JP2018132510A - Light path characteristic analysis device and light path characteristic analysis method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a variation range of characteristics of a measurable light path from becoming narrower even when a light source which varies in light power in generating linear frequency sweep light is used to analyze characteristics of the light path utilizing Brillouin scattering, according to the present invention.SOLUTION: The present disclosure relates to a light path characteristic analysis device comprising: a frequency control section 11 which sweeps a frequency of only probe light pulses or both the probe light pulses and pump light pulses; an arithmetic processing section 23 which analyzes characteristics of an optical fiber 31 to be measured using Brillouin gain intensity obtained with probe light pulses and pump light pulses made incident on the optical fiber 31 to be measured; and an intensity adjustment section 19 which makes adjustments so that a difference between maximum and minimum light intensities of the frequency-swept probe light pulses is smaller than a Brillouin gain intensity obtained with the probe light pulses and pump light pulses.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to an optical line characteristic analyzing apparatus and an optical line characteristic analyzing method for analyzing characteristics of an optical line.

  Optical sensing is a technology that monitors distortion, vibration, and temperature changes around structures by analyzing the characteristics of optical lines. BOTDA (Brillouin Optical Time Domain Analysis) described in Non-Patent Document 1, and BOCDA (Brillouin Optical Correlation-Domain Analysis) described in Non-Patent Document 2 are known.

Xiaoyi Bao, Liang Chen, “High performance BOTDA for long range sensing,” Proc. of SPIE Vol. 7982, 798206, (2011). K. Hotate and T.W. Hasegawa, IEICE Trans. Electron. E83-C, 405, (2000). H. Ohno, H .; Naruse, M.M. Kihara, and A.A. Shimada, “Industrial Applications of the BOTDR Optical Fiber Strain Sensor,” Optical Fiber Technology 7, 45-64, (2001). C. Kito, H .; Takahashi, K .; Toge, T.A. Manabe, “High-speed Dynamic Strain Measurement Based on Frequency-swept Pulsed BOTDA,” ECOC2016, Th. 2. P2. SC1.3, (2016).

  When performing optical sensing using Brillouin scattering described in Non-Patent Document 1 and Non-Patent Document 2, in order to detect the amount of change in the Brillouin Frequency Shift (BFS) of the sensing optical fiber, pump light In order to calculate the Brillouin gain spectrum while sequentially changing the relative frequency difference between the probe light and the probe light, it is necessary to perform measurement a plurality of times, and the measurement time is increased accordingly.

  In the vibration measurement, the measurable vibration frequency band is limited to a low frequency for the above reason. BOTDR (Brillouin Optical Time Domain Reflectometry) disclosed in Non-Patent Document 3 uses a spectrum of natural Brillouin scattering generated by incident pump light without changing the relative frequency difference between the pump light and the probe light. Although BFS can be detected, not only does it take time to analyze the frequency on the electrical stage or software, but also BOTDR takes a long time to average because the measurement sensitivity is significantly degraded in principle with respect to BOTDA and BOCDA. In vibration measurement, the measurable vibration frequency band is limited to the low frequency band for the above reason.

  Non-Patent Document 4 discloses a means capable of measuring BFS by sending test light one time without requiring averaging for the above problems. According to Non-Patent Document 4, a probe light pulse is probed, or a test light frequency of both the pump light pulse and the probe light pulse is linearly swept to probe an upwardly convex Brillouin gain when the pump light and the probe light collide. Can be obtained on light.

  When the BFS changes, the peak light reception time of an upwardly convex Brillouin gain changes. The change amount of the peak light receiving time can be converted into the BFS change amount by using the sweep speed of the probe light pulse or the relative sweep speed of the pump light and the probe light. At this time, since the sweep speed of the frequency requires an extremely high sweep speed of about petahertz per second, as a means for realizing it, the frequency was changed linearly by linearly modulating the injection current of the DFB laser that generates the test light. A linear frequency sweep light is generated.

  However, generally, the DFB laser has an output characteristic in which the optical power changes linearly with respect to the injection current. Therefore, the probe light whose frequency is linearly swept by the above means has a temporal intensity gradient. When the light receiving peak time of the Brillouin gain convex upward is changed from the intensity gradient of the baseline, there is a problem that the range of change in which the peak can be recognized becomes narrow. In particular, when attempting long-range measurement, the upwardly convex Brillouin gain is observed to a slight extent on the baseline of the probe light pulse, so that even a slight change in BFS due to the intensity gradient of the baseline can be observed. Disappear.

  The present disclosure has been made for the above-described circumstances, and its purpose is to use a light source whose optical power changes when generating linear frequency swept light in the analysis of the characteristics of an optical line using Brillouin scattering. Another object of the present invention is to provide an optical line characteristic analyzing apparatus that does not narrow a change range of measurable optical line characteristics.

