JP3667132B2 - Brillouin gain spectrum measurement method and apparatus - Google Patents

Brillouin gain spectrum measurement method and apparatus Download PDF

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JP3667132B2
JP3667132B2 JP37546598A JP37546598A JP3667132B2 JP 3667132 B2 JP3667132 B2 JP 3667132B2 JP 37546598 A JP37546598 A JP 37546598A JP 37546598 A JP37546598 A JP 37546598A JP 3667132 B2 JP3667132 B2 JP 3667132B2
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frequency
light
measured
continuous wave
optical fiber
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JP2000180265A (en
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寛 下田平
和夫 保立
健美 長谷川
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アンリツ株式会社
和夫 保立
健美 長谷川
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Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for measuring a Brillouin gain spectrum of an optical fiber, and more particularly to a technique for increasing 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 that uses an optical fiber to measure a physical quantity distribution such as temperature and strain in an environment in which the optical fiber is installed. Optical fiber sensors are, for example, (1) maintenance and management of optical communication paths, (2) maintenance and management of large-scale structures such as dams and dikes, and (3) materials and structures that have a self-diagnosis function for defects and failures ( Smart materials and structures) The optical fiber sensor using the present invention can increase the spatial resolution of physical quantity distribution measurement considered to be particularly important in (2) and (3).
[0003]
[Prior art]
There is a method based on the Brillouin scattering phenomenon in an optical fiber as a technique for measuring the strain or temperature in an environment where the optical fiber is installed. 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 simple configuration as compared with a method in which a large number of point sensors are arranged.
[0004]
The Brillouin scattering phenomenon is a phenomenon in which, 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. When the frequency difference between two light waves passing is ν, the moving power is approximately proportional to the Brillouin gain spectrum gB (ν) defined by Equation 1.
[0005]
[Formula 1]
[0006]
Here, νB and ΔνB are parameters that characterize the Brillouin gain spectrum, and are called Brillouin frequency shift and Brillouin gain line width (full width at half maximum), respectively. The 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 in the frequency range of the Brillouin gain line width ΔνB centered on the Brillouin frequency shift νB with respect to the frequency between the two light waves that pass in the optical fiber. Must be granted.
The Brillouin frequency shift νB is given by Equation 2 with the speed of sound in the optical fiber as va.
[0007]
[Formula 2]
[0008]
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 changing 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 temperature and strain detection means via equation (2). As a numerical example of the change sensitivity of Brillouin frequency shift νB, at an optical wavelength of 1.3 μm,
Against distortion
[0009]
[Formula 3]
[0010]
Against temperature
[0011]
[Formula 4]
[0012]
It is reported in the following document (1).
(1) M.Nikles et al., Journal of Lightwave Technology, vol.15, pp. 1842‐1851, (1997)
Accordingly, the strain or temperature distribution can be measured by measuring the Brillouin gain spectrum gB (ν) as a function of position along the optical fiber and determining the Brillouin frequency shift νB as a function of position along the optical fiber. It becomes possible.
[0013]
As a conventional technique for measuring the aforementioned Brillouin gain spectrum, pulse pump light and continuous wave probe light having a variable frequency difference, pulse pump light from one end of the optical fiber to be measured, and continuous wave probe light to the optical fiber to be measured BOTDA (Brillouin Optical Time) is a method for determining the distribution along the optical fiber of the Brillouin gain spectrum gB (ν) by measuring the change component of the probe light due to the Brillouin scattering phenomenon as a function of time. Domain Analysis) has been used.
[0014]
In addition, as a technique equivalent to BOTDA, pulse light is incident from one end of the optical fiber to be measured, and the spectrum of the scattered light due to the natural Brillouin scattering phenomenon is measured as a function of time, so that the light of the Brillouin gain spectrum gB (ν) BOTDR (Brillouin Optical Time Domain Reflectometry), which is a method for obtaining the distribution along the fiber, has also been used.
BOTDA and BOTDR are explained by the authors in the following document (2).
