumstances, and aims to realize a vibration sensing technology capable of detecting vibrations with a simple configuration. [Means for solving the problem]
[0008] Optical fibers that are not used in existing optical fiber networks usually have open ends at both ends. Therefore, we decided to utilize the phenomenon in which light input from one end of the optical fiber is reflected at the far end. In other words, we decided to input test light with a wavelength shorter than the cutoff wavelength of the single-mode optical fiber from one end of the optical fiber, and obtain the interference component of the propagation mode of the test light that is reflected from the far end and returns to the input end.
[0009] Specifically, the vibration sensing device of the present disclosure includes: A test light is input to one end of an optical fiber under test in a fundamental mode and a higher order mode; Among the light returning from one end of the optical fiber under test, light in a fundamental mode is left as it is in the fundamental mode, and light in a higher mode is converted to a fundamental mode, and the light that is left as it is in the fundamental mode is combined with the converted light in the fundamental mode; The coupled light is converted into an electrical signal, and the converted electrical signal is subjected to frequency analysis to obtain the vibration frequency of the vibration applied to the optical fiber under test.
[0010] Specifically, the vibration sensing device of the present disclosure includes: The vibration sensing device includes: a test light generating unit that generates and outputs test light; an input / output unit which leaves a part of the test light from the test light generating unit in a fundamental mode and converts a part of the test light into a higher-order mode, inputs the part into one end of the optical fiber under test, leaves the fundamental mode light of the light returning from the one end of the optical fiber under test in the fundamental mode and converts the higher-order mode light into the fundamental mode, and combines the light that has been left in the fundamental mode with the converted fundamental mode light; a receiver that converts the coupled light from the input / output unit into an electrical signal; a processor that performs frequency analysis on the converted electrical signal from the receiver to obtain a vibration frequency of the vibration applied to the optical fiber under test; The present invention is characterized by comprising:
[0011] Specifically, the vibration sensing device of the present disclosure includes: The optical fiber under test is further provided. The cutoff wavelength of the optical fiber under test is longer than the wavelength of the test light generated by the test light generating unit. It is characterized by:
[0012] Specifically, the vibration sensing device of the present disclosure includes: The other end of the optical fiber under test is provided with a reflection termination that reflects the propagating light. It is characterized by:
[0013] Specifically, the vibration sensing device of the present disclosure includes: The input / output unit includes: an offset-axis splice in which two optical fibers are connected with their cores offset from each other, and one end of the optical fiber to be tested is connected to one end of the optical fiber to be tested; an optical multiplexer / demultiplexer having a first terminal connected to the test light generating unit, a second terminal connected to the other end of the off-axis connecting unit, and a third terminal connected to the receiving unit, and outputs light input to the first terminal to the second terminal and outputs light input to the second terminal to the third terminal; The present invention is characterized by comprising:
[0014] Specifically, the vibration sensing device of the present disclosure includes: The optical fiber connecting the other end of the off-axis connection portion and the second terminal of the optical multiplexer / demultiplexer propagates only in the fundamental mode. It is characterized by:
[0015] Specifically, the vibration sensing device of the present disclosure includes: The input / output unit includes: a mode conversion coupler having a second input / output terminal connected to one end of the optical fiber under test, which keeps the fundamental mode light input to the first input / output terminal in the fundamental mode, converts the fundamental mode light input to the third input / output terminal to a higher mode, combines the light kept in the fundamental mode with the light converted to the higher mode and outputs the combined light from the second input / output terminal, outputs the fundamental mode light of the light input to the second input / output terminal in the fundamental mode from the first input / output terminal, and converts the higher mode light to the fundamental mode and outputs the combined light from the third input / output terminal; an optical branching and combining coupler having a first terminal connected to the test light generating unit, a second terminal connected to the first terminal of the mode converting coupler, a third terminal connected to the third terminal of the mode converting coupler, and a fourth terminal connected to the receiving unit, which branches light input to the first terminal and outputs the branched light to the second terminal and the third terminal, and combines the light input to the second terminal and the light input to the third terminal and outputs the combined light from the fourth terminal; The present invention is characterized by comprising:
[0016] Specifically, the vibration sensing method of the present disclosure includes: A test light is input to one end of an optical fiber under test in a fundamental mode and a higher order mode; Among the light returning from one end of the optical fiber under test, light in a fundamental mode is left as it is in the fundamental mode, and light in a higher mode is converted to a fundamental mode, and the light that is left as it is in the fundamental mode is combined with the converted light in the fundamental mode; The coupled light is converted into an electrical signal, and the converted electrical signal is subjected to frequency analysis to obtain the vibration frequency of the vibration applied to the optical fiber under test.
