Measurement device and method for controlling same

Abstract

Provided is a measurement device for measuring characteristics of an optical fiber, said measurement device comprising: a light source unit that emits frequency-modulated light; a first light generation unit that generates first light from the frequency-modulated light; an adjustment unit that is capable of adjusting the optical path length between the first light generation unit and a first end of the optical fiber; a second light generation unit that generates second light from the frequency-modulated light, the second light having a frequency different from that of the first light; a light detection unit that converts third light into an electrical signal, the third light being scattered light generated by causing the first light generated by the first light generation unit to be incident on the first end of the optical fiber through the adjustment unit and causing the second light generated by the second light generation unit to be incident on a second end of the optical fiber; and a calculation unit that calculates the frequency shift of the third light converted into the electric signal with respect to the second light. The adjustment unit includes an optical switch that switches between connection and disconnection of a delay optical path having an optical path length corresponding to the interval between correlation peaks.

Description

Measuring device and its control method

[0001] This disclosure relates to a measuring device and a method for controlling the same.

[0002] A technology is known that uses an optical fiber as a sensor to measure the strain or temperature of the location where the optical fiber is laid, and is related to an optical fiber characteristic measuring device.

[0003] For example, Patent Documents 1 and 2 describe a Brillouin Gain Spectrum (BGS) measuring device equipped with two light sources. Each of the two light sources includes a semiconductor laser and a signal generator. Therefore, the BGS measuring device can have a desired difference (in the range of approximately 10 GHz to approximately 11 GHz) between the frequencies of the light emitted from the two semiconductor lasers. The modulation frequency and phase applied to the semiconductor lasers can also be adjusted for each semiconductor laser in the BGS measuring device. As a result, the BGS measuring device can shift the correlation peak position and measure the BGS without the need for an optical frequency shifter such as a delay optical fiber and an SSB (Single Side Band) modulator.

[0004] Japanese Patent Publication No. 2013-195224 Japanese Patent Publication No. 2013-195225

[0005] The above-described BGS measuring device requires precise setting and sweeping of the frequency difference between two semiconductor lasers (with an accuracy of several MHz or less) in order to accurately measure the peak frequency of the BGS (BFS: Brillouin Frequency Shift). Generally, the frequency of semiconductor lasers is unstable because it fluctuates with the drive current and temperature. The above-described BGS measuring device is equipped with a configuration such as an optical multiplexer to constantly monitor the frequency difference between the two semiconductor lasers and corrects the semiconductor lasers during BGS measurement to achieve the desired frequency difference. As a result, the above-described BGS measuring device has a complex configuration for controlling the semiconductor lasers, which increases manufacturing costs.

[0006] Thus, conventional optical fiber characteristic measurement devices had room for improvement in terms of their complexity of configuration and manufacturing cost.

[0007] The purpose of this disclosure is to realize an optical fiber characteristic measurement device with a simpler configuration.

[0008] Some embodiments of the measuring device include: (1) a measuring device for measuring the characteristics of an optical fiber, comprising: a light source unit that emits frequency-modulated light; a first light generation unit that generates first light from the frequency-modulated light; an adjustment unit connected to the first light generation unit and the first end of the optical fiber, capable of adjusting the optical path length between the first light generation unit and the first end of the optical fiber; a second light generation unit that generates second light having a different frequency from the first light from the frequency-modulated light; a photodetection unit that converts scattered light, which is a third light generated by injecting the first light generated by the first light generation unit into the first end of the optical fiber via the adjustment unit and injecting the second light generated by the second light generation unit into the second end of the optical fiber, into an electrical signal; and a calculation unit that calculates the frequency shift of the third light converted into an electrical signal relative to the second light, wherein the adjustment unit includes an optical switch that switches whether or not to connect a delay optical path having an optical path length corresponding to the interval of correlation peaks, which is the position where the optical frequency difference between the first light and the second light is constant.

[0009] Thus, since the measuring device is equipped with an optical switch that switches the connection of a delay optical path having an optical path length corresponding to the interval of correlation peaks, if a zero-order correlation peak exists near the desired measurement position, the zero-order correlation peak can be moved away by switching the optical switch. Therefore, the measuring device can measure the characteristics at any position in the optical fiber with a simple configuration.

[0010] In one embodiment, the characteristics of the optical fiber may be measured in the measuring device of (2)(1) using the Brillouin optical correlation method.

[0011] Therefore, the measuring device can measure the strain and temperature distribution of the optical fiber.

[0012] In one embodiment, the measuring device of (3)(2) may further include a conversion unit that converts the frequency shift into strain or temperature.

