JP2007232618A - Instrument for measuring optical fiber distortion - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an instrument of high resolution for measuring an optical fiber distortion allowing long distance measurement. <P>SOLUTION: This instrument for measuring the optical fiber distortion capable of measuring the optical fiber distortion, using Brillouin scattered light generated inside an optical fiber of an measuring object, is provided with a variable wavelength light source capable of changing a light frequency of an output light, an optical branching means for branching the output light from the variable wavelength light source into two, a light frequency shifter for shifting the light frequency in one branched light to be output, a photodetector, a photomultiplexing-branching means for photomultiplexing the other branched light and the output light from the light frequency shifter to get incident into one end of the optical fiber, and for branching the output light from the one end of the optical fiber to get incident into the photodetector, and a reflecting end provided in the other end of the optical fiber, a modulation period of the variable wavelength light source is varied, and a point where a pump light propagated opposedly is brought into the same phase with a probe light returned in the reflecting end is selected to scan a shift of the light frequency by the light frequency shifter. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical fiber strain measuring apparatus for measuring strain of an optical fiber using Brillouin scattered light generated in an optical fiber to be measured, and in particular, optical fiber strain measurement capable of measuring a long distance and having high resolution. Relates to the device.

  Part of the light propagating in the optical fiber is absorbed as the kinetic energy of the glass structural molecules. The absorbed energy is released as an acoustic phonon when it exceeds a certain value. Brillouin scattering is a phenomenon in which Stokes light that is shifted by the frequency of the acoustic phonon (Brillouin frequency shift) is generated due to the mutual interference between the emitted acoustic phonon and the transmitted light.

  Prior art documents related to an optical fiber strain measuring apparatus for measuring strain of an optical fiber using Brillouin scattered light generated in an optical fiber to be measured are as follows.

Japanese Patent Laid-Open No. 02-006725 Japanese Patent Laid-Open No. 06-230091 JP 2000-180265 A Japanese Patent Laid-Open No. 2003-014584

  FIG. 5 is a block diagram showing an example of such a conventional inductive type (Brillouin Optical Time Domain Analysis: BOTDA) optical fiber strain measuring apparatus. In FIG. 5, 1 is a light source such as a semiconductor laser light source that emits laser light, 2 and 5 are optical branching means that split light, 3 is an optical modulator that modulates the intensity of incident light, and 4 is incident light such as an optical switch. Is a pulse modulator capable of pulsing and emitting optical frequency shift, 6 is an optical fiber to be measured for distortion, and 7 is a photodetector such as a photodiode.

  Light emitted from the light source 1 is incident on the light branching means 2, one branched light is incident on the incident end of the optical modulator 3, and the other branched light is incident on the incident end of the pulse modulator 4. The output light of the optical modulator 3 is incident on one end of the optical fiber 6 as probe light, and the output light of the pulse modulator 4 is incident on the other end of the optical fiber 6 through the optical branching means 5 as pulse light.

  On the other hand, the Brillouin scattered light from the optical fiber 6 is emitted from the other end of the optical fiber 6, branched by the light branching means 5, and incident on the photodetector 7.

  Here, the operation of the conventional example shown in FIG. 5 will be described with reference to FIGS. FIG. 6 is an explanatory diagram for explaining the relationship between probe light, pulsed light, and Brillouin scattered light, and FIG. 7 is a characteristic curve diagram showing an example of light intensity detected by the photodetector 7.

  The optical frequency of the probe light incident on one end of the optical fiber 6 indicated by “TM01” in FIG. 6 is “f0”, and the optical frequency of the pump light incident from the other end of the optical fiber 6 indicated by “TM02” in FIG. Is “f0 + Δf”, which is optical frequency shifted by “Δf” in the pulse modulator 4.

  If the optical frequency shift “Δf” does not coincide with the Brillouin frequency shift “fB”, Brillouin scattering does not occur, and the optical fiber 6 propagates from the other end of the optical fiber 6 indicated by “TM02” in FIG. Probe light is emitted.

  For example, if the optical frequency shift “Δf” does not coincide with the Brillouin frequency shift “fB”, the light intensity characteristic curve indicated by “CH11” in FIG. In other words, the light intensity corresponding to the light intensity of the probe light is measured over time as indicated by “CH11” in FIG.

