JP2003322588A - Reflection type method and instrument for measuring brillouin spectrum distribution - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To highly accurately measure the positional distribution of a Brillouin spectrum with a distance resolution not larger than 1 m, without cross talk, or without increasing the width of the Brillouin spectrum intrinsic to a measured medium. <P>SOLUTION: A plurality of beams are obtained, being referred to as first and second beams, by branching an incoherent continuous oscillation beam having a line width larger than the width of the Brillouin spectrum of the measured medium. The first beam is launched from one end of the medium and a beam rearward Brillouin-scattered in the medium is made to flow together with the second beam. The confluent beam is converted into an electric signal by a photo-detector to measure the frequency and power of an electric beat signal caused by an interference of a rearward Brillouin-scattered beam included in the electric signal with the second beam. Relative delay time τ<SB>12</SB>between the first beam rearward Brillouin-scattered in the medium and the second beam or the other branched beam is changed to make the relative delay time τ<SB>12</SB>concerning the beam rearward Brillouin-scattered from a desired scatter position z<SB>0</SB>T in the medium substantially zero. <P>COPYRIGHT: (C)2004,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a reflection-type Brillouin spectrum distribution measuring method and apparatus for measuring the spatial distribution of a Brillouin spectrum. In particular, the present invention
It provides a measurement technique with improved spatial resolution, not only can measure the spatial distribution of physical properties of the medium to be measured, but also when the medium to be measured is an optical waveguide such as an optical fiber, the optical waveguide Distortion due to stress applied to
Since it is possible to measure a spatial distribution such as temperature in the environment where the optical waveguide is placed, that is, a distribution along the length of the optical waveguide, it is also possible to apply the present invention to a distribution sensor.

[0002]

2. Description of the Related Art When coherent monochromatic light is incident on a medium, backscattered light whose frequency is shifted by nonlinear interaction with an acoustic wave is observed. This is called Brillouin scattering. Further, the spectrum of the scattered light has a Lorentzian shape as shown in the following equation (1). S _b (ν) ∝ (Δν _b ) ² / {4 (ν−ν _b ) ² + (Δν _b ) ² } (1) where ν is a frequency difference between incident light and Brillouin scattered light, ν
_b is the maximum intensity of the spectrum of the scattered light, the frequency difference at the center frequency (referred to as Brillouin frequency shift), and Δν _b is the full width at half maximum of the spectrum of the Brillouin scattered light. When the medium is a silica-based optical fiber and the wavelength of incident light is 1.55 μm, ν _b 11 GHz and Δν _b 2020 MHz.

Since the Brillouin scattering spectrum S _b (ν) of the above equation (1) has the same profile as a Brillouin gain spectrum indicating the optical frequency characteristic of Brillouin light amplification described later, hereafter, both will be described. Both are referred to as Brillouin spectra.

[0004] It is known that the Brillouin spectrum changes depending on the strain caused by the stress applied to the optical fiber and the temperature of the environment where the optical fiber is placed. For example,
The Brillouin frequency shift ν _b of an optical fiber made of quartz glass exhibits a dependency of about 500 MHz /% on strain and about 1 MHz / ° C. on temperature. Therefore,
By detecting these spatial changes, that is, changes in the Brillouin spectrum along the length of the optical fiber, strain / temperature distribution measurement using the optical fiber as a sensor has been realized. When ν _b changes at a pitch shorter than the spatial (distance) resolution of the measurement system, Δν _b which is apparently the full width at half maximum of the Brillouin spectrum
Therefore, by detecting a change in Δν _b , it is also possible to measure a change width such as strain and temperature.

As a technique capable of measuring the spatial distribution of the Brillouin spectrum of an optical fiber, BOTDR (Bri
llouin optical time domain reflectometry: Brillouin optical time domain reflectometry or BOTDA (Brillouin opt)
ical time domain analysis (Brillouin optical time domain analysis) has been realized (Ref. [1] T. Horiguchi et a)
l., “Development of a distributed sensing techniqu
e using Brillouin scattering ”, J. Lightwave Techn
ol., vol.13, no.7, pp.1296-1302, July 1995).

[0006] BOTDR is a technique in which a light pulse of coherent monochromatic light is incident on an optical fiber, and the spectrum of backward Brillouin scattered light generated by the light pulse is spectroscopically measured as a function of time. After the light pulse is injected, the delay time until the back Brillouin scattered light returns to the incident fiber end is proportional to the distance from the fiber end to the position in the optical fiber where the back Brillouin scattered light is generated. The spectral distribution of Brillouin scattered light, that is, the Brillouin spectral distribution, along the length direction of the optical fiber can be measured.

Similarly, in BOTDA, a coherent monochromatic light pulse is incident on an optical fiber. In the case of BOTDA, a Brillouin spectrum is measured using the Brillouin gain generated by the incident light pulse. Brillouin gain occurs only in the vicinity of the frequency at which the frequency is shifted by an amount called Brillouin frequency shift, that is, ν _b , rather than the incident light pulse, and the spectral shape of the gain is the backward Brillouin scattered light measured by the BOTDR. Is known to be identical to the spectrum of Therefore, when probe light whose frequency difference from the optical pulse is approximately ν _b is incident from the other end of the optical fiber, the probe light is optically amplified by the optical pulse propagating in the opposite direction. The Brillouin spectrum is measured by measuring the increased power change of the probe light while changing the frequency difference between the light pulse and the probe light.

The delay time before the optically amplified probe light is measured at the end of the optical pulse incident fiber is proportional to the distance to the position in the optical fiber where the probe light meets the optical pulse and is optically amplified. Therefore, the distribution of the Brillouin spectrum along the length direction of the optical fiber can be measured by measuring the power change due to the optical amplification of the probe light as a function of time as in the case of the BOTDR.

However, the distance resolution Δz _rtd of BOTDR or BOTDA is limited by the optical pulse width T and is given by the following equation (2). Δz _rtd = vT / 2 (2) where v is the speed of light in the optical fiber, which is about 2 × 10 ⁸ m / s. For example, when T = 1 μs, Δz _rtd = 100 m.
For better distance resolution, the light pulse width
It is necessary to further narrow T and broaden the bandwidth B of the optical signal receiving system to 1 / T or more. At this time, BOTDR or BOTDA
The optical signal intensity detected by the method decreases in proportion to the optical pulse width T, and the noise of the receiving system increases with the expansion of the band B, so that the S / N, which is the ratio between the signal power and the noise power, deteriorates. .

In addition, since the measured Brillouin spectrum is a superposition integral of the spectrum of the optical pulse incident on the optical fiber and the spectrum of the above equation (1), the full width at half maximum is Δν _b + (2 / T). By the way,
As described above, if Δν _b = 20 MHz, Δz _rtd = 100 m,
At 1 m, Δν _b + (2 / T) = 22 MHz and 220 MHz, respectively. That is, when the distance resolution is improved from 100 m to 1 m, the full width at half maximum of the measured Brillouin spectrum is 10
It spreads twice, and it becomes difficult to accurately measure the Brillouin frequency shift, which is the center frequency.

Further, since the full width at half maximum of the Brillouin spectrum is almost determined by the full width at half maximum of the spectrum of the optical pulse, the change in the Brillouin frequency shift, which changes at a pitch shorter than Δz _rtd, from the change in the full width at half maximum of the Brillouin spectrum It is also difficult to measure the width.

For these reasons, the range resolution of BOTDR and BOTDA up to now has been practically limited to about 1 m.

On the other hand, recently, phase modulation (Phase Modul
The method of measuring the distribution of Brillouin spectrum using the continuous wave (Continuous Wave) which has been proposed (Reference [2] K. Hotate and T. Hasegawa, “Measurement
of Brillouin Gain Spectrum Distribution along an
Optical Fiber Using a Correlation-Based Technique
− Proposal, Experiment and Simulation − ”, IEICE
TRANS. ELECTRON., Vol.E83-C, no.3, pp.405-412, Ma
rch 2000). This method (hereinafter referred to as PMCW (Phase Modulation
Continuous Wave) method), the coherent continuous light having an oscillation line width narrower than the Brillouin spectrum width of the measured optical fiber is phase-modulated at a predetermined frequency, and the phase-modulated continuous light is Pump light is generated by frequency-shifting one of the two branched light beams, and the pump light is incident from one end of the optical fiber.
Further, the other branched light is used as probe light, and the probe light is made incident from the other end of the optical fiber. The correlation between the phase of the pump light and the phase of the probe light differs depending on the position where the two lights meet in the optical fiber. The Brillouin gain increases when the correlation is high, and decreases when the correlation is low.

[0014] As has high correlation position appears periodically plurality, the modulation frequency f _m and the phase modulation, by selecting the relative time delay of the pump light and the probe light appropriately, one point in an optical fiber only, It is possible to increase the correlation. Therefore, when the frequency difference ν _c between the center frequency of the pump light and the center frequency of the probe light is swept near the Brillouin frequency shift of the optical fiber, at a position where the correlation becomes high,
Since the Brillouin gain is selectively increased, the Brillouin spectrum at the position where the correlation becomes high can be measured by measuring the power change of the probe light optically amplified by the Brillouin gain. Further, since the position where the correlation becomes high can be changed by the modulation frequency f _m of the phase modulation, the distribution measurement of the Brillouin spectrum is realized.

If the phase modulation signal is m sin (2πf _m t), the distance resolution of the PMCW method is given by the following equation (3). _{Δz rpm = (Δν b / f} m) / (v / 2πmf m) (3) As an ^{example, v = 2 × 10 8 m} / s, Δν b = 20MHz, f m = 7.5
MHz, When mf _m = 360 MHz, obtain Δz _rpm = 25cm.

