JP6686423B2 - Optical fiber characteristic measuring device and optical fiber characteristic measuring method - Google Patents

Description

  The present invention relates to an optical fiber characteristic measuring device and an optical fiber characteristic measuring method, and in particular, an optical fiber characteristic for measuring the characteristic of an optical fiber based on backscattered light generated by Brillouin scattering in an optical fiber to be measured. The present invention relates to a measuring device and an optical fiber characteristic measuring method.

  Brillouin scattering that occurs when light enters an optical fiber, which is one of optical transmission media, changes depending on the strain applied to the optical fiber and the temperature of the optical fiber. A method is known in which the strain distribution and temperature distribution in the length direction of the optical fiber are measured by measuring the frequency shift amount of light caused by the Brillouin scattering. For example, the optical fiber is stretched around a structure such as a bridge or a building, and the strained portion of the optical fiber is specified based on the method described above, whereby the strain generated in these structures can be detected. As such a measuring method, a so-called BOTDR (Brillouin Optical Time Domain Reflectometry) method and a BOCDR (Brillouin Optical Correlation Domain Reflectometry) method are known.

  The BOTDR method is a method of detecting Brillouin scattered light obtained by injecting an optical pulse from one end of an optical fiber to be measured, and measuring a frequency shift amount of the Brillouin scattered light with respect to the incident light (hereinafter referred to as Brillouin frequency shift amount ) And the time until Brillouin scattered light returns. This Brillouin scattered light is backscattered light scattered by an acoustic wave whose velocity changes depending on the strain and temperature of the optical fiber to be measured. By measuring the Brillouin frequency shift amount, it is possible to measure the magnitude and temperature of the strain of the optical fiber under measurement, and by measuring the time until the Brillouin scattered light returns, the measured light The position in the length direction of the fiber can be specified.

  On the other hand, the BOCDR measurement method measures the Brillouin frequency shift amount by detecting the Brillouin scattered light obtained by injecting the pump light that is the frequency-modulated continuous light from one end of the optical fiber to be measured. As described in Patent Document 1, in this BOCDR method, the Brillouin scattered light and the reference light are caused to interfere with each other, so that a specific position called a “correlation peak” in the measured optical fiber is obtained. Brillouin scattered light is selectively extracted. For example, when continuous light with sine wave frequency modulation is incident on the optical fiber to be measured, the interval of the correlation peak in the optical fiber to be measured is inversely proportional to the modulation frequency of the sine wave frequency modulation. Furthermore, by sweeping the modulation frequency of continuous light, the correlation peak can be moved along the length direction of the optical fiber under measurement. By obtaining the Brillouin frequency shift amount at each correlation peak point while moving the correlation peak, the strain distribution and temperature distribution in the length direction of the optical fiber to be measured can be measured.

  The correlation peak used in the BOCDR measurement method means a position where the correlation between the Brillouin scattered light generated by the pump light incident on the optical fiber to be measured and the reference light branched from the light source is high. That is, the correlation peak means a position where the frequency difference between the Brillouin scattered light and the reference light does not fluctuate with time. Multiple correlation peaks may occur on the measured optical fiber. Among the plurality of correlation peaks, the position where the optical path length of the pump light incident on the measured optical fiber and the optical path length of the reference light are equal to each other is referred to as a “zero-order correlation peak”. Further, the n-th (n is a positive integer) correlation peak generated next to the 0th-order correlation peak is referred to as an "nth-order correlation peak". The position of the nth-order correlation peak changes by sweeping the modulation frequency of the pump light incident on the optical fiber to be measured, but the position of the 0th-order correlation peak is basically fixed regardless of the modulation frequency. .

  Generally, as a method for measuring the strain distribution or temperature distribution in the length direction of the optical fiber to be measured, the above-mentioned method of using the sweep of the nth-order correlation peak or the position of the 0th-order correlation peak is used with a phase shifter. There is a method to change it. When using the sweep of the nth-order correlation peak, it is necessary to set the position of the 0th-order correlation peak at a position distant from the measured optical fiber so that the nth-order correlation peak is located on the measured optical fiber. In the prior art, as described in Patent Document 1 (FIG. 1), by providing an optical fiber called a delay fiber in the optical path of the reference light, the position of the 0th-order correlation peak is separated from the measured optical fiber. Adjusting to position.

  By providing the delay fiber in the optical path of the reference light, the optical path length of the reference light becomes long, so that the position of the 0th-order correlation peak on the optical path of the pump light incident on the measured optical fiber is located far away from the light source. Moving. In this way, if the 0th-order correlation peak is located virtually at a distance where the measured optical fiber does not physically exist, the nth-order correlation peak is used for the strain distribution and temperature distribution of the measured optical fiber. Can be measured. When pump light with sine wave frequency modulation is incident on the measured optical fiber, the modulation frequency is reduced to move the n-th order correlation peak to the end of the measured optical fiber closer to the light source, and By increasing it, the n-th order correlation peak can be moved to the end far from the light source of the optical fiber to be measured.

  The BOCDR method described above selectively outputs Brillouin scattered light in a narrow region of about several cm in the measured optical fiber as an interference output corresponding to a specific position in the length direction of the measured optical fiber. can do. Further, since not the light pulse but the continuous light is made incident on the optical fiber to be measured, the signal intensity of the backscattered light generated in the optical fiber to be measured is high, and the integration processing of the measured value is unnecessary, so that the measurement time is shortened. be able to. The spatial resolution and measurement time in this BOCDR measurement method are better than the spatial resolution (usually 1 m or more) and measurement time (several minutes to several tens of minutes) in the BOTDR measurement method in which an optical pulse is incident on the optical fiber to be measured. Are better.