[0017] The above disclosures may be combined to the greatest extent possible. Effect of the Invention
[0018] The vibration sensing device and vibration sensing method disclosed herein can realize a vibration sensing technique capable of detecting vibrations with a simple configuration. [Brief description of the drawings]
[0019] [Figure 1] FIG. 1 is a diagram illustrating an example of a configuration of a vibration sensing device according to the present disclosure. [Diagram 2] FIG. 1 is a diagram illustrating an example of a configuration of a vibration sensing device according to the present disclosure. [Diagram 3] FIG. 1 is a diagram illustrating an example of a configuration of a vibration sensing device according to the present disclosure. [Figure 4] FIG. 2 is a diagram illustrating an example of a configuration of an input / output unit according to the present disclosure. [Diagram 5] FIG. 2 is a diagram illustrating an example of a configuration of an input / output unit according to the present disclosure. [Figure 6] FIG. 1 is a diagram illustrating an example of a vibration sensing method according to the present disclosure. [Figure 7] FIG. 1 is a diagram illustrating an example of a configuration of a vibration sensing device according to the present disclosure. [Figure 8] 11A to 11C are diagrams illustrating an example of an experimental result obtained with the vibration sensing device of the present disclosure. [Figure 9] 11A to 11C are diagrams illustrating an example of an experimental result obtained with the vibration sensing device of the present disclosure. [Figure 10] 11A to 11C are diagrams illustrating an example of an experimental result obtained with the vibration sensing device of the present disclosure. [Figure 11] 11A to 11C are diagrams illustrating an example of an experimental result obtained with the vibration sensing device of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Hereinafter, the embodiments of the present disclosure will be described in detail 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 assumed to indicate the same components.
[0021] (Embodiment of Basic Configuration) The outline and operation of the vibration sensing device of this embodiment will be described with reference to Fig. 1 and Fig. 6, respectively. The vibration sensing device inputs test light in fundamental mode and higher-order mode to one end of the optical fiber 50 under test (input step 601). The light in the fundamental mode and the light in the higher-order mode are reflected at the other end of the optical fiber 50 under test and return to one end of the optical fiber 50 under test. During this time, vibration is applied to the optical fiber 50 under test. The vibration sensing device, among the light returning from one end of the optical fiber 50 under test, 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 that remains in the fundamental mode with the converted light in the fundamental mode, and converts the combined light into an electrical signal (light receiving step 602). The vibration sensing device performs AD conversion on the converted electrical signal to obtain a digitized electrical signal (measurement step 603). The vibration sensing device performs frequency analysis on the digitized electrical signal to obtain the vibration frequency of the vibration applied to the optical fiber 50 under test (calculation step 604).
[0022] The optical fiber under test 50 has a propagation constant that allows light in a higher order mode to propagate at the wavelength of the test light used by the vibration sensing device.
[0023] 1, when vibration is applied to the optical fiber 50 under test, the electric field distribution of the fundamental mode light remains unchanged but the electric field distribution of the higher mode light changes due to the vibration. When the electric field distribution changes, the conversion efficiency changes when the higher mode light among the light propagating through the optical fiber 50 under test is converted to the fundamental mode. In other words, when the fundamental mode light remains in the fundamental mode and the higher mode light is converted to the fundamental mode, and the light that remained in the fundamental mode is combined with the converted fundamental mode light, a beat is generated.
[0024] For this reason, the vibration sensing device can observe an interference wave with a beat corresponding to the vibration. By frequency analyzing the electrical signal including the beat, the vibration frequency of the vibration applied to the optical fiber 50 under test can be obtained.
[0025] As described above, it is possible to realize a vibration sensing technique capable of detecting vibrations with a simple configuration.
