Optical fiber characteristic measurement device and optical fiber characteristic measurement method

Abstract

An optical fiber characteristic measurement device (1) comprises: a light source (11) that emits laser light modulated at a prescribed modulation frequency; a photodetector (19) that detects Brillouin scattered light obtained by causing the laser light to enter an optical fiber to be measured (FUT); a synchronous detector (20) that performs synchronous detection of a detection signal output from the photodetector (19); and a computer (23) that performs control for adjusting the timing at which the laser light is caused to enter the optical fiber to be measured (FUT) in accordance with the relationship between the modulation frequency and the frequency of a synchronous signal used by the synchronous detector (20).

Description

Optical fiber characteristic measurement apparatus and optical fiber characteristic measurement method

[0001] This disclosure relates to an optical fiber characteristic measurement apparatus and an optical fiber characteristic measurement method.

[0002] An optical fiber characteristic measuring device is a device that uses the optical fiber itself as a sensor to measure characteristics such as strain, temperature, and vibration from changes in the physical quantities of the optical fiber. Vibration can be calculated from the temporal change in strain. For example, optical fiber characteristic measuring devices are used to measure the integrity (health) of structures by attaching or embedding optical fibers into them.

[0003] Some optical fiber characteristic measurement devices utilize the property that the spectrum (frequency) of Brillouin scattered light generated within an optical fiber changes with the distortion or temperature of the optical fiber. Representative examples of such optical fiber characteristic measurement devices include the BOCDR (Brillouin Optical Correlation Domain Reflectometry) method and the BOCDA (Brillouin Optical Correlation Domain Analysis) method. Patent Document 1 below discloses an example of a BOCDA-type optical fiber characteristic measurement device.

[0004] Patent No. 5043714

[0005] Incidentally, because Brillouin scattered light is weak, noise is almost always superimposed on its spectrum (Brillouin gain spectrum: BGS). Optical fiber characteristic measurement devices improve measurement accuracy by using a synchronous detector to remove unwanted noise components and detecting only the necessary signal components. However, using a synchronous detector can generate unwanted fluctuation components that are superimposed on the Brillouin gain spectrum, which can actually decrease measurement accuracy.

[0006] The fluctuation components described above are due to the direct modulation of the light source (semiconductor laser) in order to obtain frequency-modulated light. When the modulation frequency used to obtain the above-mentioned modulated light and the frequency of the synchronization signal of the synchronous detector become integer multiples (or close to integer multiples), unwanted fluctuation components are generated, which can lead to a decrease in measurement accuracy as described above.

[0007] This disclosure is made in view of the above circumstances and aims to provide an optical fiber characteristic measuring device and an optical fiber characteristic measuring method that can achieve higher measurement accuracy than conventional methods by suppressing unwanted fluctuation components.

[0008] To solve the above problems, the optical fiber characteristic measuring apparatus (1) according to the first aspect of this disclosure has a predetermined modulation frequency (f m The system comprises a light source (11) that emits a laser beam modulated by a predetermined modulation frequency, a photodetector (19) that detects Brillouin scattered light obtained by injecting the laser beam into an optical fiber (FUT) to be measured, a synchronous detector (20) that performs synchronous detection of the detection signal output from the photodetector, and a control unit (23) that controls the timing of injecting the laser beam into the optical fiber to be measured according to the relationship between the predetermined modulation frequency and the frequency (f0) of the synchronous signal used by the synchronous detector.

[0009] Furthermore, in the optical fiber characteristic measuring apparatus according to the second aspect of the present disclosure, the control unit may perform control to adjust the timing of injecting the laser light into the optical fiber to be measured when the predetermined modulation frequency and the frequency of the synchronization signal are in an integer multiple relationship.

[0010] Furthermore, in the optical fiber characteristic measuring apparatus according to the third aspect of the present disclosure, the control unit may perform control to adjust the timing of injecting the laser light into the optical fiber to be measured when the predetermined modulation frequency and the synchronization signal frequency are in a relationship close to an integer multiple, in addition to the case where the predetermined modulation frequency and the synchronization signal frequency are in a relationship close to an integer multiple.

[0011] Furthermore, the optical fiber characteristic measuring apparatus according to the fourth aspect of this disclosure is an optical fiber characteristic measuring apparatus according to any of the first to third aspects, further comprising an optical delay device (14) provided between the light source and the optical fiber to be measured, wherein the control unit adjusts the timing of injecting the laser light into the optical fiber to be measured by controlling the delay amount of the optical delay device.

[0012] Furthermore, in the optical fiber characteristic measuring apparatus according to the fifth aspect of the present disclosure, the optical delay device may include a plurality of optical fibers (33a to 33n) of different lengths, and optical switches (32, 34) that switch which of the plurality of optical fibers allows the laser light to pass through.