  On the other hand, when the optical frequency shift “Δf” coincides with the Brillouin frequency shift “fB”, the Brillouin scattered light whose optical frequency is “f0 + fB” from the other end of the optical fiber 6 indicated by “TM02” in FIG. Pulse light) is emitted together with the probe light propagating through the optical fiber 6.

  For example, when the optical frequency shift “Δf” coincides with the Brillouin frequency shift “fB”, the light intensity characteristic curve indicated by “CH12” in FIG. In other words, as indicated by “CH12” in FIG. 7, a characteristic accompanied with a pulsed light intensity change (Brillouin scattered light (pulsed light)) is measured with time at a certain time indicated by “PS11” in FIG. 7. .

  That is, if the Brillouin frequency shift “fB” of the portion where the distortion occurs in the optical fiber 6 matches the shift amount ΔΔ of the optical frequency, the Brillouin scattered light (pulse light) from the portion where the distortion occurs. ) Will come back.

  Here, the time on the horizontal axis of the characteristic curve diagram shown in FIG. 7 corresponds to the position of the optical fiber (or the distance from the other end of the optical fiber) if the propagation speed of light in the optical fiber 6 is taken into consideration. By specifying the position corresponding to a certain time indicated by “PS11” in FIG. 7, it is possible to obtain the position of the distortion occurring in the optical fiber 6.

  The Brillouin frequency shift has a property that depends on the magnitude of distortion. In other words, since the Brillouin frequency shift changes depending on the magnitude of distortion, the pulse modulator 4 scans the optical frequency shift “Δf”. As a result, Brillouin scattered light (pulsed light) sequentially returns from a portion in the optical fiber 6 where distortions having different sizes are generated.

  As a result, probe light (optical frequency “f0”) is incident from one end of the optical fiber 6, and pulsed light (optical frequency “f0 + Δf”) having a shifted optical frequency is incident from the other end of the optical fiber 6. By measuring the light emitted from the other end of the optical fiber 6 with a photodetector and scanning the optical frequency shift, it becomes possible to measure the position of various magnitudes of distortion generated in the optical fiber 6. Become.

  However, in this conventional example, if the pulse width is narrowed in order to increase the resolution, the frequency spectrum of the pulsed light is widened, which makes it difficult to detect the Brillouin frequency.

  Moreover, although the conventional correlation type (BOCDA) optical fiber strain measuring apparatus described in “Patent Document 3” achieves high resolution, the distance of the optical fiber that can be measured is limited to about a few meters. There was a problem.

On the other hand, in order to extend the measurable distance, the method described in “Patent Document 4” in which the gate is formed by pulsing is devised, but the configuration is complicated, and light is incident from both ends of the optical fiber. There was a problem that I had to do.
Therefore, the problem to be solved by the present invention is to realize an optical fiber strain measuring apparatus capable of measuring a long distance and having a high resolution.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In an optical fiber strain measuring apparatus that measures strain of an optical fiber using Brillouin scattered light generated in an optical fiber to be measured,
A variable wavelength light source capable of changing the optical frequency of the output light, an optical branching means for branching the output light of the variable wavelength light source into two, and an optical frequency shifted with respect to one of the branched lights for emission The optical frequency shifter, the photodetector, the other branched light and the output light of the optical frequency shifter are combined and incident on one end of the optical fiber, and the outgoing light from one end of the optical fiber is branched to An optical coupling means for entering the photodetector; and a reflection end provided at the other end of the optical fiber; the pump light propagating oppositely by changing the modulation period of the variable wavelength light source; and the reflection end An optical fiber strain measurement device that can measure long distances and has high resolution by selecting a point where the probe light that is turned back in phase is in phase and scanning the shift of the optical frequency by the optical frequency shifter. Realize Can. Further, since the structure is folded at the reflection end, the reference point of the position becomes easy to understand.

The invention according to claim 2
In the optical fiber strain measuring device which is the invention according to claim 1,
The variable wavelength light source is
Long distance measurement is possible by changing at a high speed over a wide frequency range above the Brillouin frequency shift.

The invention described in claim 3
In the optical fiber strain measuring device which is the invention according to claim 1,
The modulation period waveform of the variable wavelength light source is
By using a triangular wave or a sine wave, it is possible to realize an optical fiber strain measuring apparatus that can measure a long distance and has a high resolution. Further, since the structure is folded at the reflection end, the reference point of the position becomes easy to understand.

