# US 11,994,376 B2 · Distributed vibration measuring device and method

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- Number: US 11,994,376 B2
- Document ID: US11994376B2
- Status: Granted
- Kind: B2
- Application number: 17/634,435
- Filed: 2020-02-13
- Published: 2022-10-13
- Granted: 2024-05-28
- Earliest claimed: 2019-08-16
- Assignees: Nippon Telegraph and Telephone Corporation
- Applicants: Nippon Telegraph and Telephone Corporation
- Inventors: Tatsuya Okamoto; Daisuke Iida; Hiroyuki Oshida
- Web page: https://patentgrep.com/patent/US11994376B2

## CPC

- G01B 11/16: for measuring the deformation in a solid, e.g. optical strain gauge
- G01B 11/18: {using photoelastic elements}
- G01B 11/161: {by interferometric means}
- G01H 9/004: {using fibre optic sensors (light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125)}
- G01B 9/02003: using beat frequencies
- G01B 9/02004: using frequency scans
- G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01H: MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES

## Abstract

The present disclosure provides a device that accurately measures vibration at a designated position of a sensing fiber without using digital signal processing to compensate for distance fluctuation. Digital signal processing for correcting fluctuation of a measurement distance due to a frequency offset of a beat signal due to vibration, which is a measurement target, is simplified. In the present disclosure, vibration at a designated position of the sensing fiber is accurately measured without using the digital signal processing to compensate for the distance fluctuation. A spectrum analysis length of an electrical field E(τn) of backscattered light is set to be larger than a delay deviation Nd due to frequency modulation caused by dynamic strain. An index of tolerance of vibration distribution measurement to the delay deviation Nd is also clarified.

## Claims

### Claim 1

A device configured to measure dynamic strain of a sensing fiber, the device comprising:
an optical circuit unit configured to repeatedly supply frequency swept lights to the sensing fiber,
receive backscattered light from the sensing fiber, and
combine local light, the local light being a part of a frequency swept light of the frequency swept lights, with the backscattered light; and
an optical reception and analysis unit configured to generate a beat signal from the local light and the backscattered light,
execute Fourier transform on the beat signal to obtain a backscattered light waveform,
execute Fourier transform in a freely selected spectrum analysis section of the backscattered light waveform to obtain an optical spectrum, and
obtain, based on the amount of spectrum shift between a spectrum obtained in reference measurement and a spectrum obtained from the backscattered light for one sweep of the frequency swept lights, a strain amount of the dynamic strain in a section of the sensing fiber corresponding to the spectrum analysis section,
wherein a length of the spectrum analysis section is set to be larger than a distance deviation amount Nd of the spectrum analysis section caused by frequency modulation due to the dynamic strain.

### Claim 2 (depends on 1)

The device according to claim 1, wherein the distance deviation amount is obtained by Nd=T·voffset, where T is a measurement time using the frequency swept light and voffset is the amount of the frequency modulation caused by vibration.

### Claim 3 (depends on 1)

The device according to claim 1, wherein the dynamic strain is a temporal change in the strain amount in a sensing fiber section and is obtained from the strain amount for each of the sweeps of the frequency swept lights that are repeatedly supplied.

### Claim 4 (depends on 1)

The device according to claim 1, wherein the reference measurement is based on a spectrum in a state where there is no dynamic strain in the sensing fiber.

### Claim 5

A method of measuring dynamic strain of a sensing fiber, the method comprising:
repeatedly supplying frequency swept lights to the sensing fiber;
receiving backscattered light from the sensing fiber;
combining local light, the local light being a part of a frequency swept light of the frequency swept lights, with the backscattered light to generate a beat signal;
executing Fourier transform on the beat signal to obtain a backscattered light waveform;
executing Fourier transform in a freely selected spectrum analysis section of the backscattered light waveform to obtain an optical spectrum; and
obtaining, based on the amount of spectrum shift between a spectrum obtained in reference measurement and a spectrum obtained from the backscattered light for one sweep of the frequency swept lights, a strain amount of the dynamic strain in a section of the sensing fiber corresponding to the spectrum analysis section,
wherein a length of the spectrum analysis section is set to be larger than a distance deviation amount Nd of the spectrum analysis section caused by frequency modulation due to the dynamic strain.

### Claim 6 (depends on 5)

The method according to claim 5, wherein the distance deviation amount is obtained by Nd=T·voffset, where T is a measurement time using the frequency swept light and voffset is the amount of the frequency modulation caused by vibration, and the dynamic strain is a temporal change in the strain amount in a sensing fiber section and is obtained from the strain amount for each of the sweeps of the frequency swept lights that are repeatedly supplied.

### Claim 7

A device configured to measure dynamic strain, the device comprising:
an optical circuit unit configured to repeatedly supply frequency swept lights to a sensing fiber,
receive backscattered light from the sensing fiber, and
combine local light, the local light being a part of a frequency swept light of the frequency swept lights, with the backscattered light; and
an optical reception and analysis unit configured to generate a beat signal from the local light and the backscattered light,
obtain a backscattered light waveform from the beat signal,
obtain an optical spectrum in a freely selected spectrum analysis section of the backscattered light waveform, and
obtain a strain amount of the dynamic strain in a section of the sensing fiber corresponding to the spectrum analysis section,
wherein a length N of the spectrum analysis section is set with respect to an assumed distance deviation amount Nd caused by the dynamic strain such that a probability of a correlation peak exceeding a noise level has a desired value when correlation measurement between a spectrum of reference measurement and the optical spectrum is performed, the desired value being approximate to 1.

### Claim 8 (depends on 7)

The device according to claim 7, wherein the probability is expressed in an Nd-N coordinate space, the Nd-N coordinate space being acquired based on reference measurement and trial measurement, indicating the probability of the correlation peak exceeding the noise level, and the length N is in an area where the desired value is given in the Nd-N coordinate space.

### Claim 9

A method of measuring dynamic strain, the method comprising:
repeatedly supplying frequency swept lights to a sensing fiber;
receiving backscattered light from the sensing fiber;
combining local light, the local light being a part of a frequency swept light of the frequency swept lights, with the backscattered light to generate a beat signal;
obtaining a backscattered light waveform from the beat signal;
obtaining an optical spectrum in a freely selected spectrum analysis section of the backscattered light waveform; and
obtaining a strain amount of the dynamic strain in a section of the sensing fiber corresponding to the spectrum analysis section,
wherein a length N of the spectrum analysis section is set with respect to an assumed distance deviation amount Nd caused by the dynamic strain such that a probability of a correlation peak exceeding a noise level has a desired value when correlation measurement between a spectrum of reference measurement and the optical spectrum is performed, the desired value being approximate to 1.

### Claim 10 (depends on 9)

The method according to claim 9, wherein the probability is expressed in an Nd-N coordinate space, the Nd-N coordinate space being acquired based on reference measurement and trial measurement, indicating the probability of the correlation peak exceeding the noise level, and the length N is selected from an area where the desired value is given in the Nd-N coordinate space.