JP2017040576A - Optical fiber line characteristic analysis device and method for analyzing optical fiber line characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber line characteristic analysis device and a method for analyzing optical fiber line characteristic with which high sensitivity analysis is possible independently of a Brillouin frequency shift range to be compensated.SOLUTION: An optical fiber line characteristic analysis device and a method for analyzing optical fiber line characteristic according to the present invention are designed to modulate an optical frequency difference between a probe light pulse and a pump light pulse linearly and set the bandwidth of the modulation to greater than or equal to the range of Brillouin frequency shift (BFS) to be compensated. By varying the optical frequency difference between the probe light pulse and the pump light pulse linearly, it is possible to acquire BFSs differing in the longitudinal direction of a fiber being measured. Since the Brillouin gain obtained at this time is always a peak of Brillouin gain, it is possible to heighten sensitivity. Even when a fiber to be measured that has a different BFS is connected, it is possible to acquire the peak of Brillouin gain by setting the frequency sweep width of the pump light to an assumed BFS range or greater.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical line characteristic analyzer for measuring loss distribution of an optical line and an analysis method therefor.

  In an optical communication system using an optical line such as an optical fiber, a technique for confirming normality / abnormality of the optical line, detecting a failure of the optical line, or specifying a failure position is important. For confirmation of normality / abnormality, after the optical line is constructed, it is evaluated whether the connection loss value is within the standard at the connection position of the optical line.

  As a method for evaluating the connection loss value of an optical line, a far-end reflection Brillouin gain analysis method has been proposed (see, for example, Non-Patent Document 1). In Non-Patent Document 1, pulsed pump light and probe light are incident on an optical line, and Brillouin scattering generated when one light reflected at the far end and the other light collide in an optical fiber is measured. Connection loss can be evaluated. At this time, the optical frequency difference between the pump light and the probe light needs to coincide with the Brillouin frequency shift (BFS) in the optical fiber.

  On the other hand, a technique capable of individually measuring loss information of the branched lower optical fiber has been proposed (see, for example, Patent Document 1 and Non-Patent Document 2). In Patent Document 1, two test light pulses, a pump light pulse and a probe light pulse, are incident, and a Brillouin gain of each of a plurality of probe lights returning due to a difference in the branch fiber length is analyzed to obtain a branched lower optical fiber. Measure individual loss distributions. In this method as well, as with Non-Patent Document 1, it is necessary to match the BFS in the optical fiber.

  The BFS is different in BFS depending on the temperature and strain of the optical fiber or the difference in fiber parameters. Therefore, in the existing optical line, BFS is different in the longitudinal direction.

  On the other hand, there has been proposed a method (hereinafter referred to as FSAV method) for obtaining the Brillouin gain by repeatedly performing the optical frequency difference between the pump light pulse and the probe light pulse while changing each measurement. Non-Patent Document 3). However, when the Brillouin gain is obtained by the FSAV method, the measurement results in which the Brillouin gain cannot be obtained are also averaged, so the Brillouin gain after averaging is smaller than the Brillouin gain peak. Further, the obtained Brillouin gain depends on the BFS range to be compensated (BFS range that can be measured), and the Brillouin gain obtained becomes smaller as the BFS range to be compensated becomes wider.

  Non-Patent Document 4 proposes a method for compensating for BFS variations by widening the optical frequency bandwidth of the probe light pulse. However, as in Non-Patent Document 3, it can be obtained when the BFS range to be compensated is widened. The Brillouin gain that is achieved is small.

International Publication WO2012 / 165587

H. Takahashi et al. , “Centralized Measurement of Actual Splice Loss for Installed Optical Fiber Cable Networks Using End-reflection-Assisted Bridging in 63”. H. Takahashi, F.A. Ito, C.I. Kito, and K.K. Toge, “Individual loss distribution measurement in 32-branched PON using pumped pump-probe Brillouin Analysis”, Optics Express, Vol. 21, no. 6, 6739, (2013). H. Takahashi et al. , “Individual PON Monitoring Using Maintenance Band Pulsed Pump-Probe Brillouin Analysis”, Optoelectronics and Communications Conference, ThP1-4, 2012. Takahashi et al., “Brillouin loss measurement method using broadband probe light”, Society of Science Society Conference 2014.

In summary, the techniques described in the above-mentioned prior art documents have a trade-off relationship between a wide BFS range and characteristic analysis sensitivity, and can provide an optical line characteristic analysis apparatus having a wide BFS range and high characteristic analysis sensitivity. There was a problem that it was difficult.
Therefore, the present invention provides a highly sensitive optical line characteristic analyzing apparatus and optical line characteristic analyzing method that does not depend on the Brillouin frequency shift range to be compensated in the loss distribution measurement of the optical line using Brillouin scattering. Objective.

  In order to achieve the above object, an optical line characteristic analyzing apparatus and an optical line characteristic analyzing method according to the present invention linearly modulate an optical frequency difference between a probe light pulse and a pump light pulse, and compensate for the modulation bandwidth. The range was set to be equal to or greater than the range of the power Brillouin frequency shift (BFS).

Specifically, the optical line characteristic analyzing apparatus according to the present invention includes a pump light pulse that generates Brillouin scattering in a measured optical line, and a pump light pulse that is reflected at a far end of the measured optical line. An optical line characteristic analyzing apparatus for propagating a probe light pulse to be interacted with and analyzing the characteristic of the measured optical line,
An optical frequency difference sweeping means for linearly varying a set Brillouin frequency shift f _B which is an optical frequency difference between the probe light pulse and the pump light pulse;
Probe light pulse incident means for generating the probe light pulse and for injecting the probe light pulse into the measured optical line;
A pump light pulse incident means for generating the pump light pulse, and entering the pump light pulse into the measured optical line after a predetermined time when the probe light pulse is incident on the measured optical line;
A return light receiving means for receiving a return light pulse from the measured optical line and converting it to an electrical signal;
Brillouin gain acquisition means for acquiring a Brillouin gain at the predetermined time from the electrical signal converted by the return light receiving means;
Characteristic analysis means for obtaining a loss distribution with respect to the distance of the optical line to be measured from the Brillouin gain obtained by changing the predetermined time;
It is characterized by providing.