Note that the Brillouin spectrum width at the point where the correlation is maximized, measured by the PMCW method, does not spread as in BOTDR or BOTDA even if the distance resolution is increased as described above. It is the source of Brillouin scattering,
This is because the spectrum of the beat signal of the pump light and the spectrum of the beat signal of the probe light do not spread at the position where the correlation between the two lights becomes the maximum, and becomes a delta function. Since the PMCW method has such features, higher distance resolution can be easily achieved as compared with BOTDR and BOTDA.

As described above, the PMCW method is a very excellent method, but has at least the following three problems to be solved.

The first problem is the occurrence of crosstalk in distribution measurement. As described above, the phase correlation between the pump light and the probe light can be selectively increased at one point in the optical fiber, but the phase correlation at other positions except for the one point is not completely zero and has a value that cannot be ignored. Therefore, crosstalk occurs in the distribution measurement, and it becomes difficult to distinguish between the Brillouin spectrum at the measurement position and the Brillouin spectrum at another leaked location.

The second problem is spreading of the Brillouin spectrum on the frequency axis. In the above description, it is assumed that the width of the Brillouin spectrum measured by the PMCW method does not increase even when high-range resolution measurement is performed. However, it is only one point where the phase correlation between the pump light and the probe light becomes high, and as the distance from the position increases, the phase correlation gradually decreases, and the beat spectrum of both lights, that is, the Brillouin spectrum reflecting it, It spreads.

Therefore, even in the range of the distance resolution determined by the above equation (3), the width of the Brillouin spectrum is not a constant value Δν _b but increases up to several times the maximum value. In an application for measuring the distribution of the Brillouin frequency shift, which is the center frequency of the Brillouin spectrum, the increase in the width of the Brillouin spectrum does not matter. However described above, by short pitch than the distance resolution application to sensor using a variation of the observed Brillouin spectrum width .DELTA..nu _b when Brillouin frequency shift [nu _b is changed, the phase correlation is reduced Increases the Brillouin spectrum width.

As a solution to such a problem, a method of performing deconvolution integration has been proposed (see the above-mentioned reference document).
[2]). That is, regarding the first problem, there has been proposed a method of removing crosstalk by performing deconvolution integration on the measured Brillouin spectrum distribution in the length direction of the optical fiber using a computer. . Regarding the second problem, there has been proposed a method of reproducing the original Brillouin spectrum width by performing deconvolution integration on the frequency with respect to the measured Brillouin spectrum distribution using a computer.
However, although the signal processing technology using such a computer has a certain effect, it performs numerical calculation processing using a large number of measurement data affected by noise and so on. The result may be unexpected, and the problem has not been solved.

A third problem is that the PMCW method needs to input and output light at both ends of the medium to be measured. Therefore, in the PMCW method, when performing multiple measurements in order to grasp the change over time of the medium to be measured, mounting work for inputting and outputting light at both ends of the medium to be measured must be performed each time the measurement is performed. And the workability deteriorates significantly. More importantly, in the case of a medium under measurement where light input and output can be performed only at one end, the PM
This makes measurement by the CW method impossible.

[0023]

As described above,
In conventional BOTDR and BOTDA using optical pulses, the effective resolution was about 1 m in distance resolution. In addition, the conventional PMCW method, which was proposed to overcome this limitation, can achieve a distance resolution of 1 m or less. The problem that the original Brillouin spectrum width cannot be reproduced cannot be solved without complicated numerical processing. Further, there is a strict measurement condition that light must be input and output at both ends of the medium to be measured.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and an object thereof is to input and output light at one end of a medium to be measured, and to achieve a distance resolution of 1 m or less. To provide a reflection-type Brillouin spectrum distribution measuring method and apparatus capable of measuring distribution with respect to the position of the Brillouin spectrum with high accuracy without crosstalk and without expanding the original Brillouin spectrum width of the medium to be measured. is there.

A further object of the present invention is to utilize the temperature or strain dependence of the Brillouin spectrum,
It is an object of the present invention to measure the temperature or strain distribution along the length of an optical fiber with high work efficiency, high distance resolution, and high accuracy.

[0026]

In order to achieve the above object, according to the present invention, an incoherent continuous oscillation light having a line width wider than the Brillouin spectrum width of a medium to be measured is branched and converted into a first light. And the second light, etc., and the relative delay time between the first light that is backward-Brillouin scattered in the medium to be measured and the second light that is the other branched light is variable so as to be substantially zero. After adjusting with an optical delay device, the two lights are combined, and the combined light is detected by a photodetector to enhance the coherence of both lights and measure the spectrum of the beat electric signal due to the interference. By the first
The Brillouin spectrum at the position where the light is scattered can be measured selectively and with high efficiency by simply inputting and outputting light at one end of the medium to be measured.

That is, according to the reflection type Brillouin spectrum distribution measuring method of the present invention, the incoherent continuous oscillation light having a line width wider than the Brillouin spectrum width of the medium to be measured is converted into at least the first light by the optical branching means. The first light is incident from one end a of the medium to be measured, and the incident first light is Brillouin scattering backward at a position z _0T in the medium to be measured. Then, the first light reaching the one end a of the medium to be measured again and emitted from the medium to be measured is merged with the second light, and the merged light is detected by light detecting means. A power and a frequency ビ ー ト of a beat electric signal included in the electric signal, which is generated by interference between the first light and the second light, which are Brillouin scattered backward, and which are included in the electric signal. And said It said first light starting from the light branching means is Brillouin scattered backwards position z _0T in a measurement medium, the delay time until reaching the light detecting means τ
₁ and a relative delay time τ ₁₂ = τ ₁ −τ ₂ which is a difference between a delay time τ ₂ until the _second light departs from the optical branching means and reaches the light detecting means, By making the relative delay time τ ₁₂ substantially zero, the coherence between the Brillouin scattered light scattered backward at the position z _0T and the second light from other positions is increased, and the beat electric signal _Is selectively measured to measure the Brillouin spectrum at the position z _0T .

In the reflection Brillouin spectrum distribution measuring method according to a second aspect of the present invention, the incoherent continuous oscillation light having a line width wider than the Brillouin spectrum width of the medium to be measured is converted into at least a first light by the optical branching means. The first light is incident on one end a of the medium to be measured, and the incident first light is Brillouin scattered backward at a position z _0T in the medium to be measured. Then, the first light reaching the one end a of the medium to be measured again and emitted from the medium to be measured is merged with the second light, and the merged light is detected by light detection means. Convert to electrical signals,
A method for measuring a power and a frequency ビ ー ト of a beat electric signal included in the electric signal, the beat electric signal being generated by interference between the first light and the second light that have been Brillouin scattered backward, wherein the first The light departs from the light branching means, is Brillouin scattered backward at the position z _0T in the medium to be measured, and has a delay time τ ₁ until it reaches the light detecting means,
The relative delay time τ ₁₂ = τ ₁ −τ ₂ , which is a difference from the delay time τ ₂ from the time when the _second light departs from the optical branching means and reaches the light detecting means, is changed. Time τ _12
Is substantially zero, thereby increasing the coherence between the Brillouin scattered light scattered backward at the position z _0T and the second light from other positions, and selectively measuring the beat electric signal. Brillouin spectrum D obtained by
_T (ζ) is measured, and the first light branched by the light branching means is incident on a reference medium optically connected or in contact with the medium to be measured, and the first light is separated by the light branching. Brillouin scattering backward at a position z _0R in the reference medium starting from the means, a delay time τ _1R until reaching the light detecting means, and the second light starting from the light branching means and By making the relative delay time τ _12R = τ _1R −τ _2R , which is the difference from the delay time τ _2R until the light reaches the light detecting means, substantially zero, the position z _0R in the reference medium is higher than the other positions.
The Brillouin spectrum D _R (ζ) obtained by increasing the coherence between the Brillouin scattered light scattered backward and the second light and selectively measuring the beat electric signal of both lights.
And a corrected Brillouin spectrum at the position z _0T is obtained using the relationship between the Brillouin spectrum D _T (ζ) and the Brillouin spectrum D _R (ζ).

A reflection type Brillouin spectrum distribution measuring apparatus according to a third aspect of the present invention comprises: a light source for outputting incoherent continuous oscillation light having a line width wider than the Brillouin spectrum width of a medium to be measured; Light branching means for splitting light to output at least a first light and a second light; first optical means for causing the first light to enter the medium to be measured; Second optical means for extracting at least a part of the first light output from the medium to be Brillouin scattered backward by the second optical means, and Brillouin scattered backward extracted by the second optical means A light converging unit that converges the first light and the second light, a light detecting unit that converts the converged light merged by the light merging unit into an electric signal, and the electric signal as an input signal,
A filter for passing a beat electric signal generated by interference between the first light and the second light that have been subjected to the backward Brillouin scattering, and a measuring means for measuring the power and frequency の of the beat electric signal that has passed through the filter And a delay time τ ₁ until the first light starts from the light branching unit, is Brillouin scattered backward at a position z _0T in the measured medium, and reaches the light detection unit, A variable optical delay unit that changes a relative delay time τ ₁₂ = τ ₁ −τ ₂ which is a difference from a delay time τ ₂ until the light of _{No. 2} departs from the optical branching unit and reaches the light detection unit; And measuring the Brillouin spectrum distribution of the medium to be measured by changing the position z _{0T at} which the relative delay time τ ₁₂ becomes substantially zero by the variable optical delay means.

