Light measurement system and light measurement method

Abstract

The objective of the present disclosure is to enable Rayleigh scattering and Brillouin scattering to be measured simultaneously.　The present disclosure provides an optical measuring system for measuring Rayleigh scattering and Brillouin scattering in an optical fiber by causing light to enter the optical fiber from both ends, wherein incident light for the Rayleigh scattering comprises frequency-multiplexed pulses, and one of the frequency-multiplexed pulses is used as pump light for the Brillouin scattering.

Description

[Technical Field]

[0001] The present disclosure relates to a technology relating to a light measurement system and a light measurement method for measuring scattered light generated when light is incident on an optical fiber. [Background technology]

[0002] A system has been proposed that combines phase-based OTDR (Optical Time Domain Reflectometry) based on Rayleigh scattering with Brillouin Optical Time Domain Analysis (BOTDA) based on Brillouin scattering to simultaneously measure high-speed strain changes, low-speed strain changes, and temperature changes (see, for example, Non-Patent Document 1). The phase-based OTDR in Non-Patent Document 1 detects high-speed strain changes based on changes in scattered light intensity, so it is a qualitative measurement and is unable to accurately and quantitatively measure the magnitude or waveform of strain changes. In addition, there are many points where fading noise reduces the sensitivity of strain change measurements.

[0003] On the other hand, there is a technology for improving sensitivity to strain changes in a phase-shift OTDR system alone that uses frequency-division multiplexing (FDM) pulses and signal processing to remove fading noise and quantitatively measure high-speed strain changes (see, for example, Patent Document 1). In Patent Document 1, the incident pump light is an FDM pulse consisting of multiple frequencies, rather than a single-frequency pulse. A frequency filter extracts each component from Rayleigh scattering caused by the incident FDM pulse, and the average of each frequency component is calculated to determine the phase. Observing this phase change enables quantitative and highly sensitive measurement of high-speed strain changes. Since each frequency component of an FDM pulse is shifted in time by the width of the immediately preceding pulse, after extracting each component using a frequency filter, the time is shifted by the pulse width to align the starting points of each frequency component.

[0004] In the system configuration of Non-Patent Document 1, it is possible to simply combine FDM phase OTDR and BOTDA by changing the optical pulse used in phase OTDR measurement to an FDM optical pulse and calculating the phase of Rayleigh scattered light as in Patent Document 1. However, when simply combined, the frequency bands of the Brillouin gain spectrum (BGS) generated by BOTDA measurement overlap depending on the spacing and pulse width of each optical frequency component of the FDM pulse incident as pump light. As a result, it is difficult to perform BOTDA measurements simultaneously with FDM phase OTDR, making it difficult to simultaneously perform high-sensitivity measurements of high-speed strain changes due to Rayleigh scattered light and slow-speed strain and temperature changes due to Brillouin scattered light. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-169904 [Non-patent literature]

[0006] [Non-Patent Document 1] Coscetta, E. Catalano, E. Cerri, N. Cennamo, L. zeni, and A. Minardo, “Hybrid Brillouin / Rayleigh sensor for multiparameter measurements in optical fibers,” Opt. Exp., vol. 29, no. 15, pp. 24025-24031, 2021. [Non-patent document 2] J. Smith, A. Brown, M. DeMerchant, X. Bao, “Pulse width dependence of the Brillouin loss spectrum”, Opt. Com., vol. 168, Issues. 5-6, pp. 393-398, 1999. [Non-patent document 3] Yonas Muanenda, Claudio J. Oton, Fabrizio Di Pasquale, “Application of Raman and Brillouin scattering phenomena in distributed optical fiber sensing”, frontiers in Physics, vol. 7, Article 155, pp. 1-14, 2019. Summary of the Invention [Problem to be solved by the invention]

[0007] The present disclosure aims to make it possible to measure Rayleigh scattering and Brillouin scattering simultaneously. [Means for solving the problem]

[0008] The light measurement system and light measurement method disclosed herein include: An optical measurement system and an optical measurement method for measuring Rayleigh scattered light and Brillouin scattered light in an optical fiber by introducing light into both ends of the optical fiber, comprising: the incident light for Rayleigh scattering is a frequency multiplexed pulse having a predetermined frequency interval, One of the frequency-multiplexed pulses is used as pump light for the Brillouin scattering. [Effects of the Invention]

[0009] According to the present disclosure, by using, as the incident light, a frequency-multiplexed pulse having a frequency interval determined in consideration of the spread of the frequency spectrum due to the incident pulse and BGS, it is possible to simultaneously measure Rayleigh scattering and Brillouin scattering. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a system configuration diagram illustrating an example of an embodiment of the present disclosure. [Figure 2]FIG. 2 is an explanatory diagram of an FDM pulse used for pump light in the present disclosure. [Figure 3] FIG. 1 is an explanatory diagram of the frequency band of incident light and Brillouin scattering. [Figure 4] FIG. 1 is a flow diagram illustrating an example of a light measurement method according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, 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 indicate the same components.