The invention according to claim 4
In the optical fiber strain measuring device which is the invention according to claim 1,
The modulation period of the variable wavelength light source is
By being 1 μsec or longer, it is possible to realize an optical fiber strain measuring apparatus that can measure a long distance and has a high resolution. Further, since the structure is folded at the reflection end, the reference point of the position becomes easy to understand.

The invention according to claim 5
In the optical fiber strain measuring device which is the invention according to claim 1,
By providing a variable band-pass filter synchronized with the modulation period of the variable wavelength light source in front of the photodetector, it becomes possible to remove light having an unnecessary optical frequency.

The invention described in claim 6
In the optical fiber strain measuring device which is the invention according to claim 1,
Brillouin scattered light generated at an appropriately selected point by sequentially changing the modulation period of the variable wavelength light source by changing the characteristics of the optical fiber at the position around the reflection end, and at the position around the reflection end It is possible to distinguish from the Brillouin scattered light.

The invention described in claim 7
In the optical fiber strain measuring device which is the invention according to claim 5,
Brillouin scattered light generated at an appropriately selected point by sequentially changing the modulation period of the variable wavelength light source by fusing an optical fiber of another characteristic to change the characteristic of the optical fiber at a position around the reflection end. And Brillouin scattered light generated at positions around the reflection end can be distinguished.

The invention described in claim 8
In the optical fiber strain measuring device which is the invention according to claim 5,
Brillouin scattered light generated at an appropriately selected point by sequentially changing the modulation period of the variable wavelength light source by changing the characteristics of the optical fiber around the reflection end by applying distortion, and the periphery of the reflection end It is possible to distinguish from the Brillouin scattered light generated at the position.

The invention according to claim 9
In the optical fiber strain measuring device which is the invention according to claim 5,
Brillouin scattered light generated at an appropriately selected point by sequentially changing the modulation period of the variable wavelength light source by selecting the reflective end material and changing the characteristics of the optical fiber at a position around the reflective end, It becomes possible to distinguish Brillouin scattered light generated at positions around the reflection end.

The present invention has the following effects.
According to the first, second, third, and fourth aspects of the present invention, the modulation period of the variable wavelength light source is sequentially changed so that the pump light propagating in opposition and the probe light folded back at the reflection end have the same phase. By appropriately selecting a point and scanning an optical frequency shift “Δf” by an optical frequency shifter, it is possible to realize an optical fiber strain measuring apparatus capable of measuring a long distance and having a high resolution. Further, since the structure is folded at the reflection end, the reference point of the position becomes easy to understand.

  According to the invention of claim 5, it is possible to remove light having an unnecessary optical frequency by providing a variable bandpass filter synchronized with the modulation period of the variable wavelength light source in the preceding stage of the photodetector. become.

  According to the inventions of claims 6, 7, 8 and 9, the points selected as appropriate by sequentially changing the modulation period of the variable wavelength light source by changing the optical fiber characteristics around the reflection end. It is possible to distinguish the Brillouin scattered light generated at 1 and the Brillouin scattered light generated at positions around the reflection end.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a configuration block diagram showing an embodiment of a correlation type (BOTDA) optical fiber strain measuring apparatus.

  In FIG. 1, 8 is a variable wavelength light source capable of changing the optical frequency (optical wavelength) of output light, 9 is an optical branching means for branching light, and 10 is an optical frequency (optical wavelength) for incident light. An optical frequency shifter to be shifted, 11 is an optical combining / branching unit for combining and branching light, 12 is an optical fiber to be measured for distortion, 13 is a reflection end provided at the other end of the optical fiber, and 14 is a photodiode or the like. It is a photodetector.

  The output light of the variable wavelength light source 8 is incident on the optical branching unit 9, one branched light is incident on the incident end of the optical frequency shifter 10, and the other branched light is incident on the first incident end of the optical coupling / branching unit 11. The The light emitted from the optical frequency shifter 10 is incident on the second incident end of the optical combining / branching unit 11, is combined with the other branched light, and is incident on one end of the optical fiber 12.

  On the other hand, the reflected light reflected by the reflecting end 13 provided at the other end of the optical fiber 12 and the return light such as the generated Brillouin scattered light are emitted from one end of the optical fiber 12 and branched by the optical coupling / branching means 11. The light enters the photodetector 14.

  Here, the operation of the embodiment shown in FIG. 1 will be described with reference to FIGS. FIG. 2 is a characteristic curve diagram showing temporal changes in the optical frequency (optical wavelength) of the output light of the variable wavelength light source 8, and FIGS. 3 and 4 are explanatory diagrams for explaining the relationship among probe light, pulsed light, and Brillouin scattered light. .