  However, in order to arrange the 0th-order correlation peak at a distance where the optical fiber to be measured does not physically exist, that is, to arrange the 0th-order correlation peak farther from the end portion far from the light source of the measured optical fiber, It is necessary to provide a delay fiber longer than the measurement optical fiber in the optical path of the reference light. Therefore, it may be necessary to grasp the length of the optical fiber to be measured in advance and prepare a delay fiber corresponding to the length. Further, when the measured optical fiber is stretched around a large structure such as a bridge or a building, a very long delay fiber may be provided in the optical path of the reference light, which may increase the cost.

Japanese Patent No. 5105302

Fujinosuke Maruyama, "Brillouin Optical Correlation Domain Reflectometry Using Temporal Gate Method and Apodized Method", 55th Japan Society of Applied Physics, Lightwave Sensing Research Group, June 2015

    One aspect of the present invention is to perform optical fiber characteristic measurement using Brillouin scattered light even when the length of the measured optical fiber is unknown, or even when a delay fiber shorter than the measured optical fiber is used. Provided is an optical fiber characteristic measuring device and an optical fiber characteristic measuring method which enable the measurement.

An optical fiber characteristic measuring device according to an aspect of the present invention includes a light source unit that outputs frequency-modulated continuous light, a first optical branching unit that splits the continuous light into pump light and reference light, and 1 The pump light emitted from the optical branching portion is made incident from one end of the measured optical fiber, and the backscattered light generated by the Brillouin scattering of the pump light in the measured optical fiber is received from the measured optical fiber. A second light branching unit, a calculation unit that measures the characteristics of the optical fiber under measurement by causing the backscattered light and the reference light to interfere with each other may be provided. The length of a first optical path where the pump light emitted from the first optical branching portion enters the optical fiber under measurement and reaches the calculation unit as the backscattered light, and the reference light is transmitted to the calculation unit. It may be longer than the length of the second optical path to reach.
The optical fiber characteristic measuring device according to the above aspect may further include an optical delay unit that causes the pump light to have a predetermined delay on the first optical path.
In the optical fiber characteristic measuring device according to the above aspect, the optical delay unit may include an optical fiber for increasing the length of the first optical path.
In the optical fiber characteristic measuring device according to the above aspect, the optical delay unit may be arranged between the first optical branching unit and the second optical branching unit.
In the optical fiber characteristic measuring device according to the above aspect, the calculation unit is input from the combining unit that combines the backscattered light and the reference light to generate combined light, and is input from the combining unit. By causing the backscattered light included in the combined light and the reference light to interfere with each other, the backscattered light and a detection unit that detects an interference signal indicating a frequency difference between the reference light and the detection unit. Based on the input interference signal, an acquisition unit that acquires a spectrum of Brillouin scattering, from the spectrum of the Brillouin scattering input from the acquisition unit, an arithmetic control unit that calculates a Brillouin frequency shift amount, comprising: Good.
The optical fiber characteristic measuring device according to the above aspect is disposed between the first optical branching unit and the second optical branching unit, and uses the pump light input from the first optical branching unit as the second optical branching unit. A gate unit that passes or blocks the light branch unit may be further provided. The arithmetic control unit outputs a first control signal for controlling passage or blocking of the pump light emitted from the first optical branching unit to the second optical branching unit to the gate unit, and the Brillouin scattering A second control signal for controlling start and stop of acquisition of spectrum may be output to the acquisition unit.
In the optical fiber characteristic measuring apparatus according to the above aspect, the arithmetic control unit may include the data for one cycle of the frequency-modulated continuous light, the spectrum of the Brillouin scattering being output from the light source unit, The second control signal for controlling start and stop of acquisition of Brillouin scattering spectrum may be output to the acquisition unit.
The optical fiber characteristic measuring method according to an aspect of the present invention outputs frequency-modulated continuous light, branches the continuous light into pump light and reference light, and with respect to the reference light, By delaying the pump light, the pump light is made incident from one end of the optical fiber to be measured, and backscattered light generated by Brillouin scattering of the pump light in the optical fiber to be measured is received from the optical fiber to be measured. And measuring the characteristic of the measured optical fiber by causing the backscattered light and the reference light to interfere with each other.
In the optical fiber characteristic measuring method according to the above aspect, measuring the characteristic of the measured optical fiber is performed by causing the back scattered light and the reference light to interfere with each other, the back scattered light, and the reference light. Detecting an interference signal indicating a frequency difference between, and, based on the interference signal, obtaining a spectrum of Brillouin scattering, from the spectrum of the Brillouin scattering, to calculate a Brillouin frequency shift amount, You may
The optical fiber characteristic measuring method according to the above aspect is such that when a plurality of correlation peaks in which the frequency difference between the backscattered light and the reference light does not fluctuate temporally exist in the measured optical fiber, the light of the pump light The pump light may be pulsed by controlling opening / closing of a gate portion provided on the road, and may further include starting and stopping spectrum acquisition of the backscattered light received from the optical fiber under measurement. . Depending on the distance from one end of the optical fiber to be measured to a correlation peak (hereinafter, referred to as a first correlation peak) that is a measurement target among the plurality of correlation peaks, the time when the gate unit is opened and the Brillouin A time difference between the time when the acquisition of the scattering spectrum is started and a time difference between the time when the gate unit is closed and the time when the acquisition of the Brillouin scattering spectrum is stopped may be controlled.

  The optical fiber characteristic measuring device and the optical fiber characteristic measuring method according to one aspect of the present invention can be used even when the length of the measured optical fiber is unknown or when a delay fiber shorter than the measured optical fiber is used. , Optical fiber characteristic measurement using Brillouin scattered light can be performed.