(2) T. Horiguchi et al., Journal of Lightwave Technology, vol.13, pp. 1296‐1302, (1995)
[0015]
The spatial resolution δz of both the BOTDA and BOTDR prior arts is as follows: W is the optical pulse time width incident on the optical fiber as pump light, and v is the speed of light in the optical fiber.
[0016]
[Formula 5]
[0017]
Given in. Therefore, in order to increase (narrow) the spatial resolution δz, it is necessary to shorten the optical pulse time width W. However, in order to accurately measure the Brillouin gain spectrum gB (ν), particularly the Brillouin frequency shift νB, which is the maximum frequency, it is necessary to set the optical pulse time width W to the lower limit value or more shown below. For this reason, the spatial resolution δz is limited to a value larger than 2 to 3 m.
[0018]
In both conventional techniques, an optical pulse used as pump light is generated by pulsing continuous light having a frequency difference ν with respect to probe light using an optical switch or the like. Then, for example, regarding the BOTDA mentioned above, 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 centering on 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 is given by the overlap integral of the spectrum of the beat signal and the Brillouin gain spectrum gB (ν) given by Equation 1, so that light with a pulse time width W is pumped. When used as light, an integral value in a frequency range of about 2 / W centered on ν of gB (ν) is measured as the shifted power.
[0019]
The Brillouin gain spectrum gB (ν) to be measured and given by Equation 1 takes a value of 1/2 or more in the frequency range of the Brillouin gain line width (full width at half maximum) ΔνB around the Brillouin frequency shift νB. Therefore, as a result of using the pump light with the pulse time width W, if the integration range 2 / W becomes wider than twice the full width at half maximum ΔνB, the integrated value is almost constant even if the frequency difference ν is changed near the Brillouin frequency shift νB. Therefore, it becomes difficult to accurately measure the frequency difference at which the Brillouin gain spectrum takes the maximum value.
[0020]
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 greater than the lower limit given by the following equation.
[0021]
[Formula 6]
[0022]
Using the experimental value of 30 MHz for the Brillouin gain line width ΔνB 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.
[0023]
[Problems to be solved by the invention]
Since BOTDA and BOTDR, which are conventional technologies, are limited to a value larger than 2 to 3 m in spatial resolution, they are materials having maintenance and self-diagnosis functions for large-scale structures listed in the technical field to which the invention belongs. -In the structure, the spatial resolution of 1 m or less, which has been required in recent years, could not be realized.
[0024]
An object of the present invention is to provide a method and 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 apparatus capable of measuring the distribution along the optical fiber of the Brillouin gain spectrum gB (ν) 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 application fields that could not be realized by the prior art.
[0025]
[Means for Solving the Problems]
In order to solve the above-described problems, the Brillouin gain spectrum measuring method and apparatus of the present invention uses continuous light in which both pump light and probe light are modulated at a predetermined modulation frequency. That is,
The Brillouin gain spectrum measuring method according to claim 1 of the present invention includes 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. The Brillouin gain spectrum measurement method using the first continuous wave light is incident from one end face of the optical fiber to be measured, the center frequency of the second continuous wave light is frequency shifted, and the frequency shift is performed. The second continuous wave light whose center frequency is shifted by the incident light is made incident from the other end surface of the optical fiber to be measured, and the frequency shift amount of the center frequency of the second continuous wave light is changed to change the frequency of the optical fiber to be measured. The power of light emitted from one end face or the other end face is measured, and the phase of the first continuous wave light and the second continuous wave light are measured in the optical fiber to be measured. Phase and measuring the Brillouin gain spectrum at the position where the synchronization correlation value increases.
[0026]
The Brillouin gain spectrum measurement method according to claim 2 of the present invention is the Brillouin gain spectrum measurement method according to claim 1, wherein the Brillouin gain spectrum in the optical fiber to be measured is measured by changing the predetermined modulation frequency. It is characterized by changing.
