Optical fiber characteristics measuring device and optical fiber characteristics measurement method

Abstract

To provide an optical fiber characteristics measuring device and an optical fiber characteristics measurement method with which, while suppressing occurrence of giant pulses, it is possible to improve the accuracy of measuring the characteristics of the optical fiber to be measured.SOLUTION: An optical fiber characteristics measuring device 10 comprises: a first light source 13 for emitting first superimposed light being superimposed on pump light; a pump light amplifier 15 for amplifying light multiplexed from the pump light and the first superimposed light; a circulator 24 for having the amplified light outputted from the pump light amplifier 15 entered from a first terminal of the optical fiber 80 being measured, and extracting return light from the optical fiber 80 being measured; a filter 16 for attenuating the component of superimposed light included in either the amplified light or the return light; and a measurement unit 43 for detecting the return light and measuring the characteristics of the optical fiber 80 being measured.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an optical fiber characteristic measuring device and an optical fiber characteristic measuring method. [Background technology]

[0002] BACKGROUND ART Conventionally, a method for measuring the characteristics of an optical fiber by detecting scattered light generated within the optical fiber is known (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-41843 Summary of the Invention [Problem to be solved by the invention]

[0004] The characteristics of the optical fiber under test are measured based on the detected intensity of return light that is scattered or reflected by the optical fiber under test after pump light is incident on the optical fiber under test. Increasing the detected intensity of return light improves the measurement accuracy of the characteristics of the optical fiber under test.

[0005] The detected intensity of the return light can be increased by amplifying the pump light input to the optical fiber under test or by amplifying the return light itself. However, when the pump light or return light is amplified by an optical amplifier, a giant pulse can occur. A giant pulse is a pulse with excessive optical power that can destroy the photodetector.

[0006] The present disclosure has been made in consideration of the above-mentioned points, and aims to provide an optical fiber characteristic measuring device and an optical fiber characteristic measuring method that can improve the measurement accuracy of the characteristics of an optical fiber to be measured while suppressing the occurrence of giant pulses. [Means for solving the problem]

[0007] (1) In some embodiments, an optical fiber characteristic measuring device includes a first light source that emits first superimposed light to be superimposed on pump light; a pump light amplifier that amplifies light obtained by combining the pump light and the first superimposed light; a circulator that inputs the amplified light output from the pump light amplifier into a first end of an optical fiber under test and extracts returned light from the optical fiber under test; a filter that attenuates a component of the superimposed light contained in at least one of the amplified light and the returned light; and a measuring unit that detects the returned light and measures characteristics of the optical fiber under test. Equipped with.

[0008] By superimposing the first superimposed light on the pump light, excessive population inversion is less likely to occur in the pump light amplifier. As a result, the generation of giant pulses from the pump light amplifier is suppressed. Furthermore, the gain of the pump light in the pump light amplifier is increased to the extent that the frequency of giant pulse generation can be reduced. As a result, the measurement accuracy of the characteristics of the optical fiber under test is improved.

[0009] (2) In the optical fiber characteristic measuring device described in (1) above, the filter may be connected between the circulator and the measuring unit. This attenuates noise components contained in the returning light, thereby improving the measurement accuracy of the characteristics of the optical fiber under test.

[0010] (3) In the optical fiber characteristic measuring device described in (1) or (2) above, the filter may be connected between the pump optical amplifier and the circulator. This suppresses the generation of stimulated Brillouin scattering light originating from the component of the first superimposed light. The length of the measured optical fiber 80 is limited by the stimulated Brillouin scattering light originating from the component of the first superimposed light. By suppressing the generation of stimulated Brillouin scattering light originating from the component of the first superimposed light, the limit on the length of the measured optical fiber is alleviated.

[0011] (4) In the optical fiber characteristic measuring device according to any one of (1) to (3), the first light source may emit the first superimposed light as light of a DC component. When the light source emits the first superimposed light as light of a DC component, the light source can be realized with a simple configuration.

[0012] (5) In the optical fiber characteristic measuring device according to any one of (1) to (4), the first light source may emit the first superimposed light as light having a DC component and an AC component. By including the AC component in the first superimposed light, limitations on the length of the optical fiber to be measured are alleviated.

[0013] (6) In the optical fiber characteristic measuring device described in (4) or (5) above, the first light source may control the intensity of the DC component based on the detected intensity of the returned light. By controlling the intensity of the DC component of the first superimposed light based on the detected intensity of the returned light, it is possible to suppress the generation of giant pulses from the pump optical amplifier and to improve the gain of the pump light in the pump optical amplifier to the extent that the frequency of giant pulse generation can be reduced.

[0014] (7) In the optical fiber characteristic measuring device described in any one of (4) to (6) above, when the first light source emits pulsed light as the pump light, the first light source may control the intensity of the DC component based on the length of a period during which the pulsed light is turned off. By the light source 13 controlling the intensity of the DC component of the first superimposed light based also on the length of a period during which the pulsed light as the pump light is turned off, excessive population inversion is unlikely to be formed in the EDFA 15.

[0015] (8) In the optical fiber characteristic measuring device described in any one of (1) to (3) above, the first light source may emit the first superimposed light as pulsed light. By emitting the first superimposed light as pulsed light, the gain of the pump light in the pump light amplifier is increased. As a result, the measurement accuracy of the characteristics of the optical fiber under test is improved.

[0016] (9) In the optical fiber characteristic measuring device described in (8) above, the first light source may control a period during which the first superimposed light is on so that the pulsed light as the pump light is on during a period during which the pulsed light as the first superimposed light is off. In this way, formation of an excessive population inversion in the pump light amplifier is efficiently suppressed.

[0017] (10) The optical fiber characteristic measuring device according to any one of (1) to (9) above may further include a second light source that emits a second superimposed light to be superimposed on the return light, and a return light amplifier that amplifies light obtained by combining the return light and the second superimposed light. By superimposing the second superimposed light on the return light, it is possible to suppress the generation of giant pulses from the return light amplifier and to amplify the return light to a degree that reduces the frequency of giant pulse generation. As a result, the measurement accuracy of the characteristics of the optical fiber under test is improved.

[0018] (11) The optical fiber characteristic measuring device according to any one of (1) to (10) above may further include an optical coupler that splits, from the pump light, a reference light to be multiplexed with the return light in order to detect the return light. By doing so, when measuring the characteristics of the optical fiber under test using the Brillouin Optical Correlation Domain Reflectometry (BOCDR) method, it is possible to both suppress the occurrence of giant pulses and improve the gain of the pump light to the extent that the frequency of giant pulse occurrence can be reduced.

[0019] (12) The optical fiber characteristic measuring device according to any one of (1) to (10) above may further include an optical coupler that splits probe light, which is incident on a second end of the optical fiber under test opposite to the first end, from the pump light. By doing so, when measuring the characteristics of the optical fiber under test using the Brillouin Optical Correlation Domain Analysis (BOCDA) method, it is possible to both suppress the occurrence of giant pulses and improve the gain of the pump light to the extent that the frequency of giant pulse occurrence can be reduced.

