JP3408789B2 - 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 backscattered light output from an optical fiber to be measured, and more particularly to a backscattered light measuring method and apparatus for measuring back Rayleigh scattered light and back Brillouin scattered light.

[0002]

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

The main backscattered light generated in the optical fiber to be measured is the backward Rayleigh scattered light, the backward Brillouin scattered light, and the backward Raman scattered light. The powers of the backward Brillouin scattered light and the backward Raman scattered light are two to three orders of magnitude lower than the power of the backward Rayleigh scattered light.
Usually, in OTDR, back Rayleigh scattered light is measured.
However, the power of the backward Rayleigh scattered light itself is lower than the peak power of the optical pulse incident on the optical fiber to be measured by about 5 orders of magnitude, so that the OTDR requires a highly sensitive photodetection technique.

Currently, the most sensitive photodetection method is coherent detection, and its effectiveness in OTDR has already been demonstrated. In the coherent detection, a coherency, that is, a light source having excellent coherence is used, and 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 of both is measured. Since the power of this beat light increases in proportion to the power of the local oscillation light, coherent detection is performed up to the power of weak light that is power equivalent to so-called shot noise due to the quantum fluctuations that light inherently has. Can be measured by

However, when coherent detection is used for OTDR, it is known that a problem of fading noise newly 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 changes randomly depending on the position in the longitudinal direction of the optical fiber under measurement. The power of the backscattered light measured also changes with time according to the scattering position. The fluctuation of the power of the backscattered light significantly deteriorates the accuracy of the fault position search and the loss distribution measurement of the optical fiber to be measured.

Therefore, for this fading noise, it is said that it is effective to use, as a light source, a semiconductor laser to which the backward Rayleigh scattered light output from the optical fiber to be measured is fed back. In this case, the semiconductor laser becomes a light source suitable for coherent detection by narrowing the oscillation spectrum width to 100 kHz or less, and the oscillation frequency of the semiconductor laser is kept constant for a certain time and then the other frequency. The so-called frequency hopping occurs, which changes repeatedly to become constant. Then, 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 each of a number of different frequencies and taking the average thereof, the difference in the phase relationship between the light waves depending on the scattering position, that is, fading noise can be reduced. For details of this type of technique, see J. P.
"Development of a Coherent OTDR" by King and others
Instrument "(J. Lightwave Technol. Vol.LT-5 no.4 pp.
616-624 1987).

By the way, in addition to the above-described OTDR for measuring the backward Rayleigh scattered light, an OTDR for measuring the backward Brillouin scattered light (hereinafter, referred to as BOTDR, the above-mentioned OTDR for measuring the backward Rayleigh scattered light is simply O
TDR) has been proposed. The center frequency of the backward Rayleigh scattered light described above is the same as the frequency of the incident light to the measured optical fiber, but the center frequency of the backward Brillouin scattered light is shifted from the frequency of the incident light, and its spectrum Has a finite width. The frequency shift amount and the spectral 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 optical fiber, the Brillouin shift and the 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, and It has been reported that they are inversely proportional to the first power and the second power of the wavelength of the incident light, respectively.

Of these, the Brillouin shift changes depending on the constituent material of the optical fiber to be measured, the strain generated in the optical fiber to be measured, the temperature difference, and the like. Therefore, by BOTDR, the Brillouin shift in the length direction of the optical fiber to be measured is obtained. Each distribution of constituent material, strain and temperature can be measured.

By the way, as described above, the power of the backward Brillouin scattered light is lower than the power of the backward Rayleigh scattered light by two to three orders of magnitude. Therefore, the most sensitive coherent detection can be used for the measurement. desirable. Since the back Brillouin scattered light is generated by the nonlinear interaction between the incident light and the thermally induced sound wave, the phase of the back Brillouin scattered light randomly changes as the phase of the sound wave changes. Therefore, the fading noise can be reduced by measuring the backward Brillouin scattered light a number of times and averaging the measured values, without changing the frequency of the incident light as in the case of measuring the backward Rayleigh scattered light.

