Optical path characteristic analysis device

Abstract

To provide an optical path characteristic analysis device for measuring the loss distribution of an optical path, and to allow improvement in spatial resolution even in an optical path to be measured having a variation in the BFS.SOLUTION: The present disclosure is an optical path characteristic analysis device that acquires a plurality of measurement results of Brillouin scattering measuring a first area in which a specific point of an optical path to be measured is included and a second area in the first area excluding the specific point, and calculates the loss distribution at the specific point by using the plurality of measurement results of the Brillouin scattering.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical fiber line characteristic analysis device and an analysis method for measuring the loss distribution of an optical fiber line. [Background technology]

[0002] The mainstream of FTTH (Fiber To The Home) is PON (Passive Optical Network) where an optical splitter is connected between a communication building and a user's home. When constructing or maintaining a PON, technology is important to check whether the optical line is normal or abnormal, detect faults in the optical line, or identify the fault location. End reflection assisted Brillouin Optical Time Domain Reflectometry (ERA-BOTDR) has been proposed as a method for individually acquiring the loss distribution of branched optical fibers branched by an optical splitter (for example, see Non-Patent Document 1).

[0003] In measuring branched optical fibers, it is important to be able to measure the end of a large loss such as an 8-branch splitter (hereinafter sometimes abbreviated as 8SP), and to be able to perform the measurement without considering the variation of Brillouin frequency shift (hereinafter sometimes abbreviated as BFS) in the field. As a method to achieve both, ERA-BOTDR using a frequency swept pump pulse has been proposed (for example, see Non-Patent Document 2). In Non-Patent Document 2, compensation of BFS by frequency sweeping and the pre-pump effect of acoustic waves are used.

[0004] However, with the Frequency Swept Pump Pulse, there is a dead zone beyond a large loss event such as 8SP, and there are cases where the loss event of the branched optical fiber cannot be measured correctly. A method using differential double pulses (Non-Patent Document 3) has been proposed as a method to improve spatial resolution, but in a measured optical line with variation in BFS, the frequency difference between the pump light and the probe light must be changed while measuring. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6461746 (NTT) [Non-patent literature]

[0006] [Non-Patent Document 1] H. Takahashi, F. Ito, C. Kito, and K. Toge, “Individual loss distribution measurement in 32-branched PON using pulsed pump-probe Brillouin analysis,” Optics Express, vol. 21, no. 6, pp. 6739-6748 (2013). [Non-Patent Document 2] H. Takahashi, K. Toge, C. Kito and T. Manabe, “Brillouin-based PON monitoring with efficient compensation of gain profile variation using frequency-swept pump pulse,” Journal of Lightwave Technology, vol. 35, no. 12, pp. 2358-2365 (2017). [Non-Patent Document 3] W. Li, X. Bao, Y. Li, and L. Chen, “Differential pulse-width pair BOTDA for high spatial resolution sensing,” Optics Express, vol. 16, no. 26, pp. 21616-21625 (2008). Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present disclosure is to enable an improvement in spatial resolution in an optical line characteristic analysis device for measuring the loss distribution of an optical line, even in an optical line to be measured having variations in BFS. [Means for solving the problem]

[0008] The optical line characteristic analysis device disclosed herein executes the optical line characteristic analysis method disclosed herein. In the optical fiber line characteristic analysis method disclosed herein, an optical fiber line characteristic analysis apparatus disclosed herein comprises: acquiring a plurality of Brillouin scattering measurement results obtained by measuring a first region including a specific point of the optical path to be measured and a second region of the first region excluding the specific point; The loss distribution at the specific point is calculated using the results of the multiple Brillouin scattering measurements.

[0009] The multiple Brillouin scattering measurement results may be Brillouin gains in the first region and the second region. In this case, the optical line characteristic analysis device calculates a loss distribution at the specific point by calculating a difference between the Brillouin gains in the first region and the second region. In this way, the present disclosure can obtain a loss distribution from which the effects of crosstalk have been removed.

[0010] The optical path characteristic analysis device according to the present disclosure comprises: a first optical frequency modulation unit capable of generating pump light having a bandwidth wider than the frequency change in the distance direction of the Brillouin frequency shift of the optical line under test; a first light pulsing unit that generates pump light pulses by pulsating the pump light; a second optical frequency modulation unit that generates a probe light having a frequency different from that of the pump light; a second light pulsing unit that generates a probe light pulse by pulsing the probe light; a timing adjustment unit that controls a time difference between when the pump light pulse and the probe light pulse are generated by controlling the timing at which the first and second light pulsing units generate the pump light pulse and the probe light pulse; an optical combiner for combining the pump light pulse and the probe light pulse; an optical receiving unit that receives return light resulting from the combined optical pulse being reflected on the optical line under test and converts the return light into an electrical signal; a Brillouin scattering signal analysis unit that acquires a Brillouin gain of the optical line under test based on the signal obtained by the optical receiving unit; a differential pulse analyzer that calculates a loss distribution at the specific point based on Brillouin gains in the first and second regions obtained by using the pump light pulses having different pulse widths; The device may include:

[0011] The first optical pulsing unit may generate pump optical pulses having a first pulse width and a second pulse width. The first pulse width is wider than the second pulse width. In this case, the optical receiving unit receives a first return light obtained by reflecting the pump optical pulse having the first pulse width on the measured optical line, and a second return light obtained by reflecting the pump optical pulse having the second pulse width on the measured optical line. Then, the Brillouin scattering signal analyzing unit obtains a Brillouin gain of the first region based on a signal obtained from the first return light, and obtains a Brillouin gain of the second region based on a signal obtained from the second return light.