[0013] Furthermore, an optical fiber characteristic measuring apparatus according to a sixth aspect of the present disclosure may also include, in an optical fiber characteristic measuring apparatus according to a fifth aspect, the optical switch comprising a first port into which the laser light is incident and a plurality of second ports to which one end of the plurality of optical fibers is connected, and a first optical switch (32) that switches the second ports connected to the first port under the control of the control unit; and a plurality of third ports to which the other ends of the plurality of optical fibers are connected and a fourth port from which the laser light is emitted, and a second optical switch (34) that switches the third ports connected to the fourth port under the control of the control unit.

[0014] Furthermore, an optical fiber characteristic measuring apparatus according to the seventh aspect of this disclosure may include, in an optical fiber characteristic measuring apparatus according to any of the first to sixth aspects, an optical splitter (12) that splits the laser light into a first branched light (L1) and a second branched light (L2), and a shifter (13) that shifts the frequency of the first branched light or the second branched light, wherein the photodetector detects Brillouin scattered light obtained by injecting the first branched light from one end of the optical fiber to be measured and the second branched light from the other end of the optical fiber to be measured.

[0015] Furthermore, an optical fiber characteristic measuring apparatus according to the eighth aspect of this disclosure may include a processing unit (23) in an optical fiber characteristic measuring apparatus according to any of the first to seventh aspects that obtains a Brillouin gain spectrum from the signal obtained by the synchronous detector and measures the characteristics of the optical fiber to be measured based on the Brillouin gain spectrum.

[0016] A method for measuring optical fiber characteristics according to a first aspect of this disclosure is a method for measuring optical fiber characteristics with a predetermined modulation frequency (f m The method includes the steps of: emitting a laser beam modulated by (S12); detecting Brillouin scattered light obtained by injecting the laser beam into an optical fiber under test (FUT) (S14); performing synchronous detection of the detection signal obtained by detecting the Brillouin scattered light (S15); and controlling the timing of injecting the laser beam into the optical fiber under test according to the relationship between the predetermined modulation frequency and the frequency of the synchronization signal used in the synchronous detection.

[0017] In the optical fiber characteristic measurement method according to the second aspect of the present disclosure, the control for adjusting the timing may be performed between the completion of one measurement of the optical fiber under test and the start of the next measurement of the optical fiber under test.

[0018] In the optical fiber characteristic measurement method according to the third aspect of the present disclosure, the control for adjusting the timing may be performed while a single measurement of the optical fiber under test is being carried out, in the optical fiber characteristic measurement method according to the first aspect.

[0019] According to this disclosure, suppressing unwanted fluctuation components has the effect of achieving higher measurement accuracy than conventional methods.

[0020] This is a block diagram showing the main components of an optical fiber characteristic measuring device according to one embodiment of the present disclosure. This is a block diagram showing the main components of an optical delay device included in an optical fiber characteristic measuring device according to one embodiment of the present disclosure. This is a flowchart showing an example of operation of an optical fiber characteristic measuring device according to one embodiment of the present disclosure. This is a diagram showing an example of a correlation peak before the optical delay device is controlled according to one embodiment of the present disclosure. This is a diagram showing an example of a correlation peak after the optical delay device is controlled according to one embodiment of the present disclosure. This is a diagram illustrating an example of a measurement procedure in one embodiment of the present disclosure.

[0021] The optical fiber characteristic measurement apparatus and optical fiber characteristic measurement method according to embodiments of this disclosure will be described in detail below with reference to the drawings.

[0022] Figure 1 is a block diagram showing the main components of an optical fiber characteristic measuring device according to one embodiment of the present disclosure. As shown in Figure 1, the optical fiber characteristic measuring device 1 of this embodiment includes a light source 11, an optical splitter 12, an optical modulator 13 (shifter), an optical delayer 14, an optical isolator 15, a pulse modulator 16, a directional coupler 17, a timing adjuster 18, a photodetector 19, a synchronous detector 20, a signal generator 21, an A / D converter 22, and a computer 23 (control unit, processing unit).

[0023] The optical fiber characteristic measurement device 1 of this embodiment is an optical fiber characteristic measurement device that uses the so-called BOCDA method, in which probe light L1 (first branch light) is incident from one end of the optical fiber FUT to be measured, pump light L2 (second branch light) is incident from the other end of the optical fiber FUT to be measured, and the characteristics of the optical fiber FUT to be measured are measured based on the Brillouin scattered light obtained from the other end.

[0024] The light source 11 comprises a semiconductor laser 11a and a signal generator 11b, and has a predetermined modulation frequency f mA modulated laser beam is emitted. Here, the semiconductor laser 11a can be, for example, an MQW DFB LD (Multi-Quantum Well Distributed Feedback Laser Diode) which is small and emits laser beam with a narrow spectral width. The signal generator 11b modulates the laser beam emitted from the semiconductor laser 11a to a modulation frequency f under the control of the computer 23. m A frequency-modulated sinusoidal signal (modulated signal) is output to the semiconductor laser 11a. The optical splitter 12 splits the laser light emitted from the light source 11 into two, for example, two, at a 1:1 intensity ratio.