Also, the optical line characteristic analysis method according to the present invention includes a pump light pulse that generates Brillouin scattering in the optical line to be measured, and an interaction with the pump light pulse that is reflected at the far end of the optical line to be measured. An optical line characteristic analysis method for propagating a probe light pulse to be analyzed and analyzing the characteristic of the optical line to be measured,
An optical frequency difference sweeping step that linearly varies a set Brillouin frequency shift f _B that is an optical frequency difference between the probe light pulse and the pump light pulse;
Generating the probe light pulse, and injecting the probe light pulse into the measured optical line;
After the probe light pulse incident step, the pump light pulse is generated, and the pump light pulse is incident on the measured optical line after a predetermined time when the probe light pulse is incident; and
After the pump light pulse incident step, a return light receiving step for receiving a return light pulse from the measured optical line and converting it to 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 line to be measured from the Brillouin gain acquired by changing the predetermined time;
It is characterized by performing.

  By varying the optical frequency difference between the probe light pulse and the pump light pulse linearly, different BFS in the longitudinal direction of the measured fiber can be obtained. At this time, since the obtained Brillouin gain always obtains the peak of the Brillouin gain, the sensitivity can be increased. Even when fibers to be measured with different BFS are connected, it is possible to obtain the peak of the Brillouin gain by setting the frequency sweep width of the pump light to be greater than or equal to the assumed BFS range.

  Therefore, the present invention can provide a highly sensitive optical line characteristic analyzing apparatus and optical line characteristic analyzing method independent of the Brillouin frequency shift range to be compensated.

  In the optical frequency difference sweeping device of the optical line characteristic analyzing apparatus according to the present invention, the linear fluctuation range for linearly changing the set Brillouin frequency shift fB corresponds to the Brillouin frequency shift distribution width to be measured in the measured optical line. It is more than the measurement object frequency range to perform.

  The optical frequency difference sweeping means of the optical line characteristic analyzing apparatus according to the present invention, the frequency variation of the linear variation range from the frequency difference between the optical frequency sweep start frequency of the linear variation range and the median of the optical measurement target frequency range The measurement target frequency range is arranged in the linear variation range so that the frequency difference between the end frequency of the optical frequency sweep and the median value of the optical measurement target frequency range becomes smaller.

The optical line characteristic analysis method according to the present invention is
In the optical frequency difference sweep step,
Linear variation range for varying the set Brillouin frequency shift f _B linearly is the is the measured frequency range of interest than that corresponding to the Brillouin frequency shift distribution width to be measured in the measurement beam path,
From the frequency difference between the optical frequency sweep start frequency of the linear variation range and the median value of the optical measurement target frequency range, the optical frequency sweep end frequency of the linear variation range and the median value of the optical measurement target frequency range The frequency range to be measured is arranged in the linear variation range so that the frequency difference becomes smaller.

  When the bandwidth (linear variation range) for linearly modulating the optical frequency difference between the probe light pulse and the pump light pulse is equal to or greater than the BFS range (measurement target frequency range) to be compensated, the linear variation range is the measurement target frequency range. By arranging them on the higher frequency side, it is possible to reduce the frequency sweep speed of the optical frequency difference between the probe light pulse and the pump light pulse and increase the measurement sensitivity. The frequency sweep may be monotonically increasing or monotonically decreasing.

In the optical line characteristic analyzing apparatus according to the present invention, when the measured optical line has a structure in which one basic optical line and a plurality of branched optical lines are connected by an optical splitter, the difference in length between the branched optical lines is determined. Further comprising optical path length difference detecting means for detecting a minimum difference ΔL _min of
The probe light pulse incident means sets the pulse width of the probe light pulse to be narrower than 2nΔL _min / c.

In the optical line characteristic analysis method according to the present invention, when the measured optical line has a structure in which one basic optical line and a plurality of branched optical lines are connected by an optical splitter,
Before the probe light pulse incident step, an optical path length difference detecting step of detecting a minimum difference ΔL _min among the length differences of the branched optical lines is performed.
In the probe light pulse incidence step, a pulse width of the probe light pulse is set to be narrower than 2nΔL _min / c.

  The present invention can also measure the characteristics of a branched optical line.

  The optical frequency difference sweeping means of the optical line characteristic analyzing apparatus according to the present invention may vary the optical frequency of the pump light pulse by intensity-modulating the injection current of the semiconductor laser.

  The present invention can provide a highly sensitive optical line characteristic analyzing apparatus and optical line characteristic analyzing method independent of the Brillouin frequency shift range to be compensated.

It is a figure explaining the optical-line characteristic analyzer based on this invention. In the optical line characteristic analyzer which concerns on this invention, it is a figure explaining the frequency modulation means to change the optical frequency of test light. It is a conceptual diagram of the loss measurement by the linearly swept pump light pulse in the optical line characteristic analyzing apparatus according to the present invention. It is a figure explaining the comparison with the Brillouin gain acquired with the optical-line characteristic analyzer based on this invention, and the Brillouin gain of FSAV method. It is a conceptual diagram explaining crosstalk. It is a figure explaining the relationship between frequency arrangement | positioning and crosstalk. It is a figure explaining the to-be-measured fiber whose branch number is 8. SSMF is a general-purpose single-mode optical fiber (Standard Single Mode Fiber), and BIF is a low bending loss optical fiber (Bending-loss Insensitive Fiber). It is a figure explaining the result of having measured the loss of the branch optical line with the optical line characteristic analyzer concerning the present invention. It is a flowchart explaining the optical-line characteristic analysis method based on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. Embodiment shown below is an example and this invention is not restrict | limited to the following embodiment. In the present specification and drawings, the same reference numerals denote the same components.

[Constitution]
FIG. 1 is a block diagram showing a configuration of an optical line characteristic analyzing apparatus 301 of the present embodiment. The optical line characteristic analyzing apparatus 301 includes a pump light pulse that generates Brillouin scattering in the optical line to be measured, and the pump light pulse that is reflected at the far end of the optical line to be measured opposite to the optical splitter. An optical line characteristic analyzer for propagating a probe light pulse to be interacted with and analyzing the characteristic of the optical line to be measured;
An optical frequency difference sweeping means for linearly varying a set Brillouin frequency shift f _B which is an optical frequency difference between the probe light pulse and the pump light pulse;
Probe light pulse incident means for generating the probe light pulse and for injecting the probe light pulse into the measured optical line;
A pump light pulse incident means for generating the pump light pulse, and entering the pump light pulse into the measured optical line after a predetermined time when the probe light pulse is incident on the measured optical line;
A return light receiving means for receiving a return light pulse from the measured optical line via the optical splitter and converting it into an electrical signal;
Brillouin gain acquisition means for acquiring a Brillouin gain at the predetermined time from the electrical signal converted by the return light receiving means;
Characteristic analysis means for obtaining a loss distribution with respect to the distance of the optical line to be measured from the Brillouin gain obtained by changing the predetermined time;
Is provided.