[0012] The timing adjustment unit may adjust the time at which the frequency difference between the pump light pulse and the probe light pulse coincides with the Brillouin frequency shift so that the time at which the Brillouin frequency shift occurs is positioned backward within the pulse duration of the first pulse width, and may make the frequency difference between the pump light pulse and the probe light of the second pulse width coincide with the frequency difference forward in the pulse duration of the first pulse width.

[0013] The optical fiber to be measured may include a trunk optical fiber connected to the optical fiber characteristic analyzer, an optical splitter connected to the trunk optical fiber, and a plurality of branch optical fibers branched by the optical splitter. The timing adjustment unit controls a time difference between when the pump optical pulse and the probe optical pulse are generated so that a probe optical pulse reflected at a far end of the branch optical fiber collides with a pump optical pulse in the branch optical fiber.

[0014] The above disclosures may be combined to the greatest extent possible. Effect of the Invention

[0015] According to the present disclosure, in an optical line characteristic analysis device for measuring the loss distribution of an optical line, it is possible to improve the spatial resolution even in a measured optical line having variations in BFS. [Brief description of the drawings]

[0016] [Figure 1] 1 is a block diagram showing a configuration of an optical path characteristic analysis device according to an embodiment of the present invention; [Diagram 2] FIG. 2 is an explanatory diagram showing an example of a pump light pulse and a probe light pulse according to the present disclosure. [Diagram 3] 4 shows an example of frequency characteristics of a pump light pulse and a probe light pulse. [Figure 4] 2 shows an example of the configuration of a measured optical line. [Diagram 5] 4 shows an example of the signal intensity received by the optical receiving unit. [Figure 6] 1 shows an example of the intensity of the Brillouin acoustic wave obtained in the first measurement. [Figure 7] 13 shows an example of the intensity of the Brillouin acoustic wave obtained in the second measurement. [Figure 8] 13 shows an example of frequency characteristics of a pump light pulse and a probe light pulse during a first measurement with a wide pulse width. [Figure 9]13 shows an example of frequency characteristics of a pump light pulse and a probe light pulse during a second measurement with a narrow pulse width. [Figure 10] 1 shows an example of an optical path characteristic analysis method according to the present disclosure. [Figure 11] 1 shows an example of a loss distribution when large losses are present. [Figure 12] FIG. 2 is an explanatory diagram of crosstalk occurring in Brillouin scattering. [Figure 13] An example of the measurable range before and after the occurrence of a loss is shown below. [Figure 14] 5A and 5B are explanatory diagrams of pump light pulses used when measuring the branch optical fiber 23#2, where (a) shows a comparative example and (b) shows an example of the present disclosure. [Figure 15] 13 is a loss measurement result according to a comparative example of the present disclosure. [Figure 16] 13 is a loss measurement result according to an embodiment of the present disclosure. [Figure 17] 1 is a conceptual diagram of loss measurement of a measured optical fiber line performed by an optical fiber line characteristic analysis device; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, the 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 are assumed to indicate the same components.

[0018] [composition] 1 is a block diagram showing the configuration of an optical line characteristic analysis device according to this embodiment. Optical line characteristic analysis device 91 is an optical line characteristic analysis device that measures loss distribution at a specific point in a measured optical line 92. In this embodiment, an example is shown in which optical line characteristic analysis device 91 includes optical frequency modulation units 11-1 and 11-2, light sources 12-1 and 12-2, timing adjustment unit 13, pulse generators 14-1 and 14-2, optical pulsing units 15-1 and 15-2, optical multiplexer 16, optical branching unit 17, optical receiving unit 18, and calculation processing unit 19.

[0019] The optical frequency modulation unit 11-1 functions as a first optical frequency modulation unit and linearly frequency modulates the continuous light from the light source 12-1. The optical pulse generation unit 15-1 functions as a first optical pulse generation unit and pulses the continuous light from the optical frequency modulation unit 11-1 in accordance with the pulse from the pulse generator 14-1. This generates a pump light pulse.

[0020] The optical frequency modulation unit 11-2 functions as a second optical frequency modulation unit, and linearly frequency-modulates the continuous light from the light source 12-2 at a frequency different from that of the pump light. The optical pulse generation unit 15-2 functions as a second optical pulse generation unit, and pulses the continuous light from the optical frequency modulation unit 11-2. This generates a probe light pulse.

[0021] The optical multiplexer 16 multiplexes the pump light pulse and the probe light pulse. The optical branching unit 17 inputs the multiplexed light from the optical multiplexer 16 into the optical line 92 under measurement. The backscattered light in the optical line 92 under measurement is output from the optical branching unit 17 to the optical receiving unit 18. As a result, the backscattered light in the optical line 92 under measurement is received by the optical receiving unit 18.