[0025] The optical modulator 13 comprises a microwave generator 13a and an SSB (Single Sideband) modulator 13b. It modulates (shifts the optical frequency of) one of the laser beams split by the optical splitter 12 to generate a sideband (single sideband) relative to the center frequency of the laser beam. In this embodiment, it is assumed that the low-frequency single sideband is emitted from the optical modulator 13. The microwave generator 13a, under the control of the computer 23, outputs a microwave having a frequency equal to the frequency shift applied to one of the laser beams split by the optical splitter 12. The SSB modulator 13b generates a single sideband having a frequency difference equal to the frequency of the microwave output from the microwave generator 13a relative to the center frequency of the incident light. The frequency of the microwave output from the microwave generator 13a is variable.

[0026] The optical delay unit 14 delays the single-sideband of the laser light emitted from the optical modulator 13. The optical delay unit 14 can switch the delay time (delay duration) of the single-sideband of the laser light emitted from the optical modulator 13 under the control of the computer 23. The optical delay unit 14 is provided to adjust the position of the correlation peak formed in the optical fiber FUT under measurement.

[0027] Specifically, the optical delay unit 14 delays the single-sideband of the laser light emitted from the optical modulator 13, thereby setting the position where the zero-order correlation peak appears (the position where the optical path difference between the probe light L1 and the pump light L2 becomes zero) to a predetermined position outside the optical fiber FUT under test. Furthermore, under the control of the computer 23, the optical delay unit 14 switches the above-mentioned delay time, thereby shifting the positions of all correlation peaks, including the zero-order correlation peak, in the longitudinal direction of the optical fiber FUT under test.

[0028] Figure 2 is a block diagram showing the main components of an optical delay device included in an optical fiber characteristic measuring device according to one embodiment of the present disclosure. The optical delay device 14 shown in Figure 2(a) has two delay time switching stages, and the optical delay device 14 shown in Figure 2(b) has n delay time switching stages (where n is an integer of 3 or more).

[0029] The optical delay device 14 shown in Figure 2(a) comprises an optical fiber 31, an optical switch 32 (first optical switch), optical fibers 33a and 33b, and an optical switch 34 (second optical switch). The optical fiber 31 is an optical fiber having a predetermined length and delays the single-sideband of the laser light emitted from the optical modulator 13 by a predetermined time. The optical fiber 31 is provided so that the position where the zero-order correlation peak appears is at a predetermined position outside the optical fiber FUT under measurement.

[0030] The optical switch 32 has one first port through which the single-sideband laser light transmitted via the optical fiber 31 is incident, and two second ports to which one end of the optical fibers 33a and 33b are connected. Under the control of the computer 23, the optical switch 32 switches which of the two second ports is connected to the first port. In other words, the optical switch 32 can be described as a switch that switches which of the optical fibers 33a and 33b allows the single-sideband laser light transmitted via the optical fiber 31 to pass through.

[0031] Optical fibers 33a and 33b are optical fibers having mutually different lengths. The difference in length between optical fibers 33a and 33b is, for example, approximately several tens of cm to several m. Optical fibers 33a and 33b may be of the same type as optical fiber 31, or may be of different types. Since optical delay device 14 delays the single sideband of the laser light emitted from optical modulator 13, it is preferable that optical fibers 33a, 33b and optical fiber 31 have as small a transmission loss as possible for the single sideband of the laser light emitted from optical modulator 13. Optical fibers 33a and 33b are provided to shift the positions of all correlation peaks, including the 0th-order correlation peak, in the longitudinal direction of the optical fiber under test FUT.

[0032] Optical switch 34 includes two third ports to which the other ends of optical fibers 33a and 33b are connected, and one fourth port from which the single sideband of the laser light is emitted. Under the control of computer 23, optical switch 34 switches which of the two third ports is connected to the fourth port. In other words, similar to optical switch 32, optical switch 34 can also be described as a switch that switches which optical fiber among optical fibers 33a and 33b allows the passage of the single sideband of the laser light that has passed through optical fiber 31.

[0033] Here, optical switches 32 and 34 are controlled such that either one of optical fibers 33a and 33b is connected between the first port of optical switch 32 and the fourth port of optical switch 34. That is, for optical switches 32 and 34, one end of either optical fiber 33a or 33b is connected to the first port of optical switch 32, and the other end of either optical fiber 33a or 33b is connected to the fourth port of optical switch 34 under such control. Note that optical switches 32 and 34 are not controlled such that, for example, one end of either optical fiber 33a or 33b is connected to the first port of optical switch 32, and the other end of the other of optical fibers 33a and 33b is connected to the fourth port of optical switch 34.

