JP3152314B2 - Method and apparatus for measuring backscattered light - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring backscattered light output from an optical fiber to be measured, in particular, backscattered light for measuring back Rayleigh scattered light and back Brillouin scattered light.

[0002]

2. Description of the Related Art An optical pulse tester or an optical Ti
The me Domain Reflectometer (OTDR) is widely used for searching for a fault position in an optical fiber or measuring an optical loss distribution. This OTDR measures the power of backscattered light generated at scattering points in an optical fiber by injecting pulsed light into the optical fiber to be measured as a function of time corresponding to the distance between the incident point and each scattering point. It is. Therefore,
By measuring the temporal change of the backscattered light using the OTDR, it is possible to search for a fault point of the measured optical fiber or measure the loss distribution.

There are three main types of backscattered light generated in an optical fiber to be measured: rear Rayleigh scattered light, rear Brillouin scattered light, and rear Raman scattered light. The power of the backward Brillouin scattered light and the backward Raman scattered light is lower than the power of the backward Rayleigh scattered light by two to three orders of magnitude.
Normally, OTDR measures backward Rayleigh scattered light.
However, the power of the rear Rayleigh scattered light itself is about five orders of magnitude lower than the peak power of the light pulse incident on the optical fiber to be measured. Therefore, OTDR requires an extremely high-sensitivity light detection technique.

At present, the most sensitive light detection method is coherent detection, and its effectiveness in OTDR has already been demonstrated. In coherent detection, a coherency, that is, a light source having excellent coherence is used, the backscattered light that is the signal light and the local light are combined, and the power of the beat light that is the interference light between the two is measured. Since the power of the beat light increases in proportion to the power of the local oscillation light, coherent detection is performed up to the weak light power equivalent in power to so-called shot noise due to the quantum fluctuation inherent in the light. Can be measured by

However, it is known that when coherent detection is used for OTDR, a new problem of fading noise occurs. This fading noise is
It is caused by the interference between the light waves backscattered by a large number of distributed scatterers, and the phase relationship between the backscattered light waves randomly changes depending on the position in the length direction of the measured optical fiber. The power of the measured backscattered light also changes with time according to the scattering position. The fluctuation of the power of the backscattered light greatly deteriorates the accuracy of searching for a fault position and measuring the loss distribution of the measured optical fiber.

Therefore, it is said that it is effective to use a semiconductor laser that feeds back the Rayleigh scattered light output from the optical fiber to be measured as a light source against the fading noise. In this case, the semiconductor laser has its oscillation spectrum width narrowed to 100 kHz or less and becomes a light source suitable for coherent detection, and the oscillation frequency of the semiconductor laser is kept constant for a certain time, and then the other frequency In other words, so-called frequency hopping occurs. When the oscillation frequency of the semiconductor laser changes, the phase relationship between the backscattered light waves also changes. Therefore, by measuring the backscattered light for a number of different frequencies and averaging them, the difference in the phase relationship between the light waves depending on the scattering position, that is, the fading noise can be reduced. For details of this type of technology, see, for example, P.
"Development of a Coherent OTDR" by King et al.
Instrument "(J. Lightwave Technol. Vol.LT-5 no.4 pp.
616-624 1987).

By the way, in addition to the above-mentioned OTDR for measuring backward Rayleigh scattered light, an OTDR for measuring backward Brillouin scattered light (hereinafter referred to as BOTDR, and the above-described OTDR for measuring backward Rayleigh scattered light is simply an OTDR.
TDR) has been proposed. The center frequency of the above-mentioned backward Rayleigh scattered light is the same as the frequency of the light incident on the optical fiber to be measured, but the center frequency of the backward Brillouin scattered light is shifted from the frequency of the incident light, and its spectrum is Has a certain finite width. The frequency shift amount and the spectrum width of these backward Brillouin scattered light are called Brillouin shift and Brillouin line width, respectively. When the optical fiber to be measured is, for example, a silica-based optical fiber, the Brillouin shift and Brillouin line width of the backward Brillouin scattered light are about 13 GHz and about 30 MHz, respectively, when the wavelength of the incident light is 1.3 μm. It is reported that they are inversely proportional to the first and second powers of the wavelength of the incident light, respectively.

The Brillouin shift varies depending on the constituent material of the optical fiber to be measured, strain and temperature difference generated in the optical fiber to be measured, and the like. The respective distributions of constituent material, strain and temperature can be measured.

As described above, since the power of the backward Brillouin scattered light is two to three orders of magnitude lower than the power of the backward Rayleigh scattered light, the most sensitive coherent detection can be used for the measurement. desirable. Since the backward Brillouin scattered light is generated by nonlinear interaction between incident light and thermally induced sound waves, the phase of the backward Brillouin scattered lights changes randomly with the change in the phase of the sound waves. Therefore, fading noise can be reduced by measuring and averaging the backward Brillouin scattered light many times without changing the frequency of the incident light as in the measurement of the backward Rayleigh scattered light.

