Apparatus and method for measuring loss and crosstalk occurring in optical fiber transmission line

Abstract

An objective of the present disclosure is to provide a measurement device which independently measures loss and crosstalk produced in multiple locations of optical fiber along which a plurality of modes are propagated, and a method for measuring the same.　The present disclosure is provided with a light incident unit which causes frequency-swept probe light to be incident on one end of an optical fiber to be measured and pump light to be incident, for the probe light, on the other end of the optical fiber to be measured, thereby amplifying the probe light in the optical fiber to be measured; a light receiving unit which receives multiplexed light in which local light has been multiplexed with the probe light; an interference waveform measurement unit which measures the interference waveform of the probe light and the local light; a base mode time waveform analysis unit which acquires a base mode time waveform of the probe light on the basis of the interference waveform; and a control computation unit which calculates loss and crosstalk locations where loss and crosstalk are produced in the optical fiber to be measured on the basis of the base mode time waveform.

Description

[Technical field]

[0001] The present disclosure relates to a measurement device and a measurement method for independently measuring loss and crosstalk occurring in an optical fiber transmission line. [Background technology]

[0002] In recent years, with the rapid increase in transmission traffic, few-mode fiber (FMF) and multimode fiber (MMF) that can utilize multiple propagation modes instead of single-mode fiber (SMF) currently used in transmission lines have attracted much attention as a way to enable further capacity expansion. In these optical fibers, when passing through devices such as splices and mode multiplexers / demultiplexers, differential modal attenuation (DMA) occurs due to the difference in loss experienced by each mode, and crosstalk (hereinafter sometimes referred to as XT) occurs when some of the modes are coupled to different modes. Since DMA and XT are important parameters for signal processing on the receiving side, it is desirable to be able to measure the loss and XT of each mode at each point in the transmission line in order to evaluate the quality of the transmission line.

[0003] A method for acquiring the propagation characteristics for each mode using the Brillouin gain analysis method has been proposed (see, for example, Non-Patent Document 1). In this measurement method, the loss and XT suffered by the pump light can be acquired from the amount of generated gain by controlling the frequency difference between the pump light and the probe light incident on the optical fiber. However, when there are multiple points where the loss and XT occur, the XT component of the probe light occurs in the optical fiber, and the gain due to this XT component becomes an error factor, which makes it difficult to measure the loss and XT occurring at multiple points for each point. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] T. Oda et al., “MEASUREMENT OF MODAL ATTENUATION AND CROSSTALK IN SPLICED FEW-MODE FIBRES BASED ON MODE-RESOLVED BOTDA”, Proc. ECOC2019, P36 (2019). Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present disclosure is to provide a measurement device and a measurement method for independently measuring the loss and XT occurring at multiple points in an optical fiber through which multiple modes propagate. [Means for solving the problem]

[0006] The measurement device of the present disclosure is An apparatus for measuring loss and crosstalk of a test optical fiber, comprising: a light input unit that inputs a frequency-swept probe light into one end of an optical fiber under test and inputs a pump light having a frequency difference corresponding to a Brillouin frequency shift amount with respect to the probe light into the other end of the optical fiber under test, thereby amplifying the probe light in the optical fiber under test; a light receiving unit that receives a multiplexed light obtained by multiplexing the local light, to which a delay time has been imparted by being input into a delay optical fiber having a length corresponding to a group delay time when the local light is propagated through the entire section of the test optical fiber in a fundamental mode, with the probe light output from the test optical fiber; an interference waveform measurement unit that measures an interference waveform between the probe light and the local light using the signal intensity obtained from the light receiving unit; a fundamental mode time waveform analysis unit for acquiring a fundamental mode time waveform of the probe light propagated in the entire section of the test optical fiber as a fundamental mode based on the interference waveform; a control and calculation unit for calculating a loss and a crosstalk at a desired point of the optical fiber under test based on the fundamental mode time waveform; Equipped with.

