JP2008157859A - Device for measuring optical fiber characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To heighten furthermore space resolution in a device for measuring optical fiber characteristics wherein pulsed light enters from one end of an optical fiber to be measured, and continuous light enters from the other end of the optical fiber to be measured. <P>SOLUTION: The device includes the first light source devices 1, 2, 9 for generating a pulse train 2a wherein a time interval between the pulse width center of the first pulsed light and the pulse width center of the second pulsed light is a time interval below a lifetime of an acoustic wave in the optical fiber to be measured, from coherent light having the first frequency, and allowing the pulse train 2a to enter from one end 61 of the optical fiber 6 to be measured as pulsed light L1; and a variable means 9 for changing a difference between a frequency of the pulsed light L1 and a frequency of the continuous light, including a Brillouin frequency shift quantity of the optical fiber 6 to be measured. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  In the present invention, pulsed light is incident from one end of the optical fiber to be measured, continuous light is incident from the other end of the optical fiber to be measured, and continuous light emitted from one end of the optical fiber to be measured is detected. The present invention relates to an optical fiber characteristic measuring apparatus for measuring characteristics of a measuring optical fiber.

  There is a method of measuring strain and temperature distribution in an environment in which an optical fiber is installed by measuring the center frequency of Brillouin scattered light generated by incidence of pulsed light into the optical fiber. In this measuring method, since the installed optical fiber itself is used as a medium for detecting strain or temperature, strain and temperature distribution can be measured with a simple configuration as compared with a method in which a large number of point sensors are arranged.

Among such measurement methods, there are a so-called BOTDR (Brillouin Optical Time Domain Reflectometry) method and BOTDA (Brillouin Optical Time Domain Analysis) method.
The measurement method of the BOTDR method is a method of measuring the frequency shift amount of natural Brillouin scattered light (back Brillouin scattered light) reflected by an acoustic wave whose speed changes depending on strain and temperature, and from one end of the optical fiber. This is a method for detecting backward Brillouin scattered light emitted from the same end of an optical fiber by entering pulsed light (see Patent Documents 1 and 2).
On the other hand, the measurement method of the BOTDA method enters a light pulse from one end of an optical fiber, enters continuous light from the other end of the optical fiber, and changes the continuous light component due to the stimulated Brillouin scattering phenomenon caused by the interaction of both lights. This is a measurement method (see Patent Document 3).
Japanese Patent No. 2575794 Japanese Patent No. 3481494 Japanese Patent No. 2589345

Incidentally, in the BOTDR measurement method and the BOTDA measurement method, it is known that the spatial resolution is improved by reducing the pulse width of the pulsed light incident on the optical fiber. In this case, since the center frequency of the Brillouin scattered light cannot be measured with high accuracy, the spatial resolution is about 2 to 3 m.
In recent years, measurement with higher accuracy is required, and high spatial resolution of about 1 to 10 cm is desired. However, since the BOTDR measurement method detects natural Brillouin scattered light, its signal level is weak. For this reason, even if the above limitation can be overcome, it is difficult to achieve a high spatial resolution of about 1 to 10 cm.

  The present invention has been made in view of the above-described problems. Pulse light is incident from one end of an optical fiber to be measured, continuous light is incident from the other end of the optical fiber to be measured, and one end of the optical fiber to be measured. An object of the present invention is to achieve further higher spatial resolution in an optical fiber characteristic measuring apparatus that measures characteristics of an optical fiber to be measured by detecting continuous light emitted from the optical fiber.

  In order to achieve the above object, according to the present invention, the time interval between the pulse width center of the first pulse light and the pulse width center of the second pulse light is determined from the coherent light having the first frequency in the optical fiber to be measured. A pulse train having a time interval equal to or shorter than the lifetime of the acoustic wave is generated, and the first light source device incident from one end of the optical fiber to be measured is used as the pulse light, and the coherent light having the second frequency is received as the continuous light. A second light source device that is incident from the other end of the measurement optical fiber; a variable unit that varies a difference between the first frequency and the second frequency including a Brillouin frequency shift amount of the optical fiber to be measured; A light detecting means for detecting light emitted from one end of the optical fiber to be measured; and a signal processing means for measuring the characteristics of the optical fiber to be measured based on a detection result of the light detecting means. And features.

  According to the present invention having such a feature, the time interval between the center of the pulse width of the first pulse light and the center of the pulse width of the second pulse light is set to a time interval equal to or shorter than the lifetime of the acoustic wave in the optical fiber to be measured. The pulse train is incident as pulsed light from one end of the optical fiber to be measured, continuous light is incident from the other end of the optical fiber to be measured, and the difference between the frequency of the pulsed light and the frequency of the continuous light is the Brillouin frequency of the optical fiber to be measured. It is variable including the shift amount.