[0010]

In the above-described conventional method and apparatus for measuring backscattered light,
There were the following disadvantages. First, in OTDR that measures backward Rayleigh scattered light by coherent detection,
Frequency hopping in the above-described semiconductor laser is caused by disturbances such as heat or elongation of the optical fiber to be measured.
This is caused by a change in the spectrum of the backscattered light output from the feedback optical fiber attached to the semiconductor laser, so that there is a problem that frequency hopping cannot be accurately controlled.

Therefore, if the time interval at which frequency hopping occurs in the semiconductor laser is shorter than the time when the optical signal travels round the optical fiber to be measured, the frequency of the backscattered light output from near the end of the optical fiber to be measured. The frequency of the beat light obtained by combining the backscattered light and the local light deviates from the reception band of the optical receiver, so that the backscattered light cannot be detected. Also,
Contrary to the case described above, if the time interval at which frequency hopping occurs in the semiconductor laser is very long, it becomes impossible to measure backscattered light for many different frequencies within a certain measurement time, Fading noise cannot be reduced sufficiently. In general, the oscillation frequency of a laser including a semiconductor laser can be changed by changing its temperature. However, since the temperature cannot be controlled quickly, there is a problem that it takes time to measure.

On the other hand, in a BOTDR for measuring backward Brillouin scattered light by coherent detection, light emitted from a light source is branched into two, one of which is used as a probe light to be incident on an optical fiber to be measured, and the other is used as a local light. However, as described above, when the measured optical fiber is a silica-based optical fiber and the wavelength of the incident light is 1.3 μm,
Brillouin shift of backward Brillouin scattered light is about 13G
Hz. Therefore, in the optical receiver, about 13 GHz
z signal must be received. However, it is very difficult to produce a photodetector and an electric circuit that coherently detect a high-frequency signal of about 13 GHz with low noise.

Therefore, as one method for solving the above-mentioned problem, one light source having a narrow line width is used as each of the light sources for emitting the probe light and the local light, and the frequency difference between these light sources is substantially reduced to Brillouin shift. It is conceivable to control them to equal values. In this case, since the frequency difference between the backward Brillouin scattered light by the probe light and the local light is small, coherent detection with high sensitivity is possible.

However, a light source whose oscillation frequency is stabilized with a narrow line width is generally expensive, and it is not economical to use two light sources. However, the light source described in the OTDR where the semiconductor laser is provided with a feedback optical fiber as an external resonator is a relatively inexpensive light source with a narrow line width, but as described above, the oscillation frequency of the semiconductor laser is Since the hopping is performed at random in time, it is difficult to control the frequency difference between the two light sources so that the frequency hopping cannot be accurately controlled.

Since the Brillouin shift in the optical fiber to be measured differs for each optical fiber, it is necessary to measure the BOTDR by changing the frequency difference between the probe light and the local oscillation light many times near the expected Brillouin shift. There is. However, as in the case of the OTDR, the oscillation frequency of the laser including the semiconductor laser can be changed by changing its temperature. However, since the temperature cannot be controlled promptly, there is a problem that the measurement takes time. Was.

[0016] The present invention has been made under such a background, in the measurement of the backward Rayleigh scattered light, provides a measuring method and apparatus of the backscattered light can be sufficiently reduced fading noise Aim to be
You.

[0017]

According to the first aspect of the present invention,
Using a continuous wave light source having a narrow spectrum width, light emitted from the continuous wave light source is light-modulated at a modulation frequency p, and light modulated by the light modulation is made to enter an optical fiber to be measured. By receiving coherently the backward Rayleigh scattered light scattered backward and propagating in the incident direction at a part of the modulated light as local light, the power of the backward Rayleigh scattered light is measured, and the modulation frequency p Are performed a plurality of times, and the power of the backward Rayleigh scattered light is obtained from an average value of a plurality of measurement values obtained as a result of the plurality of measurements.

According to the second aspect of the present invention, the spectral width is narrow.
Continuous wave light source and a part of light emitted from the continuous wave light source
First optical means for separating light into local light,
An optical pulse modulator that modulates the emitted light into pulsed light,
Second optical system for making pulsed light incident on the optical fiber to be measured
Means, the backscattered light from the optical fiber to be measured and the
Third optical means for multiplexing the local light and the local light;
Receiving the waved light, and combining the backscattered light and the local light
An optical receiver for receiving a beat electrical signal;
A detection circuit for detecting the amplitude or power of the electrical signal;
Signal processing for processing a signal detected by the detection circuit
A measuring device for backscattered light comprising:
The continuous wave light source before being input to the first optical means
An optical modulator for modulating the emitted light of the
Variable frequency modulation signal that supplies the input modulated electrical signal
And a source.