In order to achieve the above object, the optical line characteristic analyzing apparatus of the present disclosure is
A test light pulse generator for generating a first test light pulse and a second test light pulse having different wavelengths;
A frequency control unit that temporally changes only the first test light pulse generated by the test light pulse generation unit or both the first test light pulse and the second test light pulse generated by the test light pulse generation unit. When,
A Brillouin gain intensity in the measured optical line obtained by the first test light pulse and the second test light pulse incident on the measured optical line is changed between a first state and a second state where the state of the measured optical line is different. And an arithmetic processing unit that analyzes the characteristics of the optical line to be measured using the Brillouin gain intensity in the first state and the second state,
The difference between the maximum and minimum light intensity of the first test light pulse incident on the measured optical line is smaller than the Brillouin gain intensity in both the first state and the second state. An intensity adjusting unit for adjusting the light intensity of the first test light pulse;
Is provided.

  The intensity adjusting unit may be an optical amplifier or a light intensity modulator that makes the light intensity of the first test light pulse constant. Here, the optical amplifier is a saturation gain medium, and the light intensity of the first test light pulse may be made constant by using a gain at the time of saturation.

  In the optical line characteristic analyzing apparatus of the present disclosure, the temporal change in frequency in the frequency control unit may be linear with respect to time. Here, the frequency control unit changes only the first test light pulse generated by the test light pulse generation unit or changes the frequencies of the first test light pulse and the second test light pulse generated by the test light pulse generation unit. The frequency sweep speed in the frequency control unit must be different between the first test light pulse and the second test light pulse.

  In the optical line characteristic analyzing apparatus of the present disclosure, a test light pulse generation unit that generates a test light pulse that changes a frequency of the first test light pulse and the second test light pulse is a semiconductor laser, and the frequency control The unit may change the frequency of the test light pulse with time by changing the injection current of the semiconductor laser.

The optical line characteristic analysis method of the present disclosure is:
The first test light pulse and the second test light pulse having different wavelengths are generated, and the frequency of only the generated first test light pulse or both of the generated first test light pulse and the second test light pulse is set. A test light pulse incident procedure for causing the first test light pulse and the second test light pulse to enter the optical path to be measured while changing in time;
The Brillouin gain intensity in the measured optical line obtained by the first test light pulse and the second test light pulse incident on the measured optical line is set to a first state and a first state in which the state of the measured optical line is different. And a calculation processing procedure for analyzing the characteristics of the optical line to be measured using the Brillouin gain intensity in the first state and the second state,
Is an optical line characteristic analyzing method executed by the optical line characteristic analyzing apparatus,
In the test light pulse incident procedure, the maximum and minimum light intensity difference of the first test light pulse incident on the optical line to be measured is the Brillouin gain intensity in both the first state and the second state. The light intensity of the first test light pulse is adjusted so as to be smaller.

  The above disclosures can be combined as much as possible.

  According to the present disclosure, it is possible to prevent the change range of the measurable characteristics of the optical line from being narrowed even when a light source whose optical power changes is used when generating linear frequency sweep light.

It is an example of composition of an optical line characteristic analysis device concerning this indication. It is an example of the structure of a 1st test light output part, and an example of the relationship between the injection current to a semiconductor laser, and output light intensity. It is an example of the time change of received light intensity. It is a 1st example of the conceptual diagram of the characteristic measurement by the linear sweep probe light which concerns on this embodiment. It is a 2nd example of the conceptual diagram of the characteristic measurement by the linear sweep probe light which concerns on this embodiment. It is an example of a BFS change when the light reception time of the Brillouin gain peak can be recognized. It is an example of a BFS change when it becomes impossible to recognize the light reception time of the Brillouin gain peak. It is a 1st flowchart of the procedure of the optical-line characteristic analysis method which concerns on this embodiment. It is a 2nd flowchart of the procedure of the optical-line characteristic analysis method which concerns on this embodiment.

  Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In addition, this indication is not limited to embodiment shown below. These embodiments are merely examples, and the present disclosure can be implemented in various modifications and improvements based on the knowledge of those skilled in the art. In the present specification and drawings, the same reference numerals denote the same components.

Hereinafter, the present embodiment will be described with reference to the drawings.
FIG. 1 is a block diagram illustrating a configuration of an optical line characteristic analyzing apparatus according to an embodiment of the present disclosure. The optical line characteristic analyzing apparatus shown in FIG. 1 includes a first test light output unit 12 and a second test light output unit 15, a frequency control unit 11, incident time control units 13 and 16, pulsing units 14 and 17, and intensity adjustment. A unit 19, a circulator 18, an optical receiver 21, an A / D converter 22, and an arithmetic processing unit 23 are provided.

  The first test light output unit 12, the pulse unit 14, the second test light output unit 15 and the pulse unit 17 function as a test light pulse generation unit. The first test light output unit 12 generates first test light as continuous light, and the second test light output unit 15 generates second test light as continuous light. The first test light output unit 12 and the second test light output unit 15 are, for example, semiconductor lasers such as DFB lasers. The pulse unit 14 pulses the first test light, and the pulse unit 17 pulses the second test light. Thereby, the first test light pulse and the second test light pulse are generated. The first test light pulse and the second test light pulse are incident on the measured optical fiber 31 that is the measured optical line.

