Optical fiber characteristic measurement device and optical fiber characteristic measurement method

Abstract

An optical fiber characteristic measurement device 100 according to the present disclosure comprises: a light source unit 102 that emits frequency-modulated light; a first light generation unit 103 that generates first light from the frequency-modulated light; a second light generation unit 104 that generates second light from the frequency-modulated light; a polarization generation unit 105 that generates two orthogonal linearly polarized light beams from the second light; a polarization splitting unit 108 that splits two orthogonal scattered light beams, which are generated by allowing the first light to enter one end of an optical fiber 101 and allowing the two linearly polarized light beams to enter the other end of the optical fiber 101, into scattered light in a first polarization direction and scattered light in a second polarization direction; a light detection unit that converts the scattered light in the first polarization direction and the scattered light in the second polarization direction into electrical signals; and an arithmetic unit 111 that combines the scattered light in the first polarization direction, which has been converted into an electrical signal, and the scattered light in the second polarization direction, which has been converted into an electrical signal.

Description

Optical fiber characteristic measuring device and optical fiber characteristic measuring method CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Japanese Patent Application No. 2024-038268, filed on March 12, 2024, the entire disclosure of which is incorporated herein by reference.

[0002] The present disclosure relates to an optical fiber characteristic measuring device and an optical fiber characteristic measuring method.

[0003] 2. Description of the Related Art Conventionally, there is known a technique for using an optical fiber as a sensor to measure strain and temperature at a location where the optical fiber is laid.

[0004] For example, Non-Patent Document 1 discloses an optical fiber characteristic measuring device that measures the characteristics of an optical fiber used as a sensor by Brillouin Optical Correlation Domain Analysis (BOCDA).

[0005] Kazuo Hotate, Koji Abe and Kwang Yong Song, “Suppression of Signal Fluctuation in Brillouin Optical Correlation Domain Analysis System Using Polarization Diversity Scheme”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 24, DECEMBER 15, 2006, p.2653-2655

[0006] When measuring the characteristics of an optical fiber using the BOCDA method, stimulated Brillouin scattering is locally generated by injecting pump light into one end of the optical fiber and probe light into the other end of the optical fiber, and the generated scattered light is measured.

[0007] In this case, the efficiency of stimulated Brillouin scattering depends on the polarization states of the probe and pump lights, which easily fluctuate, making it difficult to stably measure the scattered light due to stimulated Brillouin scattering.

[0008] In order to stably measure scattered light due to stimulated Brillouin scattering, Non-Patent Document 1 discloses a technique in which the polarization state of probe light is switched over time between two orthogonal linearly polarized lights, and the scattered light measured with each linearly polarized light is combined.

[0009] However, the technique described in Non-Patent Document 1 has the problem that the measurement time is doubled because two linearly polarized lights are switched over in time and measurements are performed twice. Also, the technique described in Non-Patent Document 1 has the problem that stable measurements cannot be performed if the installation environment of the optical fiber changes while the two linearly polarized lights are being switched over in time and the polarization state changes.

[0010] Therefore, an object of the present disclosure is to provide an optical fiber characteristic measuring device and an optical fiber characteristic measuring method that are capable of stably measuring the characteristics of an optical fiber in a short period of time.

[0011] According to some embodiments, an optical fiber characteristic measurement device for measuring characteristics of an optical fiber includes: a light source unit that emits frequency-modulated light; a first light generation unit that generates a first light from the frequency-modulated light; a second light generation unit that generates a second light from the frequency-modulated light, the second light having a frequency different from that of the first light; a polarization generation unit that generates two orthogonal linearly polarized lights from the second light; a polarization separation unit that separates two orthogonal scattered lights generated by inputting the first light into one end of the optical fiber and inputting the two linearly polarized lights into the other end of the optical fiber into scattered light in a first polarization direction and scattered light in a second polarization direction; a light detection unit that converts the scattered light in the first polarization direction and the scattered light in the second polarization direction into electrical signals; and a calculation unit that combines the scattered light in the first polarization direction converted into electrical signals and the scattered light in the second polarization direction converted into electrical signals. This optical fiber characteristic measurement device enables stable measurement of optical fiber characteristics in a short time.

[0012] In one embodiment, the optical fiber characteristic measurement device may measure the characteristics of the optical fiber using Brillouin optical correlation domain analysis, thereby measuring the strain and temperature distribution of the optical fiber.

[0013] In one embodiment of the optical fiber characteristic measuring device, the polarization generation unit may include a first polarization-maintaining optical fiber into which the second light is incident and a second polarization-maintaining optical fiber that outputs the two orthogonal linearly polarized light beams, and the first polarization-maintaining optical fiber and the second polarization-maintaining optical fiber may be connected such that the polarization axis of the first polarization-maintaining optical fiber and the polarization axis of the second polarization-maintaining optical fiber are inclined by 45 degrees. This makes it possible to generate two orthogonal linearly polarized light beams with a simple configuration.

[0014] In one embodiment, the polarization generation unit may include a first optical coupler into which the second light is incident and a second optical coupler that outputs the two orthogonal linearly polarized light beams, the first optical coupler and the second optical coupler being connected by a first optical path and a second optical path, the first optical path including a first polarization-maintaining optical fiber and a second polarization-maintaining optical fiber, the first polarization-maintaining optical fiber and the second polarization-maintaining optical fiber being connected such that the polarization axis of the first polarization-maintaining optical fiber and the polarization axis of the second polarization-maintaining optical fiber are aligned, and the second optical path including a third polarization-maintaining optical fiber and a fourth polarization-maintaining optical fiber, the third polarization-maintaining optical fiber and the fourth polarization-maintaining optical fiber being connected such that the polarization axis of the third polarization-maintaining optical fiber and the polarization axis of the fourth polarization-maintaining optical fiber are tilted by 90 degrees. This makes it possible to generate two orthogonal linearly polarized light beams with a simple configuration.