[0019]

[0020]

[0021]

[0022]

According to the present invention, the output light of the light source is modulated, and the rear Rayleigh scattered light whose sideband is scattered backward by the optical fiber to be measured is measured many times by changing the modulation frequency. Fading noise in the measurement of scattered light is reduced .

[0023]

Embodiments of the present invention will be described below with reference to the drawings. First Embodiment FIG. 1 is a block diagram showing a configuration of an apparatus for measuring backscattered light (backward Rayleigh scattered light) according to a first embodiment of the present invention. In FIG. A continuous wave light source that is narrow and has excellent coherency (coherency).
R semiconductor laser, semiconductor laser whose line width is narrowed by feeding back scattered light from optical fiber, single mode oscillation solid laser (YAG laser, Er laser, etc.), gas laser (He-Ne laser, Ar laser) , K
and a fiber laser using an optical fiber doped with elements such as Er, Nd and Pr.

Reference numeral 2 denotes an optical modulator for optically modulating the light emitted from the light source 1 with an electric modulation signal S _M (for example, a sine wave signal) output from a modulation signal source 3. The light modulated by the optical modulator 2 is split by an optical fiber coupler 7, one of which is used as a probe light incident on the optical fiber 5 to be measured, and the other is used as a local light for coherent reception. . The probe light emitted from the optical fiber coupler 7 is modulated into pulse light by the optical pulse modulator 4 and is incident on the measured optical fiber 5 via the optical fiber coupler 8. The rear Rayleigh scattered light scattered backward by the optical fiber 5 passes through the optical fiber coupler 8, and is separated from the local light and the optical fiber coupler 9 separated before.
And the multiplexed light is coherently received by the optical receiver 10. In the optical receiver 10,
_10a is a FET amplifier, and _10b is a photodiode.

At this time, when an acousto-optic light modulator is used as the light pulse modulator 4, the frequency of the probe pulse light is propagated through the acousto-optic modulator from the frequency of the light incident on the light pulse modulator 4. to shifted frequency f _AO of sound waves were received signal of the multiplexed light becomes a beat electrical signal of a frequency f _AO. Therefore, the frequency of the beat electric signal is down-shifted by the mixer 11, the local electric signal source 12, and the electric filter 13, and the square or wave detection is performed by the detection circuit 14, so that the power or amplitude of the beat electric signal is reduced. Is measured. The power of the beat electric signal corresponds to the power of the backward Rayleigh scattered light.

As described above, the backward Rayleigh scattered light is measured with high sensitivity by the coherent reception. However, as described above, the power of the backward Rayleigh scattered light is very weak, so that the coherent reception is performed. Nevertheless, the S / N is usually low. Therefore, the probe pulse light obtained by the optical pulse modulator 4 is incident on the optical fiber 5 to be measured many times, and the respective rear Rayleigh scattered signals measured at that time are averaged by the signal processing device 15, and S / S Improve N. At this time, the timing signal S to the optical pulse modulator 4 and the signal processing device 15
_TM is provided by the timing controller 6.

The most important feature of the first embodiment of the present invention is to measure backward Rayleigh scattered light in which a sideband generated by modulation by the optical modulator 2 is scattered by the optical fiber 5 to be measured. . Therefore, when the modulation frequency of the optical modulator 2 is changed in synchronization with the optical pulse modulator 4 by the timing signal _STM from the timing controller 6, the backward Rayleigh scattered light for the probe light having a different frequency is measured many times. Then, the average value can be obtained. At this time, as described above, if the frequency of the probe light changes, the phase relationship between the backscattered light waves also changes, so that fading noise peculiar to coherent reception is greatly reduced.

As the optical modulator 2, for example, LiNbO
_When an optical modulator made of ₃ or KTP crystal is used, modulation up to about 100 GHz is possible in principle. Therefore, using the modulation signal source 3 with frequency accuracy and stability <± 1 MHz, the modulation frequency is set to 0 in 1 MHz steps.
When varied from 100GHz from 10 ⁵ times or more, it will be possible to measure the backward Rayleigh scattered light while changing the frequency, the fading noise is sufficiently reduced. Further, when an optical modulator having a multiple quantum well structure is used as the optical modulator 2, it is possible to modulate even higher frequencies, and the fading noise is further reduced.

In the first embodiment of the present invention, as described above, since the frequency of the probe light is changed by the electric signal, the frequency can be changed reliably and quickly. Therefore, the measurement can be performed very efficiently without using a wasteful time for changing the frequency as in the past.