[0020] (13) In some embodiments, a method for measuring optical fiber characteristics includes the steps of: emitting first superimposed light to be superimposed on pump light; amplifying light obtained by combining the pump light and the first superimposed light; inputting the amplified light into a first end of an optical fiber under test; extracting return light from the optical fiber under test; attenuating components of the superimposed light contained in at least one of the amplified light or the return light; and detecting the return light to measure the characteristics of the optical fiber under test. [Effects of the Invention]

[0021] According to the optical fiber characteristic measuring device and the optical fiber characteristic measuring method of the present disclosure, the occurrence of giant pulses is suppressed and the measurement accuracy of the characteristics of the optical fiber under test is improved. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a block diagram showing the configuration of an optical fiber characteristic measuring device according to a comparative example. [Figure 2] 1 is a block diagram illustrating an example configuration of an optical fiber characteristic measuring device according to the present disclosure. [Figure 3] FIG. 10 is a block diagram showing an example of the configuration of an optical fiber characteristic measuring device in which a filter is arranged between a circulator and a feedback optical amplifier. [Figure 4] FIG. 10 is a block diagram showing an example of the configuration of an optical fiber characteristic measuring device in which a filter is arranged between a pump optical amplifier and a circulator. [Figure 5] 1 is a block diagram showing an example of the configuration of an optical fiber characteristic measuring device including a pulse generator that pulse-drives pump light and first superimposed light. [Figure 6] 10 is a graph showing an example of a waveform of the first superimposed light in each driving method. [Figure 7A] 1 is a flowchart illustrating an example of a method for measuring optical fiber characteristics, including a procedure for filtering return light. [Figure 7B]10 is a flowchart illustrating an example of a method for measuring optical fiber characteristics, including a procedure for filtering amplified light. [Figure 8] 10 is a flowchart showing an example of an optical fiber characteristic measuring method including a procedure for feeding back the intensity of the returning light to the control of the first superimposed light. [Figure 9] FIG. 1 is a block diagram showing an example of the configuration when an optical fiber characteristic measuring device according to the present disclosure is applied to the BOCDA method. [Figure 10] FIG. 10 is a block diagram showing an example of the configuration of an optical fiber characteristic measuring device that superimposes second superimposed light on return light. [Figure 11] 11 is a block diagram showing an example of the configuration of an optical fiber characteristic measuring device obtained by modifying the configuration example of FIG. 10 so that the first superimposed light is not superimposed on the pump light. FIG. [Figure 12] FIG. 1 is a block diagram showing an example of the configuration of an optical fiber characteristic measuring device in which reference light functions as superimposed light. DETAILED DESCRIPTION OF THE INVENTION

[0023] One possible method for measuring characteristics such as temperature or strain at various positions along the length of an optical fiber is to detect scattered or reflected light generated within the optical fiber in response to light incident on the optical fiber. One method for measuring optical fiber characteristics is Brillouin Optical Correlation Domain Reflectometry (BOCDR). BOCDR involves injecting pump light branched from frequency-modulated light into the optical fiber and detecting interference light between reference light branched from the modulated light and Brillouin scattered light generated within the optical fiber. Brillouin scattered light is backscattered light generated by Brillouin scattering due to pump light incident on the optical fiber, and returns to the end where the pump light entered.

[0024] (Comparative Example) As shown in FIG. 1, the optical fiber characteristic measuring apparatus 90 according to the comparative example includes a light source 911, an optical coupler 920, an optical coupler 940, a photodetector 941, a signal amplifier 942, and an ESA (Electric Spectrum Analyzer) 943.

[0025] In the optical fiber characteristic measuring device 90, a light source 911 emits frequency-modulated (FM) light. The optical fiber characteristic measuring device 90 further includes a driving power supply 912. The driving power supply 912 drives the light source 911 based on a signal corresponding to a modulation frequency of the frequency modulation. The light source 911 emits light that has been frequency-modulated based on the modulation frequency.

[0026] Light emitted from the light source 911 is split into pump light and reference light by the optical coupler 920. The reference light travels to the optical coupler 940. The pump light travels toward the optical fiber 980 under test. The optical fiber characteristic measuring device 90 further includes an optical switch 921, a delay fiber 922, a polarization scrambler (PS) 923, and a circulator 924 between the optical coupler 920 and the optical fiber 980 under test. The optical switch 921 converts the pump light into pulsed light by chopping it. The pump light converted into pulsed light travels toward the optical fiber 980 under test through the delay fiber 922, the polarization scrambler 923, and the circulator 924. The delay fiber 922 adjusts the phase of the pump light. The polarization scrambler 923 makes the polarization of the pump light more uniform. The circulator 924 directs the pump light traveling from the optical coupler 920 toward the optical fiber 980 under test.

[0027] When the pump light is incident on the optical fiber under test 980, Brillouin scattering occurs in the optical fiber under test 980. The Brillouin scattered light generated by Brillouin scattering in the optical fiber under test 980 returns to the circulator 924. The circulator 924 prevents the Brillouin scattered light, which is the return light returning from the optical fiber under test 980, from traveling toward the optical coupler 920, and instead causes the Brillouin scattered light to travel toward the EDFA 925 and optical coupler 940. The EDFA 925 is an optical amplifier that amplifies the return light input from the circulator 924 and outputs the amplified light.

[0028] The optical fiber characteristic measuring device 90 heterodyne-detects the Brillouin scattered light by detecting light obtained by combining the reference light and the Brillouin scattered light using an optical coupler 940 with a photodetector 941. The optical fiber characteristic measuring device 90 amplifies the frequency components of the Brillouin gain spectrum (BGS) contained in the signal heterodyne-detected by the photodetector 941 with a signal amplifier 942. The optical fiber characteristic measuring device 90 measures the BGS by analyzing the signal amplified by the signal amplifier 942 with an ESA 943, and calculates the Brillouin frequency shift (BFS), which is the peak frequency of the BGS. The BFS depends to the first order on the temperature or strain of the optical fiber 980 under test. Therefore, by calculating the BFS, the optical fiber characteristic measuring device 90 can measure the temperature or strain acting on the optical fiber 980 under test as a characteristic of the optical fiber 980 under test.

[0029] In order to improve the measurement accuracy of the characteristics of the optical fiber under test 980, it is effective to increase the intensity of the Brillouin scattered light, which contains information about the characteristics such as the temperature or strain of the optical fiber under test 980. The intensity of the Brillouin scattered light is proportional to the intensity of the pump light incident on the optical fiber under test 980. From the above, it is effective to increase the intensity of the pump light in order to improve the measurement accuracy of the characteristics of the optical fiber under test 980.

[0030] However, when the pump light converted into pulsed light is amplified by an optical amplifier, a giant pulse having excessive optical power is generated, which may destroy the photodetector 941.

[0031] Therefore, the present disclosure describes an optical fiber characteristic measuring device 10 (see FIG. 2) and an optical fiber characteristic measuring method that can improve the measurement accuracy of the characteristics of a measured optical fiber 80 (see FIG. 2) while suppressing the occurrence of giant pulses.

[0032] (Configuration example of optical fiber characteristic measuring device 10 according to the present disclosure) An embodiment of an optical fiber characteristic measuring device 10 for measuring the characteristics of an optical fiber will be described below with reference to FIG. 2 . The optical fiber characteristic measuring device 10 according to an embodiment of the present disclosure measures the characteristics of an optical fiber under test (FUT: Fiber Under Test) 80 using the BOCDR method. The characteristics of the optical fiber under test 80 include, for example, a temperature distribution or strain distribution in the longitudinal direction of the optical fiber under test 80, vibration of the optical fiber under test 80, or other characteristics of the optical fiber under test 80. Characteristics such as frequency, amplitude, or spectrum of scattered light or reflected light generated within the optical fiber under test 80 when pump light is input into the optical fiber under test 80 change depending on physical quantities such as temperature or strain that affect the optical fiber under test 80. Therefore, in the optical fiber characteristic measuring device 10, the optical fiber under test 80 itself is used as a sensor. The end of the optical fiber under test 80 into which the pump light is input is also referred to as a first end. The end of the optical fiber under test 80 opposite to the first end is also referred to as a second end.