[0007] The measurement method of the present disclosure includes: 1. A method for measuring loss and crosstalk of a test optical fiber, comprising the steps of: a light input unit inputs a frequency-swept probe light into one end of the optical fiber under test, and inputs a pump light having a frequency difference corresponding to a Brillouin frequency shift amount with respect to the probe light into the other end of the optical fiber under test, thereby amplifying the probe light in the optical fiber under test; a light receiving unit receives a multiplexed light obtained by multiplexing the local light, to which a delay time has been imparted by being input into a delay optical fiber having a length corresponding to a group delay time when the local light propagates through the entire section of the measured optical fiber in a fundamental mode, with the probe light output from the measured optical fiber; an interference waveform measuring unit measures an interference waveform between the probe light and the local light using the signal intensity obtained from the light receiving unit; a fundamental mode time waveform analysis unit acquires a fundamental mode time waveform of the probe light propagated in the entire section of the measured optical fiber as a fundamental mode based on the interference waveform; A control and calculation unit calculates the loss and crosstalk at a desired point of the optical fiber under test based on the fundamental mode time waveform. Effect of the Invention

[0008] According to the present disclosure, by suppressing the XT component of the probe light in the optical fiber, which is a problem in measuring loss and XT using Brillouin gain analysis, it is possible to independently measure the loss and XT occurring at each point in the optical fiber through which multiple modes propagate. [Brief description of the drawings]

[0009] [Figure 1] 1 illustrates an example of a measuring means according to an embodiment of the present disclosure. [Diagram 2] 1 shows an example of the Brillouin gain spectrum in a two-mode fiber. [Diagram 3] 1 is an explanatory diagram of a crosstalk component generated in a probe light propagated in an LPO1 mode through the entire section of a measured optical fiber. FIG. [Figure 4] 1A and 1B are diagrams illustrating a method for extracting probe light propagated in an LP01 mode through the entire section of an optical fiber to be measured. [Diagram 5] 2 shows an example of an optical fiber transmission line model. [Figure 6] An example of a method for obtaining the Brillouin gain coefficient will be described. [Figure 7] An example of a method for obtaining the Brillouin gain coefficient will be described. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings are assumed to indicate the same components.

[0011] (Brillouin gain occurring between multiple modes) The Brillouin frequency shift ν that maximizes the Brillouin amplification in the optical fiber under test b is given by the following equation: (Number 1) ν b =2n i V a / λ (1) where n i is the effective refractive index of the mode, V a is the effective acoustic wave velocity, and λ is the wavelength in a vacuum. In other words, in an optical fiber in which multiple modes propagate, the amount of Brillouin frequency shift differs depending on the propagating mode, and the Brillouin gain spectrum in each mode differs for each mode. This disclosure utilizes this fact.

[0012] For simplicity, when considering a two-mode fiber (TMF) in which only two modes propagate, the pump light and the probe light each have an LP 01 and L.P. 11If there is amplitude in both modes, (1)v 01-01 (Pump Light LP 01 Ingredients and Probe Light LP 01 (Interactions between ingredients) (2)v 01-11 (Pump Light LP 01 Ingredients and Probe Light LP 11 Between components, or pump light LP 11 Ingredients and Probe Light LP 01 (Interactions between ingredients) (3)v 11-11 (Pump Light LP 11 Ingredients and Probe Light LP 11 (Interactions between ingredients) There are three different Brillouin gain spectra:

[0013] In this way, the pump light and the probe light after TMF output have LP 01 Mode and LP 11 In this disclosure, the probe light is an LP 01 Only the components propagating in the mode are used, and the LP of the probe light is measured using the FMCW method. 01 This extracts only the mode components. This ensures that the probe light is always LP 01 Mode, pump light is LP 01 Mode or LP 11 Since the Brillouin amplification occurs in the optical fiber under test, the amount of Brillouin amplification is measured and analyzed to determine the loss and crosstalk (LP) that the pump light has experienced. 01 Loss of Mode, LP 11 It is possible to obtain the mode loss, crosstalk, etc. This will be explained in detail below.

