JP2000180265A - Brillouin gain spectrum measuring method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and device capable of measuring the Brillouin gain spectrum by the space resolving power of 1 m or less. SOLUTION: A pump light and a probe light emitted from a light source 1 and a light source 2 respectively, and subjected to frequency modulation by a prescribed modulation frequency are incident on both ends of an optical fiber 6 to be measured, when the frequency difference between the central frequency of the pump light and that of the probe light is changed so that it becomes the vicinity of the brillan-frequency shift, the power movement from the pump light to the probe light is selectively caused in the position where the phases of both lights are synchronized. After emitted from the optical fiber 6 to be measured, the power of the probe light branched by an optical turnout 4 is detected by an optical detector 5, so that the Brillouin gain spectrum in the position where the phases of both lights are synchronized can be measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for measuring a Brillouin gain spectrum of an optical fiber.
The present invention relates to a technique for improving the spatial resolution of a position where a Brillouin scattering phenomenon occurs in an optical fiber.

[0002] This technique can be applied to an optical fiber sensor or the like that uses an optical fiber to measure a physical quantity distribution such as temperature and strain in an environment where the optical fiber is installed. Optical fiber sensors include, for example, (1) maintenance and management of optical communication paths, (2) maintenance and management of large-scale structures such as dams and embankments,
(3) Used for materials and structures that have a self-diagnosis function for defects and failures (smart material structures). Especially if it is an optical fiber sensor utilizing the present invention,
The spatial resolution of the physical quantity distribution measurement considered to be important in (2) and (3) can be increased.

[0003]

2. Description of the Related Art As a technique for measuring strain or temperature distribution in an environment where an optical fiber is installed, there is a method based on a Brillouin scattering phenomenon in the optical fiber. In this method, since the installed optical fiber itself is used as a medium for detecting distortion or temperature, distribution measurement can be realized with a simpler configuration than a method of arranging many point sensors.

The Brillouin scattering phenomenon is a phenomenon that, when two light waves having different frequencies pass each other in an optical fiber, power moves from high-frequency light to low-frequency light via an acoustic wave in the optical fiber. Say. When the frequency difference between two light waves passing each other is ν, the moving power is approximately the Brillouin gain spectrum gB defined by Equation 1.
(ν).

[0005]

(Equation 1)

Here, νB and ΔνB are parameters characterizing the Brillouin gain spectrum, and are called Brillouin frequency shift and Brillouin gain line width (full width at half maximum), respectively. Brillouin gain spectrum gB (ν)
Takes a value of 1/2 or more in the frequency range ΔνB centered on the Brillouin frequency shift νB. In other words, in order to efficiently generate the Brillouin scattering phenomenon in the optical fiber, the frequency difference within the frequency range of the Brillouin gain line width ΔνB around the Brillouin frequency shift νB with respect to the frequency between two light waves passing through the optical fiber is considered. Needs to be given. The Brillouin frequency shift νB is given by Equation 2 where the sound speed in the optical fiber is va.

[0007]

(Equation 2)

Here, n is the refractive index of the optical fiber, and λ is the wavelength of light incident on the optical fiber. As a result of the sound velocity va varying depending on the temperature around the optical fiber and the strain applied to the optical fiber, the measurement of the Brillouin frequency shift νB provides a means for sensing temperature and strain via equation 2. As a numerical example of the change sensitivity of the Brillouin frequency shift νB, at a light wavelength of 1.3 μm,

[0009]

(Equation 3)

[0010] With respect to temperature

[0011]

(Equation 4)

It is reported in the following document (1). (1) M. Nikles et al., Journal of Lightwave Technol
ogy, vol. 15, pp. 1842--1851, (1997) Therefore, the Brillouin gain spectrum gB (ν) is measured as a function of the position along the optical fiber, and the Brillouin frequency shift νB is Obtaining as a function makes it possible to measure strain or temperature distribution.

As a conventional technique for measuring the above-mentioned Brillouin gain spectrum, a pulse pump light and a continuous wave probe light having a variable frequency difference are irradiated with a continuous wave probe light from one end of an optical fiber to be measured. BOTDA (a method of measuring the distribution of the Brillouin gain spectrum gB (ν) along the optical fiber by measuring the change component of the probe light caused by the Brillouin scattering phenomenon as a function of time by being incident from the other end of the measurement optical fiber, respectively. Brillouin Op
tical Time Domain Analysis) has been used.