[0027]
According to the third aspect of the present invention, the Brillouin gain spectrum measurement method branches the continuous wave light frequency-modulated at a desired modulation frequency, and uses the first continuous wave light and the second continuous wave light obtained by the branching. A method of measuring the Brillouin gain spectrum, 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 The second continuous wave light whose center frequency is shifted by the frequency shift is made incident from the other end surface of the optical fiber to be measured, and the amount of frequency shift of the center frequency of the second continuous wave light is changed. And changing the phase of the first continuous wave light in the measured optical fiber by measuring the power of the light emitted from the one end surface or the other end surface of the measured optical fiber. The Brillouin gain spectrum is measured at a position where the phase of the continuous wave light of 2 is synchronized and the correlation value is increased, and the modulation frequency is changed, so that the phase of the first continuous wave light and the second phase of the first continuous wave light are measured. The Brillouin gain spectrum is measured by changing the position where the phase of the continuous wave light is synchronized and the correlation value is increased, and the distribution of the Brillouin gain spectrum in the optical fiber to be measured is measured.
[0028]
According to a fourth aspect of the present invention, there is provided a Brillouin gain spectrum measuring apparatus, wherein a first light source that outputs a first continuous wave light frequency-modulated at a predetermined modulation frequency, and frequency-modulated at a frequency equal to the predetermined frequency. A second light source that outputs a second continuous wave light; an optical frequency converter that provides a desired frequency shift with respect to the center frequency of the second continuous wave light; The second light beam that is incident on one end of the measurement optical fiber and that has undergone a frequency shift by the optical frequency converter, is incident on the other end of the optical fiber to be measured, propagates through the optical fiber to be measured, and is emitted. Optical means for guiding at least part of the continuous wave light and a photodetector for measuring the power of the light guided by the optical means are provided. The Brillouin gain spectrum is measured 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 optical fiber to be measured.
[0029]
A Brillouin gain spectrum measuring apparatus according to a fifth aspect of the present invention includes a light source that outputs continuous wave light frequency-modulated at a desired modulation frequency, the continuous wave light that is branched, and a first output light and a second output light. An optical branching means for outputting light, an optical frequency converter for giving a desired frequency shift to a center frequency of the first output light, an optical delay for giving a predetermined delay time to the second output light, The second output light delayed by the optical delay device is received and incident on one end of the optical fiber to be measured, and after being frequency-shifted by the optical frequency converter, incident on the other end of the optical fiber to be measured And optical means for guiding at least a part of the first output light emitted through the optical fiber to be measured and a photodetector for measuring the power of the light guided by the optical means. . 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 It is possible to measure the distribution of Brillouin gain spectrum.
[0030]
[Action]
In the following, frequency modulation and phase modulation are equivalent techniques, so the operation will be described using phase modulation in order to simplify the process of deriving the theoretical formula of spatial resolution. Is possible.
[0031]
First, 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 phase-modulated at a predetermined frequency are incident from one end and the other end of the optical fiber to be measured. If the frequency difference between the center frequencies of the two lights is ν, a beat signal having a single frequency ν is generated at a position where the phases of both lights are synchronized and the correlation value is increased in the optical fiber to be measured, while the phases of both lights are asynchronous. At a position where the correlation value is low, a phase-modulated beat signal is generated.
[0032]
The power moving 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 prior art. Therefore, paying attention to the spectrum of the beat signal, a beat signal with a single frequency ν is generated at a position where the phases of both lights are synchronized, and the spectrum has a sharp peak, whereas the phase of both lights is out of phase at an asynchronous position. A modulated beat signal is produced, and its spectrum spreads around ν. When the frequency difference ν between the center frequencies of both lights is changed, the spectrum center of the beat signal is also shifted. During this time, the power that moves from the pump light to the probe light at the position where the phases of both lights are synchronized is the spectrum of the beat signal. As a result of the sharp peak shape, it changes according to the Brillouin gain spectrum gB (ν). On the other hand, at a position where the phases of the two lights are asynchronous, the moving power hardly changes even if the frequency difference ν is changed.