  The 1st test light output part 12 and the 2nd test light output part 15 are implement | achieved by the structure shown in FIG. 2, for example. The first test light output unit 12 and the second test light output unit 15 output continuous light. Hereinafter, the first test light output from the first test light output unit 12 is referred to as probe light, and the second test light output from the second test light output unit 15 is referred to as pump light below. The probe light and the pump light output optical frequencies that differ by about BFS.

  In this embodiment, the 1st test light output part 12 produces | generates the linear frequency sweep light which changed the frequency linearly temporally. The probe light modulates the optical frequency linearly with respect to time by the frequency control unit 11, and the modulation width is set to a bandwidth wider than the BFS change amount corresponding to the physical quantity change to be measured. However, the present disclosure can be implemented regardless of whether the frequency of the pump light is linearly modulated or not.

  Further, the present invention can be implemented by modulating the frequencies of both the pump light and the probe light at different modulation speeds. In the following embodiment, a case is described in which only the probe optical frequency is linearly modulated. As shown in FIG. 2, the first test light output unit 12 can be considered as a simple and inexpensive means of modulating the optical frequency of the output light by modulating the current injected into the semiconductor laser. The second test light output unit 15 may have the same configuration as the first test light output unit 12. However, the frequency sweep speed of the second test light should not match that of the first test light.

  FIG. 2 shows an example of the relationship between the output light frequency of the semiconductor laser and the output light intensity. In general, the output optical frequency of a semiconductor laser shifts to a high frequency as the injection current increases. At this time, the output power characteristic of the semiconductor laser is generally such that the output light intensity increases in proportion to the increase in injection current. Therefore, the light intensity output from the semiconductor laser whose frequency is linearly modulated is linearly amplified in time.

The pulsing unit 14 pulses the probe light, and the pulsing unit 17 pulses the pump light. The means for pulsing is arbitrary. For example, the pulsing units 14 and 17 include acoustooptic switches that drive the acoustooptic elements in pulses. Note that the pulsing units 14 and 17 may include waveguide switches that are pulse-driven using the electro-optic element LiNbO ₃ . The pulsing units 14 and 17 pulse the probe light and the pump light in the time driven by the electric pulses from the incident time control units 13 and 16. Further, when the semiconductor laser of FIG. 2 is used, the pulsing units 14 and 17 may be substituted by pulsing the current injected into the semiconductor laser.

  The pulsed probe light pulse has a gradient in output light intensity due to modulation of the injection current into the semiconductor laser. The intensity adjusting unit 19 adjusts the difference between the maximum and minimum light intensity of the probe light pulse (output gradient) to be small. By inputting the probe light to the intensity adjusting unit 19, the light intensity of the probe light after passing through the intensity adjusting unit 19 becomes a constant value in time, or the gradient of the output intensity can be reduced. As the intensity adjusting unit 19, an optical amplifier, an optical intensity modulator, or the like can be used. The optical amplifier is preferably a saturated gain medium such as a rare earth doped optical fiber, a rare earth doped optical amplifier, or a semiconductor optical amplifier. Thereby, it is possible to obtain a constant light intensity at the time of saturation.

  In the example of FIG. 1, the example in which the intensity adjusting unit 19 is inserted between the pulsing unit 14 and the optical fiber 31 to be measured has been shown, but between the first test light output unit 12 and the pulsing unit 14. The strength adjusting unit 19 may be inserted.

  In the embodiment shown in FIG. 1, since the pump light does not have an output gradient, no intensity adjusting unit is inserted in the pump light. However, the present disclosure can also be implemented when the pump light has an output gradient by sweeping the frequency of the pump light. In this case, an intensity adjusting unit may be inserted into the pump light.

  The incident time control units 13 and 16 have a function capable of changing the modulation time of the driving electric pulse in the pulsing units 14 and 17. The incident time control units 13 and 16 control the timing at which the probe light pulse and the pump light pulse are generated by changing the timing at which the pulsing units 14 and 17 modulate the pulse. More specifically, the incident time control units 13 and 16 indicate that the probe light pulse and the pump light pulse are measured light so that the probe light pulse and the pump light pulse collide at a desired position in the measured optical fiber 31. The incident time difference t incident on the fiber 31 is adjusted.

  The probe light pulse is incident on one end of the optical fiber 31 to be measured. The pump light pulse passes through the circulator 18 and enters the other end of the measured optical fiber 31. The probe light pulse and the pump light pulse collide in the measured optical fiber 31 by changing the incident time of the probe light pulse and the pump light pulse to the measured optical fiber 31 using the incident time control units 13 and 16. The position to be measured, that is, the position to be measured in the measured optical fiber 31 can be changed.