As a technique equivalent to BOTDA, a Brillouin gain spectrum g is obtained by injecting pulsed light from one end of an optical fiber to be measured and measuring the spectrum of scattered light due to the natural Brillouin scattering phenomenon as a function of time.
BO, a method of finding the distribution of B (ν) along the optical fiber
TDR (Brillouin Optical Time Domain Reflectometry
) Has also been used. BOTDA and BOTDR are described by the inventors in the following reference (2). (2) T. Horiguchi et al., Journal of Lightwave Technol
ogy, vol.13, pp. 1296--1302, (1995)

The spatial resolution δz of both BOTDA and BOTDR prior arts is as follows: W is the time width of the light pulse incident on the optical fiber as pump light, and v is the speed of light in the optical fiber.

[0016]

(Equation 5)

Is given by Therefore, in order to increase (narrow) the spatial resolution δz, it is necessary to shorten the light pulse time width W. However, the Brillouin gain spectrum gB (ν)
In particular, in order to accurately measure the Brillouin frequency shift νB, which is the frequency at which it takes the maximum value, it is necessary to set the optical pulse time width W to be equal to or greater than the lower limit shown below. Limited to values greater than ~ 3m.

In both prior arts, an optical pulse used as pump light is produced by pulsing continuous light having a frequency difference ν from probe light using an optical switch or the like. Then, for example, regarding the aforementioned BOTDA, the spectrum of the beat signal generated by the interference between the pump light having the pulse time width W and the continuous wave probe light in the optical fiber to be measured is about 2 / W around ν. Has a large value in the frequency range. The power that moves from the pump light to the probe light due to the Brillouin scattering phenomenon depends on the spectrum of the beat signal,
Brillouin gain spectrum gB (ν) given by Equation 1
When light having a pulse time width W is used as pump light, the integral value in a frequency range of about 2 / W centered on ν of gB (ν) becomes Will be measured.

The Brillouin gain spectrum gB (ν) to be measured and given by Equation 1 has a value of 以上 or more in the frequency range of the Brillouin gain line width (full width at half maximum) ΔνB around the Brillouin frequency shift νB. I take the. Therefore, if the integration range 2 / W becomes wider than twice the full width at half maximum ΔνB as a result of using the pump light having the pulse time width W, even if the frequency difference ν is changed near the Brillouin frequency shift νB, the integrated value is almost zero. Since the Brillouin gain spectrum is constant, it becomes difficult to accurately measure the frequency difference at which the Brillouin gain spectrum has the maximum value.

For the above reasons, in order to accurately determine the Brillouin frequency shift νB, it is necessary to use an optical pulse having a pulse time width W equal to or larger than the lower limit given by the following equation.

[0021]

(Equation 6)

Experimental value of Brillouin gain line width ΔνB 30 M
Hz and the speed of light in the optical fiber v = 2 × (10 8) m /
s, the spatial resolution δz given by equation 5 is 3 m
Becomes

[0023]

[Problems to be Solved by the Invention] BOTDA as prior art
And BOTDR, because the spatial resolution is limited to a value greater than 2-3m, the materials and structures having the maintenance management and self-diagnosis function of large-scale structures listed in the section of the technical field to which the invention belongs, 1m recently required
The following spatial resolution could not be realized.

An object of the present invention is to provide a method and an apparatus capable of measuring a Brillouin gain spectrum gB (ν) with a spatial resolution of 1 m or less. Another object of the present invention is to provide a method and an apparatus capable of measuring the distribution of the Brillouin gain spectrum gB (ν) along an optical fiber with a spatial resolution of 1 m or less. The provision of the method and apparatus leads to the realization of strain distribution and temperature distribution sensors in applications that could not be realized by the prior art.

[0025]

In order to solve the above-mentioned problems, a method and apparatus for measuring a Brillouin gain spectrum according to the present invention use continuous light in which both pump light and probe light are modulated at a predetermined modulation frequency. did. That is, in the Brillouin gain spectrum measuring method according to the first aspect of the present invention, the first continuous wave light frequency-modulated at a predetermined modulation frequency and the second continuous wave light frequency-modulated at a modulation frequency equal to the predetermined modulation frequency are provided. A Brillouin gain spectrum measuring method using oscillating light, wherein the first continuous oscillation light is made incident from one end face of the optical fiber to be measured, and
The center frequency of the continuous oscillation light is frequency-shifted, and the second continuous oscillation light whose center frequency is shifted by the frequency shift is incident from the other end face of the optical fiber to be measured, and the center frequency of the second continuous oscillation light is changed. And the power of light emitted from the one end face or the other end face of the measured optical fiber is measured, and the phase of the first continuous wave light and the second 2
The Brillouin gain spectrum at a position where the phase of the continuous wave light is synchronized and the correlation value increases is measured.

According to a second aspect of the present invention, in the Brillouin gain spectrum measuring method according to the first aspect, the Brillouin gain spectrum in the optical fiber to be measured is changed by changing the predetermined modulation frequency. The measurement position is changed.