[0013] Therefore, the measuring device can obtain measured values ​​of optical fiber strain and temperature from the frequency shift.

[0014] In one embodiment, in any of the measuring devices (1) to (3) of (4), the adjustment unit may include an optical path as the delay optical path having an optical path length four times the minimum value of the interval between correlation peaks.

[0015] Thus, the measuring device can be miniaturized because it is equipped with an optical path having a path length four times the minimum value of the interval between correlation peaks as a delay optical path.

[0016] In one embodiment, any of the measuring devices (5)(1) to (4) may further include a control unit for controlling the frequency of the light emitted from the light source.

[0017] In this way, the measuring device can change the position of correlation peaks other than the zeroth order by controlling the frequency of light, and measure the characteristics of the optical fiber at a desired position.

[0018] A control method for a measuring device according to several embodiments includes: (6) a control method for a measuring device for measuring the characteristics of an optical fiber, comprising a light source unit, a first light generation unit, an adjustment unit, a second light generation unit, a photodetector unit, and a calculation unit, wherein the light source unit emits frequency-modulated light; the first light generation unit generates first light from the frequency-modulated light; the adjustment unit is connected to the first light generation unit and the first end of the optical fiber and adjusts the optical path length between the first light generation unit and the first end of the optical fiber; the second light generation unit generates second light having a different frequency from the first light from the frequency-modulated light; the photodetector unit converts scattered light, which is generated by injecting the first light generated by the first light generation unit into the first end of the optical fiber via the adjustment unit, and injecting the second light generated by the second light generation unit into the second end of the optical fiber, into an electrical signal; and the calculation unit calculates the frequency shift of the third light converted into an electrical signal relative to the second light. The adjustment unit adjusts the optical path length by using an optical switch to switch between connecting or not connecting a delay optical path having an optical path length corresponding to the interval of correlation peaks, which is the position where the optical frequency difference between the first light and the second light is constant.

[0019] Thus, since the measuring device is equipped with an optical switch that switches the connection of a delay optical path having an optical path length corresponding to the interval of correlation peaks, if a zero-order correlation peak exists near the desired measurement position, the zero-order correlation peak can be moved away by switching the optical switch. Therefore, the measuring device can measure the characteristics at any position in the optical fiber with a simple configuration.

[0020] In one embodiment, in the control method of the measuring device of (7)(6), the characteristics of the optical fiber may be measured using the Brillouin optical correlation domain method.

[0021] Therefore, the strain and temperature distribution of the optical fiber can be measured.

[0022] In one embodiment, in the control method of the measuring device of (8)(7), the conversion unit may convert the frequency shift into distortion or temperature.

[0023] Therefore, from the frequency shift, measurements of optical fiber strain and temperature can be obtained.

[0024] In one embodiment, in the control method for any of the measuring devices described in (9), (6) to (8), the adjustment unit may include an optical path as the delay optical path having an optical path length four times the minimum value of the interval between the correlation peaks.

[0025] Thus, by providing an optical path with a path length four times the minimum value of the interval between correlation peaks as the delay optical path, miniaturization is possible.

[0026] In one embodiment, in the control method for any of the measuring devices described in (10), (6) to (9), the control unit may control the frequency of the light emitted from the light source unit.

[0027] In this way, by controlling the frequency of light, the position of correlation peaks other than the zeroth order can be changed, and the characteristics of the optical fiber at a desired position can be measured.

[0028] According to one embodiment of this disclosure, an optical fiber characteristic measurement device can be realized with a simpler configuration.

[0029] This figure shows an example configuration of a measurement system according to one embodiment. This figure illustrates the measurement of the Brillouin gain spectrum. This figure illustrates the probe light and the correlation peak generated by the probe light. This figure shows an example configuration of the peak adjustment unit in Figure 1. This figure illustrates the correlation peak shift by the measurement device according to one embodiment. This figure illustrates the correlation peak shift by the measurement device according to one embodiment. This figure shows the configuration of the peak adjustment unit according to a comparative example. This figure illustrates the correlation peak shift by the measurement device according to a comparative example.

[0030] <Embodiment> Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In each drawing, parts having the same configuration or function are denoted by the same reference numerals. In the description of this embodiment, redundant descriptions of the same parts may be omitted or simplified as appropriate.

[0031] Figure 1 shows an example of the configuration of a measurement system 1 according to one embodiment. The measurement system 1 includes a measuring device 10 and an optical fiber 50.

[0032] The measuring device 10 is an optical fiber characteristic measuring device that uses an optical fiber 50 as a sensor to measure characteristics such as strain or temperature from changes in the physical quantities of the optical fiber 50. The measuring device 10 can also calculate the vibration of the optical fiber 50 from the temporal change in strain. By attaching or embedding the optical fiber 50 in a structure or the like and then measuring the changes in the physical quantities of the optical fiber 50, the measuring device 10 can determine the soundness (health condition) of the structure.