[0022] The timing adjustment unit 13 adjusts the generation timing and pulse width of the pulses in the pulse generators 14-1 and 14-2. In this way, the timing adjustment unit 13 adjusts the generation timing and pulse width of the pump light pulse and the probe light pulse. In this embodiment, the pulse width is τ p1 and τ p2 A pump light pulse with a pulse width of τ p1 is the pulse width τp2 That is, the pulse width τ p1 is the “first pulse width” of the present disclosure, and the pulse width τ p2 is the "second pulse width" of the present disclosure. In this embodiment, the pulse width τ p1 A wide pulse width τ p1 , pulse width τ p2 The narrow pulse width τ p2 It is sometimes called.

[0023] The optical line characteristic analysis device 91 of the present disclosure performs the following two types of measurements. (i) First measurement: Pulse width τ p1 The pulse generator 14-2 generates a pulse with a pulse width of τ s This generates a pulse with a pulse width of τ p1 The pump light pulse Pp and pulse width τ s The probe light pulses Pr are generated in sequence. (ii) Second measurement: Pulse width τ p2 The pulse generator 14-2 generates a pulse with a pulse width of τ s This generates a pulse with a pulse width of τ p2 The pump light pulse Pp and pulse width τ s The probe light pulses Pr are generated in sequence.

[0024] 3 shows an example of the frequency characteristics of the pump light pulse and the probe light pulse. The optical frequency modulation unit 11-1 linearly sweeps the frequency of the pump light pulse. On the other hand, the frequency of the probe light pulse can also be swept using the optical frequency modulation unit 11-2. However, in this embodiment, in order to facilitate understanding of the invention, an example will be described in which the frequency in the optical frequency modulation unit 11-2 is constant and the probe light pulse has a single frequency.

[0025] 4 shows an example of the configuration of an optical line to be measured 92 of this embodiment. A trunk optical fiber 21 connected to an optical line characteristic analysis device 91 is branched into a plurality of branch optical fibers 23#1 and 23#2 by an optical splitter 22. Reflective optical filters 24#1 and 24#2 are provided at the far ends of the respective branch optical fibers 23#1 and 23#2. The reflective optical filter 24#1 is provided at a position distant from the optical line characteristic analysis device 91 by a distance L1, and the reflective optical filter 24#2 is provided at a position distant from the optical line characteristic analysis device 91 by a distance L2.

[0026] The timing adjustment unit 13 controls the time difference between the generation of the pump light pulse and the probe light pulse so that the probe light pulse reflected at the far ends of the branch optical fibers 23#1 and 23#2 collides with the pump light pulse in the branch optical fiber. At this time, when the multiplexed light from the optical multiplexer 16 is input to the measured optical line 92, the probe light pulse reflected by the reflective optical filter 24#1 collides with the pump light pulse in the branch optical fiber 23#1, causing Brillouin scattering. Similarly, Brillouin scattering occurs in other branch optical fibers 23 such as the branch optical fiber 23#2. As a result, as shown in FIG. 5, the return light pulse S#1 from the branch optical fiber 23#1 and the return light pulse S#2 from the branch optical fiber 23#2 are received by the optical receiving unit 18. The signal intensity of the return light pulse is determined depending on the time integral of the Brillouin acoustic wave.

[0027] In this embodiment, a first region of the optical line 92 under measurement is measured by the first measurement, and a second region of the optical line 92 under measurement is measured by the second measurement. For example, the optical receiving unit 18 receives a first return light reflected in the optical line 92 under measurement by the first measurement. The optical receiving unit 18 also receives a second return light reflected in the optical line 92 under measurement by the second measurement. The Brillouin scattering signal analyzing unit 19A obtains the Brillouin gain of the first region based on a signal obtained from the first return light, and obtains the Brillouin gain of the second region based on a signal obtained from the second return light.

[0028] 6 and 7 show an example of a plurality of Brillouin scattering measurement results obtained by measuring the first and second regions of the branch optical fiber 23#2. p This is an example of the Brillouin scattering measurement result in the first region including the specific point of strong interaction indicated by τ = 0. The first region shown in Figure 6 has a wide pulse width of 400 ns. p1 The Brillouin acoustic wave is shown when the frequency is swept linearly as shown in FIG. 8. FIG. 7 shows an example of the measurement result of Brillouin scattering in a second region excluding the specific point in the first region. The second region shown in FIG. 7 has a narrow pulse width τ p2 FIG. 9 shows the Brillouin acoustic wave when the frequency is swept linearly.

[0029] The calculation processing unit 19 executes the optical path characteristic analysis method of the present disclosure. Specifically, as shown in Figure 10, The Brillouin scattering signal analyzer 19A has a wide pulse width τ p1 The Brillouin gain B1 shown in FIG. 6 measured with the pump light pulse is calculated (S11). The Brillouin scattering signal analyzer 19A analyzes the narrow pulse width τ p2 The Brillouin gain B2 shown in FIG. 7 measured with the pump light pulse is calculated (S12). The differential pulse analysis unit 19B calculates the difference between the Brillouin gains B1 and B2 (S13). Using this difference, the differential pulse analysis unit 19B calculates p Calculate the loss distribution in the area including the specific point where =0.