It is a block diagram showing an example of an optical fiber characteristic measuring device in a 1st embodiment. It is a flow chart which shows an example of a flow of processing of an optical fiber characteristic measuring device in a 1st embodiment. It is a block diagram which shows an example of the optical fiber characteristic measuring device in 2nd Embodiment. It is a flow chart which shows an example of a flow of processing of an optical fiber characteristic measuring device in a 2nd embodiment.

  Hereinafter, some embodiments of an optical fiber characteristic measuring device according to the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram showing an example of an optical fiber characteristic measuring device according to the first embodiment of the present invention. As shown in FIG. 1, the optical fiber characteristic measuring device 1 of the first embodiment is, for example, a light source unit 10, a first optical branching unit 20, an optical delaying unit 30, an optical amplifying unit 35, and a second optical unit. The branch unit 40, the optical connector 50, and the calculation unit 60 are provided. The calculation unit 60 includes, for example, a multiplexing unit 70, a detection unit 80, an acquisition unit 90, a calculation control unit 100, and a modulation unit 110.

  The light source unit 10 outputs frequency-modulated continuous light. The light source unit 10 outputs continuous light whose frequency is sinusoidally frequency-modulated, for example. The light source unit 10 includes, for example, a semiconductor laser such as a distributed feedback laser diode. In order to modulate the light output from the light source unit 10, for example, a modulation signal input from the modulator 110 is used.

  The first light splitting unit 20 splits the continuous light input from the light source unit 10 into two lights having an appropriate intensity ratio (for example, 1: 1). One of the two lights is the pump light that is incident on the measured optical fiber FUT. The pump light passes through the optical delay unit 30, the optical amplification unit 35, the second optical branching unit 40, and the optical connector 50, and is incident on one end of the measured optical fiber FUT. The other of the two lights is the reference light when optical heterodyne detection is performed. The reference light passes through the second light guide section 4 and is incident on the combining section 70.

  The optical delay unit 30 causes the pump light incident from the first optical branching unit 20 to have a predetermined delay. The optical delay unit 30 includes, for example, an optical fiber having a predetermined length. The delay time can be adjusted by changing the length of the optical fiber. The optical fiber has a length of, for example, several tens of meters or more. Alternatively, the optical delay unit 30 may include a phase shifter that performs phase adjustment so that the pump light incident from the first optical branching unit 20 causes a predetermined delay. Since the optical delay unit 30 generates a predetermined delay time between the pump light branched by the first optical branching unit 20 and the reference light, the return light from the measured optical fiber FUT (backscattering of Brillouin scattering) is generated. A predetermined delay time is also generated between the light or the Stokes light) and the reference light. This delay time can be arbitrarily set in advance. In the first embodiment, by providing the optical delay unit 30, the pump light emitted from the first optical branching unit 20 is incident on the measured optical fiber FUT, and as the backscattered light from the measured optical fiber FUT. The optical path of the pump light incident on the multiplexing unit 70 via the second optical branching unit 40 (the first optical branching unit 20, the optical delay unit 30, the optical amplification unit 35, the second optical branching unit 40, the optical connector 50, An optical path that sequentially passes through the measurement optical fiber FUT, the optical connector 50, the second optical branching section 40, the first light guide section 2, and the multiplexing section 70. The length of the "first optical path 6" will be described below. The reference light split by the first light splitting unit 20 passes through the second light guiding unit 4, and enters the multiplexing unit 70 more than the optical path of the reference light (hereinafter, referred to as “second optical path 8”). It is set long. The optical delay unit 30 increases the length of the first optical path 6.

  The optical amplification unit 35 amplifies the pump light input from the optical delay unit 30. The optical amplification unit 35 may be provided between the first optical branching unit 20 and the optical delay unit 30. If the amplification of the pump light is unnecessary, the optical amplification section 35 may not be provided. A modulator (not shown) that shifts the optical frequency of the pump light by a predetermined amount may be provided before and after the optical delay unit 30. As the modulator, for example, an SSB (Single Side-Band Modulation) modulator or the like may be used. Further, an optical amplifier or the like (not shown) for amplifying the reference light may be provided on the optical path of the second light guide section 4. Further, a modulator (not shown) that shifts the optical frequency of the reference light by a predetermined amount may be provided on the optical path of the second light guide unit 4.

  The second optical branching unit 40 includes a first port, a second port, and a third port. The first port is connected to the optical amplification section 35. The second port is connected to the measured optical fiber FUT via the optical connector 50. The third port is connected to the multiplexing unit 70. Based on this configuration, the second optical branching unit 40 outputs the pump light input from the first port to the second port. Also, the backscattered light from the optical fiber under test FUT input from the second port is output to the third port. The second optical branch section 40 includes, for example, an optical circulator and the like.

  The optical connector 50 is a connector that connects a path extending from the second port of the second optical branching unit 40 and the measured optical fiber FUT. The pump light emitted from the second port of the second optical branching unit 40 is input to the measured optical fiber FUT via the optical connector 50. The backscattered light from the measured optical fiber FUT is input to the second port of the second optical branching unit 40 via the optical connector 50.

  The multiplexing unit 70 outputs the backscattered light from the measurement optical fiber F that is output from the third port of the second optical branching unit 40, passes through the first light guide unit 2 and is input to the multiplexing unit 70, and The reference light output from the first light branching unit 20 and passing through the second light guide unit 4 and input to the combining unit 70 is combined to generate combined light. Further, the combining unit 70 branches the combined light into two lights having an appropriate intensity ratio (for example, 1: 1), and outputs the two lights to the detection unit 80. For example, each of the two branched lights includes 50% of the backscattered light from the measurement optical fiber F and 50% of the reference light.