[0033] The optical fiber characteristic measuring device 10 includes a light source 11 , an optical coupler 20 , an optical coupler 40 , a photodetector 41 , a signal amplifier 42 , and an ESA 43 .

[0034] The light source 11 emits frequency-modulated light. The light source 11 may be configured to include a semiconductor laser element such as a distributed feedback laser diode (DFB-LD). The optical fiber characteristic measuring apparatus 10 further includes a driving power supply 12. The driving power supply 12 drives the light source 11 based on a modulation signal having a modulation frequency for frequency modulation. The light source 11 emits light that has been frequency-modulated based on the modulation frequency.

[0035] The optical coupler 20 splits the input light into two beams and outputs them. The optical coupler 20 causes one of the split beams to travel as pump light toward the optical fiber 80 to be measured. The optical coupler 20 causes the other of the split beams to travel as reference light toward the optical coupler 40.

[0036] The optical fiber characteristic measuring device 10 further includes an optical switch 21, a delay fiber 22, a polarization scrambler (PS) 23, a light source 13, an optical coupler 14, an EDFA (Erbium Doped Fiber Amplifier) ​​15, and a circulator 24 between the optical coupler 20 and the optical fiber 80 to be measured.

[0037] The optical switch 21 converts the pump light into pulsed light by chopping it. The pump light converted into pulsed light travels through a delay fiber 22, a polarization scrambler 23, and a circulator 24 to the optical fiber 80 under test. The delay fiber 22 delays the propagation of the pump light by a predetermined time. The delay fiber 22 may include, for example, an optical fiber of a predetermined length. The polarization scrambler 23 controls the state of polarization so that the polarization components contained in the pump light become nearly uniform in each direction of polarization.

[0038] The light source 13 emits a first superimposed light to be superimposed on the pump light. The light source 13 is also referred to as a first light source. The light source 13 generates and emits the first superimposed light such that the frequency of the first superimposed light and the frequency of the pump light are shifted by a first superimposed difference frequency. The first superimposed difference frequency is determined according to the characteristics of a filter used to separate the first superimposed light and the pump light. The optical coupler 14 outputs light obtained by combining the first superimposed light emitted from the light source 13 with the pump light to the EDFA 15.

[0039] The EDFA 15 is an optical amplifier that amplifies input light and outputs the amplified light to the circulator 24. The amplified light is light obtained by multiplexing and amplifying the first superimposed light and the pump light. The amplified light includes a component of the pump light and a component of the first superimposed light. Since the EDFA 15 amplifies the pump light, it is also called a pump light amplifier.

[0040] The circulator 24 transmits the amplified light input from the EDFA 15 toward the optical fiber under test 80. When the amplified light containing a pump light component is incident on the optical fiber under test 80, return light returns from the optical fiber under test 80 toward the end where the amplified light entered. The return light includes scattered light such as Brillouin scattered light caused by Brillouin scattering in the optical fiber under test 80, or reflected light caused by the amplified light being reflected in the optical fiber under test 80. Brillouin scattering is a phenomenon that occurs in the optical fiber under test 80 when a pump light component is incident on the optical fiber under test 80. The circulator 24 transmits the return light containing Brillouin scattered light and the like that has returned from the optical fiber under test 80 toward the optical coupler 40, rather than toward the optical coupler 20. In other words, the circulator 24 extracts the return light from the optical fiber under test 80 so that the Brillouin scattered light can be detected by the photodetector 41. The circulator 24 may be replaced with an optical coupler.

[0041] The optical fiber characteristic measuring apparatus 10 further includes an EDFA 25 and a filter 16 between the circulator 24 and the optical coupler 40 .

[0042] The EDFA 25 is an optical amplifier that amplifies and outputs the return light input from the circulator 24. The EDFA 25 is also called a return light amplifier because it amplifies the return light.

[0043] The filter 16 attenuates or blocks components originating from the first superimposed light that are contained in the return light amplified by the EDFA 25. The components originating from the first superimposed light that are contained in the return light include the first superimposed light itself, Rayleigh scattered light caused by the first superimposed light, Brillouin scattered light caused by the first superimposed light, etc.

[0044] The optical coupler 40 combines the two input light beams, that is, the return light beam and the reference light beam, into one light beam and outputs the combined light beam.

[0045] The photodetector 41 heterodyne-detects the Brillouin scattered light by detecting the intensity of the light obtained by combining the return light and the reference light, and outputs a heterodyne-detected electrical signal. The photodetector 41 may be configured to include, for example, a photodiode (PD) or a phototransistor.

[0046] The signal amplifier 42 amplifies the frequency components of the Brillouin gain spectrum (BGS) contained in the electrical signal heterodyne detected by the photodetector 41 and outputs the amplified signal to the ESA 43. The signal amplifier 42 may be configured to include, for example, an amplifier.

[0047] The ESA 43 analyzes the signal amplified by the signal amplifier 42 to measure the BGS and calculates the Brillouin frequency shift (BFS), which is the peak frequency of the BGS. The BFS depends to the first order on the temperature or strain of the optical fiber 80 under test. Therefore, by calculating the BFS, the optical fiber characteristic measuring apparatus 10 can measure the temperature or strain acting on the optical fiber 80 under test as a characteristic of the optical fiber 80 under test.

[0048] When the ESA 43 acquires the signal amplified by the signal amplifier 42 as an analog signal, the ESA 43 may convert the analog signal into a digital signal for analysis. That is, the ESA 43 may include an A / D converter that converts the analog signal into a digital signal. The ESA 43 may acquire the signal amplified by the signal amplifier 42 as a digital signal. In this case, the signal amplifier 42 may be configured to include an A / D converter, and may convert the signal obtained by amplifying the frequency component of the BGS into a digital signal and output it to the ESA 43.

[0049] The ESA 43 may include a signal interface that receives signals from the signal amplifier 42. The signal interface may be configured to communicate signals according to various standards, such as RS-232C or RS-485.

[0050] The ESA 43 may include a communication interface for connecting to other components so that they can communicate with each other. The communication interface may be configured to be able to communicate based on a communication standard such as a LAN (Local Area Network). The communication standard is not limited to LAN, and various other standards may also be used.

[0051] The ESA 43 may include a calculation unit. The calculation unit may include a processor such as a CPU (Central Processing Unit) or a dedicated circuit such as an FPGA (Field Programmable Gate Array). The calculation unit may be configured to execute a program that realizes the functions of the ESA 43.

[0052] The ESA 43 may include a storage unit. The storage unit may store various types of information used in the operation of the ESA 43, or programs that implement the functions of the ESA 43. The storage unit may function as a work memory for the calculation unit. The storage unit may be configured as, for example, a semiconductor memory. The storage unit may be configured integrally with the calculation unit or separately.

[0053] The ESA 43 or the optical fiber characteristic measuring apparatus 10 may include an input device that accepts input from a user. The input device may include, for example, a keyboard or physical keys, a touch panel or touch sensor, or a pointing device such as a mouse.

[0054] The ESA 43 or the optical fiber characteristic measuring device 10 may be provided with a display device. The display device may include various displays such as a liquid crystal display. The ESA 43 or the optical fiber characteristic measuring device 10 may be provided with an interface that outputs the information to be displayed to an external display device. The ESA 43 or the optical fiber characteristic measuring device 10 may be provided with an audio output device such as a speaker. The ESA 43 or the optical fiber characteristic measuring device 10 may be provided with an interface that outputs audio information to an external speaker or the like.