[0034] The optical delay device 14 shown in FIG. 2(b) basically has the same configuration as the optical delay device 14 shown in FIG. 2(a), except that the number of second ports of the optical switch 32, the number of third ports of the optical switch 34, and the number of optical fibers provided between the optical switch 32 and the optical switch 34 are different. Specifically, the optical switch 32 includes n second ports, and the optical switch 34 includes n third ports. Further, n optical fibers 33a to 33n having mutually different lengths are provided between the optical switch 32 and the optical switch 34. In the optical delay device 14 shown in FIG. 2(b), the optical switches 32 and 34 are controlled such that any one of the optical fibers 33a to 33n is connected between the first port of the optical switch 32 and the fourth port of the optical switch 34.

[0035] The optical isolator 15 transmits light traveling from the optical delay device 14 toward the fiber under test FUT, and blocks light traveling from the fiber under test FUT toward the optical delay device 14. The light that travels from the optical delay device 14 through the optical isolator 15 toward the fiber under test FUT enters the fiber under test FUT from one end of the fiber under test FUT as probe light L1.

[0036] The pulse modulator 16 includes a signal generator 16a and an optical intensity modulator 16b, and pulses the other laser light (continuous light) split by the optical splitter 12. Under the control of the computer 23, the signal generator 16a outputs a timing signal that defines the timing for pulsing the laser light. The optical intensity modulator 16b is, for example, an EO (Electro-Optic) switch, and pulses the laser light from the optical splitter 12 at a timing defined by the timing signal output from the signal generator 17b.

[0037] The directional coupler 17 uses the pulsed laser light emitted from the pulse modulator 16 as pump light L2 to inject into the optical fiber FUT under test from the other end, and also emits light including the probe light L1 that has propagated through the optical fiber FUT and been emitted from the other end towards the timing adjuster 18. The intensity of the light in the optical frequency band of the probe light L1 is affected by the stimulated Brillouin scattering phenomenon occurring within the optical fiber FUT under test.

[0038] The timing regulator 18 opens or closes based on the timing signal output from the signal generator 21, allowing only stimulated Brillouin scattered light generated at and near the measurement point (the point where the characteristics are to be measured) set within the optical fiber FUT under measurement to pass through. Specifically, based on the timing signal from the signal generator 21, the timing regulator 18 opens when the stimulated Brillouin scattered light generated at and near the measurement point reaches the timing regulator 18 via the directional coupler 17, and closes when the stimulated Brillouin scattered light has passed through the timing regulator 18.

[0039] The photodetector 19 is equipped with a highly sensitive photodetector, such as an avalanche photodiode, and detects the light that has passed through the timing regulator 18 and outputs a detection signal (received signal). In Figure 1, although the illustration is simplified, the photodetector 19 is equipped with an optical wavelength filter (not shown) in addition to the above-mentioned photodetector, and selects only the low-frequency sideband related to the probe light L1 from the light that has passed through the timing regulator 18 and detects its power. In this embodiment, since an SSB modulator 13b is used, the optical wavelength filter can be omitted. However, although providing an optical wavelength filter increases costs, it can suppress unwanted frequency components, thereby improving measurement accuracy.

[0040] The synchronous detector 20 synchronously detects the detection signal output from the photodetector 19 using a synchronization signal having a predetermined frequency f0 output from the signal generator 21. For example, a lock-in amplifier can be used as the synchronous detector 20. The signal generator 21 generates a timing signal to be output to the timing adjuster 18 under the control of the computer 23, and also generates a synchronization signal to be used in the synchronous detector 20.

[0041] Furthermore, the frequency of the timing signal output from the signal generator 21 to the timing adjuster 18, the frequency f0 of the synchronization signal output from the signal generator 21 to the synchronous detector 20, and the frequency of the timing signal output from the signal generator 16a of the pulse modulator 16 to the optical intensity modulator 16b are all the same. The A / D converter 22 converts the signal synchronously detected by the synchronous detector 20 into a digital signal and outputs it to the computer 23 as detection data.

[0042] The computer 23 controls the operation of the optical fiber characteristic measuring device 1 and performs various processes on the digital signal (detection data) output from the A / D converter 22 to determine the characteristics of the optical fiber FUT under test. Specifically, the computer 23 determines the modulation frequency f of the laser light emitted from the semiconductor laser 11a. m The frequency of the sine wave signal output from the signal generator 11b is set, and the frequency f0 of the synchronization signal (synchronization signal output from the signal generator 21) used in the synchronous detector 20 is set and modified.

[0043] Here, the computer 23 determines the modulation frequency f m The timing of injecting the probe light L1 into the optical fiber FUT under measurement is controlled according to the relationship between the modulation frequency f and the synchronization signal frequency f. Specifically, the computer 23 controls the modulation frequency f m When the frequency f0 of the synchronization signal is an integer multiple (or close to an integer multiple), the optical delay unit 14 is controlled to adjust the timing of the probe light L1 being incident on the optical fiber FUT under measurement. This control is performed to achieve higher measurement accuracy than conventional methods by suppressing unwanted fluctuation components.