  In the present invention, the pulse width of the first pulse light is narrower than the time interval between the pulse width center of the first pulse light and the pulse width center of the second pulse light. A configuration in which the width is narrower than half the time interval between the pulse width center of the first pulse light and the pulse width center of the second pulse light can be employed.

  Further, in the present invention, it is possible to employ a configuration including polarization plane changing means capable of changing the polarization plane of the pulsed light or the continuous light.

  Further, in the present invention, it is possible to employ a configuration including unnecessary component removing means for removing unnecessary components contained in the pulse light.

  Further, in the present invention, a configuration is adopted in which an optical frequency filter that passes the continuous light component and blocks the pulsed light component out of the components contained in the light emitted from one end of the measured optical fiber is adopted. can do.

  In the present invention, it is possible to adopt a configuration in which the lifetime of the acoustic wave is a time from the peak power to 5% or less of the peak power.

According to the present invention, a pulse train in which the time interval between the pulse width center of the first pulse light and the pulse width center of the second pulse light is equal to or shorter than the life of the acoustic wave in the optical fiber to be measured is used as the pulse light. The incident light is incident from one end of the measured optical fiber, the continuous light is incident from the other end of the measured optical fiber, and the difference between the frequency of the pulsed light and the continuous light is variable including the Brillouin frequency shift amount of the measured optical fiber. Is done.
And the intensity | strength of the Brillouin scattered light of the 2nd pulse light which pulse light contains changes greatly according to the difference of the frequency of pulse light and the frequency of continuous light. For this reason, since the Brillouin spectrum obtained in the signal processing means is narrowed and becomes steep, the detection of the Brillouin frequency shift becomes extremely easy, and the improvement of the spatial resolution can be achieved effectively.
Therefore, according to the present invention, pulsed light is incident from one end of the measured optical fiber, continuous light is incident from the other end of the measured optical fiber, and light emitted from one end of the measured optical fiber is detected. As a result, in the optical fiber characteristic measuring apparatus for measuring the characteristic of the optical fiber to be measured, it is possible to achieve further higher spatial resolution.

  Hereinafter, an embodiment of an optical fiber characteristic measuring apparatus according to the present invention will be described with reference to the drawings. In the following drawings, the scale of each member is appropriately changed in order to make each member a recognizable size.

(First embodiment)
FIG. 1 is a block diagram showing a functional configuration of the optical fiber characteristic measuring device S1 of the first embodiment. As shown in this figure, the optical fiber characteristic measuring device S1 includes a first light source 1, an optical pulse generation circuit 2, an optical amplifier 3, an optical directional coupler 4, a first optical connector 5, an optical fiber 6 to be measured, Two light sources 7, a second optical connector 8, an optical frequency control device 9 (variable means), a photodetector 10 (light detecting means), and a signal processing unit 11 (signal processing means) are provided.

  The first light source 1 emits a narrow line width coherent light 1a, and for example, a 1.55 μm band MQW-DFB (multiple quantum well-distributed feedback type) semiconductor laser or the like can be used. In the present embodiment, the frequency of the coherent light 1a emitted from the first light source 1 is represented as a frequency fp (first frequency).

  The optical pulse generation circuit 2 is a high-speed optical switch such as an acousto-optic optical modulator or a field effect optical modulator, and is a pulse that can achieve the required spatial resolution from the coherent light 1a by turning on / off the switch. Pulse light having a width of about 100 ps to several n seconds is generated and emitted toward the optical fiber 6 to be measured as pulsed light L1. In the optical fiber characteristic measuring apparatus S1 of the present embodiment, the optical pulse generation circuit 2 generates a pulse train 2a in which two pulse lights having a pulse width of 100 ps to several n seconds are continuous instead of a single pulse light. Generated. That is, the pulse train 2a is emitted from the optical pulse generation circuit 2 toward the optical fiber 6 to be measured as the pulsed light L1. At this time, the pulse widths of the two consecutive pulse lights need not be the same.