[0055] As described above, the optical fiber characteristic measuring apparatus 10 according to the present disclosure measures the characteristics of the optical fiber 80 under test by injecting pump light into the optical fiber under test 80 and heterodyne detecting the Brillouin scattered light returning from the optical fiber under test 80. The photodetector 41, signal amplifier 42, and ESA 43 are also referred to as a measurement unit because they measure the characteristics of the optical fiber under test 80 by detecting the Brillouin scattered light. The optical fiber characteristic measuring apparatus 10 can increase the intensity of the Brillouin scattered light by amplifying the pump light with the EDFA 15. The increased intensity of the Brillouin scattered light improves the calculation accuracy of the BFS. The improved calculation accuracy of the BFS improves the measurement accuracy of the characteristics of the optical fiber 80 under test.

[0056] Here, the optical fiber characteristic measuring device 10 according to the present disclosure does not amplify only the pump light by the EDFA 15, but amplifies the light obtained by combining the pump light with the first superimposed light by the EDFA 15, and attenuates or blocks, i.e., filters, the components of the first superimposed light before detecting the Brillouin scattered light by the photodetector 41.

[0057] When amplifying pulsed light, the EDFA 15 can generate giant pulses. Giant pulses are pulses with excessive optical power. The EDFA 15 optically excites ions added to the fiber core to form a population inversion, and uses stimulated emission to amplify light. When the EDFA 15 amplifies pulsed light, excessive population inversion occurs while the pulsed light is off, causing the light emitted when the pulsed light is turned on to become a giant pulse.

[0058] In other words, the cause of the giant pulse generation in the EDFA 15 is the formation of excessive population inversion. Therefore, by inputting into the EDFA 15 light obtained by multiplexing the pump light converted into pulsed light with the first superimposed light, the formation of excessive population inversion in the EDFA 15 is prevented. Specifically, a population inversion is formed in the EDFA 15 while the pump light converted into pulsed light is off, but by inputting the first superimposed light into the EDFA 15 while the pump light is off, the formation of the population inversion is delayed. By delaying the formation of the population inversion, the formation of excessive population inversion is prevented regardless of the length of time the pump light is off. As a result, the generation of giant pulses in the EDFA 15 is prevented.

[0059] Conversely, if only the pump light converted into pulsed light is input to the EDFA 15, excessive population inversion is likely to occur in the EDFA 15 while the pump light is off. As a result, if only the pump light converted into pulsed light is input to the EDFA 15, the EDFA 15 is likely to generate giant pulses. In other words, it is difficult to suppress the generation of giant pulses while amplifying only the pump light converted into pulsed light. The optical fiber characteristic measuring apparatus 10 according to the present disclosure can suppress the generation of giant pulses while amplifying the pump light converted into pulsed light by amplifying light obtained by combining the first superimposed light with the pump light converted into pulsed light.

[0060] The greater the amplification factor, i.e., the gain, of the EDFA 15, the more likely it is that excessive population inversion will be formed in the EDFA 15. Therefore, the greater the gain of the EDFA 15, the greater the effect of suppressing the generation of giant pulses caused by combining the first superimposed light with the pump light.

[0061] Note that by multiplexing the first superimposed light with the pump light and delaying the formation of a population inversion, the gain of the EDFA 15 is suppressed. However, the generation of giant pulses is more easily suppressed by multiplexing the first superimposed light than by reducing the gain of the EDFA 15.

[0062] Experiments have shown that, on the condition that the occurrence of giant pulses is suppressed, the gain of the EDFA 15 when the first superimposed light is multiplexed can be made larger than the gain of the EDFA 15 when the first superimposed light is not multiplexed. Therefore, by multiplexing the first superimposed light with the pump light, the optical fiber characteristic measuring apparatus 10 according to the present disclosure can amplify the pump light to an intensity sufficient to improve the measurement accuracy of the measured optical fiber 80, i.e., an intensity sufficient to improve the calculation accuracy of the BFS, while suppressing the occurrence of giant pulses.

[0063] Furthermore, changing the superimposed light can be achieved with a simpler configuration than changing the gain itself of the EDFA 15. For example, changing the gain itself of the EDFA 15 may reduce the stability of the EDFA 15. By setting the gain so that the EDFA 15 is stable and then adjusting the superimposed light, the stability of the EDFA 15 is ensured.

[0064] The optical fiber characteristic measuring apparatus 10 according to the present disclosure further includes an EDFA 25 that amplifies the return light. The EDFA 25 amplifies the Brillouin scattered light component contained in the return light. The amplification factor of the Brillouin scattered light component may be lower than the amplification factor of the pump light by the EDFA 15. Therefore, the amplification factor, i.e., the gain, of the EDFA 25 may be set low enough to suppress the generation of giant pulses. The optical fiber characteristic measuring apparatus 10 according to the present disclosure does not need to include the EDFA 25.

[0065] <Filtering of superimposed light> In the optical fiber characteristic measuring device 10 according to the present disclosure, the amplified light obtained by combining the first superimposed light with the pump light and amplifying the light contains a component of the first superimposed light. If the component of the first superimposed light is not filtered, the component of the first superimposed light will be included in the returned light. Then, when the reference light and the returned light are combined by the optical coupler 40 and heterodyne detected by the photodetector 41, a BGS originating from the component of the first superimposed light appears. In other words, the component of the first superimposed light affects the waveform of the BGS, reducing the calculation accuracy of the BFS, i.e., the measurement accuracy of the measured optical fiber 80. Therefore, the optical fiber characteristic measuring device 10 according to the present disclosure may attenuate the component of the first superimposed light using the filter 16. Alternatively, the optical fiber characteristic measuring device 10 may block the component of the first superimposed light using the filter 16.

[0066] In the optical fiber characteristic measuring apparatus 10, the difference between the frequency of the pump light and the frequency of the first superimposed light may be set in accordance with the frequency characteristics of the filter 16. That is, the frequency of the first superimposed light emitted by the light source 13 may be set to a frequency that is separate from the frequency of the pump light in accordance with the frequency characteristics of the filter 16 so that the attenuation rate of the pump light is less than an allowable value and the first superimposed light is sufficiently attenuated. Furthermore, the frequency of the first superimposed light emitted by the light source 13 may be set to a frequency that is separate from the frequency of the pump light in accordance with the frequency characteristics of the filter 16 so that the pump light is not attenuated and the first superimposed light can be blocked. The difference between the frequency of the pump light and the frequency of the first superimposed light may be set to, for example, several nm.

[0067] 2, the filter 16 may be connected between the EDFA 25 and the optical coupler 40. That is, the filter 16 may be connected after the EDFA 25. By being connected after the EDFA 25, the filter 16 attenuates or blocks noise components contained in the return light that has traveled to the EDFA 25. As a result, the accuracy of calculating the BFS, i.e., the accuracy of measuring the characteristics of the optical fiber 80 to be measured, is improved.

[0068] 3, the filter 16 may be connected between the circulator 24 and the EDFA 25. That is, the filter 16 may be connected before the EDFA 25. By connecting the filter 16 before the EDFA 25, the component of the first superimposed light input to the EDFA 25 is reduced. By reducing the component of the first superimposed light input to the EDFA 25, the gain of the EDFA 25 is increased.

[0069] As shown in FIG. 4, the filter 16 may be connected between the EDFA 15 and the circulator 24. That is, the filter 16 may be connected before the optical fiber 80 under test. By connecting the filter 16 before the optical fiber 80 under test, the components of the first superimposed light input to the optical fiber 80 under test are reduced. By reducing the components of the first superimposed light input to the optical fiber 80 under test, the generation of Brillouin scattered light derived from the components of the first superimposed light is suppressed. The length of the optical fiber 80 under test is limited by stimulated Brillouin scattered light derived from the components of the first superimposed light. By suppressing the generation of stimulated Brillouin scattered light derived from the components of the first superimposed light, the limit on the length of the optical fiber 80 under test is alleviated.