[0033] The measuring device 10 measures the characteristics of the optical fiber 50 using the Brillouin Optical Correlation Domain Analysis (BOCDA) method. The measuring device 10 measures the characteristics of the optical fiber 50 by injecting frequency-modulated probe light from one end of the optical fiber 50 while injecting frequency-modulated pump light from the other end. Here, at the correlation peak position, which is the position where the optical frequency difference between the probe light and the pump light is constant, the probe light gains due to stimulated Brillouin scattering.

[0034] Figure 2 illustrates the measurement of the Brillouin gain spectrum. The frequency difference ν between the pump light and the probe light. B Sweep the frequency difference ν B When the level of the probe light relative to the optical fiber is measured as the gain, a spectrum with the shape of a Lorentz function centered on the Brillouin frequency shift (BFS), a physical quantity of the optical fiber, is obtained. This spectrum is called the Brillouin gain spectrum (BGS).

[0035] The BFS depends on the material, strain, and temperature of the optical fiber 50, and is known to respond linearly to strain and temperature in particular. Therefore, the measuring device 10 can detect the relative strain or temperature at the correlation peak position by measuring the BFS in the optical fiber 50. The measuring device 10 can measure the relative strain or temperature at the measurement position by moving the correlation peak to the position to be measured in the optical fiber 50 (the optical fiber to be measured) and measuring the BFS. By repeating such processing, the measuring device 10 can measure the strain distribution or temperature distribution over the longitudinal direction of the optical fiber 50 (the optical fiber to be measured).

[0036] Note that although the BFS depends on both strain and temperature, in order to separate these contributions, the measuring device 10 may measure an unknown quantity based on the value of the BFS using a known quantity of strain and temperature. For example, when using the optical fiber 50 as a strain sensor, the measuring device 10 measures the BFS at the measurement position of the optical fiber 50 in a free state without strain in the optical fiber 50. Next, the measuring device 10 measures the BFS at the same measurement position with the optical fiber 50 attached to the measurement object. The measuring device 10 may measure the strain of the object based on the difference between these measured values of the BFS.

[0037] As shown in FIG. 1, the measuring device 10 includes a light source unit 11, a pump light generation unit 12, a probe light generation unit 13, an optical circulator 14, a peak adjustment unit 15, a light detection unit 16, a control unit 17, an amplifier 18, a BFS calculation unit 19, a setting unit 20, a measurement position list 21, a BFS distribution data generation unit 22, a conversion unit 23, a storage unit 24, a display unit 25, and a timing unit 26.

[0038] The light source unit 11 frequency-modulates the laser light output from the semiconductor laser and emits it to the pump light generation unit 12 and the probe light generation unit 13. The light source unit 11 may include at least any one of a semiconductor laser, a temperature control circuit, a drive circuit, an optical pulse generation circuit, a frequency modulation circuit, a signal generator, a directional coupler (optical coupler), etc. Such a configuration is an example, and the light source unit 11 may be any configuration capable of generating light.

[0039] The pump light generation unit 12 as the second light generation unit outputs pump light as the second light to the optical circulator 14 by amplifying the frequency-modulated light incident from the light source unit 11. The pump light generation unit 12 may include at least any one of a polarization controller, an optical switch, an optical isolator, an optical amplifier, an optical frequency shifter, an optical attenuator, a signal generator, etc. The optical frequency shifter may be configured using an SSB modulator or the like. Such a configuration is an example, and the pump light generation unit 12 may have any configuration capable of generating pump light. The pump light generation unit 12 is connected to the second end of the optical fiber 50 via the optical circulator 14.

[0040] The probe light generation unit 13 as the first light generation unit generates probe light as the first light from the frequency-modulated light incident from the light source unit 11. The probe light generation unit 13 emits the generated probe light to the peak adjustment unit 15.

[0041] The probe light is light having a frequency and amplitude different from those of the pump light. The probe light generation unit 13 shifts the frequency of the frequency-modulated light incident from the light source unit 11 and amplifies the amplitude to generate the probe light. When shifting the frequency of the frequency-modulated light incident from the light source unit 11, the probe light generation unit 13 can adjust the shift amount.