[0044] Here, the "relationship close to an integer multiple" means that the modulation frequency f m and the frequency f0 of the synchronization signal do not have an integer multiple relationship, but have a relationship where the aforementioned unnecessary fluctuation components may occur. For example, the modulation frequency f m divided by the frequency f0 of the synchronization signal, if the remainder obtained is smaller than 0.1 or larger than 0.9, it can be considered as a relationship close to an integer multiple. The details of the aforementioned control performed by the computer 23 will be described later.

[0045] The computer 23 performs processing to remove noise from the detection data output from the A / D converter 22. For example, the computer 23 performs Fourier transform on the detection data output from the A / D converter 22 to remove a predetermined frequency component (the noise frequency component), and performs inverse Fourier transform on the detection data from which this frequency component has been removed, thereby removing the noise.

[0046] Furthermore, the computer 23 approximates the noise-removed detection data using a predetermined approximation formula. For example, the computer 23 approximates the noise-removed detection data by least squares approximation. In the present embodiment, the case of approximation by the least squares method is taken as an example, but any approximation method can be used to approximate the noise-removed detection data. The computer 23 detects a peak frequency from the approximated detection data and obtains characteristics of the optical fiber (for example, the magnitude of strain and temperature) at a set measurement point.

[0047] Furthermore, the computer 23 is realized by a hardware processor such as a CPU (Central Processing Unit) executing a program (software) stored in a memory unit not shown. In addition, some or all of these functional units may be realized by hardware (including circuitry) such as LSI (Large Scale Integration), ASIC (Application Specific Integrated Circuit), FPGA (Field-Programmable Gate Array), and GPU (Graphics Processing Unit), or by the cooperation of software and hardware.

[0048] Figure 3 is a flowchart illustrating an example of operation of an optical fiber characteristic measuring device according to one embodiment of the present disclosure. The flowchart shown in Figure 3 illustrates the operation when measuring the characteristics along the length direction of the optical fiber FUT under test. The flowchart shown in Figure 3 is initiated, for example, when an instruction to start measurement is given to the optical fiber characteristic measuring device 1.

[0049] When the process shown in the flowchart in Figure 3 is initiated, first, the modulation frequency f of the laser light is determined. m The computer 23 performs the process of setting the frequency f0 of the synchronization signal (step S11). As mentioned above, the frequency f0 of the synchronization signal output from the signal generator 21 to the synchronous detector 20 is the same as the frequency of the timing signal output from the signal generator 16a of the pulse modulator 16 to the light intensity modulator 16b (the repetition frequency of the pump light L2). Therefore, by calculating the frequency f0 of the synchronization signal, the repetition frequency of the pump light L2 is also calculated.

[0050] The modulation frequency f of the laser light m And once the synchronization signal frequency f0 is set, a control signal is output from the computer 23 to the signal generator 11b. Then, a sinusoidal signal (modulated signal) is output from the signal generator 11b to the semiconductor laser 11a, and the light source 11 outputs the modulation frequency f mA frequency-modulated laser beam is emitted (step S12).

[0051] The laser light emitted from the light source 11 is incident on the optical splitter 12 and split into two laser beams. One of the laser beams split by the optical splitter 12 is incident on the optical modulator 13 and modulated by the SSB modulator 13b, thereby generating a single-sideband relative to the center frequency of the laser light. The laser light with a single sideband (continuous light) emitted from the optical modulator 13 is delayed by a predetermined amount in the optical delayer 14, and then, via the optical isolator 15, is incident into the optical fiber FUT under test from one end as probe light L1. The other laser beam split by the optical splitter 12 is incident on the pulse modulator 16 and pulsed. This pulsed light is incident into the optical fiber FUT under test from the other end as pump light L2 via the directional coupler 17 (step S13).

[0052] When a continuous probe beam L1 and a pulsed pump beam L2 are incident into the optical fiber FUT under test, at the position of the correlation peak, the probe beam L1 gains a stimulated Brillouin amplification (SBR) due to the pump beam L2. The probe beam L1 and the SBR scattered light generated within the optical fiber FUT are emitted from the other end of the optical fiber FUT and then incident into the directional coupler 17. The probe beam L1 emitted from the directional coupler 17 is incident into the timing regulator 18, but only the probe beam L1 and the SBR scattered light incident at the timing defined by the timing regulator 18 are transmitted through the timing regulator 18. In other words, by adjusting the timing of the timing regulator 18, it is possible to obtain SBR scattered light generated at or near the set measurement point.

[0053] The probe light L1 and stimulated Brillouin scattered light that have passed through the timing regulator 18 are incident on the photodetector 19, and the light on the lower frequency sideband is selected by the optical wavelength filter (not shown) of the photodetector 19 and its intensity is detected (step S14). The photodetector 19 then outputs a detection signal indicating the detection result. This detection signal is input to the synchronous detector 20 and synchronously detected, and noise is removed (step S15).

