JP2012132927A - Brillouin scattering measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Brillouin scattering measurement device capable of measuring Brillouin scattering only using a measured optical fiber.SOLUTION: A Brillouin scattering measurement device comprises: measured light generation means for generating a probe light aand a pump light b; polarization combination means 40 for combining the probe light and the pump light generated by the measured light generation means in a state that a polarization plane is different; a measured optical fiber 41 whose one end receives a combined light from the polarization combination means; rotating reflection means 42 that is arranged on the other end side of the measured optical fiber, rotates the polarization plane of the combined light and reflects only the probe light in the combined light to let it re-enter the other end of the measured optical fiber; and probe light detection means that is arranged on one end side of the measured optical fiber and detects a probe light areflected by the rotating reflection means.

Description

  The present invention relates to a Brillouin scattering measurement apparatus, and more particularly, to a Brillouin scattering measurement apparatus for measuring two Brillouin scatterings that occur when two laser beams having different wavelengths are incident on an optical fiber to be measured. is there.

  Conventionally, an optical fiber is used to measure a physical quantity distribution such as temperature and strain of an environment in which the optical fiber is installed. Such physical quantity measurement methods using optical fibers include: maintenance and management of optical communication paths, maintenance and management of large-scale structures such as dams and dikes, and materials and structures that have self-diagnosis functions for defects and failures (smart It is used for material structure.

  As a method for measuring the distribution of spatial strain, temperature, and the like in an environment where an optical fiber is installed, a method using the Brillouin scattering phenomenon in the optical fiber is known. Brillouin scattering is a phenomenon in which when two light waves with different frequencies pass in an optical fiber, the power of the light moves from high-frequency light to low-frequency light via acoustic waves in the optical fiber. To do.

  Moreover, the frequency difference at which the moving power is maximum depends on the refractive index in the optical fiber and the velocity of the acoustic wave, which depends on the temperature around the optical fiber and the strain applied to the optical fiber. By specifying the frequency difference at which the moving power due to scattering is maximized and the location where the movement is occurring, it is possible to measure the temperature and strain distribution in the space where the optical fiber is laid.

  In Patent Document 1, as a measurement method using Brillouin scattering, a first continuous wave light frequency-modulated with a predetermined modulation frequency and a second continuous wave frequency-modulated with a modulation frequency equal to the predetermined modulation frequency are used. What uses light is described. In particular, the first continuous wave light is incident from one end face of the optical fiber to be measured, the center frequency of the second continuous wave light is frequency-shifted, and the second frequency is shifted by the frequency shift. Light emitted from the one end face or the other end face of the optical fiber to be measured by making the oscillation light enter from the other end face of the optical fiber to be measured and changing the frequency shift amount of the center frequency of the second continuous wave light The Brillouin gain spectrum is measured at the position where the phase of frequency modulation of the first and second continuous wave lights is synchronized and the correlation value is increased in the measured optical fiber.

  Specifically, as shown in FIG. 1, the laser light emitted from the semiconductor laser 1 driven by the signal generation circuit 2 is branched into two by an optical branching unit 3. One of the branched laser beams is shifted in the center frequency of the laser beam by the light intensity modulator 5 driven by the microwave generator 4 in accordance with the frequency of the microwave. The frequency-shifted laser light is incident from one end of the measured optical fiber 6 as probe light L1. The probe light uses a sideband on the low frequency side generated by the light intensity modulator 5.

  The other branched laser beam is incident from the other end of the measured optical fiber 6 as pump light L2. If necessary, an optical delay device 7 is interposed to adjust the timing at which the laser light L2 enters the optical fiber 6 to be measured.

  The probe light emitted from the optical fiber 6 to be measured is led out to the photodetector side by an optical path converter 8 such as a circulator, and the frequency of the probe light L1 (corresponding to the above-described low-frequency sideband) by the optical wavelength filter 9. The light wave having the frequency of the light wave is separated and transmitted, and the light intensity of the light wave is detected by the photodetector 10.