  An optical amplifier (not shown) for amplifying the backscattered light from the measurement optical fiber F may be provided on the optical path of the first light guide unit 2.

  The detection unit 80 performs optical heterodyne detection by causing the backscattered light included in the combined light input from the combining unit 70 and the reference light to interfere with each other. The detection unit 80 is, for example, a balanced photodiode including two photodiodes (PD: Photodiodes) 82 and 84 and a detection unit 86. The detection unit 80 receives the light output by the multiplexing unit 70 and outputs a signal indicating the frequency difference between the backscattered light and the reference light to the acquisition unit 90 as an interference signal (beat signal). When the pump light or the reference light is frequency-shifted in advance, the detection unit 80 may perform optical homodyne detection.

  The acquisition unit 90 observes the frequency characteristic of the electrical beat signal input from the detection unit 80. The acquisition unit 90 may include, for example, a spectrum analyzer. Alternatively, the acquisition unit 90 may acquire temporally continuous data with a time axis measuring device such as an oscilloscope, and then separately convert the data into spectral data by using a technique such as fast Fourier transform.

  The calculation control unit 100 calculates the Brillouin frequency shift amount from the spectrum data measured by the acquisition unit 90, and outputs a control signal for controlling the measurement position on the optical fiber FUT to be measured to the modulation unit 110. The arithmetic control unit 100 may include, for example, an arithmetic device such as a personal computer. Further, the arithmetic control unit 100 may include a display unit that displays the Brillouin frequency shift amount obtained by the arithmetic operation as physical information such as strain and temperature. Further, information such as strain and temperature of the optical fiber FUT to be measured may be interpreted as information indicating the state of the object that is the measurement target and displayed on the display unit. The display unit is, for example, a liquid crystal display or an organic EL (Electroluminescence) display device.

  The modulation unit 110 outputs a modulation signal for modulating the light emitted from the light source unit 10 to the light source unit 10, based on the control signal input from the arithmetic control unit 100. The modulator 110 includes, for example, a signal generator such as an arbitrary waveform generator or a function generator. The modulation unit 110 injects into the light source unit 10 a modulation signal that converts the laser continuous light output from the light source unit 10 into continuous light whose frequency is, for example, sinusoidally frequency modulated. As the modulation signal, for example, a signal obtained by superimposing an alternating current on a direct current is used.

  Next, the operation of the optical fiber characteristic measuring device 1 of the first embodiment will be described. FIG. 2 is a flowchart showing an example of the flow of processing of the optical fiber characteristic measuring device 1 in the first embodiment.

  The light source unit 10 outputs the frequency-modulated continuous light to the first optical branching unit 20 (step S101). The light source unit 10 outputs continuous light whose frequency is sinusoidally modulated to the first optical branching unit 20, for example.

  Next, the first light branching unit 20 splits the continuous light input from the light source unit 10 into pump light and reference light (step S103). The first optical branching unit 20 outputs the pump light to the optical delay unit 30 (step S105).

  Next, the optical delay unit 30 outputs the pump light input from the first optical branching unit 20 to the optical amplification unit 35. After performing the amplification process of the pump light, the optical amplification unit 35 outputs the pump light to the optical fiber FUT to be measured via the second optical branching unit 40 and the optical connector 50 (step S107). When the frequency-modulated pump light is incident on the measured optical fiber FUT, Brillouin scattering occurs in the measured optical fiber FUT. Here, the backscattered light generated by the Brillouin scattering is affected by an acoustic wave whose velocity changes depending on the strain and temperature of the optical fiber FUT to be measured, and the frequency thereof is shifted. When the wavelength of the light source unit 10 is about 1.55 μm and a general-purpose communication single-mode fiber is used as the measured optical fiber FUT, the backscattered light from the measured optical fiber FUT is incident on the measured optical fiber FUT. The frequency is shifted by about 10.8 GHz with respect to the frequency of the generated continuous light. The Brillouin frequency shift amount varies depending on the strain and temperature applied to the optical fiber FUT to be measured.

  Next, the second optical branching unit 40 receives the backscattered light from the measured optical fiber FUT and outputs it to the first light guide unit 2 (step S109). Next, the first light guide section 2 outputs the backscattered light input from the second light branch section 40 to the multiplexing section 70.

  In parallel with the above steps S105, S107, and S109, or before or after steps S105, S107, and S109, the first optical branching unit 20 converts the reference light split by the first optical branching unit 20, It outputs to the 2nd light guide part 4 (step S111). Next, the second light guide section 4 outputs the reference light to the multiplexing section 70.

  Next, the multiplexing unit 70 multiplexes the backscattered light input from the first light guide unit 2 and the reference light input from the second light guide unit 4. Next, the combining unit 70 splits the combined light into two lights having an appropriate intensity ratio (for example, 1: 1), and outputs the two lights to the detection unit 80. The detection unit 80 performs optical heterodyne detection by interfering the light input from the multiplexing unit 70. The detection unit 80 uses, for example, two PDs 82 and 84 and the detection unit 86 to detect an interference signal (beat signal) indicating the frequency difference between the return light and the reference light input from the multiplexing unit 70. . The detection unit 80 performs appropriate level adjustment on the detected interference signal, that is, the Brillouin scattering signal, and then outputs the interference signal to the acquisition unit 90 (step S113).

  Next, the acquisition unit 90 measures the interference signal input from the detection unit 80 and measures the spectrum of Brillouin scattering (step S115). Next, the acquisition unit 90 outputs the measured Brillouin scattering spectrum to the calculation control unit 100 as spectrum data.

