WO2025197014A1 - Brillouin scattering measuring device - Google Patents

Abstract

The purpose of the present invention is to provide a Brillouin scattering measuring device capable of compensating for fluctuations in a frequency component of broadband probe light and performing highly accurate measurement.　A Brillouin scattering measurement device according to the present invention uses two broadband pulsed light beams having the same optical characteristics, acquires broadband probe light including Brillouin scattered light, and broadband probe light that does not include Brillouin scattered light, and compensates for fluctuations in a frequency component of the broadband probe light including Brillouin scattered light on the basis of the broadband probe light that does not include Brillouin scattered light.

Description

Brillouin scattering measurement device

This disclosure relates to a reflectometry technology that performs sensing by measuring scattered light from light incident on an optical fiber.

Figure 1 is a diagram explaining the configuration of a Brillouin scattering measurement device (see, for example, Patent Document 1). The Brillouin scattering measurement device 300 is equipped with a scattered light generation unit 41 that generates pulsed pump light Lpn and probe light Lpr with a wideband frequency component to generate scattered light, and a scattered light acquisition unit 42 that acquires the generated scattered light by heterodyne detection. By performing optical heterodyne detection using the wideband probe light Lpr, the Brillouin scattering measurement device 300 can acquire a BGS (Brillouin Gain Spectrum) with a single pulsed pump light Lpn, enabling vibration measurement.

Figures 2 and 3 are diagrams explaining the measurement method performed by the Brillouin scattering measurement device 300. The Brillouin scattering measurement device 300 causes Brillouin scattering by counter-propagating pulsed pump light Lpn and probe light Lpr with a wideband frequency component within the optical fiber 50 under test, and converts the probe light containing Brillouin scattering into an electrical signal in the scattered light acquisition unit 42. The scattered light acquisition unit 42 then detects the Brillouin scattered light component through frequency analysis and performs vibration measurement using Brillouin scattering by calculating the BGS at each point (Figure 3). In this way, the Brillouin scattering measurement device 300 achieves high-speed sensing by using wideband light.

International Publication WO2023/248437 Pamphlet

However, some light sources may output fluctuating light, with different intensities for each frequency, as shown in Figure 4, and with this intensity changing over time. Hereafter, this type of fluctuation will be referred to as "fluctuations in frequency components," and such light will be referred to as "light with fluctuations in frequency components." In the case of light with fluctuations in frequency components, the frequency components of the underlying broadband probe light fluctuate, causing the peak position of the scattered light to change in response to these fluctuations, making it difficult to maintain measurement accuracy.

In order to solve the above problems, the present invention aims to provide a Brillouin scattering measurement device that can compensate for fluctuations in the frequency components of broadband probe light and enable high-precision measurements.

To achieve the above objective, the Brillouin scattering measurement device of the present invention uses two broadband pulsed lights with identical optical characteristics to obtain broadband probe light that includes Brillouin scattered light and broadband probe light that does not include Brillouin scattered light, and compensates for fluctuations in the frequency components of the broadband probe light that includes Brillouin scattered light based on the broadband probe light that does not include Brillouin scattered light.

Specifically, the Brillouin scattering measurement device according to the present invention comprises:
a pump light generator that pulses continuous light of a single frequency from a laser to generate pump light;
a probe light generator that generates probe light by pulsing broadband continuous light from a broadband light source, the broadband continuous light having frequency components broader than the frequency of the continuous light output by the laser, with a time width wider than that of the pump light;
a modulator that generates local light by shifting the frequency of the continuous light from the laser by an arbitrary frequency;
a detector that generates a measurement signal by coherently detecting the probe light containing Brillouin scattered light using the local light, the probe light being generated by counter-propagating the pump light and the probe light through an optical fiber under test, and that generates a reference signal by coherently detecting the probe light not containing Brillouin scattered light using the local light;
a computing unit that compensates for fluctuations in the frequency component of the probe light included in the measurement signal based on the reference signal;
Equipped with.