[0014] The device of the present disclosure includes a light input section, an optical delay imparting section, and a light receiving section. The light input section converts the probe light and pump light into a specific mode and then inputs them into the optical fiber under test. The probe light is a continuous light that has been frequency swept. The pump light has an arbitrary frequency difference with respect to the probe light. The optical delay imparting unit imparts a delay time to the local light by inputting the local light, which is the same frequency sweep as the probe light, into a delay optical fiber. The delay optical fiber is capable of propagating only in the fundamental mode and has a length corresponding to the group delay time when the light propagates through the entire section of the measured optical fiber in the fundamental mode. The light receiving section receives the multiplexed light of the probe light and the local light propagated through the optical fiber under test.

[0015] The apparatus of the present disclosure includes an interference waveform measuring unit, a fundamental mode time waveform analyzing unit, and a control and calculation unit. The interference waveform measuring unit measures an interference waveform between the probe light propagated through the measured optical fiber and the local light propagated through the delay optical fiber. The fundamental mode time waveform analysis unit obtains the time waveform of the probe light propagated in the fundamental mode through the entire section of the measured optical fiber based on the interference waveform. The control and calculation unit controls the light input unit, and calculates the loss and XT at any point in the optical fiber under test based on the probe light time waveform acquired by the fundamental mode time waveform analysis unit.

[0016] The control and calculation unit executes the following process to obtain the Brillouin gain coefficient of the optical fiber under test. The control and calculation unit inputs pump light and probe light having an arbitrary frequency difference into a reference optical fiber having the same number of modes as the measured optical fiber and no mode coupling, in a combination of the pump light in all modes and the probe light in any one mode, for the light input unit. The control and calculation unit obtains the Brillouin gain coefficient generated in the probe light for each of the combinations of the modes from the signal intensity measured by the light receiving unit. The control and calculation unit varies the arbitrary frequency difference with respect to the light input unit, and obtains the Brillouin gain coefficient when the arbitrary frequency difference is varied from the signal intensity measured by the light receiving unit.

[0017] The control and calculation unit executes the following processes in order to measure the loss and XT of the optical fiber under test. The control and calculation unit prepares pump light and probe light having a frequency difference when the Brillouin gain coefficient is generated for the light input unit, and inputs pump light of any one mode and probe light of the mode when the Brillouin gain coefficient is obtained into the measured optical fiber. The control and calculation unit acquires, from the fundamental mode time waveform acquired by the fundamental mode time waveform analysis unit, the Brillouin amplified component generated by the probe light for each of the frequency differences as a distribution in the longitudinal direction of the measured optical fiber. The control and calculation unit obtains the ratio of the amount of amplification due to Brillouin amplification at each loss and XT occurrence point from the longitudinal distribution of the Brillouin amplified component. The control and calculation unit uses the ratio of the amplification amounts due to the Brillouin amplification and the Brillouin gain coefficient to calculate the loss and XT at the first loss and XT occurrence point from the point corresponding to the end of the measured optical fiber where the pump light is input. The loss and XT at the desired point are calculated using the ratio of the amount of amplification due to Brillouin amplification at the point where each loss and XT occurs, the Brillouin gain coefficient, and the loss and XT before the desired point.

[0018] (Example of device configuration) An example of a measurement means according to an embodiment of the present disclosure is shown in Figure 1. The device of this embodiment includes a frequency sweep light generating means 11, an optical frequency control means 31, an optical pulsing means 32, a delay optical fiber 22, a multiplexing element 13, an optical receiving means 14, a digitization processing means 15, and a numerical calculation means 16. The numerical calculation means 16 of the present disclosure can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network.

[0019] The frequency swept light generating means 11, the optical frequency control means 31, and the optical pulsing means 32 function as the optical input section of the present disclosure. The delay optical fiber 22 functions as the optical delay imparting section of the present disclosure. The optical receiving means 14 functions as the optical receiver section of the present disclosure. The digitization processing means 15 and the numerical calculation means 16 function as the interference waveform measuring section, the fundamental mode time waveform analyzing section, and the control calculation section of the present disclosure.

[0020] The device of this embodiment also includes a mode multiplexing / demultiplexing means 21 and a mode selection / multiplexing / demultiplexing means 33. The mode multiplexing / demultiplexing means 21 selects the mode of the probe light incident on the measured optical fiber 100. The mode selection / multiplexing / demultiplexing means 33 selects the mode of the pump light incident on the measured optical fiber 100.