  The probe light pulse and the pump light pulse interact in the measured optical fiber 31. Due to the interaction between the probe light pulse and the pump light pulse, the probe light pulse undergoes Brillouin amplification. The probe light pulse that has undergone Brillouin amplification passes through the circulator 18 and is led out to the optical receiver 21.

  The optical receiver 21 receives the probe light pulse subjected to Brillouin amplification in the optical fiber 31 to be measured as the derived light of the circulator 18. The optical receiver 21 converts the received derived light into a current signal.

  The A / D converter 22 converts the current signal output from the optical receiver 21 into a digital signal. The A / D converter 22 outputs the converted digital signal to the arithmetic processing unit 23. Any means capable of measuring the Brillouin amplification time change can be output to the arithmetic processing unit 23 as an analog signal at the subsequent stage of the optical receiving unit 21.

  The arithmetic processing unit 23 analyzes the characteristics of the measured optical fiber 31 using the Brillouin gain intensity obtained from the derived light. The arithmetic processing unit 23 performs arithmetic processing as described below on the current value input to the frequency control unit 11, and sends one set of probe light and pump light to one point of the measured optical fiber 31. The Brillouin gain intensity is measured, and the characteristic distribution with respect to the distance of the optical fiber 31 to be measured is obtained by repeating the sending of the probe light and the pump light.

  FIG. 3 shows an example of a temporal change in the received light intensity of the optical receiver 21. The temporal changes SM0 and SM1 of the received light intensity of the light receiving unit 21 when both the probe light pulse and the pump light pulse are incident on the optical fiber 31 to be measured are shown in FIG. In addition, a convex Brillouin gain is superimposed on PR1.

It is possible to measure the BFS change by converting the change of the time τ ₀ and τ ₁ taking the peak value of the Brillouin gain into the frequency change amount using the modulation frequency speed (sweep speed γ _r ) of the probe light. is there. For example, as shown in FIG. 4, when the frequency f _probe of the probe light is changed linearly, the frequency difference Δf between the frequency f _probe of the _probe light and the frequency f _{pump of the} pump light changes linearly with respect to time. . Therefore, it is possible to measure the relative change amount of the characteristics of the measured optical fiber 31 by measuring the measured change amount of the BFS. This utilizes the fact that the BFS change amount linearly increases or decreases with respect to the temperature change and strain change amount of the optical fiber 31 to be measured.

  Furthermore, the vibration frequency of the dynamic strain can be measured by measuring the trajectory of the BFS with time. This is because the BFS change amount and the strain change amount change linearly, so that the BFS time change frequency can be directly converted into a strain change frequency that dynamically changes dynamically. The peak value of the Brillouin gain appearing on the time axis of the received signal of the optical receiver 21 is obtained by performing a quadratic function fitting analysis on a section taking a half value or more with respect to the fitting analysis by the Lorentz function or the peak value, for example. It can be easily obtained.

Next, the operation of the optical line characteristic analyzing apparatus of the present embodiment configured as described above will be described.
First, the output light frequency and frequency bandwidth of the probe light and the pump light, the frequency sweep speed, the light receiving unit 21, and the A / D converter 22 need to satisfy the following conditions.

(Condition 1) The modulation frequency bandwidth of the probe light or the bandwidth of the modulation frequency difference between the probe light and the pump light is equal to or more than the BFS change amount corresponding to the physical quantity change to be measured.
(Condition 2) The linear sweep speed of the frequency does not match between the probe light and the pump light.
Conditions 1 and 2 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 positions in the measured optical fiber 31. 4 and 5 show a first example and a second example of a conceptual diagram of characteristic measurement using a linear sweep probe light. When the pump light is not frequency-modulated and only the probe light is linearly frequency-modulated, the frequency difference Δf between the pump light and the probe light is indicated by the Brillouin correlation in the longitudinal direction of the measured fiber 31 as shown by the broken line in FIG. The Brillouin gain is acquired at different pump light-probe light frequency differences Δf within the range.

At this time, the obtained Brillouin gain has a convex shape upward in time, and the light reception time of the Brillouin gain peak value received from the Brillouin correlation range can be converted into BFS using the probe light sweep speed γ _r. (Details will be described later).

  However, in this embodiment, the case where only the probe light is swept in the frequency is shown. However, in this principle, the frequency difference Δf between the pump light and the probe light changes with time with respect to the distance corresponding to the spatial resolution. What is necessary is just to sweep the frequency of both the pump light and the probe light.

  Condition 2 is a condition for the Brillouin gain intensity change to appear in the form of a Lorentz function on the time axis of the received signal. If the frequency sweep speeds of the probe light and the pump light are matched, the Brillouin gain is evenly placed on the probe light pulse, so that the peak of the Brillouin gain does not appear on the time axis, and the present disclosure cannot be implemented. For this reason, the frequency sweep speed in the frequency controller 11 differs between the probe light pulse and the pump light pulse.