  Specifically, the optical line characteristic analyzing apparatus 301 includes a first test light output unit 10-1, a second test light output unit 10-2, an optical frequency modulation unit 12, an incident time controller (15, 16), a pulse. Comprising means (13, 14), multiplexing element 20, circulator 21, optical receiving means 26, A / D converter 27 and arithmetic processing unit 28. Here, the optical frequency difference sweeping means is the optical frequency modulation means 12, the probe light pulse incidence means is the pulsing means 13, the pump light pulse incidence means is the pulsing means 14, the return light receiving means is the light receiving means 26, and the Brillouin gain. The acquisition unit and the characteristic analysis unit correspond to the A / D converter 27 and the arithmetic processing unit 28.

  The 1st test light output means 10-1 and the 2nd test light output means 10-2 are implement | achieved by the structure shown to Fig.2 (a) and FIG.2 (b), for example, and output continuous light. The continuous light output from the first test light output means 10-1 and the second test light output means 10-2 is hereinafter referred to as probe light and pump light. The probe light and the pump light have different optical frequency differences of about BFS. The probe light is not frequency-modulated and is narrowband light. On the other hand, the optical frequency of the pump light is linearly modulated with respect to time by the optical frequency modulation means 12. The modulation width of the pump light is set to a bandwidth wider than at least the BFS change amount in the distance direction. As shown in FIG. 2 (a), the test light output means 10 modulates the optical frequency of the output light by modulating the current injected into the laser diode, and as shown in FIG. 2 (b). In addition, either configuration in which the output light of the narrow band laser is modulated by an external modulator (SSB-LN modulator or the like) can be realized.

The pulsing means (13, 14) includes an acousto-optic switch that drives the acousto-optic element in pulses. The pulsing means (13, 14) may be provided with a waveguide switch that drives the electro-optic element using LiNbO ₃ in pulses. Furthermore, a semiconductor optical switch for driving a semiconductor optical amplifier (SOA) in a pulse may be provided. Hereinafter, an acousto-optic modulator composed of an acousto-optic switch, a LiNbO ₃ modulator composed of an electro-optic switch, or an SOA composed of a semiconductor optical switch is referred to as an optical device. The optical device pulses the probe light and the pump light at a time driven by the electric pulse. Further, when the laser diode of FIG. 2A is used for the pulsing means (13, 14), the current injected into the laser diode may be replaced by pulsing.

At this time, the pulsing unit 13 sets the pulse width of the probe light pulse to 2nΔL _min / c or less. Here, ΔL _min is the minimum value of the difference in length between the branched optical fibers included in the measured optical fiber. c is the speed of light in vacuum. n is the refractive index of the optical fiber. The pulse width of the pump light pulse adjusted by the pulsing means 14 will be described later.

  The incident time control means (15, 16) has a function capable of changing the modulation time of the driving electric pulse in the pulsing means (13, 14). The incident time control means (15, 16) controls the timing at which each optical pulse is generated by changing the timing at which the pulsing means (13, 14) modulates the pulse.

  The multiplexing element 20 combines the probe light pulse and the pump light pulse.

  The probe light pulse and the pump light pulse combined by the multiplexing element 20 pass through the circulator 21 and enter the measured optical fiber. Thereby, it becomes possible to give a time difference between the time when the probe light pulse is incident on the optical fiber to be measured and the time when the pump light pulse is incident on the optical fiber to be measured. One of the probe light pulse and the pump light pulse is always set to zero by setting the voltage of one of the electric pulse in the probe light pulsing means 13 or the electric pulse in the pump light pulsing means 14 to zero. It is also possible to enter only the optical fiber to be measured.

  The optical fiber to be measured includes an optical splitter 22 and a branched optical fiber 23 that are optically coupled to one end of the backbone optical fiber 25 (see FIG. 8). Here, a reflection type optical filter 24 is disposed at the end of each branch optical fiber 23. These reflection-type optical filters 24 have a characteristic of reflecting light of each wavelength of the probe light pulse and the pump light pulse and transmitting light of other wavelengths. The reflection means may be a reflection filter or an open end Fresnel reflection.

  The optical splitter 22 branches the supplied probe light pulse and pump light pulse into N (for example, N = 8). Then, the probe light pulse reflected by each reflection type optical filter 24 and the pump light pulse directed to the reflection type optical filter 24 interact in the branch optical fiber 23. The probe light undergoes Brillouin amplification due to the interaction between the probe light pulse and the pump light pulse. The probe light pulse and the pump light pulse that have undergone Brillouin amplification return to the circulator 21, pass through the circulator 21, and are led to the light receiving means 26.

  The light receiving means 26 receives light derived from the circulator 21 that is return light from the optical fiber to be measured. The optical receiver 26 converts the received derived light into a current signal. The A / D converter 27 converts the current signal output from the light receiving means 26 into a digital signal. The A / D converter 27 outputs a digital signal to the arithmetic processing device 28. The arithmetic processing unit 28 performs arithmetic processing as described below on the input current value to obtain a loss distribution with respect to the distance for the branched optical fiber.

First, the arithmetic processing unit 28 enters both the probe light pulse and the pump light pulse into the optical fiber to be measured with an incident time difference t ₁ . The incident time difference t ₁ is a parameter for setting a measurement point in the measured fiber. The arithmetic processing unit 28 then determines the probe light intensity when only the probe light pulse is incident on the measured optical fiber from the probe light intensity when both the probe light pulse and the pump light pulse are incident on the measured optical fiber. To find the induced Brillouin gain. Next, the arithmetic processing unit 28 changes the t ₁ repeated measurement of the induced Brillouin gain, to calculate the induced Brillouin gain with respect to the distance. The arithmetic processing unit 28 obtains a loss distribution with respect to the distance for the branched optical fiber from the induced Brillouin gain with respect to the distance.

[Operation]
Next, the operation of the optical line characteristic analyzing apparatus of the present embodiment configured as described above will be described.