[0026] (Application embodiment 1) The configuration of the vibration sensing device of this embodiment is shown in Fig. 2. In Fig. 2, 10 is a test light generating section, 20 is an input / output section, 30 is a receiving section, 40 is a calculation processing section, and 50 is an optical fiber under test.
[0027] In the vibration sensing device, a test light generating unit 10 located at one end of an optical fiber 50 under test generates and outputs test light. An input / output unit 20 leaves part of the test light from the test light generating unit 10 in the fundamental mode and converts the rest to a higher mode, and inputs it to one end of the optical fiber 50 under test. The cutoff wavelength of the optical fiber 50 under test is set to be longer than the wavelength of the test light generated by the test light generating unit 10. If the optical fiber 50 under test is already installed, the wavelength of the test light generated by the test light generating unit 10 is set to be shorter than the cutoff wavelength of the optical fiber 50 under test.
[0028] The fundamental mode light and the higher mode light propagate through the optical fiber under test 50, are reflected at the other end, and return to one end of the optical fiber under test 50. Of the light that has returned to the input / output section 20 from one end of the optical fiber under test 50, the input / output section 20 leaves the fundamental mode light as it is and converts the higher mode light to the fundamental mode, and then couples the light that has remained in the fundamental mode with the converted fundamental mode light. When vibration is applied to the optical fiber under test 50, a beat corresponding to the vibration is generated during coupling.
[0029] The receiver 30 converts the coupled light from the input / output unit 20 into an electrical signal. The converted electrical signal contains beats.
[0030] The arithmetic processing unit 40 performs frequency analysis on the time waveform of the converted electrical signal from the receiving unit 30 to obtain the vibration frequency of the vibration applied to the optical fiber 50 under test.
[0031] As described above, it is possible to realize a vibration sensing technique capable of detecting vibrations with a simple configuration.
[0032] (Application embodiment 2) The configuration of the vibration sensing device of this embodiment is shown in Fig. 3. In Fig. 3, 10 is a test light generating section, 20 is an input / output section, 30 is a receiving section, 40 is a calculation processing section, 50 is an optical fiber under test, and 51 is a reflection termination.
[0033] The difference from the previous embodiment is that a reflective termination 51 that reflects propagating light is further provided at the other end of the optical fiber under test 50. The reflective termination 51 efficiently reflects the fundamental mode light input by the input / output unit 20 and the converted higher mode light toward one end of the optical fiber under test 50. The reflective termination 51 can be configured by providing an interference film on the reflecting mirror surface at the other end of the optical fiber under test 50.
[0034] By providing the reflection termination 51, the electric signal converted by the receiving unit 30 becomes larger, and the accuracy of the frequency analysis in the arithmetic processing unit 40 is improved.
[0035] (Embodiment 1 of the input / output unit) An example of the configuration of the input / output unit 20 is shown in FIG. 4. In FIG. 4, 20 is an input / output unit, 21 is an optical circulator, and 22 is an axis-shifted connection unit. In FIG. 4, the optical circulator 21 as an optical multiplexer / demultiplexer has a first terminal connected to the test light generating unit 10 (not shown), a second terminal connected to the other end of the axis-shifted connection unit 22, and a third terminal connected to the receiving unit 30 (not shown). The optical circulator 21 outputs the test light input from the test light generating unit 10 (not shown) to the first terminal from the second terminal to the other end of the axis-shifted connection unit 22. The optical circulator 21 outputs the light input from the other end of the axis-shifted connection unit 22 to the second terminal from the third terminal to the receiving unit 30 (not shown). If such a function is available, the three terminals of a 2×2 optical multiplexer / demultiplexer may be used instead of an optical circulator.
[0036] The off-axis connection part 22 is formed by connecting two optical fibers with their core centers offset from each other. Mode conversion occurs due to the offset of the core centers. One end of the off-axis connection part 22 is connected to one end of the optical fiber 50 under test (not shown), and the other end is connected to the second terminal of the optical circulator 21.