[0010]

By the way, in the above-mentioned conventional method and apparatus for measuring backscattered light,
There were the following drawbacks. First, in OTDR that measures backward Rayleigh scattered light by coherent detection,
The frequency hopping in the semiconductor laser described above is caused by heat or disturbance such as elongation of the optical fiber to be measured,
There is a problem that frequency hopping cannot be accurately controlled because it is caused by a change in the spectrum of the backscattered light output from the feedback optical fiber attached to the semiconductor laser.

Therefore, when the time interval at which frequency hopping occurs in the semiconductor laser is shorter than the time during which the optical signal travels back and forth in the optical fiber under measurement, the frequency of the backscattered light output from the vicinity of the end of the optical fiber under measurement. The frequency of the beat light, which is a combination of the backscattered light and the local light, is out of the reception band of the optical receiver, so that the backscattered light cannot be detected. Also,
Contrary to the above case, when the time interval where frequency hopping occurs in the semiconductor laser is very long, it becomes impossible to measure the backscattered light for many different frequencies within a fixed measurement time. Fading noise cannot be reduced sufficiently. Further, in general, the oscillation frequency of a laser including a semiconductor laser can be changed by changing its temperature, but since the temperature cannot be controlled quickly, there is a problem that measurement takes time.

On the other hand, in the BOTDR for measuring the backward Brillouin scattered light by coherent detection, the light emitted from the light source is split into two, one is used as the probe light incident on the optical fiber to be measured, and the other is used as the local light. However, as described above, when the optical fiber to be measured is a silica optical fiber and the wavelength of the incident light is 1.3 μm, the Brillouin shift of the backward Brillouin scattered light is about 13 GHz.
And big. Therefore, the optical receiver must receive a signal of about 13 GHz. However, about 13
It is very difficult to manufacture a photodetector and an electric circuit for coherently detecting a high frequency signal of GHz even with low noise.

Therefore, as one method for solving the above-mentioned problem, a narrow linewidth light source is used for each of the probe light source and the local light source, and the frequency difference between these light sources is almost equal to the Brillouin shift. It is possible to control to an equal value. In this case, since the frequency difference between the backward Brillouin scattered light and the local light due to the probe light is small, highly sensitive coherent detection becomes 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. Of course, the light source described in the OTDR in which the feedback optical fiber is attached to the semiconductor laser as the external resonator is a relatively inexpensive and narrow linewidth light source, but as already explained, the oscillation frequency of the semiconductor laser is However, since hopping is performed randomly in time, it is difficult to control the frequency difference between the two light sources to be constant so that the frequency hopping cannot be accurately controlled.

Further, the Brillouin shift in the optical fiber to be measured is different for each optical fiber. Therefore, in BOTDR, it is necessary to measure the frequency difference between the probe light and the local oscillation light by changing it a number of times around the expected Brillouin shift. There is. However, as in the case of OTDR, the oscillation frequency of a laser including a semiconductor laser can be changed by changing its temperature, but since the temperature cannot be controlled quickly, there is a problem that measurement takes time. It was

The present invention has been made under such a background, and it is an object of the present invention to provide an economical, high-speed and high-performance method for measuring backscattered light and a device therefor in measuring the backscattered Brillouin light. To aim.

[0017]