  As indicated by “CH31” in FIG. 2, the output light of the variable wavelength light source 8 is swept with the output optical frequency (optical wavelength) at a constant modulation period “T”.

  Such output light is branched into two by the optical branching means 9, and the branched output light indicated by “LG 21” in FIG. 1 is directly incident on one end of the optical fiber 12 through the optical coupling / branching means 11 and the other end. 1 is reflected by the reflection end provided on the light source and propagates as indicated by “LG22” in FIG. 1 to become probe light (frequency: F1).

  On the other hand, the branched output light indicated by “LG23” in FIG. 1 undergoes an optical frequency shift by an optical frequency shifter, and then passes through the optical coupling / branching means 11 as indicated by “LG24” in FIG. It enters as one end of pump light (frequency: F0 + Δf).

  In the optical fiber 12, the probe light turned back at the reflection end 13 and the pump light subjected to the optical frequency shift propagate in opposition to each other, and are uniquely determined by the length of the optical fiber 12 and the modulation period of the variable wavelength light source 8. Brillouin amplification is performed by a strong correlation only at a specific position determined by (2), and Brillouin scattered light is emitted from one end of the optical fiber 12.

For example, the optical path lengths of the probe light and the pump light until they enter the optical coupling / branching means 11 are the same, the light propagation speed “v” in the optical fiber 12, the length “L” of the optical fiber 12, the modulation period "T"
v = 200 [m / μsec]
L = 1500 [m]
T = 10 [μsec]
And

In this case, the modulation period “10 μsec”
200 [m / μsec] × 10 [μsec] = 2000 [m]
Since the length of the optical fiber 12 is “1500 m”, the probe light turned back at the reflection end 13 at the point “1000 m” from one end (incident end) of the optical fiber 12 has propagated “2000 m”. become.

  Therefore, at a point “500 m” from one end (incidence end) of the measurement optical fiber 12, the pump light propagating oppositely and the probe light returning at the reflection end 13 have the same phase.

  Basically, the optical frequency of the output light of the variable wavelength light source 8 is swept at every modulation period as indicated by “CH31” in FIG. The optical frequency “F0” excluding “” does not match the optical frequency “F1” of the probe light.

However, at a point “500 m” from one end (incident end) of the measurement optical fiber 12, the pump light propagating oppositely and the probe light returning at the reflection end 13 have the same phase.
F0 = F1
It becomes.

  At this time, when the optical frequency shift “Δf” in the optical frequency shifter 10 and the Brillouin frequency shift “fB” at the point where the pump light and the probe light are in phase, the Brillouin amplification is caused by a strong correlation. The Brillouin scattered light is emitted from one end of the optical fiber 12, and the light intensity is detected by the photodetector 14.

  For example, the point indicated by “SP41” in FIG. 3 is the pump light “F0 + Δf” that propagates oppositely indicated by “LG41” in FIG. 3 and the probe light that is turned back at the reflection end 13 indicated by “LG42” in FIG. If it is assumed that “F1” is a point having the same phase, when the optical frequency shift “Δf” and the Brillouin frequency shift “fB” in the optical frequency shifter 10 coincide with each other, Brillouin amplification is performed by strong correlation, and Brillouin scattering is performed. Light “F0 + fb” is emitted from one end of the optical fiber 12.

  Here, since the pump light is continuous light instead of pulsed light as in the conventional example, the light intensity of the light detected by the photodetector 14 is equal to the amount of shift of the optical frequency by the optical frequency shifter 10 “Δf”. Correspondingly, the light intensity is constant.

  In other words, the Brillouin frequency at a specific position uniquely determined by the length of the optical fiber 12 and the modulation period of the variable wavelength light source 8 by sequentially scanning the optical frequency shift “Δf” by the optical frequency shifter 10. The shift “fB” is known, and the magnitude of the distortion can be measured.

  On the other hand, when the modulation period “T” of the variable wavelength light source 8 is changed, a specific position uniquely determined by the length of the optical fiber 12 and the modulation period of the variable wavelength light source 8 is shifted.

For example, the optical path lengths of the probe light and the pump light until they enter the optical coupling / branching means 11 are the same, and the modulation period “T” is
T = 12 [μsec]
The modulation period “12 μsec”
200 [m / μsec] × 12 [μsec] = 2400 [m]
Since the length of the optical fiber 12 is “1500 m”, the probe light turned back at the reflection end 13 at the point “600 m” from one end (incident end) of the optical fiber 12 has propagated “2400 m”. become.