[0070] <Driving method for the first superimposed light> The gain of the EDFA 15 is suppressed by multiplexing the first superimposed light with the pump light and inputting the resultant light to the EDFA 15. The first superimposed light may be generated to satisfy a superimposition condition. In the present disclosure, the superimposition condition is a condition for increasing the gain of the EDFA 15 to a degree that reduces the frequency at which the EDFA 15 generates giant pulses. The superimposition condition may also be a condition for maximizing the gain of the EDFA 15 to a degree that does not cause the EDFA 15 to generate giant pulses.

[0071] 5, the optical fiber characteristic measuring apparatus 10 may include a driving power supply 61. The driving power supply 61 drives the light source 13 that generates the first superimposed light and adjusts the first superimposed light.

[0072] The superposition condition can be satisfied, for example, by adjusting the intensity of the DC component included in the first superposed light. The DC component is a light component whose intensity remains constant over time. The stronger the DC component of the first superposed light, the less likely EDFA 15 is to generate a giant pulse, but the smaller the gain of EDFA 15. Conversely, the weaker the DC component of the first superposed light, the greater the gain of EDFA 15, but the more likely EDFA 15 is to generate a giant pulse. Therefore, the intensity of the DC component of the first superposed light may be adjusted as appropriate so that the superposition condition is satisfied.

[0073] The first superimposed light may further include an AC component in addition to a DC component. The AC component is a light component whose intensity periodically changes over time. The intensity of the AC component may change according to a sine wave of a single frequency or according to a waveform that is a mixture of sine waves of multiple frequencies.

[0074] 6, the light source 13 may be DC-driven to emit the first superimposed light, which is to be superimposed on the pump light converted into pulsed light, as light with a DC component. The intensity of the DC component is represented by P0. The light source 13 may adjust P0 so that the superimposition condition is satisfied. When the light source 13 emits the first superimposed light as light with a DC component, the light source 13 can be realized with a simple configuration.

[0075] The light source 13 may be AC ​​driven to emit the first superimposed light as light obtained by adding an AC component to a DC component. The intensity of the DC component is represented by P0. The light source 13 may adjust P0 so that the superimposition condition is satisfied.

[0076] When the light source 13 is DC-driven or AC-driven to emit the first superimposed light, the light source 13 may emit the first superimposed light regardless of the period during which the pulsed light as the pump light is turned on or off. The light source 13 may emit the first superimposed light regardless of the period during which the pulsed light as the pump light is turned off (ΔT OFF) the intensity of the DC component of the first superimposed light based on the length of the period in which the pulsed light as pump light is turned off. The light source 13 may strengthen the DC component of the first superimposed light as the period in which the pulsed light as pump light is turned off becomes longer so that excessive population inversion is less likely to be formed in the EDFA 15. In other words, excessive population inversion is less likely to be formed in the EDFA 15 by the light source 13 controlling the intensity of the DC component of the first superimposed light based also on the length of the period in which the pulsed light as pump light is turned off.

[0077] The first superimposed light may be pulsed light. The light source 13 may be pulsed to emit the first superimposed light. As shown in FIG. 5 , the optical fiber characteristic measuring device 10 may include a pulse generator 60. The pulse generator 60 may generate a pulse signal for pulse-driving a driving power supply 61 that drives the light source 13 that emits the first superimposed light, and may control the pulse frequency and duty ratio of the first superimposed light. The pulse generator 60 may generate a pulse signal for controlling the optical switch 21 that converts the pump light into pulsed light, and may control the pulse frequency and duty ratio of the pump light. In other words, one pulse generator 60 may generate a pulse signal for converting the pump light into pulsed light and a pulse signal for pulse-driving the light source 13. The pulse signal for converting the pump light into pulsed light and the pulse signal for pulse-driving the light source 13 may be generated by separate devices.

[0078] As shown in FIG. 6, the light source 13 is turned off during a period (ΔT OFF ) the first superimposed light so that the pulsed light as the first superimposed light is on during a period when the pulsed light as the pump light is off. The light source 13 may control the period when the pulsed light as the first superimposed light is on so that the pulsed light as the first superimposed light is on during a period when the pulsed light as the pump light is off. By controlling the first superimposed light so that the pulsed light as the first superimposed light is on during a period when the pulsed light as the pump light is off, the formation of excessive population inversion in the EDFA 15 is efficiently suppressed. The light source 13 may adjust the intensity when the pulsed light is on so that the superimposition condition is satisfied.

[0079] Regardless of whether the first superimposed light contains only a DC component, contains an AC component, or is pulsed light, i.e., regardless of the form of the first superimposed light, the superimposition condition is satisfied by appropriately adjusting the intensity of the first superimposed light.

[0080] When the first superimposed light includes an AC component, the intensity of the first superimposed light is increased compared to when the first superimposed light includes only a DC component. Furthermore, when the first superimposed light includes an AC component, restrictions on the length of the optical fiber 80 to be measured, into which the pump light is input, are relaxed compared to when the first superimposed light includes only a DC component. In other words, when the first superimposed light includes an AC component, the characteristics of a longer optical fiber 80 to be measured can be measured compared to when the first superimposed light includes only a DC component.

[0081] When the first superimposed light is pulsed light, the gain of the pump light in the EDFA 15 is increased to the extent that the frequency of giant pulses can be reduced, compared to when the first superimposed light contains only a DC component. Increasing the gain of the pump light improves the measurement accuracy of the measured optical fiber 80. In other words, emitting pulsed light as the first superimposed light improves the performance of the optical fiber characteristic measuring apparatus 10.

[0082] The first superimposed light may be adjusted based on the intensity of the returned light detected by the photodetector 41. For example, the pulse generator 60 or the drive power supply 61 may control the intensity of the DC component of the first superimposed light based on the detected intensity of the returned light. As the detected intensity of the returned light increases, the DC component of the first superimposed light may be strengthened so that the gain of the pump light can be lowered to further reduce the frequency of giant pulses. Conversely, as the detected intensity of the returned light decreases, the DC component of the first superimposed light may be weakened so that the gain of the pump light can be increased within a range that can reduce the frequency of giant pulses. By controlling the intensity of the DC component of the first superimposed light based on the detected intensity of the returned light, both the suppression of giant pulses from the EDFA 15 and the improvement of the gain of the pump light in the EDFA 15 within a range that can reduce the frequency of giant pulses can be achieved.

[0083] The optical fiber characteristic measuring apparatus 10 may further include a control device that controls the pulse generator 60 or the driving power supply 61 to adjust the first superimposed light based on the detected intensity of the returned light. The control device may include a processor, a memory unit, or a communication interface. The control device may also include an input device or a display device.

[0084] In the optical fiber characteristic measuring apparatus 10, the ESA 43 may be configured to adjust the first superimposed light based on the detected intensity of the returned light. The pulse generator 60 or the driving power supply 61 may be configured to adjust the first superimposed light based on the detected intensity of the returned light.

[0085] <Example of a flow chart for measuring optical fiber characteristics> When the filter 16 is connected after the circulator 24 as in Fig. 2 or 3, the optical fiber characteristic measuring apparatus 10 according to the present disclosure may measure the characteristics of the optical fiber 80 under test by executing an optical fiber characteristic measuring method including the steps of the flowchart shown in Fig. 7A. At least a part of the optical fiber characteristic measuring method may be realized as an optical fiber characteristic measuring program executed by a calculation unit or the like of the ESA 43. The optical fiber characteristic measuring program may be stored in a non-transitory computer-readable medium.

