JP2769185B2 - Backscattered light measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention detects a weak backward Rayleigh scattered light by generating a Brillouin light amplification state in a specific optical frequency region in an optical fiber. And a backscattered light measuring device.

(Prior Art) As a method for detecting a light loss distribution in a single-mode optical fiber transmission line or a break point in an optical fiber, light from a light source is incident on an optical fiber to be measured, and a rear Rayleigh generated in the optical fiber is generated. There are methods to detect scattered light (for example,
MKBarnoski, etc., “optical timedomain reflectom
eter ", Appl. Opt., vol. 16 (1977) pp. 2395-2379). In this method, the accuracy of determining the break point position depends on the light pulse width emitted from the light source. When the width is reduced, under the condition that the peak value of the light pulse is constant, the intensity of the backscattered light that can be received decreases in proportion to the light pulse width, and the light receiving level decreases, deteriorating the S / N,
Good signal detection becomes difficult.

Therefore, by setting the optical fiber to be measured to a state in which Brillouin light amplification is possible, the backward Rayleigh scattered light component, which was determined only by the optical loss, has an amplification component to improve the light reception level and achieve high resolution. A method for performing the measurement has been proposed.

FIG. 2 is a block diagram showing a first conventional example of a backscattered light measuring device to which this method is applied. In Figure 2, 1 is a first laser light source, backward Rayleigh scattering light probe light for generating (pulsed light) emitting a P _r. 2 is a second laser light source emits excitation light (CW light) P _m for the Brillouin light amplification status in the measured optical fiber FB. Numeral 3 is an optical frequency adjusting device such that the optical frequency difference of each emitted light from the first and second laser light sources 1 and 2 coincides with the optical frequency shift amount of Brillouin light generated in the optical fiber FB to be measured. adjust. Reference numerals 4 and 5 denote optical isolators, 6 denotes a multiplexer, and the first and second laser light sources 1 and 2 that have passed through the optical isolators 4 and 5
2 multiplexes the probe light P _r and the pumping light P _m by. Numeral 7 denotes an optical coupler formed by an optical coupler or the like. The multiplexed light from the multiplexer 6 is made incident on the optical fiber FB to be measured, and the backward Rayleigh scattered light Brillouin-amplified in the optical fiber FB to be measured is detected by a photodetector Incident on Reference numeral 9 denotes a signal processing device which inputs an electric signal corresponding to the backward Rayleigh scattered light received and detected by the photodetector 8, and calculates the intensity of the backward Rayleigh scattered light based on the input signal.

In such a configuration, each other of the optical frequency difference by the optical frequency adjusting device 3 is adjusted to match the optical frequency shift of the Brillouin light, the probe light P _r of the first and second by the laser light source 1 and 2 and the excitation light P _m through the optical isolator 4 and 5 is incident to the multiplexer 6, are combined. The multiplexed light passes through the optical coupler 7 and enters the optical fiber FB to be measured. In conjunction with this, the measured optical fiber FB is subjected to the action of the pumping light P _m, the Brillouin light amplification status. As a result, the backward Rayleigh scattered light generated in the measured optical fiber FB is amplified, and the measured optical fiber FB is amplified.
The light exits from the multiplexed light incident end of the FB. The rear Rayleigh scattered light is received by the photodetector 8 via the optical coupler 7 and converted into an electric signal, and the signal processing device 9 calculates the intensity of the rear Rayleigh scattered light. In this way, in the apparatus shown in FIG. 2, the S / N was improved and good signal detection was performed.

FIG. 3 is a configuration diagram showing a second conventional example of the backscattered light measuring device. In the second conventional example, the probe light _Pr from the first laser light source is made incident from one end face of the optical fiber FB to be measured to generate backward Rayleigh scattered light having a Brillouin-shifted frequency. the frequency adjusting unit 3, the optical frequency and the pump light P _m by the second laser light source 2 is matched, by Brillouin light amplification backward Rayleigh scattering light is made incident from the other end face of the optical fiber to be measured FB, backward Rayleigh The scattered light was detected.

(Problems to be Solved by the Invention) However, in the former device, two laser light sources 1 and 2 for generating backward Rayleigh scattered light and for amplifying Brillouin light are used.
It requires, and multiplexer 6 for multiplexing the probe light P _r and the pumping light P _m by these laser sources 1 and 2 are indispensable.