There are two types of light modulation methods using the light modulator 2 described above: intensity modulation and phase modulation. Note that frequency modulation is equivalent to phase modulation, and a description thereof will be omitted. Hereinafter, the intensity modulation and the phase modulation will be described step by step. (1) Intensity modulation Now, in the optical modulator 2, when the light emitted from the light source 1 is intensity-modulated by a sine wave signal of frequency p, the amplitude of the modulated light is e (t) = E _C ｛1 + mcos ( 2πpt)｝ × sin (2πf _C t)... Here, E _C and f _C represent the amplitude and frequency of the light incident on the optical modulator 2, respectively. In addition, m indicates the degree of modulation, and t indicates time. Now, when the equation is expanded, e (t) = E _c sin (2πf _C t) + (m / 2) E _c sin ｛2π (f _C− p) t｝ + (m / 2) E _c sin ｛2π (F _C + p) t｝. The first term on the right side of the equation is the emitted light itself from the light source 1 and corresponds to a carrier wave. The second and third terms on the right side of the equation are sidebands generated by the modulation, and their frequencies are shifted by the frequency p.

However, in the configuration of the first embodiment shown in FIG. 1, the beat frequency of the backward Rayleigh scattered light by the carrier wave received by the optical receiver 10 and the local light is different from that of the sideband and the local light. It matches the beat frequency. Therefore, it is not possible to separate and measure only the rear Rayleigh scattered light due to the sideband. Further, as can be seen from the equation, the frequency of the light of the carrier wave does not change due to the modulation by the optical modulator 2, so that the fading noise of the backward Rayleigh scattered light due to the carrier wave is not reduced.

Therefore, the ratio of the rear Rayleigh scattered light power due to the sideband that can be expected to reduce fading noise to the rear Rayleigh scattered light power due to the carrier wave that cannot be expected to reduce fading noise is 2 × (m / 2) ² = m
A ^2/2, also the value of the modulation m as a maximum value,
Only 0.5. That is, when an intensity modulator is used as the optical modulator 2, the degree of fading noise reduction is relatively small.

(2) Phase Modulation Now, in the optical modulator 2, when the light emitted from the light source 1 is phase-modulated by a sine wave signal, the amplitude of the modulated light is e (t) = E _c sin ｛2πf _C t + Δφcos (2πpt)｝. Here, Δφ indicates a phase shift. Expanding the equation, e (t) = E _C ｛J ₀ (Δφ) sin (2πf _C t) + J ₁ (Δφ) cos [2π (f _C + p) t] + J ₁ (Δφ) cos [2π (f _C -P) t] + J ₂ (Δφ) cos [2π (f _C + 2p) t] + J ₂ (Δφ) cos [2π (f _C -2p) t] + J ₃ (Δφ) cos [2π (f _C + 3p) t + J ₃ (Δφ) cos [2π (f _C −3p) t] +... The first term on the right side of the equation is a carrier having the same frequency as the light incident on the optical phase modulator, the even terms below the second term are the upper sideband, and the odd terms are the lower sideband. J _n
Denotes an n-th order Bessel function of the first kind. Here, FIG. 2 shows a Bessel function of the first kind. From FIG. 2, the phase shift Δφ
The, m-th zero of _{J 0 (x) (j 0.m} ; j 0. 1 = 2.4
05, j _0.2 = 5.520, j _0.3 = 8.654,...
It turns out that it is possible to reduce the power of the carrier wave to zero by selecting the method in ()). That is, by using a phase modulator for the optical modulator 2, unlike the case of the intensity modulation described above, only the rear Rayleigh scattered light due to the sideband can be measured, and as a result, fading noise is greatly reduced. Reduced.

[Second Embodiment] Next, a second embodiment of the present invention will be described. FIG. 3 is a block diagram showing a configuration of a device for measuring backscattered light (backward Brillouin scattered light) according to a second embodiment of the present invention. In this figure, parts corresponding to the respective parts in FIG. And description thereof is omitted. The difference between the measuring device shown in FIG. 3 and the measuring device shown in FIG. 1 is that the local light input to the optical receiver 10 is supplied not from the light modulated by the optical modulator 2 but from a part of the output light from the light source 1. That is the point.

Hereinafter, as in the case of the above-described first embodiment, the case where the modulation by the optical modulator 2 is the intensity modulation and the case where the modulation is the phase modulation will be described. (1) Intensity modulation The spectrum of the intensity-modulated light is, as can be seen from the above equation, a sideband (frequency f _C ± p) generated on both sides of the carrier (frequency f _C ). As shown in a). When the modulated light is converted into pulse light by the optical pulse modulator 4, the spectrum of the pulse light is as shown in FIG. 4B. According to this figure, the spectral line width of the pulsed light is 2
/ W (W is the light pulse width).