[0086] The optical fiber characteristic measuring device 10 outputs pump light (step S1). The optical fiber characteristic measuring device 10 according to the present disclosure outputs modulated light modulated at a modulation frequency from a light source 11, branches the modulated light into pump light and reference light using an optical coupler 20, and converts the branched pump light into pulsed light using an optical switch 21, thereby outputting the pump light.

[0087] The optical fiber characteristic measuring device 10 outputs superimposed light to be superimposed on the pump light (step S2). The superimposed light to be superimposed on the pump light is also referred to as first superimposed light in this disclosure. The optical fiber characteristic measuring device 10 causes the light source 13 to emit the first superimposed light.

[0088] The optical fiber characteristic measuring device 10 amplifies the light obtained by multiplexing the pump light and the superimposed light (step S3). The optical fiber characteristic measuring device 10 multiplexes the pump light and the superimposed light using the optical coupler 14, inputs the multiplexed light to the EDFA 15, and amplifies the multiplexed light using the EDFA 15.

[0089] The optical fiber characteristic measuring device 10 inputs the amplified light obtained by amplifying the light obtained by multiplexing the pump light with the superimposed light into the measured optical fiber 80 (step S4). The optical fiber characteristic measuring device 10 filters the return light from the measured optical fiber 80 using the filter 16 (step S5). Filtering is the attenuation or blocking of the superimposed light component contained in the return light. The optical fiber characteristic measuring device 10 may filter the return light before or after the return light is amplified by the EDFA 25.

[0090] The optical fiber characteristic measuring apparatus 10 detects the returned light with the photodetector 41 (step S6). The optical fiber characteristic measuring apparatus 10 according to the present disclosure outputs light obtained by combining the reference light and the returned light with the optical coupler 40 to the photodetector 41, and the photodetector 41 heterodyne detects the Brillouin scattered light from the returned light.

[0091] The optical fiber characteristic measuring apparatus 10 measures the characteristics of the optical fiber 80 under test based on the detection result of the photodetector 41 (step S7). The optical fiber characteristic measuring apparatus 10 according to the present disclosure generates a BGS of the Brillouin scattered light heterodyne detected by the photodetector 41 and calculates the BFS, thereby measuring the characteristics of the optical fiber 80 under test.

[0092] After completing the procedure of step S7, the optical fiber characteristic measuring apparatus 10 ends the execution of the procedure of the flowchart in FIG. 7A.

[0093] When the filter 16 is connected before the circulator 24 as in Fig. 4, the optical fiber characteristic measuring apparatus 10 according to the present disclosure may measure the characteristics of the optical fiber 80 under test by executing an optical fiber characteristic measuring method including the steps of the flowchart shown in Fig. 7B. At least a part of the optical fiber characteristic measuring method may be realized as an optical fiber characteristic measuring program executed by a calculation unit or the like of the ESA 43. The optical fiber characteristic measuring program may be stored in a non-transitory computer-readable medium.

[0094] The optical fiber characteristic measuring apparatus 10 outputs pump light (step S11). The optical fiber characteristic measuring apparatus 10 may perform the procedure of step S11 in the same manner as the procedure of step S1 in FIG. 7A.

[0095] The optical fiber characteristic measuring apparatus 10 outputs the superimposed light to be superimposed on the pump light (step S12). The optical fiber characteristic measuring apparatus 10 may perform the procedure of step S12 in the same manner as the procedure of step S2 in FIG.

[0096] The optical fiber characteristic measuring apparatus 10 amplifies the light obtained by combining the pump light with the superimposed light (step S13). The optical fiber characteristic measuring apparatus 10 may perform the procedure of step S13 in the same manner as the procedure of step S3 in FIG. 7A.

[0097] The optical fiber characteristic measuring device 10 uses the filter 16 to filter the amplified light obtained by amplifying the light obtained by multiplexing the pump light with the superimposed light (step S14). Filtering is the attenuation or blocking of the superimposed light component contained in the amplified light. In other words, the optical fiber characteristic measuring device 10 may filter the amplified light before inputting it into the optical fiber 80 under test.

[0098] The optical fiber characteristic measuring apparatus 10 inputs the filtered amplified light into the optical fiber 80 to be measured (step S15).

[0099] The optical fiber characteristic measuring apparatus 10 detects the returning light with the photodetector 41 (step S16). The optical fiber characteristic measuring apparatus 10 may perform the procedure of step S16 in the same manner as the procedure of step S6 in FIG. 7A.

[0100] The optical fiber characteristic measuring apparatus 10 measures the characteristics of the optical fiber 80 under test based on the detection result of the photodetector 41 (step S7). The optical fiber characteristic measuring apparatus 10 may execute the procedure of step S17 in the same manner as the procedure of step S7 in FIG. 7A.

[0101] After completing the procedure of step S17, the optical fiber characteristic measuring apparatus 10 ends the execution of the procedure of the flowchart in FIG. 7B.

[0102] The optical fiber characteristic measuring apparatus 10 according to the present disclosure may control the pulse generator 60 or the driving power supply 61 based on the output of the photodetector 41 as shown in Fig. 6 by executing an optical fiber characteristic measuring method including the steps of the flowchart shown in Fig. 8. At least a part of the optical fiber characteristic measuring method may be realized as an optical fiber characteristic measuring program executed by a calculation unit or the like of the ESA 43. The optical fiber characteristic measuring program may be stored in a non-transitory computer-readable medium.

[0103] The optical fiber characteristic measuring apparatus 10 outputs pump light (step S21). The optical fiber characteristic measuring apparatus 10 may perform the procedure of step S21 in the same manner as the procedure of step S1 in FIG. 7A.

[0104] The optical fiber characteristic measuring apparatus 10 outputs the superimposed light to be superimposed on the pump light (step S22). The optical fiber characteristic measuring apparatus 10 may perform the procedure of step S22 in the same manner as the procedure of step S2 in FIG. 7A.

[0105] The optical fiber characteristic measuring apparatus 10 amplifies the light obtained by combining the pump light with the superimposed light (step S23). The optical fiber characteristic measuring apparatus 10 may perform the procedure of step S23 in the same manner as the procedure of step S3 in FIG. 7A.

[0106] The optical fiber characteristic measuring apparatus 10 inputs the amplified light obtained by amplifying the light obtained by multiplexing the pump light with the superimposed light into the measured optical fiber 80 (step S24). The optical fiber characteristic measuring apparatus 10 filters the return light from the measured optical fiber 80 using the filter 16 (step S25). The optical fiber characteristic measuring apparatus 10 may execute the procedures of steps S24 and S25 in the same manner as the procedures of steps S4 and S5 in FIG. 7A.

[0107] The optical fiber characteristic measuring apparatus 10 detects the returning light with the photodetector 41 (step S26). The optical fiber characteristic measuring apparatus 10 may perform the procedure of step S26 in the same manner as the procedure of step S6 in FIG. 7A.

[0108] The optical fiber characteristic measuring device 10 determines whether the intensity of the return light detected by the photodetector 41 is within a predetermined range (step S27). The predetermined range may be specified by an upper limit and a lower limit of the intensity of the return light. The upper limit of the intensity of the return light may be set as appropriate so as not to damage the photodetector 41. The lower limit of the intensity of the return light may be set as appropriate so as to improve the detection sensitivity of the photodetector 41 to the return light.

[0109] If the intensity of the returned light is within a predetermined range (step S28: YES), the optical fiber characteristic measuring apparatus 10 measures the characteristics of the measured optical fiber 80 based on the detection result of the photodetector 41 (step S28). The optical fiber characteristic measuring apparatus 10 may execute the procedure of step S28 in the same manner as the procedure of step S7 in Fig. 7A. After completing the procedure of step S28, the optical fiber characteristic measuring apparatus 10 finishes executing the procedure of the flowchart in Fig. 7A.