[0037] The operation of the input / output unit shown in Fig. 4 will be described. Test light generating unit 10 (not shown) generates and outputs test light. Optical circulator 21 outputs test light input to a first terminal from a second terminal to the other end of off-axis connecting unit 22. Off-axis connecting unit 22 converts part of the fundamental mode test light input to the other end into a higher order mode while leaving part of the fundamental mode test light as it is, combines the light that remains in the fundamental mode with the light converted into the higher order mode, and inputs the combined light from one end to one end of optical fiber 50 (not shown) under test.
[0038] Of the light input to one end of the off-axis connecting section 22 and returned from one end of the optical fiber 50 under test (not shown), the off-axis connecting section 22 leaves a portion of the light in fundamental mode as is and converts a portion of 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 from the other end to a second terminal of the optical circulator 21. The optical circulator 21 outputs the light that remains in fundamental mode input to the second terminal and the converted light in fundamental mode from a third terminal to the receiving section 30 (not shown).
[0039] It is desirable to connect the other end of off-axis connection part 22 to the second terminal of optical circulator 21 with an optical fiber that propagates only the fundamental mode. Since unnecessary light in higher modes is removed and only light in the fundamental mode propagates, the beat can be detected with high accuracy by receiver 30 (not shown).
[0040] (Embodiment 2 of the input / output unit) An example of the configuration of the input / output unit 20 is shown in Fig. 5. In Fig. 5, 20 is an input / output unit, 23 is an optical branching / coupling coupler, and 24 is a mode conversion coupler. In Fig. 5, the optical branching / coupling coupler 23 has a first terminal connected to the test light generating unit 10 (not shown), a second terminal connected to a first input / output terminal of the mode conversion coupler 24, a third terminal connected to a third input / output terminal of the mode conversion coupler 24, and a fourth terminal connected to the receiving unit 30 (not shown). The optical branching / coupling coupler 23 branches the test light input from the test light generating unit 10 (not shown) to the first terminal to the second terminal and the third terminal, and outputs the test light to the first input / output terminal and the third input / output terminal of the mode conversion coupler 24, respectively. The optical branching and combining coupler 23 combines light input from the first input / output terminal of the mode conversion coupler 24 to the second terminal and light input from the third input / output terminal of the mode conversion coupler 24 to the third terminal, and outputs the combined light from the fourth terminal to the receiving unit 30 (not shown).
[0041] The mode conversion coupler 24 includes a first input / output terminal connected to the second terminal of the optical branching / coupling coupler 23, a second input / output terminal connected to one end of the optical fiber 50 (not shown) under test, and a third input / output terminal connected to the third terminal of the optical branching / coupling coupler 23. The mode conversion coupler 24 leaves the fundamental mode light input to the first input / output terminal as it is, converts the fundamental mode light input to the third input / output terminal to a higher mode, combines the light left in the fundamental mode with the light converted to the higher mode, and outputs the combined light from the second input / output terminal. Conversely, when light is input to the second input / output terminal, the fundamental mode light of the light input to the second input / output terminal is output from the first input / output terminal as it is in the fundamental mode, and the higher mode light is converted to the fundamental mode and output from the third input / output terminal.
[0042] The operation of the input / output unit shown in Fig. 5 will be described. The test light generating unit 10 (not shown) generates and outputs test light. The optical branching / coupling coupler 23 branches the fundamental mode test light input from the test light generating unit 10 (not shown) to a first terminal, and outputs it from a second terminal to a first input / output terminal of the mode conversion coupler 24 and from a third terminal to a third input / output terminal of the mode conversion coupler 24. The mode conversion coupler 24 keeps the fundamental mode test light input to the first input / output terminal as it is, converts the fundamental mode test light input to the third input / output terminal to a higher order mode, combines the light kept in the fundamental mode with the light converted to the higher order mode, and inputs it from the second input / output terminal to one end of the optical fiber 50 (not shown) under test.
[0043] The mode conversion coupler 24 outputs fundamental mode light from the first input / output terminal, and converts higher mode light to fundamental mode light and outputs it from the third input / output terminal, among the light input to the second input / output terminal and returned from one end of the optical fiber 50 under test. The optical branching / coupling coupler 23 combines the fundamental mode light input to the second terminal and the converted fundamental mode light input to the third terminal, and outputs it to the receiving unit 30 (not shown) from the fourth terminal.