[0015] In one embodiment, the polarization generation unit may include an optical coupler into which the second light is incident and a polarization beam combiner that outputs the two orthogonal linearly polarized light beams, the optical coupler and the polarization beam combiner being connected by a first optical path and a second optical path, the first optical path including a first polarization-maintaining optical fiber and a second polarization-maintaining optical fiber, the first polarization-maintaining optical fiber and the second polarization-maintaining optical fiber being connected such that the polarization axis of the first polarization-maintaining optical fiber and the polarization axis of the second polarization-maintaining optical fiber are aligned, and the second optical path including a third polarization-maintaining optical fiber and a fourth polarization-maintaining optical fiber, the third polarization-maintaining optical fiber and the fourth polarization-maintaining optical fiber being connected such that the polarization axis of the third polarization-maintaining optical fiber and the polarization axis of the fourth polarization-maintaining optical fiber are aligned. This makes it possible to generate two orthogonal linearly polarized light beams with a simple configuration.

[0016] In one embodiment of the optical fiber characteristic measuring apparatus, the polarization generation unit includes a first optical coupler into which the second light is incident and a second optical coupler that outputs the two orthogonal linearly polarized lights, the first optical coupler and the second optical coupler are connected by a first optical path and a second optical path, the first optical path includes a first polarization-maintaining optical fiber and a second polarization-maintaining optical fiber, the first polarization-maintaining optical fiber and the second polarization-maintaining optical fiber are connected such that the polarization axis of the first polarization-maintaining optical fiber and the polarization axis of the second polarization-maintaining optical fiber are aligned, and the second optical path includes a third polarization-maintaining optical fiber, a fourth polarization-maintaining optical fiber, a polarization controller, a fifth polarization-maintaining optical fiber ... and a sixth polarization-maintaining optical fiber, wherein the third polarization-maintaining optical fiber and the fourth polarization-maintaining optical fiber are connected such that the polarization axis of the third polarization-maintaining optical fiber and the polarization axis of the fourth polarization-maintaining optical fiber are aligned, the fourth polarization-maintaining optical fiber and the fifth polarization-maintaining optical fiber are connected such that the polarization axis of the fourth polarization-maintaining optical fiber and the polarization axis of the fifth polarization-maintaining optical fiber are tilted by 90 degrees by the polarization controller, and the fifth polarization-maintaining optical fiber and the sixth polarization-maintaining optical fiber are connected such that the polarization axis of the fifth polarization-maintaining optical fiber and the polarization axis of the sixth polarization-maintaining optical fiber are aligned. This makes it possible to generate two orthogonal linearly polarized light beams with a simple configuration.

[0017] In one embodiment of the optical fiber characteristic measuring apparatus, the scattered light may be light scattered by stimulated Brillouin scattering.

[0018] The optical fiber characteristic measuring device according to one embodiment may further include an amplifier that amplifies the scattered light having the first polarization direction and the scattered light having the second polarization direction and the scattered light having the second polarization direction, both of which have been converted into electrical signals, thereby enabling amplification of weak electrical signals.

[0019] According to some embodiments, an optical fiber characteristic measurement method for measuring characteristics of an optical fiber includes the steps of: emitting frequency-modulated light; generating a first light from the frequency-modulated light; generating a second light from the frequency-modulated light, the second light having a frequency different from that of the first light; generating two orthogonal linearly polarized lights from the second light; inputting the first light into one end of the optical fiber and inputting the two linearly polarized lights into the other end of the optical fiber, thereby generating two orthogonal scattered lights into scattered light of a first polarization direction and scattered light of a second polarization direction; converting the scattered light of the first polarization direction and the scattered light of the second polarization direction into electrical signals; and combining the scattered light of the first polarization direction converted into electrical signals and the scattered light of the second polarization direction converted into electrical signals. This optical fiber characteristic measurement method enables stable measurement of optical fiber characteristics in a short time.

[0020] According to the present disclosure, it is possible to provide an optical fiber characteristic measuring device and an optical fiber characteristic measuring method that are capable of stably measuring the characteristics of an optical fiber in a short period of time.

[0021] 1 is a diagram showing a schematic configuration of an optical fiber characteristic measurement device according to an embodiment; FIG. 2 is a diagram showing a first configuration example of a polarized light generation unit; FIG. 3 is a diagram showing a second configuration example of a polarized light generation unit; FIG. 4 is a diagram showing a fourth configuration example of a polarized light generation unit; FIG. 5 is a diagram showing a schematic configuration of an optical fiber characteristic measurement device according to a comparative example; FIG. 6 is a diagram showing an example of a Brillouin gain spectrum when switching to linearly polarized light in the x-axis direction in an optical fiber characteristic measurement device according to a comparative example; FIG. 7 is a diagram showing an example of a Brillouin gain spectrum when switching to linearly polarized light in the y-axis direction in an optical fiber characteristic measurement device according to a comparative example; and FIG. 8 is a diagram showing an example of a Brillouin gain spectrum obtained by combining two Brillouin gain spectra in an optical fiber characteristic measurement device according to a comparative example.

[0022] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings.

[0023] 1 is a diagram showing a schematic configuration of an optical fiber characteristic measurement apparatus 100 according to one embodiment. The optical fiber characteristic measurement apparatus 100 is an apparatus for measuring the characteristics of an optical fiber 101 by Brillouin optical correlation domain analysis (BOCDA).

[0024] The optical fiber characteristic measuring device 100 comprises an optical fiber 101, a light source unit 102, a pump light generating unit 103, a probe light generating unit 104, a polarization generating unit 105, an optical isolator 106, an optical circulator 107, a polarization splitting unit 108, an optical detection unit 109, an amplification unit 110, and an arithmetic unit 111.