According to a third aspect of the present invention, there is provided a method for measuring a Brillouin gain spectrum, comprising branching a continuous wave light frequency-modulated at a desired modulation frequency, and obtaining a first continuous wave light and a second continuous wave light obtained by the branch. A Brillouin gain spectrum measuring method using light, wherein the first continuous wave light is delayed, the delayed first continuous wave light is incident from one end face of the optical fiber to be measured, and the second continuous wave light is measured. The center frequency of the oscillation light is frequency-shifted, and the second continuous oscillation light whose center frequency is shifted by the frequency shift is made incident from the other end face of the optical fiber to be measured, and the frequency of the center frequency of the second continuous oscillation light is changed. By changing the shift amount and measuring the power of the light emitted from the one end face or the other end face of the measured optical fiber, the first continuous light is emitted from the measured optical fiber. By measuring the Brillouin gain spectrum at a position where the phase of the light and the phase of the second continuous oscillation light are synchronized and the correlation value increases, and by changing the modulation frequency, the first continuous oscillation light of the first continuous oscillation light is measured in the optical fiber to be measured. The Brillouin gain spectrum is measured by changing the position where the phase and the phase of the second continuous wave light are synchronized to increase the correlation value, and the distribution of the Brillouin gain spectrum in the measured optical fiber is measured.

According to a fourth aspect of the present invention, there is provided a Brillouin gain spectrum measuring apparatus comprising: a first light source for outputting a first continuous oscillation light frequency-modulated at a predetermined modulation frequency; and a frequency equal to the predetermined frequency. A second light source that outputs a modulated second continuous oscillation light, an optical frequency converter that provides a desired frequency shift with respect to a center frequency of the second continuous oscillation light, and the first continuous oscillation light. While receiving and entering one end of the measured optical fiber, and after undergoing a frequency shift by the optical frequency converter, the incident on the other end of the measured optical fiber is propagated through the measured optical fiber and emitted. Optical means for guiding at least a part of the second continuous oscillation light, and a photodetector for measuring the power of the light guided by the optical means are provided. Then, a Brillouin gain spectrum at a position where the phase of the first continuous wave light and the phase of the second continuous wave light are synchronized in the measured optical fiber is measured.

According to a fifth aspect of the present invention, there is provided a Brillouin gain spectrum measuring apparatus, comprising: a light source for outputting continuous oscillation light frequency-modulated at a desired modulation frequency; Optical output means for outputting a second output light, an optical frequency converter for giving a desired frequency shift to a center frequency of the first output light, and an optical delay for giving a predetermined delay time to the second output light Receiving the second output light delayed by the optical delay device and entering one end of the optical fiber to be measured, receiving the frequency shift by the optical frequency converter, and Optical means for guiding at least a part of the first output light which has been incident on the end and propagated through the optical fiber to be measured and emitted, and a photodetector for measuring the power of the light guided by the optical means. Have. Then, by changing the desired modulation frequency, the position where the phase of the first output light and the phase of the second output light incident from both ends of the optical fiber to be measured are synchronized is changed, and the optical fiber to be measured is changed. Is characterized in that the distribution of Brillouin gain spectrum can be measured.

[0030]

In the following, since frequency modulation and phase modulation are equivalent technologies, the operation will be described using phase modulation in order to simplify the process of deriving the theoretical equation of spatial resolution. A similar description is possible.

First, the means for measuring the Brillouin gain spectrum gB (ν) at a predetermined position of the optical fiber to be measured will be described. Pump light and probe light that have been phase-modulated at a predetermined frequency are incident on one end and the other end of the measured optical fiber, respectively. The frequency difference between the center frequencies of the two lights is ν
Then, a beat signal of a single frequency ν is generated at a position where the phases of both lights are synchronized and the correlation value increases in the optical fiber to be measured, while a phase modulation is performed at a position where the phases of both lights are asynchronous and the correlation value is low. A beat signal is generated.

The power that moves from the pump light to the probe light due to the Brillouin scattering phenomenon is given by the overlap integral of the spectrum of the beat signal of both lights and the Brillouin gain spectrum gB (ν), as described in the background art. Focusing on the spectrum of the beat signal, a beat signal having a single frequency ν is generated at a position where the phases of both lights are synchronized,
While the spectrum has a sharp peak, a phase-modulated beat signal is generated at a position where the phases of the two lights are asynchronous, and the spectrum spreads around ν. When the frequency difference ν between the center frequencies of the two lights is changed, the center of the spectrum of the beat signal also shifts. During this time, the power moving from the pump light to the probe light at the position where the phases of the two lights are synchronized is the spectrum of the beat signal. Are sharp peaks, and change according to the Brillouin gain spectrum gB (ν). On the other hand, at a position where the phases of both lights are asynchronous, even if the frequency difference ν is changed, the moving power hardly changes.