[0021] For example, the probe light input to the input 21i1 of the mode multiplexing / demultiplexing means 21 is LP 01 The probe light input to the input 21i2 of the mode multiplexing / demultiplexing means 21 is LP 11 After being converted into a mode, the light is input to the optical fiber 100 to be measured.

[0022] For example, the probe light input to the input 33i1 of the mode multiplexing / demultiplexing means 33 is LP 01 The probe light input to the input 33i2 of the mode multiplexing / demultiplexing means 33 is input as an LP 11 After being converted into a mode, the light is input to the optical fiber 100 to be measured.

[0023] 1, the light output from a frequency swept light generating means 11 that generates coherent frequency swept light is branched into three by a branching element 12, which are probe light, pump light, and local light. The probe light is made fundamental mode by the mode multiplexing / demultiplexing means 21 in FIG. 1 and is input to the optical fiber 100 to be measured.

[0024] The pump light is given a frequency difference corresponding to the Brillouin frequency shift of the test optical fiber 100 on the higher frequency side by the optical frequency control means 31, and then pulsed by the optical pulsing means 32. The pump light is then converted to an arbitrary mode by the mode selection / multiplexing / demultiplexing means 33, and then is input to the test optical fiber 100 from the probe light output side.

[0025] The pulsed pump light and probe light incident on the measured optical fiber 100 generate Brillouin interactions throughout the entire section of the measured optical fiber 100, which causes the probe light to be Brillouin amplified. Since the pump light is pulsed, the amount of amplification of the probe light at each time based on the incident time of the pump light corresponds to the optical intensity of the pump light at each point on the measured optical fiber 100.

[0026] The local light is given a delay amount equivalent to that of the optical fiber 100 under test by the delay optical fiber 22. Then, the local light and the probe light emitted from the optical fiber 100 under test are multiplexed by the multiplexing element 13 and converted into an electrical signal by the optical receiving means 14. The optical receiving means 14 is any device capable of converting an optical signal into an electrical signal, and may be, for example, a balanced photodetector (BPD). The received electrical signal is digitized by the digitization processing means 15, and the time waveform of the probe light component propagated in the fundamental mode through the entire section of the optical fiber under test is extracted by the numerical calculation means 16. Then, the numerical calculation means 16 obtains the Brillouin gain distribution from the time waveform of the probe light component.

[0027] Specifically, the Brillouin gain is analyzed by obtaining a reference intensity of the probe light when no pump light is input, and then obtaining the signal intensity when both the pump light and the probe light are input. The increase in the reference intensity is calculated from the signal intensity, and the Brillouin gain distribution can be obtained.

[0028] This configuration is one example, and any means may be used as long as the device configuration can provide a frequency difference and an incidence time difference corresponding to a frequency (wavelength) shift according to the mode between the pump light and the probe light, and can extract a time waveform when the probe light propagated in the fundamental mode through the entire section of the measured optical fiber 100 is amplified in an arbitrary mode. In addition, since the present disclosure can be applied to a general SMF by shortening the incident wavelength, the measured optical fiber 100 may be any one that satisfies the condition for propagating multiple modes.

[0029] (Get Brillouin gain coefficient) The Brillouin gain spectrum generally has a full width at half maximum (FWHM) of about 30 MHz, so the Brillouin frequency shift for each mode, ν b If the modes are not far apart enough, the Brillouin effect will occur simultaneously for other mode components. In other words, when observing the gain, gain will occur for modes other than the desired one, resulting in a gain that is a mixture of loss and XT components. To resolve the gain component into a loss component and an XT component, the Brillouin gain coefficient between each mode is used. The resolution into loss and XT components using the Brillouin gain coefficient will be described later in the connection point model where loss and XT occur at multiple connection points. The procedure for obtaining the Brillouin gain coefficient between each mode is shown below.