Of the two pulse lights included in the pulse train 2a, the time interval between the pulse width center of the first pulse light 2a1 generated first and the pulse width center of the second pulse light 2a2 generated later is the light to be measured. The time interval is equal to or shorter than the lifetime of the acoustic wave in the fiber 6. The term “acoustic wave lifetime” as used herein refers to the lifetime of an acoustic wave generated in the optical fiber 6 to be measured in a broad sense, and includes the time from when a predetermined acoustic wave is generated until it disappears. However, the time interval is set for the purpose of allowing the second pulsed light 2a2 to reach before the acoustic wave induced by the first pulsed light 2a1 disappears. For this reason, considering that the second pulsed light 2a2 can reach the time interval before the acoustic wave induced by the first pulsed light 2a1 disappears, the energy of the acoustic wave changes from the peak power to the peak power. It is preferable that the time is 5% or less. For example, when the energy of the acoustic wave is attenuated based on the following formula (1), the time until the peak power reaches 5% or less is the time until (t> 3 ＞ a). be able to. In Equation (1), Τa is the acoustic wave decay time.
exp [-t / Τa] (1)
The pulse width center here means the center in the pulse width direction.

  Further, the generation cycle of the pulse train 2a depends on the length (that is, the distance range) of the optical fiber 6 to be measured. For example, if the distance range is 10 km, the generation cycle is about 200 μsec, In the distance range, the generation cycle is 20 μs.

  The optical amplifier 3 is an optical fiber amplifier using an Er (erbium) doped fiber, and amplifies the pulsed light so that the pulsed light L1 has a desired optical pulse power. In the case where the pulsed light L1 emitted from the optical pulse generation circuit 2 achieves a desired optical pulse power, a configuration in which the optical amplifier 3 is not provided is also possible.

  The optical directional coupler 4 is an optical circulator or the like. The optical directional coupler 4 emits pulsed light L1 incident on the incident port 41 from the emission / incident port 42 and emits emission light L3 from the measured optical fiber 6 incident on the emission / incident port 42. The light is emitted from the port 43.

  The optical fiber 6 to be measured is an optical fiber having a predetermined Brillouin frequency shift amount fB, one end 61 is connected to the optical directional coupler 4 via the first optical connector 5, and the other end 62 is a second one. The second light source 7 is connected via the optical connector 8.

  The second light source 7 emits narrow line-width coherent light and emits it as continuous light L2. For example, as in the first light source 1, a 1.55 μm band MQW-DFB (multiple quantum well − (Distributed feedback type) semiconductor laser or the like can be used. In the present embodiment, the frequency of the coherent light emitted from the second light source 7, that is, the frequency of the continuous light L2 is represented as a frequency fs (second frequency).

The optical frequency control device 9 can change the frequency of the coherent light 1a emitted from the first light source 1, that is, the frequency of the pulsed light L1 by controlling the first light source 1, and controls the second light source 7. Thus, the frequency of the coherent light emitted from the second light source 7, that is, the frequency of the continuous light L2 can be varied. Note that either the first light source 1 or the second light source 7 may be variable in frequency.
Then, the optical frequency control device 9 controls the first light source 1 and / or the second light source 7 so that the difference between the frequency of the pulsed light L1 and the frequency of the continuous light L2 is the Brillouin of the measured optical fiber 6. Variable so as to include the frequency shift amount fB.

As described above, in the optical fiber characteristic measuring device S1 of the present embodiment, the first light source 1, the optical pulse generation circuit 2, and the optical frequency control device 9 use the pulse of the first pulsed light 2a1 from the coherent light 1a having the frequency fp. A pulse train 2a is generated in which the time interval between the width center and the pulse width center of the second pulse light 2a2 is equal to or shorter than the lifetime of the acoustic wave in the measured optical fiber 6, and the pulse train 2a is received as the pulse light L1. The light enters from one end 61 of the measurement optical fiber 6. That is, in the optical fiber characteristic measuring device S1 of the present embodiment, the first light source of the present invention is constituted by the first light source 1, the optical pulse generation circuit 2, and the optical frequency control device 9.
Further, in the optical fiber characteristic measuring device S1 of the present embodiment, the second light source 7 and the optical frequency control device 9 cause the coherent light having the frequency fs to be incident from the other end 62 of the optical fiber 6 to be measured as continuous light L2. . That is, in the optical fiber characteristic measuring device S1 of the present embodiment, the second light source of the present invention is configured by the second light source 7 and the optical frequency control device 9.

The photodetector 10 detects the emitted light L3 emitted from the emission port 43 of the optical directional coupler 4, converts it into an electric signal L4, and outputs it.
The signal processing unit 11 measures the characteristics of the optical fiber 6 to be measured based on the detection result of the photodetector 10, that is, the electric signal L4.

  Next, the operation of the thus configured optical fiber characteristic measuring device S1 of the present embodiment will be described.

  First, when coherent light 1 a having a frequency fp is emitted from the first light source 1, the coherent light 1 a enters the optical pulse generation circuit 2. In the optical pulse generation circuit 2, a pulse train composed of the first pulsed light 2 a 1 and the second pulsed light 2 a 2 from the coherent light 1 a whose interval between the centers of the pulse widths is equal to or less than the lifetime of the acoustic wave in the optical fiber 6 to be measured. 2a is generated.