An optical line characteristic analysis method using the present disclosure when the above conditions are satisfied will be described.
The first test light output unit 12 and the second test light output unit 15 output two test lights having different wavelengths (first test light and second test light). The first test light is probe light, and the second test light is pump light.

  First, the pulsing unit 14 causes the probe light pulse to enter the optical fiber 31 to be measured. Then, the pulsing unit 17 makes the pump light pulse incident on the measured optical fiber 31 t seconds after the probe light pulse is made incident.

(A) Brillouin gain by linearly swept probe light When the frequency difference Δf between the probe light and the pump light coincides with the Brillouin frequency shift f _B , when the probe light pulse and the pump light pulse interact, the probe is located at the interacted position. The light pulse is Brillouin amplified. When a linearly swept probe light is used and the frequency bandwidth of the probe light is wider than the BFS change band corresponding to the physical quantity change to be measured, the Brillouin correlation range in which the frequency difference Δf between the probe light and the pump light is approximately f _B is set. Arise.

The frequency control unit 11 sweeps the probe light frequency with time, so that the probe light intensity obtained Brillouin gain accompanies time variation of the Lorentz function shape, measures the light reception time of the peak value, and sweeps the probe light frequency Convert to BFS using velocity γ _r [Hz / s]. Hereinafter, a BFS change analysis method according to the present disclosure will be described.

First, the first measurement for specifying the time τ ₀ is performed. The peak time (reference time) of the Brillouin gain corresponding to the BFS (reference BFS) in a state where there is no change in physical quantity to be measured (reference state) is measured. That is, with respect to the time change of the Brillouin gain measured in a state where there is no change in the physical quantity, the time for taking the peak value obtained as a result of performing the quadratic function fitting on the band taking the half value from the peak value is the reference time, that is, time τ ₀ To do.

で表される。このΔｆ_ＢＦＳを測定すべき物理量変化に換算することで所望の測定を実施可能である。 Next, a second measurement for specifying the time τ ₁ is performed. In a state where the physical quantity to be measured changes, the time τ _{1 at} which the peak value of the Brillouin gain with time is obtained is measured in the same manner. At this time, the BFS change amount Δf _BFS is

It is represented by A desired measurement can be performed by converting this Δf _BFS into a change in physical quantity to be measured.

となる。すなわち、本開示においてＢＦＳ変化量は光受信部２１の受信信号の時間に対する強度変化を解析することで測定可能であり、光受信部２１での受信信号の周波数解析などの煩雑かつ時間を要する処理は演算処理部２３において必要ない。 For example, about 1 MHz / ° C. in the case of temperature and about 0.05 MHz / με in the case of strain can be used to convert the BFS change amount into a physical quantity. Note that when the pump light has a frequency sweep speed γ _p [Hz / s], the BFS variation Δf _BFS is

It becomes. That is, in the present disclosure, the BFS change amount can be measured by analyzing a change in intensity with respect to time of the received signal of the optical receiving unit 21, and complicated and time-consuming processing such as frequency analysis of the received signal in the optical receiving unit 21 Is not required in the arithmetic processing unit 23.

ここで、ηは微分量子効率、ｈνは光子エネルギー、Ｉは注入電流、Ｉ_ｔｈはレーザの閾値電流、ｅは電荷素量である。 (B) Enlargement effect of measurable BFS change width by equalizing probe light intensity Hereinafter, the effect of adjusting the light intensity of probe light according to the present disclosure will be described. The fact that the output power of the first test light output unit 12 is modulated by the injection current modulation of the semiconductor laser used in the present disclosure is generally expressed by the following equation (3).

Here, eta is the differential quantum efficiency, hv is the photon energy, I is the injection current, I _th is the threshold current of the laser, e is an elementary charge.

The above equation, it can be seen that the output light intensity in proportion to the current value I-I _th increases. For example, if when the current value I-I _th doubled, the output light intensity is doubled, the baseline of the probe light pulse has a large inclination. On the other hand, the analysis of the BFS change according to the present disclosure needs to be able to recognize the light reception peak time of the convex Brillouin gain obtained on the probe light pulse.

6 and 7 show how the peak light reception time of the convex Brillouin gain changes due to the change in BFS in the first measurement and the second measurement. 6 and 7, the broken line indicates the first measurement, and the solid line indicates the second measurement. 6 and 7, p ₀ indicates the light intensity at which the Brillouin gain intensity at the time τ ₀ obtained by the first measurement reaches a peak, and p ₁ indicates the time at the time τ ₁ obtained by the second measurement. The light intensity at which the Brillouin gain intensity reaches its peak is indicated, p _max indicates the maximum light intensity of the probe light pulse, p _min indicates the minimum light intensity of the probe light pulse, and ΔP indicates the maximum and minimum of the probe light pulse. The light intensity difference (p _max -p _min ) is shown.