Therefore, in order to set the laser oscillation frequency, a complicated device for adjusting the optical frequency by monitoring each oscillation light is required, and excessive loss and reflected light due to the multiplexer 6 are generated, and high resolution is required. However, there has been a problem that the break point cannot be detected and the optical loss measurement cannot be performed with high accuracy. Further, an optical isolator is required for each laser light source in order to suppress the return light to the laser light sources 1 and 2, and there is a disadvantage that the number of components is increased.

In the latter device, since the probe light _Pr and the excitation light _Pm are made to be incident on both end faces of the optical fiber FB to be measured, for example, the characteristics of an optical fiber cable laid on a pipeline or on the seabed are required. To evaluate, you need to be far away (for example, 100k
m) The disadvantage is that measurements must be taken simultaneously from both ends. Further, when the optical fiber FB to be measured is broken in the middle, it is impossible to perform Brillouin optical amplification in principle, and there is a disadvantage that applicable measurement is limited.

The present invention has been made in view of such circumstances,
The purpose is to generate backward Rayleigh scattered light and induce the Brillouin light amplification state of the optical fiber by the light pulse from one light source, and high-resolution break inspection without causing S / N deterioration of the detected backward Rayleigh scattered light. It is an object of the present invention to provide a backscattered light measuring device capable of performing high-precision light loss measurement.

(Means for Solving the Problems) In order to achieve the above object, in claim (1), an optical pulse for generating backward Rayleigh scattered light in an optical fiber to be measured and a Brillouin light in the optical fiber to be measured are described. A light source that generates an excitation light pulse for inducing an amplification state, and an optical frequency difference between the light pulse and the excitation light pulse generated by the light source is an optical frequency shift of the Brillouin light generated in the measured optical fiber. Light pulse adjusting means for adjusting the amount of light emitted from the optical fiber to be measured, and the backward Rayleigh scattered light generated in the optical fiber to be measured and emitted from the optical fiber to be measured, in a direction different from the incident direction of the emitted light pulse by the light source. Light separating means, a detecting means for detecting the intensity of the rear Rayleigh scattered light separated by the light separating means, and a detecting signal based on the detection signal by the detecting means. Signal processing means for calculating the intensity of the backward Rayleigh scattered light.

Further, in claim (2), there is provided an optical amplifying means for amplifying a light pulse emitted from the light source and causing the amplified light pulse to enter the measured optical fiber.

In claim (3), an optical amplifier for amplifying the rear Rayleigh scattered light is arranged between the optical separator and the detector.

(Operation) According to claim (1), the light pulse generated by the light source and the excitation light pulse are changed by the light pulse adjusting means so that the optical frequency difference between the light pulse and the excitation light pulse is changed by the light of the Brillouin light generated in the optical fiber to be measured. The light is adjusted and output so as to match the frequency shift amount. The light pulse and the excitation light pulse emitted from these light sources are incident on one end face of the optical fiber to be measured. Along with this, the optical pulse propagates through the optical fiber to be measured,
Back Rayleigh scattered light is generated.

At substantially the same time, the measured optical fiber is induced into a Brillouin light amplification state by the excitation light pulse. Thereby, the backward Rayleigh scattered light generated in the measured optical fiber is subjected to Brillouin light amplification in the measured optical fiber, and is emitted from one end surface of the measured optical fiber.

The rear Rayleigh scattered light emitted from the optical fiber to be measured is separated by the light separating means in a direction different from the incident direction of each emitted light pulse by the light source, and is incident on the detecting means. Thereby, the intensity of the backward Rayleigh scattered light is detected by the detection means, and the detection signal is input to the signal processing means. The signal processing means calculates the intensity of the backward Rayleigh scattered light based on the input detection signal.

Further, according to claim (2), the light pulse and the excitation light pulse adjusted by the light pulse adjusting means and emitted from the light source are amplified by the light amplifying means and then incident on the optical fiber to be measured. .

According to claim (3), the backward Rayleigh scattering generated in the optical fiber to be measured, subjected to the Brillouin light amplifying action, emitted to the optical fiber to be measured and separated by the light separating means as described above. After the light is amplified by the light amplifying means, it is incident on the detecting means.