[0033]
This is because the spectrum of the beat signal at a non-synchronized position has a large value in the frequency range determined according to the distance from the position where the closest phase is synchronized with ν as the center, and gB (ν) This is because the integrated power moves. Therefore, by changing the frequency difference ν between the center frequencies of the two lights in the vicinity of the Brillouin frequency shift νB and measuring the change in the power of the outgoing probe light or the power of the outgoing pump light, The Brillouin gain spectrum gB (ν) at a position where the phase of the pump light and the phase of the probe light are synchronized can be measured.
[0034]
There are a plurality of positions where the phase of the pump light and the phase of the probe light are synchronized (hereinafter referred to as synchronization points) at intervals determined according to the modulation frequency of the phase modulation, but there are synchronization points existing in the measured fiber. It is possible to set the modulation frequency of the phase modulation so that it is limited to one point. Furthermore, by changing the modulation frequency of the phase modulation, the synchronization point, that is, the position where the Brillouin gain spectrum is measured can be changed. For these implementation means, a theoretical formula of spatial resolution is derived below. Explain in the process.
[0035]
Next, it is shown that a spatial resolution of 1 m or less can be obtained by deriving a theoretical formula of spatial resolution obtained in the present invention and substituting numerical values used in experimental results described later in the examples into the theoretical formula.
The center frequencies of the pump light and the probe light are ν1 and ν2, respectively, and sine wave phase modulation having a modulation index m and a modulation frequency νm is applied to both lights. Let E1 and E2 be the complex amplitudes of the optical fields of the pump light and probe light, respectively.
[0036]
[Formula 7]
[0037]
[Formula 8]
[0038]
It expresses. Here, t is time, z is position, and v is the group velocity of both lights.
The beat signal E1 × (complex conjugate of E2) of both lights is
[0039]
[Formula 9]
[0040]
[Formula 10]
[0041]
Given in. From Equation 10, the position where the phase of the pump light and the probe light is synchronized, that is, the synchronization point is
[0042]
[Formula 11]
[0043]
In the equation (10), the position where the component that temporally oscillates at the modulation frequency νm disappears, that is,
[0044]
[Formula 12]
[0045]
Is defined as the position where Therefore, in order to limit the synchronization point existing in the optical fiber to be measured 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 optical fiber to be measured. 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 so that a non-zero integer multiple (N ≠ 0 in Equation 11) synchronization point exists in the optical fiber to be measured. Thus, means for measuring the Brillouin gain spectrum distribution in the optical fiber to be measured is obtained by changing the synchronization point position by changing the modulation frequency νm.
[0046]
In the prior art, as described above, pulse light is used as pump light. Therefore, by changing the time from when pulse pump light is output until Brillouin gain is measured, the Brillouin gain in the optical fiber to be measured is changed. It was possible to change the measurement position. In the present invention, it is necessary to change the synchronization point position of the pump light and the probe light. However, the change of the synchronization point position by the optical delay device having a variable delay amount is very small at present and is not practical. Therefore, a method of changing the synchronization point position by using the non-zero integer multiple synchronization point position and changing the modulation frequency νm as described above is adopted.
[0047]
Since the spectrum of the beat signal at the synchronization point is a single frequency of ν = [absolute value of (ν1 −ν2)], the frequency difference ν between the center frequencies of the pump light and the probe light is close to the Brillouin frequency shift νB. By changing the power of the pump light or the probe light emitted from the optical fiber to be measured, the Brillouin gain spectrum at the synchronization point is measured.
Here, in order to obtain the spatial resolution, a beat signal in the vicinity of the synchronization point is considered, and z = vN / νm + ε (where ε represents a slight deviation from the synchronization point). The beat signal is a function of ε, t
[0048]
[Formula 13]
[0049]
[Formula 14]
[0050]
It becomes. The double sign takes + and-, respectively, according to the even and odd of N. From Equation 14, the instantaneous frequency ν (t) of the beat signal is
[0051]
[Formula 15]
[0052]
Therefore, the beat signal spectrum at a position ε away from the synchronization point is
[0053]
[Formula 16]
[0054]
Will occupy the frequency band.