[0012] The system configuration of an embodiment of the present disclosure is shown in Figure 1. The optical measurement system according to this embodiment includes a scattered light generator 17 that generates Rayleigh scattering and Brillouin scattering in an FUT (Fiber Under Test) 7, a scattered light acquirer 18 that acquires the Rayleigh scattered light and Brillouin scattered light generated in the FUT 7, and an arithmetic processing unit 20. The FUT 7 is any medium capable of generating Rayleigh scattering and Brillouin scattering, such as a silica single-mode fiber. Using this configuration, the optical measurement system of this embodiment simultaneously measures both high-speed strain changes using an FDM phase-shift OTDR and low-speed strain and temperature changes using a BOTDA.

[0013] The scattered light generating unit 17 uses frequency-multiplexed pulses (hereinafter referred to as FDM pulses) as incident light, and simultaneously generates Rayleigh scattering and Brillouin scattering in the FUT 7. As a result, the optical measurement system of the present disclosure simultaneously performs phase OTDR using the Rayleigh scattered light generated by the FDM pulses in the FUT 7, and BOTDA based on Brillouin scattering using the FDM pulses as pump light. Here, because the FDM pulses have a frequency spread, only one of the FDM pulses is used as pump light to generate Brillouin scattering.

[0014] Specifically, the scattered light generation unit 17 is the part that acquires Rayleigh scattering and Brillouin scattering, and includes a CW light source 1 that emits continuous light such as a laser, a modulation signal generator 2, a modulator 3, a pulse generation unit 4, a polarization scrambler 5, a circulator 6 that extracts only the light scattered in the incident direction, and an FUT 7.

[0015] The scattered light generator 17 splits the continuous light from the CW light source 1 into two beams: pump light and probe light. The modulation signal generator 2 and modulator 3 sweep the frequency of the probe light within the range of Brillouin frequency shifts typically used in optical fiber BOTDA. The pulse generator 4 converts the pump light into FDM pulses with different frequencies, as shown in Figure 2. In this embodiment, an example of an FDM pulse is shown in which the differences between the optical frequency of the CW light source 1 and the center frequency of each frequency component are f1, f2, and f3. The polarization scrambler 5 randomizes the polarization of the FDM pulse between pulses with different timings and inputs it to the circulator 6. This averages the polarization dependence of the stimulated Brillouin scattering light. The circulator 6 inputs the FDM pulses from the polarization scrambler 5 to the FUT 7.

[0016] In the FUT7, Rayleigh scattering occurs due to the FDM pulse. Also, in the FUT7, Brillouin scattering occurs due to the probe light and FDM pulse. The circulator 6 outputs the backscattered light of the FDM pulse to the scattered light acquisition unit 18. As a result, the Rayleigh scattered light generated in the FUT7 is output to the scattered light acquisition unit 18. Also, the circulator 6 adds stimulated Brillouin scattered light to the probe light and outputs the light that has passed through the FUT7 to the scattered light acquisition unit 18. As a result, the Brillouin scattered light generated in the FUT7 is output to the scattered light acquisition unit 18.

[0017] The scattered light acquisition unit 18 includes a separation unit 19 that separates and detects the two types of scattered light acquired, a PD (Photo Detector) 10, a 90° hybrid 13, BPDs (Balanced Photo Detectors) 14 and 15, and a data acquisition unit 16. The data acquisition unit 16 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.

[0018] The separator 19 includes circulators 8 and 11 and FBGs (Fiber Bragg Gratings) 9 and 12. The scattered light from the FUT 7 is separated into Brillouin scattered light and Rayleigh scattered light by the separator 19 using the FBGs 9 and 12. For example, the wavelength corresponding to the Brillouin scattering is reflected by the FBG 9 and detected by a PD (Photo Detector) 10. Furthermore, the FBG 12 reflects the wavelength corresponding to the Rayleigh scattered light of the FDM pulse from the light transmitted through the FBG 9 and makes it incident on the 90° hybrid 13. Here, the wavelength reflected by the FBG 9 may be the wavelength of the Stokes light generated by the Brillouin scattering.