[0044] (Example) A specific example of a vibration sensing device is shown in Fig. 7. In Fig. 7, 11 is an FL (Fiber Laser), 21 is an optical circulator, 22 is an axially offset connection section, 30 is a receiving section, 31 is a PD (Photo Diode), 32 is an HPF (High Pass Filter), 33 is an LPF (Low Pass Filter), 34 is an ADC (A / D Converter), 41 is a PC (Personal Computer), 50 is an optical fiber under test, and 60 is an AWG (Arbitrary Wave Generator).
[0045] FL11 was used as the test light generator. The test light generator may be any one that generates and outputs laser light as the test light. The central wavelength of FL11 was set to 1050 nm, which allows the optical fiber 50 under test to propagate the LP01 fundamental mode and the LP11 higher order mode. The optical fiber cord (HI1060 manufactured by Corning) capable of transmitting only the fundamental mode was used between FL11 and the optical circulator 21, and between the optical circulator 21 and the off-axis connection part 22. A single mode optical fiber with a total length of 2 km conforming to the ITU-T standard G.652 was used as the optical fiber 50 under test. The other end of the optical fiber 50 under test was an open end cut perpendicular to the longitudinal direction, and one end was a fusion connection point with the core center shifted between the optical fiber cord connected to the optical circulator 21. This fusion connection point functions as the off-axis connection part 22.
[0046] The test light in the LP01 fundamental mode from FL11 passed through the optical circulator 21, and at the off-axis splicing section 22, part of the light remained in the LP01 fundamental mode, and part of the light was excited into the LP11 higher-order mode. Light in both the LP01 fundamental mode and the LP11 higher-order mode was input to one end of the optical fiber 50 under test. The light in the LP01 fundamental mode and the light in the LP11 higher-order mode propagated through the optical fiber 50 under test, were reflected at the other end of the optical fiber 50 under test, and returned to the one end. During this time, the AWG 50 installed near the center of the optical fiber 50 under test applied vibration to the optical fiber 50 under test, affecting the light in the LP11 higher-order mode propagating through the optical fiber 50 under test. At the off-axis splicing section 22, part of the light in the LP01 fundamental mode remained in the LP01 fundamental mode, and part of the light in the LP11 higher-order mode was converted into the LP01 fundamental mode, and the light that remained in the LP01 fundamental mode and the converted light in the LP01 fundamental mode were coupled together. The combined light passes through the optical circulator 21 and is input to the receiving section 30 .
[0047] In the receiving section 30, the coupled light is converted into an electric signal by the PD 31, and the converted electric signal passes through the HPF 32 and the LPF 33 to remove unnecessary direct current and high frequency components. The HPF 32 and the LPF 33 may be replaced with one BPF (Band Pass Filter). Then, the signal is sampled at a sampling rate of 100 kHz by the ADC 34, digitized, and input to the PC 41.
[0048] The PC 41 processes the digitized electrical signal as a time waveform. The time waveform contains beats corresponding to the vibration. The PC 41 performed frequency analysis of the time waveform. The obtained experimental results are shown in Figs. 8 to 11. Fig. 8 shows the time waveform when the AWG 60 applied a vibration of 50 Hz to the optical fiber 50 under test, and Fig. 9 shows the frequency characteristics at that time. Fig. 10 shows the time waveform when the AWG 60 applied a vibration of 100 Hz to the optical fiber 50 under test, and Fig. 11 shows the frequency characteristics at that time.
[0049] 8 to 11, the vibration sensing device can detect the same frequency as the frequency of the vibration applied to the tested optical fiber 50. In other words, by converting the light returning from the tested optical fiber 50 into an electrical signal and analyzing the frequency, it is possible to infer whether or not vibration has been applied to the tested optical fiber 50, and further the vibration frequency of the applied vibration.
[0050] As described above, the vibration sensing device and vibration sensing method disclosed herein can realize vibration sensing capable of detecting vibrations with a simple configuration.