[0110] If the intensity of the returned light is not within the predetermined range (step S28: NO), i.e., if the intensity of the returned light is outside the predetermined range, the optical fiber characteristic measurement apparatus 10 controls the superimposed light by the pulse generator 60 or the drive power supply 61 (step S29). When increasing the intensity of the returned light, the optical fiber characteristic measurement apparatus 10 may control the pulse generator 60 or the drive power supply 61 to decrease the intensity of the superimposed light. When decreasing the intensity of the returned light, the optical fiber characteristic measurement apparatus 10 may control the pulse generator 60 or the drive power supply 61 to increase the intensity of the superimposed light. After performing the procedure of step S29, the optical fiber characteristic measurement apparatus 10 performs the procedures of steps S21 to S26 to redetect the returned light.

[0111] The optical fiber characteristic measuring apparatus 10 may execute the procedure of determining the intensity of the returned light in step S27 or the procedure of controlling the superimposed light in step S29 using the ESA 43. The optical fiber characteristic measuring apparatus 10 may further include a control device, separate from the ESA 43, for executing the procedure of determining the intensity of the returned light in step S27 or the procedure of controlling the superimposed light in step S29.

[0112] (summary) As described above, according to the optical fiber characteristic measuring apparatus 10 and the optical fiber characteristic measuring method according to the present disclosure, by superimposing and amplifying the superimposed light on the pump light, the generation of giant pulses from the optical amplifier is suppressed. Further, the gain of the pump light in the optical amplifier is increased within a range where the generation frequency of giant pulses from the optical amplifier can be reduced. By increasing the gain of the pump light input to the measured optical fiber 80, the intensity of the Brillouin scattered light returning from the measured optical fiber 80 increases. By increasing the intensity of the Brillouin scattered light, the detection accuracy of BGS and the calculation accuracy of BFS are improved. By improving the calculation accuracy of BFS, the measurement accuracy of the characteristics of the measured optical fiber 80 is improved. From the above, according to the optical fiber characteristic measuring apparatus 10 and the optical fiber characteristic measuring method according to the present disclosure, both suppression of the generation of giant pulses from the optical amplifier and improvement of the measurement accuracy of the characteristics of the measured optical fiber 80 within a range where the generation frequency of giant pulses from the optical amplifier can be reduced are realized.

[0113] (Other Embodiments) Hereinafter, other embodiments will be described.

[0114] <Configuration Example Applicable to BOCDA> As shown in FIG. 9, an optical fiber characteristic measuring apparatus 70 according to another embodiment measures the characteristics of a measured optical fiber 80 by a Brillouin optical correlation domain analysis (BOCDA) method. In the BOCDA method, the optical fiber characteristic measuring apparatus 70 inputs pump light and probe light from both sides of the measured optical fiber 80, detects the BGS of the Brillouin scattered light generated from the pump light and the probe light inside the measured optical fiber 80, and calculates the BFS to measure the characteristics of the measured optical fiber 80. In the present disclosure, the end for inputting pump light to the measured optical fiber 80 corresponds to the first end. The end for inputting probe light to the measured optical fiber corresponds to the second end.

[0115] The optical fiber characteristic measuring device 70 includes a light source 11, a driving power supply 12, an optical coupler 20, a circulator 24, a photodetector 41, a signal amplifier 42, and an ESA 43. The light source 11 and the driving power supply 12 may be configured similarly to the light source 11 and the driving power supply 12 of the optical fiber characteristic measuring device 10 illustrated in Fig. 2 etc. The photodetector 41, the signal amplifier 42, and the ESA 43 may be configured similarly to the photodetector 41, the signal amplifier 42, and the ESA 43 of the optical fiber characteristic measuring device 10 illustrated in Fig. 2 etc.

[0116] The optical coupler 20 may be configured similarly to the optical coupler 20 of the optical fiber characteristic measuring apparatus 10 exemplified in Fig. 2 etc. The optical coupler 20 splits input light into two beams and outputs the two beams as pump light and probe light, respectively.

[0117] The optical fiber characteristic measuring device 70 further includes a frequency shifter 71, a delay fiber 72, and an isolator 73 between a terminal of the optical coupler 20 that outputs the probe light and one end of the optical fiber 80 under test. The probe light is input from the optical coupler 20 to one end of the optical fiber 80 under test through the frequency shifter 71, the delay fiber 72, and the isolator 73. The frequency shifter 71 shifts the frequency of the probe light relative to the frequency of the pump light. The delay fiber 72 may have a configuration similar to the delay fiber 22 of the optical fiber characteristic measuring device 10 illustrated in FIG. 2 and other figures. The isolator 73 allows the probe light to travel toward the optical fiber 80 under test while attenuating or blocking light returning from the optical fiber 80 under test, thereby preventing light from flowing back to the light source 11.

[0118] The optical fiber characteristic measuring device 70 further includes an optical coupler 75, an EDFA 76, and a filter 77 between the terminal of the optical coupler 20 that outputs the pump light and the optical fiber 80 under test. The circulator 24 is connected between the filter 77 and one end of the optical fiber 80 under test. The pump light is input from the optical coupler 20 to one end of the optical fiber 80 under test through the optical coupler 75, the EDFA 76, the filter 77, and the circulator 24.

[0119] The circulator 24 may be configured in the same manner as the circulator 24 of the optical fiber characteristic measuring device 10 illustrated in FIG. 2 and the like. The circulator 24 causes the pump light to travel from the filter 77 toward one end of the optical fiber under measurement 80. On the other hand, the circulator 24 causes the return light including the Brillouin scattered light generated inside the optical fiber under measurement 80 to travel from the optical fiber under measurement 80 toward the photodetector 41.

[0120] The optical fiber characteristic measuring device 70 further includes a light source 74 connected to the optical coupler 75. The light source 74 emits a first superimposed light superimposed on the pump light. The light source 74 may be configured in the same manner as the light source 13 of the optical fiber characteristic measuring device 10 illustrated in FIG. 2 and the like.

[0121] The EDFA 76 is an optical amplifier that amplifies the input light and outputs it as amplified light. The EDFA 76 may be configured in the same manner as the EDFA 15 of the optical fiber characteristic measuring device 10 illustrated in FIG. 2 and the like. The filter 77 attenuates or blocks the component of the first superimposed light included in the amplified light and outputs it to the circulator 24. The filter 77 may be configured in the same manner as the filter 16 of the optical fiber characteristic measuring device 10 illustrated in FIG. 2 and the like.

[0122] From the above, in the optical fiber characteristic measuring device 70 that measures the characteristics of the optical fiber under measurement 80 by the BOCDA method, by inputting the light obtained by multiplexing the pump light and the first superimposed light into the EDFA 76, both suppression of the generation of giant pulses and improvement of the gain of the pump light in a range where the generation frequency of giant pulses can be reduced are achieved.

[0123] <Multiplexing of Superimposed Light with Respect to EDFA25> As shown in FIG. 10, the optical fiber characteristic measuring device 10 may further include a light source 17 and an optical coupler 18 for superimposing a second superimposed light on the return light. The light source 17 is also referred to as a second light source. The optical coupler 18 is connected between the circulator 24 and the EDFA 25. That is, the optical coupler 18 is connected before the EDFA 25 and inputs the light obtained by multiplexing the return light and the second superimposed light into the EDFA 25.

[0124] Light source 17 generates and emits second superimposed light such that the frequency of the second superimposed light is shifted by a second superimposed difference frequency from the frequency of the Brillouin scattered light contained in the returned light. The second superimposed difference frequency is determined according to the characteristics of a filter used to separate the second superimposed light from the Brillouin scattered light. Optical coupler 18 outputs light obtained by combining the second superimposed light emitted from light source 17 with the returned light to EDFA 25.