[0042] The probe light generation unit 13 may include, for example, an optical frequency shifter and an optical amplifier. The optical frequency shifter shifts the frequency of the frequency-modulated light incident from the light source unit 11. The optical frequency shifter may be, for example, an SSB modulator. The optical amplifier amplifies the amplitude of the frequency-modulated light incident from the light source unit 11. The probe light generation unit 13 may further include at least any one of a polarization controller, an optical switch, an optical isolator, an optical attenuator, a signal generator, etc. Such a configuration is an example, and the probe light generation unit 13 may have any configuration capable of generating probe light. The probe light is incident on the optical fiber 50 through the peak adjustment unit 15. The probe light generation unit 13 is connected to the first end of the optical fiber 50 via the peak adjustment unit 15.

[0043] The optical circulator 14 directs the pump light, which is incident from the pump light generation unit 12, into the optical fiber 50. As a result, the pump light is incident on one end of the optical fiber 50, and the probe light is incident on the other end of the optical fiber 50. This causes scattered light due to stimulated Brillouin scattering to be locally generated in the optical fiber 50.

[0044] The optical circulator 14 emits scattered light incident from the optical fiber 50 to the photodetector 16. The optical circulator 14 may also be equipped with a directional coupler (optical coupler).

[0045] The peak adjustment unit 15 adjusts the correlation peak position, which is the position where the optical frequency difference between the probe light and the pump light is constant, by delaying the probe light incident from the probe light generation unit 13. The peak adjustment unit 15 has any configuration to adjust the optical path length of the probe light. For example, the peak adjustment unit 15 may include a delay optical fiber and a phase adjuster. Alternatively, the peak adjustment unit 15 may include a spatial optical system.

[0046] The position of the correlation peak in the optical fiber 50 (optical fiber under measurement) can be moved by changing the modulation frequency applied to the semiconductor laser of the light source unit 11. However, as will be described later, the position of the zeroth-order correlation peak does not move even if the modulation frequency is changed. Therefore, the measuring device 10 is equipped with a peak adjustment unit 15 for adjusting the optical path length of the probe light so that there is no zeroth-order correlation peak in the measurement range of the optical fiber 50 (optical fiber under measurement).

[0047] The light detection unit 16 converts the light incident from the light circulator 14 into an electrical signal. The light detection unit 16 may include a photodiode for converting the incident light into an electrical signal. The photodiode may have any configuration, such as an avalanche photodiode, a PIN type photodiode, or a differential balanced photodiode incorporating two photodiodes with matched characteristics. The light detection unit 16 may further include an optical attenuator, a trans-impedance amplifier circuit, etc. Such a configuration is just an example, and the light detection unit 16 may have any configuration for converting incident light into an electrical signal.

[0048] The control unit 17, based on commands from the setting unit 28, calculates each setting parameter so that the measuring device 10 can perform BFS distribution measurement according to the measurement conditions, and controls the light source unit 11, the pump light generation unit 12, the probe light generation unit 13, the photodetection unit 16, and the amplifier 18. The control unit 17 may also output drive signals to the pump light generation unit 12, the probe light generation unit 13, the photodetection unit 16, and the amplifier 18. The control unit 17 also performs processing to ensure that the measuring device 10 operates normally, such as starting and stopping the measurement. The relationship between the measurement position and the modulation frequency is calculated by the control unit 17.

[0049] The BFS calculation unit 19 analyzes the power spectrum of the Brillouin scattered light returning from each position of the optical fiber 50 and calculates the peak frequency (BFS). The BFS calculation unit 19 extracts the frequency of the Brillouin frequency shift at which the Brillouin gain peaks. Since the frequency of the Brillouin frequency shift at which the Brillouin gain peaks depends on the strain and temperature of the optical fiber 50, the BFS calculation unit 19 can measure the strain or temperature of the optical fiber 101 by extracting the frequency of the Brillouin frequency shift at which the Brillouin gain peaks. The BFS calculation unit 19 may also perform spectral analysis using a spectrum analyzer, oscilloscope, voltmeter, etc.

[0050] Furthermore, the location in the optical fiber 50 at which scattered light due to stimulated Brillouin scattering is generated depends on the modulation frequency of the frequency-modulated light emitted by the light source unit 11. Therefore, by sweeping and measuring the modulation frequency, the measuring device 10 can measure the strain or temperature distribution of the optical fiber 50.

[0051] The control unit 17 includes at least one processor, at least one dedicated circuit, or a combination thereof. The processor is a general-purpose processor such as a CPU (Central Processing Unit) or GPU (Graphics Processing Unit), or a dedicated processor specialized for a specific process. The dedicated circuit is, for example, an FPGA (Field-Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

[0052] The amplifier 18 amplifies the scattered light, which has been converted into an electrical signal and supplied from the photodetector 16. The amplifier 18 outputs the amplified electrical signal of the scattered light to the BFS calculation unit 19. The amplifier 18 may be an electronic circuit including an operational amplifier, a resistor, a capacitor, etc. The amplifier 18 may further include a lock-in amplifier, a noise filter, etc. This configuration is just one example, and the amplifier 18 may be any configuration capable of amplifying an electrical signal based on scattered light.