The invention according to claim 1 is
A light source having a narrow spectral width is used, and light emitted from the light source is optically modulated to generate a sideband having a spectrum separated from the spectrum of the emitted light, and the modulated light is used as an optical fiber to be measured. The power of the backward Brillouin scattered light is obtained by coherently receiving the backward Brillouin scattered light which is scattered in the optical fiber to be measured and propagates in the incident direction. The invention according to claim 2 uses a light source having a narrow spectrum width, makes the light emitted from the light source incident on the optical fiber to be measured, and is backscattered by being scattered backward in the optical fiber to be measured and propagating in the incident direction. The power of the backward Brillouin scattered light is obtained by optically modulating the light and coherently receiving the sideband of the backward Brillouin scattered light generated by the light modulation. According to a third aspect of the present invention, a continuous wave light source having a narrow spectrum width is used, light emitted from the continuous wave light source is incident on the optical fiber to be measured, and the light is scattered backward in the optical fiber to be measured and is incident in the incident direction. The backward Brillouin scattered light that propagates, part of the emitted light of the continuous wave light source, or the emitted light of another light source is coherently received as the sideband wave of the light-modulated light as local light, whereby the backward Brillouin is obtained. The feature is that the power of scattered light is obtained. According to a fourth aspect of the present invention, a continuous wave light source having a narrow spectrum width, a first optical means for separating a part of light emitted from the continuous wave light source into local light, and the emitted light are provided. An optical pulse modulator for modulating the pulsed light, and a second optical means for making the pulsed light incident on the optical fiber to be measured,
Third optical means for combining the backscattered light from the optical fiber to be measured and the local light, and the beat of the backscattered light and the local light by receiving the combined light. Measurement of backscattered light including an optical receiver for receiving an electric signal, a detection circuit for detecting the amplitude or power of the beat electric signal, and a signal processing device for processing the signal detected by the detection circuit In the device, for modulating the emitted light from the continuous wave light source separated from the local oscillation light, or for modulating the pulsed light, or for modulating the backscattered light, or, It is characterized by comprising an optical modulator for modulating local light and a variable frequency modulation signal source for supplying a modulated electric signal to be inputted to the optical modulator.

According to the present invention, the probe light, or
The backward Brillouin scattered light scattered backward by the optical fiber to be measured or the local oscillation light is modulated, and the backward Brillouin scattered light is coherently detected, and the beat frequency of the backward Brillouin scattered light and the local oscillation light is set to a low frequency. Therefore, it is possible to economically measure the backward Brillouin scattered light at high speed and with high accuracy.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION 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 a backscattered light (backward Rayleigh scattered light) measuring device according to a first embodiment of the present invention. In FIG. 1, 1 denotes an oscillation spectrum width. It is a continuous wave light source that is narrow and has excellent coherency.
DFB semiconductor laser, DBR semiconductor laser, semiconductor laser whose line width is narrowed by feeding back backscattered light from an optical fiber, single mode oscillation solid-state laser (YAG laser, Er laser, etc.), gas laser (He-
Ne laser, Ar laser, Kr laser, etc.), Er, N
It is a fiber laser using an optical fiber doped with an element such as d or Pr.

Reference numeral 2 denotes an optical modulator, which optically modulates the light emitted from the light source 1 with an electric modulation signal S _M (for example, a sine wave signal) output from the modulation signal source 3. The light modulated by the optical modulator 2 is branched by the optical fiber coupler 7, one of which is used as the probe light incident on the optical fiber 5 to be measured, and the other of which is used as the local light for coherent reception. . The probe light emitted from the optical fiber coupler 7 is modulated into pulsed light by the optical pulse modulator 4 and is incident on the measured optical fiber 5 via the optical fiber coupler 8. Then, the backward Rayleigh scattered light scattered backward by the optical fiber 5 passes through the optical fiber coupler 8 to separate the local oscillation light and the optical fiber coupler 9 previously separated.
Are combined by the optical receiver 10, and the combined light is coherently received by the optical receiver 10. In the optical receiver 10,
10 _a is a FET amplifier and 10 _b is a photodiode.

At this time, if an acousto-optical modulator is used as the optical pulse modulator 4, the frequency of the probe pulse light is propagated through the acousto-optical modulator from the frequency of the incident light of the optical pulse modulator 4. Since the frequency f _{AO of} the sound wave is shifted, the reception signal of the combined light becomes a beat electric signal of the frequency f _AO . Therefore, the mixer 11, the local electric signal source 12, and the electric filter 13 downshift the frequency of the beat electric signal, and the detection circuit 14 squares or envelope-detects the power or amplitude of the beat electric signal. To measure. The power of this 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 already explained, the power of the backward Rayleigh scattered light is very weak, and therefore 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 made to enter the optical fiber 5 to be measured a number of times, and the respective backward Rayleigh scattered signals measured at that time are averaged by the signal processing device 15, and S / Improve N. Timing signal S to optical pulse modulator 4 and signal processing device 15 at this time
_TM is provided by the timing controller 6.