  Here, in the first embodiment, since the optical delay unit 30 is provided in the optical path (first optical path 6) of the pump light branched by the first optical branching unit 20, the length of the first optical path 6 is reduced. , Longer than the second optical path 8 which is the optical path of the reference light. Therefore, the position of the zero-order correlation peak, which is the position where the optical path length of the pump light and the optical path length of the reference light are equal, is between the first optical branching section 20 and the measured optical fiber FUT, that is, the first optical branch. It is adjusted to the position of the front stage of the optical fiber FUT to be measured in the optical path 6. For example, the position of the 0th-order correlation peak occurs on the optical path in the optical delay unit 30. As a result, not the 0th-order correlation peak but the nth-order correlation peak (n is a positive integer) used for the characteristic measurement of the measured optical fiber FUT is generated on the measured optical fiber FUT.

  Next, the arithmetic control unit 100 calculates the Brillouin frequency shift amount from the spectrum data input from the acquisition unit 90 to calculate the characteristics of the measured optical fiber FUT (distortion distribution in the length direction of the measured optical fiber FUT, The temperature distribution, etc.) is measured (step S117).

  Next, the arithmetic control unit 100 outputs a control signal for controlling the measurement position on the measured optical fiber FUT to the modulation unit 110. For example, when the continuous light to which the sine wave frequency modulation is applied is incident on the measured optical fiber FUT, the modulation frequency is reduced so that the nth-order correlation peak is located on the side far from the light source unit 10 of the measured optical fiber FUT. It can be moved. Further, by increasing the modulation frequency, the n-th order correlation peak can be moved to the side of the measured optical fiber FUT closer to the light source unit 10. The relationship between the moving direction of the n-th order correlation peak and the modulation frequency in the first embodiment is the conventional technique in which a delay unit is provided on the optical path of the reference light, that is, the 0th-order correlation peak is far from the optical fiber under measurement when viewed from the light source. The relationship between the moving direction of the n-th order correlation peak and the modulation frequency in the positioned configuration is opposite. Furthermore, by sweeping the modulation frequency of continuous light, the correlation peak can be moved along the length direction of the optical fiber under measurement. By obtaining the Brillouin frequency shift amount at each correlation peak point while moving the correlation peak, the strain distribution and temperature distribution in the length direction of the optical fiber to be measured can be measured.

  Next, the modulation unit 110 outputs a modulation signal for modulating the light emitted from the light source unit 10 to the light source unit 10 based on the control signal input from the calculation control unit 100, and the processing of this flowchart ends. To do.

  The optical fiber characteristic measuring device 1 according to the first embodiment described above includes a light source unit 10 that outputs frequency-modulated continuous light, and a first optical branching unit that branches the continuous light into pump light and reference light. 20 and the pump light emitted from the first optical branching unit 20 is incident from one end of the measured optical fiber FUT, and the backscattered light generated by the Brillouin scattering of the pump light in the measured optical fiber FUT is generated. A second optical branching unit 40 that receives from the measured optical fiber FUT, an arithmetic unit 60 that measures the characteristics of the measured optical fiber FUT by causing the backscattered light and the reference light to interfere with each other. The length of the first optical path, in which the pump light emitted from the first optical branching unit 20 is incident on the measured optical fiber FUT and reaches the calculating unit 60 as the backscattered light, is the reference value. Longer than the length of the second optical path where the light reaches to the arithmetic unit 60. That is, since the optical delay unit 30 is provided in the optical path (first optical path 6) of the pump light branched by the first optical branching unit 20, the length of the first optical path 6 is the optical path of the reference light. 2 is longer than the optical path 8. Therefore, the position of the zero-order correlation peak, which is the position where the optical path length of the pump light and the optical path length of the reference light are equal, is between the first optical branching section 20 and the measured optical fiber FUT, that is, the first optical branch. It is adjusted to the position of the front stage of the optical fiber FUT to be measured in the optical path 6. Since it is not necessary to change the length of the optical delay section (for example, the optical fiber) according to the length of the measured optical fiber FUT, even if the length of the measured optical fiber FUT is unknown, the measured The characteristics of the optical fiber FUT can be measured. Further, even when using a delay fiber shorter than the measured optical fiber FUT, the characteristics of the measured optical fiber FUT can be measured.

  In the above-described first embodiment, the optical delay unit 30 is described as being arranged between the first optical branching unit 20 and the second optical branching unit 40, but the optical delaying unit 30 is the second delay unit. It may be arranged between the optical branching section 40 and the multiplexing section 70. The optical delay unit 30 may be provided at an appropriate position on the first optical path 6 of pump light. In this case, an optical amplifier or the like for amplifying the backscattered light may be provided between the optical delay unit 30 and the multiplexing unit 70.

  In addition, the length of the first optical path 6 which is the optical path of the pump light is longer than that of the second optical path 8 which is the optical path of the reference light by lengthening the optical path in the optical amplification section 35 without providing the optical delay section 30. A longer structure may be adopted.

(Second embodiment)
Next, a second embodiment of the present invention will be described. Compared with the first embodiment, the optical fiber characteristic measuring device according to the second embodiment has a point where a gate unit is provided on the optical path of pump light, and a gated acquisition unit between the detection unit 80 and the arithmetic control unit 100. The difference is that 92 is provided. Therefore, in the description of the second embodiment, the same parts as those in the first embodiment are given the same reference numerals, and the description thereof will be omitted or simplified.

  In the first embodiment, the description has been given mainly for the example in which only one n-th order correlation peak occurs in the measured optical fiber FUT. However, since the interval between two adjacent correlation peaks changes depending on the modulation frequency of the light source, when the modulation frequency of the light source is high and the interval between correlation peaks is shorter than the length of the measured optical fiber FUT, the measured optical fiber FUT Multiple correlation peaks may occur above. In this case, when measuring the Brillouin scattering spectrum in the acquisition unit 90, it is necessary to determine at which point on the measured optical fiber FUT the correlation peak is present.

