CA2615327A1 - Optical fiber characteristic measuring system - Google Patents
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Abstract
An optical fiber characteristic measuring system includes a device for generating a pulse train from coherent light having a first frequency, and launching the pulse train as pulsed light into one end of a target optical fiber to be measured, wherein the pulse train includes a first light pulse and a second light pulse, and the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse is less than or equal to the lifetime of an acoustic wave in the target optical fiber; a device for launching coherent light having a second frequency as continuous light into another end of the target optical fiber; a device for varying the difference between the first frequency and the second frequency within a range which includes a Brillouin frequency shift with respect to the target optical fiber; a device for detecting light, which is emitted from said one end of the target optical fiber;
and a device for measuring characteristics of the target optical fiber based on a result of the light detection.
and a device for measuring characteristics of the target optical fiber based on a result of the light detection.
Description
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OPTICAL FIBER CHARACTERISTIC MEASURING SYSTEM
BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to an optical fiber characteristic measuring system, in which pulsed light is launched into one end of a target optical fiber to be measured, continuous light is launched into the other end of the target optical fiber, and continuous light, emitted from the one end of the optical fiber, is detected so as to measure the characteristics of the target optical fiber.
Priority is claimed on Japanese Patent Application No. 2006-349391, filed December 26, 2006, the content of which is incorporated herein by reference.
Description of the Related Art In a known method of measuring strain or the temperature distribution along an optical fiber which is presently installed, the center frequency of Brillouin scattered light is measured, which is generated by pulsed light and acoustic wave in the optical fiber.
In this measurement method, the installed optical fiber itself is used as a medium for measuring strain or temperature, so that strain or the temperature distribution can be measured using a simpler structure in comparison with a method of arranging a number of point sensors.
The above measurement method includes a BOTDR (Brillouin optical time domain reflectometry) method and a BOTDA (Brillouin optical time domain analysis) method.
BOTDR is a measurement method of measuring a frequency shift of spontaneous Brillouin scattered light (i.e., Brillouin back-scattered light), generated by pulsed light and acoustic wave, whose velocity varies depending on strain or temperature. In this method, pulsed light is launched into one end of an optical fiber, and Brillouin back-scattered light, emitted from the same end of the optical fiber, is detected (see Patent Documents 1 and 2).
On the other hand, in the BOTDA measurement method, pulsed light is launched into one end of an optical fiber, continuous light is launched into the other end of the target optical fiber, and a variation in the continuous light is measured, which is caused by stimulated Brillouin scattering caused by interaction between the pulsed light and the continuous light (see Patent Document 3).
Patent Document 1: Japanese Patent No. 2575794.
Patent Document 2: Japanese Patent No. 3481494.
Patent Document 3: Japanese Patent No. 2589345.
With respect to the BOTDR and BOTDA measurement methods, it is known that spatial resolution can be improved by narrowing the pulse width of pulsed light, which is launched into an optical fiber. However, if the pulse width becomes smaller than or equal to a specific value, the center frequency of Brillouin scattered light cannot be measured with desired accuracy. Therefore, it is also known that the spatial resolution is 2 to 3 m.
In recent years, more accurate measurement is required, and a higher spatial resolution of approximately 1 to 10 cm is desired. However, in the BOTDR
method, as spontaneous Brillouin scattered light is detected, the signal level is low.
Therefore, if the above restriction (i.e., when the pulse width becomes smaller than or equal to a specific value) can be released, it is difficult to obtain a high spatial resolution of approximately 1 to 10 cm.
SUMMARY OF THE INVENTION
In light of the above circumstances, an object of the present invention is to provide an optical fiber characteristic measuring system, in which pulsed light is launched into one end of a target optical fiber to be measured, continuous light is launched into the other end of the target optical fiber, and continuous light, emitted from the one end of the optical fiber, is detected so as to measure the characteristics of the target optical fiber, and in which a higher spatial resolution in comparison with conventional systems is produced.
Therefore, the present invention provides an optical fiber characteristic measuring system comprising:
a first light source device for generating a pulse train from coherent light having a first frequency, and launching the pulse train as pulsed light into one end of a target optical fiber to be measured, wherein the pulse train includes a first light pulse and a second light pulse, and the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse is less than or equal to the lifetime of an acoustic wave in the target optical fiber;
a second light source device for launching coherent light having a second frequency as continuous light into another end of the target optical fiber;
a varying device for varying the difference between the first frequency and the second frequency within a range which includes a Brillouin frequency shift with respect to the target optical fiber;
OPTICAL FIBER CHARACTERISTIC MEASURING SYSTEM
BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to an optical fiber characteristic measuring system, in which pulsed light is launched into one end of a target optical fiber to be measured, continuous light is launched into the other end of the target optical fiber, and continuous light, emitted from the one end of the optical fiber, is detected so as to measure the characteristics of the target optical fiber.
Priority is claimed on Japanese Patent Application No. 2006-349391, filed December 26, 2006, the content of which is incorporated herein by reference.
Description of the Related Art In a known method of measuring strain or the temperature distribution along an optical fiber which is presently installed, the center frequency of Brillouin scattered light is measured, which is generated by pulsed light and acoustic wave in the optical fiber.
In this measurement method, the installed optical fiber itself is used as a medium for measuring strain or temperature, so that strain or the temperature distribution can be measured using a simpler structure in comparison with a method of arranging a number of point sensors.
The above measurement method includes a BOTDR (Brillouin optical time domain reflectometry) method and a BOTDA (Brillouin optical time domain analysis) method.
BOTDR is a measurement method of measuring a frequency shift of spontaneous Brillouin scattered light (i.e., Brillouin back-scattered light), generated by pulsed light and acoustic wave, whose velocity varies depending on strain or temperature. In this method, pulsed light is launched into one end of an optical fiber, and Brillouin back-scattered light, emitted from the same end of the optical fiber, is detected (see Patent Documents 1 and 2).
On the other hand, in the BOTDA measurement method, pulsed light is launched into one end of an optical fiber, continuous light is launched into the other end of the target optical fiber, and a variation in the continuous light is measured, which is caused by stimulated Brillouin scattering caused by interaction between the pulsed light and the continuous light (see Patent Document 3).
Patent Document 1: Japanese Patent No. 2575794.
Patent Document 2: Japanese Patent No. 3481494.
Patent Document 3: Japanese Patent No. 2589345.
With respect to the BOTDR and BOTDA measurement methods, it is known that spatial resolution can be improved by narrowing the pulse width of pulsed light, which is launched into an optical fiber. However, if the pulse width becomes smaller than or equal to a specific value, the center frequency of Brillouin scattered light cannot be measured with desired accuracy. Therefore, it is also known that the spatial resolution is 2 to 3 m.
In recent years, more accurate measurement is required, and a higher spatial resolution of approximately 1 to 10 cm is desired. However, in the BOTDR
method, as spontaneous Brillouin scattered light is detected, the signal level is low.
Therefore, if the above restriction (i.e., when the pulse width becomes smaller than or equal to a specific value) can be released, it is difficult to obtain a high spatial resolution of approximately 1 to 10 cm.
SUMMARY OF THE INVENTION
In light of the above circumstances, an object of the present invention is to provide an optical fiber characteristic measuring system, in which pulsed light is launched into one end of a target optical fiber to be measured, continuous light is launched into the other end of the target optical fiber, and continuous light, emitted from the one end of the optical fiber, is detected so as to measure the characteristics of the target optical fiber, and in which a higher spatial resolution in comparison with conventional systems is produced.
Therefore, the present invention provides an optical fiber characteristic measuring system comprising:
a first light source device for generating a pulse train from coherent light having a first frequency, and launching the pulse train as pulsed light into one end of a target optical fiber to be measured, wherein the pulse train includes a first light pulse and a second light pulse, and the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse is less than or equal to the lifetime of an acoustic wave in the target optical fiber;
a second light source device for launching coherent light having a second frequency as continuous light into another end of the target optical fiber;
a varying device for varying the difference between the first frequency and the second frequency within a range which includes a Brillouin frequency shift with respect to the target optical fiber;
an optical detection device for detecting light, which is emitted from said one end of the target optical fiber; and a signal processing device for measuring characteristics of the target optical fiber based on a result of detection performed by the optical detection device.
In accordance with the above structure, the pulse train, in which the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse is less than or equal to the lifetime of an acoustic wave in the target optical fiber, is launched as pulsed light into one end of the target optical fiber, and the continuous light is launched into another end of the target optical fiber. The difference between the frequency of the pulsed light and the frequency of the continuous light is varied in a range which includes the Brillouin frequency shift with respect to the target optical fiber. Therefore, the intensity of Brillouin scattered light with respect to the second light pulse greatly varies depending on the difference between the frequency of the pulsed light and the frequency of the continuous light.
Accordingly, the Brillouin spectrum obtained by the signal processing device is narrowed and becomes steep, so that the Brillouin frequency shift can be detected very easily, and the spatial resolution can be effectively improved.