The greatest feature of the first embodiment of the present invention is to measure the backward Rayleigh scattered light in which the sideband wave generated by the 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 S _TM from the timing controller 6, the backward Rayleigh scattered light with respect to the probe light having different frequencies is measured many times. Then, it becomes possible to obtain the average value. At this time, as already described, when the frequency of the probe light changes, the phase relationship between the backscattered light waves also changes, so that fading noise specific to coherent reception is significantly reduced.

As the optical modulator 2, for example, LiNbO is used.
_When an optical modulator made of ₃ or KTP crystal is used, in principle, modulation up to about 100 GHz is possible. Therefore, by using the modulation signal source 3 with frequency accuracy and stability <± 1 MHz, the modulation frequency is set to 0 in 1 MHz steps.
From 100 GHz to 100 GHz, the backward Rayleigh scattered light can be measured 10 ⁵ or more times while changing the frequency, and 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 fading noise can be further reduced.

In the first embodiment of the present invention, as described above, the frequency of the probe light is changed by the electric signal, so that the frequency can be changed surely and quickly. Therefore, the measurement can be performed very efficiently without wasting time for changing the frequency as in the past.

There are two methods of optical modulation by the optical modulator 2 described above, intensity modulation and phase modulation. Since frequency modulation is equivalent to phase modulation, its description is omitted. Hereinafter, the intensity modulation and the phase modulation will be described in order. (1) Intensity Modulation Now, in the optical modulator 2, when the emission light from the light source 1 is intensity-modulated by the sine wave signal of the 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 respectively represent the amplitude and frequency of the incident light on the optical modulator 2. Further, m represents the modulation factor and t represents time. Now, expanding the equation, 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 the carrier wave. The second and third terms on the right side of the equation are sideband waves 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 frequencies of the backward Rayleigh scattered light due to the carrier wave received by the optical receiver 10 and the local oscillation light are the sideband wave and the local oscillation light. It matches the beat frequency. Therefore, only the backward Rayleigh scattered light due to the sideband cannot be separated and measured. Further, as can be seen from the formula, the frequency of the light of the carrier wave does not change even by the modulation by the optical modulator 2, so the fading noise of the backward Rayleigh scattered light due to the carrier wave is not reduced.

Therefore, the ratio of the backward Rayleigh scattered light power due to the sideband that can be expected to reduce fading noise and the backward Rayleigh scattered light power due to the carrier 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,
It is 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 emitted light from the light source 1 is phase-modulated with a sine wave signal, the amplitude of the modulated light is e (t) = E _C sin {2πf It can be expressed as _C t + Δφ cos (2πpt)} ... Here, Δφ represents the 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 wave having the same frequency as the incident light to the optical phase modulator, the even terms and the second term and below are the upper sideband waves, and the odd terms are the lower sideband waves. J _n
Indicates the n-th order Bessel function of the first kind. Here, FIG. 2 shows the 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 is understood that it is possible to make the carrier power zero by selecting *). That is, by using a phase modulator for the optical modulator 2, unlike the case of the intensity modulation described above, it is possible to measure only the backward Rayleigh scattered light due to sidebands, and as a result, fading noise is significantly reduced. Will be reduced.

[Second Embodiment] Next, a second embodiment of the present invention will be described. FIG. 3 is a block diagram showing the configuration of a backscattered light (backward Brillouin scattered light) measuring apparatus according to the second embodiment of the present invention. In this figure, parts corresponding to the parts in FIG. , And the description is omitted. 3 is different from that of FIG. 1 in 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 of the light source 1. That is the point.

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

The backscattered light obtained by scattering the pulsed light backward by the optical fiber 5 to be measured is composed of rear Rayleigh scattered light and rear Brillouin scattered light. The backward Rayleigh scattered light is not accompanied by frequency shift, and therefore its spectrum is the same as that of the incident light and remains as shown in FIG. 4 (b). 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) Will be things. Further, the spectral line width of the backward Brillouin scattered light is further widened by the Brillouin line width Δf _B.