  As a method for discriminating a plurality of correlation peaks existing on the measured optical fiber, the temporal gate method and the like shown in Patent Document 1 and Non-Patent Document 1 are known. In this method, a gate portion is provided on the optical path of the pump light so that only one of the plurality of correlation peaks generated on the optical fiber under measurement can be taken out. Since the pump light is sent out in pulses only while the gate is open, the Brillouin scattered light (backscattered light) returns only from the measured optical fiber at the portion through which the light pulse passes. . The calculation unit provided in the optical fiber characteristic measuring device continuously receives the backscattered light that is the return light, but it does not detect the backward scattered light from a specific portion (the position where the optical pulse of the pump light has passed). Correlation peaks can be selected by performing gate processing on the received light signal input to the acquisition unit so that only scattered light is selectively received.

  The temporal gate method is similar to the BOTDR method in that the backscattered light at a specific portion is determined by making an optical pulse incident on the optical fiber to be measured and continuously sampling the backscattered light. However, the spatial resolution limit in the BOTDR system is about 1 m, whereas the spatial resolution in the BOCDR system using the temporal gate method is several cm. Therefore, the BOCDR method using the temporal gate method has a higher spatial resolution than the BOTDR method.

  Therefore, in the second embodiment of the present invention, it is possible to observe only a specific part from a plurality of correlation peaks generated on the optical fiber under measurement while maintaining the advantages of the first embodiment. The optical fiber characteristic measuring device will be described.

  FIG. 3 is a block diagram showing an example of an optical fiber characteristic measuring device according to the second embodiment. The optical fiber characteristic measuring apparatus 1A shown in FIG. 3 includes a gate unit 120 between the optical amplification unit 35 and the second optical branching unit 40. The gate unit 120 may be provided between the first optical branching unit 20 and the optical delay unit 30, or between the optical delay unit 30 and the optical amplification unit 35. Further, the optical fiber characteristic measuring apparatus 1A includes a gated acquisition unit 92 between the detection unit 80 and the arithmetic control unit 100. The arithmetic control unit 100 outputs a first gate signal S1 (first control signal) for controlling the operation of the gate unit 120 to the gate unit 120. Further, the arithmetic control unit 100 outputs a second gate signal S2 (second control signal) for controlling the operation of the gated acquisition unit 92 to the gated acquisition unit 92.

  The gate unit 120 performs a gate process of passing the pump light input from the optical amplification unit 35 to the second optical branching unit 40 (open state) or blocking the pump light (closed state). The gate unit 120 includes an optical switch having a high extinction ratio, an intensity modulator, a semiconductor optical amplifier (SOA), and the like. For example, the semiconductor optical amplifier also has a function of amplifying pump light.

  The open / closed state of the gate unit 120 is controlled by the first gate signal S1 from the arithmetic control unit 100. When the first gate signal S1 indicating that the gate unit 120 is to be opened is input from the arithmetic control unit 100, the gate unit 120 is opened. In this case, the pump light input from the optical amplification unit 35 passes through the gate unit 120 and is output to the second optical branching unit 40. On the other hand, when the first gate signal S1 indicating that the gate unit 120 is closed is input from the arithmetic and control unit 100, the gate unit 120 is closed. In this case, the pump light input from the optical delay unit 30 is blocked without passing through the gate unit 120, and nothing is output from the gate unit 120.

  The gated acquisition unit 92 starts or stops the measurement of the interference signal input from the detection unit 80 based on the second gate signal S2 from the calculation control unit 100. The reason why the measurement start and measurement stop of the gated acquisition unit 92 are controlled is to measure only a specific portion on the measured optical fiber FUT, of the backscattered light arranged in time series. Performing measurement at a predetermined time means performing measurement at a specific position (predetermined distance) on the optical fiber FUT to be measured.

  Next, the operation of the optical fiber characteristic measuring device 1A of the second embodiment will be described. FIG. 4 is a flowchart showing an example of the processing flow of the optical fiber characteristic measuring device 1A in the second embodiment.

  The light source unit 10 outputs the frequency-modulated continuous light to the first light branching unit 20 (step S201).

  Next, the first optical branching unit 20 splits the continuous light input from the light source unit 10 into pump light and reference light (step S203). The first optical branching unit 20 outputs the pump light to the optical delay unit 30 (step S205).

  Next, the optical delay unit 30 outputs the pump light input from the first optical branching unit 20 to the optical amplification unit 35. The optical amplification unit 35 outputs the pump light to the gate unit 120 after performing the amplification process of the pump light. The open / closed state of the gate unit 120 is controlled based on the first gate signal S1 input from the arithmetic control unit 100. As a result, the gate unit 120 outputs the pulsed pump light to the second optical branch unit 40. Each optical pulse pulsed by the gate section 120 needs to include one cycle of the original modulated continuous light. This one cycle is the time corresponding to one correlation peak.

  Next, the second optical branching unit 40 outputs the pulsed pump light input from the gate unit 120 to the measured optical fiber FUT via the optical connector 50 (step S209). The pulsed pump light propagates through the optical fiber under test FUT. The backscattered light returns from each part of the measured optical fiber FUT through which the optical pulse has passed, in time series in the order in which the optical pulse has passed.

  Next, the second optical branching unit 40 receives the backscattered light from the measured optical fiber FUT and outputs it to the first light guide unit 2 (step S211). Next, the first light guide section 2 outputs the backscattered light input from the second light branch section 40 to the multiplexing section 70.