In the conventional technique, the frequency band occupied by the beat signal (given by 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 Equation 16 is twice the full width at half maximum ΔνB,
[0055]
[Formula 17]
[0056]
Is obtained. From Equation 17, it can be seen that the spatial resolution is improved by increasing the modulation frequency νm of the phase modulation. Experimental value of Brillouin gain line width ΔνB 30 MHz, speed of light in the fiber v = 2 × (10 8) m / s, modulation index m = 50 used in the experimental results shown later in the embodiment, modulation frequency of phase modulation The spatial resolution δz obtained by substituting νm = 7.5 MHz into Equation 17 is 35 cm, and a spatial resolution of 1 m or less is realized.
Furthermore, the beat signal of Equation 13 is expressed using the first kind Bessel function.
[0057]
[Formula 18]
[0058]
[Formula 19]
[0059]
Using the fact expressed as follows, the modulation frequency νm of phase modulation
[0060]
[Formula 20]
[0061]
Under these conditions, a theoretical equation of spatial resolution can be obtained using only the amplitude J0 (y) of the zeroth-order spectral component of the beat signal. This is because, under the conditional expression 20, the non-zero order spectral component exists outside the frequency band where the Brillouin gain spectrum gB (ν) takes a large value of 1/2 or more. As a result of the suppression of the zero-order component J0 (y) to 1/2 or less, 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) ≈0.5 Using
[0062]
[Formula 21]
[0063]
Is obtained. A spatial resolution δz = 5 cm is obtained under the conditions of νm = 50 MHz> ΔνB / 2 = 15 MHz and the modulation index m = 20.
[0064]
Note that the correlation value in the description that the phases of the pump light and the probe light used in the above description are synchronized and the correlation value of both lights increases is the zero-order spectral component of the beat signal given by Equation 18. Means the amplitude J0 (y). The first-order Bessel function of the zero order takes a maximum value when y = 0, and therefore, ε means a slight deviation from the synchronization point in y defined by Equation 19, so that the correlation value at the synchronization point is It can be seen that the correlation value decreases as the maximum value is taken and deviates from the synchronization point.
[0065]
DETAILED DESCRIPTION OF THE INVENTION
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 that generates pump light that is frequency-modulated at a predetermined modulation frequency, and a second light source that generates probe light that is frequency-modulated at a predetermined modulation frequency. The light source 2, the optical frequency converter 3 for shifting the center frequency of the probe light output from the second light source 2 to a desired frequency, pump light is incident from one end of the optical fiber 6 to be measured, and Optical branching device 4 as optical means for guiding at least part of the probe light emitted from the measurement optical fiber 6 to the optical detector 5, and light for detecting the power of the probe light branched by the optical branching device 4 It comprises a detector 5.
[0066]
The first light source 1 and the second light source 2 that generate light modulated at a predetermined frequency are means for generating frequency-modulated light by modulating the injection current of the semiconductor laser at a predetermined modulation frequency. Etc. 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-shifted light is generated, and the amount of frequency shift is equal to the frequency of 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, so that an optical circulator, a beam splitter, a half mirror or the like is used in addition to the optical branching device. May be.
[0067]
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 frequency converter 3 is used to change the center frequency of the probe light so that the frequency difference between the center frequencies of the probe light and the pump light is close to the Brillouin frequency shift νB, The power shift from the pump light to the probe light occurs selectively at the position where the correlation value of light increases. Accordingly, the Brillouin gain spectrum gB (ν) at the position where the phases of the two lights are synchronized 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. Is measured.
