Method of measuring acoustic distribution and distributed acoustic sensor

Patent metadata

Field	Value
Publication number	US-9228890-B2
Application number	US-201313907371-A
Country	US
Kind code	B2
Filing date	May 31, 2013
Priority date	Jun 1, 2012
Publication date	Jan 5, 2016
Grant date	Jan 5, 2016

Abstract

Optical pulses are output from a light source to an optical fiber at one selected emission wavelength, a reflection light reflected at a fiber Bragg grating at the optical fiber is received at an optical receiver and is converted to a reflection signal by an optical to electrical conversion, a fiber Bragg grating which reflects the reflection light is identified by the signal processing unit when an intensity of the reflection signal obtained by the optical receiver is over a predetermined threshold value, and a measurement step that calculates acoustic frequencies at the fiber Bragg grating based on a temporal change of the intensity of the reflection signal at the fiber Bragg grating is repeated more than two times while changing an emission wavelength of the light source. The acoustic frequency at each fiber Bragg grating is calculated to determine an acoustic distribution along a longitudinal direction of the fiber.

First claim

What is claimed is:

1. A method of measuring an acoustic distribution, the method comprising:
using a distributed acoustic sensor which comprises:
a light source unit which is capable of emitting optical pulses at two or more emission wavelengths;
an optical fiber which comprises two or more fiber Bragg gratings disposed between a first end of the optical fiber and a second end of the optical fiber along a longitudinal direction of the optical fiber, the first end being close to the light source unit and the second end being far from the light source unit;
an optical receiver unit, which receives a reflection light emitted from the light source unit and reflected at one or more of the two or more fiber Bragg gratings and which converts the reflection light to a reflection signal by an optical to electrical conversion;
a light branching unit which separates a light output from the light source unit into a measurement light which is directed to the two or more fiber Bragg gratings and a reference light which is to be converted to a reference signal by an optical to electrical conversion without reaching any of the two or more fiber Bragg gratings; and
a signal processing unit analyzing the reflection signal and the reference signal; and

calculating acoustic frequencies for all of the two or more fiber Bragg gratings disposed at the optical fiber to determine an acoustic distribution along the longitudinal direction of the optical fiber, by repeating a measurement step more than two times while changing an emission wavelength of the light source unit, the measurement step comprising:
outputting the two or more optical pulses from the light source unit to the optical fiber at intervals, two or more optical pulses having one emission wavelength selected from among the two or more emission wavelengths;
identifying a fiber Bragg grating, which reflects the reflection light corresponding to the reflection signal, among the two or more fiber Bragg gratings by the signal processing unit based on a time difference between when the reflection signal is detected and when the reference signal is detected in a case where a reflection signal intensity obtained by the optical receiver unit is over a predetermined threshold value, and obtaining a temporal change of a reflection signal intensity at the identified fiber Bragg grating; and
calculating an acoustic frequency at the identified fiber Bragg grating based on the temporal change of the reflection signal intensity, wherein:

at the light source unit, a difference of two different emission wavelengths arbitrarily selected from among the two or more fiber Bragg gratings is greater than a maximum value Δλa of a wavelength width of an optical pulse output at each of the two or more emission wavelengths; and
at the optical fiber, a minimum value Δλb of a reflection bandwidth of the two or more fiber Bragg gratings is greater than the maximum value Δλa of the wavelength width.

2. The method of measuring an acoustic distribution according to claim 1 , wherein the measurement step is repeated so as to use all of the two or more emission wavelengths.

3. The method of measuring an acoustic distribution according to claim 1 , comprising determining whether or not the signal processing unit calculates the acoustic frequencies at all of the two or more fiber Bragg gratings disposed at the optical fiber,
wherein when acoustic frequencies at all of the two or more fiber Bragg gratings are calculated by the signal processing unit, the measurement step is not repeated.

4. The method of measuring an acoustic distribution according to claim 1 , wherein:
the distributed acoustic sensor comprises a second optical receiver unit which receives the reference light and converts the reference light to the reference signal by an optical to electrical conversion; and
the light branching unit is a light splitting unit which is disposed between the light source unit and the first end and capable of branching a light output from the light source unit into the reference light which is directed to the second optical receiver unit and the measurement light which is directed to the first end.

5. The method of measuring an acoustic distribution according to claim 1 , wherein:
the light branching unit is a partial reflection unit which is disposed at the optical fiber and capable of partially reflecting a light as the reference light at all of the two or more emission wavelengths; and
the optical receiver unit receives the reference light and the reflection light.

6. A distributed acoustic sensor comprising:
a light source unit which is capable of emitting optical pulses at two or more emission wavelengths and outputting two or more optical pulses at each of the emission wavelengths, the two or more optical pulses being output at intervals;
an optical fiber which comprises two or more fiber Bragg gratings disposed between a first end of the optical fiber and a second end of the optical fiber along a longitudinal direction of the optical fiber, the first end being close to the light source unit and the second end being far from the light source unit;
an optical receiver unit, which receives a reflection light emitted from the light source unit and reflected at one or more of the two or more fiber Bragg gratings and which converts the reflection light to a reflection signal by an optical to electrical conversion;
a light branching unit which separates a light output from the light source unit into a measurement light which is directed to the two or more fiber Bragg gratings and a reference light which is to be converted to a reference signal by an optical to electrical conversion without reaching any of the two or more fiber Bragg gratings;
a signal processing unit being configured to identify a fiber Bragg grating, which reflects the a reflection light corresponding to the reflection signal, among the two or more fiber Bragg gratings based on a time difference between when the reflection signal is detected and when the reference signal is detected in a case where a reflection signal intensity obtained by the optical receiver unit is over a predetermined threshold value, and to obtain a temporal change of a reflection signal intensity at the identified fiber Bragg grating; and
a control unit being configured to calculate acoustic frequencies for all of the two or more fiber Bragg gratings disposed at the optical fiber to determine an acoustic distribution along the longitudinal direction of the optical fiber, by repeating a measurement step more than two times while changing an emission wavelength of the light source unit, the measurement step comprising:
outputting two or more optical pulses from the light source unit to the optical fiber at intervals, the two or more optical pulses having one emission wavelength selected from among the two or more emission wavelengths;
identifying a fiber Bragg grating, which reflects the reflection light corresponding to the reflection signal, among the two or more fiber Bragg gratings by the signal processing unit in a case where a reflection signal intensity obtained by the optical receiver unit is over a predetermined threshold value, and obtaining a temporal change of a reflection signal intensity at the identified fiber Bragg grating; and
calculating an acoustic frequency at the identified fiber Bragg grating based on the temporal change of the reflection signal intensity, wherein:

at the light source unit, a difference of two different emission wavelengths arbitrarily selected from among the two or more fiber Bragg gratings is greater than a maximum value Δλa of a wavelength width of an optical pulse emitted at each of the two or more emission wavelengths; and
at the optical fiber, a minimum value Δλb of a reflection bandwidth of

Assignees

• Fujikura Ltd

Inventors

• Omichi Koji
• Terada Yoshihiro
• Tanaka Daiichiro

Classifications

• G01H9/004 Primary

  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) · CPC title

• G01H9/00

  Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means · CPC title