[0025] The optical fiber 101 is an optical fiber used as a sensor for measuring strain and temperature. The optical fiber 101 may be attached to or embedded in a measurement object whose strain and temperature are to be measured. The measurement object may be, for example, a building, a road, a bridge, a dam, a tunnel, an aircraft, etc. The optical fiber 101 may be a single-mode optical fiber or a multi-mode optical fiber.

[0026] The light source unit 102 emits frequency-modulated light to the pump light generating unit 103 and the probe light generating unit 14 .

[0027] The light source unit 102 includes, for example, a semiconductor laser and a frequency modulation circuit. The semiconductor laser emits laser light. The frequency modulation circuit frequency-modulates the laser light emitted by the semiconductor laser. The light source unit 102 may further include a temperature control circuit, a drive circuit, a signal generator, a directional coupler, etc. Note that the above-described configuration is an example, and the light source unit 102 may have any configuration that is capable of emitting frequency-modulated light.

[0028] The pump light generating unit 103 generates pump light from the frequency-modulated light input from the light source unit 102. The pump light generating unit 103 outputs the generated pump light to the optical circulator 107. The pump light generating unit 103 amplifies the amplitude of the frequency-modulated light input from the light source unit 102 to generate pump light.

[0029] The pump light generating unit 103 includes, for example, an optical amplifier. The optical amplifier amplifies the amplitude of the frequency-modulated light incident from the light source unit 102. The pump light generating unit 103 may further include an optical pulse generating circuit, an optical switch, a delay fiber, a phase adjuster, an optical frequency shifter, an optical attenuator, a signal generator, etc. The optical frequency shifter may be, for example, an SSB (Single Side-Band) modulator. Note that the above-described configuration is an example, and the pump light generating unit 103 may have any configuration capable of generating pump light.

[0030] The probe light generating unit 104 generates probe light from the frequency-modulated light incident from the light source unit 102. The probe light generating unit 104 outputs the generated probe light to the polarization generating unit 105.

[0031] The probe light is light having a different frequency and amplitude from the pump light. The probe light generation unit 104 generates the probe light by shifting the frequency of the frequency-modulated light incident from the light source unit 102 and amplifying the amplitude of the frequency-modulated light incident from the light source unit 102. When shifting the frequency of the frequency-modulated light incident from the light source unit 102, the probe light generation unit 104 can adjust the amount of shift.

[0032] The probe light generating unit 104 includes, 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 102. 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 102. The probe light generating unit 104 may further include an optical pulse generating circuit, an optical switch, a delay fiber, a phase adjuster, an optical attenuator, a signal generator, etc. Note that the above-described configuration is an example, and the probe light generating unit 104 may have any configuration capable of generating probe light.

[0033] The polarization generation unit 105 simultaneously generates two orthogonal linearly polarized lights from the probe light incident from the probe light generation unit 104. The polarization generation unit 105 outputs the generated two orthogonal linearly polarized lights to the optical isolator 106.

[0034] 1, linearly polarized light polarized in the x direction is denoted as Ex. Linearly polarized light polarized in the y direction is denoted as Ey. Since the x direction and the y direction are orthogonal to each other, linearly polarized light Ex and linearly polarized light Ey are orthogonal to each other.

[0035] As shown in FIG. 1, when the polarized light generating unit 105 receives the probe light containing the linearly polarized light Ex, the polarized light generating unit 105 outputs the probe light containing the linearly polarized light Ex and the linearly polarized light Ey.

[0036] The polarization generation unit 105 may include a polarization-maintaining optical fiber (hereinafter also referred to as a "polarization-maintaining optical fiber"), an optical coupler, a polarization beam combiner, a polarization controller, etc. An example of the configuration of the polarization generation unit 105 will be described later.

[0037] The optical isolator 106 is an optical component that allows light to pass only in one direction. The optical isolator 106 outputs the probe light incident from the polarization generation unit 105 to the optical fiber 101. The probe light incident from the polarization generation unit 105 is probe light containing linearly polarized light Ex and linearly polarized light Ey. The optical isolator 106 outputs the probe light containing linearly polarized light Ex and linearly polarized light Ey to the optical fiber 101. The optical isolator 106 does not allow light incident from the optical fiber 101 to pass through.

[0038] The optical circulator 107 outputs the pump light input from the pump light generating unit 103 to the optical fiber 101 .

[0039] As a result, the pump light is input to one end of the optical fiber 101, and the probe light including the linearly polarized light Ex and the linearly polarized light Ey is input to the other end of the optical fiber 101. Then, scattered light is locally generated in the optical fiber 101 due to stimulated Brillouin scattering.

[0040] The scattered light caused by stimulated Brillouin scattering includes two orthogonal scattered lights. The scattered light generated by stimulated Brillouin scattering using pump light and linearly polarized probe light Ex is scattered light polarized in the x direction. The scattered light generated by stimulated Brillouin scattering using pump light and linearly polarized probe light Ey is scattered light polarized in the y direction. Hereinafter, "scattered light polarized in the x direction" may be referred to as "linearly polarized Ex scattered light." Furthermore, "scattered light polarized in the y direction" may be referred to as "linearly polarized Ey scattered light." The scattered light includes two orthogonal scattered lights, namely, linearly polarized Ex scattered light and linearly polarized Ey scattered light.

[0041] The optical circulator 107 outputs the scattered light incident from the optical fiber 101 to the polarization splitter 108. As described above, the scattered light includes scattered light of linearly polarized light Ex and scattered light of linearly polarized light Ey.

[0042] The polarization separation unit 108 separates the scattered light incident from the optical circulator 107 into linearly polarized scattered light Ex and linearly polarized scattered light Ey. The polarization separation unit 108 outputs the separated linearly polarized scattered light Ex and linearly polarized scattered light Ey to the light detection unit 109. The polarization separation unit 108 may include, for example, a polarizing beam splitter that can separate the scattered light containing linearly polarized light Ex and linearly polarized Ey into linearly polarized scattered light Ex and linearly polarized scattered light Ey.