  The pulse train 2 a emitted from the optical pulse generation circuit 2 is amplified by the optical amplifier 3 to an optical pulse power that generates stimulated Brillouin scattering in the optical fiber 6 to be measured, and then incident on the optical directional coupler 4. The light enters the port 41. Thereafter, the pulse train 2 a is emitted from the emission / incident port 42 of the optical directional coupler 4, and enters the measured optical fiber 6 from one end 61 as the pulsed light L <b> 1 through the first optical connector 5.

  On the other hand, when coherent light having a frequency fs is emitted from the second light source, the coherent light is incident on the other end 62 of the optical fiber 6 to be measured as continuous light L <b> 2 via the second optical connector 8.

  In this way, the pulsed light L1 is incident from the one end 61 of the measured optical fiber 6, and the continuous light L2 having a frequency difference of the Brillouin frequency shift amount fB of the measured optical fiber 6 with respect to the pulsed light L1 is the measured optical fiber 6. When the light is incident from the other end 62, an acoustic wave is strongly induced and strong scattered light is obtained. That is, strong energy is exchanged between the pulsed light L1 and the continuous light L2.

  Here, when the phase velocities between the acoustic wave and the two light waves (pulse light L1 and continuous light L2) are mismatched, that is, the difference between the frequency fp of the pulse light L1 and the frequency fs of the continuous light L2 is the light to be measured. When deviating from the Brillouin frequency shift amount fB of the fiber 6 (fp−fs = fa ≠ fB), a phase difference is generated between the acoustic wave induced at time t = t1 and the acoustic wave induced at time t = t2. .

  Here, in the optical fiber characteristic measuring device S1 of the present embodiment, the pulsed light L1 includes the first pulsed light 2a1 whose time interval between the centers of the pulse widths is equal to or less than the lifetime of the acoustic wave in the measured optical fiber 6. Second pulsed light 2a2. For this reason, the acoustic wave induced by the first pulsed light 2a1 and the acoustic wave induced by the second pulsed light 2a2 interfere with each other, and the amplitude of the acoustic wave formed by superimposing both is equal to the frequency fp of the pulsed light L1 and the continuous light. It changes according to the difference in the frequency fs of L2.

FIGS. 3 and 4 show acoustic waves when an acoustic wave is induced by a pulse train having a time interval of 5 nsec between the pulse width center of the first pulsed light 2a1 and the pulse width center of the second pulsed light 2a2 as shown in FIG. It is a figure which shows the amplitude change of a wave. FIG. 3 is a diagram showing the change in the amplitude of the acoustic wave when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 matches the Brillouin frequency shift amount fB of the optical fiber 6 to be measured. FIG. 4 is a diagram illustrating the change in the amplitude of the acoustic wave when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift amount fB of the optical fiber 6 to be measured.
As shown in FIG. 3, when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 matches the Brillouin frequency shift amount fB of the optical fiber 6 to be measured, that is, (fp−fs = fa = When the phase velocities match at fB), the acoustic wave induced by the first pulsed light 2a1 and the acoustic wave induced by the second pulsed light 2a2 are added in the same phase. That is, the amplitude induced by the first pulsed light 2a1 is further increased by the second pulsed light 2a2.
On the other hand, as shown in FIG. 4, when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift amount fB of the measured optical fiber 6, that is, (fp−fs = fB + 100 MHz). ), The phase difference between the acoustic wave induced by the first pulsed light 2a1 and the acoustic wave induced by the second pulsed light 2a2 is π (2π · 100 MHz · 5 nsec). Counteract each other. That is, the acoustic wave induced by the first pulsed light 2a1 is canceled by the second pulsed light 2a2, the amplitude of the acoustic wave once becomes zero, and then rises again.
Even when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift amount fB of the optical fiber 6 to be measured, the phase velocity is matched (the phase difference is 2π). In the case of an integral multiple of the acoustic wave, the acoustic wave induced by the first pulsed light 2a1 and the acoustic wave induced by the second pulsed light 2a2 are added in phase.