This Brillouin scattering measurement device generates two broadband probe beams with pulse shapes and identical optical characteristics, separated by a time lag, as shown in Figure 5. The method for generating the two probe beams will be described later. The first probe beam, Lpr1, is used to measure Brillouin scattering by causing it to interact with pump beam Lpn. The second probe beam, Lpr2, is measured directly (without Brillouin scattering) and is used as reference light to compensate for fluctuations in the frequency components of the broadband beam.
The pulse width of the broadband probe light has the following limitations.
(1) The upper limit of the pulse width of the broadband probe light is the maximum width at which the probe light Lpr1 and the probe light Lpr2 do not overlap.
(2) The lower limit of the pulse width of the broadband probe light is wider than the pulse width of the pump light Lpn.
(3) When the broadband probe BOTDA (see Patent Document 1) is applied to the entire fiber, the pump light and the probe light must propagate counter-propagatingly throughout the entire range of the optical fiber 50 under test. Therefore, the pulse width of the broadband probe light must be twice the width of the time it takes for the pump light Lpn to propagate through the optical fiber 50 under test.

Figure 6 is a diagram illustrating the measured waveform Mpr1 of probe light Lpr1 and the measured waveform Mpr2 of probe light Lpr2 measured by the scattered light acquisition unit 42, as well as their frequency analysis results (FA1, FA2). As shown in Figure 6, the difference between the two lights measured by the scattered light acquisition unit 42 is the presence or absence of Brillouin scattering due to the presence or absence of pump light Lpn. Therefore, by calculating the difference between the two, it is possible to eliminate fluctuations in the frequency component of the probe light and obtain highly accurate Brillouin scattering information.

Therefore, the present invention can provide a Brillouin scattering measurement device that can compensate for fluctuations in the frequency components of broadband probe light and enable high-precision measurements.

As one method of generating two probe lights, the Brillouin scattering measurement device of the present invention further includes an optical delay device that splits the probe light generated by the probe light generator into two, delays one of the probe lights, and inputs it together with the other probe light into the optical fiber under test, causing Brillouin scattering between the pump light and one of the probe lights.

As another method for generating two probe lights, the Brillouin scattering measurement device of the present invention further includes an optical splitter that splits the probe light generated by the probe light generator into two, and inputs one of the probe lights into the optical fiber under test and the other into a reference optical fiber, and the detector generates the reference signal from the local light that has passed through the reference optical fiber.

Furthermore, to further improve measurement accuracy, the probe light generator of the Brillouin scattering measurement device according to the present invention may generate the probe light by randomizing the polarization using a polarization scrambler, and the detector may perform polarization diversity heterodyne detection as the coherent detection.

The present invention is a program for causing a computer to function as the computing unit. The computing unit of the Brillouin scattering measurement device of the present invention can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network.

The above inventions can be combined as much as possible.

The present invention can provide a Brillouin scattering measurement device that can compensate for fluctuations in the frequency components of broadband probe light and enable highly accurate measurements.
According to the present invention, by applying an experimental system and a signal processing method that compensate for the frequency instability of broadband light, it is possible to prevent a decrease in measurement performance and to enable measurements with higher accuracy.

FIG. 1 is a diagram illustrating the configuration of a Brillouin scattering measurement device. FIG. 1 is a diagram illustrating a method for measuring Brillouin scattered light. FIG. 1 is a diagram illustrating a method for measuring Brillouin scattered light. FIG. 1 is a diagram illustrating a problem to be solved by the present invention. 1 is a diagram illustrating the measurement principle of a Brillouin scattering measurement device according to the present invention. 1 is a diagram illustrating the measurement principle of a Brillouin scattering measurement device according to the present invention. 1 is a diagram illustrating a Brillouin scattering measurement device according to the present invention. 1 is a diagram illustrating a Brillouin scattering measurement device according to the present invention. 10A and 10B are diagrams illustrating a method for generating two probe beams having the same pulse shape characteristics at different times in the Brillouin scattering measurement device according to the present invention. 1 is a diagram illustrating a method for measuring Brillouin scattered light using a Brillouin scattering measurement device according to the present invention. FIG. 1 is a diagram illustrating a Brillouin scattering measurement device according to the present invention. 1 is a diagram illustrating a Brillouin scattering measurement device according to the present invention. 1 is a diagram illustrating the operation of a Brillouin scattering measurement device according to the present invention.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components with the same reference numerals in this specification and drawings are considered to represent the same components.