[0019] The 90° hybrid 13 and BPDs 14 and 15 function as receivers for Rayleigh scattered light. The 90° hybrid 13 performs phase diversity homodyne detection using local light. The BPDs (Balanced Photo Detectors) 14 and 15 acquire the I and Q component signals detected by the 90° hybrid 13. The PD 10 and BPDs 14 and 15 are connected to a data acquisition unit 16. The data acquisition unit 16 converts the input signals from analog to digital signals and stores them.

[0020] The data acquisition unit 16 stores the signal strength of the frequency components after BFS (Brillouin Frequency Shift) from the PD 10. The calculation processing unit 20 can calculate the BGS using the signal strength of the frequency components after BFS stored in the data acquisition unit 16.

[0021] The signal intensities of the I and Q components of the Rayleigh scattered light are also stored in the data acquisition unit 16. The calculation processing unit 20 can determine the phase in the FUT 7 using the signal intensities of the I and Q components of the Rayleigh scattered light stored in the data acquisition unit 16.

[0022] Therefore, the present disclosure makes it possible to simultaneously perform highly sensitive measurement of high-speed strain changes using Rayleigh scattered light and measurement of slow strain and temperature changes using Brillouin scattered light.

[0023] Here, the Brillouin scattering in the simultaneous measurement will be explained with reference to Fig. 3. The Brillouin scattering occurs when the incident pump light L U This scattered light has components in the frequency band 10-11 GHz away from the pump light L U The frequency 10-11 GHz away from the BFS is called Brillouin Frequency Shift (BFS). By sweeping the frequency near the BFS with a probe light, BGSL R By observing the change in the position of this BGS peak on the frequency axis, we can measure the change in slow strain and temperature.

[0024] The shape of the BGS depends on the characteristics of the incident pump light. U is an FDM pulse as shown in Fig. 2, so the pump light L U The frequency spread is the pulse width T of the FDM pulse. P It depends on the pulse width T P The smaller the FDM pulse shape used in this disclosure, the greater the frequency spread. Any shape can be used as long as the intended BOTDA and phase OTDR can be performed. For example, a rectangular pulse can be used, as shown in Figure 2.

[0025] In this case, the frequency spread of the FDM pulse (hereinafter also referred to as the pulse spectrum) is expressed by the following equation from the Fourier transform X(ω) of the pulse with width 2a:

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[0026] Here, ω represents angular frequency, and the value of X(ω) indicates the extent to which the pulse light contains a component of a certain angular frequency ω. The first point from the peak where X(ω) = 0 is ω = -π / a, π / a. From equation (1), we can see that the frequency band already has a spread corresponding to the pulse width at the time of the FDM pulse before it enters the FUT7.

[0027] This FDM pulse is input to the FUT7 to obtain the Brillouin scattered light. U According to Non-Patent Document 2, the pump light L U Pulse width T P The BGS that takes this into account is expressed by the following equation, which convolves the pulse spectrum of the FDM pulse and the gain spectrum when the pump light is assumed to be a continuous wave at a single frequency.

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[0028] In equation (2), P CW is the power of the probe light, P P is the pump light L U where L is the total length of the optical fiber, A is the power of the eff is the core area of ​​the optical fiber FUT7, T P is the pulse width of the FDM pulse on the time axis, α is the attenuation coefficient of the optical fiber FUT7, and g B is the Brillouin gain, Δν B is the full width at half maximum of the BGS, ν is the frequency at a certain point in the BGS, and ν0 is the frequency at which Brillouin scattering occurs. α can be the attenuation coefficient of a linear optical fiber. L eff is called the interaction length and is expressed by equations (2.1) and (2.2). c represents the speed of light in a vacuum, and n represents the refractive index of the core of the optical fiber, which is FUT7. When the pulse width on the time axis of the FDM pulse is long, equation (2.1) is used. When the pulse width on the time axis of the FDM pulse is short, equation (2.2) is used.

[0029] In BGS measurements, the frequency interval of the incident FDM pulse must be sufficiently wide, taking into account the frequency spread caused by the pump light being a pulse and the frequency spread when acquiring Brillouin scattering. That is, when only measuring Rayleigh scattered light using an FDM pulse, the interval between each optical frequency component needs to be greater than the frequency spread of the FDM pulse. However, in this disclosure, which implements both FDM phase OTDR and BOTDA, the frequency spread is calculated from the convolution of the pulse spectrum of the FDM pulse and the gain spectrum when the pump light is assumed to be a continuous wave at a single frequency (using Equation (2) in the case of a square wave).