[0030] In an optical fiber in which multiple modes propagate, the Brillouin frequency shift and the amount of gain differ depending on the combination of modes used. Using this difference, a reference optical fiber that exhibits the same characteristics as the measured optical fiber 100 and does not have mode coupling in the optical fiber is prepared in advance, and the Brillouin gain coefficient of each mode corresponding to the frequency difference between the pump light and the probe light in that optical fiber is acquired. Here, the reference optical fiber is an optical fiber that has the same number of modes as the measured optical fiber 100, and has the same Brillouin gain spectrum (the frequency difference between the pump light and the probe light at which gain occurs and the Brillouin gain coefficient at that frequency difference) generated between each mode, but does not have mode coupling. In other words, the reference optical fiber is used to acquire in advance the Brillouin gain coefficient in a state without mode coupling.

[0031] The Brillouin gain coefficient is obtained by combining pump light of all modes propagating through the measured optical fiber 100 and probe light of any one mode. In other words, in an optical fiber (TMF) through which only two modes propagate, if the probe light mode includes LP 01 When using the mode, the combinations for obtaining the Brillouin gain coefficient are as follows: (1)v 01-01 (Pump Light LP 01 Ingredients and Probe Light LP 01 (Interactions between ingredients) (2)v 01-11 (Pump Light LP 11 Ingredients and Probe Light LP 01 (Interactions between ingredients) An example of the Brillouin gain spectrum in a two-mode fiber is shown in Figure 2. From Figure 2, it can be seen that the Brillouin gain coefficients of the modes that are generated due to the frequency difference between the pump light and the probe light differ.

[0032] The Brillouin gain coefficient can be obtained by fixing the power of the pump light and probe light modes, changing the mode of the incident pump light while keeping the mode of the incident probe light fixed, and measuring the amount of gain at that time.

[0033] (Suppression of XT component of probe light by FMCW method) When there are multiple loss and XT generating points in the transmission line, gain occurs due to components (XT of the probe light) other than the mode of the probe light used to obtain the Brillouin gain coefficient, which makes it difficult to accurately measure loss and XT. Therefore, we use frequency swept optical interferometry (FMCW method) and differential group delay (DMD) between each mode to remove the XT component of the probe light in the optical fiber.

[0034] Specifically, in TMF, the mode of the probe light for measuring loss and XT is LP. 01 In this case, the entire section of the optical fiber 100 to be measured is LP 01 The concept of this extraction method is shown in Figures 3 and 4.

[0035] In this extraction, as shown in Fig. 3, a frequency sweep light source 11 is first prepared, and the light from the frequency sweep light source 11 is split into two by a splitter element 12. One of the split lights is a probe light, and the other is a local light. The probe light is fed to a TMF 101, and the local light is fed to an SMF 102, both of which are LPs. 01 At each connection point z1 to z n Since XT occurs at , part of the probe light is 11 The light emitted from the TMF 101 and the SMF 102 is multiplexed by the multiplexing element 13, and then converted into an electrical signal by the optical receiving means 14.

[0036] The received electrical signal is an interference waveform between the probe light and the local light as shown in Fig. 4(a), and by Fourier transforming this waveform, the probe light intensity versus the beat frequency of the probe light and the local light can be obtained as shown in Fig. 4(b), where the beat frequency corresponds to the delay time difference between the probe light and the local light.

[0037] In an optical fiber in which multiple modes propagate, each mode has a different group delay time. Here, LP 01 The group delay time of LP 11Assuming that the optical fiber is smaller than LP, the peak Pb with the smallest beat frequency as shown in FIG. 4(b) is the peak Pb that runs through the entire section of the optical fiber 100 to be measured. 01 In other words, the peaks greater than this frequency are the components propagated as LP in a part of the optical fiber 100 under test. 11 Here, for the waveform in FIG. 4(b), only the peak Pb is extracted using a frequency filter, and this peak Pb is shifted to DC, that is, to the point where the frequency is zero. After that, by performing an inverse Fourier transform on this waveform, the entire section of the optical fiber 100 to be measured is transformed into an LP 01 It is possible to obtain the time waveform of the probe light of the component propagated as a fundamental mode, i.e., the fundamental mode time waveform.

[0038] (Measurement of loss and XT using Brillouin gain and Brillouin loss) For the purpose of explaining the embodiment of the present disclosure, an optical fiber transmission line model having n connection points (points where loss and XT occur) as shown in FIG. 5 is considered as an example. In this model, LP 11 Mode, probe light to LP 01 By utilizing the mode, the amount of Brillouin amplification generated in the probe light upon collision is compared in the longitudinal direction of the optical fiber under test 100, and the amount of loss and XT generated at each connection point is measured.