The intensity of the Brillouin scattered light generated in the measured optical fiber 6 is proportional to the intensity of the acoustic wave, that is, the square of the amplitude. The sign is positive when the acoustic wave amplitude is increasing and negative when the acoustic wave amplitude is decreasing. Here, the sign is positive means that the energy is transferred from the second pulse light 2a2 to the continuous light L2, and the emitted light L3 emitted from the one end 61 of the optical fiber 6 to be measured is increased, and the sign is negative. Means that the energy is transferred from the continuous light L2 to the second pulsed light 2a2, and the emitted light L3 emitted from the one end 61 of the measured optical fiber 6 is reduced.
For this reason, for example, when the phase velocities match because the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 matches the Brillouin frequency shift amount fB of the optical fiber 6 to be measured (fp−fs = (FB, fp−fs = fB + 200 MHz, fp−fs = fB + 400 MHz), as shown in FIG. Is strongly included.
On the other hand, when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift amount fB of the optical fiber 6 to be measured, and the phase velocities are mismatched (fp−fs = fB, fp− fs = fB + 100 MHz, fp−fs = fB + 300 MHz, fp−fs = fB + 500 MHz), as shown in FIG. Scattered light is weakly included.
As described above, the intensity of the Brillouin scattered light of the first pulsed light 2a1 among the pulsed light included in the pulsed light L1 hardly depends on the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2. The intensity of the Brillouin scattered light of the second pulsed light 2a2 periodically increases and decreases depending on the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2.

In the optical fiber characteristic measuring device S1 of the present embodiment, the optical frequency control device 9 causes the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 to be the Brillouin frequency shift of the measured optical fiber 6. Variable to include quantity fB. Specifically, for example, the optical frequency control device 9 is configured so that the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 is in a range of −500 to 500 MHz with respect to the Brillouin frequency shift amount fB. The first light source 1 and / or the second light source 7 are controlled.
Thus, by changing the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2, the second pulse that fluctuates with the frequency difference fp−fs from the one end 61 of the optical fiber 6 to be measured. The emission light L3 including the Brillouin scattered light of the light 2a2 is emitted.

Such emitted light L3 is converted into an electric signal L4 by intensity detection in the photodetector 10, and the electric signal L4 is input to the signal processing unit 11. A Brillouin spectrum is obtained by measuring the intensity of the emitted light L3 as a function of time t at each frequency difference.
The Brillouin spectral gain fluctuates greatly periodically as the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 is varied. For this reason, the Brillouin spectrum is narrowed and steep. Then, the signal processing unit 11 measures the characteristics of the measured optical fiber 6 using the narrowed and steep Brillouin spectrum. By using such a narrowed and steep Brillouin spectrum, it becomes possible to detect the Brillouin frequency shift with high accuracy, and the spatial resolution is improved.

FIG. 7 shows an optical fiber A having a length of 1 m and a Brillouin frequency shift amount fB = 0 (relative value) and an optical fiber having a length of 20 cm and a Brillouin frequency shift amount fB = 50 MHz (relative value). B and the two-dimensional (time (distance), frequency shift amount) of the Brillouin scattered light power when the optical fiber C having a length of 1 m and a Brillouin frequency shift amount fB = 0 (relative value) optical fiber C is connected in order. ) Distribution. 8 shows the Brillouin spectrum at the center point of the optical fiber A, and FIG. 9 shows the Brillouin spectrum at the center point of the optical fiber B. FIG. 10 shows the distribution of the Brillouin frequency shift amount. 7 to 10 are based on simulation.
As shown in these drawings, in the optical fiber characteristic measuring device S1 of the present embodiment, it can be seen that the Brillouin spectrum is narrowed and steep. Therefore, it was found that the detection of the Brillouin frequency shift becomes extremely easy, and the spatial resolution can be effectively improved.
In addition, since the optical fiber characteristic measuring apparatus S1 of the present embodiment uses the stimulated Brillouin scattering phenomenon, it has a higher dynamic range than the optical fiber characteristic measuring apparatus that performs the measurement using the natural Brillouin scattering phenomenon. Measurement is possible.

According to the optical fiber characteristic measuring apparatus S1 of this embodiment, the time interval between the pulse width center of the first pulsed light 2a1 and the pulse width center of the second pulsed light 2a2 is the acoustic wave in the measured optical fiber 6. The pulse train 2a having a time interval equal to or shorter than the lifetime of the light is incident as the pulsed light L1 from one end 61 of the measured optical fiber 6, and the continuous light L2 is incident from the other end 62 of the measured optical fiber 6, and the frequency of the pulsed light L1. The difference between fp and the frequency fs of the continuous light L2 is varied including the Brillouin frequency shift amount fB of the optical fiber 6 to be measured.
The intensity of the Brillouin scattered light of the second pulsed light 2a2 included in the pulsed light L1 varies greatly according to the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2. For this reason, since the Brillouin spectrum obtained in the signal processing unit 11 is narrowed and becomes steep, the detection of the Brillouin frequency shift becomes extremely easy, and the improvement of the spatial resolution can be achieved effectively.
Therefore, according to the optical fiber characteristic measuring device S1 of the present embodiment, the pulsed light L1 is incident from one end 61 of the measured optical fiber 6, and the continuous light L2 is incident from the other end 62 of the measured optical fiber 6, In the optical fiber characteristic measuring apparatus that measures the characteristic of the optical fiber 6 to be measured by detecting the emitted light L3 emitted from the one end 61 of the optical fiber 6 to be measured, it is possible to achieve further higher spatial resolution. . It should be noted that the Brillouin frequency shift amount can be detected with higher accuracy by performing a filtering process utilizing the periodic variation of the Brillouin spectrum.