[0053] The setting unit 20 accepts settings for various measurement conditions, such as spatial resolution, measurement distance range, measurement position step (distance sampling), measurement speed, and number of averaging cycles.

[0054] The measurement position list 21 stores a list of measurement positions set in the measurement condition setting unit in a memory or other storage device.

[0055] The BFS distribution data generation unit 22 associates the BFS calculated by the BFS calculation unit 19 with the corresponding measurement location.

[0056] The conversion unit 23 converts BFS into strain or temperature using a pre-set BFS-temperature dependence or BFS-strain dependence relationship. Through processing by the conversion unit 23, the measurement result of the BFS distribution can be expressed as a strain distribution or a temperature distribution.

[0057] The storage unit 24 stores various data such as Brillouin scattered light spectrum data for each measurement location, BFS, converted strain, converted temperature, measurement date and time, and measurement conditions. The storage unit 24 may be equipped with a storage device such as an HDD (Hard Disk Drive), SSD (Solid State Drive), or USB (Universal Serial Bus) memory.

[0058] The display unit 25 displays the measured strain distribution or temperature distribution measurement results on the measurement screen. The display unit 25 may be composed of, for example, a liquid crystal display or an organic EL display device.

[0059] The timing unit 26 measures the date and time of measurement. The timing unit 26 may be composed of any clock.

[0060] The optical fiber 50 is the optical fiber under test for measuring strain or temperature. The optical fiber 50 is composed of single-mode fiber, multimode fiber, multicore fiber, etc. The type of sheathing to protect the optical fiber 50 from damage may be appropriately selected depending on the environment in which the optical fiber 50 is laid.

[0061] Figure 3 illustrates the probe light and the correlation peaks generated by the probe light. Figure 3 shows how the correlation peaks shift when the modulation frequency of the signal generator in the light source unit 11 is changed. Figure 3 also shows an example of the correlation peak distribution when the optical path length has not been adjusted by the peak adjustment unit 15. When the modulation frequency fm of the signal generator is changed, the interval dm of the correlation peaks changes and the correlation peaks shift. In the example in Figure 3, when the modulation frequency is changed from fm0 to fm1, the interval of the nth-order (n is an integer other than 0) correlation peaks changes from dm0 to dm1. However, the 0th-order correlation peak does not shift even when the modulation frequency is changed. When continuously measuring BFS along the optical fiber 50 (optical fiber under measurement) (distribution measurement), the displacement distance of the correlation peaks must be less than or equal to the spatial resolution of the measuring device 10. This is because if the displacement distance of the correlation peaks becomes greater than the spatial resolution, it becomes impossible to continuously measure the characteristics. Therefore, the minimum modulation frequency resolution that can be varied by the signal generator of the light source unit 11 must be an amount that can sufficiently reduce the displacement distance of the correlation peak with respect to the spatial resolution.

[0062] Next, the details of the procedure for calculating the measurement position performed by the control unit 17 will be explained. The interval dm of the correlation peaks is given by the speed of light c and the refractive index n of the optical fiber 50. ref It is known that the frequency is given by the following equation, depending on the modulation frequency fm of the light source unit 11: dm = c / (2 × fm × n ref ) (Formula 1)

[0063] Therefore, if the correlation peak is one in which the optical path difference between the pump light and the probe light is not zero (zero-order correlation peak), then by adjusting the modulation frequency fm that modulates the light source unit 11 (Equation 1), the correlation peak interval dm can be changed and the correlation peak position can be moved (see Figure 3).

[0064] The 0th-order correlation peak where the optical path difference between the pump light and the probe light is zero exists at the midpoint of a series of optical paths from the end point where the probe light is emitted, through the optical fiber 50, to the end point where the pump light is emitted. That is, if the conditions shown in (Equation 2) or in (Equation 3) are satisfied, there is no 0th-order correlation peak in optical fiber 50. L1 > L2 + L3 (Equation 2)L1 + L2 < L3 (Equation 3) Here, as shown in FIG. 3, L1 is the length (distance) of the optical fiber from the end point where the probe light is emitted to the incident end (0 m position) of the optical fiber 50 for the probe light. L2 is the length (distance) of the optical fiber 50. L3 is the length (distance) of the optical fiber from the end point where the pump light is emitted to the incident end of the optical fiber 50 for the pump light.