  Patent Document 2 discloses the following optical fiber characteristic measuring device. A light source that emits laser light, an incident means that makes a part of the laser light emitted from the light source incident as a probe light of continuous light from one end of the optical fiber to be measured, and a part of the laser light emitted from the light source A pulse modulator that pulses the remainder of the optical fiber to be measured and enters as pump light from the other end of the optical fiber to be measured, and is set to the optical fiber to be measured among light emitted from the other end of the optical fiber to be measured An optical fiber comprising: a timing adjuster that allows only light from the vicinity of a measurement point to pass; and a detector that detects light that has passed through the timing adjuster and measures characteristics of the optical fiber to be measured. It is a characteristic measuring device.

  Specifically, as shown in FIG. 2, the laser light emitted from the semiconductor laser 1 driven by the signal generation circuit 2 is branched into two by an optical branching unit 3. One of the branched laser beams is shifted in the center frequency of the laser beam by the light intensity modulator 5 driven by the microwave generator 4 corresponding to the frequency of the microwave, as in FIG. The frequency-shifted laser light is incident from one end of the measured optical fiber 6 as probe light L1.

  The other branched laser beam is incident from the other end of the measured optical fiber 6 as pump light L2. The laser beam L2 is pulsed by the pulse modulator 11.

  The probe light emitted from the optical fiber to be measured 6 is led to the optical detector side by the optical path converter 8 and enters the timing adjuster 12 that allows only light from the vicinity of the measurement point set in the optical fiber to be measured 6 to pass. Is done. The light wave having passed through the timing adjuster 12 is selected by the optical wavelength filter 9 as the light wave having the frequency of the probe light L 1, and the light intensity of the light wave that has passed through the optical wavelength filter 9 is detected by the photodetector 10.

  However, in the measurement method of Patent Document 1, since the first and second continuous lights are periodically frequency-modulated, a correlation peak is exhibited at a periodic point of about 10 to 50 m. For this reason, even if Brillouin scattering is measured, these periodic points are measured simultaneously. In order to solve this problem, it is necessary to limit the measurement distance to the period of the correlation peak, that is, 10-50 m or less.

  In the measurement method of Patent Document 2, the above limitation can be removed by a pulse modulator and a timing adjuster. However, in this method, it is necessary to adjust the timing each time the measurement sample is changed, and the measurement work becomes complicated and automatic adjustment is difficult.

  The present applicant solves such a problem and can measure the Brillouin scattering of the optical fiber under measurement with a resolution of 1 m or less while being able to measure over a wide area. A Brillouin scattering measurement device that can be easily automated was proposed in a previous patent application (Patent Document 3).

  In an example of the Brillouin scattering measurement device described in Patent Document 3, a light source that emits laser light, a modulation unit that periodically changes the wavelength of the laser light, and a branch that branches the laser light emitted from the light source Part, a frequency converting means for converting the frequency of one branched laser light, and a light wave output from the frequency converting means is incident on one end of the optical fiber to be measured, and the other branched laser light is An optical fiber to be measured configured to be incident on the other end of the optical fiber for measurement; and provided between the branch portion and the other end of the optical fiber for measurement. In a Brillouin scattering measurement apparatus having a deriving unit for deriving a light wave emitted from an end other than the branching unit, and a photodetector for detecting the light wave derived from the deriving unit, continuous light is an integer of the period of the modulation unit Double cycle Pulse converting means for pulsing, characterized in that provided between the light source and the branch unit.

  Specifically, as shown in FIG. 3, a signal having a predetermined frequency f1 is input to the semiconductor laser 20 from a signal generator 21 that is a modulation unit that periodically changes the wavelength of the laser light. Emits a laser beam frequency-modulated at a predetermined frequency f1.

  The laser light is converted into a predetermined cycle T1 (usually, T1 = N / f1, N is a natural number by a pulse converter 22 which is a pulse conversion unit. )). The laser beam converted into a pulse shape is branched into two laser beams by an optical branching device 23 constituting a branching unit.

  One of the branched laser lights is modulated by the specific frequency f2 by the frequency converting means 24 for converting the frequency, and the center frequency of the laser light is shifted by a predetermined amount.

  The light wave output from the frequency converting means 24 enters one end of the optical fiber 25 to be measured as the probe light L1. The other branched laser beam is configured to enter the other end of the optical fiber 25 to be measured as pump light L2.