(Embodiment) FIG. 1 is a block diagram showing a first embodiment of the backscattered light measuring device according to the present invention, and the same components as those of FIG. 2 showing the conventional example are denoted by the same reference numerals. That is, FB is an optical fiber to be measured, 1a is a laser light source, for example, a semiconductor laser (for example, DF
B-LD) made were based on injection current is adjusted by the later-described optical pulse adjuster 3a, the light pulse P _R and the measured optical fiber FB for generating the backward Rayleigh scattered light in the measurement optical fiber FB generating an excitation light pulse P _M for inducing Brillouin light amplification status in. 3a is an optical pulse adjuster, the optical frequency difference between the optical pulse P _R and the excitation light pulse P _M generated by the laser light source 1a is measured optical fiber FB
Adjust the current value of the pulse applied to the laser light source 1a to match the optical frequency shift amount of the Brillouin light generated in the pulse pattern generated by the laser light source 1a,
Adjust the pulse light frequency, pulse repetition conditions, etc.

4 is an optical isolator, 7 is an optical coupler (optical separation means)
Then, the optical pulse (P _R + P _M ) from the laser light source 1a is made incident on the optical fiber FB to be measured, and the optical fiber FB
In which the backward Rayleigh scattered light amplified by Brillouin light is separated. Reference numeral 8 denotes a photodetector, which receives the backward Rayleigh scattered light separated by the optical coupler 7 and converts the scattered light into a detection signal DT which is an electric signal corresponding to the intensity thereof. Reference numeral 9 denotes a signal processing device that calculates the intensity of backward Rayleigh scattered light based on the detection signal DT from the photodetector 8. The light pulse adjusting device 3a operates in synchronization with the signal processing device 9.

Next, the operation of the above configuration will be described step by step based on the light pulse pattern diagrams of FIGS.

First, the optical pulse adjustment device 3a, so as to emit light pulse pattern having a specific optical frequency for every optical pulse of the optical pulse P _R and the excitation light pulse P _M by the laser light source 1a,
Modulation is performed. For example, as shown in FIG. 4, the optical pulse width optical pulse P _R to the excitation light pulse P _M W1, W2,
It is generated and emitted with optical frequencies ν ₁ and ν ₂ .

The light pulse P _R and the excitation light pulse P _M thus set
Is incident on the measured optical fiber FB via the optical isolator 4 and the optical coupler 7. In conjunction with this, the light pulse P _R is propagated through the optical fiber to be measured FB, it generates backward Rayleigh scattering light. Backward Rayleigh scattering light generated is emitted from the optical fiber to be measured FB undergoing Brillouin light amplification effect which will be described later, by the optical coupler 7, the light pulse P _R and the excitation light pulses
Is separated in a direction different from the incident direction of the P _M, it is emitted to the photodetector 8.

At this time, the power P of the backward Rayleigh scattered light detected
_BS is: It is expressed as However, P1 is the peak value of the light pulse P _R, Vq is the group velocity of light in the measured optical fiber FB, alpha _R is Rayleigh scattering coefficient, S is the proportion of the amount of light that is scattered backward of the total Rayleigh scattering amount , Α indicate the loss of the measured optical fiber FB, and Z indicates the distance from the incident end of the measured optical fiber FB to the position where the rear Rayleigh scattering occurs.

On the other hand, when laser light that oscillates in a single longitudinal mode and has a narrow oscillation spectrum line width is incident, backward Brillouin amplified light is easily generated with incident excitation light power of several mW. In that case the gain G by the Brillouin light amplification is expressed by the following equation _{G = exp (gZ e P pump} / A eff) ... (2). Here, g is the Brillouin gain coefficient, Ze is the effective length for the guided mode, and is expressed by the following equation: Ze = (1−exp−α2) / α (3) Further, backward Brillouin scattering occurs in a region where the frequency is shifted to a lower frequency side with respect to the excitation light frequency. When the Brillouin shift amount is ν _B, wavelength λ =
Ν _B = 21,13,11GHz for each wavelength of 0.8,1.3,1.65μm
It is.

Now, Brillouin light amplification gain G by the excitation light pulse P _M is generally, W1, W2 and the width of the light pulse P _R each excitation light pulse P _M, as shown in the following equation, It is expressed as However, P2 represents the peak value of the excitation light pulse P _M.