Therefore, in accordance with the present invention, a higher spatial resolution in comparison with conventional systems can be produced in an optical fiber characteristic measuring system in which pulsed light is launched into one end of a target optical fiber to be measured, continuous light is launched into the other end of the target optical fiber, and continuous light, emitted from the one end of the optical fiber, is detected so as to measure the characteristics of the target optical fiber.
In a typical example, the pulse width of the first light pulse is smaller than the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse; and the pulse width of the second light pulse is smaller than half the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse.
In accordance with the above structure, the pulse train, in which the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse is less than or equal to the lifetime of an acoustic wave in the target optical fiber, is launched as pulsed light into one end of the target optical fiber, and the continuous light is launched into another end of the target optical fiber. The difference between the frequency of the pulsed light and the frequency of the continuous light is varied in a range which includes the Brillouin frequency shift with respect to the target optical fiber. Therefore, the intensity of Brillouin scattered light with respect to the second light pulse greatly varies depending on the difference between the frequency of the pulsed light and the frequency of the continuous light.
Accordingly, the Brillouin spectrum obtained by the signal processing device is narrowed and becomes steep, so that the Brillouin frequency shift can be detected very easily, and the spatial resolution can be effectively improved.
Therefore, in accordance with the present invention, a higher spatial resolution in comparison with conventional systems can be produced in an optical fiber characteristic measuring system in which pulsed light is launched into one end of a target optical fiber to be measured, continuous light is launched into the other end of the target optical fiber, and continuous light, emitted from the one end of the optical fiber, is detected so as to measure the characteristics of the target optical fiber.
In a typical example, the pulse width of the first light pulse is smaller than the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse; and the pulse width of the second light pulse is smaller than half the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse.
5 The optical fiber characteristic measuring system may further comprise (i) a polarization control device, by which the polarization state with respect to the pulsed light or the continuous light can be changed, (ii) an undesired element removing device for removing an undesired element, which is included in the pulsed light, or (iii) an optical frequency filter for transmitting an element originated in the continuous light, and blocking an element originated in the pulsed light, wherein both elements are included in the light emitted from said one end of the target optical fiber.
Preferably, the lifetime of the acoustic wave is a time period from when the energy of the acoustic wave has its peak power value to when it has decreased to 5% or smaller of the peak power value.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a first embodiment in accordance with the present invention.
Fig. 2 is a diagram showing an example of the waveform of the pulsed light.
Fig. 3 is a diagram showing an amplitude variation of an acoustic wave, obtained when the difference between the frequency of the pulsed light and the frequency of the continuous light coincides with the Brillouin frequency shift of the target optical fiber.
Preferably, the lifetime of the acoustic wave is a time period from when the energy of the acoustic wave has its peak power value to when it has decreased to 5% or smaller of the peak power value.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a first embodiment in accordance with the present invention.
Fig. 2 is a diagram showing an example of the waveform of the pulsed light.
Fig. 3 is a diagram showing an amplitude variation of an acoustic wave, obtained when the difference between the frequency of the pulsed light and the frequency of the continuous light coincides with the Brillouin frequency shift of the target optical fiber.
Fig. 4 is a diagram showing an amplitude variation of an acoustic wave, obtained when the difference between the frequency of the pulsed light and the frequency of the continuous light deviates from the Brillouin frequency shift of the target optical fiber.
Fig. 5 is a diagram showing intensity of Brillouin scattered light produced by an acoustic wave, when the difference between the frequency of the pulsed light and the frequency of the continuous light coincides with the Brillouin frequency shift of the target optical fiber.
Fig. 6 is a diagram showing intensity of Brillouin scattered light produced by an acoustic wave, when the difference between the frequency of the pulsed light and the frequency of the continuous light deviates from the Brillouin frequency shift of the target optical fiber.
Fig. 7 is a diagram showing a two-dimensional distribution (time (distance) versus frequency shift) with respect to the power of Brillouin scattered light obtained when the target optical fiber 6 in the first embodiment, which consists of (i) optical fiber A having a length of 1 m and a Brillouin frequency shift fB of 0 (relative value), (ii) optical fiber B having a length of 20 cm and a Brillouin frequency shift fB of 50 MHz (relative value), and (iii) optical fiber C having a length of 1 m and a Brillouin frequency shift fB of 0 (relative value), wherein these fibers are connected in this order.
Fig. 8 is a diagram showing a Brillouin spectrum at the center point of the optical fiber A.
Fig. 9 is a diagram showing a Brillouin spectrum at the center point of the optical fiber B.
Fig. 10 is a graph showing a distribution of the Brillouin frequency shift (fB).
Fig. 11 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a second embodiment in accordance with the present invention.
Fig. 12 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a third embodiment in accordance with the present invention.
Fig. 13 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a fourth embodiment in accordance with the present invention.
Fig. 14 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a fifth embodiment in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the optical fiber characteristic measuring system in accordance with the present invention will be described with reference to the appended figures. In the drawings, the scale of each element is appropriately modified so that the element can be recognized.
First embodiment Fig. 1 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S 1 in a first embodiment. As shown in Fig. 1, the optical fiber characteristic measuring system S 1 includes a first light source 1, an optical pulse generator 2, an optical amplifier 3, an optical directional coupler 4, a first optical connector 5, an optical fiber 6 to be measured, a second light source 7, a second optical 1 ~
Fig. 5 is a diagram showing intensity of Brillouin scattered light produced by an acoustic wave, when the difference between the frequency of the pulsed light and the frequency of the continuous light coincides with the Brillouin frequency shift of the target optical fiber.
Fig. 6 is a diagram showing intensity of Brillouin scattered light produced by an acoustic wave, when the difference between the frequency of the pulsed light and the frequency of the continuous light deviates from the Brillouin frequency shift of the target optical fiber.
Fig. 7 is a diagram showing a two-dimensional distribution (time (distance) versus frequency shift) with respect to the power of Brillouin scattered light obtained when the target optical fiber 6 in the first embodiment, which consists of (i) optical fiber A having a length of 1 m and a Brillouin frequency shift fB of 0 (relative value), (ii) optical fiber B having a length of 20 cm and a Brillouin frequency shift fB of 50 MHz (relative value), and (iii) optical fiber C having a length of 1 m and a Brillouin frequency shift fB of 0 (relative value), wherein these fibers are connected in this order.
Fig. 8 is a diagram showing a Brillouin spectrum at the center point of the optical fiber A.
Fig. 9 is a diagram showing a Brillouin spectrum at the center point of the optical fiber B.
Fig. 10 is a graph showing a distribution of the Brillouin frequency shift (fB).
Fig. 11 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a second embodiment in accordance with the present invention.
Fig. 12 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a third embodiment in accordance with the present invention.
Fig. 13 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a fourth embodiment in accordance with the present invention.
Fig. 14 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system in a fifth embodiment in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the optical fiber characteristic measuring system in accordance with the present invention will be described with reference to the appended figures. In the drawings, the scale of each element is appropriately modified so that the element can be recognized.
First embodiment Fig. 1 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S 1 in a first embodiment. As shown in Fig. 1, the optical fiber characteristic measuring system S 1 includes a first light source 1, an optical pulse generator 2, an optical amplifier 3, an optical directional coupler 4, a first optical connector 5, an optical fiber 6 to be measured, a second light source 7, a second optical 1 ~
connector 8, an optical frequency control device 9 (as the varying device of the present invention), an optical detector 10 (as the optical detection device of the present invention), and a signal processor 11 (as the signal processing device of the present invention).
The first light source 1 emits narrow line-width coherent light 1 a, which may be a MQW-DFB semiconductor laser (i.e., of a multi-quantum well and distributed-feedback type) for a wavelength band of 1.55 gm. In the present embodiment, the frequency of the coherent light l a, launched from the first light source 1, is indicated by "fp" (as the first frequency of the present invention).
The optical pulse generator 2 is a high-speed optical switch such as an acoustooptic (optical) modulator or electro-optical modulator. The optical pulse generator 2 generates pulsed light, having a pulse width from 100 psec to a few nsec for realizing a required spatial resolution, from the coherent light 1 a, by means of an ON/OFF operation of the switch. The generated light is launched as pulsed light L1 into the target optical fiber 6 to be measured. In the optical fiber characteristic measuring system S 1 of the present embodiment, not a single light pulse, but a pulse train 2a including two subsequent light pulses is generated by the optical pulse generator 2, each having a pulse width from 100 psec to a few nsec. That is, the pulse train 2a is launched as the pulsed light Ll from the optical pulse generator 2 into the target optical fiber 6. The pulse widths of the subsequent light pulses do not need to be the same.