  In parallel with the above steps S205, S207, S209, and S211, or before or after steps S205, S207, S209, and S211, the first optical branching unit 20 is branched by the first optical branching unit 20. The reference light is output to the second light guide unit 4 (step S213). Next, the second light guide section 4 outputs the reference light to the multiplexing section 70.

  Next, the multiplexing unit 70 multiplexes the backscattered light input from the first light guide unit 2 and the reference light input from the second light guide unit 4. Next, the combining unit 70 outputs the combined light to the detection unit 80. The detection unit 80 performs optical heterodyne detection by interfering the light input from the multiplexing unit 70. The detection unit 80 performs appropriate level adjustment and the like on the detected interference signal, that is, the signal of Brillouin scattering, and then outputs it to the gated acquisition unit 92 (step S215).

  Next, the gated acquisition unit 92 receives the interference signal input from the detection unit 80. Next, based on the second gate signal S2 input from the arithmetic and control unit 100, the measurement start and measurement stop of the gated acquisition unit 92 are controlled. The gated acquisition unit 92 measures the Brillouin scattering spectrum at a time delayed by a predetermined time after the gate unit 120 is opened based on the first gate signal S1 (step S217). The arithmetic control unit 100 uses the first gate signal S1 and the second gate signal S2 to delay timing between the opening / closing process of the gate unit 120 and the opening / closing process of the gated acquisition unit 92 (measurement start and measurement stop control). Is controlled to sweep the observation position of the optical fiber FUT to be measured. For example, by sweeping the delay timing between the opening / closing process of the gate unit 120 and the opening / closing process of the gated acquisition unit 92 little by little, the sweep measurement can be performed while gradually changing the observation position on the optical fiber FUT to be measured. .

  Here, as in the first embodiment, since the optical delay unit 30 is provided in the optical path (first optical path 6) of the pump light branched by the first optical branching unit 20, the length of the first optical path 6 is increased. Is longer than the second optical path 8 which is the optical path of the reference light. Therefore, the position of the zero-order correlation peak, which is the position where the optical path length of the pump light and the optical path length of the reference light are equal, is between the first optical branching section 20 and the measured optical fiber FUT, that is, the first optical branch. It is adjusted to the position of the front stage of the optical fiber FUT to be measured in the optical path 6. As a result, not the 0th-order correlation peak but the nth-order correlation peak used for the characteristic measurement of the measured optical fiber FUT is generated on the measured optical fiber FUT.

  Next, the gated acquisition unit 92 outputs the measured Brillouin scattering spectrum to the arithmetic control unit 100 as spectrum data. The calculation control unit 100 calculates the Brillouin frequency shift amount from the spectrum data input from the gated acquisition unit 92 to calculate the characteristics of the measured optical fiber FUT (strain distribution in the length direction of the measured optical fiber FUT, temperature Distribution) is measured (step S219).

  Next, the arithmetic control unit 100 outputs a control signal for controlling the measurement position on the measured optical fiber FUT to the modulation unit 110. Next, the modulation unit 110 outputs a modulation signal for modulating the light emitted from the light source unit 10 to the light source unit 10 based on the control signal input from the calculation control unit 100, and the processing of this flowchart ends. To do. In this way, by controlling the modulation signal for modulating the light emitted from the light source unit 10, it is possible to control the measurement position on the measured optical fiber FUT in more detail.

  According to the optical fiber characteristic measuring device 1A of the second embodiment described above, since the optical delay unit 30 is provided in the optical path (first optical path 6) of the pump light branched by the first optical branching unit 20, The length of the first optical path 6 is longer than the second optical path 8 which is the optical path of the reference light. Therefore, the position of the zero-order correlation peak, which is the position where the optical path length of the pump light and the optical path length of the reference light are equal, is between the first optical branching section 20 and the measured optical fiber FUT, that is, the first optical branch. It is adjusted to the position of the front stage of the optical fiber FUT to be measured in the optical path 6. Since it is not necessary to change the length of the delay section (for example, the optical fiber) according to the length of the measured optical fiber FUT, the measured light is measured even when the measured optical fiber FUT has an unknown length. The characteristics of the fiber FUT can be measured. Further, even when using a delay fiber shorter than the measured optical fiber FUT, the characteristics of the measured optical fiber FUT can be measured.

  Further, according to the optical fiber characteristic measuring apparatus 1A of the second embodiment, by incorporating the BOCDR system temporal gate method, for example, when the length of the optical delay unit 30 is shorter than the length of the measured optical fiber FUT, etc. Even if a plurality of correlation peaks occur on the optical fiber FUT to be measured, only one correlation peak corresponding to the position of the observation target can be selectively measured from the plurality of correlation peaks. That is, when there are a plurality of correlation peaks in which the frequency difference between the backscattered light and the reference light does not fluctuate with time in the measured optical fiber FUT, the opening / closing of the gate section 120 provided on the optical path of the pump light is controlled. By doing so, the pump light is pulsed. Further, according to the distance from one end of the measured optical fiber FUT to the correlation peak (first correlation peak) that is the measurement target among the plurality of correlation peaks, the time when the gate unit 120 is opened and the Brillouin scattering spectrum By controlling the time difference between the time when the acquisition is started and the time when the gate unit 120 is closed and the time when the acquisition of the Brillouin scattering spectrum is stopped, the measurement target is selected from the plurality of correlation peaks. Only one correlation peak corresponding to the position can be selectively measured.

  In the above-described second embodiment, the optical delay unit 30 is described as being arranged between the first optical branching unit 20 and the optical amplifying unit 35, but the optical delay unit 30 does not include the second optical branching unit. It may be arranged between the section 40 and the multiplexing section 70. In this case, an optical amplifier or the like for amplifying the backscattered light may be provided between the optical delay unit 30 and the multiplexing unit 70.