Here, the optical frequency of the light pulse P _R and the excitation light pulse P _M [nu ₁
Setting and [nu ₂ the ν ₂ -ν _{1 =} ν _B become as, backward Rayleigh scattering light of the light pulse P _R is, will be Brillouin light amplification in the measured optical fiber FB by the excitation light pulse P _R . The Brillouin-amplified backward Rayleigh scattered light pulse P _{BS, Br} is generally expressed by the following equation: It is represented by

For example, the Brillouin shift amount ν _B has a wavelength λ = 1.55 μm.
Since about 11GHz respect, setting the optical frequency of the light pulse P _R in the 11GHz lower frequency than that of the excitation light pulse P _M.
Brillouin gain amplification Δν _B is usually about 100 MHz (N. Shib
ata, et al. Opt. Lett. vol. 13, p. 595, 1988)
In order to perform Brillouin optical amplification, it is necessary to set the optical frequency with an accuracy within 100 MHz.

As the laser light source 1a, a semiconductor laser having a narrow spectral line width oscillating in a single mode as described above (for example, DFB-L
D), it is possible to easily generate an optical pulse shifted by an optical frequency of 11 GHz for each optical pulse by setting the current difference between the pulses applied to the laser to about 11 mA from the FM modulation characteristics of the laser.

As described above, the rear Rayleigh scattered light generated in the measured optical fiber FB and subjected to the Brillouin light amplification action is received by the photodetector 8 and converted into a detection signal DT which is an electric signal corresponding to the intensity. Is done. The detection signal DT is input to the signal processing device 9, where the intensity of the backward Rayleigh scattered light is calculated.

Next, detection of a break point position will be described. When the length of the measured optical fiber FB and L, the light pulse P backward Rayleigh scattered light _R is, the width W2 of the excitation light pulse P _M from the time to return to the entrance end of the length L is shown by the following formula Thus, W2 ≧ znL / C (6) can be expressed. Where n is the refractive index of the optical fiber,
C indicates the speed of light. For example, for a measured optical fiber FB of 300 km, W2 ≧ 3 × 10 ⁻³ (S), and for a measured optical fiber FB of 500 km, W1 ≧ 5 × 10 ⁻³ (S). Usually, the light pulse P _R backward Rayleigh scattering light occurs, the optical pulse width is approximately W1 = 4 .mu.s. Therefore, the light pulse P _R and the excitation light pulse P _M emitted from the laser light source 1a
Is W1 = 4 μs, W2 = 3 ms, (L = 300 km), W1 = 4 μs, W2 =
A fixed pattern of 5 ms (L = 500 km) is repeated.
At this time, the repetition frequencies correspond to about 300 Hz and 200 Hz, respectively. Thus, by both optical pulse of the optical frequency [nu ₁ and [nu ₂ to set the pulse interval, to measure the position of the break by the waveform of backward Rayleigh scattering light of the light pulse P _R which is Brillouin light amplification Can be.

At this time, backward Rayleigh scattering light of the light pulse P _R from the time when the excitation light pulse P _M is incident on the measured optical fiber FB, to the far end of the backward Rayleigh scattering light of the optical pulse P _R returns to end the incident, Since the Brillouin light is always amplified, the Brillouin-amplified rear Rayleigh scattered light power P
_{BS, Br} can be represented by the following equation.

This means that the measured optical fiber FB is excited to the Brillouin light amplification state with CW light, and that there is no deterioration due to the use of the optical pulse (P _M ) for the Brillouin light amplification. Means

Next, measurement of the optical loss value α will be described. In the configuration of Figure 1, the use of part of the pulse pattern obtained by shifting the optical frequency of the pumping light pulse P _M, the portion of frequency shift backward Rayleigh scattering light of the light pulse P _R is not subjected to the Brillouin light amplification. Therefore, optical frequency [nu ₂ the excitation light pulse P _M as shown in FIG. 5, when generating a pulse pattern obtained by dividing into small pulse P _M2, P _M3 of [nu ₃ example alternately backward Rayleigh which is Brillouin light amplification Scattered light power P _{BS, Br}
Is It is represented by Here, the excitation light pulse P _M2 of the optical frequency ν ₂
Is the total pulse width of W2 'and the excitation light pulse P _M3 of the optical frequency ν ₃
Is W3, and it is assumed that there is a relationship of W2 = W2 '+ W3. From equation (8), the backward Rayleigh scattered light power P _{BS, Br}
It can be seen that the waveform of does not simply show exponential decay.