[0054] The detection signal, from which noise has been removed by the synchronous detector 20, is input to the A / D converter 22, converted into a digital signal, and input to the computer 23 as detection data. Then, the computer 23 performs a process to determine the Brillouin gain spectrum based on the input detection data (step S16). Once the Brillouin gain spectrum is determined, the computer 23 performs a process to determine the Brillouin frequency shift amount by finding its peak frequency. Then, the computer 23 performs a process to measure the characteristics of the optical fiber FUT by converting the obtained Brillouin frequency shift amount into the magnitude of distortion and temperature change applied to the optical fiber FUT under test (step S17).

[0055] Once the above processes are complete, the computer 23 determines whether the measurement is finished or not (step S18). For example, it determines whether the measurement is finished for all measurement points along the length of the optical fiber FUT under measurement. If the computer 23 determines that the measurement is not finished (if the result of the determination in step S18 is "NO"), the process returns to step S11, the modulation frequency or the synchronization signal frequency is reset, and then the processes in steps S11 to S17 are performed.

[0056] While the above processing is being performed, the computer 23 sets the modulation frequency f set in step S11. m Determine whether the modulation frequency f0 and the synchronization signal frequency are integer multiples (or close to integer multiples). mIf it is determined that the modulation frequency f0 and the synchronization signal frequency are in an integer multiple relationship (or a relationship close to an integer multiple relationship), the optical delay unit 14 is controlled to adjust the timing of injecting the probe light L1 into the optical fiber FUT under test (a step of controlling the timing of injecting the laser light into the optical fiber under test). The computer 23 determines the modulation frequency f m The optical delay unit 14 may be controlled if there are no combinations of the synchronization signal frequency f0 that are not integer multiples (or close to integer multiples).

[0057] Figure 4 shows an example of correlation peaks before the optical delay device is controlled in one embodiment of the present disclosure. Figure 5 shows an example of correlation peaks after the optical delay device is controlled in one embodiment of the present disclosure. In Figures 4 and 5, the optical delay device 14 and the optical fiber FUT under measurement are shown, but other components of the optical fiber characteristic measurement device 1 are not shown.

[0058] As shown in Figure 4, before the optical delay unit 14 is controlled, the optical switches 32 and 34 are configured so that the probe light L1 passes through the optical fiber 33a. The probe light L1 is incident on one end of the optical fiber FUT under test via the optical delay unit 14, and the pump light L2 is incident on the other end of the optical fiber FUT under test via the optical delay unit 14. For the sake of simplicity, the frequency f0 of the synchronization signal is assumed to be fixed and not changed.

[0059] First, the modulation frequency f m When fm0 is set, for example, the interval between correlation peaks becomes dm0. As a result, as shown in Figure 4, the 0th-order correlation peak P0 appears at position Z0 inside the optical fiber characteristic measuring device 1, and multiple correlation peaks P1 to Pn appear with intervals of dm0 along the direction of propagation of the probe light L1. Therefore, for example, the characteristics of the optical fiber FUT under test at position Z11 where the kth-order correlation peak Pk appears can be measured.

[0060] Next, the modulation frequency f mWhen fm1 is set, for example, the interval between correlation peaks becomes dm1. As a result, as shown in Figure 4, the 0th-order correlation peak P0 appears at position Z0 inside the optical fiber characteristic measuring device 1, and multiple correlation peaks P1 to Pn appear along the direction of propagation of the probe light L1 with intervals of dm1. Therefore, for example, the characteristics of the optical fiber FUT under test at position Z12 where the kth-order correlation peak Pk appears can be measured.

[0061] Thus, the modulation frequency f m By changing this, the interval between correlation peaks changes, and the positions of correlation peaks P1 to Pn, excluding the 0th-order correlation peak P0, can be altered. Therefore, the modulation frequency f m By detecting stimulated Brillouin scattered light emitted from the optical fiber FUT under test while changing the parameters, the characteristics of the optical fiber FUT in the longitudinal direction can be measured.

[0062] Suppose computer 23 determines that the modulation frequency fm1 and the synchronization signal frequency f0 are integer multiples (or nearly integer multiples). Then, as shown in Figure 5, computer 23 controls the optical switches 32 and 34 of the optical delay unit 14 so that the probe light L1 passes through the optical fiber 33b. As a result, the zeroth-order correlation peak P0 appears at a position Z1 different from position Z0 inside the optical fiber characteristic measurement device 1. Then, all correlation peaks P0 to Pn, including the zeroth-order correlation peak P0, are shifted by (Z1 - Z0) in the direction of propagation of the probe light L1.