The back scattered light obtained by scattering the pulse light backward by the measured optical fiber 5 is composed of backward Rayleigh scattered light and backward Brillouin scattered light. Since the backward Rayleigh scattered light has no frequency shift, its spectrum is the same as that of the incident light, and is as shown in FIG. 4B. On the other hand, backward Brillouin scattered light and Stokes light frequency is only downshift f _B, consists of a anti-Stokes light frequency is only upshift f _B, respectively shown in FIG. 4 the spectrum (c) and (d) It will be. The spectral line width of the backward Brillouin scattered light is
It is further expanded by the Brillouin line width Δf _B.

The backscattered light is multiplexed with the local light having the frequency f _C (see FIG. 4E) by the optical fiber coupler 9 and coherently detected. At this time, the spectrum of the beat electric signal received by the optical receiver 10 is as shown in FIG. In FIG. 5A, signal a is a received signal corresponding to backward Brillouin scattered light due to sideband waves, signals b and d are received signals corresponding to backward Brillouin scattered light due to a carrier wave, respectively.

The signals c and d have the frequency f
_B (up to 13 GHz) or higher, and it is difficult for the optical receiver 10 to receive this signal with high sensitivity. However, by selecting the modulation frequency p to a value close to the frequency f _B , the signal b becomes a low-frequency signal of the frequency | p−f _B |, and the optical receiver 10 can receive the signal with high sensitivity. further,
When the low cut-off frequency f _LC and the high cut-off frequency f _UC of the optical receiver 10 are selected as shown by the formulas and equations, the strong rear Rayleigh scattered light is blocked, and only the signal b corresponding to the rear Brillouin scattered light is removed. Reception is enabled. 1 / W ≦ f _LC ≦ | p−f _B | − {1 / W + (Δf _B / 2)}... | P−f _B | + {1 / W + (Δf _B / 2)} ≦ f _UC ·・ ・

This signal b may be inputted as it is to the detection circuit 14, but in consideration of performing digital signal processing in the detection circuit 14 or the subsequent signal processing device 15, this signal b is converted into a baseband signal. It is desirable to do. Therefore, the signal b is further mixed with the electric signal of the frequency f _L = | p−f _B | from the local electric signal source 12 by the mixer 11, and the bandwidth B [1 / W + (Δ
_{f B / 2) ≦ B ≦} f L + | p-f B | - {1 / W + (Δf B
/ 2)｝] to obtain a baseband signal b1 (see FIG. 5B).

As can be understood from the above description, the Brillouin shift f _B of the measured optical fiber 5 is determined by the modulation frequency p when the power of the received signal is maximized and the frequency f _{L of the} local electric signal source 12. using preparative given by f _B = p-f _L (when p-f _B> 0) (when _{p-f B <0) f} B = p + f L.

As can be seen from the above description, the signal b shown in FIG. 5A is a signal in which Stokes light and anti-Stokes light overlap on the frequency spectrum of the electric signal. Therefore, the above-described second embodiment is suitable for increasing the signal intensity of the backward Brillouin scattered light. However, on the other hand, there is also a case where it is desired to separately measure the Stokes light and the anti-Stokes light (for example, a temperature measurement using a difference in the temperature dependence of the scattering coefficient of the Stokes light and the anti-Stokes light). In such a case, the frequency of the signal light or the local light may be shifted. For example, if an acousto-optic light modulator is used for the light pulse modulator 4, the frequency of the modulated pulse light is shifted by the frequency f _{AO of the} sound wave transmitted through the acousto-optic light modulator. At this time,
The spectra of the backward Rayleigh scattered light, the backward Brillouin scattered light Stokes light and the anti-Stokes light are shown in FIG.
In (d), it is given by being shifted by the frequency f _AO . FIG. 6 shows the spectrum of the beat electric signal received by the optical receiver 10 when the frequency f _AO or p is selected so as to satisfy the expression. f _AO = | p−f _B | At this time, the signal b ′ received as the baseband signal
Means that when p−f _B > 0, anti-Stokes light and p−f _B <
When it is 0, it corresponds to Stokes light. Therefore, by shifting the frequency of signal light, backscattered light, or local light, Stokes light and anti-Stokes light of backward Brillouin scattered light can be measured separately.

As can be seen from the above description, the Brillouin shift f _B of the measured optical fiber 5 is determined by the modulation frequency p when the power of the received signal is maximized and the frequency shift f _AO of the acousto-optic light modulator. with given by f _B = p-f _AO (when p-f _B> 0) (when _{p-f B <0) f} B = p + f AO.