Then, the argument φ of the exponential function of Expression (8) is Becomes For two different X1 and X2, assuming that the longitudinal positions of the measured optical fiber FB satisfying the extreme value condition for the distance Z of the argument φ are Z1 and Z2, respectively, the optical loss α is as follows. like, It is expressed as

Therefore, by obtaining the positions Z1 and Z2 that give the extreme values from the backward Rayleigh scattered light waveform obtained from the equation (10),
Ratio X obtained by shifting a portion of the optical frequency of the pumping light pulse P _M
The light loss value α can be evaluated using 1, X2. At this time, since the backward Rayleigh scattering light of the optical pulse P _R from being amplified, the optical frequency [nu ₃ of the excitation light pulse P _M3 must be shifted or ± 100 MHz by the optical frequency P2 of the pumping light pulse P _M2. It is also effective to use a pattern in which the light output is zero in the portion of the light pulse width W3. Thus, in this configuration, the excitation light pulse P _M, further small pulse P
_The optical loss can be measured by using a pulse pattern decomposed into _M2 and _PM3 .

In addition to this evaluation method, it is also possible to calculate the optical loss value α by performing a least-squares approximation of the obtained waveform using Expression (7).

Further, by generating an optical pulse pattern having a code suitable for correlation processing such as an M-sequence of the optical frequency ν ₁ , ν ₂ (ν ₂ −ν ₁ = ν _B ) from the laser light source by the optical pulse adjusting device 3a, Pulse pattern of Brillouin-amplified backward Rayleigh scattered light in optical fiber and laser light source 1a
The backscattering amount can be obtained by delaying a bit of the pulse pattern applied to the reference signal as a reference code in the signal processing device 9 and measuring the correlation value at each distance. From the distribution of the backward Rayleigh scattered light, the position of the break point and the loss value of the optical fiber can be measured.

Next, measurement of the characteristics of the optical fiber when the optical fibers are cascaded in multiple stages will be described. In optical fibers, the structure of an optical fiber, a material or the like of the core shape and the distribution of the Brillouin shift value [nu _B and Brillouin gain values .DELTA..nu _B differ. Furthermore, these values differ depending on the manufacturing conditions even for fibers having the same configuration parameters (N.shibata et al.
al. OPTICS LETTER, pp. 595-597, 1988.). In particular, S
In a silica-based optical fiber using iO ₂ and a core material, a Brillouin gain is one, but in a dispersion-shifted fiber using GeO ₂ —SiO ₂ as a core material, a plurality of acoustic wave modes guided in the core and HE11 are used. A plurality of gain peaks occur due to the interaction with the mode (for example, about three).

Accordingly, these optical fibers, the transmission line which connects the elongated changes the set frequency of the light pulse P _R and the excitation light pulse P _M in the configuration of FIG. 1, to adjust the optical frequency difference ([nu _B) By setting only the desired optical fiber to the Brillouin light amplification state and detecting the rear Rayleigh scattered light there, not only the type and structure of the optical fiber can be determined, but also the breaking point and loss value in the desired optical fiber can be determined. There is a great advantage that it can be measured.

FIG. 6 is a characteristic diagram showing a relationship between a back Rayleigh scattered light waveform obtained by the back scattered light measuring device according to the present invention and a distance from an optical fiber incidence end, and as an example,
Wavelength 1.55μm, optical loss α = 0.18dB / km, optical pulse width W1 =
A value obtained by calculating the backward Rayleigh scattered light waveform with respect to the parameter X when 4 μs and W2 = 3 ms is shown. Here, X = 1 is a Brillouin-amplified backward Rayleigh scattered light waveform corresponding to the excitation light pulse input P _M = 10 mW,
Hereinafter, a waveform corresponding to when each changing the ratio W ₂ of the optical frequency [nu ₂ of the optical pulse width W2 '. X = 0
Is a backward Rayleigh scattered light waveform not due to Brillouin light amplification.

In the present invention, since the Brillouin light amplification effect is used, as can be seen from FIG. 6, a simple exponential decay with respect to the distance Z is not exhibited. For example, the distance difference for the local maximum in the backward Rayleigh scattered light waveform for X = 0.8 and 0.6 is Z1-Z2 = 0.08km. This value is calculated by equation (10)
To the optical loss value α of the optical fiber.
It can be evaluated as 0.18dB / km.

FIG. 7 is a block diagram showing a second embodiment of the backscattered light measuring device according to the present invention. The second embodiment and the first embodiment
The different point of the embodiment is that the optical isolator 4 and the optical coupler 7
The optical amplifiers 10 and 11 made of a semiconductor laser or an optical fiber are arranged between the optical coupler 7 and the photodetector 8, respectively.