The reason is that the spectrum of the beat signal at the asynchronous position has a large value in the frequency range centered on ν and determined according to the distance from the position where the closest phase is synchronized, so that gB (ν) is This is because the power integrated in the frequency range moves. Therefore, by changing the frequency difference ν between the center frequencies of the two lights near the Brillouin frequency shift νB and measuring the change in the power of the output probe light or the change in the power of the output pump light, The Brillouin gain spectrum gB (ν) at the position where the phase of the pump light and the phase of the probe light are synchronized can be measured.

A plurality of positions where the phase of the pump light and the phase of the probe light are synchronized (hereinafter referred to as a synchronization point) exist at intervals determined according to the modulation frequency of the phase modulation, but exist in the fiber to be measured. It is possible to set the modulation frequency of the phase modulation so that the synchronization point is limited to one point. Further, by changing the modulation frequency of the phase modulation, the synchronization point, that is, the position at which the Brillouin gain spectrum is measured can be changed. For these implementation means, the theoretical formula of the spatial resolution is derived below. It will be explained in the process.

Next, by deriving the theoretical formula of the spatial resolution obtained in the present invention, and substituting the numerical values used in the experimental results described later in the examples into the theoretical formula, it is confirmed that the spatial resolution of 1 m or less can be obtained. Show. The center frequencies of the pump light and the probe light are denoted by ν1 and ν2, respectively.
A sinusoidal phase modulation with a modulation frequency νm is applied. The complex amplitudes of the optical electric fields of the pump light and the probe light are E1 and E1, respectively.
As 2

[0036]

Equation 7

[0037]

(Equation 8)

## EQU1 ## Here, t is time, z is position, and v is the group velocity of both lights. Beat signal E1 of both lights
(The complex conjugate of E2) is

[0039]

[Equation 9]

[0040]

(Equation 10)

Is given by From Equation 10, the position where the phase of the pump light and the phase of the probe light are synchronized, that is, the synchronization point is

[0042]

[Equation 11]

In the equation (10), the position where the temporally oscillating component at the modulation frequency νm disappears, that is,

[0044]

(Equation 12)

Is defined as the position where Therefore, in order to limit the number of synchronization points existing in the measured optical fiber to one point, the modulation frequency νm may be set so that the interval v / νm between the synchronization points is longer than the length of the measured optical fiber. Further, a predetermined delay time is given between the pump light and the probe light using an optical delay device, and a setting is made such that a non-integer multiple (N ≠ 0 in Equation 11) synchronization point exists in the optical fiber to be measured. By doing so, by changing the modulation frequency νm and changing the position of the synchronization point, a means for measuring the Brillouin gain spectrum distribution in the measured optical fiber can be obtained.

In the prior art, as described above, pulsed light is used as pump light. Therefore, by changing the time from when the pulsed pump light is output to when the Brillouin gain is measured, the time in the optical fiber to be measured is changed. Was able to change the Brillouin gain measurement position. In the present invention, the position of the synchronization point between the pump light and the probe light must be changed,
At present, the change of the synchronization point position due to the variable delay amount of the optical delay device is very small and not practical. Therefore, as described above, the modulation frequency ν is used by using the synchronization point position of a non-zero integer multiple.
The method of changing the synchronization point position by changing m is taken.

Since the spectrum of the beat signal at the synchronization point has a single frequency of ν = [absolute value of (ν 1 −ν 2)], the frequency difference ν between the center frequencies of the pump light and the probe light is calculated by the Brillouin frequency shift ν B Change near
By measuring the power of the pump light or the probe light emitted from the measured optical fiber, the Brillouin gain spectrum at the synchronization point is measured. Here, in order to obtain the spatial resolution, considering a beat signal near the synchronization point and setting z = vN / νm + ε (where ε represents a small deviation from the synchronization point), z The beat signal is a function of ε, t

[0048]

(Equation 13)

[0049]

(Equation 14)

Is as follows. The decryption takes + and-depending on the evenness of N, respectively. From equation 14, the instantaneous frequency ν of the beat signal
(T) is

[0051]

(Equation 15)

Thus, the spectrum of the beat signal at a position ε away from the synchronization point is

[0053]

(Equation 16)