(2) Phase Modulation A case where a phase modulator is used as the optical modulator 2 in the configuration of FIG. 3 will be described. Phase-modulated light, as can be seen from the equation, the carrier component of the frequency f _O, in addition to first-order sideband of the frequency (f _O ± p), frequency (f _O ± 2
p), (f _O ± 3p),...
However, the frequency of the beat electrical signals and sideband of these higher order, the local light frequency f _O, when the (p~f _B),
Since the frequency becomes very high (〜2f _B ), (〜3f _B ),..., Substantially no backscattered light signal due to these higher-order sideband waves is received by the optical receiver 10.

Therefore, when the phase is modulated at the frequency (p to f _B ), the spectrum of the electric signal received by the optical receiver 10 becomes the same as that when the intensity is modulated.
Given in (a). However, in the case of the phase modulation, the carrier component of the modulated light can be made zero by selecting the phase shift Δφ as j _0.m (the root of the Bessel function of the first kind). The signal a corresponding to the strong backward Rayleigh scattered light in FIG. 5A can be eliminated.
Therefore, by employing the phase modulation, the condition required for the low cut-off frequency characteristic in the optical receiver 10 can be greatly reduced.

Further, since the Stokes light and the anti-Stokes light of the backward Brillouin scattered light are measured separately, it can be seen from FIG. 6 even when the frequency of the probe light, the back scattered light, or the local light is shifted. As described above, in order to cut off the rear Rayleigh scattered light signal, the condition required for the frequency characteristics of the optical receiver 10 can be greatly reduced.

Further, the phase modulation is different from the intensity modulation in that
It also has the following advantages. That is, by selecting an appropriate value for the phase shift Δφ, it is possible to increase the power of the sideband wave, which is the measurement signal, as compared with the carrier wave, as can be seen from FIG. This is particularly advantageous when optically amplifying the probe light in order to increase the dynamic range of the measurement system for backscattered light. This is because the optical power that can be obtained by optical amplification or the optical power that can be input to the optical fiber to be measured has an upper limit, so that when the power of the sideband is larger than the power of the carrier, the maximum Is obtained by optical amplification, and can be incident on the optical fiber 5 to be measured.

Third Embodiment Next, a third embodiment of the present invention will be described. FIG. 7 is a block diagram showing a configuration of a device for measuring backscattered light (backward Brillouin scattered light) according to the third embodiment of the present invention. In this figure, parts corresponding to the respective parts in FIG. And description thereof is omitted. The difference of the measuring device shown in FIG. 7 from that of FIG. 3 is that the back scattered light from the measured optical fiber 5 is not reflected by the optical modulator 2 instead of the probe light incident on the measured optical fiber 5.
, And coherent detection is performed on the sideband of the modulated backward Brillouin scattered light. The spectrum of the backscattered light combined with the local light by the optical fiber coupler 9 is the same as that of the second embodiment described above (FIG. 4).
(B) to (d)). Therefore,
Also in this embodiment, similarly to the case of the second embodiment, it is possible to measure the backward Brillouin scattered light with high sensitivity.

"Fourth Embodiment" Next, a fourth embodiment of the present invention will be described. FIG. 8 is a block diagram showing a configuration of a backscattered light (back Brillouin scattered light) measuring apparatus according to a fourth embodiment of the present invention. In this figure, parts corresponding to the respective parts in FIG. And description thereof is omitted. The difference between the measuring device shown in FIG. 8 and the measuring device shown in FIG. The point lies in that light is modulated and coherent detection of backward Brillouin scattered light is performed using the modulated sideband of the local light.

Here, FIGS. 9 and 10 show the spectra of each light wave and electric signal in this embodiment. FIG. 9 (a)
(E) shows the spectrum of the light emitted from the light source, the rear Rayleigh scattered light, the Stokes light of the rear Brillouin scattered light, the anti-Stokes light of the rear Brillouin scattered light, and the local light whose intensity is modulated at the frequency p. Also, the optical receiver 10
FIG. 10 shows the spectrum of the beat electric signal received at the step S1. This spectrum is the same as the spectrum in the second and third embodiments shown in FIG. 5A except for the signal e of the frequency p corresponding to the backward Rayleigh scattered light.

Further, similarly to the case of the second embodiment, when the frequency of the probe light is shifted by f _AO using an acousto-optic modulator as the light pulse modulator 4, the light is received by the optical receiver 10. The spectrum of the electric signal is the sum of the spectrum (see FIG. 6) in the second and third embodiments and the signal of the frequency (p ± f _AO ). Therefore, also in this embodiment, similarly to the second and third embodiments, it is possible to measure the back Brillouin scattered light with high sensitivity. Even when phase modulation is used instead of intensity modulation, backward Brillouin scattered light can be measured in the same manner as in the previous embodiments.