First, the output optical frequency and frequency bandwidth of the probe light and the pump light, the pulsing unit 13, the optical receiving unit 26, and the A / D converter 27 need to satisfy the following conditions.
(Condition 1) The modulation frequency bandwidth of the pump light is equal to or greater than the frequency change amount in the distance direction of the BFS change amount assumed for the optical fiber to be measured.
(Condition 2) The pulse width τ of the probe light pulse output from the optical pulse forming means 13 is narrower than the time difference 2nΔL / c of the return light from the reflection type optical filter at the end of the branch optical fiber.
(Condition 3) The band of the optical receiver 16 and the band of the A / D converter 27 are bands that can receive the pulse width τ.

  Conditions 1 to 3 have the following meanings.

  Condition 1 is a condition necessary for the probe light pulse and the pump light pulse to cause stimulated Brillouin scattering at all distances in the optical fiber to be measured. That is, in the optical frequency difference sweeping means, the linear fluctuation range for linearly changing the set Brillouin frequency shift fB is equal to or larger than the measurement target frequency range corresponding to the Brillouin frequency shift distribution width to be measured in the measured optical line. It is characterized by that.

  FIG. 3 is a conceptual diagram of the loss measurement of the fiber under measurement performed by the optical line characteristic analyzing apparatus 301. FIG. 3A is a diagram for explaining the optical frequency difference between the pump light and the probe light when only the pump light is linearly modulated without modulating the frequency of the probe light. FIG. 3B is a diagram for explaining that the optical frequency of Brillouin scattering varies depending on the position of the measured fiber. As shown by the shaded area in FIG. 3B, the frequency difference between the pump light and the probe light obtains different BFS in the longitudinal direction of the measured fiber. At this time, since the obtained Brillouin gain always obtains the peak of the Brillouin gain, it is possible to obtain the Brillouin gain without waste.

  Since the optical frequency of Brillouin scattering differs depending on the tensile strain applied to the optical line, temperature change, etc., the optical frequency difference sweeping means sets the frequency sweep width of the pump light to a range that can cover the BFS range to be measured. Even when fibers to be measured with different BFS are connected, it is possible to obtain the peak of the Brillouin gain by setting the frequency sweep width of the pump light to be greater than or equal to the assumed BFS range.

  In this embodiment, the case where the frequency of only the pump light is swept is shown. However, in this principle, it is sufficient that the optical frequency difference between the pump light and the probe light is changed with respect to the distance. One or both of the lights may be frequency swept.

Condition 2 is a condition for preventing the stimulated Brillouin scattered light for each branch optical fiber from overlapping. When the pulse width τ of the probe light pulse is wider than the minimum value 2nΔL / c of the time difference of the return light from the reflection type optical filter at the end of each branch optical fiber, the stimulated Brillouin scattered light for each branch optical fiber overlaps. As a result, the stimulated Brillouin scattered light for each branch optical fiber cannot be separated in terms of time. However, this is not the case when there is no branch (N = 1). When there are three or more branch optical fibers, the minimum difference ΔL _min is detected in advance among the length differences of the branch optical lines, and the probe light pulse incident means sets the pulse width τ of the probe light pulse to 2nΔL _min. Set narrower than / c.

  Condition 3 is a condition for accurately measuring an optical pulse having a pulse width τ. That is, it means that the band of the optical receiving means 26 and the band of the A / D converter 27 need to be wider than 1 / τ.

[Measuring method]
Next, a branched optical line characteristic analysis method performed by the branched optical line characteristic analysis 301 that satisfies the above conditions will be described. The branching optical line characteristic analysis method includes a pump light pulse that generates Brillouin scattering in a measured optical line, and a pump light pulse that is reflected at a far end of the measured optical line opposite to the optical splitter. An optical line characteristic analysis method for propagating a probe light pulse to be interacted with and analyzing the characteristic of the optical line to be measured,
An optical frequency difference sweeping step that linearly varies a set Brillouin frequency shift f _B that is an optical frequency difference between the probe light pulse and the pump light pulse;
Generating the probe light pulse, and injecting the probe light pulse into the measured optical line;
After the probe light pulse incident step, the pump light pulse is generated, and the pump light pulse is incident on the measured optical line after a predetermined time when the probe light pulse is incident; and
After the pump light pulse incident step, a return light receiving step for receiving a return light pulse from the optical path to be measured through the optical splitter and converting it into an electric 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 line to be measured from the Brillouin gain acquired by changing the predetermined time;
I do.

Here, in the optical frequency difference sweep step,
Linear variation range for varying the set Brillouin frequency shift f _B linearly is the is the measured frequency range of interest than that corresponding to the Brillouin frequency shift distribution width to be measured in the measurement beam path,
The frequency difference between the maximum optical frequency of the linear variation range and the maximum optical frequency of the measurement target frequency range is smaller than the frequency difference between the minimum optical frequency of the linear variation range and the minimum optical frequency of the measurement target frequency range. As described above, the measurement target frequency range is arranged in the linear variation range.

Further, when the measured optical line has a structure in which one basic optical line and a plurality of branched optical lines are connected by an optical splitter,
Before the probe light pulse incident step, an optical path length difference detecting step of detecting a minimum difference ΔL _min among the length differences of the branched optical lines is performed.
In the probe light pulse incidence step, a pulse width of the probe light pulse is set to be narrower than 2nΔL _min / c.

First, the optical line characteristic analyzing apparatus causes a probe light pulse to enter the optical fiber to be measured. Then, the optical line characteristic analyzing apparatus enters the pump light pulse into the optical fiber to be measured t ₁ second after the probe light pulse is incident.

  The probe light pulse and the pump light pulse incident on the optical fiber to be measured are branched into N (for example, N = 8) pieces by the optical splitter.

(A) when the frequency difference between the Brillouin gain probe light and pumping light with a linear sweep pumping light coincides with the Brillouin frequency shift f _B, when the probe light pulse and the pump light pulse interaction, the probe light pulse is Brillouin amplified . When linearly swept pump light is used and the frequency bandwidth is wider than the assumed BFS range, there is always a distance z at which the frequency difference between the probe light and the pump light becomes f _B.

  That is, it is possible to always obtain a Brillouin gain peak in all the measurements to be averaged, and it is possible to obtain a large Brillouin gain after the average measurement. FIG. 4 shows a comparison of the Brillouin gain between the linear sweep pump light and the FSAV method. The horizontal axis represents the modulation frequency bandwidth, and the vertical axis represents the Brillouin gain normalized by the Brillouin gain when the pump light is not modulated. In the linear sweep pump light, the frequency sweep speed γ was kept constant, and the modulation frequency bandwidth was changed by changing the pump light pulse width.