Occupies the frequency band of In the prior art, the condition that the frequency band occupied by the beat signal (given as 2 / W from the pulse time width W) is twice the Brillouin gain line width (full width at half maximum) ΔνB of the Brillouin gain spectrum gB (ν). As the spatial resolution is determined, also in the present invention, when the spatial resolution δz is determined from the condition that the frequency band of the beat signal given by Expression 16 is twice the full width at half maximum ΔνB,

[0055]

(Equation 17)

Is obtained. From Equation 17, it can be seen that increasing the modulation frequency νm of the phase modulation improves the spatial resolution. Experimental value of Brillouin gain linewidth ΔνB 30MH
z, the speed of light in the fiber v = 2 × (10 8) m /
s, modulation index m = 50, modulation frequency νm = 7.5 MHz of phase modulation used in the experimental results described later in Examples.
Is substituted into Equation 17, the spatial resolution δz obtained is 35 cm, and a spatial resolution of 1 m or less is realized. Further, Equation 13
Using the Bessel function of the first kind

[0057]

(Equation 18)

[0058]

(Equation 19)

Using the fact expressed as follows, the modulation frequency νm of the phase modulation

[0060]

(Equation 20)

Under the following conditions, a theoretical formula of the spatial resolution can be obtained using only the amplitude J0 (y) of the zero-order spectral component of the beat signal. Because, under the conditional expression 20, the non-zero order spectral component is the Brillouin gain spectrum gB (ν)
Is outside the frequency band having a large value of 1/2 or more. When the zero-order component J0 (y) is suppressed to 1/2 or less, and the spatial resolution δz is obtained from the condition that the power transfer from the pump light to the probe light is suppressed, J0 (π / 2) ≒
Using 0.5

[0062]

(Equation 21)

Is obtained. νm = 50MHz> ΔνB / 2 =
A spatial resolution δz = 5 cm is obtained at 15 MHz and a modulation index m = 20.

The correlation value in the description that the phase of the pump light and the probe light used in the description above is synchronized and the correlation value of the two lights increases is the zero order of the beat signal given by the equation (18). Means the amplitude J0 (y) of the spectral component of Since the zero-order Bessel function of the first kind takes the maximum value at y = 0, since ε means a small deviation from the synchronization point at y defined by Equation 19, the correlation value at the synchronization point is It can be seen that the correlation value takes the maximum value and decreases as the position deviates from the synchronization point.

[0065]

FIG. 1 shows a Brillouin gain spectrum measuring apparatus according to a first embodiment of the present invention. The Brillouin gain spectrum measuring apparatus according to the first embodiment includes a first light source 1 for generating pump light frequency-modulated at a predetermined modulation frequency, and a second light source 1 for generating probe light frequency-modulated at a predetermined modulation frequency. Light source 2, second light source 2
An optical frequency converter 3 for shifting the center frequency of the probe light output from the optical fiber by a desired frequency, and pump light incident from one end of the measured optical fiber 6 and emitted from the measured optical fiber 6 A light splitter 4 as an optical means for guiding at least a part of
And a photodetector 5 for detecting the power of the probe light split by the optical splitter 4.

The first light source 1 and the second light source 2, which generate light modulated at a predetermined frequency, modulate the injection current of the semiconductor laser at a predetermined modulation frequency to convert the frequency-modulated light. It is realized by means for generating. As the optical frequency converter 3 for giving a desired frequency shift to the center frequency of the probe light output from the second light source 2, a field effect type light intensity modulator or the like is used. When amplitude modulation is applied using a light intensity modulator, frequency shift light is generated, and the amount of frequency shift is equal to the frequency of the amplitude modulation.
As the optical means, at least a part of the probe light emitted from the optical fiber 6 to be measured may be guided to the photodetector 5, and thus, in addition to the optical splitter, an optical circulator, a beam splitter, a half mirror, or the like is used. You may.

Pump light and probe light frequency-modulated at a predetermined modulation frequency are incident from both ends of the optical fiber 6 to be measured. When the center frequency of the probe light is changed using the frequency converter 3 so that the frequency difference between the center frequencies of the probe light and the pump light becomes close to the Brillouin frequency shift νB, the position where the phases of the two lights are synchronized, that is, Power is selectively transferred from the pump light to the probe light at a position where the light correlation value increases. Therefore, by detecting the power of the probe light branched by the optical branching device 4 after being emitted from the optical fiber 6 to be measured by the photodetector 5, the Brillouin gain spectrum gB (ν) at the position where the phases of the two lights are synchronized. Is measured.

