Brillouin gain analyzer and Brillouin gain analyzer

Abstract

The objective of the present invention is to provide a Brillouin gain analysis device and a Brillouin gain analysis method with which it is possible for high-speed strain changes in an optical fiber to be measured distributionally.　A Brillouin gain analysis device 301 according to the present invention comprises: a laser 11 for outputting continuous light having a single frequency; a pulse generator 12 for pulsing the continuous light from the laser and inputting the same into one end of an optical fiber 50 being measured; an ASE light source 13 for generating continuous light having frequency components over a wider band than the frequency of the continuous light output by the laser 11, and inputting the same into the other end of the optical fiber 50; a modulator 14 for generating local light obtained by shifting the frequency of the continuous light from the laser 11 by an arbitrarily defined frequency; and a wave detector 15 for performing heterodyne detection between Brillouin scattered light generated by the optical fiber and the local light.

Description

Technical Field

[0001] The present disclosure relates to reflectometry that performs sensing by measuring scattered light of light incident on an optical fiber.

Background Art

[0002] As an optical fiber sensing technique, there exists a measurement method called BOTDA (Brillouin Optical Time Domain Analysis) as shown in FIG. 1. This method acquires a Brillouin gain spectrum (BGS) as shown in FIG. 2 in order to measure the strain and temperature of an optical fiber 50. The strain and temperature of the optical fiber can be acquired distributively from the change 21 of the peak of the BGS. In order to capture the Brillouin frequency shift (BFS), which is the amount of deviation of the change 21 of the peak of the BGS, it is necessary to perform frequency sweeping of the probe light Lpr. When a single pulsed pump light Lpn is incident on the optical fiber 50, in order to fix the frequency of the probe light Lpr, it is necessary to change the frequency of the probe light Lpr and make the pulsed pump light Lpn incident a plurality of times in order to measure the entire BGS.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Non-Patent Document 2

[0004] BOTDA acquires Brillouin scattered light of arbitrary frequencies, as shown by the dashed line 22 in Figure 2, over the entire length of the fiber in a single measurement. To obtain optical information at all frequencies within the BGS range, the frequency sweep range 23 of the probe light must be within the BGS range, which makes the measurement time-consuming. Therefore, BOTDA has the challenge of being unable to measure rapid strain changes in optical fibers.

[0005] To measure the rapid strain changes of optical fibers, for example, Non-Patent Document 1 discloses a measurement method that avoids changing the frequency of the probe light and injecting pulsed pump light multiple times. Figure 3 is a diagram illustrating the BOTDA disclosed in Non-Patent Document 1. The measurement target of the BOTDA disclosed in Non-Patent Document 1 is a network NW where optical fibers branch. A reflector Mr that reflects light is installed at the far end (the end face opposite to the input end) of the optical fiber after branching. The BOTDA disclosed in Non-Patent Document 1 is a device configuration that measures the loss of each optical fiber after branching from the input end. The BOTDA disclosed in Non-Patent Document 1 measures BOTDA using the probe light reflected by the reflector Mr and the pump light that is incident with a timing delay relative to the probe light.

[0006] The BOTDA described in Non-Patent Literature 1 uses Amplified Spontaneous Emission (ASE), which includes broadband frequency components, as the probe light to extract the intensity of Brillouin gain independently of BFS (Brillouin Frequency Shift) variations and measure the loss of the optical fiber. However, the BOTDA described in Non-Patent Literature 1 is intended for measuring branched optical fibers within a network, and since both the pump light and probe light are pulsed, it cannot measure the distribution of the entire optical fiber. In other words, while the BOTDA described in Non-Patent Literature 1 can measure the loss of the optical fiber at high speed, it has the problem of being unable to measure the rapid distortion changes of the optical fiber in a distributed manner.

[0007] Therefore, the present invention aims to provide a Brillouin gain analyzer and a Brillouin gain analyzer that can measure high-speed strain changes in optical fibers in a distributed manner in order to solve the above problems. [Means for solving the problem]

[0008] To achieve the above objective, the BOTDA according to the present invention utilizes broadband continuous light ASE instead of pulsed probe light, and acquires Brillouin scattering by optical heterodyne detection.

[0009] Specifically, the Brillouin gain analyzer according to the present invention is A laser that outputs continuous light of a single frequency, An ASE (Amplified Spontaneous Emission) light source is provided, which generates continuous light with a broader bandwidth of frequency components than the frequency of the continuous light output by the aforementioned laser, and is incident on one end of the optical fiber to be measured. A pulse generator that pulses the continuous light from the laser and injects it into the other end of the optical fiber, A modulator that generates local light obtained by shifting the frequency of the continuous light from the laser by an arbitrary frequency, A detector that performs heterodyne detection of the Brillouin scattered light generated in the optical fiber and the local light, It is equipped with.