(Embodiment 1)
7 is a diagram illustrating a Brillouin scattering measurement device 301 according to this embodiment. The Brillouin scattering measurement device 301 includes:
a pump light generator 10 that pulses continuous light of a single frequency from a laser 11 to generate pump light Lpn;
a probe light generator 20 for generating probe light Lpr by pulsing broadband continuous light from a broadband light source 13, the broadband continuous light having frequency components broader than the frequency of the continuous light output by the laser 11, with a time width wider than that of the pump light Lnp;
a modulator 14 that generates local light by shifting the frequency of the continuous light from the laser 11 by an arbitrary frequency;
a detector 15 that coherently detects the probe light containing Brillouin scattered light generated by counter-propagating pump light Lpn and probe light Lpr through the measured optical fiber 50, using the local light, to generate a measurement signal, and that coherently detects the probe light not containing Brillouin scattered light, using the local light, to generate a reference signal;
a calculator (43) that compensates for fluctuations in the frequency component of the probe light included in the measurement signal based on the reference signal;
Equipped with.

In this embodiment, the Brillouin scattering measurement device 301 includes:
The optical delay device 17 further includes: a first branch of the probe light Lpr generated by the probe light generator 20; a delay circuit 17 for delaying one of the first probe light Lpr; and an optical delay circuit 17 for delaying the first probe light Lpr and inputting the second probe light Lpr into the optical fiber to be measured together with the second probe light Lpr.
The pump light Lpn and one of the probe lights Lpr are subjected to Brillouin scattering.

The Brillouin scattering measurement device 301 is composed of a scattered light generation unit 41 and a scattered light acquisition unit 42.
The scattered light generator 41 generates pump light Lpn consisting of a pulsed single frequency and probe light Lpr consisting of two pulsed broadband frequencies. The pump light Lpn is generated by modulating single-frequency light emitted from a laser 11 into a pulsed shape using an intensity modulator 12 and amplifying the light using an optical amplifier 61 as needed. The optical filter 18a removes noise due to the optical amplifier 61. On the other hand, the probe light Lpr is generated by pulsing light emitted from a broadband light source 13 using an intensity modulator 16. For example, the broadband light source 13 outputs light obtained by exciting ASE light using an EDFA.

Figure 9 is a diagram illustrating a method for generating two probe beams with the same pulse shape characteristics but with a time lag in this embodiment. In this embodiment, an optical delay device 17 is used. In this embodiment, a delay optical fiber 17b is placed in one of the branched paths, generating two probe beams with the same pulse shape characteristics but with a time lag. Any delay amount can be achieved by changing the length of the delay optical fiber 17b.

Specifically, optical delayer 17 splits one pulsed probe light Lpr into two using coupler 17a, adds a delay to one probe light Lpr using delay optical fiber 17b arranged on one path R1, and recombines the delayed probe light Lpr with the non-delayed probe light Lpr using coupler 17c. Optical delayer 17 outputs pulse-shaped probe light Lpr (Lpr1, Lpr2) with the same characteristics, with a time difference.

The scattered light generating unit 41 inputs the pump light Lpn into one end of the measured optical fiber 50 and inputs both probe light Lpr into the other end of the measured optical fiber 50. Here, the scattered light generating unit 41 adjusts the input timing of the pump light Lpn and both probe light Lpr into the measured optical fiber 50 so that one probe light Lpr and the pump light Lpn do not collide within the measured optical fiber 50, and the other probe light Lpr and the pump light Lpn do not collide within the measured optical fiber 50.

The scattered light acquisition unit 42 is a device that uses local light to perform coherent detection of the probe light Lpr output from one end of the optical fiber 50 under test. The local light is obtained by modulating light branched from the laser 11 to near BFS using the frequency modulator 14. If necessary, the local light may be amplified using an optical amplifier. Coherent detection is performed by combining the probe light Lpr and the local light and detecting them using a BPD. The detected signal is then converted to digital data by an ADC, and frequency analysis processing is performed on a PC.

In order to generate Brillouin scattering stably and independently of polarization, a configuration like that shown in Figure 8 may be used. The Brillouin scattering measurement device 302 in Figure 8 differs from the Brillouin scattering measurement device 301 in Figure 7 in that the probe light generator 20 generates probe light Lpr by randomizing polarization using the polarization scrambler 19, and the detector 15 performs polarization diversity heterodyne detection as the coherent detection. The optical filter 18b removes light of unnecessary optical frequencies based on changes in the Brillouin frequency shift (BFS) (extracting only light with frequency components necessary to cause Brillouin scattering). Specifically, the optical filter 18b extracts light of a predetermined band that is approximately the same as the optical frequency of the BFS from the optical frequency of the pump light as the probe light.