  In this case, the comparison was made by matching the spatial resolution at the 10 dB loss point of the FSAV method and the linear sweep pump light. While the Brillouin gain obtained with the FSAV method increases as the modulation frequency bandwidth increases, the linear swept pump light does not depend on the modulation frequency bandwidth, and a larger Brillouin gain than the FSAV method can be obtained. Recognize.

(B) Frequency sweep speed γ of linear sweep pump light and performance of loss distribution measurement (b-1) Sensitivity When sweep speed γ is small in linear sweep pump light, the vicinity of the Brillouin gain peak is measured for a long time. The Brillouin gain obtained is large. When the sweep rate γ is high, the Brillouin gain obtained is small because the Brillouin gain bandwidth is measured only for a short time. That is, the sensitivity of the loss distribution measurement is improved as the sweep speed γ is decreased.

(B-2) When a spatial resolution linear sweep pump is used, since the Brillouin gain is obtained only when the frequency difference between the probe light and the pump light is near f _B , the spatial resolution is different from that of Non-Patent Documents 1-4. It is determined not by the pulse width but by the sweep speed γ of the frequency modulation of the pump light. Therefore, the pump light pulse width may be equal to or greater than the time width corresponding to the spatial resolution. As the spatial resolution, it is necessary to evaluate (1) the spatial resolution at the point where fibers having different BFS are connected and (2) the spatial resolution at the loss occurrence point.

である。ここで、νは光ファイバ中の光速、ｆ_Ｂ１，ｆ_Ｂ２は被測定ファイバのＢＦＳ、Δｆ_Ｂ１、Δｆ_Ｂ２は被測定ファイバのブリルアン利得帯域の半値半幅を示している。この場合、掃引速度γを高くすることで空間分解能が向上する。 (B-2-1) The position where the Brillouin gain is obtained differs in that the fibers having different spatial resolution BFS are connected at the point where the fibers having different BFS are connected. The spatial resolution Δz at this time is

It is. Here, ν is the speed of light in the optical fiber, f _B1 and f _B2 are the BFS of the measured fiber, and Δf _B1 and Δf _B2 are the half-widths at half maximum of the Brillouin gain band of the measured fiber. In this case, the spatial resolution is improved by increasing the sweep speed γ.

(B-2-2) Spatial resolution at loss occurrence point When the pump pulse width is wide, Brillouin gain after receiving loss (desired Brillouin gain) and Brillouin gain (crosstalk) before receiving connection loss The sum is measured. A conceptual diagram of crosstalk is shown in FIG.

  When the loss of the splitter or the like is large, the Brillouin gain (crosstalk) before receiving the loss is relatively large with respect to the desired Brillouin gain. When the crosstalk is large, the spatial resolution deteriorates as shown by the broken line in FIG. This crosstalk depends on the sweep speed as shown in FIGS. When the sweep speed is low, crosstalk increases because the vicinity of the Brillouin gain peak is measured even on the upper side of the loss. However, as shown in FIG. 5 (c), by setting the sweep speed high, the gain sufficiently separated from the Brillouin gain peak on the upper side of the loss is measured, so that the crosstalk can be reduced. Therefore, the spatial resolution is improved by setting the sweep speed high.

(B-2-3) Relationship between Frequency Arrangement in Pump Light Pulse and Crosstalk The optical frequency difference sweeping means includes an optical frequency sweep start frequency in the linear variation range and a median value in the optical measurement target frequency range. The measurement target frequency range is arranged in the linear variation range so that the frequency difference between the optical frequency sweep end frequency of the linear variation range and the median value of the optical measurement target frequency range is smaller than the frequency difference. It is characterized by that.

  The Brillouin gain, which is the main component of crosstalk, is always on the near side (the optical line characteristic analyzer side) of the point where the loss is always received. Therefore, by arranging the BFS range to be compensated behind the frequency-swept pump light in time, the range that receives the Brillouin gain on the lower loss side is made wider than the range that receives the Brillouin gain on the upper loss side. Can do. That is, it is possible to minimize crosstalk by arranging the BFS range to be compensated behind the pump light in terms of time.

FIG. 6 shows the relationship between the frequency arrangement of the BFS range to be compensated and crosstalk. FIG. 6A shows an arrangement of optical frequencies corresponding to the BFS range to be compensated within the pump pulse width. The right end corresponds to the rising edge of the pump pulse, and the left end corresponds to the falling edge of the pump pulse. ΔF is the total frequency modulation bandwidth of the pump optical frequency, ΔF _comp is the BFS range to be compensated, X is the deviation of the ΔF _comp arrangement, and X = 0 is the coincidence of the falling edge of the pump pulse and the left end of the ΔF _comp arrangement Shows the case. “Placing the BFS range to be compensated behind the frequency-swept pump light in terms of time” means “behind the optical frequency range of Brillouin scattering to be measured in the pump light whose optical frequency varies linearly” It means “place on the side”.

FIG. 6B shows the result of simulating the relationship between the ΔF _comp arrangement and the necessary sweep speed for each crosstalk. From the lower part of FIG. 6, it can be seen that at X = 0, the required sweep rate is low for any crosstalk. Therefore, in relation to (b-2-2), when the crosstalk is small, the sweep speed γ can be set small, and the sensitivity of the loss distribution measurement can be increased. Further, as described above, it can be seen that the necessary sweep speed γ is the lowest at X = 0 in FIG.

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

ここで、ｇ_Ｂはブリルアン利得係数、ｚは分岐光ファイバの入射端から、プローブ光パルスとポンプ光パルスとがインタラクションした位置までの距離である。α_Ｂ（ｚ）は、入射端からの位置ｚのときの誘導ブリルアン利得である。ｇ_Ｂは、誘導ブリルアン散乱係数である。Ｉ_ｐｕｍｐ（ｚ）は、分岐光ファイバの入射端から距離ｚだけ離れた位置におけるポンプ光パルスの強度である。νは光ファイバ中の光速、Δｆ_Ｂは被測定ファイバのブリルアン利得の半値半幅である。 If the frequency difference between the measuring probe light and pumping light line loss of (c) the measured optical fiber is f _B, when the probe light pulse and the pump light pulse interaction, Table probe light pulses by the formula (2) Undergo amplification.