[0043] The photodetector 109 receives the scattered light of linearly polarized light Ex incident from the polarization splitter 108 and converts it into an electrical signal. The photodetector 109 receives the scattered light of linearly polarized light Ey incident from the polarization splitter 108 and converts it into an electrical signal.

[0044] The photodetector 109 outputs the linearly polarized scattered light Ex converted into an electric signal to the amplifier 110. The photodetector 109 outputs the linearly polarized scattered light Ey converted into an electric signal to the amplifier 110.

[0045] The photodetector 109 may include, for example, a photodiode that receives scattered light of linearly polarized light Ex and converts it into an electrical signal, and a photodiode that receives scattered light of linearly polarized light Ey and converts it into an electrical signal. The photodiode may be, for example, an avalanche photodiode or a PIN-type photodiode. Alternatively, the photodiode may be a differential photodiode incorporating photodiodes with uniform characteristics. The photodetector 109 may further include an optical attenuator, a transimpedance amplifier circuit, etc. Note that the above-described configuration is merely an example, and the photodetector 109 may have any configuration that is capable of receiving scattered light and converting it into an electrical signal.

[0046] The amplifier 110 amplifies the scattered light of linearly polarized light Ex that has been converted into an electrical signal and that is supplied from the photodetector 109. The amplifier 110 amplifies the scattered light of linearly polarized light Ey that has been converted into an electrical signal and that is supplied from the photodetector 109.

[0047] The amplifier 110 outputs the amplified electrical signal of the scattered light of linearly polarized light Ex to the calculation unit 111. The amplifier 110 outputs the amplified electrical signal of the scattered light of linearly polarized light Ey to the calculation unit 111.

[0048] The amplifier 110 may be an electronic circuit including an operational amplifier, a resistor, a capacitor, etc. The amplifier 110 may further include a lock-in amplifier, a noise filter, etc. The above-described configuration is an example, and the amplifier 110 may have any configuration that is capable of amplifying an electrical signal based on scattered light.

[0049] The calculation unit 111 combines an electrical signal based on the scattered light of linearly polarized light Ex supplied from the amplification unit 110 with an electrical signal based on the scattered light of linearly polarized light Ey supplied from the amplification unit 110 .

[0050] The calculation unit 111 performs frequency analysis on the combined electrical signal to calculate the Brillouin gain spectrum, where the horizontal axis represents the Brillouin frequency shift and the vertical axis represents the Brillouin gain.

[0051] The calculation unit 111 extracts the frequency of the Brillouin frequency shift at which the Brillouin gain peaks. The frequency of the Brillouin frequency shift at which the Brillouin gain peaks depends on the strain and temperature of the optical fiber 101. Therefore, the calculation unit 111 can measure the strain and temperature of the optical fiber 101 by extracting the frequency of the Brillouin frequency shift at which the Brillouin gain peaks.

[0052] Furthermore, the position in the optical fiber 101 at which scattered light is generated by stimulated Brillouin scattering depends on the modulation frequency of the frequency-modulated light emitted by the light source unit 102. Therefore, by sweeping and measuring the modulation frequency, the optical fiber characteristic measuring device 100 can measure the strain and temperature distribution of the optical fiber 101.

[0053] The calculation unit 111 includes at least one processor, at least one dedicated circuit, or a combination thereof. The processor is a general-purpose processor such as a central processing unit (CPU) or a graphics processing unit (GPU), or a dedicated processor specialized for a specific process. The dedicated circuit is, for example, a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC).

[0054] In this way, the optical fiber characteristic measuring device 100 inputs probe light containing two orthogonal linearly polarized lights into the optical fiber 101, and simultaneously receives and combines two orthogonal scattered lights, thereby enabling the characteristics of the optical fiber 101 to be measured stably in a short time.

[0055] 1, the light detection unit 109 is shown as one component, but the light detection unit 109 may be divided into two components. When the light detection unit 109 is divided into two components, one may process the scattered light of linearly polarized light Ex, and the other may process the scattered light of linearly polarized light Ey. Similarly, the amplification unit 110 may be divided into two components.

[0056] <Configuration Example of Polarized Light Generation Unit> Configuration examples of the polarized light generation unit 105 will be described with reference to Figures 2A to 2D. Figure 2A is a diagram showing a first configuration example of the polarized light generation unit 105. Figure 2B is a diagram showing a second configuration example of the polarized light generation unit 105. Figure 2C is a diagram showing a third configuration example of the polarized light generation unit 105. Figure 2D is a diagram showing a fourth configuration example of the polarized light generation unit 105.

[0057] 2A, a first configuration example of the polarization generating unit 105 will be described. The polarization generating unit 105 according to the first configuration example includes a polarization-maintaining optical fiber 301 and a polarization-maintaining optical fiber 302. The polarization-maintaining optical fiber 301 and the polarization-maintaining optical fiber 302 are optical fibers that have a polarization axis and maintain the polarization direction of propagating light.

[0058] The polarization-maintaining optical fiber 301 and the polarization-maintaining optical fiber 302 are fusion-spliced ​​together with the polarization axis of the polarization-maintaining optical fiber 301 and the polarization axis of the polarization-maintaining optical fiber 302 tilted by 45 degrees.

[0059] Probe light containing linearly polarized light Ex is incident on the polarization-maintaining optical fiber 301 from the probe light generation unit 104. Then, probe light containing linearly polarized light Ex and linearly polarized light Ey is generated at the fusion splice between the polarization-maintaining optical fiber 301 and the polarization-maintaining optical fiber 302.

[0060] The polarization-maintaining optical fiber 302 outputs the probe light containing the linearly polarized light Ex and the linearly polarized light Ey to the optical isolator 106 .