[0065] The length (distance) L from the end point where the probe light is emitted to the 0th-order correlation peak position Z is given by (Equation 4). L z = (L1 + L2 + L3) / 2 (Equation 4)

[0066] Note that (Equations 2) to (4) are premised on the refractive indices of the optical fiber from the end point where the probe light is emitted to the incident end (0 m position) of the optical fiber 50 for the probe light, the refractive index of the optical fiber 50, and the refractive index of the optical fiber from the end point where the pump light is emitted to the incident end of the optical fiber 50 for the pump light being the same, which is n. ref

[0067] The measurement position can be expressed by (Equation 5) using the correlation peak interval dm, the order n of the correlation peak (n is a natural number), and L1, L Z . Measurement position = dm × n - (L1 - L2) (Equation 5)

[0068] From (Equation 1) and (Equation 5), the measurement position is expressed by (Equation 6) using the frequency fm, the speed of light c, and the refractive index n of the optical fiber 50. Measurement position = c × n / (2 × fm × n ref ) - (L1 - L2) (Equation 6) ref

[0069] The measuring device 10 can measure BFS by setting an arbitrary measurement position based on the modulation frequency and correlation peak order according to (Equation 6). For example, by setting the modulation frequency to fm0 and the order to n, the measuring device 10 can measure the BFS at position A in Figure 3. Similarly, by setting the modulation frequency to fm1 and the order to n, the measuring device 10 can measure the BFS at position B. Furthermore, by setting the modulation frequency fm and the correlation peak order n according to the measurement position, the measuring device 10 can also measure the distribution of BFS in the optical fiber 50. Positions A and B are expressed by the following (Equations 7) and (Equations 8): Position A = c × n / (2 × fm0 × n) ref ) - (L1 - L2) (Formula 7) B position = c x n / (2 x fm1 x n ref )-(L1-L2) (Formula 8)

[0070] In this way, the measuring device 10 can move the correlation peak to a desired position in the optical fiber 50 (optical fiber under test) and measure the BFS, thereby measuring the relative strain, temperature, or vibration at that measurement position. Furthermore, by repeating this process, the measuring device 10 can measure the strain distribution or temperature distribution along the longitudinal direction of the optical fiber 50 (optical fiber under test).

[0071] However, as shown in Figure 3, if there is a zero-order correlation peak indicated by the Z position in the optical fiber 50 (optical fiber under measurement), the measuring device 10 cannot measure the strain or temperature near the zero-order correlation peak position (in the range of less than ±dm). There are two possible reasons why measurement becomes impossible not only at the zero-order correlation peak position (Z position) but also in the vicinity of it. The first reason is that as the modulation frequency increases, dm becomes shorter, so the range of less than ±dm can be narrowed, but there is an upper limit to the range in which the modulation frequency can be varied, so there is a lower limit to the dm that can be set. The second reason is that as dm becomes shorter, multiple correlation peak positions will exist within the optical fiber 50 (optical fiber under measurement), making it difficult to separate the measured values ​​at each correlation peak position.

[0072] Therefore, the measuring device 10 according to this embodiment moves the zero-order correlation peak position by adjusting the length L1 with the peak adjustment unit 15. As a result, even if the zero-order correlation peak position exists within the optical fiber 50, the measuring device 10 can measure the BFS at any position in the optical fiber 50 by moving the zero-order correlation peak position with the peak adjustment unit 15.

[0073] As the peak adjustment unit 15, it is also conceivable to use an optical fiber having a length greater than or equal to the length of the optical fiber 50 (optical fiber under measurement), as shown in the comparative example described later with reference to Figures 7 and 8. With such a configuration, the position of the zero-order correlation peak that was located in the optical fiber 50 can be moved to the probe light generation side in the measuring device 10. However, the optical fiber 50 may have a length of several hundred meters or more, or several kilometers or more, and accordingly, the length of the optical fiber in the peak adjustment unit 15 must also be several hundred meters or more, or several kilometers or more. Thus, with the configuration of the comparative example, as the length (distance range) of the optical fiber 50 increases, the device becomes larger and the manufacturing cost also increases.

[0074] Therefore, the measuring device 10 according to this embodiment includes a peak adjustment unit 15 that allows switching via an optical switch between connecting a delay optical path of a certain length (approximately several tens of meters or more) instead of an optical fiber longer than the length of the optical fiber 50. When a zero-order correlation peak position exists near a desired measurement position within the optical fiber 50, the measuring device 10 moves the zero-order correlation peak position away from the measurement position by switching the optical switch. As a result, the measuring device 10 makes it possible to measure the BFS at any position in the optical fiber 50 and measure strain, temperature, or vibration with a configuration that is small in size and has low manufacturing costs.