When the time interval between the pulse width center of the first pulsed light 2a1 and the pulse width center of the second pulsed light 2a2 is less than or equal to the lifetime of the acoustic wave, the acoustic wave induced by the first pulsed light 2a1 is large. Since the acoustic wave is induced by the second pulsed light 2a2 after being attenuated, sufficient interference occurs between the acoustic wave induced by the first pulsed light 2a1 and the acoustic wave induced by the second pulsed light 2a2. Absent. For this reason, even if the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 is varied, the Brillouin spectral gain does not vary periodically or even if it varies periodically, the amplitude of the periodically varying component is It is very small and the Brillouin spectrum is not narrowed and does not become steep. Therefore, even if such a Brillouin spectrum is used, the Brillouin frequency shift cannot be detected with high accuracy.
In order to generate sufficient interference between the acoustic wave induced by the first pulsed light 2a1 and the acoustic wave induced by the second pulsed light 2a2, the pulse width of the first pulsed light 2a1 is the first pulse. The pulse width of the second pulse light 2a2 is narrower than the time interval between the pulse width center of the light 2a1 and the pulse width center of the second pulse light 2a2, and the pulse width of the second pulse light 2a1 and the pulse width of the second pulse light 2a2 It is preferably narrower than half the time interval with the center. Thus, sufficient interference occurs between the acoustic wave induced by the first pulsed light 2a1 and the acoustic wave induced by the second pulsed light 2a2, and the Brillouin spectrum is narrowed, so that the Brillouin frequency shift is increased. It can be detected with accuracy.

  As can be seen from FIG. 7 to FIG. 10, according to the optical fiber characteristic measuring device S <b> 1 of the present embodiment, it is possible to measure a strain distribution that is close to the strain distribution formed in the optical fiber to be measured. Therefore, according to the optical fiber characteristic measuring device S1 of the present embodiment, the spatial resolution can be effectively improved.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. In the description of the second embodiment, the description of the same parts as in the first embodiment will be omitted or simplified.

FIG. 11 is a block diagram showing a functional configuration of the optical fiber characteristic measuring device S2 of the second embodiment.
As shown in this figure, in the optical fiber characteristic measuring device S2 of the second embodiment, a polarization control device 20 (polarization control means) is installed between the optical amplifier 3 and the optical directional coupler 4. Yes. This polarization control device 20 changes randomly by changing the polarization plane of the pulsed light L1 at high speed.

In the first embodiment, it is assumed that the relationship between the polarization states of the pulsed light L1 and the continuous light L2 is constant. However, such a condition is satisfied only by a special optical fiber such as a polarization maintaining optical fiber or a multimode optical fiber whose polarization plane is randomized. That is, when a general optical fiber is used as the optical fiber 6 to be measured, the above condition is not satisfied.
On the other hand, the stimulated Brillouin scattering phenomenon has a polarization dependency that takes a maximum value when the polarization directions of the pulsed light L1 and the continuous light L2 coincide with each other and becomes zero when they are orthogonal.

  For this reason, like the optical fiber characteristic measuring device S2 of this embodiment, the polarization dependence is canceled by changing the polarization plane of the pulsed light L1 at high speed by the polarization control device 20 at high speed. be able to.

  Note that the polarization dependence is also canceled by a method in which the polarization control device 20 changes the polarization plane of the pulsed light L1 by 90 ° for each predetermined unit time and takes the square sum of the measurement results in a plurality of unit times. Can do.

  Further, in the optical fiber characteristic measuring device S2 of the present embodiment, a configuration in which the polarization control device 20 is installed between the optical amplifier 3 and the optical directional coupler 4 is adopted. However, the present invention is not limited to this, and a similar effect can be obtained by installing a polarization control device between the second light source 7 and the optical fiber 6 to be measured and changing the polarization state of the continuous light L2. .

(Third embodiment)
Next, a third embodiment of the present invention will be described. In the description of the third embodiment, the description of the same parts as those of the first embodiment will be omitted or simplified.