  By adjusting the predetermined period of the pulse converter 22, the number of standing wave nodes (locations where Brillouin scattering is measured) due to the pump light and probe light generated in the optical fiber to be measured is set to one. To do. Next, the node can be moved in the optical fiber to be measured by changing the driving frequency f1.

  In order to measure the probe light L1 emitted from the optical fiber for measurement 25, it is provided between the optical splitter 23 and the other end of the optical fiber for measurement 25, and is emitted from the other end of the optical fiber for measurement. Deriving means 26 for deriving the probe light L1 other than the optical splitter 23 is provided. As the derivation means, an optical path changer 26 such as a circulator can be used.

  The probe light L1 derived by the optical path changer 26 is input to the photodetector 27, and the light intensity of the probe light is measured. In addition, in order to measure the wavelength light which concerns on probe light correctly, it is also possible to arrange | position a wavelength filter in the front | former stage of the photodetector 27 as needed.

  In Patent Document 3, the probe light and the pump light are generated separately using a method in which different pulse converters are used in synchronization with the probe light and the pump light, or a plurality of light sources and a plurality of pulse converters. The method of making it also is disclosed.

  On the other hand, in any of the above-described techniques of Patent Documents 1 to 3, it is necessary to make the probe light and the pump light enter the opposite ends of the optical fiber to be measured so as to face each other. Therefore, in order to actually measure the Brillouin scattering, as shown in FIG. 4A, the measuring device 30 is arranged on one side of the optical fiber 25 to be measured arranged on the structure 31 or the like. It is indispensable to guide either the probe light or the pump light with another optical fiber on the other side of the optical fiber for measurement.

  Also, as shown in FIG. 4B, when the probe light and the pump light are generated by different light sources, when the measuring device (33, 34) is separated into two, both are connected by an electric cable or the like. It is essential to connect and control each measuring device (33, 34) with high accuracy.

  In this way, laying optical fibers other than the optical fiber to be measured or laying electrical cables lowers the measurement efficiency and complicates and complicates the work process and causes problems in measurement reliability. Will occur.

Japanese Patent No. 3667132 Japanese Patent No. 3607930 Japanese Patent Application No. 2007-193323 (filing date: July 25, 2007)

Kazuo Hotate and two others, “Exploration of temperature and strain separation measurement method for BOCDA system”, 21st Society of Instrument and Control Engineers Sensing Forum, Toyo University, 2B1-2, pp. 261-266, September 15, 2004

  The problem to be solved by the present invention is to provide a Brillouin scattering measuring apparatus that solves the above-described problems and can measure Brillouin scattering using only the optical fiber to be measured.

  In order to solve the above-mentioned problem, the invention according to claim 1 is characterized in that the measurement light generating means for generating the probe light and the pump light, and the probe light and the pump light generated by the measurement light generating means are Polarization combining means for combining with different wave fronts, optical fiber for measurement into which the combined light from the polarization combining means is incident on one end, and the other end of the optical fiber for measurement are combined and combined Rotating / reflecting means for rotating the polarization plane of the light and reflecting at least the probe light in the combined light and re-entering the other end of the optical fiber to be measured, and one end side of the optical fiber to be measured And a Brillouin scattering measuring apparatus comprising probe light detecting means for detecting probe light reflected by the rotary reflecting means.

  In particular, the invention according to claim 1 is characterized in that the rotary reflection means re-enters only the probe light to the other end of the optical fiber to be measured.

  The invention according to claim 2 is the Brillouin scattering measuring apparatus according to claim 1, wherein the optical fiber for measurement is a polarization maintaining fiber.

  The invention according to claim 3 is the Brillouin scattering measurement apparatus according to claim 1 or 2, characterized in that the rotation of the polarization plane of the combined light rotates the polarization plane by 90 °.

  The invention according to claim 4 is the Brillouin scattering measurement device according to any one of claims 1 to 3, wherein the Brillouin scattering measurement device measures strain of an object on which the optical fiber for measurement is arranged. It is characterized by being a distortion measuring apparatus.

  The invention according to claim 5 is the Brillouin scattering measurement device according to any one of claims 1 to 3, wherein the Brillouin scattering measurement device measures a temperature distribution of an object on which the optical fiber for measurement is arranged. It is the temperature distribution measuring device for this.