The reflection type briller according to claim 4 of the present invention.
The spectral distribution measuring device uses the Brillouin
Incoherent with a line width wider than the spectral width
A light source that outputs a continuous wave light,
Optical branching means for outputting at least a first light and a second light
And optically connected to or in contact with the medium to be measured
A reference medium and the first light in and before the medium to be measured.
First optical means for entering the reference medium;
Brillouin behind in the constant medium and in the reference medium
A second extracting at least a portion of the scattered first light
Optical means, and a blister scattered backward by the medium to be measured.
Luang scattered light and brill scattered back by the reference medium
Optical combining for combining the unscattered light and the second light respectively
And a converging light which is merged by the optical converging means.
Signal, and the electric signal is inputted,
The rearward blow in the measured medium and the reference medium
The interference between the first light that has been scattered by Luang and the second light
A filter that passes and outputs a beat electrical signal
Filter and the power of the beat electrical signal passing through the filter.
Measuring means for measuring the frequency and frequency ζ, and the first light
Starting from the optical branching means, the position z in the medium to be measured_0T
Brillouin scattering backwards and reaches the light detecting means
Delay time τ₁And the second light is the light splitting means.
Delay time from departure to reaching the light detection means
τ_TwoRelative delay time τ₁₂= τ ₁−τ_TwoChanged
And the first light starts from the light splitting means.
Position z in the reference medium_0RBrillouin scattered backward by
And a delay time τ until the light reaches the light detecting means._1RAnd before
The second light starts from the light branching means and the light detection means
Delay time τ to reach the stage_2RRelative delay which is the difference between
Time τ_12R= τ_1R−τ_2RAdjust, at least one is acceptable
One or more optical delay means which are variable delay means;
The rear optical path from the medium to be measured is changed by the variable optical delay means.
The relative delay time τ related to Lilouin scattered light₁₂But almost
The position z which becomes b_0TAnd the optical delay hand
Steps to convert back Brillouin scattered light from within the reference medium
Relative delay time τ involved_12RAt the position z_0RWith almost zero
By doing so, the position z in the medium to be measured_0TAnd before
Position z in the reference medium_0RIn each of the above phases
Delay time τ₁₂And τ_12RIs almost zero.
The rilluan spectrum was measured, and the measured Brilliance was measured.
D for each of the vectors_T(ζ) and D_R(ζ),
The D_T(ζ) and D_RUsing the relationship (ζ), the position z_0TIn
Data processing to obtain a corrected Brillouin spectrum
And a control means.

Preferably, in the configuration of claim 4, a second light branching means for branching the first light, which is one of the output lights from the light branching means, and the second light branching means. A second variable optical delay unit having a variable delay amount, which receives one output light of the optical branching unit as an input and changes and outputs a delay amount of the input light, and a delay amount by the second variable optical delay unit. The changed light is used as an input and the input light is used as the frequency f
_A first light modulating means for modulating and outputting the modulated light with the modulation signal of _SD1 , and a variable delay amount for changing the delay amount of the inputted light with the other output light of the second optical branching means as an input and outputting the same; The third variable optical delay means and the light whose delay amount has been changed by the third variable optical delay means are input, and the input light is modulated by a modulation signal having a frequency f _SD2 (≠ f _SD1 ). A second light modulating means for outputting, and a second light merging means for merging the output light of the first light modulating means and the output light of the second light modulating means and outputting the same as the first light again. And optical means for guiding the first light combined by the second light combining means to the reference medium and the medium to be measured, and extracting light Brillouin scattered backward in the medium. And a third optical branching unit, wherein the measuring unit includes:
A synchronous detection receiver for measuring the power of the beat electric signal in synchronization with the synchronous signals of the frequencies f _{SD1 and} f _SD2 is included.

Further, this is preferably in the configuration of claim 3, wherein the first further comprising a light modulating means for modulating at a frequency f _SD of the modulation signal light, the said measuring means, of the frequency f _SD A synchronous detection receiver for measuring the power of the beat electric signal in synchronization with the synchronous signal is included.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Principle and Operation of the Invention) Before describing a specific embodiment of the present invention, the principle and operation of the present invention will be described with reference to the drawings to facilitate understanding of the present invention. This is described below.

In the present invention, as described above, incoherent continuous oscillation light having a line width wider than the Brillouin spectrum width of the medium to be measured is used. Back Brillouin scattering in an optical fiber when monochromatic light is used as continuous wave light will be described. Further, the backward Brillouin scattering is combined with the continuous wave light, and a beat electric signal based on the interference will be described. This description will be made using the configurations shown in FIGS. 1, 2 and 3 which are the basis of the present invention. 1, 2 and 3 are the basis of the present invention,
FIG. 9 is a diagram illustrating a first example, a second example, and a third example of a configuration for obtaining a beat electric signal by backward Brillouin scattered light. Reference numeral 17 in FIG. 1 is a reflection mirror that reflects a second light described later.

As shown in the configurations of FIGS. 1, 2 and 3, the output light from the light source 1 is split by the optical splitter 2, and one of the split first light is measured. The light enters from one end of the optical fiber as the medium 5. The light propagation direction is
Expressed on the z-axis. The first light is monochromatic light having a frequency F and a propagation constant K, and assuming that the light propagates in the + z direction, the electric field is represented by E ₁ exp [i (2πFt−Kz)]. Now, an acoustic wave over a very wide band is thermally excited in the optical fiber 5, and the frequency of one of the components is changed to F-
f, when the speed is _{V a, K + k = 2π} (Ff) / V a satisfies the phase matching condition (4), the frequency is f, SIZE propagation constant k,
Scattered light whose propagation direction is −z direction, E _BS exp [i (2πf t
+ kz)] occurs efficiently. This is called Brillouin scattering.

The frequency difference satisfying the above equation (4), F−
If f is ν _b , this is called a Brillouin frequency shift. This Brillouin scattering can be interpreted as follows.

That is, the acoustic wave having a frequency of ν _b modulates the refractive index of the optical fiber in its length direction by the photoelastic effect, and therefore travels in the same direction (+ z direction) as the first light.
And the pitch to form a diffraction grating is V _a / ν _b in the optical fiber. Due to this moving grating, the frequency F
Is subjected to a Doppler shift by the frequency ν _b = Ff and is backscattered to become light of the frequency f.

The backward Brillouin scattered light is combined with the second light, which is another branched light, as shown in the configurations of FIGS. Think about it. Since the backward Brillouin scattered light has a frequency shifted by ν _b as described above, a beat electric signal having a frequency of ν _b is observed at the output of the photodetector 7,
The power is proportional to | E ₁ | ² | E ₂ | ² , which is the product of the powers of the first light and the second light.

In practice, the power spectrum density of the backward Brillouin scattering increases in the optical frequency range of ± Δν _b / 2 around F-ν _b due to the attenuation of the acoustic wave. The power spectral density P _B (ζ) is
The Lorentz type given by the following equation (5) is shown. P _B (ζ) = H (Δν _b ) ² / {4 (ζ−ν _b ) ² + (Δν _b ) ² } (5) where ζ is the frequency of the beat electric signal. H is a coefficient proportional to the sensitivity of the detection system and the power of the first light and the second light, and represents a peak value of P _B (ζ) when と き = ν _b . Δν _b is the full width at half maximum of the Brillouin spectrum as described above. The above equation (5) is given by ζ = Ff =
Assuming that ν, the above equation (1) representing the Brillouin spectrum
With the exception of the proportionality coefficient. Therefore, here, the power spectral density P _B (ζ) of the beat electric signal described above is also referred to as a Brillouin spectrum. Further, as already described, when the optical fiber is made of silica glass, and the wavelength of the light source and 1.55 .mu.m, [nu _b is approximately 11 GHz, .DELTA..nu _b almost
20 MHz.

Next, based on the theory of Brillouin scattering when monochromatic light described above is used, an incoherent light having a wide line width equal to or larger than the Brillouin spectrum width Δν _{b of the} medium to be measured according to the present invention is described. Brillouin scattering when using continuous wave light will be described. Now, an example of the spectrum of the continuous wave light used in the present invention is shown in FIG.
Shown in Here, the first light and the second light are both obtained from the same continuous wave light source 1, and the center frequency and the full width at half maximum of the spectrum are respectively _Fc ,
And Δν _S (> Δν _b ).

First, the electric field component E ₁ (F
Think only about). In this case, since it can be considered as in the case of the monochromatic light, when _{Ff = ν b, E 1 (} F)
Is converted into backward Brillouin scattered light of frequency f by an acoustic wave of frequency ν _b . The electric field component E _BS
(f) is proportional to E ₁ (F) as in the following equation. E _BS (f) ∝E ₁ (F) ∝E _LS (F) (6.1) f = F−ν _b (6.2) where E _LS (F) is the first light and the second light Is the electric field component of the output light of the light source from which the light was generated.

Since the above equations (6.1) and (6.2) hold for all the frequency components of the first light, the spectrum of the backward Brillouin scattered light is as shown in FIG. First
Can be considered to have shifted the frequency of the light by −ν _b (actually, the spectrum spreads by Δν _b as shown by the broken line in FIG. 4 due to the attenuation of the acoustic wave. Will be omitted.) The backward Brillouin scattered light and the second light having the same spectrum as the first light (the electric field component of the frequency F is E ₂ (F)) are combined, and converted into an electric signal by the photodetector 7. Then, both beat electric signals are observed.