FIG. 12 is a block diagram showing a functional configuration of the optical fiber characteristic measuring device S3 of the third embodiment.
As shown in this figure, in the optical fiber characteristic measuring device S3 of the third embodiment, an ASE light removing optical switch 30 (unnecessary component removing means) is installed between the optical amplifier 3 and the optical directional coupler 4. Has been. This ASE light removal optical switch 30 is for amplifying the pulse train 2a by the optical amplifier 3 to remove noise components (ASE light) riding on the pulse train 2a.

In the first embodiment, it has been assumed that the noise component (unnecessary component) generated in the optical amplifier 3 can be ignored, but in reality, there is a risk of causing S / N degradation of the pulsed light L1 and the emitted light L3. Therefore, it is preferable to remove them.
Therefore, by installing the ASE light removal optical switch 30 as in the optical fiber characteristic measuring device S3 of this embodiment, it is possible to suppress S / N deterioration of the pulsed light L1 and the emitted light L3.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described. In the description of the fourth embodiment, the description of the same parts as those of the first embodiment will be omitted or simplified.

FIG. 13 is a block diagram showing a functional configuration of the optical fiber characteristic measuring device S4 according to the fourth embodiment.
As shown in this figure, in the optical fiber characteristic measuring device S4 of the fourth embodiment, an optical frequency filter 40 is installed between the optical directional coupler 4 and the photodetector 10. The optical frequency filter 40 passes a continuous light component (frequency component of the continuous light L2) out of the components included in the emitted light L3 from the one end 61 of the optical fiber 6 to be measured, and a pulsed light component (frequency of the pulsed light L1). Component).

  The pulse light component included in the emitted light L3 becomes noise in the photodetector 10. For this reason, according to the optical fiber characteristic measuring device S4 of the present embodiment, the pulsed light component can be removed from the emitted light L3 by the optical frequency filter 40, so that it is possible to perform more accurate measurement.

(Fifth embodiment)
Next, a fifth embodiment of the present invention will be described. In the description of the fifth embodiment, the description of the same parts as those of the first embodiment will be omitted or simplified.

FIG. 14 is a block diagram showing a functional configuration of the optical fiber characteristic measuring device S5 of the fifth embodiment.
As shown in this figure, the optical fiber characteristic measuring apparatus S5 of the fifth embodiment includes a single light source 51, a branch coupler 52 that branches the coherent light emitted from the light source 51, and the frequency of the branched coherent light. Includes a modulation unit 53 that modulates the light intensity with a variable modulation signal.
The modulation unit 53 includes a microwave generator 531 that generates a modulation signal and a light intensity modulator 532 that modulates the light intensity of the coherent light based on the modulation signal. Of the sidebands generated by the light intensity modulation, One side of the coherent light is incident from one end 61 of the optical fiber 6 to be measured as continuous light L2.
The other coherent light branched by the branch coupler 52 is incident on the optical pulse generation circuit 2.

According to such an optical fiber characteristic measuring device S5 of this embodiment, coherent light emitted from a single light source is branched, one is continuous light L2, and the other is pulsed light L1. Further, the difference between the frequency of the pulsed light L1 and the frequency of the continuous light L2 is made variable by controlling the frequency of the modulation signal generated by the microwave generator 531.
That is, in the optical fiber characteristic measuring device S5 of the present embodiment, the first light source of the present invention is configured by the light source 51 and the optical pulse generation circuit 2, and the second light source of the present invention is the light source 51 and the modulation unit 53. The modulation unit 53 also has a function as a variable means.

  Also in the optical fiber characteristic measuring device S5 of this embodiment, the same effects as those of the optical fiber characteristic measuring device S1 of the first embodiment can be obtained.

  As mentioned above, although preferred embodiment of the optical fiber characteristic measuring apparatus based on this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to the said embodiment. Various shapes, combinations, and the like of the constituent members shown in the above-described embodiments are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

In the above-described embodiment, it is assumed that the optical frequency of continuous light is approximately 10 GHz lower than the optical frequency of pulsed light, energy is transferred from pulsed light to continuous light, and continuous light is amplified. The Brillouin spectrum obtained at this time is a gain spectrum.
However, when the optical frequency of continuous light is approximately 10 GHz higher than the optical frequency of pulsed light, energy transfer occurs from the continuous light to the pulsed light, and the continuous light suffers loss. The Brillouin spectrum obtained at this time is a loss spectrum, but the effect that the spectrum is narrowed and high range resolution can be achieved is the same.