  According to the first aspect of the present invention, the measurement light generation means for generating the probe light and the pump light, and the probe light and the pump light generated by the measurement light generation means are combined with different polarization planes. Polarization combining means, an optical fiber to be measured on which one of the combined lights from the polarization combining means is incident, and the other end of the optical fiber to be measured are arranged to rotate the polarization plane of the combined light And a rotary reflection means for reflecting at least the probe light in the combined light and re-entering the other end of the optical fiber for measurement, and disposed on one end side of the optical fiber for measurement and reflected by the rotary reflection means A Brillouin scattering measuring device characterized by having a probe light detecting means for detecting the probe light that has been detected, so that no optical fiber other than the optical fiber for measurement is required, and the measuring device is connected to the optical fiber for measurement. one It is possible to measure a state arranged on the side.

  In particular, since the rotary reflecting means re-enters only the probe light to the other end of the optical fiber to be measured, Brillouin scattering can be measured only during the return path of the probe light. The measurement distance is short and the measurement time can be shortened.

  According to the invention of claim 2, since the optical fiber to be measured is a polarization maintaining fiber, when the probe light and the pump light travel in the same direction, both interfere with each other, and the Brillouin scattering is performed with higher accuracy. Can be measured.

  According to the third aspect of the present invention, since the polarization plane of the synthesized light is rotated by 90 °, when the probe light or the pump light is turned back at the other end of the optical fiber to be measured, each light has a different polarization. When converted to a wavefront and travels in one direction through the optical fiber to be measured, each light has a different polarization plane when it travels in the opposite direction. This makes it possible to obtain substantially the same effect as when a folded optical fiber is laid with only one optical fiber to be measured.

  According to the invention according to claim 4, since the Brillouin scattering measuring device is a strain measuring device for measuring the strain of the object on which the optical fiber to be measured is arranged, the optical fiber arranged inside and outside the object is Only the optical fiber to be measured arranged in the object is sufficient, and the strain measurement can be performed with high measurement efficiency and high measurement reliability.

  According to the invention of claim 5, since the Brillouin scattering measuring device is a temperature distribution measuring device for measuring the temperature distribution of the object on which the optical fiber to be measured is arranged, the optical fiber arranged inside and outside the object. Only the optical fiber to be measured arranged in the object is sufficient, and the temperature measurement can be performed with high efficiency of measurement work and high reliability of measurement.

It is the schematic of the conventional Brillouin scattering measuring apparatus. It is the schematic of the other conventional Brillouin scattering measuring apparatus. It is the schematic which shows one Example of the Brillouin scattering measuring apparatus disclosed by patent document 3. FIG. It is the schematic explaining the problem of the conventional Brillouin scattering measuring apparatus. It is the schematic which shows the Example of the Brillouin scattering measuring apparatus which concerns on this invention. It is a figure which shows the structure of the rotation reflection means which reflects probe light and pump light. It is a figure which shows the structure of the rotation reflection means to reflect only probe light.

Hereinafter, the present invention will be described in detail using preferred examples.
FIG. 5 is a diagram showing an embodiment of the Brillouin scattering measuring apparatus of the present invention.
The Brillouin scattering measurement apparatus of the present invention includes a measurement light generation unit that generates probe light (a _{0 to} a ₃ ) and pump light (b _{0 to} b ₃ ), and a probe generated by the measurement light generation unit. Polarization combining means 40 for combining the light and the pump light with different polarization planes, an optical fiber 41 to be measured on which the combined light from the polarization combining means is incident on one end, and the light to be measured A rotary reflection means 42 disposed on the other end of the fiber, rotating the polarization plane of the combined light, reflecting at least the probe light in the combined light and re-entering the other end of the optical fiber to be measured; And a probe light detection unit that is disposed on one end side of the optical fiber to be measured and detects the probe light (a ₃ ) reflected by the rotary reflection unit.