FIG. 10 is a diagram illustrating a method for measuring Brillouin scattered light performed by the scattered light acquisition unit 42.
Of the two probe lights obtained by the optical delay device 17, one (e.g., Lpr1) is made to collide with the pump light Lpn within the measured optical fiber 50, and the other (e.g., Lpr2) is made to pass through the measured optical fiber 50 without colliding with the pump light Lpn, and both probe lights are coherently detected by the detector 15.

Figure 10(A) shows a waveform obtained by frequency analysis of probe light (e.g., Lpr2) that has passed through the measured optical fiber 50 without colliding with pump light Lpn. This waveform contains information about the broadband probe light only. Figure 10(B) shows a waveform obtained by frequency analysis of probe light (e.g., Lpr1) that has been made to collide with pump light Lpn within the measured optical fiber 50. This waveform contains information about the broadband probe light that includes scattered light.

The purpose is to obtain information about the Brillouin gain corresponding to the strain in the optical fiber under test 50. The waveform in Figure 10(B) contains information about both the Brillouin gain and the broadband probe light, but the peak position fluctuates due to fluctuations in the frequency components of the broadband probe light. Therefore, the waveform in Figure 10(A) obtained for reference (information about fluctuations in the frequency components of the broadband probe light) is used to obtain the signal in Figure 10(C) (information about the Brillouin gain) by removing the fluctuation components of the frequency components of the broadband probe light from the waveform in Figure 10(B). Note that this calculation is performed by the signal processing unit 43 by unifying the vertical axes of Figures 10(A) and (B) into units of power on a dB scale, and dividing or deconvolving the measurement signal for the broadband probe light that includes Brillouin scattered light by the reference signal for the broadband probe light that does not include Brillouin scattered light.

(Embodiment 2)
11 is a diagram illustrating a Brillouin scattering measurement device 303 of this embodiment. The Brillouin scattering measurement device 303 further includes an optical branching device 31 that branches the probe light Lpr generated by the probe light generator 20 into two beams, and inputs one of the probe light beams into the optical fiber under test 50 and the other probe light beam into the reference optical fiber 51. This differs from the Brillouin scattering measurement device 301 of FIG. 7 in that the detector 15 generates the reference signal from the probe light that has passed through the reference optical fiber 51.

The measurement of Brillouin scattered light described in the first embodiment can also be performed in a similar manner using a Brillouin scattering measurement device 303 having separate systems for the measurement signal passing through the optical fiber 50 under test and the reference signal passing through the reference optical fiber 51.
The optical branching unit 31 branches the probe light Lpr into two, and inputs one (e.g., probe light Lpr1) into the measured optical fiber 50, and the other (e.g., probe light Lpr2) into the reference optical fiber 51. The scattered light acquiring unit 42 has two sets of detectors 15. One detector 15 coherently detects the probe light Lpr1 that has collided with the pump light Lpn in the measured optical fiber 50 using local light to generate a measurement signal. The other detector 15 coherently detects the probe light Lpr2 that has passed through the reference optical fiber 51 using local light to generate a reference signal. As in the first embodiment, the calculator 43 compensates for fluctuations in the frequency component of the probe light included in the measurement signal based on the reference signal.

Even when generating a reference signal using a reference optical fiber 51, a Brillouin scattering measurement device 304 configuration such as that shown in Figure 12 may be used to generate stable Brillouin scattering independent of polarization.

(Embodiment 3)
13 is a flowchart illustrating the operation of the Brillouin scattering measurement apparatus (301 to 304) described in the first and second embodiments. In this operation, pulsed pump light Lpn is repeatedly input into the fiber under test 50 until a desired measurement time is reached.

Step S01
A probe beam is generated by pulsing broadband light in a frequency range that takes into account the BFS of the BGS. The probe beam is then split into two beams. In the Brillouin scattering measurement device (301-302) of the first embodiment, one of the probe beams is delayed, and both beams are incident on the fiber under test 50 so as to counter-propagate with the pulsed pump beam Lpn. In the Brillouin scattering measurement device (303-304) of the second embodiment, one of the probe beams is incident on the reference optical fiber 51, and the other probe beam is incident on the fiber under test 50 so as to counter-propagate with the pulsed pump beam Lpn.