Here, g _B is the Brillouin gain coefficient, and z is the distance from the incident end of the branch optical fiber to the position where the probe light pulse and the pump light pulse interact. α _B (z) is the induced Brillouin gain at the position z from the incident end. g _B is the stimulated Brillouin scattering coefficient. I _pump (z) is the intensity of the pump light pulse at a position away from the incident end of the branch optical fiber by a distance z. ν is the speed of light in the optical fiber, and Δf _B is the half width at half maximum of the Brillouin gain of the measured fiber.

If the loss coefficient of the branch optical fiber (#i) is α _i , the total loss when reciprocating through the branch optical fiber (#i) is 2L _i , and the incident power of the probe light pulse is I _probe (0), the reflection at the end The intensity I _probe (2L _i) at the incident end of the branch optical fiber of the probe light pulse that collides with the pump light pulse at the position z ₁ from the incident end of the measured fiber after being reflected by the type optical filter (#i) , Z ₁ ) is expressed by equation (3).

From Expression (3), the intensity I _probe (2L _i , z) of the probe light at the incident end of the branched optical fiber is a function of g _B and I _pump (z). Here, I _pump (z) is expressed by Expression (4), where the incident power of the pump light pulse is I _pump (0).

Further, the reflected probe light intensity I _ref (2L _i ) that returns to the incident end of the branch optical fiber when only the probe light is incident is expressed by Expression (5).

Therefore, Formula (3) is expressed as Formula (6) when Formula (4) and Formula (5) are used.

のみの関数となる。そこで、以下の演算を行うことで、ある地点ｚ_０からの損失分布を取得することができる。

よって、誘導ブリルアン散乱光の特性を解析すれば、ある地点ｚ_０を基準にした被測定光ファイバの線路損失を測定することができる。 From the equation (6), the gain of the stimulated Brillouin scattered light is a value obtained by integrating the product of the loss to the interacted location and the stimulated Brillouin scattering coefficient with the width of the Brillouin interaction distance. Here, the induced Brillouin gain coefficient g _B (f) has a finite frequency band, and the value integrated in the frequency direction is a constant. Therefore, when the conditions 2 and 3 are satisfied in the broadband probe light and the broadband pump light, the integrated value in the frequency direction of the induced Brillouin gain coefficient can always be acquired. That is, the above formula (6) is

Is only a function. Therefore, by performing the following calculation, it is possible to obtain the loss distribution from a point z _0.

Therefore, by analyzing the characteristics of the stimulated Brillouin scattered light, it is possible to measure the line loss of optical fiber to be measured relative to the certain point z _0.

となる。また、プローブ光パルスが被測定光ファイバに入射されてから光スプリッタにより分岐され、分岐光ファイバの終端の反射型光フィルタ（＃ａ）で反射されて、被測定光ファイバの入射端からの距離ｌ_ｘ１に到達する時間ｔは、

である。 (D) Measurement of Brillouin scattered light distribution with respect to the distance of the branched optical fiber Length from the incident end of the measured optical fiber to the end of the branched optical fiber (#a) (1 ≦ a ≦ N, here N = 8) _Let L be _La . The probe light pulse is reflected by a reflection type optical filter (#a) installed at the end of the branch optical fiber (#a). Here, the distance from the end of the branch optical fiber l, the refractive index n of the optical fiber to be measured, when the speed of light in a vacuum is c, the reflected probe light pulses t _1/2 seconds after the l = c / Since it proceeds by n × t _1/2 , if the distance from the incident end of the measured optical fiber is l _x1 , the distance l _x1 is

It becomes. Further, after the probe light pulse is incident on the optical fiber to be measured, it is branched by the optical splitter, reflected by the reflection type optical filter (#a) at the end of the branched optical fiber, and the distance from the incident end of the optical fiber to be measured. The time t to reach l _x1 is

It is.

The time when the pump light is incident on the optical fiber to be measured is t ₁ second after the probe light is incident. Assuming that the distance from the incident end of the optical fiber to be measured that the pump light reaches after t seconds is l _x2 , the distance l _x2 is expressed by Expression (10).

From the expressions (8) and (10), the probe light pulse and the pump light pulse interact at the position of c · t ₁ / 2n. Also, time for interaction is t _1/2 seconds after the time that is reflected by the reflection type optical filter at the end of the branch optical fiber (#a). That is, the position where the probe light and the pump light interact can be controlled by changing the time difference t _{1 at} which the probe light pulse and the pump light pulse are incident on the optical fiber to be measured. For this reason, the characteristic distribution of stimulated Brillouin scattering with respect to the distance can be obtained.

(E) Measurement of the time when the probe light pulse reflected by the branched optical fiber (#a) reaches the light receiving means 26 The time when the probe light pulse reaches the light receiving means 26 is defined as _tda . The probe light pulse is reflected by the reflection type optical filter (#a) at the end of the branch optical fiber and returns to the light receiving means 26. Therefore, the arrival time is expressed by the equation (11).

よって、光受信手段２６に戻る時間差は、式（１３）で表される。

Here, the time t _db when the probe light pulse returned from the other branch optical fiber (#b) (integer of 1 ≦ b ≦ N, here N = 8) reaches the light receiving means 26 is expressed by the equation (12). ).

Therefore, the time difference for returning to the optical receiving means 26 is expressed by equation (13).

であること。 When L _a ≠ L _b, the time for reaching the light receiving means 26 is different. When the fiber to be measured includes a branched optical fiber, the following conditions must also be satisfied.
(Condition 4)
If the pulse width of the probe light pulse is τ,

Be.

  When the condition 4 is satisfied, the probe light pulses returned from the branch optical fibers (# 1) to (# 8) do not overlap with the optical splitter. Therefore, by measuring the arrival time of the optical receiving means 26, it is possible to temporally determine which of the branched optical fibers of the branched optical fibers (# 1) to (# 8) is the probe light pulse. it can.

  From the above (a) to (e), the optical line characteristic analyzing apparatus 301 can compensate for the BFS variation and can measure the loss distribution of each highly sensitive branched optical fiber. FIG. 8 shows the result of the optical line characteristic analyzer 301 measuring the measured fiber of FIG. The branched optical fibers # 1 and # 2 are bent, and the types of the optical fibers of the branched optical fibers # 2 and # 8 are changed on the way.