As can be understood from the above description, since the spectra of both lights have a spread of Δν _S , the power spectral density P _B (ζ) of the beat electric signal having the frequency 、 has a center frequency of ν _b and a width of Is very wide, about 2Δν _S.

The total power P _BT of the beat electric signal is given by the following equation (7).
Given by _{P BT = ∫P B (ζ)} dζ α | ∫E 2 (F) dF ∫E BS (f) * df | 2 α (| E 1 | 2 | E 2 | 2 / | E LS | 4) | ∫ ^{_{| E LS (F) | 2}} dF | 2 = | E 1 | 2 | E 2 | 2 (7) ^{_{However, | E 1 | 2 = ∫}} | E 1 (F) | 2 dF (8.1) | E ₂ | ² = ∫ | E ₂ (F) | ² dF (8.2) | E _LS | ² = ∫ | E _LS (F) | ² dF (8.3).

In deriving the above equation (7), the fact that different frequency components of the output light of the light source are independent of each other is used. As can be seen from the above equation (7), the total power of the beat electric signal is proportional to the product of the total power | E ₁ | ² of the first light and the total power | E ₂ | ² of the second light. This is the same as for monochromatic light. However, as described above, the spectrum is about 2Δν _S, which is much wider than Δν _b for monochromatic light. Therefore, the power spectrum density is reduced to about Δν _b / (2Δν _S ) times that of the case of monochromatic light. Incidentally, assuming that Δν _b = 20 MHz and 2Δν _S = 2 GHz, this value is 1/100.

Further, the spectrum of the beat electric signal described above can be obtained without depending on the position where the Brillouin scattering occurs backward in the optical fiber. This feature is explained next,
Significantly different from the spectral features of another beat electrical signal, and as will become apparent, is one of the important features of the present invention.

In deriving the above power spectrum density P _B (ζ) (this is referred to as P _BI (ζ) or simply P _BI ), the first light and the second light are generated by the same light source 1 It was assumed that the output light was obtained by branching. But in fact,
As can be seen from the above derivation process, the same result can be obtained for P _BI (ζ) even when the first light and the second light are obtained from different independent light sources. In fact, as in the present invention, if they obtain the first light and the second light from the same light source, in addition to the above P _BI, it appear a new power spectral density P _BII. This power spectral density P
The source of _BII is due to the fact that beat electric signals having a large amplitude are formed when the phases of a plurality of beat electric signals having the same frequency are aligned and overlapped.

Now, as in the case where _PBI was first derived,
The backward Brillouin scattered light whose electric field component E _BS (f) is expressed by the above equation (6) and the _second light whose electric field component E ₂ (F) is E ₂ (F) ∝E ₁ (F) Consider the beat electrical signal that appears at the photodetector output due to the interference of Now, attention is paid to a plurality of beat electrical signal generated at the output of the photodetector by both light components of which satisfy the relationship of Ff = ν _b. The backward Brillouin scattered light is the first light obtained by Brillouin scattering in the optical fiber, and the first light and the second light are obtained from the same light source. Therefore, ignoring the original spectrum width Δν _b of the backward Brillouin scattered light, the backward Brillouin scattered light and the second light can be regarded as those obtained by shifting each other's frequency by ν _b . 1
Light propagates through the optical fiber starting from the optical splitter 2 and is Brillouin-scattered backward at a certain position (hereinafter, this position is referred to as position z _{0 T} ), and merges with the second light to form a photodetector. in delays between the delay time tau ₁ until it is converted into an electric signal, until converted into an electrical signal by a photodetector 7 by the second light starting from the optical splitter 2 joins the backward Brillouin scattered light Time τ ₂
When the relative delay time τ ₁₂ = τ ₁ −τ ₂ , which is the difference from the above, becomes zero, the phases between the frequency components of the backward Brillouin scattered light and the second light are synchronized.

Here, when Brillouin scattering is performed,
Also note the phase change that occurs when interacting with acoustic waves.
Need to be However, the present invention is focused on and used in the present invention.
Acoustic waves that interact with all waves of the first light
Frequency ν _bOnly acoustic waves
Therefore, all waves only experience a common phase shift
No. Therefore, the frequency ν, which appears at the photodetector output,_bof
A plurality of beat electric signals overlap in phase with each other.
It can be seen that the amplitude of Like this
Frequency ν_bThe power of the beat electrical signal is given by the following equation
It is proportional to (9).     ｜ ∫E_Two(F) E_BS* (F−ν_b ) d F ｜^Two       ∝ ｜ E₁ E_Two {∫ ｜ E_LS(F) ｜^TwodF / ｜ E_LS|^Two } |^Two        = ｜ E₁|^Two｜ E_Two|^Two                                    (9)

That is, the power of the beat electric signal of the frequency ν _b is the same as that of the monochromatic light without being reduced as in the case of the above-mentioned _PBI . The power spectrum density (hereinafter, P _BI (ζ)
_BII (ζ) or simply _PBII ) is the same as in the case of monochromatic light. However, it should be noted that the power of the beat electric signal of the above frequency ν _b is only for the light Brillouin scattered backward at the position z _0T in the optical fiber where the relative delay time τ ₁₂ becomes zero. It is to be realized. position
If the first light is backward-Brillouin-scattered at a distance z from z _0T , the phase difference between the second light and the backward Brillouin-scattered light is given by the following equation (10) due to the difference in frequency. Becomes _{2π (F-F c) z} / v + 2π (f-f c) z / v = 2π (F-F c) (2z / v) (10) where, v is the velocity of light in the optical fiber . In consideration of this phase difference, the above equation (9) is modified as the following equation (11).

| ∫E ₂ (F) E _BS * (F−ν _b ) exp [i 2π (F−F _c ) (2z / v)] d F | ² ｜| E ₁ E ₂ {∫ | E _LS ^{(F) | 2 exp [i} 2π (F-F c) (2z / v)] dF / | E LS | 2} | 2 = | E 1 | 2 | E 2 | 2 | γ (2z / v) | ² (11) where, γ (2z / v) = ∫ | E LS (F) | 2 exp [i 2π (F-F c) (2z / v)] dF / | E LS | 2 (12) is , A function representing the coherence of the light source, which plays an important role in interferometry and the like, and a coherence function. That is, it was found that the magnitude of the amplitude of the beat electric signal having the frequency ν _b is proportional to the coherence function of the light source 1.

In general, | γ (2z / v) | ² is 1 when z = 0, and has a property of rapidly approaching zero as | z | increases. That is, the relative delay time τ
The power of the beat electric signal when _{12 is set} to zero is a large value as in the case of using monochromatic light, but the position where it is obtained is due to the backward Brillouin scattering in the optical fiber where the relative delay time τ ₁₂ becomes zero. It can be seen that it is limited to the vicinity of the position _z0T . The present invention positively utilizes this characteristic, and uses a variable optical delay device to set the relative delay time τ ₁₂ between the first light subjected to the backward Brillouin scattering and the second light which is the other branched light. To change the Brillouin spectrum at an arbitrary position in the optical fiber. At this time, the Brillouin spectrum by P _BI is also determined are superimposed, as described above, the Brillouin spectrum by P _BI is very wide band, because power change is small due to the frequency of the beat electrical signals difference is measured It is relatively easy to extract only those based on _PBII from the Brillouin spectrum.

The detailed calculation process has been omitted, and only the essence of the principle of the present invention has been described. However, in order to use a light source having a large line width, there are many combinations of the frequency of the backward Brillouin scattered light and the frequency of the second light,
At first glance, it is considered that there exists a beat electric signal spectrum other than the _PBI and _{PBI I} described above. Therefore, the inventors of the present application performed a strict analysis and revealed that there was no other large beat electric signal spectrum, and in the Brillouin spectrum distribution measuring method and apparatus according to the present invention, a unit length optical fiber was used. The following simple equations (13.1) and (13.2) representing the power spectral density of the beat electrical signal obtained when the backward Brillouin-scattered light interferes with the second light were derived. . P _BI (ζ, z) = H (Δν _b + 2Δν _S ) (Δν _b ) / {4 (ζ−ν _b ) ² + (Δν _b + 2Δν _S ) ² } (13.1) P _BII (ζ , z) = H | γ (2z / v) | ² (Δν _b ) ² / {4 (ζ−ν _b ) ² + (Δν _b ) ² } (13.2) where H is the first and It is a coefficient proportional to the power of the second light and the sensitivity of the photodetector and the like. As described above, the Brillouin frequency shift ν _b in the equation varies depending on strain or temperature, and is thus not explicitly written, but is a function of the position z.

When the spectrum shape of the light source is a Lorentzian shape having a full width at half maximum of Δν _S , the following equation (14) is obtained. | Γ (2z / v) | ² = exp {−2πΔν _S | 2z / v |} (14) As described above, at the position z _0T (z = 0), | γ | ² = 1, but z _0T It approaches zero rapidly as you move away from it. Therefore, _assuming that the distance between the points on both sides of z _0T where | γ | ² = 0.5 is the distance resolution Δz _r in the distribution measurement of the present invention, Δz _r is given by the following equation (15) (see FIG. 5). ). Δz _r = 0.11 v / Δν _S (15) As shown in the above equations (13.1) and (13.2), in the measurement method according to the present invention, two types of interference are caused by the back Brillouin scattered light and the interference of the second light. Spectrums _PBI and _PBII are measured. Both spectra can be measured by using a spectrum analyzer or the like.