In FIG. 6, since the light intensities p ₀ and p ₁ are equal to or higher than the light intensity p _max , the light intensity p ₁ at the Brillouin gain peak in the second measurement can be detected even when the BFS changes. On the other hand, in FIG. 7, the BFS greatly changes with respect to the convex Brillouin gain Δp ₀ and the convex Brillouin gain Δp ₁ , so that the maximum light intensity p _max of the probe light pulse becomes the Brillouin gain peak of the second measurement. The light receiving power becomes larger than the light intensity p ₁ . In this case, regardless of the probe light pulse width, the measurable BFS change band is limited by the intensity gradient of the probe light. Specifically, according to equation (3), when the intensity gradient is doubled, the measurable BFS change range is halved.

In order to solve this problem, the present disclosure adjusts the intensity gradient so that the light intensities p ₀ and p ₁ are _{equal to} or higher than the light intensity p _max . For example, so as to be smaller than both the convex Brillouin gain Delta] p ₁ light intensity difference ΔP of the probe light pulses in convex Brillouin gain Delta] p ₀ and the time tau ₁ at time tau _0, adjusting the light intensity of the probe light pulses . Furthermore, it is preferable to make the intensity gradient of the probe light as small as possible. Furthermore, it is preferable that the intensity adjusting unit 19 makes the light intensity of the probe light pulse whose frequency is changed by the frequency control unit 11 constant. If the baseline of the probe light pulse can be completely flattened by using the above-described gain saturation medium or intensity modulator as the intensity adjuster 19, the measurable BFS variation band becomes the probe light pulse width swept by the linear frequency. It is determined by the frequency band to be included.

  Even if the baseline of the probe light pulse cannot be completely flattened by using the above-described gain saturation medium or intensity modulator in the intensity adjusting unit 19, if the gradient can be halved, the same Brillouin gain can be obtained. It becomes possible to cope with a double BFS change band.

  As described above, the optical line characteristic analyzing apparatus according to the present disclosure frequency-sweeps only the probe light or both the probe light and the pump light, reduces the gradient of the light intensity change due to the frequency sweep, and changes the BFS with respect to the reference BFS. Measurement can be performed at a high speed by sending test light once, and the measurement band corresponding to the BFS change can be expanded.

  8 and 9 are flowcharts showing the optical line characteristic analysis method of the present embodiment. Here, a case where only the probe light pulse is swept will be described as an example. In the optical line characteristic analyzing method according to the present embodiment, the optical line characteristic analyzing apparatus sequentially executes a test light pulse incident procedure and an arithmetic processing procedure. Steps S101 to S103, S107, and S108 are executed in the test light pulse incident procedure, and steps S104 to S106 and S109 to S113 are executed in the calculation processing procedure.

First, the optical line characteristic analyzing apparatus sets a probe light pulse sweep speed γ _r and a pulse width τ (step S101). Then, the optical line characterization system sets the incident time difference t between the probe light pulse and the pump light pulse to an initial value t ₁ (step S102). Then, the optical line characteristic analyzer performs the first measurement (S103 to S105).

  The optical line characteristic analyzing apparatus inputs the probe light pulse and the pump light pulse to the measured optical fiber 31 in the reference state (state 0) (step S103).

The arithmetic processing unit 23 obtains the time change of the stimulated Brillouin scattered light from the received signal of the light receiving unit 21, analyzes the time τ ₀ that takes the maximum value p ₀ of the stimulated Brillouin scattered light, and stores it in association with the incident time difference t. (Step S104).

  Subsequently, the arithmetic processing unit 23 determines whether the incident time difference t is equal to or greater than nL / c (step S105). Here, n is the refractive index of the optical fiber 31 to be measured, L is the fiber length of the optical fiber 31 to be measured, and c is the speed of light in vacuum. When the incident time difference t is less than nL / c (No in step S105), the arithmetic processing unit 23 sets the incident time difference t to t = t + Δt (step S106) and repeats the analysis process from the process in step S103. . When the incident time difference t is greater than or equal to nL / c in step S105 (Yes in step S105), the following processing after step S107 is executed.

In step S107, the optical line characterization system, again set to an initial value t ₁ the incident time difference t between the probe light pulse and the pump light pulse. Then, the optical line characteristic analyzer performs the second measurement (S108 to S112).

The optical line characteristic analyzing apparatus inputs the probe light pulse and the pump light pulse to the measured optical fiber 31 in a state where the physical quantity to be measured has changed (state 1) (step S108). The arithmetic processing unit 23 obtains the time change of the stimulated Brillouin scattered light from the received signal of the light receiving unit 21, analyzes the time τ ₁ that takes the maximum value p ₁ of the stimulated Brillouin scattered light, and stores it in association with the incident time difference t. (Step S109). The arithmetic processing unit 23 analyzes the BFS change amount with respect to the BFS (reference BFS) at the incident time difference t in the reference state from the sweep speed γ _r (step S110). The arithmetic processing unit 23 outputs the analysis result to the subsequent stage (step S111).