FIG. 2 shows a Brillouin gain spectrum measuring apparatus according to a second embodiment of the present invention. The Brillouin gain spectrum measuring apparatus according to the second embodiment includes a light source 1 that generates light frequency-modulated at a desired modulation frequency,
A first optical splitter 7 as an optical splitting unit for splitting the output light of the light source 1, an optical frequency converter 3 for shifting the center frequency of the split probe light to a desired frequency, and a splitter An optical delay unit 8 for giving a predetermined delay time between the other pump light and the probe light,
A second optical splitter 4 as an optical means for injecting pump light from one end of the measured optical fiber 6 and guiding at least a part of the probe light emitted from the measured optical fiber 6 to the photodetector 5. , And a photodetector 5 for detecting the power of the probe light split by the second optical splitter 4.

The light source 1 for generating light that is frequency-modulated at a desired modulation frequency, the optical frequency converter 3 for giving a desired frequency shift to the center frequency of the probe light, and the second optical splitter 4 are as follows. This is the same as the embodiment. Since the first optical splitter 7 only needs to be able to split the output light in order to generate the pump light and the probe light from one output light, a beam splitter or a half mirror may be used. An optical fiber or the like is used as the optical delay unit 8 for providing a predetermined delay time between the probe light and the pump light.

The pump light and the probe light frequency-modulated at the desired modulation frequency are incident from both ends of the optical fiber 6 to be measured in a state where a predetermined delay time is given between the two lights by using the optical delay unit 8. I do. The position where the phases of the two lights are synchronized, that is, the synchronization point is given by Expression 11 using the modulation frequency and the speed of light in the optical fiber 6 to be measured. The delay time given between the two lights is set to a non-zero integer multiple (N ≠ in Expression 11) in the optical fiber 6 to be measured.
Set so that the synchronization point of 0) exists. The frequency shift amount of the center frequency of the probe light is changed, and the Brillouin gain spectrum gB at a position where the phases of both lights are synchronized.
The point at which (ν) is measured is the same as in the first embodiment. Further, the distribution of the Brillouin gain spectrum gB (ν) along the measured optical fiber is measured by changing the modulation frequency to change the position where the phases of both lights are synchronized in the measured optical fiber. .

Here, the insertion position of the optical frequency converter 3 will be added. As described above, the optical frequency converter 3 measures a Brillouin gain spectrum in order to provide a frequency difference such that a Brillouin scattering phenomenon occurs between the center frequency of the pump light and the center frequency of the probe light. In the first embodiment, instead of the portion between the optical fiber 6 to be measured and the second light source 2 shown in FIG.
May be inserted between the light source 1 and the optical splitter 4. In the second embodiment, the first optical splitter 7 shown in FIG.
Instead of between the optical fiber 6 to be measured and the optical fiber 6 to be measured, it may be inserted between the first optical splitter 7 and the optical delay unit 8 or between the optical delay unit 8 and the second optical splitter 4.

[0072]

FIG. 3 shows an embodiment of the present invention. The Brillouin gain spectrum measuring apparatus of the embodiment includes a semiconductor laser 11 and a signal generator 12, which constitute a light source 1, a first optical splitter 7, an optical delay unit 8, and an optical intensity modulator 31 which constitutes an optical frequency converter 3. And the microwave generator 32, the second optical splitter 4,
It comprises an optical wavelength filter 9 and a photodetector 5.

The LD 11 emits frequency-modulated light by modulating the injection current of the semiconductor laser (hereinafter, referred to as LD) 11 with a periodic signal generated by the signal generator 12. The frequency-modulated output light of the LD 11 is split by the first optical splitter 7 into pump light and probe light, respectively, and the probe light is input to the light intensity modulator 31. By inputting the microwave generated by the microwave generator 32 to the light intensity modulator 31 and applying amplitude modulation,
A sideband having a frequency difference equal to the microwave frequency with respect to the center frequency of the input light is generated, and is incident on the optical fiber to be measured 6. Here, the sideband wave on the low frequency side is used as the probe light. After passing through the optical delay unit 8 and the second optical branch unit 4, the pump light enters the optical fiber 6 to be measured. The optical delay unit 8 sets a predetermined delay time between the pump light and the probe light. The probe light emitted from the measured optical fiber 6 is split by the second optical splitter 4, only the low-frequency sideband is selected by the optical wavelength filter 9, and the power is measured by the photodetector 5.

The modulation frequency of the LD 11 is selected such that only one position where the phases of the pump light and the probe light are synchronized exists in the optical fiber 6 to be measured. Further, the amount of delay by the optical delay unit is set so that a synchronization point having a non-zero integer multiple of the synchronization point interval determined by the modulation frequency exists in the optical fiber 6 to be measured.