Among the two light pulses included in the pulse train 2a, the first light pulse 2a1 is first generated, and the second light pulse 2a2 is generated thereafter. The temporal interval between the center of the pulse width of the first light pulse 2a1 and the center of the pulse width of the second light pulse 2a2 is set to be less than or equal to the lifetime of an acoustic wave in the target optical fiber 6. In a broad sense, the "lifetime of an acoustic wave" is the time period from generation of a specific acoustic wave in the target optical fiber 6 to its disappearance. Here, the above temporal interval is set so that the second light pulse 2a2 can reach the acoustic wave, induced by the first light pulse 2a1, before it disappears. Therefore, so that the second light pulse 2a2 can reach the acoustic wave induced by the first light pulse 2al before it disappears, it is preferable that the above temporal interval be a time period from when the energy of the above acoustic wave has its peak power value to when it has decreased to 5% or smaller of the peak power value. For example, when the energy of the acoustic wave decays based on the following formula (1), the time period necessary for the transition from the peak power to 5% or smaller thereof can be indicated by time t>3Ta, where Ta (in formula (1)) indicates the damping factor of the acoustic wave.
exp (-t / Ta) . . . . . . (1) In addition, the above center of the pulse width indicates the center along the pulse-width axis.
The period with respect to the generation of the pulse train 2a depends on the length of the target optical fiber 6 (i.e., a distance range). For example, the pulse train period is (i) approximately 200 sec for a distance range of 10 km, and (ii) approximately 20 sec for a distance range of 1 km.
The optical amplifier 3 may be an optical fiber amplifier using an Er (erbium)-doped fiber, and amplifies the pulsed light Li so that it acquires desired optical pulse power. If the pulsed light L1, emitted from the optical pulse generator 2, already has desired optical pulse power, the optical amplifier 3 may be omitted.
The optical directional coupler 4 may be an optical circulator. Through the optical directional coupler 4, the pulsed light L1, input to an input port 41 of the optical directional coupler 4, is outputted from an input/output port 42.
Simultaneously, light L3, which is emitted from the target optical fiber 6 and is launched into the input/output port 42, is emitted from an output port 43 of the optical directional coupler 4.
5 The target optical fiber 6 to be measured is an optical fiber having a predetermined Brillouin frequency shift fB (i.e., predetermined fB of the Brillouin frequency shift). One end 61 of the optical fiber 6 is connected via the first optical connector 5 to the optical directional coupler 4, and the other end 62 is connected via the second optical connector 8 to the second light source 7.
The first light source 1 emits narrow line-width coherent light 1 a, which may be a MQW-DFB semiconductor laser (i.e., of a multi-quantum well and distributed-feedback type) for a wavelength band of 1.55 gm. In the present embodiment, the frequency of the coherent light l a, launched from the first light source 1, is indicated by "fp" (as the first frequency of the present invention).
The optical pulse generator 2 is a high-speed optical switch such as an acoustooptic (optical) modulator or electro-optical modulator. The optical pulse generator 2 generates pulsed light, having a pulse width from 100 psec to a few nsec for realizing a required spatial resolution, from the coherent light 1 a, by means of an ON/OFF operation of the switch. The generated light is launched as pulsed light L1 into the target optical fiber 6 to be measured. In the optical fiber characteristic measuring system S 1 of the present embodiment, not a single light pulse, but a pulse train 2a including two subsequent light pulses is generated by the optical pulse generator 2, each having a pulse width from 100 psec to a few nsec. That is, the pulse train 2a is launched as the pulsed light Ll from the optical pulse generator 2 into the target optical fiber 6. The pulse widths of the subsequent light pulses do not need to be the same.
Among the two light pulses included in the pulse train 2a, the first light pulse 2a1 is first generated, and the second light pulse 2a2 is generated thereafter. The temporal interval between the center of the pulse width of the first light pulse 2a1 and the center of the pulse width of the second light pulse 2a2 is set to be less than or equal to the lifetime of an acoustic wave in the target optical fiber 6. In a broad sense, the "lifetime of an acoustic wave" is the time period from generation of a specific acoustic wave in the target optical fiber 6 to its disappearance. Here, the above temporal interval is set so that the second light pulse 2a2 can reach the acoustic wave, induced by the first light pulse 2a1, before it disappears. Therefore, so that the second light pulse 2a2 can reach the acoustic wave induced by the first light pulse 2al before it disappears, it is preferable that the above temporal interval be a time period from when the energy of the above acoustic wave has its peak power value to when it has decreased to 5% or smaller of the peak power value. For example, when the energy of the acoustic wave decays based on the following formula (1), the time period necessary for the transition from the peak power to 5% or smaller thereof can be indicated by time t>3Ta, where Ta (in formula (1)) indicates the damping factor of the acoustic wave.
exp (-t / Ta) . . . . . . (1) In addition, the above center of the pulse width indicates the center along the pulse-width axis.
The period with respect to the generation of the pulse train 2a depends on the length of the target optical fiber 6 (i.e., a distance range). For example, the pulse train period is (i) approximately 200 sec for a distance range of 10 km, and (ii) approximately 20 sec for a distance range of 1 km.
The optical amplifier 3 may be an optical fiber amplifier using an Er (erbium)-doped fiber, and amplifies the pulsed light Li so that it acquires desired optical pulse power. If the pulsed light L1, emitted from the optical pulse generator 2, already has desired optical pulse power, the optical amplifier 3 may be omitted.
The optical directional coupler 4 may be an optical circulator. Through the optical directional coupler 4, the pulsed light L1, input to an input port 41 of the optical directional coupler 4, is outputted from an input/output port 42.
Simultaneously, light L3, which is emitted from the target optical fiber 6 and is launched into the input/output port 42, is emitted from an output port 43 of the optical directional coupler 4.
5 The target optical fiber 6 to be measured is an optical fiber having a predetermined Brillouin frequency shift fB (i.e., predetermined fB of the Brillouin frequency shift). One end 61 of the optical fiber 6 is connected via the first optical connector 5 to the optical directional coupler 4, and the other end 62 is connected via the second optical connector 8 to the second light source 7.
10 The second light source 7 emits narrow line-width coherent light as continuous light L2. Similar to the first light source 1, the second light source 7 may be a MQW-DFB semiconductor laser for a wavelength band of 1.55 m. In the present embodiment, the frequency of coherent light, emitted from the second light source 7, that is, the frequency of the continuous light L2, is indicated by "fs" (as the second frequency of the present invention).
The optical frequency control device 9 controls (i) the first light source 1 so that the frequency of the coherent light 1 a, emitted from the first light source 1, that is, the frequency of the pulsed light L1, is variable, and (ii) the second light source 7 so that the frequency of the coherent light, emitted from the second light source 7, that is, the frequency of the continuous light L2, is variable. In a variation, either one of the first light source 1 and the second light source 7 can be frequency-variable.
The optical frequency control device 9 also controls the first light source 1 and the second light source 7 in a manner such that the difference between the frequencies of the pulsed light L1 and the continuous light L2 varies within a range which includes the Brillouin frequency shift fB of the target optical fiber 6.
The optical frequency control device 9 controls (i) the first light source 1 so that the frequency of the coherent light 1 a, emitted from the first light source 1, that is, the frequency of the pulsed light L1, is variable, and (ii) the second light source 7 so that the frequency of the coherent light, emitted from the second light source 7, that is, the frequency of the continuous light L2, is variable. In a variation, either one of the first light source 1 and the second light source 7 can be frequency-variable.
The optical frequency control device 9 also controls the first light source 1 and the second light source 7 in a manner such that the difference between the frequencies of the pulsed light L1 and the continuous light L2 varies within a range which includes the Brillouin frequency shift fB of the target optical fiber 6.
As described above, in the optical fiber characteristic measuring system S 1 of the present embodiment, the pulse train 2a is generated by the first light source 1, the optical pulse generator 2, and the optical frequency control device 9, where the temporal interval between the centers of each pulse width with respect to the first light pulse 2a1 and the second light pulse 2a2 is less than or equal to the lifetime of an acoustic wave in the target optical fiber 6. The pulse train 2a is launched as the pulsed light L1 into one end 61 of the target optical fiber 6.
That is, in the optical fiber characteristic measuring system S 1 of the present embodiment, the first light source 1, the optical pulse generator 2, and the optical frequency control device 9 form the first light source device of the present invention.
Also in the optical fiber characteristic measuring system S 1 of the present embodiment, coherent light having a frequency fs is launched as the continuous light L2 into the other end 62 of the target optical fiber 6, by means of the second light source 7 and the optical frequency control device 9.
That is, in the optical fiber characteristic measuring system S 1 of the present embodiment, the second light source 7 and the optical frequency control device 9 form the second light source device of the present invention.
The optical detector 10 detects the light L3 emitted from the output port 43 of the optical directional coupler 4, and converts the input light L3 to an electrical signal L4 to be output.
Based on the result of detection performed by the optical detector 10, that is, the electrical signal L4, the signal processor 11 measures characteristics of the target optical fiber 6.