[0030] According to Non-Patent Document 2, when the total pulse width of an FDM pulse composed of three frequencies is 150 ns, the BGS full width at half maximum has a frequency spread of approximately 38 MHz. In this case, the intervals between the optical frequencies of the FDM pulse must be 38 MHz or more. Furthermore, if a frequency bandwidth up to approximately 90% of the maximum BGS value is to be secured instead of the full width at half maximum, the intervals between the FDM pulse frequencies must be approximately 100 MHz. In this case, the three frequencies of the FDM pulse are designed so that the differences from the optical frequency of the CW light source 1 are 200 MHz, 300 MHz, and 400 MHz. This makes it possible to simultaneously measure both FDM phase OTDR and BOTDA.

[0031] Furthermore, the probe light is frequency swept within a range that is the sum of the Brillouin frequency shift range typically used in BOTDA in optical fiber and the frequency offset of the FDM pulse, as described below. Regarding the Brillouin frequency shift range, for example, referring to Non-Patent Document 1, in a typical silica single-mode fiber, the modulation signal frequency is swept within the range of 10650 MHz to 10850 MHz. If frequency sweeping is performed within this frequency range using the above-mentioned FDM pulse frequency design, stimulated Brillouin scattering between a specific frequency component of the FDM pulse and the BOTDA probe light can be generated.

[0032] At this time, it is possible to prevent the stimulated Brillouin scattering phenomenon between other frequency components of the FDM pulse and the probe light. As a result, the intensity of the probe light is modulated by the signal of only the Brillouin scattered light generated by stimulated Brillouin scattering between the specific frequency component and the BOTDA probe light. Note that the change in the peak frequency of the BGS, v B changes according to the following formula depending on the strain Δε applied to the optical fiber of FUT7 and the temperature change ΔT.

number

[0033] For Rayleigh scattering, the same processing as in Patent Document 1 is performed. Each frequency component of the Rayleigh scattered light generated by the FDM pulse is extracted using a frequency filter, and the phase of the Rayleigh scattered light at each frequency is averaged to determine the phase of the entire FDM pulse. By observing this phase change, high-speed strain changes can be quantitatively indicated.

[0034] In the FDM pulses of the present disclosure, the incidence time differs slightly for each frequency. Therefore, as shown in Patent Document 1, the present disclosure may also correct the time difference and calculate the phase. Furthermore, when the frequencies of the FDM pulses are separated, the frequency band included in the Rayleigh scattered light may increase. In this case, by utilizing the aliasing effect, it is possible to make the reception band of the receiver of the Rayleigh scattered light smaller than the total occupied frequency width of the FDM optical pulses.

[0035] For example, if the number of frequencies making up the FDM pulse is three and the differences between the optical frequency of the CW light source 1 and the center frequency of each frequency component are f1, f2, and f3 (sometimes referred to as frequency offsets), the differences between the center frequencies of each pulse, such as f2-f1 and f3-f2, must be at least the interval (denoted X) determined by the design. Let Y be the frequency bandwidth of the Rayleigh scattered light for each frequency component, and Z be the frequency band of the receiver for the Rayleigh scattered light. If X = 200 MHz and Y = 10 MHz, the frequencies of the FDM pulse must be separated by X, so the frequency differences between the optical frequency of the CW light source 1 and the FDM pulse are f1 = 100 MHz, f2 = 300 MHz, and f3 = 500 MHz. Since Z must include all frequencies f1 through f3, taking into account a ±100 MHz spread, we add 100 MHz to the maximum value of f1, f2, and f3, resulting in Z = 600 MHz.

[0036] However, even with the same X and Y, if f1 = 100 MHz, f2 = 300 MHz, f3 = 900 MHz, and Z = 400 MHz, aliasing can be used to receive Rayleigh scattered light in the narrower reception band Z. Since f1 and f2 seen at the receiver are below the reception band Z, f1 = 100 MHz and f2 = 300 Hz. Since f3 exceeds the reception band Z = 400 MHz, aliasing occurs, and f3' seen at the receiver becomes f3' = Z - |f3 - Z| = -100 MHz.

[0037] In this case, even if aliasing occurs, the components do not overlap, allowing the frequency components to be accurately separated, and the phase calculation can be performed using the method described in Patent Document 1. Even if the frequency becomes negative when aliasing occurs, the IQ components can be checked and the direction of phase rotation can be determined to determine that clockwise rotation is a negative frequency and counterclockwise rotation is a positive frequency. Therefore, even if the occupied bands of different optical frequency components overlap when their absolute values ​​are taken, they can be separated as long as the positive and negative signs are different. As a result, the reception bandwidth Z of the receiver for Rayleigh scattered light, which originally required 600 MHz (200 MHz x 3 (the number of FDM pulse frequencies)), can be observed at 400 MHz by utilizing aliasing.