These backscattered lights are combined with local light of frequency f _C (see FIG. 4 (e)) by the optical fiber coupler 9 and coherent detection is performed. At this time, the beat electric signal spectrum received by the optical receiver 10 is as shown in FIG. In FIG. 5A, a signal a is a backward Rayleigh scattered light, signals b and d are backward Brillouin scattered light due to sidebands, and a signal c is a received signal corresponding to backward Brillouin scattered light due to a carrier wave.

Of these signals, the signals c and d have a frequency f.
It is a high frequency signal of _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 be a value close to the frequency f _B , the signal b becomes a low frequency signal of frequency | p−f _B |, and the optical receiver 10 can receive it with high sensitivity. further,
If the low cutoff frequency f _LC and the high cutoff frequency f _UC of the optical receiver 10 are selected as shown in the formulas and formulas, the strong backward Rayleigh scattered light is blocked and only the signal b corresponding to the backward Brillouin scattered light is obtained. It becomes possible to receive. _{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 input to the detection circuit 14 as it is, but in consideration of performing digital signal processing in the detection circuit 14 or the signal processing device 15 thereafter, the signal b is converted into a baseband signal. It is desirable to do. Therefore, the signal b is further mixed by the mixer 11 with the electric signal of the frequency f _L = | p−f _B | from the local electric signal source 12, and the bandwidth B [1 / W + (Δ
f _B / 2) ≦ B ≦ f _L + | p−f _B | − {1 / W + (Δf _B
/ 2)}] is passed through to obtain a baseband signal b1 (see FIG. 5B).

As can be seen from the above description, the Brillouin shift f _B of the optical fiber 5 to be measured is the modulation frequency p when the power of the received signal is maximum and the frequency f _{L of the} local electric signal source 12. And, f _B = p−f _L (when p−f _B > 0) f _B = p + f _L (when p−f _B <0)

As can be seen from the above description, the signal b shown in FIG. 5 (a) is a signal in which the Stokes light and the anti-Stokes light overlap on the frequency spectrum of the electric signal. Therefore, the second embodiment described above is suitable for increasing the signal intensity of the backward Brillouin scattered light. However, on the other hand, there are cases where it is desired to measure the Stokes light and the anti-Stokes light separately (for example, in the case of temperature measurement in which the difference in the temperature dependence of the scattering coefficient of the Stokes light and the anti-Stokes light is applied). In such a case, the frequency of the signal light or the local light may be shifted. For example, when an acousto-optic light modulator is used as the optical pulse modulator 4, the frequency of the modulated pulsed 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 Stokes light and the anti-Stokes light of the backward Rayleigh scattered light and the backward Brillouin scattered 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 equation. f _AO = | p−f _B | ... At this time, the signal b ′ received as the baseband signal
When p−f _B > 0, the anti-Stokes light has p−f _B <
When 0, it corresponds to Stokes light. Therefore, by shifting the frequency of the signal light, the backscattered light, or the local light, the Stokes light of the backward Brillouin scattered light and the anti-Stokes light can be separately measured.

As can be seen from the above description, the Brillouin shift f _B of the optical fiber 5 to be measured is the modulation frequency p at which the power of the received signal is maximum and the frequency shift f _AO of the acousto-optical modulator. F _B = p−f _AO (when p−f _B > 0) f _B = p + f _AO (when p−f _B <0)

(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), etc., including higher sidebands.
However, when the frequency of the beat electric signal between these higher-order sidebands and the local oscillation light of frequency f _O is (p to f _B ),
Since the frequencies are very high as (˜2f _B ), (˜3 f _B ), ..., Backscattered light signals due to these higher-order sidebands are not substantially received by the optical receiver 10.