[0068]
FIG. 2 shows a Brillouin gain spectrum measuring apparatus according to the 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, and a first optical branch as an optical branching unit for branching output light from the light source 1. 7. An optical frequency converter 3 for shifting the center frequency of one of the branched probe lights to a desired frequency, a predetermined delay time is provided between the other branched pump light and the probe light. Optical delay unit 8 for receiving the pump light from one end of the optical fiber 6 to be measured and optical means for guiding at least part of the probe light emitted from the optical fiber 6 to be measured to the photodetector 5 It comprises a second optical branching device 4 and a photodetector 5 for detecting the power of the probe light branched by the second optical branching device 4.
[0069]
The light source 1 that generates light modulated at a desired modulation frequency, the optical frequency converter 3 that gives a desired frequency shift to the center frequency of the probe light, and the second optical branching device 4 are the same as those in the first embodiment. It is the same as the form. As for the first optical branching unit 7, in order to produce pump light and probe light from one output light, it is only necessary to branch the output light. Therefore, a beam splitter or a half mirror may be used. An optical fiber or the like is used as the optical delay device 8 that gives a predetermined delay time between the probe light and the pump light.
[0070]
The pump light and the probe light that are frequency-modulated at a 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 both lights using the optical delay device 8. The position where the phases of both lights are synchronized, that is, the synchronization point, is given by Equation 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 so that there is a non-zero integer multiple (N ≠ 0 in Equation 11) in the optical fiber 6 to be measured. Similar to the first embodiment, the Brillouin gain spectrum gB (ν) is measured at a position where the phase shift of the center frequency of the probe light is changed and the phases of the two lights are synchronized. Further, the distribution of the Brillouin gain spectrum gB (ν) along the measured optical fiber is measured by changing the position at which the phases of the two lights are synchronized in the measured optical fiber by changing the modulation frequency. .
[0071]
Here, the insertion position of the optical frequency converter 3 is added. As described above, the optical frequency converter 3 has a frequency difference that causes a Brillouin scattering phenomenon between the center frequency of the pump light and the center frequency of the probe light, and measures the Brillouin gain spectrum for the frequency difference. In the first embodiment, the first light source 1 and the second light source 2 instead of the optical fiber to be measured 6 and the second light source 2 shown in FIG. You may insert between the optical branching devices 4. FIG. Further, in the second embodiment, instead of between the first optical branching device 7 and the optical fiber 6 to be measured shown in FIG. 2, between the first optical branching device 7 and the optical delay device 8 or an optical delay. It may be inserted between the optical device 8 and the second optical branching device 4.
[0072]
【Example】
An embodiment of the present invention is shown in FIG.
The Brillouin gain spectrum measuring apparatus of the embodiment includes a semiconductor laser 11 and a signal generator 12 constituting a light source 1, a first optical branching device 7, an optical delay device 8, and an optical intensity modulator 31 constituting an optical frequency converter 3. And a microwave generator 32, a second optical branching device 4, an optical wavelength filter 9, and a photodetector 5.
[0073]
The LD 11 generates frequency-modulated light by modulating the injection current of the semiconductor laser (hereinafter referred to as LD) 11 with the periodic signal generated by the signal generator 12. The frequency-modulated output light of the LD 11 is divided into two by the first optical branching unit 7 to be pump light and probe light, respectively, and the probe light is input to the light intensity modulator 31. A microwave generated by the microwave generator 32 is input to the light intensity modulator 31 and amplitude modulation is applied to generate a sideband having a frequency difference equal to the microwave frequency with respect to the center frequency of the input light. Then, it enters the optical fiber 6 to be measured. Here, the sideband wave on the low frequency side is used as the probe light. The pump light passes through the optical delay device 8 and the second optical branching device 4 and then enters the measured optical fiber 6. A predetermined delay time is set between the pump light and the probe light by the optical delay device 8. The probe light emitted from the optical fiber 6 to be measured is branched 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.
[0074]
The modulation frequency of the LD 11 is selected so that there is only one position in the measured optical fiber 6 where the phases of the pump light and the probe light are synchronized. Furthermore, the delay amount by the optical delay device is set so that a synchronization point that is a non-zero integer multiple of the synchronization point interval determined by the modulation frequency exists in the measured optical fiber 6.