[0030] In this embodiment, the second region is a part of the first region, region A. B In step S13, the second region A is determined from the measurement result of the Brillouin scattering in the first region. A By subtracting the Brillouin scattering measurement results of the first region A A and A B The second area A B Different area A A This derives the Brillouin gain at the time t p= 0, it is possible to obtain the Brillouin scattering characteristics.

[0031] The time when strong interaction occurs can be adjusted by adjusting the time when the frequency difference between the pump light and the probe light coincides with the BFS. The time when the frequency difference between the pump light and the probe light coincides with the BFS can be adjusted, for example, by adjusting Δt shown in Figure 3. Δt is the time from the end of the pulse duration of the pump light pulse to the end of the pulse duration of the probe light pulse.

[0032] In this embodiment, the wide pulse width τ p1 When generating a pump light pulse, the time t at which BFS occurs can be adjusted by adjusting Δt shown in Fig. 3, as shown in Fig. 6. p = 0 is placed after the pulse duration of 400 ns, and the distribution of Brillouin gain from -300 ns to 100 ns is measured. Then, the narrow pulse width τ p2 Using this pump light pulse, the distribution of the Brillouin gain from -300 ns to -100 ns is measured as shown in Fig. 7. This makes it possible to obtain the Brillouin scattering characteristics from -100 ns to 100 ns, where the interaction is strong, in step S13.

[0033] In this manner, in this embodiment, the pulse width τ p1 Adjust Δt so that the pulse duration is placed backwards and the wide pulse width τ p1 Then, measure the Brillouin gain at the narrow pulse width τ p2 The frequency difference between the pump pulse and the probe pulse is expressed as a wide pulse width τ p1 The frequency difference in the pulse duration is then matched to the forward frequency difference.

[0034] Here, a pulse width of 200 ns corresponds to information for a section of about 20 m in an optical fiber. Therefore, the narrow pulse width τ p2 When measuring the Brillouin gain at , the measurement point of the Brillouin gain is set to a value with a wide pulse width τ p1 The measurement point for the Brillouin gain may be set 10 m before the measurement point for the Brillouin gain.

[0035] It should be noted that distribution measurement is also possible by changing and calculating the time difference between the incidence of the pump light and the probe light (Δt shown in FIG. 3). The specific measurement procedure is the same as that of Patent Document 1. Furthermore, either step S11 or S12 may be executed first.

[0036] Here, we will explain the dead zone. In the case of a large loss (here, X=10 dB) such as an 8-way splitter, ideally there would be a steep change at a distance of 0, as shown by ● in Fig. 11. However, the measured signal has low spatial resolution, changing from -δz to δz, as shown by ○. This is thought to be due to the fact that the optical pulse has a spatial spread and that Brillouin scattering becomes crosstalk. This crosstalk can occur in both Brillouin scattering after a loss of X dB at a distance z=-δz, as shown in Fig. 12(a), and Brillouin scattering before a loss of X dB at a distance z=δz, as shown in Fig. 12(b).

[0037] (Loss measurement results) Fig. 13 shows an example of the results of measuring the branch optical fiber 23#2 shown in Fig. 4. In this case, the optical splitter 22 was connected at a distance of about 1050 nm, and a bend was provided in the branch optical fiber 23#2 at a position 3 m away from the optical splitter 22. The frequency sweep rate γ in the optical frequency modulation unit 11-1 was set to 1.0 [PH / s].

[0038] In this disclosure, for comparison, a region where strong interactions occur is set at the midpoint of the total frequency modulation bandwidth of 400 nm of the pump light frequency, as shown in Fig. 14(a). In the comparative example, even if there is a loss event between distances 0 m and 25 m, the loss cannot be measured and becomes a dead zone, as shown by the dashed line in Fig. 13. Therefore, as shown in Fig. 15, there is a gentle loss distribution between distances 1030 nm and 1080 nm, and it can be seen that the bending loss event is buried in the dead zone of the optical splitter 22.

[0039] On the other hand, in the present disclosure, as shown in FIG. 14(b), a pulse width τp1 Measure the Brillouin gain B1 using a pulse width of 200 nm τ p2 The Brillouin gain B2 was measured using the above and the process of calculating the difference between them was performed for each point in the optical fiber 92 under test. As a result, it can be seen that a loss of 5 dB due to bending was measured near 1060 m, as shown in FIG.

[0040] [Measurement conditions] The probe light and the pump light must satisfy the following conditions: First condition: The modulation frequency bandwidth of the pump light is equal to or greater than the frequency change in the distance direction of the BFS change expected in the optical line under test. Second condition: Pulse width τ of the probe light pulse s is narrower than the time difference 2nΔL / c of ​​the return light from the reflective optical filter 24 at the end of the branch optical fiber. Third condition: The bandwidth of the optical receiver 18 and the bandwidth of the A / D conversion in the Brillouin scattering signal analyzer 19A are set to a value smaller than the pulse width τ p1 and τ p2 It is a band that can receive light. Fourth condition: The probe light pulses returning from the branch optical fibers 23#1 and 23#2 do not overlap in the optical splitter.