  As described above, the optical line characteristic analyzing apparatus of the present embodiment is a high-speed, high-sensitivity branched optical fiber even when the BFS changes due to temperature, strain, and fiber parameter differences in the distance direction of the optical fiber to be measured. Each loss distribution can be acquired.

  FIG. 8 is a flowchart showing a measurement procedure by the optical line characteristic analyzer 301.

First, the pump light pulse sweep speed γ and pulse width τ are set. (Step S1)
Then, set the incident time difference t ₁ between the probe light pulse and the pump light pulse (step S2).

  Next, the optical line characteristic analyzing apparatus 301 outputs only the probe light pulse to the optical fiber to be measured (step S3). The arithmetic processing unit 28 specifies which of the branched optical fibers is the digital signal of the probe light pulse reflected by the reflection type optical filter from the arrival time of the return light (step S4). Moreover, the arithmetic processing unit 28 acquires the reflected probe light intensity (step S5).

  Next, the optical line characteristic analyzer 301 outputs the probe light pulse and the pump light pulse to the optical fiber to be measured (step S6). The arithmetic processing unit 28 analyzes the stimulated Brillouin scattered light from the probe light pulse reflected by the reflection type optical fiber of the branch optical fiber (step S7). The arithmetic processing unit 28 outputs the analysis result to the subsequent stage (step S8).

Subsequently, the arithmetic processing unit 28 determines whether or not the pump light pulse reflected by the reflective optical fiber of the longest branched optical fiber has arrived (step S9). If the pump light pulse has arrived (Yes in step S9), the arithmetic processing unit 28 determines whether or not the input time difference is equal to 2 nL / c (step S10). When the input time difference is not equal to 2 nL / c (No in step S10), the arithmetic processing unit 28 sets the input time difference t as t = t ₁ + Δt (step S11), and repeats the analysis process from the process in step S3. Do. When the input time difference is equal to 2 nL / c in Step S10 (Yes in Step S10), the arithmetic processing unit 28 ends the series of measurement operations.

  Here, the optical frequency difference sweep step is step S1, the probe light pulse injection step and the pump light pulse injection step are step S6, the return light receiving step, the Brillouin gain acquisition step and the characteristic analysis step are step S7, and the optical path length difference detection step is This corresponds to steps S3 to S5.

To summarize the above processing, the analysis method according to the present embodiment has the following processing procedure.
Procedure 1: Set frequency sweep speed γ and pulse width τ of pump light pulse.
Procedure 2: An incident time difference t ₁ between the probe light pulse and the pump light pulse is set.
Step 3: Specify which branch optical fiber reflects the probe light pulse according to the return time of the probe light pulse.
Procedure 4: Output stimulated Brillouin gain according to the light intensity of the probe light pulse.
Procedure 5: The procedure 2 to the procedure 4 are repeated while changing the incident time difference t ₁ between the probe light pulse and the pump light pulse, and the process is completed at t = 2 nL / c.

As described above, in this embodiment, the optical line characteristic analyzing apparatus 301 has a probe light whose probe frequency has a modulation frequency bandwidth larger than the amount of change in BFS in the longitudinal direction of the optical fiber to be measured and whose frequency difference is about the same as BFS. Prepare a pump light. The optical line characteristic analyzing apparatus 301 pulses the probe light so that the pulse width of the probe light satisfies the condition using the minimum value ΔL _min of the difference between the lengths of the branched optical fibers, and pulses the pump light. . The optical line characteristic analyzer 301 makes an incident time difference between the probe light pulse and the pump light pulse and enters the optical fiber to be measured. Then, the optical line characteristic analyzer 301 enters the probe light pulse and the pump light pulse into the optical fiber to be measured while changing the difference in incident time, and generates the stimulated Brillouin backscattered light generated by the interaction of the probe light pulse and the pump light pulse. To analyze. As a result, the optical line characteristic analyzing apparatus 301 can always perform Brillouin amplification at the Brillouin gain peak at any point of any branch optical fiber, and identify which Brillouin scattering from which branch optical fiber. For this reason, the optical line characteristic analyzer 301 can obtain | require the loss distribution for every branch optical fiber with high sensitivity.

  Therefore, even when the BFS is different in the distance direction of the branched optical fiber, the peak value of the Brillouin gain can always be obtained by one measurement, so that the loss characteristic of the branched optical fiber can be measured with high sensitivity. .

  In the above embodiment, the case where the optical line characteristic analyzing apparatus 301 includes the circulator 21 has been described as an example. However, it is not limited to this. For example, the optical line characteristic analyzing apparatus 301 may include a coupler instead of the circulator 21.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

[Appendix]
The following describes the optical line characteristic analyzing apparatus according to the present invention.
(1):
An optical line characteristic analyzer for analyzing the characteristics of a measured optical line having reflection at the far end,
Probe light pulses of different wavelengths and a second light source;
A probe light pulse and a second pulse generator for pulsing the probe light pulse and the light from the second light source to generate the probe light pulse;
The time difference at which the probe light pulse and the pump light pulse are generated is controlled by controlling the timing at which the probe light pulse and the pump light pulse are generated with respect to the probe light pulse and the second pulse generator. An incident time controller;
Combining the probe light pulse and the pump light pulse, and combining the probe light pulse and the pump light pulse that are incident on the optical line of the measured optical line;
An optical receiver that receives the return light emitted from the incident end of the optical line and converts it into an electrical signal;
A converter for converting the electrical signal into a digital signal;
The frequency difference range F experienced when the reflected probe light and pump light intersect with the Brillouin frequency shift distribution width B of the measured line is set to be equal to or greater than
Set a time difference for making the probe light pulse and the pump light pulse incident on the measured optical line,
Utilizing the light intensity that the probe light pulse is reflected back from the optical line, Brillouin gain is calculated,
The time difference is changed, and the Brillouin gain is calculated for each changed time difference.
Utilizing the Brillouin gain calculated while changing the time difference, an arithmetic processing device for obtaining a loss distribution of the optical line;
An optical line characteristic analyzing apparatus comprising:

(2):
The optical frequency modulation means for modulating only the pump light linearly at a frequency sweep speed γ in order to give a frequency difference range F when the reflected probe light and the pump light intersect each other is described in (1). Optical line characteristic analyzer.