As shown in the above equation (13.1), the spectrum P _BI (ζ, z) is almost the same at any position in the length direction of the optical fiber. Its band is very wide, Δν _b + 2Δν
_S. When the beat frequency ζ is ν _b, the spectrum P
_BI (ζ, z) takes the maximum value, which is the value H when the line width of the light source is negligible with respect to the Brillouin spectrum width.
Δν _b / (Δν _b + 2Δν _S ) times that of Note that this coefficient Δν _b / (Δν _b + 2Δν _S ) is different from Δv _b / (2Δν _S ) in the description of the essence of the present invention. In the rigorous analysis, this difference is due to the spread of the spectrum due to the attenuation of the acoustic wave, Δν _b
Is considered.

Now, for the above P _BI (ζ, z), the spectrum P _BII (ζ, z) is obtained as shown by the above equation (13.2).
Takes a large value only at and near the scattering position z _0T where the relative delay time τ ₁₂ between the backward Brillouin scattered light by the first light and the second light is zero, and the maximum value is H Matches. At other positions, it approaches exponentially rapidly to zero as it moves away from z _{0T according} to equation (14). This is shown in FIG. FIG. 5 shows a Brillouin spectrum (a) at a position z _0T where the relative delay time τ ₁₂ is zero,
9 shows Brillouin spectra (b), (c), and (d) at positions gradually away from z _0T .

For comparison with the Brillouin spectrum P _BII (ζ, z) observed according to the present invention, FIG.
4 shows a spectrum of Brillouin gain obtained by the W method. In FIG. 6, the spectrum (a) of the Brillouin gain at a position z _{0 pm} where the correlation between the phase-modulated and frequency-shifted two lights is maximum, and the spectrum of the Brillouin gain at a position gradually distant from z _0pm ( (b), (c) and (d) are shown. The horizontal axis of shows the spectrum shows the frequency difference [nu _c of the center frequency of the center frequency and the probe light of the pump light used in PMCW method.

FIG. 6 _{clearly shows} that the maximum gain g ₀ is obtained at the position z _0pm even in the PMCW method, and the gains at other positions are smaller than g ₀ . However, when gradually away from z _{0 pm,} Brillouin gain PMCW method decreases slowly in proportion to the distance away (Ref
See).

On the other hand, P _BII (ζ, z) of the present invention is obtained by the above formula (1)
According to 4), it decreases exponentially as it moves away from _z0T . This indicates that the present invention is much better in crosstalk characteristics than the PMCW method.

Further, as can be seen from a comparison between FIG. 5 and FIG. 6, the spectrum shape of P _BII (ζ, z) has a full width at half maximum Δν _b.
Is Lorentz-type and does not depend on the position, but the spectral shape of the gain of the PMCW method depends on the position, and its full width at half maximum is z
It increases almost in proportion to the distance from _0pm . This is that the present invention measures in that the original Brillouin spectrum shape of the measured optical fiber is measured without deformation.
It shows that it is superior to the PMCW method.

Therefore, in the present invention, the relative delay time between the first light subjected to the backward Brillouin scattering and the second light which is the other branched light is utilized by utilizing the characteristic of the spectrum of the beat electric signal. τ ₁₂ is a variable optical delay (FIGS. 7 to 9)
Of the optical fiber, it is possible to accurately measure the Brillouin spectrum at an arbitrary position in the optical fiber and to input and output light at one end of the optical fiber. It is configured as follows. Also,
The distance resolution at that time is given by the above equation (15).

Actually, as can be seen from FIG. 5, in the present invention, a signal based on P _BI (ζ, z), D _I (ζ) = C ₁ ∫P _BI (ζ, z
) dz and P _BII (ζ, z) signal, D _II (ζ) = C ₁ ∫P _BII
The sum of (ζ, z) dz, D (ζ) = D _I (ζ) + D _II (ζ), is measured (where C ₁ is the proportionality factor). Therefore, in order to measure the original Brillouin spectrum of the medium to be measured, D _{I I}
It can be seen that (ζ) must be separated and extracted from D (ζ). However, as suggested by the above equations (13.1) and (13.2), when Δν _S > Δν _b , the change of D _I (ζ) with respect to the difference of ζ is very slow. D _II (ζ) shows a steep peak change. That is,
In the range of ζ where D _II (ζ) takes a large value, D _I
(ζ) can be regarded as almost constant, so by subtracting that constant value from the entire signal D (ζ),
It is possible to extract only _II (ζ). Further, even when the change of D _I (ζ) with respect to the difference of ζ is gradual but not negligible, D _II (ζ) takes a large value.
It is possible to obtain an approximate function for ζ from data outside the range of, and to obtain an estimated value of D _II (ζ) in the range of ζ where D _II (ζ) takes a large value based on the approximate function. It is possible.

Therefore, by subtracting the estimated value from the entire signal D (ζ), only D _II (全体) can be extracted. These signal processings are based on D _I (ν _b ) <D _II
(ν _b ) or D _I (ν _b ) to D _II (ν _b ).

However, the power spectral density P_BI(ζ, z)
Is P_BIIAlthough it is smaller than the maximum value of (ζ, z),
Since the value is almost the same over the entire length of the optical fiber,
If the fiber is long, the integral over that length
A D_IBecomes larger and D_I (ν _b ) >> D_II (ν_b)
You. At this time, the error is not detected in the signal processing as described above.
Always grow.

Therefore, in the present invention, as shown in FIGS. 8 and 9 described later, measurement is performed by optically connecting the optical fiber 5 to be measured and the reference medium 11 having a known Brillouin spectrum. At this time, the measurement is performed when the relative delay time τ ₁₂ between the first light that is backward-Brillouin scattered at a point in the medium 5 to be measured and the second light that is the other branched light is set to zero. The signal is set to D _T (ζ), and the relative delay time τ _12R of the first light that is backward-Brillouin-scattered at a point in the reference medium 11 and the second light that is the other branched light is set to zero. When the measured signal at this time is D _R (ζ),
_T (ζ) and D _R (ζ) are given by the following equations (16) and (17). D _T (ζ) = D _ITR (ζ) + C ₁ ∫ _T P _BII (ζ, z) dz (16) D _R (ζ) = C ₂ [D _ITR (ζ) + C ₁ ∫ _R P _BII (ζ , z) dz] (17) where it is _{D ITR (ζ) = C 1} ∫ T P BI (ζ, z) dz + C 1 ∫ R P BI (ζ, z) dz (18). Also, ∫ _T and ∫ _R each represent an integration of the measured optical fiber section and the reference medium segment. C ₂ is a proportional coefficient for calibrating the difference between the measurement systems when measuring D _T (ζ) and D _R (ζ). In the case of the same measurement system, C ₂ = 1.

From the above equations (16) and (17), the following equation (1)
9) is obtained. _{D DIF (ζ) = D T} (ζ) - [D R (ζ) / C 2] = C 1 ∫ T P BII (ζ, z) dz - C 1 ∫ R P BII (ζ, z) dz (19 Since C ₂ can be obtained in advance, the above equation (19)
, The calculation process of D _T (−) − [D _R (/) / C ₂ ] can be easily performed. Also, since the Brillouin spectrum of the reference medium is known, the second term C ₁ ∫ _{R on} the right side of the above equation (19)
P _BII (ζ, z) dz is easily obtained. In particular, the reference medium
In the range of the measurement frequency, when the power of the backward Brillouin scattered light using a negligibly small medium, since the second term of the equation (19) can be regarded as zero, D _T
The calculation process of (ζ) − [D _R ( ₂ ) / C ₂ ] is further simplified.
As such a reference medium, air, an optical fiber having a different material or material component ratio from the measured optical fiber, or the like can be used. If it is clear that the tip of the end 5-b opposite to the end 5-a from which light is incident on the optical fiber is air, the section where the air is located is the same as the reference medium described above. You should consider it.

As described above, according to the present invention, D _{T is} not affected by the power spectral density P _BI (ζ, z).
(zeta) and by using the relation of the equation (19) and D _R (zeta), be measured is a Brillouin spectrum of the optical fiber to be measured, a _{C 1 ∫ T P BII (ζ} , z) dz It becomes possible. Further, similarly, the measurement is performed by changing the scattering position in the optical fiber to be measured in which the relative delay time between the first light and the second light is Brillouin-scattered backward and the second light is zero. The distribution of the Brillouin spectrum of the fiber can be measured.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment) FIG. 7 shows a configuration of a reflection-type Brillouin spectrum distribution measuring apparatus according to a first embodiment of the present invention. This reflection-type Brillouin spectrum distribution measuring apparatus includes a light source 1, an optical splitter 2, an optical modulator 10, a variable optical delay unit 3, a medium to be measured 5, an optical splitter 6, a photodetector 7, , A filter 8 and a synchronous detection receiver 9.

The light source 1 outputs incoherent continuous oscillation light having a line width wider than the Brillouin spectrum width of the medium 5 to be measured. The optical splitter 2 splits output light from the light source 1 and outputs first light, second light, and the like. The optical modulator 10 modulates the first light at the frequency f _SD of the modulated signal. The variable optical delay device 3 having a variable delay amount receives the second light as input, changes the delay amount, and outputs the changed light. The optical splitter 6 is configured such that the first light is incident from one end 5-a of the measured medium 5 and is Brillouin scattered backward in the measured medium 5 and a second light whose delay amount is changed. Merge with light. The photodetector 7 converts the combined light output from the optical splitter 6 into an electric signal. The filter 8 selects and passes a beat electric signal based on the interference between the backward Brillouin scattered light and the second light having the delayed amount changed from the electric signal. Synchronous detection receiver 9 in synchronism with the frequency f _SD of the synchronization signal, measures the power of the beat electrical signals.