[0075]
By sweeping the microwave frequency applied to the light intensity modulator 31 in the vicinity of the Brillouin frequency shift νB, the Brillouin gain spectrum gB (at the position where the phases of the pump light and the probe light are synchronized in the measured optical fiber 6. ν) is measured. Furthermore, the Brillouin gain spectrum along the measured optical fiber 6 is changed by changing the modulation frequency of the injection current to the LD 11 to change the position where the phases of the pump light and the probe light are synchronized in the measured optical fiber 6. Measure.
[0076]
FIG. 4 shows the configuration of the optical fiber 6 to be measured measured using the embodiment of the present invention. The measured optical fiber 6 includes a single mode optical fiber 61, a dispersion shifted optical fiber 62, and a single mode optical fiber 63. The Brillouin frequency shifts ν B of the single mode optical fibers 61 and 63 and the dispersion shifted optical fiber 62 are 10.83 GHz and 10.56 GHz, respectively.
[0077]
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 from Equation 11. Since a 40 m optical fiber is used for the optical delay device 8, only the synchronization point corresponding to N = 2 in the equation 11 is included in the measured optical fiber 6 in consideration of the delay amount due to optical components such as an optical branching device. 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.
[0078]
FIG. 5 shows the measurement result of the Brillouin gain spectrum distribution.
In FIG. 5, the x-axis is the measurement position, and a dispersion-shifted optical fiber exists in the vicinity of 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, where z = z (x, y). Here, the gain (amplification rate (%)) based on the power of the probe light measured in advance without connecting the optical fiber to be measured is plotted on the Brillouin gain on the z axis.
[0079]
FIG. 6 shows the distribution of the Brillouin frequency shift ν B obtained from FIG.
FIG. 7 shows the Brillouin gain distribution 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, the zx plane at y = 10.83 GHz in FIG. , Y = 10.56 GHz shows the result obtained as a cross-sectional view by the zx plane.
[0080]
From FIG. 7, the spatial resolution is estimated to be 40 cm as a half of the length of the transition region between the single mode optical fibers 61 and 63 and the dispersion shifted 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 360 MHz of light generated from the LD 11 used in the experiment 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]
【The invention's effect】
The Brillouin gain spectrum measurement method and apparatus according to the present invention makes it possible to enter pump light and probe light, which are frequency-modulated at a predetermined modulation frequency, from both ends of the optical fiber to be measured, and the Brillouin gain spectrum gB at a position where the phases of both lights are synchronized. Since (ν) is selectively measured, in the prior art, the time width of the pump light pulse has a lower limit value determined by the frequency spread ΔνB of the Brillouin gain spectrum gB (ν). The problem that the resolution is 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 could be realized.
[0082]
Also, by setting a delay between the pump light and the probe light incident on the optical fiber to be measured using an optical delay device, the modulation frequency for modulating the frequency of both lights is changed, so that The position where the phases of both lights are synchronized, that is, the position where the Brillouin gain spectrum is measured can be changed by a practical distance, and a method and apparatus capable of measuring the distribution of the Brillouin gain spectrum along the optical fiber can be realized. It was.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing a first embodiment of the present invention.
FIG. 2 is a block diagram showing a second embodiment of the present invention.
FIG. 3 is a block 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.
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 optical fiber to be measured, obtained from FIG. 5. FIG.
7 is a diagram showing the Brillouin gain distribution along the Brillouin frequency shift of each of the single-mode optical fiber and the dispersion-shifted optical fiber obtained from FIG. 5. FIG.