[0041] (First condition) The first condition is a condition required for the probe light pulse and the pump light pulse to cause stimulated Brillouin scattering over the entire distance in the optical line under test. That is, the optical frequency modulation unit 11-1 performs the set Brillouin frequency shift f B is characterized in that the linear variation range in which the frequency is linearly varied is equal to or greater than the measurement target frequency range corresponding to the Brillouin frequency shift distribution width to be measured in the optical line under test.

[0042] Fig. 17 is a conceptual diagram of loss measurement of a measured optical line 92 performed by an optical line characteristic analysis device 91. Fig. 17(a) is a diagram explaining the optical frequency difference between pump light and probe light when the frequency of the probe light is not modulated and only the pump light is linearly frequency modulated. Fig. 17(b) is a diagram explaining that the optical frequency of Brillouin scattering varies depending on the position of the measured optical line 92. As shown by the hatched area in Fig. 17(b), the frequency difference between the pump light and the probe light results in different BFSs being obtained in the longitudinal direction of the measured optical line 92. At this time, the obtained Brillouin gain always obtains the peak of the Brillouin gain, so it is possible to obtain the Brillouin gain without waste.

[0043] Since the optical frequency of Brillouin scattering varies depending on the tensile strain applied to the optical line, temperature change, etc., the optical frequency modulation unit 11-1 sets the frequency sweep width of the pump light to a range that can cover the BFS range of the measurement target. Even when a measurement target optical line 92 with a different BFS is connected, it is possible to obtain the peak of the Brillouin gain by setting the frequency sweep width of the pump light to be equal to or larger than the expected BFS range.

[0044] In this embodiment, the case where only the pump light is frequency swept is shown. However, in this principle, it is sufficient that the optical frequency difference between the pump light and the probe light changes with distance, and either or both of the pump light and the probe light may be frequency swept.

[0045] (Second condition) The second condition is a condition for preventing overlapping of stimulated Brillouin scattering light for each branch optical fiber 23. The pulse width τ of the probe light pulse s is wider than the minimum value of the time difference of the return light from the reflective optical filter 24 at the end of each branch optical fiber 23, the stimulated Brillouin scattering light for each branch optical fiber 23 will overlap. As a result, it becomes impossible to separate the stimulated Brillouin scattering light for each branch optical fiber 23 in terms of time. However, this does not apply when there is no branch (N=1). Also, when there are three or more branch optical fibers 23, the minimum difference ΔL among the differences in the lengths of the branch optical fibers 23 is set in advance. minThe pulse generator 14-2 detects the pulse width τ s 2nΔL min Set narrower than / c. ΔL min is the minimum difference in length between the branched optical fibers included in the optical fiber under test. n is the refractive index of the optical fiber. c is the speed of light in a vacuum.

[0046] (Third condition) The third condition is the pulse width τ p1 and τ p2 That is, the bandwidth of the optical receiver 18 and the bandwidth of the A / D converter in the Brillouin scattering signal analyzer 19A are 1 / τ p1 , 1 / τ p2 and 1 / τ s This means it needs to be wider.

[0047] (Fourth condition) The fourth condition is a condition for the probe light pulses returning from the branch optical fibers 23#1 and 23#2 not to overlap at the optical splitter. da The probe light pulse is reflected by the reflective optical filter 24 (#a) at the end of the branch optical fiber 23 and returns to the optical receiver 18. Therefore, the arrival time t da is expressed by equation (11). (Number 11) t da =2nL a / c (11) However, L a is the distance from the optical line characteristic analyzer 91 to the reflective optical filter 24 (#a).

[0048] Here, the time t db is expressed by equation (12). (Number 12) t db =2nL b / c (12) However, L b is the distance from the optical line characteristic analyzer 91 to the reflective optical filter 24(#b).

[0049] Therefore, the time difference for the light to return to the optical receiving unit 18 is expressed by equation (13). (Number 13) |t da -t db |=2n|L a -L b | / c (13)

[0050] L a ≠L b When the probe light pulse is set to τ, the time it takes for the probe light pulse to reach the light receiving unit 18 is different. s Then, the condition is that the following formula is satisfied. (Number 14) τ s ≦|t da -t db | (14)

[0051] [Measurement method] Next, a measurement method that satisfies the above conditions will be described. The measurement method disclosed herein is an optical line characteristic analysis method that propagates a pump light pulse and a probe light pulse and analyzes the characteristics of the optical line to be measured, comprising the steps of: The set Brillouin frequency shift f, which is the optical frequency difference between the probe optical pulse and the pump optical pulse B a linearly varying optical frequency difference sweep step; a probe light pulse injection step of generating the probe light pulse and injecting the probe light pulse into the optical line under test; a pump light pulse injection step of generating the pump light pulse after the probe light pulse injection step, and injecting the pump light pulse into the optical line under test a predetermined time after the probe light pulse is injected; a return light receiving step of receiving a return light pulse from the optical line under test via the optical splitter after the pump light pulse injection step and converting the return light pulse into an electrical signal; a Brillouin gain acquisition step of acquiring a Brillouin gain at the predetermined time from the electrical signal converted in the return light receiving step; a characteristic analysis step of acquiring a loss distribution with respect to the distance of the optical fiber path under test from the Brillouin gain acquired by changing the predetermined time; Do the following.