  Subsequently, the arithmetic processing unit 23 determines whether or not the incident time difference t is not less than nL / c (step S112). When the incident time difference t is less than nL / c (No in step S112), the arithmetic processing unit 23 sets the incident time difference t to t = t + Δt (step S113) and repeats the analysis process from the process in step S108. .

  When the incident time difference t is nL / c or more in step S112 (Yes in step S112), the arithmetic processing unit 23 ends a series of measurement operations.

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 _r and pulse width τ of probe light pulse.
Procedure 2: The incident time difference t between the probe light pulse and the pump light pulse is set to the initial value.
Procedure 3: Record the time τ ₀ for taking the stimulated Brillouin gain peak from the time change of the Brillouin scattered light intensity of the probe light pulse in the reference state (state 0) in association with the incident time difference t.

Procedure 4: The above procedure 3 is repeated while changing the incident time difference t between the probe light pulse and the pump light pulse, and the following procedure 5 and subsequent steps are executed at t ≧ nL / c.
Procedure 5: The incident time difference t between the probe light pulse and the pump light pulse is set to the initial value.

Procedure 6: Record the time τ ₁ for taking the stimulated Brillouin gain peak from the time change of the Brillouin scattered light intensity of the probe light pulse in the state (state 1) in which the physical quantity to be measured changes, in association with the incident time difference t. The frequency difference between the first measurement and the second measurement is obtained from the time difference between the time τ ₁ and the time τ ₀ at the same incident time difference t and the sweep speed, and is converted into the BFS variation.
Procedure 7: The above procedure 6 is repeated by changing the incident time difference t between the probe light pulse and the pump light pulse, and the process is completed when t ≧ nL / c.

  As described above, in this embodiment, the optical line characteristic analyzing apparatus corresponds to the physical quantity to be measured in the measured optical fiber 31 in which the modulation frequency bandwidth of the probe light or the bandwidth of the modulation frequency difference between the probe light and the pump light. Probe light and pump light that are larger than the BFS change amount and have the same frequency difference as BFS are prepared.

The optical line characteristic analyzer pulsates probe light and pump light. The optical line characteristic analyzer uniformizes the output intensity of the probe light. The optical line characteristic analysis apparatus gives an incident time difference t to the probe light pulse and the pump light pulse and enters the measured optical fiber 31. Then, the optical line characteristic analyzer is configured so that a probe light pulse and a pump light pulse are incident on the measured optical fiber 31 once with respect to an arbitrary position on the measured optical fiber 31 determined by the incident time difference t. The time τ ₀ or τ _{1 in} which the stimulated Brillouin backscattered light generated by the interaction between the probe light pulse and the pump light pulse takes a peak is analyzed from the intensity waveform, and the time change | τ ₀ −τ ₁ | The BFS change amount is converted using the frequency sweep speed of the test light to be swept in frequency among the pulse and the pump light pulse.

  This feature is clearly different from the method described in Non-Patent Document 1 in which measurement is performed a plurality of times while changing the frequency difference between the pump light and the probe light. Further, Non-Patent Document 2 modulates the frequencies of the pump light and the probe light as in the present embodiment, but it is necessary to sequentially change the relative frequency difference between the two test lights in order to measure BFS. Obviously different.

  Furthermore, by repeatedly measuring the probe light pulse and the pump light pulse while changing the incident time difference t incident on the optical fiber 31 to be measured, the optical line characteristic analyzing apparatus can perform the reference state at any point of the optical fiber 31 to be measured. The BFS change with respect to the BFS (reference BFS) of the measured optical fiber 31 in (state 0) can be measured by sending a pair of pump light and probe light.

  In the above measurement, since the BFS can be analyzed from the temporal change of the received light intensity in the arithmetic processing unit 23, there is no need for frequency analysis. For this reason, the optical line characteristic analyzing apparatus can obtain the characteristic distribution of the measured optical fiber 31 at high speed. Furthermore, since the output gradient of the probe test light that has been swept in frequency is made uniform, the BFS change range in which the Brillouin gain peak light receiving time can be recognized with respect to the BFS change can be expanded.

  In the above embodiment, the case where the optical line characteristic analyzing apparatus includes the circulator 18 has been described as an example. However, it is not limited to this. For example, the optical line characteristic analyzing apparatus may include a coupler instead of the circulator 18. In the light receiving section, the example in which the A / D converter 22 converts the peak value of the Brillouin gain after being converted into a digital signal has been described. However, it is also possible to analyze the change in the time when the Brillouin gain takes the peak value with the analog signal. It is.

  In the above embodiment, the pump light and the probe light are incident from both ends of the measured optical fiber 31, but the probe light reflected from the far end of the measured optical fiber 31 is used for the analysis. It can be applied even if it exists.

  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. Furthermore, you may combine suitably the component covering different embodiment.

  In addition, the arithmetic processing unit of the optical line characteristic analyzing apparatus according to the present disclosure can be realized by a computer and a program, and the program can be recorded on a recording medium or provided through a network.