[0061] Next, a second configuration example of the polarization generation unit 105 will be described with reference to Fig. 2B. The polarization generation unit 105 according to the second configuration example includes polarization-maintaining optical fibers 303 to 308, a first optical coupler 401, and a second optical coupler 402. The polarization-maintaining optical fibers 303 to 308 are optical fibers that have polarization axes and maintain the polarization direction of propagating light. The first optical coupler 401 and the second optical coupler 402 are optical couplers that maintain the polarization direction.

[0062] The first optical coupler 401 and the second optical coupler 402 are connected by a first optical path and a second optical path. The first optical path includes a polarization-maintaining optical fiber 304 and a polarization-maintaining optical fiber 306. The polarization-maintaining optical fiber 304 and the polarization-maintaining optical fiber 306 are fusion-spliced ​​together such that the polarization axis of the polarization-maintaining optical fiber 304 and the polarization axis of the polarization-maintaining optical fiber 306 are aligned. The second optical path includes a polarization-maintaining optical fiber 305 and a polarization-maintaining optical fiber 307. The polarization-maintaining optical fiber 305 and the polarization-maintaining optical fiber 307 are fusion-spliced ​​together such that the polarization axis of the polarization-maintaining optical fiber 305 and the polarization axis of the polarization-maintaining optical fiber 307 are inclined by 90 degrees.

[0063] The probe light including linearly polarized light Ex is input to the first optical coupler 401 from the probe light generation unit 104 via the polarization-maintaining optical fiber 303. The first optical coupler 401 outputs the probe light including linearly polarized light Ex to the polarization-maintaining optical fiber 304 and the polarization-maintaining optical fiber 305 at a branching ratio of 50:50.

[0064] The probe light containing linearly polarized light Ex output to the polarization-maintaining optical fiber 304 is input to the second optical coupler 402 via the polarization-maintaining optical fiber 306 .

[0065] The probe light including linearly polarized light Ex output to the polarization-maintaining optical fiber 305 is converted into probe light including linearly polarized light Ey at the fusion splice between the polarization-maintaining optical fiber 305 and the polarization-maintaining optical fiber 307. The probe light including linearly polarized light Ey is incident on the second optical coupler 402 via the polarization-maintaining optical fiber 307.

[0066] The second optical coupler 402 combines the probe light containing linearly polarized light Ex incident from the polarization-maintaining optical fiber 306 with the probe light containing linearly polarized light Ey incident from the polarization-maintaining optical fiber 307, and outputs the probe light containing linearly polarized light Ex and linearly polarized light Ey to the optical isolator 106.

[0067] Next, a third configuration example of the polarization generation unit 105 will be described with reference to Fig. 2C. The polarization generation unit 105 according to the third configuration example includes polarization-maintaining optical fibers 309 to 314, an optical coupler 403, and a polarization beam combiner 404. The polarization-maintaining optical fibers 309 to 314 are optical fibers that have polarization axes and maintain the polarization direction of propagating light. The optical coupler 403 is an optical coupler that maintains the polarization direction.

[0068] The optical coupler 403 and the polarization beam combiner 404 are connected by a first optical path and a second optical path. The first optical path includes a polarization-maintaining optical fiber 310 and a polarization-maintaining optical fiber 312. The polarization-maintaining optical fiber 310 and the polarization-maintaining optical fiber 312 are fusion-spliced ​​together such that the polarization axis of the polarization-maintaining optical fiber 310 and the polarization axis of the polarization-maintaining optical fiber 312 are aligned. The second optical path includes a polarization-maintaining optical fiber 311 and a polarization-maintaining optical fiber 313. The polarization-maintaining optical fiber 311 and the polarization-maintaining optical fiber 313 are fusion-spliced ​​together such that the polarization axis of the polarization-maintaining optical fiber 311 and the polarization axis of the polarization-maintaining optical fiber 313 are aligned.

[0069] The probe light including linearly polarized light Ex is input to the optical coupler 403 from the probe light generation unit 104 via the polarization-maintaining optical fiber 309. The optical coupler 403 outputs the probe light including linearly polarized light Ex to the polarization-maintaining optical fiber 310 and the polarization-maintaining optical fiber 311 at a branching ratio of 50:50.

[0070] The probe light containing linearly polarized light Ex output to the polarization-maintaining optical fiber 310 is incident on the polarization beam combiner 404 via the polarization-maintaining optical fiber 312 .

[0071] The probe light containing linearly polarized light Ex output to the polarization-maintaining optical fiber 311 is incident on the polarization beam combiner 404 via the polarization-maintaining optical fiber 313 .

[0072] The polarization beam combiner 404 generates probe light containing linearly polarized light Ex and linearly polarized light Ey when the probe light containing linearly polarized light Ex is incident from the polarization-maintaining optical fiber 312 and the polarization-maintaining optical fiber 313. The polarization beam combiner 404 outputs the probe light containing linearly polarized light Ex and linearly polarized light Ey to the optical isolator 106.

[0073] Next, a fourth configuration example of the polarization generation unit 105 will be described with reference to Fig. 2D. The polarization generation unit 105 according to the fourth configuration example includes polarization-maintaining optical fibers 315-322, a first optical coupler 405, a second optical coupler 406, and a polarization controller 407. The polarization-maintaining optical fibers 315-322 are optical fibers that have a polarization axis and maintain the polarization direction of the propagating light. The first optical coupler 405 and the second optical coupler 406 are optical couplers that maintain the polarization direction.

[0074] The first optical coupler 405 and the second optical coupler 406 are connected by a first optical path and a second optical path. The first optical path includes a polarization-maintaining optical fiber 316 and a polarization-maintaining optical fiber 318. The polarization-maintaining optical fiber 316 and the polarization-maintaining optical fiber 318 are fusion-spliced ​​together such that the polarization axis of the polarization-maintaining optical fiber 316 and the polarization axis of the polarization-maintaining optical fiber 318 are aligned.