[0052] Here, in the optical frequency difference sweep step, Set Brillouin frequency shift f B is equal to or greater than a measurement target frequency range corresponding to a Brillouin frequency shift distribution width to be measured in the optical fiber under test, placing the frequency range to be measured in the linear variation range such that a frequency difference between a maximum optical frequency of the linear variation range and a maximum optical frequency of the frequency range to be measured is smaller than a frequency difference between a minimum optical frequency of the linear variation range and a minimum optical frequency of the frequency range to be measured; It is characterized by:

[0053] In addition, when the optical fiber path to be measured 92 has a structure in which one main optical fiber path and a plurality of branch optical fibers are connected by an optical splitter, Before the probe light pulse injection step, the minimum difference ΔL among the differences in the lengths of the branch optical fibers min performing an optical path length difference detection step of detecting In the probe light pulse injection step, the pulse width of the probe light pulse is set to 2nΔL min It is characterized by being set narrower than / c.

[0054] First, the optical line characteristic analysis device 91 inputs a probe light pulse into the optical line 92 under measurement. Then, the optical line characteristic analysis device 91 inputs a pump light pulse into the optical line 92 under measurement t1 seconds after inputting the probe light pulse.

[0055] The probe light pulse and the pump light pulse incident on the measured optical line 92 are split by the optical splitter 22 into N (for example, N=8) pulses.

[0056] (a) Brillouin gain with linearly swept pump light The frequency difference between the probe light and the pump light is the Brillouin frequency shift f B When the frequency difference between the probe and pump pulses is equal to f, the probe pulse is amplified by the interaction of the probe and pump pulses. When a linearly swept pump pulse is used and the frequency bandwidth is wider than the expected BFS range, the frequency difference between the probe and pump pulses is f B In other words, it is possible to always obtain a Brillouin gain peak in all measurements that are averaged, and it is possible to obtain a large Brillouin gain after averaging.

[0057] (b) Frequency sweep rate γ of linearly swept pump light and performance of loss distribution measurement (b-1) Sensitivity In the case of linearly swept pump light, when the frequency sweep rate γ is small, the vicinity of the peak of the Brillouin gain is measured for a long time, and the obtained Brillouin gain is large. When the frequency sweep rate γ is large, the Brillouin gain bandwidth is measured for only a short time, and the obtained Brillouin gain is small. In other words, the smaller the frequency sweep rate γ, the better the sensitivity of the loss distribution measurement.

[0058] (b-2) Spatial resolution When a linearly swept pump is used, the frequency difference between the probe light and the pump light is f B Since Brillouin gain is obtained only in the vicinity, the spatial resolution is determined by the frequency sweep rate γ of the frequency modulation of the pump light, not the pulse width of the pump light. Therefore, the pump light pulse width can be equal to or greater than the time width corresponding to the spatial resolution. The spatial resolution must be evaluated in terms of (1) the spatial resolution at the point where optical fibers with different BFSs are connected and (2) the spatial resolution at the point where loss occurs.

[0059] (b-2-1) Spatial resolution at the point where different optical fibers of BFS are connected When optical fibers with different BFSs are connected, the position where the Brillouin gain is obtained is different. The spatial resolution Δz in this case is expressed by the following equation. (Number 1) Δz=ν(|f B1 -f B2 |+Δf B1 +Δf B2 ) / 2γ (1) where ν is the speed of light in the optical fiber, f B1 ,f B2 is the BFS of the optical line 92 under test, Δf B1 , Δf B2 indicates the half-width at half maximum of the Brillouin gain band of the optical line under test 92. In this case, the spatial resolution is improved by increasing the frequency sweep rate γ.

[0060] (b-2-2) Spatial resolution at loss occurrence points When the pump pulse width is wide, the sum of the Brillouin gain after the loss (desired Brillouin gain) and the Brillouin gain before the splice loss (crosstalk) is measured.

[0061] When losses such as splitters are large, the Brillouin gain (crosstalk) before the loss becomes relatively large compared to the desired Brillouin gain. When the crosstalk is large, the spatial resolution deteriorates. This crosstalk depends on the sweep speed. When the sweep speed is low, the vicinity of the peak of the Brillouin gain is measured even above the loss, resulting in large crosstalk. However, by setting the sweep speed high, the gain is measured sufficiently far from the peak of the Brillouin gain above the loss, making it possible to reduce the crosstalk. Therefore, by setting the sweep speed high, the spatial resolution improves.

[0062] (b-2-3) Relationship between frequency arrangement in pump light pulse and crosstalk The Brillouin gain, which is the main component of crosstalk, is always located just before the point where loss occurs (on the optical line characteristic analyzer side). Therefore, the BFS range to be compensated is placed behind the frequency-swept pump light in terms of time. This makes it possible to make the range that receives Brillouin gain on the lower loss side wider than the range that receives Brillouin gain on the upper loss side. In other words, by placing the BFS range to be compensated behind the pump light in terms of time, it is possible to minimize crosstalk.