Step S02
In the Brillouin scattering measurement device (301-302) of embodiment 1, pump light Lpn consisting of a single frequency is incident on the measured optical fiber 50 at a timing when it counter-propagates with only one of the two probe light Lpr. Two signals are generated from the two probe light Lpr output from the measured optical fiber 50: a measurement signal including Brillouin scattering and a reference signal not including Brillouin scattering. In the Brillouin scattering measurement device (303-304) of embodiment 2, pump light Lpn consisting of a single frequency is incident on the measured optical fiber 50 at a timing when it counter-propagates with the probe light Lpr. The pump light Lpn is not incident on the reference optical fiber 51. A measurement signal including Brillouin scattering is generated from the probe light Lpr output from the measured optical fiber 50, and a reference signal not including Brillouin scattering is generated from the probe light Lpr output from the reference optical fiber 51.

Step S03
The scattered light acquisition unit 41 coherently detects the measurement signal and the reference signal using local light to acquire the respective detection signals. In other words, two detection signals, one for Brillouin scattering measurement and one for reference, can be acquired by a single injection of pump light Lpn, thereby acquiring detection signals that can form a distributed BGS throughout the entire fiber.

Step S04
If the desired measurement time has not yet been reached, measurement is restarted from step S01, whereas if the desired measurement time has been reached, step S05 is carried out.

Step S05
The acquired detection signal is subjected to frequency analysis for each distance on the optical fiber 50 under test, and the BGS at each point is calculated. At this time, fluctuations of the broadband probe light are compensated for using the method described in Fig. 10. Then, by observing the time-series changes in the peaks of the calculated BGS, the vibration at each point on the optical fiber 50 under test can be obtained. Note that, because step S05 is an independent process for each pump light, it can also be executed during the loop of steps S01 to S04 if the calculation time is sufficient.

(effect)
The present invention can compensate for fluctuations in the frequency components of broadband light and reduce the peak estimation error of BGS.

(definition)
The following abbreviations are used in this specification and drawings:
ASE: Amplified spontaneous emission
PD: Photo Detector
AOM: Acousto optic modulator
EDFA: Erbium doped fiber amplifier
A/D: Analog Digital
BGS: Brillouin Gain Spectrum
BFS: Brillouin Frequency Shift
BPD: Balanced Photo Detector
SSB: Single Side Band

10: Pump light generator 11: Laser 12: Pulse generator 13: Broadband light source 14: Modulator 15: Detector 15a: 50: 50 coupler 15b: Balanced photodiode 16: Intensity modulator 17: Delay device 18a, 18b: Optical filter 19: Polarization scrambler 31: Optical branching device 41: Scattered light generator 42: Scattered light acquirer 43: Computing unit 50: Optical fiber to be measured 51: Reference optical fibers 61, 62: Optical amplifier 63: Optical circulator 64: Optical amplifiers 300 to 304: Brillouin scattering measurement device

Claims (4)

a pump light generator that pulses continuous light of a single frequency from a laser to generate pump light;
a probe light generator that generates probe light by pulsing broadband continuous light from a broadband light source, the broadband continuous light having frequency components broader than the frequency of the continuous light output by the laser, with a time width wider than that of the pump light;
a modulator that generates local light by shifting the frequency of the continuous light from the laser by an arbitrary frequency;
a detector that generates a measurement signal by coherently detecting, using the local light, the probe light containing Brillouin scattered light generated by counter-propagating the pump light and the probe light through an optical fiber under test, and that generates a reference signal by coherently detecting, using the local light, the probe light not containing Brillouin scattered light;
a computing unit that compensates for fluctuations in the frequency component of the probe light included in the measurement signal based on the reference signal;
A Brillouin scattering measurement device comprising: an optical delay device that splits the probe light generated by the probe light generator into two, delays one of the probe lights, and inputs the delayed probe light and the other probe light into the optical fiber under test;
2. The Brillouin scattering measurement device according to claim 1, wherein the pump light and one of the probe lights are subjected to Brillouin scattering. an optical branching unit that branches the probe light generated by the probe light generator into two, and inputs one of the probe lights into the optical fiber under test and the other probe light into a reference optical fiber;
2. The Brillouin scattering measurement apparatus according to claim 1, wherein the detector generates the reference signal from the local light that has passed through the reference optical fiber. 4. The Brillouin scattering measurement device according to claim 1, wherein the probe light generator generates the probe light by randomizing polarization using a polarization scrambler, and the detector performs polarization diversity heterodyne detection as the coherent detection.