[0075] The second optical path includes a polarization-maintaining optical fiber 317, a polarization-maintaining optical fiber 321, a polarization controller 407, a polarization-maintaining optical fiber 322, and a polarization-maintaining optical fiber 319. The polarization-maintaining optical fiber 317 and the polarization-maintaining optical fiber 321 are fusion-spliced ​​together such that the polarization axis of the polarization-maintaining optical fiber 317 and the polarization axis of the polarization-maintaining optical fiber 321 are aligned. The polarization-maintaining optical fiber 322 and the polarization-maintaining optical fiber 319 are fusion-spliced ​​together such that the polarization axis of the polarization-maintaining optical fiber 322 and the polarization axis of the polarization-maintaining optical fiber 319 are aligned. The polarization controller 407 is an element that can control the polarization plane of light. The polarization controller 407 tilts the polarization axis by 90 degrees. The polarization-maintaining optical fiber 321 and the polarization-maintaining optical fiber 322 are connected together such that the polarization axis of the polarization-maintaining optical fiber 321 and the polarization axis of the polarization-maintaining optical fiber 322 are tilted by 90 degrees by the polarization controller 407.

[0076] The probe light including linearly polarized light Ex is input to the first optical coupler 405 from the probe light generation unit 104 via the polarization-maintaining optical fiber 315. The first optical coupler 405 outputs the probe light including linearly polarized light Ex to the polarization-maintaining optical fiber 316 and the polarization-maintaining optical fiber 317 at a branching ratio of 50:50.

[0077] The probe light containing linearly polarized light Ex output to the polarization-maintaining optical fiber 316 is input to the second optical coupler 406 via the polarization-maintaining optical fiber 318 .

[0078] The probe light including linearly polarized light Ex output to the polarization-maintaining optical fiber 317 is input to the polarization controller 407 via the polarization-maintaining optical fiber 321. The polarization controller 407 converts the probe light including linearly polarized light Ex into probe light including linearly polarized light Ey. The polarization controller 407 outputs the probe light including linearly polarized light Ey to the second optical coupler 406 via the polarization-maintaining optical fiber 322 and the polarization-maintaining optical fiber 319.

[0079] The second optical coupler 406 combines the probe light containing linearly polarized light Ex incident from the polarization-maintaining optical fiber 318 with the probe light containing linearly polarized light Ey incident from the polarization-maintaining optical fiber 319, and outputs the probe light containing linearly polarized light Ex and linearly polarized light Ey to the optical isolator 106.

[0080] 2A to 2D show examples in which the polarization-maintaining optical fibers are fusion-spliced ​​together, the polarization-maintaining optical fibers may also be connected to each other using a connector. For example, the polarization-maintaining optical fiber 301 and the polarization-maintaining optical fiber 302 shown in Fig. 2A may be connected to each other using a connector with their polarization axes tilted by 45 degrees. The same applies to the configuration examples shown in Fig. 2B to 2D.

[0081] The optical fiber characteristic measuring apparatus 100 according to the embodiment described above makes it possible to stably measure the characteristics of the optical fiber 101 in a short time. More specifically, the optical fiber characteristic measuring apparatus 100 includes a light source unit 102 that emits frequency-modulated light, a pump light generator 103 that generates pump light from the frequency-modulated light, a probe light generator 104 that generates probe light having a frequency different from that of the pump light, a polarization generator 105 that generates two orthogonal linearly polarized lights from the probe light, a polarization separator 108 that separates two orthogonal scattered lights generated by inputting the pump light into one end of the optical fiber 101 and inputting the two linearly polarized lights into the other end of the optical fiber 101 into scattered light in a first polarization direction and scattered light in a second polarization direction, a light detector 109 that converts the scattered light in the first polarization direction and the scattered light in the second polarization direction into electrical signals, and a calculation unit 111 that combines the scattered light in the first polarization direction converted into an electrical signal and the scattered light in the second polarization direction converted into an electrical signal. This allows the calculation unit 111 to calculate the Brillouin gain spectrum based on the scattered light in the first polarization direction and the scattered light in the second polarization direction that are simultaneously acquired. Therefore, the optical fiber characteristic measurement apparatus 100 according to one embodiment can stably extract the frequency of the Brillouin frequency shift at which the Brillouin gain peaks in a short time, and can stably measure the characteristics of the optical fiber 101 in a short time.

[0082] Comparative Example Fig. 3 is a diagram showing a schematic configuration of an optical fiber characteristic measuring apparatus 200 according to a comparative example. The optical fiber characteristic measuring apparatus 200 according to the comparative example will be described with reference to Fig. 3 .

[0083] The optical fiber characteristic measuring device 200 of the comparative example includes an optical fiber 101, a light source unit 102, a pump light generating unit 103, a probe light generating unit 104, an optical isolator 106, an optical circulator 107, a light detecting unit 109, an amplifying unit 110, a calculating unit 111, and a polarization switch 201.

[0084] The optical fiber characteristic measuring device 200 of the comparative example differs significantly from the optical fiber characteristic measuring device 100 shown in Figure 1 in that it has a polarization switch 201 instead of the polarization generating unit 105 and does not have the polarization separating unit 108.

[0085] The probe light including linearly polarized light Ex is incident on the polarization switch 201 from the probe light generation unit 104. The polarization switch 201 switches the polarization direction of the probe light including linearly polarized light Ex at predetermined time intervals, and outputs the probe light including linearly polarized light Ex or the probe light including linearly polarized light Ey to the optical isolator 106.

[0086] When the polarization switch 201 emits probe light containing linearly polarized light Ex, the calculation unit 111 acquires an electrical signal based on the scattered light of the linearly polarized light Ex. The calculation unit 111 performs frequency analysis on the acquired electrical signal based on the scattered light of the linearly polarized light Ex to calculate a Brillouin gain spectrum.