By sweeping the microwave frequency applied to the light intensity modulator 31 in the vicinity of the Brillouin frequency shift νB, the Brillouin gain at the position where the phases of the pump light and the probe light are synchronized in the optical fiber 6 to be measured. The spectrum gB (v) is measured. Further, by changing the modulation frequency of the injection current into the LD 11, the position where the phases of the pump light and the probe light are synchronized in the measured optical fiber 6 is changed, and the Brillouin gain spectrum along the measured optical fiber 6 is changed. Is measured.

FIG. 4 shows the configuration of the measured optical fiber 6 measured using the embodiment of the present invention. Optical fiber under test 6
Is composed of a single mode optical fiber 61, a dispersion shift optical fiber 62, and a single mode optical fiber 63. The Brillouin frequency shifts νB of the single mode optical fibers 61 and 63 and the dispersion shift optical fiber 62 are 10.83 GHz and 10.56 GHz, respectively.

In the experiment, the modulation frequency of the current injected into the LD 11 was swept between 7 and 8 MHz. At this time, the synchronization point interval varies between 25 and 29 m according to equation (11). Since an optical fiber of 40 m was used for the optical delay unit 8, considering the amount of delay caused by optical components such as an optical splitter, only the synchronization point corresponding to N = 2 in the equation 11 is included in the optical fiber 6 to be measured. Exists, and the optical path length difference between the pump light and the probe light in the measured optical fiber 6 changes in the range of 50 to 58 m by sweeping the modulation frequency of the injection current into the LD 11.

FIG. 5 shows the measurement results of the Brillouin gain spectrum distribution. In FIG. 5, the x-axis is the measurement position, and a dispersion-shifted optical fiber exists near 25 to 27 m. The y-axis is the frequency, and the z-axis is the Brillouin gain at the measurement position x and frequency y when z = z (x, y). Here, as the Brillouin gain on the z-axis, a gain [amplification rate (%)] based on the power of the probe light measured in advance without connecting the optical fiber to be measured is plotted.

FIG. 6 shows the distribution of the Brillouin frequency shift νB obtained from FIG. FIG. 7 shows the distribution of Brillouin gain along the Brillouin frequency shift of each of the single-mode optical fibers 61 and 63 and the dispersion-shifted optical fiber 62 obtained from FIG. 5, that is, y = 1 in FIG.
The results obtained as a cross-sectional view of the zx plane at 0.83 GHz and the zx plane at y = 10.56 GHz are shown.

From FIG. 7, the spatial resolution is estimated to be 40 cm as half the length of the transition region between the single mode optical fibers 61 and 63 and the dispersion shift optical fiber 62. The spatial resolution obtained by substituting the experimental conditions into the theoretical formula 17 is 38 to 33 cm, which is in good agreement with the experimental value of 40 cm.
For the calculation of the theoretical resolution, the frequency modulation amplitude of light generated from the LD 11 used in the experiment, 360 MHz, and the Brillouin gain line width ΔνB = 30 MHz measured in advance were used. Since the experiment was performed using frequency modulation, the frequency modulation amplitude of 360 MHz may be substituted for mνm in Equation 17 derived using phase modulation.

[0081]

According to the Brillouin gain spectrum measuring method and apparatus of the present invention, a pump light and a probe light frequency-modulated at a predetermined modulation frequency are incident from both ends of an optical fiber to be measured, and a position where the phases of the two lights are synchronized. In the prior art, the lower limit determined by the frequency spread ΔνB of the Brillouin gain spectrum gB (ν) is determined in the prior art because the Brillouin gain spectrum gB (ν) is selectively measured. For this reason, the problem that the spatial resolution was limited to 2 to 3 m or more was solved, and a method and apparatus capable of measuring a Brillouin gain spectrum with a spatial resolution of 1 m or less were realized.

Further, by setting a delay between the pump light and the probe light incident on the optical fiber to be measured by using an optical delay device, the modulation frequency for modulating the frequency of both lights is changed, thereby obtaining the measured light. It becomes possible to change the position where the phases of both lights are synchronized in the optical fiber, that is, the position where the Brillouin gain spectrum is measured, by a practical distance,
A method and apparatus that can measure the distribution of the Brillouin gain spectrum along the optical fiber have been realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing a second embodiment of the present invention.

FIG. 3 is a configuration diagram showing an embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of an optical fiber measured according to an embodiment of the present invention.