The light source 1 includes an ASE (Amplified Spontaneous Emissi
on) A light source, or a continuous wave light source obtained by cutting out the output light of the light source with an optical filter of a desired band can be used. Also, a semiconductor laser having a wide line width can be used as the light source 1.

The variable optical delay device 3 enables the relative delay time τ ₁₂ of the first light subjected to backward Brillouin scattering and the second light, which is the other light branched, to be set to zero. is there. The variable optical delay device 3 for fine adjustment can be realized by a combination of a reflection mirror or a prism reflector and a moving optical stage. In addition, the variable optical delay unit 3
As a coarse adjustment type, a type in which a plurality of optical fibers having different lengths are switched by an optical switch can be used.

The photodetector 7 is for obtaining a beat electric signal based on the interference between the backward Brillouin scattered light and the second light. The frequency of the beat electric signal is substantially Brillouin frequency shift ν _b , and when the medium to be measured 5 is a silica-based optical fiber, the frequency is ν _b 1111 GHz, so that the photodetector 7 has a very high frequency. Use a photodiode that can receive signals.

The filter 8 is for selectively passing the beat electric signal, and by making the passing frequency variable, the frequency of the Brillouin spectrum can be measured.

The optical modulator 10 modulates the light intensity or the phase of the first light or the light Brillouin scattered backward to perform synchronous detection of the light Brillouin scattered backward by the first light. If the modulation frequency _fSD is low, an optical intensity modulator using a mechanical chopper or an optical phase modulator in which an optical fiber is wound around a piezoelectric element can be used. When the modulation frequency f _SD is high, an acousto-optic light modulator or a field-effect light modulator using a lithium niobate crystal can be used as the light modulator 10.

As the optical splitter 6, in addition to the optical power splitter, an optical circulator or an optical demultiplexer for separating and outputting lights having different wavelengths can be used.

A spectrum analyzer or the like can be used as a measuring device integrating the filter 8 and the synchronous detection receiver 9 described above.

When the medium 5 to be measured in the embodiment of the present invention is an optical fiber, strain and temperature distribution can be measured as described above. Needless to say, it will not be done. This is the same for various embodiments of the present invention described below.

In the first embodiment of the present invention configured as described above, the output light of the light source 1 is split by the optical splitter 2, and the first light incident on the medium to be measured 5 and the medium to be measured Obtain a second light to interfere with backward Brillouin scattering from 5. A part of the first light is Brillouin scattered backward in the medium 5 to be measured, and the backward Brillouin scattered light is separated from the output light from the light source 1 by the optical splitter 2 and guided to the optical splitter 6. .

On the other hand, the second light is given a predetermined delay amount by the variable optical delay device 3 and is guided to the optical branch device 6. In this way, the rear Brillouin scattered light and the second light that have been input to the optical branching device 6 and merged cause interference, and a beat electric signal due to the interference is obtained by the photodetector 7.

Now, the first light is reflected backward in the medium 5 to be measured.
The position scattered by Lilouin is represented by _0TAnd And
The first light departs from the optical splitter 2 at its position z_0TAt
The light is Brillouin scattered backward and is combined with the second light by the light branching device 6.
Delay time τ from flowing to the photodetector 7₁And the second
Starts from the optical branching device 2 and passes through the optical branching device 6 to the rear side.
It is combined with Brillouin scattered light and reaches the photodetector 7
Delay time τ_TwoRelative delay time τ₁₂= τ₁−τ_TwoBut
The delay amount of the variable optical delay unit 3 was adjusted to be zero.
And At this time, as described above, the beat electric signal is large.
And the spectrum is calculated according to the above equation (13.2).
P_BII As shown by (ζ, z), the position z_0TIn
Gives the Brillouin spectrum. So this bee
The frequency and power of the electrical signal through the filter 8
By measuring with the synchronous detection receiver 9,
Use a spectrum analyzer (not shown)
The position z_0TBrillua in
Spectrum can be measured selectively.

Further, the spatial distribution of the Brillouin spectrum in the medium 5 to be measured is measured by changing the delay amount of the variable optical delay device 3 and repeating the same measurement.
Note that the range in which the large beat electric signal P _BII (, z) is generated is the range of the width Δz _r shown in the above equation (15) with the above z _0T as the center. Therefore, Δz _r gives the spatial (distance) resolution of the Brillouin spectral distribution measurement. For example, when Δν _S = 2 GHz, when the measured medium 5 is quartz glass fibers, because it is ^{v = 2x10 8 m / s,} Δz r
= 1.1 cm.

In the above description, together with P _BII (ζ, z),
P _BI (ζ, z) generated at all positions in the measured medium 5
However, the influence on the measurement of P _BII (ζ, z) was small and ignored. As described above, this assumption is based on the fact that the length of the medium 5 to be measured is relatively short and the detection signal D by P _{B I} (ζ, z)
_I (ζ) and the detection signal D _II (ζ) based on P _BII (ζ, z)
D _I (ν _b ) <D _II (ν _b ) or D _I (ν _b ) to D _II (ν _b )
It is effective when This assumption does not hold, D _I (ν _b )>
The case of> D _II (ν _b ) will be described in the following embodiment.

(Second Embodiment) FIG. 8 shows a second embodiment of the present invention.
1 shows a configuration of a reflection-type Brillouin spectrum distribution measuring apparatus which is an embodiment of the present invention. In the present embodiment, a reference medium 11 is optically connected to one end 5-a of the medium 5 to be measured. If the medium including the reference medium 11 is regarded as the medium 5 to be measured in FIG. 7, the other components in FIG. 8 are the same as the components in FIG. A data processing device 13 processes data (measurement signal) output from the synchronous detection receiver 9 and performs Brillouin spectrum distribution measurement. For example, a general-purpose computer such as a personal computer can be used. it can.

Here, a reference medium 11 whose Brillouin spectrum is known is used. Reference medium 1
Reference numeral 1 denotes a stable medium whose Brillouin spectrum does not change during the measurement time, or a medium placed in a stable environment free from unnecessary stress application and temperature change.

In the second embodiment of the present invention configured as described above, the Brillouin spectrum distribution in the measured medium 5 and the reference medium 11 is measured in the same manner as in the first embodiment. At this time, the measurement signal when the relative delay time τ ₁₂ of the first light subjected to the backward Brillouin scattering at the point in the medium 5 to be measured and the second light which is the other branched light is set to zero. Is defined as D _T (ζ), and the relative delay time τ _12R of the first light that is backward-Brillouin-scattered at a point in the reference medium 11 and the second light that is the other branched light becomes zero. Assuming that the measured signal at this time is D _R (ζ), D _T (ζ) and D _R (ζ) are given by the above equations (16) and (17), as already described. Both measurement signals D _T (ζ) and D _R (ζ) have the beat electric signal power P
Unnecessary measurement signal D _I (ζ) by _BI (ζ, z) is included,
Since they are common, the simple calculation process shown in the above equation (19) is performed by the data processing device 13 to obtain the measurement signal D _II (ζ) to be obtained by the beat electric signal power P _BII (ζ, z). ) Can be separated and extracted.

As can be seen from the above description, this embodiment is particularly effective in the case of D _I (ν _b ) >> D _II (ν _b ).

(Third Embodiment) FIG. 9 shows a third embodiment of the present invention.
1 shows a configuration of a reflection-type Brillouin spectrum distribution measuring apparatus which is an embodiment of the present invention. In the present embodiment, the scattering position of the backward Brillouin scattered light at which the relative delay time with respect to the second light becomes zero is the point light in the measured medium 5 and the point light in the reference medium 11 at the same time. To generate.

For this purpose, in the present embodiment, the optical splitter 2
An optical splitter 14 for splitting a first light, which is one output light from the optical splitter, and one output light of the optical splitter 14 is input and the output light is output while changing the delay amount of the input light. A variable optical delay unit 3-1 having a variable delay amount and light whose delay amount is changed by the variable optical delay unit 3-1 are input, and the input light is modulated by a modulation signal having a frequency f _SD1 and output. An optical modulator 10-1; a variable optical delay unit 3-2 which receives the other output light of the optical splitter 14 as an input, changes the delay amount of the input light, and outputs the variable delay amount; An optical modulator 10-2 that receives light whose delay amount is changed by the delay unit 3-2 as an input, modulates the input light with a modulation signal of a frequency f _SD2 (≠ f _SD1 ), and outputs the modulated light; Modulator 10
-1 and the output light of the optical modulator 10-2 are combined and output again as the first light, and the combined first light is transmitted to the reference medium 11 and the receiving medium. An optical splitter 1 which is an optical means for guiding light to the measurement medium 5 and extracting light Brillouin scattered backward in the medium.
6 as well as other components in FIG. However, the synchronous detection receiver 9 in FIG. 9 has a different modulation frequency (f
_SD1, f _SD2 ) Two types of signals can be detected.

The difference between the configuration in FIG. 8 and the configuration in FIG. 9 is as follows. In FIG. 8, the optical splitter 2 functions to split the output light of the light source 1 into a first light and a second light, and guides the first light to the medium 5 to be measured, and backward in the medium. Although the optical means also serves to take out the Brillouin-scattered light, in FIG. 9, the optical splitter 2 only serves to split the output light of the light source 1 into the first light and the second light. The optical means which guides the first light to the medium 5 to be measured and plays the role of taking out the Brillouin-scattered light backward in the medium is an optical branch provided separately as described above. That is, it is a container 16. With such a configuration, in the present embodiment, different delay amounts are received and different modulation frequencies (f _SD1 and f
_SD2 . However, the first two types identified by f _SD2 ≠ f _SD1 )
Are simultaneously incident on the reference medium 11 and the measured medium 5,
Back Brillouin scattered light from those media can be separated and extracted from the incident first light.