Fifth Embodiment Next, a fifth embodiment of the present invention will be described. As can be seen from the above description of the first to fourth embodiments, in the present invention, the same system is used for the measurement system of the rear Rayleigh scattered light and the measurement system of the rear Brillouin scattered light. Its configuration is different. Therefore, in the fifth embodiment, by using an optical switch, both measurement systems can be easily switched to perform measurement. FIGS. 11 to 13 are block diagrams showing the configuration of a backscattered light measuring device according to a fifth embodiment of the present invention. In these drawings, the same reference numerals are given to parts corresponding to the respective parts in FIG. , The description of which is omitted. In 11 to 13, light having an optical switch 16 having input and output terminals x _1, x ₂ and y ₁ ~y _3, input and output terminals x ₁ ', x _2' and y ₁ '~y _3' a switch 17, 'and the optical fiber 18 connecting the terminal y ₃ and y _3' terminal y ₁ and y ₁ optical fiber 19 connecting the are newly provided.

FIG. 11 shows a configuration in which the configuration of the above-described first embodiment (see FIG. 1) and the configuration of the second embodiment (see FIG. 3) can be switched. In FIG. 11, the optical pulse modulator 4 is connected to the terminal x ₁ ′ of the optical switch 17 and the optical fiber coupler 8.
, But may be arranged between the optical fiber coupler 7 and the terminal x ₁ of the optical switch 16. The operation of the measuring device shown in FIG. 11 is as follows.
To configure the first embodiment of the system shown in FIG. 1, the optical switch 16, as indicated by the solid line in FIG. 11, the terminal x ₁ and y _1, The terminal x ₂ and y ₂ is connected. At the same time, in the optical switch 17,
As shown by the solid line in FIG. 2, terminals x ₁ ′ and y ₁ ′ are
The terminals x ₂ ′ and y ₂ ′ are connected.

Next, to configure a system of the second embodiment shown in FIG. 3, the optical switch 16, as indicated by a broken line in FIG. 11, and the terminal x ₁ and y _2, also switched so that the terminal x ₂ and y ₃ in a connected state. At the same time, the optical switch 17 is switched so that the terminals x ₁ ′ and y ₂ ′ and the terminals x ₂ ′ and y ₃ ′ are connected as shown by the broken line in FIG.

FIG. 12 shows a configuration in which the configuration of the first embodiment (see FIG. 1) and the configuration of the third embodiment (see FIG. 7) can be switched, and FIG. 13 shows the configuration of the first embodiment. The configuration of the example (see FIG. 1) and the configuration of the fourth embodiment (see FIG. 8) are switchable. The operation of these devices is almost the same as that of the above-described device shown in FIG. 11, and a description thereof will be omitted.

In FIGS. 11 to 13, various types of optical switch 16 or 17 such as a mechanical switch, a thermal switch, an electro-optical switch, and an acousto-optical switch can be used.
In the present invention, since high speed switching time is not so required, a mechanical type which is economical, has little insertion loss, and is excellent in crosstalk is suitable. As one example, a configuration of a slide optical switch is shown in FIG. As shown in FIG. 14, this optical switch has a structure in which two optical waveguides or optical fibers 20 (U and V) arranged at equal intervals are opposed to each other. As can be seen from FIG. 14, switching can be performed by sliding U or V in the alignment direction. FIG. 15 shows an optical switch 1.
6 shows a configuration of a slide optical switch in which 6 and 17 are integrated. The above-described switching can be performed by sliding the optical switches in both FIGS. 15A and 15B by the alignment interval.

In each of the first to fifth embodiments described above, examples are described in which the optical pulse modulator 4 is used. This is because the backscattered light from each position of the optical fiber 5 to be measured is This is because the measurement is performed separately in time and the distribution of the measured optical fiber 5 in the length direction is obtained. Therefore,
When measuring the total power of the backscattered light from the entire optical fiber 5 to be measured, the optical pulse modulator 4 is not necessary.

In the first configuration of the second to fourth embodiments and the fifth embodiment described above (see FIGS. 3, 7, 8 and 11), the light emitted from the light source 1 , After being modulated by the optical modulator 2, the optical pulse modulator 4
However, it is needless to say that these orders may be reversed. Further, in the present invention, since a coherent light source having a small optical output is usually used for the light source 1, the use of an optical amplifier such as an optical fiber amplifier or a semiconductor laser amplifier together with the measuring dynamics of the present invention It is very effective in improving the range. The insertion position of this optical amplifier is
1, 3, 7, 8 and 11 to 13 described above.
, Between the light source 1 and the optical fiber coupler 7, between the optical fiber coupler 7 and the optical fiber coupler 8, between the optical fiber coupler 8 and the optical fiber 5 to be measured, between the optical fiber coupler 8 and the optical fiber coupler 9 And any position between the optical fiber coupler 7 and the optical fiber coupler 9.