[0125] The EDFA 25 is an optical amplifier that amplifies and outputs input light. When amplifying pulsed light, the EDFA 25 may generate giant pulses, just like the EDFA 15. The Brillouin scattered light component contained in the returned light is generated in accordance with the period when the pump light converted into pulsed light is on. On the other hand, the Brillouin scattered light component decreases in the period when the pump light is off. As a result, when the EDFA 25 amplifies the Brillouin scattered light component, excessive population inversion is formed while the Brillouin scattered light component is decreasing, generating a giant pulse.

[0126] Therefore, by inputting light obtained by combining the second superimposed light with the return light containing a Brillouin scattered light component into the EDFA 25, the formation of excessive population inversion in the EDFA 25 can be prevented, and as a result, the generation of giant pulses in the EDFA 25 can be prevented.

[0127] 11, the optical fiber characteristic measuring device 10 does not need to include the light source 13 and the optical coupler 14 for superimposing the first superimposed light on the pump light. In this case, although the gain of the EDFA 15 is limited, the Brillouin scattered light component contained in the return light from the optical fiber 80 under test is amplified by the EDFA 25. Furthermore, the generation of giant pulses in the EDFA 25 is suppressed. As a result, the accuracy of calculating the BFS, i.e., the accuracy of measuring the characteristics of the optical fiber 80 under test, is improved.

[0128] 12 , the optical fiber characteristic measuring apparatus 10 may not be provided with a configuration for superimposing the first superimposed light and the second superimposed light, but may instead include an EDFA 44 between the optical coupler 40 and the photodetector 41. In other words, the EDFA 44 may be connected after the optical coupler 40.

[0129] In this case, light obtained by multiplexing return light and reference light in the optical coupler 40 is input to the EDFA 44. The reference light is light branched from the pump light by the optical coupler 20. Therefore, the frequency of the reference light is the same as that of the pump light. On the other hand, the frequency of the Brillouin scattered light component contained in the return light is different from the frequencies of the pump light and the reference light. Therefore, by superimposing the reference light on the return light, the formation of excessive population inversion is prevented when the EDFA 44 amplifies the Brillouin scattered light component contained in the return light. In other words, the reference light functions as superimposed light. As a result, the generation of giant pulses from the EDFA 44 is suppressed.

[0130] The reference light is used to detect BGS from the Brillouin scattered light, and therefore does not need to be attenuated by a filter. In other words, when the reference light is superimposed on the return light to function as superimposed light, the optical fiber characteristic measuring apparatus 10 does not need to include a filter that attenuates or blocks the superimposed light. As a result, the configuration of the optical fiber characteristic measuring apparatus 10 is simplified.

[0131] Furthermore, when the EDFA 44 is connected after the optical coupler 40, the optical fiber characteristic measuring apparatus 10 does not need to include the light source 13 and the optical coupler 14 for superimposing the first superimposed light before the EDFA 15. In this case, although the gain of the EDFA 15 is limited, the Brillouin scattered light component contained in the return light from the optical fiber 80 under test is amplified by the EDFA 44. Furthermore, the generation of giant pulses in the EDFA 44 is suppressed. As a result, the calculation accuracy of the BFS, i.e., the measurement accuracy of the characteristics of the optical fiber 80 under test is improved.

[0132] The above describes an embodiment of the present disclosure with reference to the drawings, but the specific configuration is not limited to this embodiment, and various modifications are also included within the scope that does not deviate from the spirit of the present disclosure. [Explanation of symbols]

[0133] 10 Optical fiber characteristic measuring device (11: light source, 12: driving power supply, 13: light source, 14: optical coupler, 15: EDFA, 16: filter, 20: optical coupler, 21: optical switch, 22: delay fiber, 23: polarization scrambler, 24: circulator, 25: EDFA, 40: optical coupler, 41: photodetector, 42: signal amplifier, 43: ESA, 60: pulse generator, 61: driving power supply) 70 Optical fiber characteristic measuring device (11: light source, 12: driving power supply, 20: optical coupler, 24: circulator, 71: frequency shifter, 72: polarization scrambler, 73: isolator, 74: light source, 75: optical coupler, 76: EDFA, 77: filter) 80 Optical fiber under test 81 Optical fiber characteristic measuring device (11: light source, 12: driving power supply, 15: EDFA, 16: filter, 17: light source, 18: optical coupler, 20: optical coupler, 21: optical switch, 22: delay fiber, 23: polarization scrambler, 24: circulator, 25: EDFA, 40: optical coupler, 41: photodetector, 42: signal amplifier, 43: ESA) 82 Optical fiber characteristic measuring device (11: light source, 12: driving power supply, 15: EDFA, 20: optical coupler, 21: optical switch, 22: delay fiber, 23: polarization scrambler, 24: circulator, 40: optical coupler, 41: photodetector, 42: signal amplifier, 43: ESA, 44: EDFA)

Claims

1. a first light source that emits a first superimposed light to be superimposed on the pump light; a pump light amplifier that amplifies light obtained by combining the pump light and the first superimposed light; a circulator that inputs the amplified light output from the pumping optical amplifier into a first end of an optical fiber under test and extracts return light from the optical fiber under test; a filter that attenuates a component of the superimposed light included in at least one of the amplified light and the returned light; a measurement unit that detects the returned light and measures the characteristics of the optical fiber under test; An optical fiber characteristic measuring device comprising:

2. 2. The optical fiber characteristic measuring device according to claim 1, wherein the filter is connected between the circulator and the measuring unit.

3. 2. The optical fiber characteristic measuring device according to claim 1, wherein the filter is connected between the pump optical amplifier and the circulator.

4. 2. The optical fiber characteristic measuring device according to claim 1, wherein the first light source emits the first superimposed light as light having a DC component.

5. 2. The optical fiber characteristic measuring device according to claim 1, wherein the first light source emits the first superimposed light as light having a DC component and an AC component.

6. 5. The optical fiber characteristic measuring device according to claim 4, wherein the first light source controls the intensity of the DC component based on the detected intensity of the returned light.

7. 5. The optical fiber characteristic measuring device according to claim 4, wherein, when the first light source emits pulsed light as the pump light, the intensity of the DC component is controlled based on the length of a period during which the pulsed light is turned off.

8. 2. The optical fiber characteristic measuring device according to claim 1, wherein the first light source emits the first superimposed light as pulsed light.

9. 9. The optical fiber characteristic measuring device according to claim 8, wherein the first light source controls a period during which the first superimposed light is on so that the pulsed light as the first superimposed light is on during a period during which the pulsed light as the pump light is off.

10. a second light source that emits second superimposed light to be superimposed on the return light; a return light amplifier that amplifies light obtained by combining the return light and the second superimposed light; The optical fiber characteristic measuring device according to claim 1 , further comprising:

11. 11. The optical fiber characteristic measuring device according to claim 1, further comprising an optical coupler that splits, from the pump light, a reference light that is to be combined with the return light in order to detect the return light.

12. 11. The optical fiber characteristic measuring device according to claim 1, further comprising an optical coupler that branches probe light, which is incident on a second end of the optical fiber under test opposite to the first end, from the pump light.

13. emitting a first superimposed light to be superimposed on the pump light; amplifying light obtained by combining the pump light and the first superimposed light; A step of inputting the amplified light into a first end of an optical fiber under test; extracting return light from the optical fiber under test; attenuating a component of the superimposed light included in at least one of the amplified light and the returned light; detecting the returned light and measuring the characteristics of the optical fiber under test; An optical fiber characteristic measuring method comprising:

Images