Therefore, when the phase modulation is performed at the frequency of (p to f _B ), the spectrum of the electric signal received by the optical receiver 10 becomes the same as the case of the intensity modulation, and FIG.
Given in (a). However, in the case of phase modulation, by selecting the phase shift Δφ as j _0.m (root of the Bessel function of the first kind), it is possible to make the carrier component of the modulated light zero. The signal a corresponding to the strong backward Rayleigh scattered light in FIG. 5A can be extinguished.
Therefore, by adopting the phase modulation, it is possible to significantly relax the condition required for the low cutoff frequency characteristic in the optical receiver 10.

Further, since the Stokes light of the backward Brillouin scattered light and the anti-Stokes light are separately measured, 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 block the backward Rayleigh scattered light signal, it is possible to greatly relax the condition required for the frequency characteristic of the optical receiver 10.

Furthermore, the phase modulation has a
It also has the following advantages. That is, by selecting an appropriate value for the phase shift Δφ, as can be seen from FIG. 2, the power of the sideband which is the measurement signal can be made larger than that of the carrier wave. This enhances the dynamic range of the measurement system for backscattered light, and thus works particularly advantageously when the probe light is optically amplified. This is because there is an upper limit on the optical power that can be obtained by optical amplification or the optical power that can be input to the optical fiber under test, so when the sideband power is larger than the carrier power, the maximum This is because the sideband power of 1 can be 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 the configuration of a backscattered light (backward Brillouin scattered light) measuring apparatus according to the third embodiment of the present invention. In this figure, parts corresponding to those in FIG. , And the description is omitted. The measuring apparatus shown in FIG. 7 differs from that of FIG. 3 in that the backscattered light from the measured optical fiber 5 is not the probe light incident on the measured optical fiber 5 but the optical modulator 2
It is a point where the sideband wave of the modulated backward Brillouin scattered light is coherently detected by the light modulation by. The spectrum of the backscattered light that is combined with the local light by the optical fiber coupler 9 is the same as in the case of the second embodiment (see FIG. 4).
(See (b) to (d)). Therefore,
Also in this embodiment, it is possible to measure the backward Brillouin scattered light with high sensitivity as in the case of the second embodiment.

[Fourth Embodiment] Next, a fourth embodiment of the present invention will be described. FIG. 8 is a block diagram showing the configuration of a backscattered light (backward Brillouin scattered light) measuring apparatus according to the fourth embodiment of the present invention. In this figure, parts corresponding to those in FIG. , And the description is omitted. The measurement apparatus shown in FIG. 8 is different from that shown in FIG. 3 in that the local light generated by the optical modulator 2 is not the probe light incident on the measured optical fiber 5 or the backscattered light from the measured optical fiber 5. This is a point where the light is modulated and the backward Brillouin scattered light is coherently detected using the sideband wave of the modulated local light.

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

Further, as in the case of the second embodiment, when an acousto-optic modulator is used for the optical pulse modulator 4 to shift the frequency of the probe light by f _AO , it is received by the optical receiver 10. The spectrum of the electric signal is only the spectrum of the second and third embodiments (see FIG. 6) plus the signal of frequency (p ± f _AO ). Therefore, also in the present embodiment, it is possible to measure the backward Brillouin scattered light with high sensitivity as in the second and third embodiments. Note that the backward Brillouin scattered light can be measured in the same manner even when phase modulation is used instead of intensity modulation, which is exactly the same as in the above-described 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, the present invention uses the same components for both the backward Rayleigh scattered light measurement system and the backward Brillouin scattered light measurement system. Its configuration is different. Therefore, the fifth embodiment uses an optical switch to easily switch between the two measurement systems to enable measurement. 11 to 13 are block diagrams showing the configuration of a backscattered light measuring apparatus according to a fifth embodiment of the present invention. In these figures, parts corresponding to those in FIG. , The description is omitted. 11 to 13, an optical switch 16 having input / output terminals x ₁ , x ₂ and y _{1 to} y ₃ and an optical switch 16 having input / output terminals x ₁ ′, x ₂ ′ and y ₁ ′ to y ₃ ′. A switch 17, an optical fiber 18 connecting the terminals y ₁ and y ₁ ′, and an optical fiber 19 connecting the terminals y ₃ and y ₃ ′ 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.
Although it is arranged in the optical path between the optical fiber coupler 7 and the optical switch 16, it 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.
In order to configure the system of the first embodiment shown in FIG. 1, in the optical switch 16, as shown by the solid line in FIG. 11, the terminals x ₁ and y ₁ and the terminals x ₂ and y are provided. ₂ and are connected. At the same time, in the optical switch 17, as shown in FIG.
, The terminals x ₁ ′ and y ₁ ′ are
Also, the terminals x ₂ ′ and y ₂ ′ are connected.