Since the third embodiment of the present invention has such a configuration, a part of the first light modulated at the frequency f _SD1 is used for measuring the medium 5 to be measured. In use, the remaining light of the first light modulated at the frequency f _SD2 can be used simultaneously for the measurement of the reference medium 11. Therefore, the above-described measurement of D _T (ζ) and D _R (ζ) can be simultaneously performed. This gives D _T (ζ) and D _R
It is possible to suppress the occurrence of a measurement error due to the drift of the light source power or the light source frequency due to the difference in the measurement time in (ζ).

(Modifications and Other Embodiments) Here, some supplementary explanations will be given for the description of the embodiments of the present invention thus far.

First, the variable optical delay device 3, the variable optical delay device 3-
1 and the position of the variable optical delay unit 3-2 will be supplementarily described. Since the purpose of these variable optical delay devices is to change the relative delay time of the first light that has been back-Brillouin scattered and the second light that is the other light that has been branched, its position is shown in FIG. 9 in addition to the positions shown in FIG. 9, any position on the optical path from the optical splitter 2 via the optical splitter 6 to the end 5-a of the medium 5 to be measured, or light from the optical splitter 2 Any location on the optical path leading to the splitter 6 or any location on the optical path from the optical splitter 16 to the optical splitter 6 may be used.

The variable optical delay units 3-1 and 3-1 shown in FIG.
One of the variable optical delay unit 3-2 and the reference medium 1
1 is used for the measurement, it is needless to say that when the position of the reference medium 11 is fixed, the delay amount of the optical delay unit for measuring the reference medium 11 does not need to be variable. There is no. Similarly, the delay amount of the variable optical delay unit 3 in FIG. 9 may be fixed.

Next, the positions of the optical modulator 10-1 and the optical modulator 10-2 will be supplementarily described. Since the purpose of these optical modulators is to modulate the first light, their positions are not only the output side of the variable optical delay unit shown in FIG. 9 but also the input side of the variable optical delay unit. May be.

Next, the synchronous detection receiver 9 will be supplementarily described. The purpose of the synchronous detection receiver 9, by modulating the first light incident on the measured medium by using an optical modulator 10 at a modulation frequency f _SD, or generated by modulating a backward Brillouin scattered light directly by synchronously detects the frequency f _SD component of backward Brillouin scattered light, and improve the signal-to-noise power ratio is to measure the power of the backward Brillouin scattered light.

[0096] Thus, from the synchronous detection, although performance is inferior, can be expected improvement of the same signal-to-noise power ratio, an electric signal receiver with a built-in band-pass electrical filter having a center frequency f _SD, synchronous detection receiver 9 may be used instead. In this case, no synchronization signal is required to be input to the electric signal receiver. Further, when a large signal backward Brillouin scattering light is obtained, such and synchronous detection receiver 9, band-pass electrical filter having a center frequency f _SD is unnecessary. In this case, the synchronous detection receiver 9 or
The optical modulator 10 used as a pair with an electric signal receiver having a built-in band-pass electric filter having the center frequency f _SD described above.
Is also unnecessary.

Next, the position of the reference medium 11 will be supplementarily described. The purpose of the use of the reference medium 11 is to measure the totality of the reference medium 11 and the medium to be measured 5 so that the first light that has been back-Brillouin scattered and the second light that is the other light that has been branched off may be used. is to remove the commonly signal generated regardless of the position where the delay time is zero (the aforementioned D _I (ζ)). Therefore, the position may be on the other end 5-b side of the medium 5 to be measured in addition to the positions shown in FIGS. Further, the medium 5 to be measured may be divided, and the medium may be located at an intermediate position of the divided medium.

Next, the use of an optical amplifier (not shown) will be supplementarily described. Since a signal to be measured in the present invention is very weak, it is effective to use an optical amplifier to perform highly accurate measurement. The optical amplifier can be used at any position on the optical path in FIGS.

Next, the Brillouin frequency shift, ν _b = F _c
-Supplementary explanation about f _{c is} given. Until now, as shown in FIG. 4, the present invention has been described as measuring backward Brillouin scattered light down-shifted with respect to the frequency of the first light. In fact, at this time, not only down-shifted backward Brillouin scattered light but also up-shifted backward Brillouin scattered light is generated. However, in any of the backward Brillouin scattered lights, the frequency of the beat electric signal with the second light is the same. Therefore, the above description is equally valid for the up-shifted backward Brillouin scattered light.

Finally, the use of an optical frequency converter (not shown) will be described. As can be seen from the above description, the frequency of the beat electric signal is very high. Therefore, in order to be able to measure this Brillouin spectrum using a low-speed photodiode, the frequency of the output light of the light source 1, the first light, the second light, or the backward Brillouin scattered light is converted, and the beat is converted. It is effective to downshift the frequency of the electric signal to a low frequency. The position of such an optical frequency converter may be any position on the optical path from the light source 1 to the optical splitter 6 in FIGS. Further, as such an optical frequency converter, a field effect type light intensity modulator using lithium niobate crystal or the like, an optical phase modulator, or the like can be used. This is because one or a plurality of sideband lights generated by the modulation of the light intensity modulator or the phase modulator can be regarded as having converted the frequency of the light incident on the light modulator.

An acousto-optic frequency shifter (not shown) for shifting the frequency can also be used. If the shift amount is smaller than the Brillouin frequency shift ν _b of the medium 5 to be measured, the acousto-optic optical frequency shifter, the optical amplifier, the optical branching circuit, etc. are connected in a ring shape, and a large amount of light is A ring optical circuit that can pass through an acousto-optic optical frequency shifter and extract light that has undergone a large frequency shift of about ν _b (Ref. [3] K. Shimizu et
al., “Technique for translating light-wavefrequen
cy by using an optical ring circuit containing af
requency shifter ”, Opt. Lett., vol.17, no.18, pp.
1307-1309, Sept. 1992) can be used.

When an optical frequency converter is used, the frequencies of the beat electric signals of the down-shifted backward Brillouin scattered light and the up-shifted backward Brillouin scattered light differ from each other. It is also possible to measure separately.

[0103]

As described above, according to the present invention,
The incoherent continuous oscillation light having a line width wider than the Brillouin spectrum width of the medium to be measured is branched to obtain a plurality of lights, which are used as a first light and a second light. The light incident from one end of the measurement medium and the Brillouin-scattered light backward in the medium to be measured is combined with the second light,
The combined light is converted into an electric signal by a photodetector, and the frequency and power of the beat electric signal included in the electric signal due to the interference between the backward Brillouin scattered light and the second light are measured. First Brillouin backscattered in a medium
And the relative delay time τ ₁₂ of the backward Brillouin scattered light from the desired scattering position z _0T in the medium to be measured by changing the relative delay time τ _{12 between} To be substantially zero, the beat electric signal relating to the backward Brillouin scattered light from the position z _0T is more efficiently detected than the other positions. Brillouin spectrum distribution can be measured with an excellent spatial resolution of 1 m or less, which is determined by the measured value.

According to the present invention, a reference medium having a known Brillouin spectrum optically connected to a medium to be measured is provided, and a beat electric signal due to backward Brillouin scattered light in the reference medium can be measured. As a result, unnecessary signals common to respective beat electric signals due to backward Brillouin scattered light from the medium to be measured and the reference medium are removed, and the Brillouin spectrum of the medium to be measured can be accurately measured. .

Further, according to the present invention, the crosstalk is less than in the conventional high spatial resolution Brillouin spectrum distribution measurement technique, so that the measurement position selection characteristics are significantly improved and the original Brillouin spectrum of the medium to be measured is deformed. The Brillouin spectrum distribution can be measured with extremely high accuracy without the need for complicated arithmetic processing by a complicated computer required in the related art.

Further, in the present invention, since light is input and output using only one end of the medium to be measured, the efficiency of measurement is lower than that of the prior art which requires input and output of light at both ends of the medium to be measured. As it rises, the restrictions on the object to be measured are greatly relaxed.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a first example of a configuration for obtaining a beat electric signal based on backward Brillouin scattered light, which is a basis of the present invention.

FIG. 2 is a schematic diagram illustrating a second example of a configuration for obtaining a beat electric signal based on backward Brillouin scattered light, which is a basis of the present invention.

FIG. 3 is a schematic diagram illustrating a third example of a configuration for obtaining a beat electric signal based on backward Brillouin scattered light, which is a basis of the present invention.

FIG. 4 is a waveform diagram illustrating an example of spectra of first light, second light, and backward Brillouin scattered light used in the present invention.

FIG. 5 is a waveform diagram showing a relationship between spectra of two types of beat electric signals due to backward Brillouin scattered light measured in the present invention and scattering positions of the backward Brillouin scattered light.

FIG. 6 is a waveform diagram showing a relationship between a Brillouin gain spectrum measured by a conventional high spatial resolution Brillouin spectrum distribution measurement technique and a generation position thereof.

FIG. 7 is a block diagram illustrating a configuration of a reflection-type Brillouin spectrum distribution measuring apparatus according to the first embodiment of the present invention.

FIG. 8 is a block diagram illustrating a configuration of a reflection-type Brillouin spectrum distribution measuring apparatus according to a second embodiment of the present invention.

FIG. 9 is a block diagram showing a configuration of a reflection-type Brillouin spectrum distribution measuring apparatus according to a third embodiment of the present invention.