[0063] After controlling the optical switches 32 and 34 of the optical delay unit 14, the computer 23 controls, for example, the modulation frequency f m Let's assume that we set fm2 and the interval between correlation peaks to dm2. Then, as shown in Figure 5, the k-th correlation peak Pk can be made to appear at position Z12. In this way, when the optical switches 32 and 34 of the optical delay unit 14 are controlled, the modulation frequency f mBy setting this to an appropriate value (modulation frequency fm2 in the example shown in Figure 5), a specific correlation peak (the k-th order correlation peak Pk in the example shown in Figure 5) can be made to appear at its original position (position Z12 in the example shown in Figure 5). Such a modulation frequency fm2 can be determined in advance from the length of the optical fiber FUT under test, etc.

[0064] For the sake of clarity, here we will explain that after the computer 23 controls the optical switches 32 and 34 of the optical delay unit 14, the modulation frequency f m An example of setting the modulation frequency f has been described, but this disclosure is not limited thereto. For example, computer 23 sets the modulation frequency f m After setting the optical switches 32 and 34 of the optical delay unit 14 may be controlled, and the control of the optical switches 32 and 34 of the optical delay unit 14 and the modulation frequency f m You may perform this setting at the same time as the other setting.

[0065] Figure 6 is a diagram illustrating an example of a measurement procedure in one embodiment of the present disclosure. Figure 6(a) shows a case in which the measurement of the optical fiber FUT under test is performed multiple times, and Figure 6(b) shows a case in which the measurement of the optical fiber FUT under test is performed in a single step. In Figure 6, the points labeled m1 to m9 are a plurality of measurement points set along the longitudinal direction of the optical fiber FUT under test.

[0066] In Figure 6, "OK" indicates the modulation frequency f m This means that the measurement was successful because the frequency of the synchronization signal f0 was not an integer multiple (or close to an integer multiple). In Figure 6, "NG" indicates the modulation frequency f m The frequency of the synchronization signal f0 is an integer multiple (or close to an integer multiple), which means that the measurement could not be performed correctly.

[0067] In the measurement procedure shown in Figure 6(a), the computer 23 acquires the measurement results for the measurement points (m1 to m4, m6, m7, m9) from among the measurement points m1 to m9 that were successfully measured during the first measurement of the optical fiber FUT under test. For the measurement points (m5, m8) that were not successfully measured, the computer 23 records that the measurement was not successful at those measurement points (m5, m8).

[0068] Once the first measurement of the optical fiber FUT under test is complete, the computer 23 controls the optical delay unit 14 before the second measurement of the optical fiber FUT under test begins. Once control of the optical delay unit 14 is complete, the computer 23 starts the second measurement of the optical fiber FUT under test. In the second measurement of the optical fiber FUT under test, the computer 23 performs measurements only on the measurement points m1 to m9 that were recorded as not being measured correctly (measurement points m5 and m8). If the measurements performed on the measurement points (measurement points m5 and m8) are successful, the computer 23 terminates the measurement of the optical fiber FUT under test.

[0069] In the measurement procedure shown in Figure 6(b), the computer 23 sequentially measures the measurement points m1 to m9 set on the optical fiber FUT under test. If a measurement is not performed correctly, the computer 23 controls the optical delay unit 14 while the first measurement of the optical fiber FUT is in progress. Once the control of the optical delay unit 14 is complete, the computer 23 performs a remeasurement of the measurement point that was not measured correctly (for example, measurement point m5). If the measurement is performed correctly, the computer 23 proceeds to measure the next measurement point (for example, measurement point m6).

[0070] The measurement procedure shown in Figure 6(a) is used, for example, when the time required to switch the optical delay unit 14 is relatively long, and the measurement procedure shown in Figure 6(b) is used, for example, when the time required to switch the optical delay unit 14 is relatively short. In other words, when the time required to switch the optical delay unit 14 is relatively long, controlling the optical delay unit 14 while the first measurement of the optical fiber FUT under test is being performed will increase the measurement time. For this reason, it is preferable to control the optical delay unit 14 after the first measurement of the optical fiber FUT under test is completed and then perform a second measurement of the optical fiber FUT under test. On the other hand, when the time required to switch the optical delay unit 14 is relatively short, controlling the optical delay unit 14 while the first measurement of the optical fiber FUT under test is being performed will not increase the measurement time. For this reason, it is preferable to control the optical delay unit 14 while performing the measurement during the first measurement of the optical fiber FUT under test.

[0071] As described above, in this embodiment, laser light modulated at a predetermined modulation frequency is emitted from the light source 11, the Brillouin scattered light obtained by injecting the laser light into the optical fiber FUT to be measured is detected by the photodetector 19, and the detection signal output from the photodetector 19 is synchronously detected by the synchronous detector 20. The computer 23 then adjusts the timing of injecting the laser light into the optical fiber FUT to be measured according to the relationship between the modulation frequency of the laser light and the frequency of the synchronization signal used in the synchronous detector 20. This makes it possible to suppress unwanted fluctuation components, thereby achieving higher measurement accuracy than in conventional methods.