Next, the system of the second embodiment shown in FIG. 3 is constructed.
To achieve this, the optical switch 16 has the structure shown in FIG.
, The terminal x₁And y₂But also the terminal
x₂And y₃Switch so that and become connected. simultaneous
Also in the optical switch 17, the broken line in FIG.
As shown, terminal x₁'And y₂'Is also the terminal
x ₂'And y₃′ Is switched to the connected state.

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 first implementation. The configuration of the example (see FIG. 1) and the configuration of the fourth embodiment (see FIG. 8) are switchable configurations. The operation of these devices is almost the same as that of the device of FIG. 11 described above, and thus the description thereof is omitted.

11 to 13, various types of optical switches 16 or 17 such as mechanical type, thermal effect type, electro-optical effect type, acousto-optical type can be used.
In the present invention, a mechanical type that is economical, has a small insertion loss, and is excellent in crosstalk is suitable because high speed switching time is not required so much. As an example thereof, FIG. 14 shows the configuration of a slide type optical switch. As shown in FIG. 14, this optical switch has a structure in which a plurality of optical waveguides or optical fibers 20 aligned at equal intervals (U and V) 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. In addition, the optical switch 1 shown in FIG.
The structure of the slide type optical switch in which 6 and 17 are integrated is shown. Both of the optical switches shown in FIGS. 15A and 15B can be switched by sliding for the alignment interval.

In the above-described first to fifth embodiments, all examples using the optical pulse modulator 4 are shown. 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 by temporally separating and the distribution in the length direction of the measured optical fiber 5 is obtained. Therefore,
The optical pulse modulator 4 is not necessary when measuring the total power of the backscattered light from the entire measured optical fiber 5.

In the first configuration of the second to fourth embodiments and the fifth embodiment (see FIGS. 3, 7, 8 and 11), the light emitted from the light source 1 is first emitted. , The optical pulse modulator 4 after being modulated by the optical modulator 2.
However, it is needless to say that the order may be reversed. Furthermore, in the present invention, since a coherent light source having a small optical output is usually used for the light source 1, it is not necessary to use an optical fiber amplifier or an optical amplifier such as a semiconductor laser amplifier together. It is very effective in improving the range. The insertion position of this optical amplifier is
1, FIG. 3, FIG. 7, FIG. 8 and FIG.
In the section, 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 measured optical fiber 5, and between the optical fiber coupler 8 and the optical fiber coupler 9. , And between the optical fiber coupler 7 and the optical fiber coupler 9.

In addition, in the above-mentioned second to fourth embodiments, only the case where a part of the emitted light of the light source 1 is used for the local light emission has been described, but the light source 1 can be used by means such as injection locking. Light emitted from another light source that oscillates at the same frequency as the above may be used for local light. In this case, although there is a demerit that many light sources are used, there is an advantage that a local oscillation power required for coherent reception can be sufficiently obtained without impairing the feature of high-speed frequency control. .

[0055]

As described above, according to the present invention,
It has the following effects. (B) When measuring the backward Brillouin scattered light from the optical fiber to be measured, since the backward Brillouin scattered light from the optical fiber to be measured is coherently received by the sideband that is optically modulated probe light, the received electrical signal at the optical receiver is received. The frequency of can be lowered, and highly sensitive measurement is possible. In addition, since the frequency of the probe light can be changed electrically quickly, high-speed measurement is possible.