Below, the operation of the optical fiber characteristic measuring system S 1 in the present embodiment, having the above-described structure, will be explained.
That is, in the optical fiber characteristic measuring system S 1 of the present embodiment, the first light source 1, the optical pulse generator 2, and the optical frequency control device 9 form the first light source device of the present invention.
Also in the optical fiber characteristic measuring system S 1 of the present embodiment, coherent light having a frequency fs is launched as the continuous light L2 into the other end 62 of the target optical fiber 6, by means of the second light source 7 and the optical frequency control device 9.
That is, in the optical fiber characteristic measuring system S 1 of the present embodiment, the second light source 7 and the optical frequency control device 9 form the second light source device of the present invention.
The optical detector 10 detects the light L3 emitted from the output port 43 of the optical directional coupler 4, and converts the input light L3 to an electrical signal L4 to be output.
Based on the result of detection performed by the optical detector 10, that is, the electrical signal L4, the signal processor 11 measures characteristics of the target optical fiber 6.
Below, the operation of the optical fiber characteristic measuring system S 1 in the present embodiment, having the above-described structure, will be explained.
First, when the coherent light 1 a of frequency fp is emitted from the first light source 1, it is launched into the optical pulse generator 2. In the optical pulse generator 2, the pulse train 2a consisting of the first light pulse 2a1 and the second light pulse 2a2 is generated using the coherent light 1 a, where the interval between the centers of each pulse width of the light pulses is less than or equal to the lifetime of the relevant acoustic wave in the target optical fiber 6.
The pulse train 2a emitted from the optical pulse generator 2 is amplified by the optical amplifier 3 to a value by which stimulated Brillouin scattering occurs in the target optical fiber 6. The amplified pulse train 2a is launched into the input port 41 of the optical directional coupler 4, and then output from the input/output port 42 thereof.
The output light is then launched as the pulsed light L1 via the first optical connector 5 into one end 61 of the target optical fiber 6.
On the other hand, when coherent light of frequency fs is emitted from the second light source 7, it is incident as continuous light L2 via the second optical connector 8 into the other end 62 of the target optical fiber 6.
Accordingly, when the pulsed light L1 is launched into one end 61 of the target optical fiber 6, and the continuous light L2, having a frequency difference (with respect to the pulsed light L1) of Brillouin frequency shift fB of the target optical fiber 6, is launched into the other end 62, an acoustic wave is strongly induced, and strong scattered light is obtained. That is, strong energy transfer is performed between the pulsed light L1 and the continuous light L2.
When there is phase-velocity mismatching between the acoustic wave and the frequency difference, that is, when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB of the target optical fiber 6 (i.e., fp-fs = fa #fB), a phase difference occurs between an acoustic wave, which is induced at time t=t1, and an acoustic wave, which is induced at time t=t2.
In the optical fiber characteristic measuring system S1 of the present embodiment, the pulsed light L1 includes the first light pulse 2a1 and the second light pulse 2a2, where the interval between the centers of each pulse width of the light pulses is less than or equal to the lifetime of the relevant acoustic wave in the target optical fiber 6. Therefore, the acoustic wave induced by the first light pulse 2a1 interferes with the acoustic wave induced by the second light pulse 2a2, so that the amplitude of an acoustic wave, produced by superimposition of both acoustic waves, varies in accordance with the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2.
Fig. 2 is a diagram showing an example of waveforms of the first light pulse 2a1 and the second light pulse W. Figs. 3 and 4 are diagrams, each showing an amplitude variation of an acoustic wave, which is induced by the light pulses shown in Fig. 2, having an temporal interval of 5 nsec between the centers of each pulse width.
More specifically, Fig. 3 is an amplitude variation of the acoustic wave obtained when the difference between the frequency fp of the pulsed light Ll and the frequency fs of the continuous light L2 coincides with the Brillouin frequency shift fB of the target optical fiber 6, and Fig. 4 is an amplitude variation of the acoustic wave obtained when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB.
As shown in Fig. 3, when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 coincides with the Brillouin frequency shift fB of the target optical fiber 6, that is, when phase-velocity matching is provided at "fp-fs = fa = fB", the acoustic wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2 are added to each other at the same phase. Accordingly, the amplitude, which is induced by the first light pulse 2a1, is amplified by the second light pulse 2a2.
In contrast, as shown in Fig. 4, when the difference between the frequency fp of the pulsed light Ll and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB of the target optical fiber 6, more specifically, when phase-velocity mismatching occurs at "fp-fs = fB + 100 MHz", a phase difference of rL
(27c= 100MHz=5nsec) occurs between the acoustic wave induced by the first light pulse 2al and the acoustic wave induced by the second light pulse 2a2, so that a cancellation effect occurs between both acoustic waves. That is, the acoustic wave, induced by the first light pulse 2a1, is cancelled by the second light pulse 2a2, so that the amplitude of the acoustic wave becomes zero, and then the amplitude again rises.
Even when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB
of the target optical fiber 6, if phase-velocity matching is provided (i.e., when the phase difference is an integer multiple of 27c), then the acoustic wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2 are added to each other at the same phase.
The intensity of the Brillouin scattered light, generated in the target optical fiber 6, is in proportion to the intensity (i.e., amplitude ) of the relevant acoustic wave.
When the amplitude of the acoustic wave is increasing, the sign assigned to the intensity of the Brillouin scattered light is positive, and when the amplitude of the acoustic wave is decreasing, the sign assigned to the intensity of the Brillouin scattered light is negative.
The "positive" sign indicates that energy is transferred from the second light pulse 2a2 to the continuous light L2, and thus the light L3 emitted from one end 61 of the target optical fiber 6 increases. In contrast, the "negative" sign indicates that energy is transferred from the continuous light L2 to the second light pulse 2a2, and thus the light L3 emitted from one end 61 of the target optical fiber 6 decreases.
5 Therefore, when the difference between the frequency fp of the pulsed light and the frequency fs of the continuous light L2 coincides with the Brillouin frequency shift fB of the target optical fiber 6, and thus phase-velocity matching is provided (i.e., fp-fs = fB, fp-fs = fB + 200 MHz, fp-fs = fB + 400 MHz, or the like), then as shown in Fig. 5, the Brillouin scattered light with respect to the second light pulse 2a2 is strongly 10 included in the light L3, which is emitted from one end 61 of the target optical fiber 6.
In contrast, when the difference between the frequency fp of the pulsed light Ll and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB of the target optical fiber 6, and thus phase-velocity mismatching occurs (i.e., fp-fs = fB + 100 MHz, fp-fs = fB + 300 MHz, fp-fs = fB + 500 MHz, or the like), then 15 as shown in Fig. 6, the Brillouin scattered light with respect to the second light pulse 2a2 is weakly included in the light L3, which is emitted from one end 61 of the target optical fiber 6.
Accordingly, among the light pulses included in the pulsed light L1, the intensity of the Brillouin scattered light with respect to the first light pulse 2a1 depends very little upon the difference between the frequency fp of the pulsed light L
1 and the frequency fs of the continuous light L2, while the intensity of the Brillouin scattered light with respect to the second light pulse 2a2 depends greatly upon the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2, and periodically increases and decreases in accordance with a variation in the difference.
In the optical fiber characteristic measuring system S 1 of the present embodiment, the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 is varied by the optical frequency control device 9 within a range which includes the Brillouin frequency shift fB of the target optical fiber 6. In a specific example, the optical frequency control device 9 controls the first light source 1 and/or the second light source 7 in such a manner that the difference between the frequency fp of the pulsed light L 1 and the frequency fs of the continuous light L2 varies within a range from -500 to +500 MHz with respect to the Brillouin frequency shift fB.
When the difference between the frequency fp of the pulsed light L 1 and the frequency fs of the continuous light L2 varies as described above, the light L3, which includes Brillouin scattered light with respect to the second light pulse 2a2, is emitted from one end 61 of the target optical fiber 6, where the Brillouin scattered light varies in accordance with the frequency difference fp-fs.
The intensity of the above light L3 is measured by the optical detector 10, which converts the light L3 into the electrical signal L4. The electrical signal L4 is input into the signal processor 11. A Brillouin spectrum is obtained by measuring the intensity of the emitted light L3, as a function of time t for each frequency difference.
When the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 varies, the Brillouin spectrum periodically and greatly varies, so that the Brillouin spectrum is narrowed and becomes steep.
The signal processor 11 measures the characteristics of the target optical fiber 6 by using the narrowed and steep Brillouin spectrum. In this case, it is possible to measure the Brillouin frequency shift with high accuracy, thereby improving the spatial resolution.
Fig. 7 is a diagram showing a two-dimensional distribution (time (distance) versus frequency shift) with respect to the power of Brillouin scattered light obtained when the target optical fiber 6 consists of (i) optical fiber A having a length of 1 m and a Brillouin frequency shift fB of 0 (relative value), (ii) optical fiber B
having a length of 20 cm and a Brillouin frequency shift fB of 50 MHz (relative value), and (iii) optical fiber C having a length of 1 m and a Brillouin frequency shift fB of 0 (relative value), wherein these fibers are connected in this order.