[0072] Although an optical fiber characteristic measurement apparatus and optical fiber characteristic measurement method according to embodiments of the present disclosure have been described above, the present disclosure is not limited to the above embodiments and can be freely modified within the scope of the present disclosure. For example, in the above embodiment, the frequency of the laser light that becomes the probe light L1 from the laser light branched by the optical splitter 12 was shifted. However, the frequency of the laser light that becomes the pump light L2 from the laser light branched by the optical splitter 12 may also be shifted.

[0073] Furthermore, in the example shown in Figure 3, we have described an example in which the Brillouin gain spectrum, the Brillouin frequency shift amount, and the characteristics of the optical fiber FUT under test are determined at each measurement point. However, these processes may also be performed all at once after all measurements at the measurement points along the length of the optical fiber FUT under test have been completed (after detection data for all measurement points has been obtained).

[0074] Furthermore, although the above-described embodiment described a BOCDA-type optical fiber characteristic measurement device, it can also be applied to devices other than BOCDA-type optical fiber characteristic measurement devices. Examples of devices other than BOCDA-type optical fiber characteristic measurement devices include BOCDR-type optical fiber characteristic measurement devices.

[0075] 1 Optical fiber characteristic measurement device 11 Light source 12 Optical splitter 13 Optical modulator 14 Optical delay unit 19 Photodetector 20 Synchronous detector 23 Computer 32 Optical switch 33a-33n Optical fiber 34 Optical switch FUT Optical fiber under measurement L1 Probe light L2 Pump light

Claims

1. An optical fiber characteristic measuring apparatus comprising: a light source that emits laser light modulated at a predetermined modulation frequency; a photodetector that detects Brillouin scattered light obtained by injecting the laser light into an optical fiber to be measured; a synchronous detector that performs synchronous detection of the detection signal output from the photodetector; and a control unit that controls the timing of injecting the laser light into the optical fiber to be measured according to the relationship between the predetermined modulation frequency and the frequency of the synchronous signal used in the synchronous detector.

2. The optical fiber characteristic measuring apparatus according to claim 1, wherein the control unit performs control to adjust the timing of injecting the laser light into the optical fiber to be measured when the predetermined modulation frequency and the frequency of the synchronization signal are in an integer multiple relationship.

3. The optical fiber characteristic measuring apparatus according to claim 2, wherein the control unit performs control to adjust the timing of injecting the laser light into the optical fiber to be measured when the predetermined modulation frequency and the frequency of the synchronization signal are in a relationship that is close to an integer multiple, in addition to the case where the predetermined modulation frequency and the frequency of the synchronization signal are in a relationship that is close to an integer multiple.

4. The optical fiber characteristic measuring apparatus according to claim 1, further comprising an optical delay device provided between the light source and the optical fiber to be measured, wherein the control unit adjusts the timing of injecting the laser light into the optical fiber to be measured by controlling the delay amount of the optical delay device.

5. The optical fiber characteristic measuring device according to claim 4, further comprising: a plurality of optical fibers having different lengths; and an optical switch that switches which of the plurality of optical fibers allows the laser light to pass through.

6. The optical fiber characteristic measuring apparatus according to claim 5, further comprising: a first optical switch that has a first port into which the laser light is incident and a plurality of second ports to which one end of the plurality of optical fibers is connected, and which switches the second ports connected to the first port under the control of the control unit; and a second optical switch that has a plurality of third ports to which the other ends of the plurality of optical fibers are connected and a fourth port from which the laser light is emitted, and which switches the third ports connected to the fourth port under the control of the control unit.

7. An optical fiber characteristic measuring device according to claim 1, comprising: an optical splitter that splits the laser light into a first branched beam and a second branched beam; and a shifter that shifts the frequency of the first branched beam or the second branched beam, wherein the photodetector detects Brillouin scattered light obtained by injecting the first branched beam from one end of the optical fiber to be measured and the second branched beam from the other end of the optical fiber to be measured.

8. The optical fiber characteristic measuring apparatus according to claim 1, further comprising a processing unit that obtains a Brillen gain spectrum from the signal obtained by the synchronous detector and measures the characteristics of the optical fiber to be measured based on the Brillen gain spectrum.

9. A method for measuring the characteristics of an optical fiber, comprising: emitting laser light modulated at a predetermined modulation frequency; detecting Brillouin scattered light obtained by injecting the laser light into an optical fiber to be measured; performing synchronous detection of the detection signal obtained by detecting the Brillouin scattered light; and performing control to adjust the timing of injecting the laser light into the optical fiber to be measured according to the relationship between the predetermined modulation frequency and the frequency of the synchronization signal used in the synchronous detection.

10. The optical fiber characteristic measurement method according to claim 9, wherein the control for adjusting the timing is performed between the completion of one measurement of the optical fiber under test and the start of the next measurement of the optical fiber under test.

11. The optical fiber characteristic measurement method according to claim 9, wherein the control for adjusting the timing is performed while one measurement of the optical fiber to be measured is being performed.

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