[0087] 4A shows an example of the Brillouin gain spectrum when the polarization switch 201 emits probe light containing linearly polarized light Ex. In the case of the Brillouin gain spectrum shown in FIG. 4A, the Brillouin gain peaks when the frequency of the Brillouin frequency shift is f0.

[0088] When the polarization switch 201 emits probe light containing linearly polarized light Ey, the calculation unit 111 acquires an electrical signal based on the scattered light of the linearly polarized light Ey. The calculation unit 111 performs frequency analysis on the acquired electrical signal based on the scattered light of the linearly polarized light Ey to calculate a Brillouin gain spectrum.

[0089] 4B shows an example of the Brillouin gain spectrum when the polarization switch 201 emits probe light containing linearly polarized light Ey. In the case of the Brillouin gain spectrum shown in FIG. 4B, the Brillouin gain spectrum does not have a clear peak, so it is not possible to extract the frequency of the Brillouin frequency shift at which the Brillouin gain peaks.

[0090] The calculation unit 111 combines the Brillouin gain spectrum calculated when the polarization switch 201 emits probe light including linearly polarized light Ex and the Brillouin gain spectrum calculated when the polarization switch 201 emits probe light including linearly polarized light Ey.

[0091] FIG. 4C shows a Brillouin gain spectrum obtained by combining the Brillouin gain spectrum shown in FIG. 4A and the Brillouin gain spectrum shown in FIG. 4B.

[0092] In the case of the Brillouin gain spectrum shown in FIG. 4C, the Brillouin gain reaches a peak when the frequency of the Brillouin frequency shift is f0.

[0093] 4A and 4B, the Brillouin gain spectrum is highly dependent on the polarization direction of the probe light. Therefore, if the Brillouin gain spectrum is measured using only probe light with one polarization direction, the Brillouin gain spectrum cannot be measured stably. The optical fiber characteristic measurement apparatus 200 according to the comparative example switches the polarization direction of the probe light at predetermined time intervals and combines two Brillouin gain spectra measured, thereby enabling stable measurement of the Brillouin gain spectrum.

[0094] However, the optical fiber characteristic measurement apparatus 200 according to the comparative example switches the polarization direction of the probe light at predetermined time intervals and performs two measurements, which takes a long time to measure the characteristics of the optical fiber 101. In contrast, the optical fiber characteristic measurement apparatus 100 according to the present embodiment has the polarization generation unit 105 simultaneously emitting probe light including linearly polarized light Ex and linearly polarized light Ey, which makes it possible to simultaneously measure the Brillouin gain spectrum when the probe light including linearly polarized light Ex is incident on the optical fiber 101 and the Brillouin gain spectrum when the probe light including linearly polarized light Ey is incident on the optical fiber 101. Therefore, the optical fiber characteristic measurement apparatus 100 according to the present embodiment can measure the characteristics of the optical fiber 101 in approximately half the time required by the optical fiber characteristic measurement apparatus 200 according to the comparative example.

[0095] Furthermore, in the optical fiber characteristic measuring apparatus 200 according to the comparative example, if the installation environment of the optical fiber 101 changes while the polarization direction of the probe light is being switched and the polarization state changes, even if the two Brillouin gain spectra are combined, it may not be possible to stably extract the frequency of the Brillouin frequency shift at which the Brillouin gain peaks. In contrast, the optical fiber characteristic measuring apparatus 100 according to the present embodiment simultaneously emits probe light containing linearly polarized light Ex and linearly polarized light Ey to the optical fiber 101, and therefore can stably extract the frequency of the Brillouin frequency shift at which the Brillouin gain peaks even if the installation environment of the optical fiber 101 changes.

[0096] It will be apparent to those skilled in the art that the present disclosure can be embodied in other specific forms other than the above-described embodiments without departing from the spirit or essential characteristics thereof. Therefore, the foregoing description is illustrative and not limiting. The scope of the disclosure is defined not by the foregoing description but by the appended claims. All modifications within the range of equivalents of any modifications are intended to be encompassed therein.

[0097] For example, the arrangement and number of each component described above are not limited to the above description and the illustrations in the drawings, and may be arbitrarily configured as long as the functions thereof can be realized.

[0098] For example, in the above-described embodiment, the polarization generation unit 105 is installed downstream of the probe light generation unit 104, but the polarization generation unit 105 may also be installed downstream of the pump light generation unit 103.

[0099] For example, in the above-described embodiment, the probe light generating unit 104 shifts the frequency of the frequency-modulated light incident from the light source unit 102 to make the pump light and the probe light have different frequencies. However, without being limited to this, the pump light generating unit 103 may shift the frequency of the frequency-modulated light incident from the light source unit 102 to make the pump light and the probe light have different frequencies.

[0100] REFERENCE SIGNS LIST 100 Optical fiber characteristic measuring device 101 Optical fiber 102 Light source unit 103 Pump light generating unit (first light generating unit) 104 Probe light generating unit (second light generating unit) 105 Polarization generating unit 106 Optical isolator 107 Optical circulator 108 Polarization separating unit 109 Optical detecting unit 110 Amplifying unit 111 Calculating unit 200 Optical fiber characteristic measuring device 201 Polarization switch 301 to 322 Polarization maintaining optical fiber 401 First optical coupler 402 Second optical coupler 403 Optical coupler 404 Polarization beam combiner 405 First optical coupler 406 Second optical coupler 407 Polarization controller

Claims

1. An optical fiber characteristic 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 a first light from the frequency-modulated light; a second light generation unit that generates a second light from the frequency-modulated light, the second light having a different frequency from the first light; a polarization generation unit that generates two orthogonal linearly polarized light beams from the second light; a polarization separation unit that separates two orthogonal scattered light beams generated by inputting the first light into one end of the optical fiber and inputting the two linearly polarized light beams into the other end of the optical fiber into scattered light in a first polarization direction and scattered light in a second polarization direction; a light detection unit that converts the scattered light in the first polarization direction and the scattered light in the second polarization direction into electrical signals; and a calculation unit that combines the scattered light in the first polarization direction converted into an electrical signal and the scattered light in the second polarization direction converted into an electrical signal.