[0039] As shown in FIG. 5, each of the connection points z1 to z n In this embodiment, when the pump light and the probe light pass through, two types of loss occur: loss and XT, which is coupling to other mode components. In this embodiment, the probe light after being emitted from the optical fiber under test 100 is LP-FMCW ... 01 Only the components that propagate as modes are extracted. The extracted LP 01 Using the probe light component of the LP mode, 01 Mode probe light and LP 11 Mode or LP 01 The amount of amplification due to Brillouin amplification that occurs due to the Brillouin interaction with the pump light of the mode is obtained.

[0040] The optical intensities of the pump light and the probe light incident on the test optical fiber 100 are respectively P r , P p When this is done, the LP that occurs at the connection point z of the transmission line is 01 Amplification amount of probe light by Brillouin amplification ΔPs 01 can be expressed as follows:

number

[0041] When comparing the amount of amplification by the Brillouin amplification in the longitudinal direction of the equations (2) to (5), n The ratio of the amplification amounts of the probe light observed at

number

number

[0042] Here, from equations (7) to (10), it can be seen that the observed ratio is a value including the loss component and XT component that the pump light receives. Also, L2 in equation (8) and L3 in equation (9) are the values ​​of the previous connection points z1 to z nThis value includes the loss generated by and XT, and is expressed as L n It can be seen that the form is obtained by substituting 2 and 3 for n.

[0043] Next, each connection point z1~z n The loss and XT information is obtained by using the ratio of the gains observed at L2 to L3 and the Brillouin gain coefficient. n Regarding all L n Therefore, the following describes the measurement procedure for the loss and XT that occurs at the first connection point z1 using L1 and the measurement procedure for L n The nth connection point z n This article describes the measurement procedure for the losses and XT that occur in the

[0044] (i) Loss and XT at the first connection point z1 As shown in equation (7), L1 is the LP 11 Here, L1 and the Brillouin gain coefficient depend on the frequency difference ν between the pump light and the probe light. Taking the frequency difference into account, L1 in equation (7) can be expressed as follows:

number

[0045] where K(ν) is g 01-01 (ν) and g 01-11 Using the results obtained at the two frequency differences ν1 and ν2, the loss and XT occurring at the connection point z1 can be expressed as follows:

number

[0046] (ii) nth connection point z n Losses and XT As in the process of (i), we utilize the property that Ln and the Brillouin gain coefficient depend on the frequency difference ν between the pump light and the probe light. Taking the frequency difference into consideration, Ln in equation (10) can be expressed as follows:

number

[0047] Here, if the denominator of equation (14) is set to I(ν), it can be transformed as follows:

number

[0048] Using the results obtained for the four frequency differences ν1, ν2, ν3, and ν4, this can be expressed in the following form:

number

[0049] Here, when the inverse matrix of the matrix including the Brillouin gain coefficients is multiplied in equation (16), equation (16) can be transformed as follows:

number

[0050] Also η 1~n-1 is from connection point z1 to connection point z n-1 Since it can be calculated from the measured loss and XT at all connection points up to , it is also possible to obtain I(ν). Therefore, using equation (17), n The loss and XT that occurs can be obtained.

[0051] (Measurement Procedure of the Present Disclosure) The loss and XT measurement method disclosed herein is a method for independently measuring the loss and XT occurring at multiple points in a test optical fiber 100, and includes a first step S1 of acquiring a Brillouin gain coefficient, and a second step S2 of measuring the loss and XT occurring at the nth connection point z n and a second step S2 of calculating the losses in and XT.

[0052] FIG. 6 shows an example of the first step S1. A reference optical fiber without mode coupling that exhibits the same characteristics as the measured optical fiber 100 is prepared (S11). The reference optical fiber has the same number of modes as the measured optical fiber 100 and exhibits the same characteristics as the measured optical fiber 100. The frequency difference between the probe light and the pump light is set (S12). The modes of the pump light and the probe light are converted (S13). For example, the pump light is converted to an LP 01 Mode to LP 11 Both the pump and probe beams are converted into LP mode. 01 In this mode, this step is not necessary. The pump light and the probe light are input to a reference optical fiber (S14), and the Brillouin gain coefficient of the gain generated in the probe light is obtained (S15).