Fig. 8 shows a Brillouin spectrum at the center point of the optical fiber A, and Fig. 9 shows a Brillouin spectrum at the center point of the optical fiber B.
Fig. 10 is a graph showing a distribution of the relevant Brillouin frequency shift.
Figs. 7 to 10 are obtained through a simulation.
As shown in these figures, in the optical fiber characteristic measuring system S 1 of the present embodiment, the Brillouin spectrum is narrowed and has a steep form.
Accordingly, detection of the Brillouin frequency shift can be performed very easily, thereby effectively improving the spatial resolution.
In addition, as the optical fiber characteristic measuring system S 1 of the present embodiment uses stimulated Brillouin scattering, measurement in a higher dynamic range can be performed in comparison with an optical fiber characteristic measuring system, which performs measurement by using spontaneous Brillouin scattering.
In accordance with the above-described optical fiber characteristic measuring system S 1 of the present embodiment, (i) the pulse train 2a consisting of the first light pulse 2a1 and the second light pulse 2a2 is launched as the pulsed light L1 into one end 61 of the target optical fiber 6, wherein the temporal interval between the centers of each pulse width with respect to the first light pulse 2a1 and the second light pulse 2a2 is less than or equal to the lifetime of an acoustic wave in the target optical fiber 6, (ii) the continuous light L2 is launched into the other end 62 of the target optical fiber 6, and (iii) the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 is varied in a range which includes the Brillouin frequency shift fB of the target optical fiber 6 The intensity of the Brillouin scattered light with respect to the second light pulse 2a2, which is included in the pulsed light L1, greatly varies in accordance with the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2. Therefore, the Brillouin spectrum obtained by the signal processor 11 is narrowed, and thus becomes steep, so that the Brillouin frequency shift can be detected very easily, and the spatial resolution can be effectively improved.
Therefore, in accordance with the above-described optical fiber characteristic measuring system S 1 of the present embodiment, a high spatial resolution can be obtained in a optical fiber characteristic measuring system in which pulsed light L 1 is launched into one end 61 of the target optical fiber 6, continuous light L2 is launched into the other end 62 of the target optical fiber 6, and light L3 emitted from the one end 61 so as to measure the characteristics of the target optical fiber 6. In addition, due to a filtering process using a periodic variation in the Brillouin spectrum, the Brillouin frequency shift can be further accurately detected.
When the temporal interval between the centers of each pulse width with respect to the first light pulse 2a1 and the second light pulse 2a2 is greater than the lifetime of an acoustic wave in the target optical fiber 6, an acoustic wave is induced by the second light pulse 2a2 after an acoustic wave, induced by the first light pulse 2a1 is greatly decayed. Therefore, there is no sufficient interference between the acoustic . ~ ' wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2. Accordingly, even when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 is varied, (i) the Brillouin spectrum is not periodically varied, or (ii) even if it periodically varies, the amplitude of the periodical variation is very small, and thus the Brillouin spectrum is not narrowed or steep. Therefore, when using such a Brillouin spectrum, the Brillouin frequency shift cannot be detected with high accuracy.
So that there is sufficient interference between the acoustic wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2, it is preferable that (i) the pulse width of the first light pulse 2a1 is smaller than the temporal interval between the center of the pulse width of the first light pulse 2a1 and the center of the pulse width of the second light pulse 2a2, and (ii) the pulse width of the second light pulse 2a2 is smaller than half the temporal interval between the center of the pulse width of the first light pulse 2a1 and the center of the pulse width of the second light pulse W. Under these conditions, sufficient interference occurs between the acoustic wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2, and the Brillouin spectrum is narrowed, so that the Brillouin frequency shift can be detected with high accuracy.
As understood by Figs. 7 to 10, in accordance with the optical fiber characteristic measuring system S 1 of the present embodiment, accurate characteristics of a strain distribution formed along the target optical fiber can be measured, thereby effectively improving the spatial resolution.
Second embodiment Below, a second embodiment of the present invention will be explained. In the following, explanations with respect to parts identical to those in the first embodiment are omitted or simplified.
Fig. 11 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S2 of the second embodiment.
5 As shown in Fig. 11, in the optical fiber characteristic measuring system S2 of the second embodiment, a polarization control device 20 (as the polarization control device of the present invention) is provided between the optical amplifier 3 and the optical directional coupler 4. The polarization control device 20 changes the polarization state with respect to the pulsed light L1 at a high speed, so as to change the 10 polarization state at random.
In the above first embodiment, it is assumed that polarization conditions between the pulsed light L1 and the continuous light L2 are constant. However, such conditions are satisfied by only special optical fibers such as a polarization-maintaining optical fiber, or multimode optical fibers in which the polarization state is randomized.
15 That is, when an ordinary optical fiber is used as the target optical fiber 6, the above conditions are not satisfied.
On the other hand, stimulated Brillouin scattering depends on polarization, such that when maximum scattering occurs when the polarization axes of the pulsed light L1 and the continuous light L2 coincide with each other, and the scattering becomes zero 20 when the polarization axes thereof are orthogonal to each other.
Therefore, as performed in the optical fiber characteristic measuring system of the second embodiment, when the polarization state with respect to the pulsed light L1 is changed at high speed by the polarization control device 20, so as to change the polarization state at random, the polarization dependency can be cancelled.
The polarization dependency can also be cancelled when the polarization state with respect to the pulsed light Ll is changed by 90 degrees by the polarization control device 20 at specific intervals, and the root of the sum of squares of measured results is computed.
Also in the optical fiber characteristic measuring system S2 of the second embodiment, the polarization control device 20 is provided between the optical amplifier 3 and the optical directional coupler 4. However, this is not a limiting condition. For example, similar effects can be obtained when a polarization control device is provided between the second light source.7 and the target optical fiber 6, so as to change the polarization conditions of the continuous light L2.
Third embodiment Below, a third embodiment of the present invention will be explained. Also in the following description with respect to the third embodiment, explanations with respect to parts identical to those in the first embodiment are omitted or simplified.
Fig. 12 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S3 of the third embodiment.
As shown in Fig. 12, in the optical fiber characteristic measuring system S3 of the third embodiment, an ASE light removing optical switch 30 (as the undesired element removing device of the present invention) is provided between the optical amplifier 3 and the optical directional coupler 4. The ASE light removing optical switch 30 removes noise elements (i.e., ASE (amplified spontaneous emission) light), which are imposed on the pulse train 2a due to amplification of the pulse train 2a through the optical amplifier 3.
In the above first embodiment, it is assumed that noise elements (undesired elements) generated in the optical amplifier 3 can be discounted. However, such noise elements may degrade the SN ratio of the pulsed light L1 or the emitted light L3, and thus it is preferable to remove them.
Accordingly, when providing the ASE light removing optical switch 30 as in the optical fiber characteristic measuring system S3 of the present embodiment, it is possible to prevent the SN ratio of the pulsed light L1 or the emitted light L3 from being degraded.
Fourth embodiment Below, a fourth embodiment of the present invention will be explained. Also in the following description with respect to the fourth embodiment, explanations with respect to parts identical to those in the first embodiment are omitted or simplified.
Fig. 13 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S4 of the fourth embodiment.
As shown in Fig. 13, in the optical fiber characteristic measuring system S4 of the fourth embodiment, an optical frequency filter 40 is provided between the optical directional coupler 4 and the optical detector 10. Among elements included in the light L3 emitted from one end 61 of the target optical fiber 6, the optical frequency filter 40 transmits a continuous-light element (i.e., the frequency element with respect to the continuous light L2), and blocks a pulsed-light element (i.e., the frequency element with respect to the pulsed light Ll).
The pulsed-light element included in the emitted light L3 functions as a noise in the optical detector 10. In accordance with the optical fiber characteristic measuring system S4 of the fourth embodiment, such a pulsed-light element can be removed from the emitted light L3 by the optical frequency filter 40, thereby performing the measurement with higher accuracy.
The pulse train 2a emitted from the optical pulse generator 2 is amplified by the optical amplifier 3 to a value by which stimulated Brillouin scattering occurs in the target optical fiber 6. The amplified pulse train 2a is launched into the input port 41 of the optical directional coupler 4, and then output from the input/output port 42 thereof.
The output light is then launched as the pulsed light L1 via the first optical connector 5 into one end 61 of the target optical fiber 6.
On the other hand, when coherent light of frequency fs is emitted from the second light source 7, it is incident as continuous light L2 via the second optical connector 8 into the other end 62 of the target optical fiber 6.