[0075] Figure 4 shows an example of the configuration of the peak adjustment unit 15 in Figure 1. The peak adjustment unit 15 comprises two optical switches 151 (151a, 151b), an optical path 152, and a delay optical path 153.

[0076] The optical switches 151 (151a, 151b) switch the optical path connected between the probe light generation unit 13 and the optical fiber 50 between the optical path 152 and the delay optical path 153. In the example in Figure 4, the optical switches 151 (151a, 151b) are 1x2 switches, but any NxM switches such as 2x2 switches may also be used.

[0077] The optical path 152 has the shortest possible optical path length to guide the light incident from the probe light generation unit 13 to the optical fiber 50. The optical path 152 may be an optical fiber including a single-mode fiber such as a polarization-maintaining fiber.

[0078] The delay optical path 153 has the minimum optical path length required to enable BFS measurement at a desired position within the optical fiber 50. When the length L1 increases by d, the length (distance) L from the probe light emission end to the zero-order correlation peak position increases according to (Equation 4). Z It moves by d / 2. Therefore, the optical path length of the delay optical path 153 is, for example, the minimum value dm of the correlation peak interval dm. MIN Based on, 4 × dm MIN This may be considered as above. Here, dm MIN This is determined by (Equation 1) based on the range of possible values ​​for the modulation frequency fm of the light source unit 11. Therefore, the optical path length of the delay optical path 153 is determined by whether the modulation frequency fm that moves the correlation peak takes a value around 10 MHz, around 20 MHz, or around 5 MHz. For example, the optical path length of the delay optical path 153 is 4 × dm MIN Therefore, it would be around several tens of meters.

[0079] The delay optical path 153 can be implemented by any configuration having a constant optical path length. For example, the delay optical path 153 may be an optical fiber including a single-mode fiber such as a polarization-maintaining fiber. The delay optical path 153 may also be empty space. The delay optical path 153 may be made of a material with a higher refractive index than an optical fiber.

[0080] When the measuring device 10 measures the BFS over the entire length of the optical fiber 50, it switches the optical switch 151 (151a, 151b) of the peak adjustment unit 15 once during each measurement, at the timing when measuring the range near the zeroth order that cannot be measured. Therefore, it is sufficient for the optical switch 151 (151a, 151b) to have a switching speed of the order of msec.

[0081] Figures 5 and 6 illustrate the correlation peak shift in a measuring device 10 according to one embodiment. In the example of Figures 5 and 6, regardless of which state the optical switch 151 of the peak adjustment unit 15 is switched to, the optical path length in the peak adjustment unit 15 is shorter than the optical fiber 50, and the zero-order correlation peak position is located within the optical fiber 50.

[0082] Figure 5 shows the state in which the optical switches 151 (151a, 151b) of the peak adjustment unit 15 are not connected to the delay optical path 153 between the optical fiber 50 and the probe light generation unit 13. In Figure 5, in the A-B section of the optical fiber 50, the zero-order correlation peak does not move even when the modulation frequency is changed. As a result, in the state shown in Figure 5, the measuring device 10 cannot measure the strain and temperature in the A-B section. Therefore, when measuring the A-B section, the measuring device 10 switches the optical switches 151 (151a, 151b) to connect to the delay optical path 153.

[0083] Figure 6 shows the state in which the optical switches 151 (151a, 151b) of the peak adjustment unit 15 connect the delay optical path 153 between the optical fiber 50 and the probe light generation unit 13. In the example of Figure 6, the optical path length of the delay optical path 153 is 4 × dm MIN That concludes the explanation. In Figure 6, when the modulation frequency of the light source unit 11 is fm0, the primary correlation peak is located at position A. The measuring device 10 can measure strain or temperature in the AB section by changing the modulation frequency fm and shifting the primary or secondary correlation peak.

[0084] Thus, according to the measuring device 10 of this embodiment, it is possible to measure the strain or temperature at any position of the optical fiber 50, regardless of the length of the optical fiber 50, due to its compact size and low manufacturing cost.

[0085] (Comparative Example) Figure 7 shows the configuration of the peak adjustment unit 15z according to the comparative example. The peak adjustment unit 15z includes a delay optical fiber 159z having a length equal to or greater than the length of the optical fiber 50z.

[0086] Figure 8 illustrates the correlation peak shift in the measuring device 10z according to the comparative example. As shown in Figure 8, in the configuration of the comparative example, since the delay optical fiber 159z has a length greater than or equal to the length of the optical fiber 50z, the zero-order correlation peak is located outside the optical fiber 50. As a result, the measuring device 10z according to the comparative example can measure strain or temperature at any position in the optical fiber 50.