In addition, in the above-described second to fourth embodiments, only the case where a part of the light emitted from the light source 1 is used for the local light is described. Light emitted from another light source that oscillates at the same frequency may be used for local light. In this case, although there is a demerit of using a large number of light sources, there is an advantage that it is possible to sufficiently obtain the local light power required for coherent reception without deteriorating the feature of high-speed frequency control. .

[0059]

As described above, according to the present invention ,
In measuring the backward Rayleigh scattered light from the optical fiber to be measured, the sideband generated by the optical modulation is used as the probe light and the local light, so that the frequency of the probe light can be quickly, accurately, and in a wide frequency range. , The measurement of the backward Rayleigh scattered light by the coherent detection can be performed, and the measurement in which the fading noise is greatly reduced can be performed at high speed.

[0060]

[0061]

[0062]

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a backscattered light measuring device according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a Bessel function of a first type.

FIG. 3 is a block diagram showing a configuration of a backscattered light measuring device according to a second embodiment of the present invention.

FIG. 4 is a diagram showing an example of the spectrum of each light wave in the second embodiment of the present invention.

FIG. 5 is a diagram illustrating an example of a spectrum of an electric signal according to the second embodiment of the present invention.

FIG. 6 shows an optical receiver 1 according to a second embodiment of the present invention, in which an acousto-optic light modulator is used as the light pulse modulator 4.
FIG. 3 is a diagram illustrating an example of a spectrum of an electric signal received at 0.

FIG. 7 is a block diagram showing a configuration of a backscattered light measuring device according to a third embodiment of the present invention.

FIG. 8 is a block diagram showing a configuration of a backscattered light measuring device according to a fourth embodiment of the present invention.

FIG. 9 is a diagram showing an example of the spectrum of each light wave in the fourth embodiment of the present invention.

FIG. 10 is a diagram illustrating an example of a spectrum of an electric signal according to a fourth embodiment of the present invention.

FIG. 11 is a block diagram showing a first configuration of an apparatus for measuring backscattered light according to a fifth embodiment of the present invention.

FIG. 12 is a block diagram showing a second configuration of the backscattered light measuring device according to the fifth embodiment of the present invention.

FIG. 13 is a block diagram showing a third configuration of the backscattered light measuring device according to the fifth embodiment of the present invention.

FIG. 14 is a diagram illustrating the operation and configuration of a slide optical switch.

FIG. 15 is a diagram showing a configuration of a slide optical switch.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Light source 2 Optical modulator 3 Modulation signal source 4 Optical pulse modulator 5 Optical fiber to be measured 6 Timing controller 7, 8, 9 Optical fiber coupler 10 Optical receiver 10 _a FET amplifier 10 _b Photodiode 11 Mixer 12 Local power generation Signal source 13 Electric filter 14 Detection circuit 15 Signal processing device 16, 17 Optical switch 18, 19 Optical fiber

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kaoru Shimizu Nippon Telegraph and Telephone Corporation, 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo (56) References JP-A-4-370732 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01M 11/00-11/02 H04B 10/08

Claims (2)

(57) [Claims] 1. A continuous wave light source having a narrow spectrum width is used, light emitted from the continuous wave light source is light-modulated at a modulation frequency p, and light modulated by the light modulation is made incident on an optical fiber to be measured. The power of the rear Rayleigh scattered light is measured by receiving the rear Rayleigh scattered light scattered backward in the measured optical fiber and propagating in the incident direction by coherently receiving a part of the modulated light as local light. Performing the measurement with a different modulation frequency p a plurality of times;
A method of measuring backscattered light, comprising determining the power of the back Rayleigh scattered light from an average value of a plurality of measured values obtained as a result of the plurality of measurements. 2. A continuous wave light source having a narrow spectrum width, and a part of light emitted from the continuous wave light source is separated to generate a local light.
Optical means for modulating the emitted light into pulsed light, and second light for causing the pulsed light to be incident on the optical fiber to be measured.
Biological means, backscattered light from the measured optical fiber and the local light
A third optical means for multiplexing the light, receiving the multiplexed light,
An optical receiver for receiving a beat electric signal with light, and a detector for detecting the amplitude or power of the beat electric signal
Circuit, and signal processing for processing a signal detected by the detection circuit
The continuous oscillation light before being input to the first optical means.
An optical modulator for modulating light emitted from a source, and a frequency modulator for supplying a modulated electric signal to be input to the optical modulator.
Backscatter comprising a variable modulation signal source
Light measuring device.