Accordingly, when the pulsed light L1 is launched into one end 61 of the target optical fiber 6, and the continuous light L2, having a frequency difference (with respect to the pulsed light L1) of Brillouin frequency shift fB of the target optical fiber 6, is launched into the other end 62, an acoustic wave is strongly induced, and strong scattered light is obtained. That is, strong energy transfer is performed between the pulsed light L1 and the continuous light L2.
When there is phase-velocity mismatching between the acoustic wave and the frequency difference, that is, when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB of the target optical fiber 6 (i.e., fp-fs = fa #fB), a phase difference occurs between an acoustic wave, which is induced at time t=t1, and an acoustic wave, which is induced at time t=t2.
In the optical fiber characteristic measuring system S1 of the present embodiment, the pulsed light L1 includes the first light pulse 2a1 and the second light pulse 2a2, where the interval between the centers of each pulse width of the light pulses is less than or equal to the lifetime of the relevant acoustic wave in the target optical fiber 6. Therefore, the acoustic wave induced by the first light pulse 2a1 interferes with the acoustic wave induced by the second light pulse 2a2, so that the amplitude of an acoustic wave, produced by superimposition of both acoustic waves, varies in accordance with the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2.
Fig. 2 is a diagram showing an example of waveforms of the first light pulse 2a1 and the second light pulse W. Figs. 3 and 4 are diagrams, each showing an amplitude variation of an acoustic wave, which is induced by the light pulses shown in Fig. 2, having an temporal interval of 5 nsec between the centers of each pulse width.
More specifically, Fig. 3 is an amplitude variation of the acoustic wave obtained when the difference between the frequency fp of the pulsed light Ll and the frequency fs of the continuous light L2 coincides with the Brillouin frequency shift fB of the target optical fiber 6, and Fig. 4 is an amplitude variation of the acoustic wave obtained when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB.
As shown in Fig. 3, when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 coincides with the Brillouin frequency shift fB of the target optical fiber 6, that is, when phase-velocity matching is provided at "fp-fs = fa = fB", the acoustic wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2 are added to each other at the same phase. Accordingly, the amplitude, which is induced by the first light pulse 2a1, is amplified by the second light pulse 2a2.
In contrast, as shown in Fig. 4, when the difference between the frequency fp of the pulsed light Ll and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB of the target optical fiber 6, more specifically, when phase-velocity mismatching occurs at "fp-fs = fB + 100 MHz", a phase difference of rL
(27c= 100MHz=5nsec) occurs between the acoustic wave induced by the first light pulse 2al and the acoustic wave induced by the second light pulse 2a2, so that a cancellation effect occurs between both acoustic waves. That is, the acoustic wave, induced by the first light pulse 2a1, is cancelled by the second light pulse 2a2, so that the amplitude of the acoustic wave becomes zero, and then the amplitude again rises.
Even when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB
of the target optical fiber 6, if phase-velocity matching is provided (i.e., when the phase difference is an integer multiple of 27c), then the acoustic wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2 are added to each other at the same phase.
The intensity of the Brillouin scattered light, generated in the target optical fiber 6, is in proportion to the intensity (i.e., amplitude ) of the relevant acoustic wave.
When the amplitude of the acoustic wave is increasing, the sign assigned to the intensity of the Brillouin scattered light is positive, and when the amplitude of the acoustic wave is decreasing, the sign assigned to the intensity of the Brillouin scattered light is negative.
The "positive" sign indicates that energy is transferred from the second light pulse 2a2 to the continuous light L2, and thus the light L3 emitted from one end 61 of the target optical fiber 6 increases. In contrast, the "negative" sign indicates that energy is transferred from the continuous light L2 to the second light pulse 2a2, and thus the light L3 emitted from one end 61 of the target optical fiber 6 decreases.
5 Therefore, when the difference between the frequency fp of the pulsed light and the frequency fs of the continuous light L2 coincides with the Brillouin frequency shift fB of the target optical fiber 6, and thus phase-velocity matching is provided (i.e., fp-fs = fB, fp-fs = fB + 200 MHz, fp-fs = fB + 400 MHz, or the like), then as shown in Fig. 5, the Brillouin scattered light with respect to the second light pulse 2a2 is strongly 10 included in the light L3, which is emitted from one end 61 of the target optical fiber 6.
In contrast, when the difference between the frequency fp of the pulsed light Ll and the frequency fs of the continuous light L2 deviates from the Brillouin frequency shift fB of the target optical fiber 6, and thus phase-velocity mismatching occurs (i.e., fp-fs = fB + 100 MHz, fp-fs = fB + 300 MHz, fp-fs = fB + 500 MHz, or the like), then 15 as shown in Fig. 6, the Brillouin scattered light with respect to the second light pulse 2a2 is weakly included in the light L3, which is emitted from one end 61 of the target optical fiber 6.
Accordingly, among the light pulses included in the pulsed light L1, the intensity of the Brillouin scattered light with respect to the first light pulse 2a1 depends very little upon the difference between the frequency fp of the pulsed light L
1 and the frequency fs of the continuous light L2, while the intensity of the Brillouin scattered light with respect to the second light pulse 2a2 depends greatly upon the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2, and periodically increases and decreases in accordance with a variation in the difference.
In the optical fiber characteristic measuring system S 1 of the present embodiment, the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 is varied by the optical frequency control device 9 within a range which includes the Brillouin frequency shift fB of the target optical fiber 6. In a specific example, the optical frequency control device 9 controls the first light source 1 and/or the second light source 7 in such a manner that the difference between the frequency fp of the pulsed light L 1 and the frequency fs of the continuous light L2 varies within a range from -500 to +500 MHz with respect to the Brillouin frequency shift fB.
When the difference between the frequency fp of the pulsed light L 1 and the frequency fs of the continuous light L2 varies as described above, the light L3, which includes Brillouin scattered light with respect to the second light pulse 2a2, is emitted from one end 61 of the target optical fiber 6, where the Brillouin scattered light varies in accordance with the frequency difference fp-fs.
The intensity of the above light L3 is measured by the optical detector 10, which converts the light L3 into the electrical signal L4. The electrical signal L4 is input into the signal processor 11. A Brillouin spectrum is obtained by measuring the intensity of the emitted light L3, as a function of time t for each frequency difference.
When the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 varies, the Brillouin spectrum periodically and greatly varies, so that the Brillouin spectrum is narrowed and becomes steep.
The signal processor 11 measures the characteristics of the target optical fiber 6 by using the narrowed and steep Brillouin spectrum. In this case, it is possible to measure the Brillouin frequency shift with high accuracy, thereby improving the spatial resolution.
Fig. 7 is a diagram showing a two-dimensional distribution (time (distance) versus frequency shift) with respect to the power of Brillouin scattered light obtained when the target optical fiber 6 consists of (i) optical fiber A having a length of 1 m and a Brillouin frequency shift fB of 0 (relative value), (ii) optical fiber B
having a length of 20 cm and a Brillouin frequency shift fB of 50 MHz (relative value), and (iii) optical fiber C having a length of 1 m and a Brillouin frequency shift fB of 0 (relative value), wherein these fibers are connected in this order.
Fig. 8 shows a Brillouin spectrum at the center point of the optical fiber A, and Fig. 9 shows a Brillouin spectrum at the center point of the optical fiber B.
Fig. 10 is a graph showing a distribution of the relevant Brillouin frequency shift.
Figs. 7 to 10 are obtained through a simulation.
As shown in these figures, in the optical fiber characteristic measuring system S 1 of the present embodiment, the Brillouin spectrum is narrowed and has a steep form.
Accordingly, detection of the Brillouin frequency shift can be performed very easily, thereby effectively improving the spatial resolution.
In addition, as the optical fiber characteristic measuring system S 1 of the present embodiment uses stimulated Brillouin scattering, measurement in a higher dynamic range can be performed in comparison with an optical fiber characteristic measuring system, which performs measurement by using spontaneous Brillouin scattering.
In accordance with the above-described optical fiber characteristic measuring system S 1 of the present embodiment, (i) the pulse train 2a consisting of the first light pulse 2a1 and the second light pulse 2a2 is launched as the pulsed light L1 into one end 61 of the target optical fiber 6, wherein the temporal interval between the centers of each pulse width with respect to the first light pulse 2a1 and the second light pulse 2a2 is less than or equal to the lifetime of an acoustic wave in the target optical fiber 6, (ii) the continuous light L2 is launched into the other end 62 of the target optical fiber 6, and (iii) the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 is varied in a range which includes the Brillouin frequency shift fB of the target optical fiber 6 The intensity of the Brillouin scattered light with respect to the second light pulse 2a2, which is included in the pulsed light L1, greatly varies in accordance with the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2. Therefore, the Brillouin spectrum obtained by the signal processor 11 is narrowed, and thus becomes steep, so that the Brillouin frequency shift can be detected very easily, and the spatial resolution can be effectively improved.