[0010] Furthermore, the Brillouin gain analysis method according to the present invention is The process involves irradiating one end of the optical fiber being measured with ASE (Amplified Spontaneous Emission) continuous light, which has a broader frequency band than single-frequency laser light, as probe light. The laser light is pulsed and incident as pump light at the other end of the optical fiber. The process involves shifting the frequency of the laser light by an arbitrary frequency to generate local light, and The Brillouin scattered light generated in the optical fiber and the local light are subjected to heterodyne detection. It is characterized by the following.

[0011] In BOTDA, by broadening the bandwidth of the probe light and using optical heterodyne detection, the Brillouin scattered light on the optical fiber can be acquired in a distributed manner with a single pulsed pump light. Therefore, there is no need to sweep the frequency of the probe light or to inject the pump light multiple times. Accordingly, the present invention can provide a Brillouin gain analysis apparatus and a Brillouin gain analysis method that can measure high-speed strain changes in an optical fiber in a distributed manner.

[0012] The Brillouin gain analysis apparatus according to the present invention is further characterized by comprising a signal processing unit that performs a Fourier transform on the signal heterodyne detected by the detector to detect the Brillouin gain spectrum (BGS), and acquires the vibration distribution of the optical fiber from the time-series change of the peak of the BGS.

[0013] Furthermore, the Brillouin gain analysis method according to the present invention is The heterodyne-detected signal is Fourier-transformed to detect the Brillouin gain spectrum (BGS), and The vibration distribution of the optical fiber is obtained from the time-series change of the BGS peak. Further steps are taken.

[0014] The obtained Brillouin scattered light contains a plurality of frequency components. Therefore, by setting a window function for the Brillouin scattered light and performing a process of obtaining frequency components such as Fourier transform, the BGS is calculated. By spectrally analyzing the Brillouin scattered light, the vibration applied to the optical fiber can be obtained.

[0015] The Brillouin gain analysis device of the present invention can also be realized by a computer and a program, and it is also possible to record the program on a recording medium or provide it through a network.

[0016] In addition, the above inventions can be combined as much as possible.

Effect of the Invention

[0017] The present invention can provide a Brillouin gain analysis device and a Brillouin gain analysis method capable of distributively measuring a high-speed strain change of an optical fiber.

Brief Description of the Drawings

[0018] [Figure 1] It is a diagram for explaining BOTDA. [Figure 2] It is a diagram for explaining the measurement principle of BOTDA. [Figure 3] It is a diagram for explaining an application example of BOTDA. [Figure 4] It is a diagram for explaining the Brillouin gain analysis device according to the present invention. [Figure 5] It is a diagram for explaining the comparison of probe lights. [Figure 6] It is a diagram for explaining the configuration of a detector. [Figure 7] It is a diagram for explaining the measurement principle of the Brillouin gain analysis device according to the present invention. [Figure 8] [[ID=ID=44]]It is a diagram for explaining the Brillouin gain analysis method according to the present invention. <0= [Figure 9] It is a diagram for explaining an experimental system for obtaining Brillouin scattered light by ASE light. [Figure 10]This figure illustrates the experimental results obtained by acquiring Brillouin scattered light using ASE light. [Modes for carrying out the invention]

[0019] Embodiments of the present invention will be described with reference to the attached drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to these embodiments. In this specification and in the drawings, components with the same reference numerals refer to the same components.

[0020] Figure 4 is a diagram illustrating the Brillouin gain analyzer 301 of this embodiment. The Brillouin gain analyzer 301 is A laser 11 that outputs continuous light of a single frequency, An ASE (Amplified Spontaneous Emission) light source 13 generates continuous light with a broader bandwidth of frequency components than the frequency of the continuous light output by the laser 11, and incidents it onto one end of the optical fiber 50 to be measured. A pulse generator 12 pulses the continuous light from the laser 11 and injects it into the other end of the optical fiber 50, A modulator 14 generates local light obtained by shifting the frequency of the continuous light from the laser 11 by an arbitrary frequency, A detector 15 that performs heterodyne detection of the Brillouin scattered light generated in the optical fiber and the local light, It is equipped with.