  For this reason, at a point “300 m” from one end (incident end) of the measurement optical fiber 12, the pump light propagating oppositely and the probe light returning at the reflection end 13 have the same phase.

  That is, by sequentially changing the modulation period “T” of the variable wavelength light source 8, a specific position uniquely determined by the length of the optical fiber 12 and the modulation period of the variable wavelength light source 8 can be selected as appropriate. .

  For example, a point where the pump light “F0 + Δf” propagating oppositely indicated by “LG51” in FIG. 4 and the probe light “F1” turned back at the reflection end 13 indicated by “LG52” in FIG. 4 will move to the point indicated by “SP51” in FIG.

  At this point, when the optical frequency shift “Δf” and the Brillouin frequency shift “fB” in the optical frequency shifter 10 coincide with each other, Brillouin amplification is performed by a strong correlation, and the Brillouin scattered light “F0 + fb” is transmitted to the optical fiber 12. The light is emitted from one end.

  As described above, by sequentially scanning the optical frequency shift “Δf” by the optical frequency shifter 10, the Brillouin frequency shift “fB” at the point indicated by “SP51” in FIG. The size can be measured.

  That is, the modulation period of the variable wavelength light source 8 is sequentially changed, and a point where the pump light propagating oppositely and the probe light turned back at the reflection end 13 are in the same phase is appropriately selected, and the optical frequency by the optical frequency shifter 10 is selected. By scanning the shift amount “Δf”, the Brillouin frequency shift “fB” at the point can be known, and the magnitude of the distortion can be measured.

  As a result, the modulation period of the variable wavelength light source 8 is sequentially changed, and a point where the pump light propagating oppositely and the probe light folded back at the reflection end 13 are in the same phase is appropriately selected, and the light by the optical frequency shifter 10 is selected. By scanning the frequency shift amount “Δf”, it is possible to realize an optical fiber strain measuring apparatus capable of measuring a long distance and having a high resolution. Further, since the structure is folded at the reflection end, the reference point of the position becomes easy to understand.

  Further, by sequentially changing the modulation period of the variable wavelength light source 8, it is possible to measure a distance of about 1500 m, for example, and it is possible to measure a long distance as compared with the conventional example. Furthermore, although the spatial resolution depends on the wavelength fluctuation width of the variable wavelength light source 8 as in the conventional example, the modulation frequency is “10 μsec” and the wavelength fluctuation width of the variable wavelength light source 8 is “˜1.6 nm (200 GHz). A high resolution of about "several cm" can be obtained.

  That is, by setting the modulation period of the variable wavelength light source 8 to “1 μsec” or more, it is possible to realize an optical fiber strain measuring apparatus that can measure a long distance and has a high resolution.

  The position around the reflection end 13 is a position where the probe light and the pump light incident on one end of the optical fiber 12 with the same phase are turned back and correspond to the same phase point. When the optical frequency shift “Δf” and the Brillouin frequency shift “fB” in the optical frequency shifter 10 coincide with each other, the Brillouin amplification is performed by a strong correlation, and the Brillouin scattered light “F0 + fb” is emitted from one end of the optical fiber 12. It will be.

  However, on the photo detector 14 side, the Brillouin scattered light generated at a position around the reflection end 13 is generated at a point selected as appropriate by sequentially changing the modulation period of the variable wavelength light source 8 as it is. It cannot be distinguished whether it is light.

  Therefore, in order to distinguish between the Brillouin scattered light generated at an appropriately selected point by sequentially changing the modulation period of the variable wavelength light source 8 and the Brillouin scattered light generated at positions around the reflective end 13, There is a need to change the optical fiber characteristics at positions around 13.

  Specifically, the optical fiber characteristics (in other words, the Brillouin frequency shift) at the position around the reflection end 13 are obtained by fusing an optical fiber with another characteristic, adding a separate strain, or appropriately selecting the reflection end material. ) Are sequentially changed to the optical fiber characteristics (in other words, Brillouin frequency shift) different from the appropriately selected point by changing the modulation period of the variable wavelength light source 8.

  As a result, the Brillouin scattered light generated at an appropriately selected point by sequentially changing the modulation period of the variable wavelength light source 8 by changing the optical fiber characteristics around the reflection end 13 and the periphery of the reflection end 13 It becomes possible to distinguish the Brillouin scattered light generated at the position.