Therefore, in accordance with the above-described optical fiber characteristic measuring system S 1 of the present embodiment, a high spatial resolution can be obtained in a optical fiber characteristic measuring system in which pulsed light L 1 is launched into one end 61 of the target optical fiber 6, continuous light L2 is launched into the other end 62 of the target optical fiber 6, and light L3 emitted from the one end 61 so as to measure the characteristics of the target optical fiber 6. In addition, due to a filtering process using a periodic variation in the Brillouin spectrum, the Brillouin frequency shift can be further accurately detected.
When the temporal interval between the centers of each pulse width with respect to the first light pulse 2a1 and the second light pulse 2a2 is greater than the lifetime of an acoustic wave in the target optical fiber 6, an acoustic wave is induced by the second light pulse 2a2 after an acoustic wave, induced by the first light pulse 2a1 is greatly decayed. Therefore, there is no sufficient interference between the acoustic . ~ ' wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2. Accordingly, even when the difference between the frequency fp of the pulsed light L1 and the frequency fs of the continuous light L2 is varied, (i) the Brillouin spectrum is not periodically varied, or (ii) even if it periodically varies, the amplitude of the periodical variation is very small, and thus the Brillouin spectrum is not narrowed or steep. Therefore, when using such a Brillouin spectrum, the Brillouin frequency shift cannot be detected with high accuracy.
So that there is sufficient interference between the acoustic wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2, it is preferable that (i) the pulse width of the first light pulse 2a1 is smaller than the temporal interval between the center of the pulse width of the first light pulse 2a1 and the center of the pulse width of the second light pulse 2a2, and (ii) the pulse width of the second light pulse 2a2 is smaller than half the temporal interval between the center of the pulse width of the first light pulse 2a1 and the center of the pulse width of the second light pulse W. Under these conditions, sufficient interference occurs between the acoustic wave induced by the first light pulse 2a1 and the acoustic wave induced by the second light pulse 2a2, and the Brillouin spectrum is narrowed, so that the Brillouin frequency shift can be detected with high accuracy.
As understood by Figs. 7 to 10, in accordance with the optical fiber characteristic measuring system S 1 of the present embodiment, accurate characteristics of a strain distribution formed along the target optical fiber can be measured, thereby effectively improving the spatial resolution.
Second embodiment Below, a second embodiment of the present invention will be explained. In the following, explanations with respect to parts identical to those in the first embodiment are omitted or simplified.
Fig. 11 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S2 of the second embodiment.
5 As shown in Fig. 11, in the optical fiber characteristic measuring system S2 of the second embodiment, a polarization control device 20 (as the polarization control device of the present invention) is provided between the optical amplifier 3 and the optical directional coupler 4. The polarization control device 20 changes the polarization state with respect to the pulsed light L1 at a high speed, so as to change the 10 polarization state at random.
In the above first embodiment, it is assumed that polarization conditions between the pulsed light L1 and the continuous light L2 are constant. However, such conditions are satisfied by only special optical fibers such as a polarization-maintaining optical fiber, or multimode optical fibers in which the polarization state is randomized.
15 That is, when an ordinary optical fiber is used as the target optical fiber 6, the above conditions are not satisfied.
On the other hand, stimulated Brillouin scattering depends on polarization, such that when maximum scattering occurs when the polarization axes of the pulsed light L1 and the continuous light L2 coincide with each other, and the scattering becomes zero 20 when the polarization axes thereof are orthogonal to each other.
Therefore, as performed in the optical fiber characteristic measuring system of the second embodiment, when the polarization state with respect to the pulsed light L1 is changed at high speed by the polarization control device 20, so as to change the polarization state at random, the polarization dependency can be cancelled.
The polarization dependency can also be cancelled when the polarization state with respect to the pulsed light Ll is changed by 90 degrees by the polarization control device 20 at specific intervals, and the root of the sum of squares of measured results is computed.
Also in the optical fiber characteristic measuring system S2 of the second embodiment, the polarization control device 20 is provided between the optical amplifier 3 and the optical directional coupler 4. However, this is not a limiting condition. For example, similar effects can be obtained when a polarization control device is provided between the second light source.7 and the target optical fiber 6, so as to change the polarization conditions of the continuous light L2.
Third embodiment Below, a third embodiment of the present invention will be explained. Also in the following description with respect to the third embodiment, explanations with respect to parts identical to those in the first embodiment are omitted or simplified.
Fig. 12 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S3 of the third embodiment.
As shown in Fig. 12, in the optical fiber characteristic measuring system S3 of the third embodiment, an ASE light removing optical switch 30 (as the undesired element removing device of the present invention) is provided between the optical amplifier 3 and the optical directional coupler 4. The ASE light removing optical switch 30 removes noise elements (i.e., ASE (amplified spontaneous emission) light), which are imposed on the pulse train 2a due to amplification of the pulse train 2a through the optical amplifier 3.
In the above first embodiment, it is assumed that noise elements (undesired elements) generated in the optical amplifier 3 can be discounted. However, such noise elements may degrade the SN ratio of the pulsed light L1 or the emitted light L3, and thus it is preferable to remove them.
Accordingly, when providing the ASE light removing optical switch 30 as in the optical fiber characteristic measuring system S3 of the present embodiment, it is possible to prevent the SN ratio of the pulsed light L1 or the emitted light L3 from being degraded.
Fourth embodiment Below, a fourth embodiment of the present invention will be explained. Also in the following description with respect to the fourth embodiment, explanations with respect to parts identical to those in the first embodiment are omitted or simplified.
Fig. 13 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S4 of the fourth embodiment.
As shown in Fig. 13, in the optical fiber characteristic measuring system S4 of the fourth embodiment, an optical frequency filter 40 is provided between the optical directional coupler 4 and the optical detector 10. Among elements included in the light L3 emitted from one end 61 of the target optical fiber 6, the optical frequency filter 40 transmits a continuous-light element (i.e., the frequency element with respect to the continuous light L2), and blocks a pulsed-light element (i.e., the frequency element with respect to the pulsed light Ll).
The pulsed-light element included in the emitted light L3 functions as a noise in the optical detector 10. In accordance with the optical fiber characteristic measuring system S4 of the fourth embodiment, such a pulsed-light element can be removed from the emitted light L3 by the optical frequency filter 40, thereby performing the measurement with higher accuracy.
Fifth embodiment Below, a fifth embodiment of the present invention will be explained. Also in the following description with respect to the fifth embodiment, explanations with respect to parts identical to those in the first embodiment are omitted or simplified.
Fig. 14 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S5 of the fifth embodiment.
As shown in Fig. 14, in the optical fiber characteristic measuring system S5 of the fourth embodiment, a single light source 51, a branch coupler 52 for branching coherent light, which is launched from the light source 51, into two portions, and a modulation part 53 for subjecting branched coherent light to light intensity modulation using a modulation signal whose frequency is variable.
The modulation part 53 includes a microwave generator 531 for generating a modulating signal, and a light intensity modulator 532 for subjecting coherent light to light intensity modulation using the modulation signal. Among optical sideband signals generated by the light intensity modulation, the modulation part 53 makes coherent light on one sideband signal incident as the continuous light L2 on one end 61 of the target optical fiber 6.
The other coherent light branched through the branch coupler 52 is launched into the optical pulse generator 2.
In accordance with the optical fiber characteristic measuring system S5 of the fifth embodiment, coherent light launched from a single light source is branched into two portions: one is transformed into the continuous light L2, and the other functions as the pulsed light L1. In addition, the frequency difference between the pulsed light L1 and the continuous light L2 can be varied by controlling the frequency of the . 1 ~
Fig. 14 is a block diagram showing the structure and functions of an optical fiber characteristic measuring system S5 of the fifth embodiment.
As shown in Fig. 14, in the optical fiber characteristic measuring system S5 of the fourth embodiment, a single light source 51, a branch coupler 52 for branching coherent light, which is launched from the light source 51, into two portions, and a modulation part 53 for subjecting branched coherent light to light intensity modulation using a modulation signal whose frequency is variable.
The modulation part 53 includes a microwave generator 531 for generating a modulating signal, and a light intensity modulator 532 for subjecting coherent light to light intensity modulation using the modulation signal. Among optical sideband signals generated by the light intensity modulation, the modulation part 53 makes coherent light on one sideband signal incident as the continuous light L2 on one end 61 of the target optical fiber 6.
The other coherent light branched through the branch coupler 52 is launched into the optical pulse generator 2.
In accordance with the optical fiber characteristic measuring system S5 of the fifth embodiment, coherent light launched from a single light source is branched into two portions: one is transformed into the continuous light L2, and the other functions as the pulsed light L1. In addition, the frequency difference between the pulsed light L1 and the continuous light L2 can be varied by controlling the frequency of the . 1 ~
modulating signal, which is generated by the microwave generator 531.
That is, in the optical fiber characteristic measuring system S5 of the fifth embodiment, the first light source device of the present invention is formed by the light source 51 and the optical pulse generator 2, and the second light source device of the present invention is formed by the light source 51 and the modulation part 53.