FIG. 5 is a diagram showing a Brillouin gain spectrum distribution obtained by measuring the optical fiber shown in FIG. 4;

FIG. 6 is a diagram showing a Brillouin frequency shift distribution of the measured optical fiber obtained from FIG. 5;

FIG. 7 is a diagram showing the distribution of Brillouin gain along the Brillouin frequency shift of each of a single mode optical fiber and a dispersion shifted optical fiber obtained from FIG. 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st light source 2 2nd light source 3 optical frequency converter 4 optical means, optical splitter 5 photodetector 6 optical fiber to be measured 7 optical splitting means, optical splitter 8 optical delay 9 optical wavelength filter 11 semiconductor laser , LD 12 signal generator 31 optical intensity modulator 32 microwave generator 61 single mode optical fiber 62 dispersion shift optical fiber 63 single mode optical fiber

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroshi Shimoda 5-10-27 Minamiazabu, Minato-ku, Tokyo Anritsu Corporation F-term (reference) 2F056 VF02 VF11 VF16 VF17 2G066 BA18 CA20

Claims (5)

[Claims] 1. A Brillouin gain spectrum measuring method using a first continuous wave light frequency-modulated at a predetermined modulation frequency and a second continuous wave light frequency-modulated at a modulation frequency equal to the predetermined modulation frequency. Wherein the first
Of the second continuous oscillation light is incident from one end face of the optical fiber to be measured, the center frequency of the second continuous oscillation light is frequency-shifted, and the second continuous oscillation light whose center frequency is shifted by the frequency shift is measured. The power of the light emitted from the one end face or the other end face of the optical fiber to be measured is measured by making the light incident from the other end face of the optical fiber and changing the frequency shift amount of the center frequency of the second continuous oscillation light. Measuring the Brillouin gain spectrum at a position where the phase of the first continuous oscillation light and the phase of the second continuous oscillation light are synchronized and the correlation value increases in the optical fiber to be measured. Method. 2. A Brillouin gain spectrum measuring method according to claim 1, wherein the position at which the Brillouin gain spectrum is measured in the measured optical fiber is changed by changing the predetermined modulation frequency. Spectrum measurement method. 3. A Brillouin gain spectrum measuring method using a first continuous oscillation light and a second continuous oscillation light obtained by branching continuous oscillation light frequency-modulated at a desired modulation frequency. Delaying the first continuous wave light, inputting the delayed first continuous wave light from one end face of the optical fiber to be measured, shifting the center frequency of the second continuous wave light, The second continuous oscillation light whose center frequency is shifted by the shift is made incident from the other end face of the optical fiber to be measured, and the frequency shift amount of the center frequency of the second continuous oscillation light is changed to change the frequency of the optical fiber to be measured. By measuring the power of the light emitted from the one end face or the other end face, the phase of the first continuous oscillation light and the phase of the second continuous oscillation light are synchronized in the measured optical fiber, and the correlation value increases. By measuring the Brillouin gain spectrum at the position and changing the modulation frequency,
The Brillouin gain spectrum in the measured optical fiber is measured by changing the position where the phase of the first continuous wave light and the phase of the second continuous wave light are synchronized and the correlation value increases in the measured optical fiber. A Brillouin gain spectrum measuring method, characterized by measuring the distribution of Brillouin. 4. A first light source (1) for outputting a first continuous wave light frequency-modulated at a predetermined modulation frequency, and a second continuous wave light frequency-modulated at a frequency equal to the predetermined frequency. A second light source (2) for outputting a second continuous wave light; an optical frequency converter (3) for giving a desired frequency shift to a center frequency of the second continuous wave light; While being incident on one end of the measuring optical fiber (6),
After being subjected to a frequency shift by the optical frequency converter, an optical beam that is incident on the other end of the optical fiber under measurement, propagates through the optical fiber under measurement, and guides at least a part of the second continuous wave light emitted therefrom. Means (4) and a photodetector (5) for measuring the power of the light guided by the optical means, wherein the phase of the first continuous oscillation light and the phase of the second continuous oscillation light are measured. An apparatus for measuring a Brillouin gain spectrum at a position synchronized in an optical fiber. 5. A light source (1) for outputting continuous oscillation light frequency-modulated at a desired modulation frequency, and an optical branch for splitting the continuous oscillation light and outputting a first output light and a second output light. Means (7), an optical frequency converter (3) for giving a desired frequency shift to a center frequency of the first output light,
An optical delay unit (8) for providing a predetermined delay time to the second output light, receiving the second output light delayed by the optical delay unit, entering one end of an optical fiber to be measured, and After being frequency-shifted by the frequency converter, the optical means (4) for guiding at least a part of the first output light which is incident on the other end of the measured optical fiber, propagates through the measured optical fiber, and is emitted. ), And a photodetector (5) for measuring the power of the light guided by the optical means, and changing the desired modulation frequency so that the first light incident from both ends of the optical fiber to be measured is provided. A Brillouin gain spectrum measuring apparatus, wherein a position where the phase of the output light and the phase of the second output light are synchronized is changed to enable measurement of the distribution of the Brillouin gain spectrum in the measured optical fiber.