As the measurement light generating means for generating the probe light and the pump light according to the present invention, various techniques shown in FIGS. 1 to 3 can be adopted, and an optical system for generating the probe light L1 and the pump light L2 is used. It can be used as it is.
Further, the probe light a ₀ and the pump light b ₀ are preferably adjusted in advance so that their polarization planes are orthogonal to the fiber 41 to be measured. For this purpose, the light waves L1, L1 generated in FIGS. A method of adjusting the polarization plane of L2 through a polarizer or the like, or between the semiconductor laser 1 (20) and the optical splitter 3 (23) or from the optical splitter 3 (23) to the optical fiber 6 to be measured ( A polarizer, a Faraday rotator, and the like are arranged until 25), and the probe light L1 (a ₀ ) and the pump light L2 (b ₀ ) that are finally generated are adjusted so that their planes of polarization are orthogonal to each other. It is also possible.

The probe light a ₀ and the pump light b ₀ are combined by the polarization beam combiner 40 while maintaining the polarization planes. The light wave emitted from the polarization beam combiner 40 includes probe light and pump light. However, since the polarization planes are orthogonal to each other, they do not interfere with each other and propagate in the optical fiber 41 to be measured in the same direction. Go.
By configuring the optical fiber for measurement 41 with a polarization maintaining fiber (PMF), the probe light a ₁ and the pump light b ₁ propagate while maintaining the plane of polarization with each other. For example, as shown in FIG. 5, the interference between the probe light a ₁ and the pump light (reflected light) b ₂ and the interference between the probe light (reflected light) a ₂ and the pump light b ₁ Only Brillouin scattering can be measured with higher accuracy.

Next, the rotational reflection means 42 on the end face of the optical fiber 41 to be measured will be described.
The rotation reflecting means 42 has a function of rotating the polarization plane of the combined light and reflecting at least the probe light in the combined light so as to re-enter the other end of the optical fiber for measurement. Specifically, in adjusting the reflection means 42, the probe light a ₁ but is always reflected pump light b ₁ becomes to be reflected as required.

  Further, when the polarization plane is rotated by 90 ° in the rotation of the polarization plane of the combined light of the rotary reflection means 42, each light is different when the probe light or the pump light is folded at the other end of the optical fiber for measurement. When the light is converted into a polarization plane and travels in one direction through the optical fiber to be measured, each light has a different polarization plane when returning to the opposite direction. This makes it possible to obtain substantially the same effect as when a folded optical fiber is laid with only one optical fiber to be measured.

  Brillouin scattering can be measured when the probe light reciprocates in the optical fiber to be measured by causing both the probe light and the pump light to reenter the other end of the optical fiber for measurement 41 in the rotary reflection means 42. Thus, the same part of the optical fiber to be measured can be measured both when the probe light travels and when it returns, and more accurate measurement is possible. In addition, by using a polarization maintaining fiber, it is possible to compare the Brillouin scattering state between the forward path and the return path of the probe light and specify whether the Brillouin scattering due to temperature change or the Brillouin scattering due to strain load.

  In particular, when a polarization maintaining fiber is used, the peak frequency of the Brillouin gain spectrum is different for each polarization maintaining surface (slow axis, fast axis) as disclosed in Non-Patent Document 1, In addition, the temperature and strain dependence of the Brillouin shift with respect to each polarization plane is also different, so at the same location of the optical fiber to be measured, the probe light's forward and backward travel times are measured and calculated from the result values. It is possible to separate and judge change and distortion.

In order to reflect both the probe light and the pump light, as shown in FIG. 6, the polarization plane of the probe light a ₁ or the pump light b ₁ is rotated by 45 ° by a Faraday rotator 43 in which a Faraday rotator and a magnet are combined. Then, the rotated probe light a ₄ and pump light b ₄ are reflected by the next total reflection mirror 44. Next, the probe light a ₅ and the pump light b ₅ reflected by the reflection mirror 44 are rotated again by 45 degrees in the polarization plane by the Faraday rotator. The reflected light is reflected (a ₂ or b ₂ ).

Next, when only the probe light is reincident on the other end of the optical fiber to be measured by the rotary reflecting means 42, the Brillouin scattering can be measured only when the probe light (a ₂ ) is returned, and also during the forward pass. Compared to the case of measurement on the return path, the measurement distance is short and the measurement time can be shortened.