[0021] The Brillouin gain analyzer 301 includes a scattered light generation unit 41 and a scattered light acquisition unit 42. The scattered light generation unit 41 generates two types of light, pump light Lpn and probe light Lpr, and propagates them in opposite directions through the optical fiber 50 under test. Pump light Lpn is generated by pulsed single-frequency continuous light output by laser 11 through intensity modulation by AOM in pulse generator 12. The power of pump light Lpn is amplified by EDFA 61. On the other hand, probe light Lpr is generated by amplified broadband light generated using ASE light source 13 by EDFA 62. Figure 5 is a comparison of probe light Lpr of Brillouin gain analyzer 300 (Figure 5(A)) and probe light Lpr of Brillouin gain analyzer 301 (Figure 5(B)). The pump light Lpn and the probe light Lpr interact within the optical fiber 50 under test. Due to this interaction, the light in which the Brillouin scattered light from the pump light Lpn is superimposed on the probe light Lpr is sent to the scattered light acquisition unit 42 by the circulator 63.

[0022] The scattered light acquisition unit 42 converts the light sent from the scattered light generation unit 41 into an electrical signal. First, the Rayleigh scattered light component is removed by the BPF 64, resulting in a signal light Lsig consisting only of Brillouin scattered light. In addition, the scattered light acquisition unit 42 also receives a single-frequency continuous light branched from the laser 11. The SSB modulator 14 modulates this continuous light to near the Brillouin frequency shift (BFS) and outputs it as local light Lo. Detector 15 performs heterodyne detection using the signal light Lsig and the local light Lo. Figure 6 is a comparison of the detector of the Brillouin gain analyzer 300 (Figure 6(A)) and the detector 15 of the Brillouin gain analyzer 301 (Figure 5(B)). The detector of the Brillouin gain analyzer 300 is a photodiode, while the detector 15 of the Brillouin gain analyzer 301 is equipped with a 50:50 coupler 15a and a balanced photodiode (BPD; Balanced Photo Detector) 15b.

[0023] Figure 7 illustrates the measurement principle of the Brillouin gain analyzer 301. Compared to the conventional BOTDA shown in Figure 2, the probe light Lpr has a wider bandwidth, adequately covering the BGS frequency range 23. Therefore, while conventional BOTDA required multiple measurements by changing the frequency of the probe light Lpr, the BGS can be obtained in a single measurement with the Brillouin gain analyzer 301.

[0024] The Brillouin gain analyzer 301 further includes a signal processing unit 43 that performs a Fourier transform on the signal heterodyne detected by the detector 15 to detect the Brillouin gain spectrum (BGS), and acquires the vibration distribution of the optical fiber 50 from the time-series change of the peaks of the BGS.

[0025] Figure 8 is a flowchart illustrating the operation of the Brillouin gain analyzer 301, including the signal processing unit 43. This Brillouin gain analysis method is as follows: Step S01: Continuous ASE (Amplified Spontaneous Emission) light, which has a broader frequency band than single-frequency laser light, is incident as probe light Lpr on one end of the optical fiber 50 to be measured. The laser light is pulsed and injected as pump light Lpn into the other end of the optical fiber 50 (step S02). (Step S03) to generate local light Lo by shifting the frequency of the laser light by an arbitrary frequency, and The Brillouin scattered light Lsig generated in the optical fiber 50 and the local light Lo are subjected to heterodyne detection (step S04). It is characterized by the following.

[0026] Then, in the signal processing unit 43, Step S05: The heterodyne detected signal is Fourier transformed to detect the Brillan gain spectrum (BGS), and Obtain the vibration distribution of the optical fiber 50 from the time-series change of the BGS peak (Step S06). Further steps are taken.

[0027] First, in step S01, the light from the ASE light source 13, within the range considering the BFS of the BGS, is continuously incident on one end of the optical fiber 50 as probe light Lpr. Next, in step S02, a pump light Lpn consisting of a single frequency is injected into the other end of the optical fiber 50, and the probe light Lpr and the pump light Lpn interact within the optical fiber 50 to generate Brillouin scattered light. In step S03, the frequency of the light branched from the pump light source is shifted by an arbitrary frequency to generate a local light Lo with a frequency near the BFS. In step S04, the Brillouin scattered light Lsig and the local light Lo are heterodyned and detected. At this point, the signals necessary to form a distributed BGS with a single pump light pulse are received. Steps S02 to S04 are repeated for the duration for which vibration detection is desired. After the specified time has elapsed, in step S05, a Fourier transform is performed on the acquired signal to form a BGS. Finally, in step S06, oscillations are obtained by observing the time-series changes in the BGS peaks.

[0028] In other words, the pump light is pulsed, while the probe light is continuous, and one incidence of the pump light corresponds to one measurement of the entire optical fiber 50. Then, this pulsed pump light is incident for the desired measurement time, and vibration measurement is performed.