  Further, the triangular wave as shown in FIG. 2 is illustrated as the modulation period waveform of the variable wavelength light source 8, but it is needless to say that a modulation period waveform such as a sine wave may be used as the modulation waveform.

  Further, if necessary, an optical amplifier may be used as appropriate, and a variable bandpass filter synchronized with the modulation period of the variable wavelength light source 8 is provided in the preceding stage of the photodetector 14 to remove light having an unnecessary optical frequency. It doesn't matter.

  Further, by changing the modulation period “T” of the variable wavelength light source 8, instead of appropriately selecting a point where the pump light propagating oppositely and the probe light turned back at the reflection end 13 have the same phase, the probe light is selected. The point may be appropriately selected by changing the optical path length of light.

  Further, as separate light sources that operate the probe light and the pump light with the same period and the same variable wavelength (frequency) width, the pump light propagating oppositely by changing the phase difference between the two light sources and the reflection end The point where the probe light turned back at 13 is in phase may be selected as appropriate.

  In addition, by using a MEMS (Micro Electro Mechanical Systems) type external resonator type light source as the variable wavelength light source 8, a wide frequency (wavelength) range more than the Brillouin frequency shift can be changed at high speed, and a long distance can be measured. become.

1 is a configuration block diagram showing an embodiment of a correlation type optical fiber strain measurement device. FIG. It is a characteristic curve figure which shows the time change of the optical frequency (optical wavelength) of the output light of a variable wavelength light source. It is explanatory drawing explaining the relationship between probe light, pulsed light, and Brillouin scattered light. It is explanatory drawing explaining the relationship between probe light, pulsed light, and Brillouin scattered light. It is a block diagram which shows an example of the conventional correlation type optical fiber distortion measuring apparatus. It is explanatory drawing explaining the relationship between probe light, pulsed light, and Brillouin scattered light. It is a characteristic curve figure which shows an example of the light intensity detected with a photodetector.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2,5,9 Optical branching means 3 Optical modulator 4 Pulse modulator 6,12 Optical fiber 7,14 Photo detector 8 Variable wavelength light source 10 Optical frequency shifter 11 Optical coupling / branching means 13 Reflecting end

Claims (9)

In an optical fiber strain measuring apparatus that measures strain of an optical fiber using Brillouin scattered light generated in an optical fiber to be measured,
A variable wavelength light source capable of changing the optical frequency of the output light; and
A light branching means for branching the output light of the variable wavelength light source into two;
An optical frequency shifter that emits light with a shifted optical frequency with respect to one branched light; and
A photodetector;
An optical combining / branching unit for combining the other branched light and the output light of the optical frequency shifter to be incident on one end of the optical fiber and for branching the outgoing light from one end of the optical fiber to be incident on the photodetector; ,
A reflection end provided at the other end of the optical fiber,
By changing the modulation period of the variable wavelength light source, the point where the pump light propagating in opposition and the probe light folded back at the reflection end are in phase is selected, and the optical frequency shift by the optical frequency shifter is selected. An optical fiber strain measuring device characterized by scanning. The variable wavelength light source is
2. The optical fiber strain measuring device according to claim 1, wherein the optical fiber strain measuring device is changed at high speed in a wide frequency range equal to or greater than the Brillouin frequency shift. The modulation period waveform of the variable wavelength light source is
2. The optical fiber strain measuring apparatus according to claim 1, wherein the apparatus is a triangular wave or a sine wave. The modulation period of the variable wavelength light source is
2. The optical fiber strain measuring device according to claim 1, wherein the optical fiber strain measuring device is 1 [mu] sec or longer. 2. The optical fiber strain measuring apparatus according to claim 1, wherein a variable bandpass filter synchronized with a modulation period of the variable wavelength light source is provided in a front stage of the photodetector. 2. The optical fiber strain measuring apparatus according to claim 1, wherein a characteristic of the optical fiber at a position around the reflection end is changed. 6. The optical fiber strain measuring apparatus according to claim 5, wherein an optical fiber having a different characteristic is fused to change the characteristic of the optical fiber at a position around the reflection end. 6. The optical fiber strain measuring apparatus according to claim 5, wherein distortion is applied to change the characteristics of the optical fiber at a position around the reflection end. 6. The optical fiber strain measuring apparatus according to claim 5, wherein the reflection end material is selected to change the characteristics of the optical fiber at a position around the reflection end.

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