[0087] However, the measuring device 10z in the comparative example has a delay optical fiber 159z as the peak adjustment unit 15z, which has a length greater than or equal to the length of the optical fiber 50z, making it large and costly to manufacture. In contrast, the delay optical path 153 in the peak adjustment unit 15 of the measuring device 10 according to this embodiment only needs to be of a length corresponding to the correlation peak interval dm. Therefore, the measuring device 10 according to this embodiment can be manufactured in a small size and at low cost.

[0088] This disclosure is not limited to the embodiments described above. For example, multiple blocks shown in the block diagram may be merged, or a single block may be divided. Other modifications are possible without departing from the spirit of this disclosure.

[0089] 1: Measurement system 10: Measurement device 10z: Measurement device (comparative example) 11: Light source unit 12: Pump light generation unit 13: Probe light generation unit 14: Optical circulator 15: Peak adjustment unit 15z: Peak adjustment unit (comparative example) 151a, 151b: Optical switch 152: Optical path 153: Delay optical path 159z: Delay optical fiber (comparative example) 16: Photodetection unit 17: Control unit 18: Amplifier 19: BFS calculation unit 20: Setting unit 21: Measurement position list 22: BFS distribution data generation unit 23: Conversion unit 24: Storage unit 25: Display unit 26: Timing unit 50: Optical fiber 50z: Optical fiber (comparative example)

Claims

1. A measuring device for measuring the characteristics of an optical fiber, comprising: a light source unit that emits frequency-modulated light; a first light generation unit that generates first light from the frequency-modulated light; an adjustment unit connected to the first light generation unit and the first end of the optical fiber, capable of adjusting the optical path length between the first light generation unit and the first end of the optical fiber; a second light generation unit that generates second light having a different frequency from the first light from the frequency-modulated light; a photodetection unit that converts scattered light, which is a third light generated by injecting the first light generated by the first light generation unit into the first end of the optical fiber via the adjustment unit and injecting the second light generated by the second light generation unit into the second end of the optical fiber, into an electrical signal; and a calculation unit that calculates the frequency shift of the third light converted into an electrical signal relative to the second light, wherein the adjustment unit is equipped with an optical switch that switches whether or not to connect a delay optical path having an optical path length corresponding to the interval of correlation peaks, which are positions where the optical frequency difference between the first light and the second light is constant.

2. The measuring apparatus according to claim 1, which measures the characteristics of the optical fiber using the Brillouin optical correlation domain method.

3. The measuring device according to claim 2, further comprising a conversion unit that converts the frequency shift into distortion or temperature.

4. The measuring device according to any one of claims 1 to 3, wherein the adjustment unit includes an optical path having an optical path length four times the minimum value of the interval between correlation peaks as the delay optical path.

5. The measuring device according to any one of claims 1 to 3, further comprising a control unit for controlling the frequency of the light emitted from the light source unit.

6. A control method for a measuring device for measuring the characteristics of an optical fiber, comprising a light source unit, a first light generation unit, an adjustment unit, a second light generation unit, a photodetector unit, and a calculation unit, the method comprising: the light source unit emitting frequency-modulated light; the first light generation unit generating first light from the frequency-modulated light; the adjustment unit connecting to the first light generation unit and the first end of the optical fiber and adjusting the optical path length between the first light generation unit and the first end of the optical fiber; the second light generation unit generating second light having a different frequency from the first light from the frequency-modulated light; the photodetector unit converting scattered light, which is generated by injecting the first light generated by the first light generation unit into the first end of the optical fiber via the adjustment unit and the second light generated by the second light generation unit into the second end of the optical fiber, into an electrical signal; and the calculation unit calculating the frequency shift of the third light converted into an electrical signal relative to the second light. A control method for a measuring device, wherein the adjustment unit adjusts the optical path length by using an optical switch to switch whether or not to connect a delay optical path having an optical path length corresponding to the interval of correlation peaks, which is the position where the optical frequency difference between the first light and the second light is constant.

7. A control method for the measuring apparatus according to claim 6, wherein the characteristics of the optical fiber are measured using the Brillouin optical correlation domain method.

8. A control method for a measuring device according to claim 7, wherein the conversion unit converts the frequency shift into distortion or temperature.

9. The control method for a measuring device according to any one of claims 6 to 8, wherein the adjustment unit includes an optical path having an optical path length four times the minimum value of the interval between correlation peaks as the delay optical path.

10. A control method for a measuring device according to any one of claims 6 to 8, wherein the control unit controls the frequency of the light emitted from the light source unit.

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