(B) In measuring the backward Brillouin scattered light from the optical fiber to be measured, the sideband wave obtained by optically modulating the backward Brillouin scattered light from the optical fiber to be measured is coherently received. The frequency of can be lowered, and highly sensitive measurement is possible. Also,
Since the frequency of the backscattered light can be changed electrically quickly,
High-speed measurement is possible.

(C) In measuring the backward Brillouin scattered light from the optical fiber to be measured, since the sideband wave in which the local light is optically modulated and the backward Brillouin scattered light from the optical fiber to be measured are received, The frequency of the electric signal received by the optical receiver can be lowered, and highly sensitive measurement is possible. Moreover, since the frequency of local light can be changed electrically rapidly, high-speed measurement is possible.

[Brief description of drawings]

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

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

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

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

FIG. 5 is a diagram showing an example of a spectrum of an electric signal in the second embodiment of the present invention.

FIG. 6 is a diagram showing an example of a spectrum of an electric signal received by the optical receiver 10 when an acousto-optical modulator is used as the optical pulse modulator 4 in the second embodiment of the present invention. .

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

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

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

FIG. 10 is a diagram showing an example of a spectrum of an electric signal in the fourth embodiment of the present invention.

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

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

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

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

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

[Explanation of symbols]

1 Light Source 2 Optical Modulator 3 Modulated Signal Source 4 Optical Pulse Modulator 5 Optical Fiber Under Test 6 Timing Controller 7, 8, 9 Optical Fiber Coupler 10 Optical Receiver 10 _a FET Amplifier 10 _b Photodiode 11 Mixer 12 Local Electricity Signal source 13 Electric filter 14 Detection circuit 15 Signal processing device 16,17 Optical switch 18,19 Optical fiber.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kaoru Shimizu Kaoru Shimizu 2-3-1, Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP 61-201133 (JP, A) JP Sho 62-217135 (JP, A) JP 62-123327 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01M 11 / 00-11 / 08

Claims (4)

(57) [Claims] 1. A light source having a narrow spectral width is used, and light emitted from the light source is optically modulated to generate a sideband having a spectrum separated from the spectrum of the emitted light, and the modulated light is generated. To the power of the rear Brillouin scattered light by coherently receiving the rear Brillouin scattered light in which the sideband is scattered backward in the optical fiber to be measured and propagates in the incident direction. And a method for measuring backscattered light. 2. A light source having a narrow spectral width is used, light emitted from the light source is incident on an optical fiber to be measured, and backscattered light that is scattered backward in the optical fiber to be measured and propagates in an incident direction is emitted. A method for measuring backscattered light, characterized in that the power of the backscattered Brillouin scattered light is obtained by modulating and coherently receiving the sidebands of the back Brillouin scattered light generated by the light modulation. 3. A backward oscillation light source having a narrow spectrum width is used, light emitted from the continuous oscillation light source is incident on an optical fiber to be measured, and is scattered backward in the optical fiber under measurement and propagates in an incident direction. Brillouin scattered light, a part of the emitted light of the continuous wave light source, or the emitted light of another light source, by coherently receiving the sideband of the light-modulated light as local light, the rear Brillouin scattered light of A method for measuring backscattered light , characterized by obtaining power. 4. A continuous wave light source having a narrow spectrum width, a first optical means for separating a part of light emitted from the continuous wave light source into local light, and the emitted light into pulsed light. An optical pulse modulator for modulating, a second optical means for making the pulsed light incident on the optical fiber to be measured, and a third means for multiplexing the backscattered light from the optical fiber to be measured and the local light. And an optical receiver for receiving the combined light and receiving a beat electric signal of the backscattered light and the local light, and detecting the amplitude or power of the beat electric signal. In a backscattered light measuring device comprising a detection circuit and a signal processing device for processing a signal detected by the detection circuit, for modulating the emitted light from the continuous wave light source separated from the local oscillation light. , Or the pulse An optical modulator for modulating light, or for modulating the backscattered light, or for modulating the local light, and a variable frequency generator for supplying a modulated electric signal to be input to the optical modulator. A device for measuring backscattered light, comprising: a modulation signal source.