In the second embodiment, by adopting such a configuration, in addition to the effects of the first embodiment, light emitted from the laser light source 1a pulse P _R, P _M or backward Rayleigh scattering light,
Alternatively, by amplifying them by the optical amplifiers 10 and 11 at the same time, the backward Rayleigh scattered light incident on the photodetector 8 is amplified, and furthermore, high-resolution break point detection and high-precision light in a long optical fiber transmission line are performed. There is an effect that loss measurement can be performed.

(Effect of the Invention) As described above, according to claim (1), an optical pulse for generating backward Rayleigh scattered light in an optical fiber to be measured and a Brillouin light amplification state in the optical fiber to be measured are described. A light source that generates an excitation light pulse for inducing, and an optical frequency difference between the light pulse generated by the light source and the excitation light pulse coincides with an optical frequency shift amount of the Brillouin light generated in the measured optical fiber. An optical pulse adjusting means for adjusting so that the rear Rayleigh scattered light generated in the measured optical fiber and emitted from the measured optical fiber is separated in a direction different from the incident direction of the emitted light pulse by the light source. Light separating means, detecting means for detecting the intensity of the rear Rayleigh scattered light separated by the light separating means, and a rear Rayleigh based on a detection signal from the detecting means. Since the signal processing means for calculating the intensity of the scattered light is provided, the rear Rayleigh scattered light can be detected with high resolution and high sensitivity without causing S / N deterioration.

In addition, by adjusting each pulse pattern of the light pulse generated by the light source and the excitation light pulse to a desired pattern by the light pulse adjusting means, measurement of the light loss distribution of the optical fiber transmission line, and detection of the position of a break point or a connection point. Further, there is an advantage that the type and structure of the optical fiber can be obtained, and the position of the break point can be detected with high accuracy.

Further, according to claim (2), after the output light pulse from the light source is amplified by the optical amplifier, it can be made to enter the optical fiber to be measured, so that in addition to the effect of claim (1), High-resolution break point detection and high-precision optical loss measurement in a long-distance optical fiber transmission line can be performed.

Further, according to claim (3), after the rear Rayleigh scattered light is amplified by the optical amplifier, it can be detected by the detector, so that the effect of claim (1) or (2) can be obtained. Accordingly, it is possible to perform higher-resolution break point detection and higher-precision light loss measurement.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a backscattered light measuring device according to the present invention, FIG. 2 is a block diagram showing a first conventional example of the backscattered light measuring device, and FIG. FIG. 4 and FIG. 5 are light pulse pattern diagrams according to the present invention, and FIG. 6 is a backward Rayleigh scattered light waveform obtained by the back scattered light measuring device according to the present invention. FIG. 7 is a characteristic diagram showing a relationship with respect to the distance from the optical fiber entrance end, and FIG. 7 is a configuration diagram showing a second embodiment of the backscattered light measuring device according to the present invention. In the figure, 1a: laser light source, 3a: light pulse adjusting device, 4
…… Optical isolator, 7 …… Optical coupler (light separating means)
8 photodetector (detection means) 9 signal processing device 1
0,11 …… Optical amplifier.

Continuation of the front page (72) Inventor Nobu Shibata 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (58) Field surveyed (Int.Cl. ⁶ , DB name) G01M 11/00 G01M 11 / 02

Claims (3)

(57) [Claims] A light source for generating an optical pulse for generating backward Rayleigh scattered light in an optical fiber to be measured and an excitation light pulse for inducing a Brillouin light amplification state in the optical fiber to be measured; An optical pulse adjusting means for adjusting an optical frequency difference between an optical pulse generated by a light source and an excitation light pulse so as to coincide with an optical frequency shift amount of Brillouin light generated in the optical fiber to be measured; and Light separating means for separating the rear Rayleigh scattered light generated in the fiber and emitted from the optical fiber to be measured in a direction different from the incident direction of the emitted light pulse from the light source; and a rear Rayleigh separated by the light separating means. Detecting means for detecting the intensity of the scattered light; and signal processing means for calculating the intensity of the rear Rayleigh scattered light based on a detection signal from the detecting means. A backscattered light measuring device, characterized in that: 2. The backscattered light measuring device according to claim 1, further comprising an optical amplifier for amplifying an emitted light pulse from the light source and causing the amplified light pulse to enter the measured optical fiber. 3. The backscattered light measuring device according to claim 1, wherein a light amplifying means for amplifying the backward Rayleigh scattered light is arranged between the light separating means and the detecting means.