It is the block diagram which showed the function structure of the optical fiber characteristic measuring apparatus in 1st Embodiment of this invention. It is a wave form diagram of pulsed light. It is a figure which shows the amplitude change of an acoustic wave in case the difference of the frequency of pulsed light and the frequency of continuous light corresponds with the Brillouin frequency shift amount of a to-be-measured optical fiber. It is a figure which shows the amplitude change of the acoustic wave when the difference of the frequency of pulsed light and the frequency of continuous light remove | deviates from the Brillouin frequency shift amount of a to-be-measured optical fiber. It is a figure which shows the intensity | strength of the Brillouin scattered light by an acoustic wave in case the difference of the frequency of pulsed light and the frequency of continuous light corresponds with the Brillouin frequency shift amount of a to-be-measured optical fiber. It is a figure which shows the intensity | strength of the Brillouin scattered light by an acoustic wave when the difference of the frequency of pulsed light and the frequency of continuous light remove | deviates from the Brillouin frequency shift amount of a to-be-measured optical fiber. The optical fiber A to be measured of the optical fiber characteristic measuring apparatus according to the first embodiment of the present invention includes an optical fiber A having a length of 1 m and a Brillouin frequency shift amount fB = 0 (relative value), and a Brillouin frequency shift amount of 20 cm in length. Two Brillouin scattered light powers when an optical fiber B of fB = 50 MHz (relative value) and an optical fiber C having a length of 1 m and a Brillouin frequency shift amount fB = 0 (relative value) are sequentially connected are used. It shows a dimensional (time (distance), frequency shift amount) distribution. 3 is a diagram showing a Brillouin spectrum at the center point of the optical fiber A. FIG. 3 is a diagram showing a Brillouin spectrum at the center point of the optical fiber B. FIG. It is a figure which shows distribution of Brillouin frequency shift amount (fB). It is the block diagram which showed the function structure of the optical fiber characteristic measuring apparatus in 2nd Embodiment of this invention. It is the block diagram which showed the function structure of the optical fiber characteristic measuring apparatus in 3rd Embodiment of this invention. It is the block diagram which showed the function structure of the optical fiber characteristic measuring apparatus in 4th Embodiment of this invention. It is the block diagram which showed the function structure of the optical fiber characteristic measuring apparatus in 5th Embodiment of this invention.

Explanation of symbols

  S1 to S5: Optical fiber characteristic measuring device, 1 ... First light source, 2 ... Optical pulse generation circuit, 6 ... Optical fiber to be measured, 61 ... One end, 62 ... Other end, 7 ... Second Light source, 9: Optical frequency control device (variable means), 10: Photo detector (light detection means), 11: Signal processing unit (signal processing means), 20: Polarization control device (polarization changing means) ), 30... ASE light removal optical switch (unnecessary component removal means), 40... Optical frequency filter, L1... Pulsed light, L2... Continuous light, L3.

Claims (6)

From the coherent light having the first frequency, a pulse train in which the time interval between the pulse width center of the first pulse light and the pulse width center of the second pulse light is set to be equal to or less than the lifetime of the acoustic wave in the optical fiber to be measured. A first light source device that is incident from one end of the optical fiber to be measured as the pulsed light,
A second light source device that injects coherent light having a second frequency as continuous light from the other end of the optical fiber to be measured;
Variable means for varying the difference between the first frequency and the second frequency including the Brillouin frequency shift amount of the optical fiber to be measured;
Light detecting means for detecting light emitted from one end of the optical fiber to be measured;
An optical fiber characteristic measuring apparatus comprising: signal processing means for measuring characteristics of the optical fiber to be measured based on a detection result of the light detecting means. The pulse width of the first pulse light is narrower than the time interval between the pulse width center of the first pulse light and the pulse width center of the second pulse light, and the pulse width of the second pulse light is the first pulse light. 2. The optical fiber characteristic measuring device according to claim 1, wherein the optical fiber characteristic measuring device is narrower than half of the time interval between the pulse width center of the second pulse light and the pulse width center of the second pulse light. 3. The optical fiber characteristic measuring apparatus according to claim 1, further comprising a polarization plane changing unit capable of changing a polarization plane of the pulsed light or the continuous light. The optical fiber characteristic measuring apparatus according to claim 1, further comprising an unnecessary component removing unit that removes an unnecessary component contained in the pulsed light. The optical frequency filter which passes the said continuous light component among the components contained in the light inject | emitted from the end of the said optical fiber to be measured, and blocks | prevents the said pulsed light component is provided. An optical fiber characteristic measuring device according to claim 1. The optical fiber characteristic measurement according to any one of claims 1 to 5, wherein the acoustic wave lifetime is a time from the peak power to 5% or less of the peak power. apparatus.

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