To reflect only the probe light a _1, as shown in FIG. 7, 45 ° rotates the polarization plane by the Faraday rotator 43 probe light a ₁ or the pump light b _1. Then, the rotated probe light a ₄ and pump light b ₄ are incident on a 45 ° polarizer 45, and at this time, only the probe light a _{4 is} allowed to pass, and the pump light b ₄ is shielded. 45 is arranged. Then, only the probe light a ₄ is reflected by the total reflection mirror 44. Probe light a ₅ reflected by the reflection mirror 44, again passes through the polarizer 45, is rotated plane of polarization 45 ° by the Faraday rotator is rotated change in polarization and 90 ° different planes of polarization of the time of the incident end In this state, the light is reflected (a ₂ ).

  The rotation reflection means 42 is not limited to the configuration shown in FIG. 6 or 7, and any rotation reflection means 42 may be used as long as the function of the rotation reflection means 42 necessary for the present invention is realized.

Next, the reflected probe light a ₂ propagates in the optical fiber for measurement 41 in the reverse direction, and further propagates in the polarization synthesizer 40 in the reverse direction to obtain the probe light a _3. It is emitted from. In order to detect such probe light, probe light detection means such as those in various Brillouin scattering measurement devices disclosed in FIGS. 1 to 3 can be used. Specifically, as shown in FIGS. 1 to 3, the probe light a ₃ is separated from the optical path of the pump light b ₀ via the optical splitter 8 (26), and the separated probe light is detected by the photodetector 10 (27). ) To detect. Moreover, an optical wavelength filter, a timing adjuster, etc. are arrange | positioned as needed.

  The Brillouin scattering measuring device of the present invention described above is a strain measuring device for measuring strain of an object in which an optical fiber for measurement is arranged, or a temperature distribution of an object in which an optical fiber for measurement is arranged. It can be used as a temperature distribution measuring device for measurement. Moreover, in such an apparatus, the optical fiber to be arranged inside and outside the object need only be the optical fiber to be measured arranged in the object, and the measurement operation is highly efficient and the measurement reliability is high. Measurement is possible.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the Brillouin scattering measuring apparatus which can measure a Brillouin scattering only using the to-be-measured optical fiber.

DESCRIPTION OF SYMBOLS 1,20 Semiconductor laser 2,21 Signal generator 3,23 Optical branching device 4 Microwave generator 5 Optical intensity modulator 6,25 Optical fiber 7 to be measured 7 Optical delay device 8, 26 Optical path changer (derivation means)
DESCRIPTION OF SYMBOLS 9 Optical wavelength filter 10, 27 Photo detector 11 Pulse modulator 12 Timing adjuster 22 Pulse converter 24 Frequency converter 30, 33, 34 Measuring means 31 Object 32 Optical fiber 35 Electric cable 40 Polarization synthesizer 41 Polarization Holding fiber 42 Rotating reflection means 43 Faraday rotator 44 Reflecting mirror 45 Polarizer

Claims (5)

Measurement light generating means for generating probe light and pump light;
Polarization combining means for combining the probe light and the pump light generated by the measurement light generating means with different polarization planes;
An optical fiber for measurement in which the combined light from the polarization beam combining means is incident on one end;
Rotational reflection that is arranged on the other end of the optical fiber to be measured, rotates the polarization plane of the combined light, reflects only the probe light in the combined light, and re-enters the other end of the optical fiber to be measured Means,
A Brillouin scattering measuring apparatus, comprising probe light detecting means arranged on one end side of the optical fiber for measurement and detecting probe light reflected by the rotary reflecting means.   2. The Brillouin scattering measurement apparatus according to claim 1, wherein the optical fiber for measurement is a polarization maintaining fiber.   3. The Brillouin scattering measurement apparatus according to claim 1, wherein the rotation of the polarization plane of the combined light rotates the polarization plane by 90 °. 4.   4. The Brillouin scattering measurement device according to claim 1, wherein the Brillouin scattering measurement device is a strain measurement device for measuring strain of an object on which the optical fiber to be measured is arranged. 5. Brillouin scattering measurement device.   4. The Brillouin scattering measuring device according to claim 1, wherein the Brillouin scattering measuring device is a temperature distribution measuring device for measuring a temperature distribution of an object on which the optical fiber for measurement is arranged. A Brillouin scattering measuring device characterized by the above.

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