Additionally, the modulation part 53 also functions as the varying device of the present invention.
Also in the optical fiber characteristic measuring system S5 of the fifth embodiment, similar effects to those obtained by the optical fiber characteristic measuring system S 1 of the first embodiment can be obtained.
While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the scope of the present invention.
Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.
The above embodiments have a prior condition such that the optical frequency of the continuous light is lower than that of the pulsed light by approximately 10 GHz.
In this case, energy transition occurs from the pulsed light to the continuous light, thereby amplifying the continuous light. In this case, the obtained Brillouin spectrum is a "gain spectrum".
In contrast, when the optical frequency of the continuous light is higher than that of the pulsed light by approximately 10 GHz, energy transition occurs from the continuous light to the pulsed light, and thus the continuous light has a loss. In this case, the obtained Brillouin spectrum is a "loss spectrum". However, also in this case, the same effect of narrowing the spectrum and realizing a high spatial resolution can be obtained.
That is, in the optical fiber characteristic measuring system S5 of the fifth embodiment, the first light source device of the present invention is formed by the light source 51 and the optical pulse generator 2, and the second light source device of the present invention is formed by the light source 51 and the modulation part 53.
Additionally, the modulation part 53 also functions as the varying device of the present invention.
Also in the optical fiber characteristic measuring system S5 of the fifth embodiment, similar effects to those obtained by the optical fiber characteristic measuring system S 1 of the first embodiment can be obtained.
While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the scope of the present invention.
Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.
The above embodiments have a prior condition such that the optical frequency of the continuous light is lower than that of the pulsed light by approximately 10 GHz.
In this case, energy transition occurs from the pulsed light to the continuous light, thereby amplifying the continuous light. In this case, the obtained Brillouin spectrum is a "gain spectrum".
In contrast, when the optical frequency of the continuous light is higher than that of the pulsed light by approximately 10 GHz, energy transition occurs from the continuous light to the pulsed light, and thus the continuous light has a loss. In this case, the obtained Brillouin spectrum is a "loss spectrum". However, also in this case, the same effect of narrowing the spectrum and realizing a high spatial resolution can be obtained.
Claims (6)
1. An optical fiber characteristic measuring system comprising:
a first light source device for generating a pulse train from coherent light having a first frequency, and launching the pulse train as pulsed light into one end of a target optical fiber to be measured, wherein the pulse train includes a first light pulse and a second light pulse, and the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse is less than or equal to the lifetime of an acoustic wave in the target optical fiber;
a second light source device for launching coherent light having a second frequency as continuous light into another end of the target optical fiber;
a varying device for varying the difference between the first frequency and the second frequency within a range which includes a Brillouin frequency shift with respect to the target optical fiber;
an optical detection device for detecting light, which is emitted from said one end of the target optical fiber; and a signal processing device for measuring characteristics of the target optical fiber based on a result of detection performed by the optical detection device.
a first light source device for generating a pulse train from coherent light having a first frequency, and launching the pulse train as pulsed light into one end of a target optical fiber to be measured, wherein the pulse train includes a first light pulse and a second light pulse, and the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse is less than or equal to the lifetime of an acoustic wave in the target optical fiber;
a second light source device for launching coherent light having a second frequency as continuous light into another end of the target optical fiber;
a varying device for varying the difference between the first frequency and the second frequency within a range which includes a Brillouin frequency shift with respect to the target optical fiber;
an optical detection device for detecting light, which is emitted from said one end of the target optical fiber; and a signal processing device for measuring characteristics of the target optical fiber based on a result of detection performed by the optical detection device.
2. The optical fiber characteristic measuring system in accordance with Claim 1, wherein:
the pulse width of the first light pulse is smaller than the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse; and the pulse width of the second light pulse is smaller than half the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse.
the pulse width of the first light pulse is smaller than the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse; and the pulse width of the second light pulse is smaller than half the temporal interval between the center of the pulse width of the first light pulse and the center of the pulse width of the second light pulse.
3. The optical fiber characteristic measuring system in accordance with any one of Claims 1 and 2, further comprising:
a polarization control device, by which the polarization state with respect to the pulsed light or the continuous light can be changed.
a polarization control device, by which the polarization state with respect to the pulsed light or the continuous light can be changed.
4. The optical fiber characteristic measuring system in accordance with any one of Claims 1 to 3, further comprising:
an undesired element removing device for removing an undesired element, which is included in the pulsed light.
an undesired element removing device for removing an undesired element, which is included in the pulsed light.
5. The optical fiber characteristic measuring system in accordance with any one of Claims 1 to 4, further comprising:
an optical frequency filter for transmitting an element originated in the continuous light, and blocking an element originated in the pulsed light, wherein both elements are included in the light emitted from said one end of the target optical fiber.
an optical frequency filter for transmitting an element originated in the continuous light, and blocking an element originated in the pulsed light, wherein both elements are included in the light emitted from said one end of the target optical fiber.
6. The optical fiber characteristic measuring system in accordance with any one of Claims 1 to 5, wherein:
the lifetime of the acoustic wave is a time period from when the energy of the acoustic wave has its peak power value to when it has decreased to 5% or smaller of the peak power value.
the lifetime of the acoustic wave is a time period from when the energy of the acoustic wave has its peak power value to when it has decreased to 5% or smaller of the peak power value.
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| Application Number | Priority Date | Filing Date | Title |
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| JP2006349391A JP4904148B2 (en) | 2006-12-26 | 2006-12-26 | Optical fiber characteristic measuring device |
| JP2006-349391 | 2006-12-26 |
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| CA2615327A1 true CA2615327A1 (en) | 2008-06-26 |
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| CA2615327A Expired - Fee Related CA2615327C (en) | 2006-12-26 | 2007-12-20 | Optical fiber characteristic measuring system |
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| CA (1) | CA2615327C (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2014012411A1 (en) * | 2012-07-19 | 2014-01-23 | 南京大学 | Botda system based on pulse coding and coherent detection |
| CN109000157A (en) * | 2018-10-01 | 2018-12-14 | 江苏亨通光纤科技有限公司 | A kind of pipeline on-Line Monitor Device and monitoring method |
| CN117969043A (en) * | 2024-03-28 | 2024-05-03 | 德州振飞光纤技术有限公司 | Detection device for polarization axis of polarization maintaining optical fiber |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
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| JP5264659B2 (en) * | 2009-09-07 | 2013-08-14 | 日本電信電話株式会社 | Optical line characteristic measuring method and apparatus |
| CN101832856A (en) * | 2010-04-19 | 2010-09-15 | 北京交通大学 | Method for measuring luminous period by utilizing variable frequency light source with constant amplitude and pulse width |
| JP6097712B2 (en) * | 2014-03-06 | 2017-03-15 | 日本電信電話株式会社 | Apparatus and method for measuring propagation constant of optical fiber |
| CN104634503B (en) * | 2015-02-10 | 2017-03-08 | 北京航空航天大学 | A kind of method and device of measurement flow field pressure field |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP2589345B2 (en) * | 1988-06-24 | 1997-03-12 | 日本電信電話株式会社 | Method and apparatus for evaluating characteristics of optical fiber |
| JP3072865B2 (en) * | 1991-09-17 | 2000-08-07 | 日本電信電話株式会社 | Optical amplifier for optical pulse tester |
| JP3237745B2 (en) * | 1996-07-31 | 2001-12-10 | 日本電信電話株式会社 | Strain / temperature distribution measuring method and its measuring device |
| JP2003185534A (en) * | 2001-12-17 | 2003-07-03 | Kansai Electric Power Co Inc:The | Dispersion distribution-measuring method and apparatus |
-
2006
- 2006-12-26 JP JP2006349391A patent/JP4904148B2/en not_active Expired - Fee Related
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Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2014012411A1 (en) * | 2012-07-19 | 2014-01-23 | 南京大学 | Botda system based on pulse coding and coherent detection |
| CN109000157A (en) * | 2018-10-01 | 2018-12-14 | 江苏亨通光纤科技有限公司 | A kind of pipeline on-Line Monitor Device and monitoring method |
| CN109000157B (en) * | 2018-10-01 | 2024-03-29 | 江苏亨通光纤科技有限公司 | Online monitoring device and method for pipeline |
| CN117969043A (en) * | 2024-03-28 | 2024-05-03 | 德州振飞光纤技术有限公司 | Detection device for polarization axis of polarization maintaining optical fiber |
| CN117969043B (en) * | 2024-03-28 | 2024-05-31 | 德州振飞光纤技术有限公司 | Detection device for polarization axis of polarization maintaining optical fiber |
Also Published As
| Publication number | Publication date |
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| JP4904148B2 (en) | 2012-03-28 |
| CA2615327C (en) | 2013-04-16 |
| JP2008157859A (en) | 2008-07-10 |
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