(3):
A fiber to be measured is a branched optical line formed by branching one end of a basic optical line into a plurality by an optical branching unit, and optically coupling one end of the branched optical line to each branching end portion of the optical branching unit,
The pulse width of the probe light is pulsed so as to satisfy the condition using the minimum value of the difference in length between the plurality of branched optical lines,
From the time when the probe light pulse is reflected back from the branch optical line, the probe light pulse is identified from which of the plurality of branch optical lines is reflected,
Utilizing the light intensity of the returned probe light pulse, the Brillouin gain is calculated,
The time difference is changed, and the Brillouin gain is calculated for each changed time difference.
The optical line characteristic analysis apparatus according to (1) and (2), wherein the loss distribution of each of the plurality of branched optical lines is acquired using the Brillouin gain calculated while changing the time difference.

(4):
The optical frequency difference between the pump light and the first test light corresponding to the Brillouin frequency shift range to be compensated for the fiber to be measured is placed behind the pump light pulse in time to minimize crosstalk at the loss occurrence point The optical line characteristic analyzing apparatus according to (1) and (2) above, characterized in that:

(5):
The apparatus for analyzing optical line characteristics according to (2) above, wherein the second test light is generated by intensity-modulating the injection current of the semiconductor laser.

[effect]
As described above, according to the present invention, two types of test lights having different wavelengths are prepared, and after being pulsed, an incident time difference is given to the two types of pulsed test lights to be incident on the optical line to be measured. The reflected Brillouin backscattered light generated by the opposite propagation of the reflected pulse test light (probe light) and the later incident pulse test light (pump light) is received by the optical receiver, and the optical frequency of the pump light is determined. By linearly modulating and setting the modulation bandwidth to be equal to or greater than the BFS range to be compensated, a Brillouin gain peak can always be obtained regardless of the BFS range to be compensated, enabling high-sensitivity measurement. Become.
Therefore, according to the present invention, it is possible to provide an optical line characteristic measuring apparatus and a measuring method thereof that can measure loss distribution of optical lines having different BFSs with high sensitivity.

10: light source 10-1: first test light output means 10-2: second test light output means 12: optical frequency changing means 13, 14: optical pulse converting means 15, 16: incident time control means 20: multiplexing element 21: circulator 22: optical splitter 23: branching optical line 24: light reflection filter 25: backbone optical fiber 26: optical receiving means 27: A / D converter 28: arithmetic processing unit

Claims (8)

Propagating a pump light pulse that generates Brillouin scattering in the optical line to be measured and a probe light pulse that is reflected at the far end of the optical line to be measured and interacts with the pump light pulse, and the measured light An optical line characteristic analyzer for analyzing the characteristics of an optical line,
An optical frequency difference sweeping means for linearly varying a set Brillouin frequency shift f _B which is an optical frequency difference between the probe light pulse and the pump light pulse;
Probe light pulse incident means for generating the probe light pulse and for injecting the probe light pulse into the measured optical line;
A pump light pulse incident means for generating the pump light pulse, and entering the pump light pulse into the measured optical line after a predetermined time when the probe light pulse is incident on the measured optical line;
A return light receiving means for receiving a return light pulse from the measured optical line and converting it to an electrical signal;
Brillouin gain acquisition means for acquiring a Brillouin gain at the predetermined time from the electrical signal converted by the return light receiving means;
Characteristic analysis means for obtaining a loss distribution with respect to the distance of the optical line to be measured from the Brillouin gain obtained by changing the predetermined time;
An optical line characteristic analyzing apparatus comprising: The optical frequency difference sweeping means, it linear variation range for varying the set Brillouin frequency shift f _B linearly is, the is measured frequency range of which corresponds to the Brillouin frequency shift distribution width to be measured in the measurement beam path The optical line characteristic analyzing apparatus according to claim 1.   The optical frequency difference sweeping means determines the optical frequency sweep end frequency of the linear variation range and the optical measurement based on the frequency difference between the optical frequency sweep start frequency of the linear variation range and the median value of the optical measurement target frequency range. 3. The optical line characteristic analyzing apparatus according to claim 2, wherein the measurement target frequency range is arranged in the linear variation range so that a frequency difference from a median of the target frequency range is smaller. When the measured optical line has a structure in which one basic optical line and a plurality of branched optical lines are connected by an optical splitter, an optical path for detecting the minimum difference ΔL _min among the length differences of the branched optical lines It further comprises a length difference detection means,
4. The optical line characteristic analyzing apparatus according to claim 1, wherein the probe light pulse incident unit sets a pulse width of the probe light pulse to be narrower than 2nΔL _min / c. 5.   5. The optical line characteristic analyzing apparatus according to claim 1, wherein the optical frequency difference sweeping means varies the optical frequency of the pump light pulse by intensity-modulating an injection current of a semiconductor laser. . Propagating a pump light pulse that generates Brillouin scattering in the optical line to be measured and a probe light pulse that is reflected at the far end of the optical line to be measured and interacts with the pump light pulse, and the measured light An optical line characteristic analysis method for analyzing the characteristics of an optical line,
An optical frequency difference sweeping step that linearly varies a set Brillouin frequency shift f _B that is an optical frequency difference between the probe light pulse and the pump light pulse;
Generating the probe light pulse, and injecting the probe light pulse into the measured optical line;
After the probe light pulse incident step, the pump light pulse is generated, and the pump light pulse is incident on the measured optical line after a predetermined time when the probe light pulse is incident; and
After the pump light pulse incident step, a return light receiving step for receiving a return light pulse from the measured optical line and converting it to 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 line to be measured from the Brillouin gain acquired by changing the predetermined time;
The optical line characteristic analysis method characterized by performing. In the optical frequency difference sweep step,
Linear variation range for varying the set Brillouin frequency shift f _B linearly is the is the measured frequency range of interest than that corresponding to the Brillouin frequency shift distribution width to be measured in the measurement beam path,
From the frequency difference between the optical frequency sweep start frequency of the linear variation range and the median value of the optical measurement target frequency range, the optical frequency sweep end frequency of the linear variation range and the median value of the optical measurement target frequency range 6. The optical line characteristic analysis method according to claim 5, wherein the measurement target frequency range is arranged in the linear variation range so that a frequency difference becomes smaller. When the measured optical line has a structure in which one basic optical line and a plurality of branched optical lines are connected by an optical splitter,
Before the probe light pulse incident step, an optical path length difference detecting step of detecting a minimum difference ΔL _min among the length differences of the branched optical lines is performed.
8. The optical line characteristic analysis method according to claim 6, wherein a pulse width of the probe light pulse is set to be narrower than 2nΔL _min / c in the probe light pulse incident step.

Images