[Explanation of symbols]

1 light source 2 Optical branching device 3,3-1,3-2 Variable optical delay unit 5 Medium to be measured 6 Optical branching device 7,7-1 Photodetector 8 Filter 9 Synchronous detection receiver 10, 10-1, 10-2 optical modulator 11 Reference medium 13 Data processing device 14, 15, 16 Optical splitter 17 Reflection mirror

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Akika Shimada             2-3-1 Otemachi, Chiyoda-ku, Tokyo Sun             Within the Telegraph and Telephone Corporation (72) Inventor Toshio Oikawa             2-3-1 Otemachi, Chiyoda-ku, Tokyo Sun             Within the Telegraph and Telephone Corporation (72) Inventor Fumi Izumida             2-3-1 Otemachi, Chiyoda-ku, Tokyo Sun             Within the Telegraph and Telephone Corporation F term (reference) 2F103 BA08 BA10 BA37 BA43 CA07                       CA08 EB06 EB12 EC09 EC10                       EC11 EC16                 2G086 CC04 DD04 DD05

Claims (6)

[Claims] 1. The Brillouin spectrum width of a medium under measurement
Incoherent continuous wave light with wide line width
At least the first light and the second light by the light branching means
Branch, The first light is incident from one end a of the medium to be measured, and
The incident first light is located at a position z in the measured medium._0TBack
Is Brillouin scattered and reaches one end a of the medium to be measured again.
The first light that has reached and exited from the measured medium;
Merge with 2 lights, The combined light is detected by the light detecting means and converted into an electric signal.
Converted, Brillouin scattering back into the electrical signal
The video generated by the interference between the first light and the second light
By measuring the power and frequency お よ び of the electrical signal
So, The first light is emitted from the optical branching means and the measured light
Position z in the medium_0TBrillouin scattering backward at the light
Delay time τ to reach the detection means₁And the second
Light departs from the light branching means and reaches the light detection means
Delay time τ_TwoRelative delay time τ₁₂=
τ₁−τ_TwoTo change The relative delay time τ₁₂By making zero nearly zero, other
Position z rather than position _0TBrillouin scattered backward by
The coherence between the scattered light and the second light is increased, and the beat
By selectively measuring the electrical signal, the position z_0T
Measuring the Brillouin spectrum at
Reflective Brillouin spectrum distribution measurement method. 2. The Brillouin spectrum width of the measured medium
Incoherent continuous wave light with wide line width
At least the first light and the second light by the light branching means
Branch, The first light is incident from one end a of the medium to be measured, and
The emitted first light is located at a position z in the measured medium._0TBack
Brillouin scattering occurs again at one end a of the medium to be measured.
A first light which reaches and is emitted from the medium to be measured;
Merge with 2 lights, The combined light is detected by the light detecting means and converted into an electric signal.
Converted, Brillouin scattering back into the electrical signal
The video generated by the interference between the first light and the second light
By measuring the power and frequency お よ び of the electrical signal
So, The first light starts from the light branching means, and
Position z in the medium_0TBrillouin scattering backward at the light
Delay time τ to reach the detection means₁And the second
Light departs from the light splitting means and reaches the light detecting means
Delay time τ_TwoRelative delay time τ₁₂=
τ₁−τ_TwoTo change The relative delay time τ₁₂By making zero nearly zero, other
Position z rather than position _0TBrillouin scattered backward by
The coherence between the scattered light and the second light is increased, and the beat
Brills obtained by selectively measuring electrical signals
Unspectrum D_T(ζ) is measured, Further, the first light split by the light splitting means is forward.
The reference medium that is optically connected to or in contact with the medium to be measured
And the first light is emitted from the light splitting means and
Position z in the reference medium_0RBrillouin scattering backward at
Delay time τ until the light reaches the light detecting means_1RAnd the said
A second light starting from the light branching means and the light detecting means
Τ until it reaches_2RRelative delay, which is the difference from
Interval τ_12R= τ_1R−τ_2RBy making zero nearly zero, other
Position z in the reference medium rather than position_0RBackscattered by
The coherence between the Brillouin scattered light and the second light.
By selectively measuring the beat electrical signals of both lights.
Brillouin spectrum D obtained_R(ζ) is measured, And the Brillouin spectrum D_T(ζ) and the Brillouin
Spectrum D_RUsing the relationship (ζ), the position z_0TIn
To obtain a corrected Brillouin spectrum
A reflection type Brillouin spectrum distribution measuring method. 3. A light source for outputting an incoherent continuous oscillation light having a line width wider than the Brillouin spectrum width of a medium to be measured, and branching the continuous oscillation light into at least a first light and a second light. A light branching means for outputting; a first optical means for causing the first light to enter the medium to be measured; and a second optical means for performing Brillouin scattering backward in the medium to be measured and outputting from the medium to be measured. Second optical means for extracting at least a part of the first light; light for merging the second light extracted by the second optical means and backwardly Brillouin-scattered; and the second light Converging means; light detecting means for converting the converging light merged by the light converging means into an electric signal; interference between the backward Brillouin-scattered first light and the second light using the electric signal as an input signal; Beat caused by A filter for passing the electric signal; a measuring means for measuring the power and frequency ζ of the beat electric signal passing through the filter; and the first light starting from the optical branching means, and Brillouin scattering backward at the position z _0T , and a delay time τ ₁ from when the light reaches the light detecting means,
Variable light delay means for changing a relative delay time τ ₁₂ = τ ₁ −τ ₂ which is a difference from the delay time τ ₂ until the light of the light departs from the light branch means and reaches the light detection means. A reflection type Brillouin spectrum, wherein the position z _{0T at} which the relative delay time τ ₁₂ becomes substantially zero is changed by the variable optical delay means, and the Brillouin spectrum distribution of the medium to be measured is measured. Distribution measuring device. 4. A light source for outputting an incoherent continuous oscillation light having a line width wider than the Brillouin spectrum width of the medium to be measured, and branching the continuous oscillation light into at least a first light and a second light. An output light branching unit, a reference medium optically connected to or in contact with the measured medium, and a first optical means for causing the first light to be incident on the measured medium and on the reference medium. Second optical means for extracting at least a part of the first light Brillouin scattered backward in the medium to be measured and the reference medium; and Brillouin scattered light scattered backward in the medium to be measured. Light converging means for converging the Brillouin scattered light scattered backward by the reference medium with the second light; light detecting means for converting the converged light converged by the light converging means into an electric signal; An electric signal is input, and a beat electric signal generated by interference between the first light that has been Brillouin scattered backward in the medium to be measured and the reference medium and the second light passes therethrough and is output. A filter, measuring means for measuring the power and frequency ビ ー ト of the beat electric signal passing through the filter, and the first light starting from the light branching means and rearward at a position z _0T in the medium to be measured, Brillouin. scattered, relative delay time tau ₁ and the second light to reach the light detecting means is a difference between the delay time tau ₂ to reach the light detecting means, starting from said light branching means Delay time τ ₁₂ = τ
₁₋ τ _2, and a delay time until the first light is Brillouin scattered backward from the light branching means at a position z _0R in the reference medium and reaches the light detecting means. tau _1R and the second light is adjusted relative delay time τ _12R = τ _1R -τ _2R is the difference between the delay time tau _2R to reach the light detecting means, starting from said light branching means, At least one of which is a variable optical delay means, one or more optical delay means, and the variable optical delay means makes the relative delay time τ ₁₂ relating to backward Brillouin scattered light from within the measured medium substantially zero. By changing the position z _0T, and by making the relative delay time τ _12R related to the backward Brillouin scattered light from within the reference medium by the optical delay means substantially zero at the position z _0R , position z _0T and the reference medium In the position z _0R in the Brillouin spectrum when each of the relative delay time tau ₁₂ and tau _12R substantially zero is measured, respectively D _T of the measured the Brillouin spectrum (zeta) and D _R (zeta) Then, by utilizing the relationship between D _T (ζ) and D _R (ζ), the position z _0T
And a data processing means for obtaining a corrected Brillouin spectrum. 5. A second light splitting means for splitting the first light which is one output light from the light splitting means, and one output light of the second light splitting means as an input. A second variable optical delay means for changing the delay amount of the input light and outputting the same, and the light having the delay amount changed from the second variable optical delay means is input as the input. A first light modulating means for modulating light with a modulation signal of a frequency f _SD1 and outputting the light, and the other output light of the second light branching means as an input and changing the delay amount of the inputted light to output A third variable optical delay means having a variable delay amount, and a light having a delay amount changed by the third variable optical delay means as an input and modulating the input light with a frequency f _SD2 (≠ f _SD1 ) signal. Second light modulating means for modulating and outputting the light, and output light of the first light modulating means and the second light modulating means. Second output as an output light and a first light again joins the light modulation means
A light converging means for guiding the first light converged by the second light converging means to the reference medium and the medium to be measured, and extracting light Brillouin scattered backward in the medium. A third optical branching unit that is an optical unit, wherein the measuring unit includes a synchronous detection receiver that measures the power of the beat electric signal in synchronization with the synchronous signals of the frequencies f _{SD1 and} f _SD2. 5. The reflection-type Brillouin spectrum distribution measuring device according to claim 4, wherein: 6. further comprising a light modulating means for modulating the modulation signal of the first optical frequency f _SD, the said measuring means, said beat electrical signals in synchronism with the frequency f _SD of the synchronization signal The reflection-type Brillouin spectrum distribution measuring apparatus according to claim 3, further comprising a synchronous detection receiver for measuring power.