[0051] The arithmetic processing unit 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. [Industrial Applicability]
[0052] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]
[0053] 10: Test light generation unit 11: FL (Fiber Laser) 20: Input / output section 21: Optical circulator 22: Off-axis connection 23: Optical branching coupler 24: Mode conversion coupler 30: Receiving section 31: PD (Photo Diode) 32:HPF (High Pass Filter) 33: LPF (Low Pass Filter) 34: ADC (A / D Converter) 40: Processing unit 41: PC (Personal Computer) 50: Optical fiber under test 51: Reflective termination 60:AWG(Arbitrary Wave Generator)
Claims
1. A test light is input to one end of an optical fiber under test in a fundamental mode and a higher order mode; Among the light returning from one end of the optical fiber under test, light in a fundamental mode is left as it is in the fundamental mode, and light in a higher mode is converted to a fundamental mode, and the light that is left as it is in the fundamental mode is combined with the converted light in the fundamental mode; The vibration sensing device converts the coupled light into an electrical signal, and performs frequency analysis on the converted electrical signal to obtain the vibration frequency of the vibration applied to the optical fiber under test.
2. The vibration sensing device includes: a test light generating unit that generates and outputs test light; an input / output unit which leaves part of the test light from the test light generating unit in a fundamental mode and converts part of the test light into a higher-order mode, inputs the converted test light into one end of the optical fiber under test, leaves the fundamental mode light in the fundamental mode and converts the higher-order mode light into the fundamental mode among the light returned from the one end of the optical fiber under test, and combines the light left in the fundamental mode with the converted fundamental mode light; a receiver that converts the coupled light from the input / output unit into an electrical signal; a processor that performs frequency analysis on the converted electrical signal from the receiver to obtain a vibration frequency of the vibration applied to the optical fiber under test; The vibration sensing device according to claim 1 , further comprising:
3. The optical fiber under test is further provided.
3. The vibration sensing device according to claim 2, wherein a cutoff wavelength of the optical fiber under test is longer than a wavelength of the test light generated by the test light generating section.
4. 4. The vibration sensing device according to claim 3, further comprising a reflection termination at the other end of the optical fiber under test, the reflection termination reflecting the propagating light.
5. The input / output unit includes: an offset-axis splice in which two optical fibers are connected with their cores offset from each other, and one end of the offset-axis splice is connected to one end of the optical fiber under test; an optical multiplexer / demultiplexer having a first terminal connected to the test light generating unit, a second terminal connected to the other end of the off-axis connection unit, and a third terminal connected to the receiving unit, and outputs light input to the first terminal to the second terminal and outputs light input to the second terminal to the third terminal; The vibration sensing device according to claim 2 , further comprising:
6. 6. The vibration sensing device according to claim 5, wherein an optical fiber connecting the other end of the off-axis connection portion and the second terminal of the optical multiplexer / demultiplexer propagates only a fundamental mode.
7. The input / output unit includes: a mode conversion coupler having a second input / output terminal connected to one end of the optical fiber under test, which leaves light in a fundamental mode input to the first input / output terminal in the fundamental mode, converts light in the fundamental mode input to the third input / output terminal into a higher-order mode, combines the light in the fundamental mode with the light converted into the higher-order mode and outputs the combined light from the second input / output terminal, outputs the light in the fundamental mode of the light input to the second input / output terminal from the first input / output terminal while leaving it in the fundamental mode, and converts light in the higher-order mode into the fundamental mode and outputs the combined light from the third input / output terminal; an optical branching and combining coupler having a first terminal connected to the test light generating unit, a second terminal connected to the first terminal of the mode converting coupler, a third terminal connected to the third terminal of the mode converting coupler, and a fourth terminal connected to the receiving unit, which branches light input to the first terminal to the second terminal and the third terminal and outputs the branched light, and combines the light input to the second terminal and the light input to the third terminal and outputs the combined light from the fourth terminal; The vibration sensing device according to claim 2 , further comprising:
8. A test light is input to one end of an optical fiber under test in a fundamental mode and a higher order mode; Among the light returning from one end of the optical fiber under test, light in a fundamental mode is left as it is in the fundamental mode, and light in a higher mode is converted to a fundamental mode, and the light that is left as it is in the fundamental mode is combined with the converted light in the fundamental mode; A vibration sensing method for converting the coupled light into an electrical signal, and performing frequency analysis on the converted electrical signal to obtain a vibration frequency of the vibration applied to the optical fiber under test.