[0063] From the above, it can be said that there is a trade-off between sensitivity and spatial resolution, and the minimum γ that satisfies the desired resolution is optimal.

[0064] (c) Measurement of the line loss of the optical line 92 under test The frequency difference between the probe light and the pump light is f B If , when the probe light pulse interacts with the pump light pulse, the probe light pulse undergoes amplification represented by equation (2).

number

[0065] The loss coefficient of the branch optical fiber (#i) is α i The total loss when the branched optical fiber (#i) travels back and forth is 2L i , the incident power of the probe light pulse is I probeIf (0), the intensity I of the probe light pulse at the input end of the branch optical fiber is the intensity I of the probe light pulse that is reflected by the reflective optical filter (#i) at the end and collides with the pump light pulse at a distance z1 from the input end of the optical line 92 to be measured. probe (2L i , z1) is expressed by equation (3).

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[0066] From equation (3), the intensity of the probe light at the input end of the branched optical fiber I probe (2L i ,z) is g B and I pump (z) where I pump (z) is the input power of the pump light pulse, I pump If (0), it is expressed by equation (4). (Number 4) I pump (z)=I pump (0)exp(-α i z) (4)

[0067] In addition, the reflected probe light intensity I ref (2L i ) is expressed by equation (5). (Number 5) I ref (2L i )=I probe (0)·2L i (5)

[0068] Therefore, equation (3) can be expressed as equation (6) using equations (4) and (5).

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[0069] From equation (6), the gain of stimulated Brillouin scattering light is calculated by integrating the product of the loss to the interaction point and the stimulated Brillouin scattering coefficient over the Brillouin interaction distance. Here, the stimulated Brillouin gain coefficient g B (f) has a finite frequency band, and the value integrated in the frequency direction is a constant. Therefore, if the second and third conditions are satisfied for the broadband probe light and broadband pump light, the frequency-direction integral value of the stimulated Brillouin gain coefficient can always be obtained. In other words, the above formula (6) becomes a function of only the following formula.

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[0070] Therefore, the loss distribution from a certain point z0 can be obtained by performing the following calculation.

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[0071] (d) Measurement of Brillouin scattering distribution versus branch optical fiber length The length from the input end of the measured optical line 92 to the end of the branch optical fiber (#a) (1≦a≦N, an integer, here N=8) is defined as L. a The probe light pulse is reflected by the reflective optical filter (#a) installed at the end of the branch optical fiber (#a). Here, if the distance from the end of the branch optical fiber is l, the refractive index of the optical line 92 to be measured is n, and the speed of light in a vacuum is c, the reflected probe light pulse travels l=c / n×t1 / 2 after t1 / 2 seconds, so the distance from the input end of the optical line to be measured is l. x1 Then, the distance l x1 is expressed by the following equation.

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[0072] In addition, the probe light pulse is incident on the measured optical line 92, branched by the optical splitter, reflected by the reflective optical filter (#a) at the end of the branch optical fiber, and is reflected at a distance l from the incident end of the measured optical line. x1 The time t to reach is expressed by the following equation.

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[0073] The time when the pump light is injected into the measured optical line 92 is t1 seconds after the injection of the probe light. The distance from the input end of the measured optical line 92 that the pump light arrives at after t seconds is l. x2 Then, the distance l x2 is expressed by equation (10).

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[0074] From equations (8) and (10), the probe light pulse and the pump light pulse interact at a position of c t1 / 2n. The time of interaction is t1 / 2 seconds after the probe light pulse is reflected by the reflective optical filter (#a) at the end of the branch optical fiber. In other words, by changing the time difference t1 at which the probe light pulse and the pump light pulse are incident on the optical line 92 under test, the position at which the probe light and the pump light interact can be controlled. This makes it possible to obtain the characteristic distribution of stimulated Brillouin scattering with respect to distance.

[0075] (e) Measurement of the time it takes for the probe light pulse reflected by the branch optical fiber (#a) to reach the optical receiver 18 When the fourth condition is satisfied, the probe light pulses returning from the branch optical fibers (#1) to (#8) do not overlap at the optical splitter. Therefore, by measuring the arrival time at the optical receiver 18, it is possible to distinguish in terms of time which of the branch optical fibers (#1) to (#8) the probe light pulse was output from.

[0076] By the above (a) to (e), the optical fiber characteristic analyzer 91 can compensate for BFS variations and measure the loss distribution of each branched optical fiber with high sensitivity while reducing dead zones.

[0077] As described above, the optical path characteristic analysis device 91 according to the present disclosure has the following features: a first optical frequency modulation unit 11-1 that generates pump light whose frequency is swept over a range equal to or greater than the frequency change in the distance direction of the Brillouin frequency shift of the optical line 92 to be measured; a first light pulsing unit 15-1 that generates a pump light pulse by pulsating the pump light; a second optical frequency modulation unit 11-2 that generates a probe light having a different frequency from that of the pump light; a second light pulsing unit 15-2 that generates a probe light pulse by pulsing the probe light so as to satisfy "Condition 4" described in Patent Document 1; a timing adjustment unit 13 that controls the generation timing of the pump light pulse and the probe light pulse in the first and second light pulsing units; an optical multiplexing unit that multiplexes the pump light pulse and the probe light pulse and inputs the multiplexed pulse into a trunk optical line of the optical line to be measured; and an optical receiving unit 18 that receives return light emitted from the trunk optical line and converts it into an electrical signal; a Brillouin scattering signal analyzer 19A for acquiring a loss distribution of each branch optical fiber 23 by measuring the Brillouin gain for each branch optical fiber 23 while changing a time difference between generating the pump light pulse and the probe light pulse; a differential pulse analyzer 19B that calculates a loss distribution from which the influence of crosstalk has been removed based on the loss distribution measured using two types of pump light pulses with different pulse widths; Equipped with.