[0038] FIG. 4 is a flow chart showing a series of procedures related to this embodiment. Step S11: Taking into consideration the overlap of BGS by FDM phase OTDR and BOTDA, the pulse generator 4 generates pump light with sufficiently separated frequencies of FDM pulses. Step S12: The pump light of the FDM pulse and the probe light are propagated in a counter-propagating manner, and the Rayleigh scattered light and the Brillouin scattered light are respectively obtained. Step S13: The scattered light acquired by the scattered light generation unit 17 is sent to the scattered light acquisition unit 18, which acquires each scattered light. At this time, the IQ components of the fiber length of the FUT 7 can be acquired from the Rayleigh scattering generated by one FDM pulse. In contrast, all BGS information can be acquired in the length direction from the Brillouin scattering generated by one FDM pulse, and in the frequency direction, the frequency of a certain point corresponding to the probe light can be acquired. Step S14: The calculation processing unit 20 acquires the signal intensities of the I and Q components of Rayleigh scattering, finds the phase using the signal processing method of Patent Document 1, and calculates the vibration. The calculation processing unit 20 forms a BGS from the signals stored in the data acquisition unit 16, and acquires the strain and temperature by observing changes in the BGS peaks.

[0039] (Effects of the present disclosure) According to the formulas explained in the above embodiment, by separating the frequencies of the FDM pulses, which are the incident pump light, sufficiently and sweeping the frequency with the probe light in the corresponding frequency range, it is possible to avoid interference between BGSs generated by different FDM pulse frequencies in the BOTDA. This makes it possible to simultaneously measure high-speed strain changes caused by Rayleigh scattered light and slow strain and temperature changes caused by Brillouin scattered light in simultaneous measurements with the FDM phase OTDR and BOTDA. [Industrial Applicability]

[0040] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0041] 17: Scattered light generation section 1:CW light source 2: Modulation signal generator 3: Modulator 4: Pulse generation section 5: Polarization scrambler 6: Circulator 7: FUT (Fiber Under Test) 18: Scattered light acquisition section 8, 11: Circulator 9, 12:FBG(Fiber Bragg Grating) 10:PD (Photo Detector) 13:90° Hybrid 14, 15:BPD(Balanced Photo Detector) 16: Data acquisition section 17: Scattered light generation section 18: Scattered light acquisition section 19: Separation part 20: Processing unit

Claims

1. An optical measurement system for measuring Rayleigh scattered light and Brillouin scattered light in an optical fiber by introducing light into both ends of the optical fiber, comprising: the incident light for Rayleigh scattering is a frequency multiplexed pulse having a predetermined frequency interval, the predetermined frequency interval is wider than the spread of a Brillouin gain spectrum caused by the time width of each pulse included in the frequency multiplexed pulse, one pulse of the frequency-multiplexed pulses is used as pump light for the Brillouin scattering; Light measurement system.

2. Local light is generated by branching continuous light, performing phase detection of Rayleigh scattered light generated in the optical fiber using the local light; The optical measurement system of claim 1 .

3. sweeping the frequency of the probe light in a range in which the Brillouin gain spectrum of the pump light can be extracted; 3. The optical measurement system according to claim 1 or 2.

4. the predetermined frequency interval is determined using a convolution of a pulse spectrum of the frequency-multiplexed pulse and a gain spectrum when the pump light is assumed to be a continuous wave at a single frequency; 4. The optical measurement system according to claim 1.

5. a frequency band of the receiving section of the Rayleigh scattered light is narrower than a frequency band of the entire pulse spectrum of the frequency-multiplexed pulse; 5. The optical measurement system according to claim 1.

6. The Rayleigh scattered light and the Brillouin scattered light generated in the optical fiber are separated using an FBG (Fiber Bragg Grating).

6. A light measurement system according to any one of claims 1 to 5.

7. 1. An optical measurement method for measuring Rayleigh scattered light and Brillouin scattered light in an optical fiber by introducing light into both ends of the optical fiber, the method comprising: the incident light for Rayleigh scattering is a frequency multiplexed pulse having a predetermined frequency interval, the predetermined frequency interval is wider than the spread of a Brillouin gain spectrum caused by the time width of each pulse included in the frequency multiplexed pulse, one pulse of the frequency-multiplexed pulses is used as pump light for the Brillouin scattering; Light measurement method.

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