2. An optical fiber characteristics measuring device according to claim 1, wherein the optical fiber characteristics measuring device measures the characteristics of the optical fiber by Brillouin optical correlation domain analysis.

3. An optical fiber characteristic measuring device according to claim 1, wherein the polarization generating unit comprises a first polarization-maintaining optical fiber into which the second light is incident, and a second polarization-maintaining optical fiber that outputs the two orthogonal linearly polarized light beams, and the first polarization-maintaining optical fiber and the second polarization-maintaining optical fiber are connected such that the polarization axis of the first polarization-maintaining optical fiber and the polarization axis of the second polarization-maintaining optical fiber are inclined by 45 degrees.

4. An optical fiber characteristics measuring device according to claim 1, wherein the polarization generation unit comprises: a first optical coupler into which the second light is incident; and a second optical coupler that outputs the two orthogonal linearly polarized light beams; the first optical coupler and the second optical coupler are connected by a first optical path and a second optical path; the first optical path includes a first polarization-maintaining optical fiber and a second polarization-maintaining optical fiber, and the first polarization-maintaining optical fiber and the second polarization-maintaining optical fiber are connected such that the polarization axis of the first polarization-maintaining optical fiber and the polarization axis of the second polarization-maintaining optical fiber are aligned; and the second optical path includes a third polarization-maintaining optical fiber and a fourth polarization-maintaining optical fiber, and the third polarization-maintaining optical fiber and the fourth polarization-maintaining optical fiber are connected such that the polarization axis of the third polarization-maintaining optical fiber and the polarization axis of the fourth polarization-maintaining optical fiber are inclined by 90 degrees.

5. An optical fiber characteristics measuring device according to claim 1, wherein the polarization generation unit comprises an optical coupler into which the second light is incident, and a polarized beam combiner that outputs the two orthogonal linearly polarized lights, wherein the optical coupler and the polarized beam combiner are connected by a first optical path and a second optical path, wherein the first optical path includes a first polarization-maintaining optical fiber and a second polarization-maintaining optical fiber, and the first polarization-maintaining optical fiber and the second polarization-maintaining optical fiber are connected such that the polarization axis of the first polarization-maintaining optical fiber and the polarization axis of the second polarization-maintaining optical fiber are aligned, and the second optical path includes a third polarization-maintaining optical fiber and a fourth polarization-maintaining optical fiber, and the third polarization-maintaining optical fiber and the fourth polarization-maintaining optical fiber are connected such that the polarization axis of the third polarization-maintaining optical fiber and the polarization axis of the fourth polarization-maintaining optical fiber are aligned.

6. In the optical fiber characteristic measuring device according to claim 1, the polarization generation unit comprises: a first optical coupler into which the second light is incident; and a second optical coupler that outputs the two orthogonal linearly polarized lights; the first optical coupler and the second optical coupler are connected by a first optical path and a second optical path; the first optical path includes a first polarization-maintaining optical fiber and a second polarization-maintaining optical fiber, and the first polarization-maintaining optical fiber and the second polarization-maintaining optical fiber are connected such that the polarization axis of the first polarization-maintaining optical fiber and the polarization axis of the second polarization-maintaining optical fiber are aligned; the second optical path includes a third polarization-maintaining optical fiber, a fourth polarization-maintaining optical fiber, a polarization controller, a fifth polarization-maintaining optical fiber, and a sixth polarization-maintaining optical fiber, and the third polarization-maintaining optical fiber and the fourth polarization-maintaining optical fiber are connected such that the polarization axis of the third polarization-maintaining optical fiber and the polarization axis of the fourth polarization-maintaining optical fiber are aligned; the fourth polarization-maintaining optical fiber and the fifth polarization-maintaining optical fiber are connected by the polarization controller such that the polarization axis of the fourth polarization-maintaining optical fiber and the polarization axis of the fifth polarization-maintaining optical fiber are tilted by 90 degrees, and the fifth polarization-maintaining optical fiber and the sixth polarization-maintaining optical fiber are connected such that the polarization axis of the fifth polarization-maintaining optical fiber and the polarization axis of the sixth polarization-maintaining optical fiber are aligned.

7. An optical fiber characteristic measuring device according to claim 1, wherein the scattered light is scattered light due to stimulated Brillouin scattering.

8. An optical fiber characteristic measuring device according to claim 1, further comprising an amplifier section for amplifying the scattered light in the first polarization direction converted into an electrical signal and the scattered light in the second polarization direction converted into an electrical signal.

9. An optical fiber characteristic measuring method for measuring the characteristics of an optical fiber, comprising the steps of: emitting frequency-modulated light; generating a first light from the frequency-modulated light; generating a second light from the frequency-modulated light, the second light having a different frequency from the first light; generating two orthogonal linearly polarized light beams from the second light; inputting the first light into one end of the optical fiber and inputting the two linearly polarized light beams into the other end of the optical fiber, thereby generating two orthogonal scattered lights, into scattered light in a first polarization direction and scattered light in a second polarization direction; converting the scattered light in the first polarization direction and the scattered light in the second polarization direction into electrical signals; and combining the scattered light in the first polarization direction converted into electrical signals and the scattered light in the second polarization direction converted into electrical signals.

10. The optical fiber characteristic measuring method according to claim 9, wherein the optical fiber characteristic is measured by Brillouin optical correlation domain analysis.