  In addition, the length of the first optical path 6 which is the optical path of the pump light is longer than that of the second optical path 8 which is the optical path of the reference light by lengthening the optical path in the optical amplification section 35 without providing the optical delay section 30. A longer structure may be adopted.

  The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but it goes without saying that the present invention is not limited to the above embodiments. The shapes, combinations, and the like of the constituent members shown in the above-described embodiments are examples, and various modifications can be made based on design requirements and the like without departing from the spirit of the present invention.

  1, 1A ... Optical fiber characteristic measuring device, 2 ... First light guide section, 4 ... Second light guide section, 6 ... First light path, 8 ... Second light path, 10 ... Light source section, 20 ...... First optical branching unit, 30 ...... Optical delaying unit, 35 ...... Optical amplifying unit, 40 ...... Second optical branching unit, 50 ...... Optical connector, 60 ...... Computing unit, 70 ...... Multiplexing unit, 80 ... Detection unit, 82, 84 ... Photodiode, 86 ... Detection unit, 90 ... Acquisition unit, 92 ... Gated acquisition unit, 100 ... Arithmetic control unit, 110 ... Modulation unit, 120 ... Gate, FUT ... Optical fiber to be measured

Claims (8)

A light source unit that outputs frequency-modulated continuous light,
A first light branching unit for branching the continuous light into pump light and reference light;
The pump light emitted from the first optical branching portion is made incident from one end of the measured optical fiber, and backscattered light generated by Brillouin scattering of the pump light in the measured optical fiber is emitted from the measured optical fiber. A second optical branching unit for receiving,
By causing the backscattered light and the reference light to interfere with each other, an arithmetic unit that measures the characteristics of the optical fiber under measurement ,
An optical delay unit arranged between the first optical branching unit and the second optical branching unit or between the second optical branching unit and the arithmetic unit, and causing a predetermined delay in the pump light;
Equipped with
The length of a first optical path where the pump light emitted from the first optical branching portion enters the optical fiber under measurement and reaches the calculation unit as the backscattered light, and the reference light is transmitted to the calculation unit. rather long than the length of the second optical path,
The position of the zero-order correlation peak is adjusted to the position of the preceding stage of the optical fiber under measurement in the first optical path,
Optical fiber characteristic measuring device. The optical delay unit includes an optical fiber for increasing the length of the first optical path,
The optical fiber characteristic measuring device according to claim 1 . The arithmetic unit is
A multiplexing unit that multiplexes the backscattered light and the reference light to generate combined light,
By interfering the backscattered light included in the combined light input from the combiner with the reference light, an interference signal indicating a frequency difference between the backscattered light and the reference light is detected. A detector,
Based on the interference signal input from the detection unit, an acquisition unit that acquires a spectrum of Brillouin scattering,
From the spectrum of the Brillouin scattering input from the acquisition unit, a calculation control unit that calculates the Brillouin frequency shift amount,
With
The optical fiber characteristic measuring device according to claim 1 . A gate unit is disposed between the first optical branching unit and the second optical branching unit, and further includes a gate unit that passes or blocks the pump light input from the first optical branching unit to the second optical branching unit. Prepare,
The arithmetic control unit,
Outputting a first control signal to the gate unit for controlling passage or blocking of the pump light emitted from the first light branching unit to the second light branching unit;
Outputting a second control signal for controlling the start and stop of the acquisition of the spectrum of the Brillouin scattering to the acquisition unit,
The optical fiber characteristic measuring device according to claim 3 . The calculation control unit controls start and stop of acquisition of the Brillouin scattering spectrum so that the Brillouin scattering spectrum includes data for one cycle of the frequency-modulated continuous light output from the light source unit. Outputting the second control signal to the acquisition unit,
The optical fiber characteristic measuring device according to claim 4 . Outputting continuous light that has been frequency-modulated,
Splitting the continuous light into a pump light and a reference light,
Delaying the pump light with respect to the reference light to adjust the position of the zero-order correlation peak to a position at which the pump light reaches the optical fiber under measurement ;
And said pump light to be incident from one end of the optical fiber to be measured, receives the backscattered light generated by the Brillouin scattering of the pump light in said optical fiber to be measured from the optical fiber to be measured,
By measuring the characteristics of the measured optical fiber by causing the backscattered light and the reference light to interfere with each other,
An optical fiber characteristic measuring method comprising: Measuring the characteristics of the optical fiber under test includes
By causing the backscattered light and the reference light to interfere with each other, detecting the interference signal indicating the frequency difference between the backscattered light and the reference light,
Acquiring a spectrum of Brillouin scattering based on the interference signal;
Calculating a Brillouin frequency shift amount from the spectrum of the Brillouin scattering;
Equipped with
The optical fiber characteristic measuring method according to claim 6 . When there are a plurality of correlation peaks in which the frequency difference between the backscattered light and the reference light does not fluctuate with time in the measured optical fiber, delaying the pump light, and measuring the backscattered light in the measured light Between receiving from an optical fiber, further comprising pulsing the pump light by controlling the opening and closing of a gate portion provided on the optical path of the pump light,
To obtain the spectrum of the Brillouin scattering, the gate unit is opened according to the distance from one end of the measured optical fiber to the position of the first correlation peak that is the measurement target among the plurality of correlation peaks. A time difference between the time and the time when the acquisition of the Brillouin scattering spectrum is started, and the time when the gate part is closed, and the time difference between the time when the acquisition of the Brillouin scattering spectrum is stopped is controlled,
The optical fiber characteristic measuring method according to claim 7 .

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