[0078] According to the present disclosure, loss events near optical splitters can be accurately measured. Moreover, the present disclosure makes it possible to measure the loss characteristics of the branch optical fibers 23 individually. Furthermore, the present disclosure makes it possible to measure precise loss characteristics without changing the optical frequency difference of the test optical pulses even when the Brillouin frequency shift differs in the distance direction of the branch optical fiber 23.

[0079] The Brillouin scattering signal analysis unit 19A and the differential pulse analysis unit 19B provided in the optical line characteristic analysis device 91 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. [Explanation of symbols]

[0080] 11-1, 11-2: Optical frequency modulation section 12-1, 12-2: Light source 13: Timing adjustment section 14-1, 14-2: Pulse generator 15-1, 15-2: Optical pulse generator 16: Optical multiplexer 17: Optical branching section 18: Optical receiver 19: Processing unit 19A: Brillouin scattering signal analysis section 19B: Differential pulse analysis section 22: Optical splitter 23: Branch optical fiber 24: Reflective optical filter 91: Optical line characteristic analysis device 92: Optical line under test

Claims

1. acquiring a plurality of Brillouin scattering measurement results obtained by measuring a first region including a specific point of the measured optical line and a second region of the first region excluding the specific point; calculating a loss distribution at the specific point using the plurality of Brillouin scattering measurement results; Optical line characteristic analysis device.

2. the plurality of Brillouin scattering measurements are Brillouin gains in the first region and the second region; Calculating a difference between the Brillouin gains in the first region and the second region to calculate a loss distribution at the specific point.

2. The optical path characteristic analysis apparatus according to claim 1.

3. a first optical frequency modulation unit capable of generating pump light having a bandwidth wider than a frequency change amount in a distance direction of the Brillouin frequency shift of the optical line under test; a first light pulsing unit that generates a pump light pulse by pulsating the pump light; a second optical frequency modulation unit that generates a probe light having a frequency different from that of the pump light; a second light pulsing unit that generates a probe light pulse by pulsing the probe light; a timing adjustment unit that controls a time difference between when the pump light pulse and the probe light pulse are generated by controlling a timing at which the first and second light pulsing units generate the pump light pulse and the probe light pulse; an optical combiner for combining the pump light pulse and the probe light pulse; an optical receiving unit that receives return light resulting from the combined optical pulse being reflected on the optical line under test and converts the return light into an electrical signal; a Brillouin scattering signal analysis unit that acquires a Brillouin gain of the optical line under test based on the signal obtained by the optical receiving unit; a differential pulse analyzer that calculates a loss distribution at the specific point based on Brillouin gains in the first region and the second region acquired by using the pump light pulses having different pulse widths; The optical path characteristic analysis apparatus according to claim 1 ,

4. the first light pulsing unit generates pump light pulses having a first pulse width and a second pulse width; the first pulse width is greater than the second pulse width; the optical receiving unit receives a first return light obtained by reflecting the pump light pulse having the first pulse width from the measured optical line, and a second return light obtained by reflecting the pump light pulse having the second pulse width from the measured optical line, the Brillouin scattering signal analysis unit acquires a Brillouin gain of the first region based on a signal obtained from the first return light, and acquires a Brillouin gain of the second region based on a signal obtained from the second return light.

4. The optical path characteristic analysis device according to claim 3.

5. The timing adjustment unit is adjusting a time at which the frequency difference between the pump light pulse and the probe light pulse coincides with the Brillouin frequency shift so that the time at which the Brillouin frequency shift occurs is located at the rear of the pulse duration of the first pulse width; a frequency difference between the pump light pulse and the probe light having the second pulse width is matched to a forward frequency difference in a pulse duration of the first pulse width; 5. An optical path characteristic analysis apparatus according to claim 4.

6. the optical fiber under test includes a trunk optical fiber connected to the optical fiber characteristic analysis device, an optical splitter connected to the trunk optical fiber, and a plurality of branch optical fibers branched by using the optical splitter, the timing adjustment unit controls a time difference between generation of the pump light pulse and the probe light pulse so that the probe light pulse reflected at the far end of the branch optical fiber collides with the pump light pulse in the branch optical fiber.

5. An optical path characteristic analysis apparatus according to claim 4.

7. An optical path characteristic analysis method executed by an optical path characteristic analysis device, comprising: acquiring a plurality of Brillouin scattering measurement results obtained by measuring a first region including a specific point of the measured optical line and a second region of the first region excluding the specific point; calculating a loss distribution at the specific point using the plurality of Brillouin scattering measurement results; Optical line characteristic analysis method.

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