[0029] (Examples) This example describes experimental results obtained by acquiring Brillouin scattered light using ASE light. Figure 9 illustrates the experimental setup in this embodiment. The optical fiber 50 under test in this experimental setup consists of a single-mode optical fiber SSMF and a low-bending-loss optical fiber BIF connected in series. In the Brillouin gain analyzer 301 of the embodiment, probe light Lpr was incident on one end of the optical fiber 50 and pump light Lpn on the other end. However, in this experimental setup, probe light Lpr, which is ASE light, and pump light Lpn, which is pulsed light consisting of a single frequency, are incident on one end of the optical fiber 50. To create the same conditions as the Brillouin gain analyzer 301, a mirror Mr is placed at the other end of the optical fiber 50, and probe light Lpr is incident on the pump light Lpn in time before it. The pump light Lpn, which is incident later, interacts with the probe light Lpr reflected by the mirror Mr at the other end, causing Brillouin scattering.

[0030] Figure 10 illustrates the result (waveform RL2) of heterodyne detection (waveform RL2) performed by detector 15 on the return light containing Brillouin scattered light from optical fiber 50 in the experimental setup shown in Figure 9. When only the pump light Lpn is incident on the optical fiber 50, and the ASE probe light Lpr is not incident on the optical fiber 50, the returned light has the waveform RL1, indicating that Brillouin scattering does not occur.

[0031] Because two different types of optical fibers with different characteristics are used as the optical fiber 50 under test, two Brillouin scattering peaks can be observed in the waveform RL2. In Figure 10, the information of the entire optical fiber 50 is shown in a single figure to confirm whether Brillouin scattering can be acquired with ASE light. By dividing the waveform RL2 into time segments and converting it into a frequency spectrum, the vibration of the optical fiber 50 can be acquired in a distributed manner. In other words, this experimental system shows that by broadening the bandwidth of the probe light and using optical heterodyne detection, Brillouin scattered light on the optical fiber can be acquired in a distributed manner with a single pulsed pump light.

[0032] (effect) In the prior art (Non-Patent Literature 1), pulsed ASE light was used as the probe light to acquire the BGS of a branched optical fiber. In the present invention, for high-speed measurement of an unbranched optical fiber, continuous ASE light is used as the probe light, and the acquired signal is converted to BGS using a method such as Fourier transform. This method makes it possible to acquire a distributed BGS with a single pulsed pump light. A feature of the present invention is that, in order to shorten the measurement time required for vibration measurement, continuous ASE light is used as the probe light, and the acquired signal is converted to BGS using a frequency analysis method.

[0033] (definition) The abbreviations used in this specification and in the drawings are as follows: 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 [Explanation of Symbols]

[0034] 11: Laser 12: Pulse Generator 13:ASE light source 14: Modulator 15: Detector 15a:50:50 coupler 15b: Balanced photodiode 21: Changes in BGS peak 22: Arbitrary frequency 23: Sweep range 41:Scattered light generation section 42: Scattered light acquisition section 50: Optical fiber under test 61, 62: Optical amplifier 63: Light Circulator 73: Frequency range of probe light 91: Optical multiplexer 300, 301: Brillouin gain analyzer

Claims

1. A laser that outputs continuous light of a single frequency, An ASE (Amplified Spontaneous Emission) light source generates continuous light with a broader bandwidth of frequency components than the frequency of the continuous light output by the aforementioned laser, and directly incidents it onto one end of the optical fiber to be measured as probe light, A pulse generator that pulses the continuous light from the laser and injects it into the other end of the optical fiber as pump light, A modulator that generates local light obtained by shifting the frequency of the continuous light from the laser by an arbitrary frequency, A detector that performs heterodyne detection of the Brillouin scattered light generated in the optical fiber and the local light, A Brillouin gain analyzer equipped with the following features.

2. The Brillouin gain analyzer according to claim 1, further comprising a signal processing unit that performs a Fourier transform on the signal heterodyne detected by the detector to detect the Brillouin gain spectrum (BGS), and acquires the vibration distribution of the optical fiber from the time-series change of the peak of the BGS.

3. The method involves directly injecting ASE (Amplified Spontaneous Emission) continuous light, which has a broader frequency band than single-frequency laser light, into one end of the optical fiber being measured as probe light. The laser light is pulsed and incident as pump light at the other end of the optical fiber. The process involves shifting the frequency of the laser light by an arbitrary frequency to generate local light, and The Brillouin scattered light generated in the optical fiber and the local light are subjected to heterodyne detection. A Brillouin gain analysis method characterized by the following.

4. The heterodyne-detected signal is Fourier-transformed to detect the Brillouin gain spectrum (BGS), and The vibration distribution of the optical fiber is obtained from the time-series change of the BGS peak. The Brillouin gain analysis method according to claim 3, further characterized by performing the following steps.

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