[Explanation of symbols]
1 First light source
2 Second light source
3 Optical frequency converter
4 Optical means, optical splitter
5 photodetectors
6 Optical fiber to be measured
7 Optical branching means, optical branching unit
8 Optical delay device
9 Optical wavelength filter
11 Semiconductor laser, LD
12 Signal generator
31 Light intensity modulator
32 microwave generator
61 Single mode optical fiber
62 Dispersion shifted optical fiber
63 Single mode optical fiber

Claims (5)

  1. A Brillouin gain spectrum measurement method using a first continuous wave light frequency-modulated with a predetermined modulation frequency and a second continuous wave light frequency-modulated with a modulation frequency equal to the predetermined modulation frequency,
    The first continuous wave light is incident from one end face of the optical fiber to be measured,
    The center frequency of the second continuous wave light is frequency-shifted, and the second continuous wave light whose center frequency is shifted by the frequency shift is incident from the other end surface of the optical fiber to be measured.
    By changing the frequency shift amount of the center frequency of the second continuous wave light and measuring the power of light emitted from the one end surface or the other end surface of the optical fiber to be measured, A Brillouin gain spectrum measuring method, wherein a Brillouin gain spectrum is measured at a position where the phase of the first continuous wave light and the phase of the second continuous wave light are synchronized to increase the correlation value.
  2. 2. The Brillouin gain spectrum measurement method according to claim 1, wherein a position where the Brillouin gain spectrum is measured in the optical fiber to be measured is changed by changing the predetermined modulation frequency.
  3. A method for measuring a Brillouin gain spectrum using a first continuous wave light and a second continuous wave light obtained by branching continuous wave light frequency-modulated at a desired modulation frequency,
    Delaying the first continuous wave light, causing the delayed first continuous wave light to enter from one end face of the optical fiber to be measured;
    The center frequency of the second continuous wave light is frequency-shifted, and the second continuous wave light whose center frequency is shifted by the frequency shift is incident from the other end surface of the optical fiber to be measured.
    By changing the frequency shift amount of the center frequency of the second continuous wave light and measuring the power of light emitted from the one end surface or the other end surface of the optical fiber to be measured, Measuring the 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 to increase the correlation value;
    By changing the modulation frequency, the Brillouin gain spectrum 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 is increased in the optical fiber to be measured. A Brillouin gain spectrum measurement method, comprising: measuring a Brillouin gain spectrum distribution in an optical fiber to be measured.
  4. A first light source (1) for outputting a first continuous wave light frequency-modulated at a predetermined modulation frequency;
    A second light source (2) for outputting a second continuous wave light frequency-modulated at a frequency equal to the predetermined frequency;
    An optical frequency converter (3) for giving a desired frequency shift to the center frequency of the second continuous wave light;
    The first continuous wave light is received and incident on one end of the optical fiber to be measured (6), and after being subjected to a frequency shift by the optical frequency converter, the optical fiber is incident on the other end of the optical fiber to be measured. Optical means (4) for guiding at least a part of the second continuous wave light emitted through the measurement optical fiber;
    A photodetector (5) for measuring the power of the light guided by the optical means,
    A Brillouin gain spectrum measuring apparatus for measuring 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 optical fiber to be measured.
  5. A light source (1) that outputs continuous wave light frequency-modulated at a desired modulation frequency;
    Optical branching means (7) for branching the continuous wave light and outputting first output light and second output light;
    An optical frequency converter (3) for providing a desired frequency shift with respect to the center frequency of the first output light;
    An optical delay device (8) for giving a predetermined delay time to the second output light;
    The second output light delayed by the optical delay device is received and incident on one end of the optical fiber to be measured, and after being frequency-shifted by the optical frequency converter, incident on the other end of the optical fiber to be measured An optical means (4) for guiding at least a part of the first output light emitted through the measured optical fiber;
    A photodetector (5) for measuring the power of the light guided by the optical means,
    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 Brillouin in the optical fiber to be measured is changed. A Brillouin gain spectrum measuring apparatus characterized by enabling distribution measurement of a gain spectrum.
JP37546598A 1998-12-14 1998-12-14 Brillouin gain spectrum measurement method and apparatus Expired - Fee Related JP3667132B2 (en)

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