(Effect of the invention)
As described above, the present disclosure prepares two types of test light having different wavelengths, and after pulsing, gives an incident time difference t to the two types of pulsed test light and enters the measured optical line first. The reflected light of the incident pulse test light (first test light) and the later incident pulse test light (second test light) are propagated in opposite directions and the stimulated Brillouin backscattered light is received by the light receiving unit. The injection current of the DFB laser that generates the optical frequencies of only one test light or both the first test light and the second test light is linearly modulated. Thus, the present disclosure modulates the output frequency linearly in time, and sets the modulation bandwidth to be equal to or greater than the range of BFS change corresponding to the physical quantity change to be measured, on the time axis of the received signal. Change of the Brillouin gain peak is converted into a BFS change, and the BFS change can be analyzed into a physical quantity change to be measured.

  Here, the present disclosure relates to an optical line characteristic analyzer that enables high-speed measurement, and adjusts the intensity for test light whose output power is also linearly modulated by linearly modulating the injection current of the DFB laser. By reducing the intensity gradient of the baseline in a state where there is no Brillouin gain by passing the part, it is possible to expand the range of BFS change that can analyze the change of the Brillouin gain peak on the time axis of the received signal Is possible.

  Therefore, according to the present disclosure, the BFS change is measured by intensity analysis of the received signal without performing frequency analysis of the received signal at the light receiving unit, and the characteristic distribution (temperature, static strain and dynamic strain vibration) of the optical line is measured. It is possible to expand the change range of the characteristics of the optical line that can be measured by the optical line characteristic measuring apparatus capable of measuring the frequency and the distortion amount at high speed.

  The present disclosure can be applied to the information communication industry.

11: Frequency control unit 12: First test light output unit 13, 16: Incidence time control unit 14, 17: Pulse unit 15: Second test light output unit 18: Circulator 19: Intensity adjustment unit 21: Light reception unit 22 A / D converter 23 Arithmetic processing unit 31 Optical fiber to be measured

Claims (7)

A test light pulse generator for generating a first test light pulse and a second test light pulse having different wavelengths;
A frequency control unit that temporally changes only the first test light pulse generated by the test light pulse generation unit or both the first test light pulse and the second test light pulse generated by the test light pulse generation unit. When,
A Brillouin gain intensity in the measured optical line obtained by the first test light pulse and the second test light pulse incident on the measured optical line is changed between a first state and a second state where the state of the measured optical line is different. And an arithmetic processing unit that analyzes the characteristics of the optical line to be measured using the Brillouin gain intensity in the first state and the second state,
The difference between the maximum and minimum light intensity of the first test light pulse incident on the measured optical line is smaller than the Brillouin gain intensity in both the first state and the second state. An intensity adjusting unit for adjusting the light intensity of the first test light pulse;
An optical line characteristic analyzing apparatus comprising: The intensity adjusting unit is an optical amplifier or a light intensity modulator that makes the light intensity of the first test light pulse constant.
The optical line characteristic analyzing apparatus according to claim 1. The optical amplifier is a saturation gain medium, and the light intensity of the first test light pulse is made constant by using a gain at the time of saturation.
The optical line characteristic analysis apparatus according to claim 2. The frequency change in the frequency control unit is linear with respect to time.
The optical line characteristic analysis apparatus according to any one of claims 1 to 3. The frequency control unit changes only the first test light pulse generated by the test light pulse generation unit, or the frequency of the first test light pulse and the second test light pulse generated by the test light pulse generation unit,
The first test light pulse and the second test light pulse have different frequency sweep speeds in the frequency controller.
The optical line characteristic analyzer of Claim 4. Of the first test light pulse and the second test light pulse, the test light pulse generating unit that generates a test light pulse whose frequency is changed by the frequency control unit is a semiconductor laser,
The frequency control unit changes the frequency of the test light pulse with time by changing the injection current of the semiconductor laser.
The optical line characteristic analysis apparatus according to claim 1. The first test light pulse and the second test light pulse having different wavelengths are generated, and the frequency of only the generated first test light pulse or both of the generated first test light pulse and the second test light pulse is set. A test light pulse incident procedure for causing the first test light pulse and the second test light pulse to enter the optical path to be measured while changing in time;
The Brillouin gain intensity in the measured optical line obtained by the first test light pulse and the second test light pulse incident on the measured optical line is set to a first state and a first state in which the state of the measured optical line is different. And a calculation processing procedure for analyzing the characteristics of the optical line to be measured using the Brillouin gain intensity in the first state and the second state,
Is an optical line characteristic analyzing method executed by the optical line characteristic analyzing apparatus,
In the test light pulse incident procedure, the maximum and minimum light intensity difference of the first test light pulse incident on the optical line to be measured is the Brillouin gain intensity in both the first state and the second state. Adjusting the light intensity of the first test light pulse to be smaller than
Optical line characteristic analysis method.

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