[0053] It is determined whether or not all the Brillouin gain coefficients to be used in the second step have been acquired (S16), and if all the Brillouin gain coefficients to be used in the second step have not been acquired, steps S12 to S15 are repeated. If all the Brillouin gain coefficients to be used in the second step have been acquired, the process proceeds to the second step. In this embodiment, the measured optical fiber 100 is determined to have a propagating mode of LP. 01 Mode and LP 11 Because it is a mode, g 01-01 and g 01-11 Steps S12 to S15 are repeated until both of the above are acquired.

[0054] FIG. 7 shows an example of the second step S2. S21: Set the frequency difference between the probe light and the pump light. At this time, set it to the frequency difference between the probe light and the pump light when any one Brillouin gain coefficient is obtained. For example, the LP shown in FIG. 11 Mode and LP 01 Brillouin gain coefficient of the mode g 01-11 The frequency difference is set to 10.795 GHz, which is the frequency difference at which the peak of the gain coefficient is obtained. In this disclosure, at least two or more gain coefficients are required. Therefore, a frequency difference of two or more is set. S22: Set the time difference between the incidence of the probe light and the pump light. Since the time difference between the incidence of the probe light and the pump light corresponds to the point where they collide in the optical fiber, the time difference is set so that the gain before and after the splice point can be obtained.

[0055] S23: Convert the mode of the pump light and the probe light. For example, convert the pump light into an LP 01 Mode to LP 11 Both the pump and probe beams are converted into LP mode. 01 In this mode, this step is not necessary. S24: Extract only the probe light that has propagated in the fundamental mode through the entire section of the test optical fiber 100 by the FMCW method. The specific method is as described in FIG.

[0056] S25: The pump light is injected. S26: From the distribution of the amplification amount of the probe light when the pump light is input, each connection point z1 to z n The ratio of the gains in S27: Determine whether the distribution of the amount of amplification in all frequency differences for which the gain coefficients have been obtained has been acquired. If the distribution of the amount of amplification in all frequency differences for which the gain coefficients have been acquired has not been acquired (No in S27), steps S21 to S26 are repeated. If the distribution of the amount of amplification in all frequency differences for which the gain coefficients have been acquired has been acquired (Yes in S27), the following process is executed.

[0057] S28: Using the ratio between the Brillouin gain coefficient obtained in the first step S1 and the amount of amplification observed, the loss and XT at the first connection point viewed from the pump light input side are obtained. S29: It is determined whether the connection point for which loss and XT are to be obtained is the first connection point. S30: The i-th loss and XT are obtained using the losses and XT from 1 to i-1, the Brillouin gain coefficient obtained in step S1, and the ratio of the i-th amplification amount.

[0058] In the present embodiment, the measured optical fiber is a TMF in which only two modes propagate, but the present disclosure is not limited to this. For example, the present disclosure can be applied to a measured optical fiber in which three or more modes propagate by setting the propagation modes of the probe light and the pump light to three or more modes and setting the frequency difference in step S21 to three or more modes. [Industrial Applicability]

[0059] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0060] 11: Frequency swept light source 12, 17: Branching element 13: Wave combining element 14: Optical receiving means 15: Digitalization processing means 16: Numerical calculation means 21: Mode multiplexing / demultiplexing means 22: Delay optical fiber 31: Optical frequency control means 32: Light pulsing means 100: Optical fiber under test 101:TMF

Claims

1. An apparatus for measuring loss and crosstalk of a test optical fiber, comprising: a light input unit that inputs a frequency-swept probe light into one end of an optical fiber under test and inputs a pump light having a frequency difference corresponding to a Brillouin frequency shift amount with respect to the probe light into the other end of the optical fiber under test, thereby amplifying the probe light in the optical fiber under test; a light receiving unit that receives a multiplexed light obtained by multiplexing the local light, to which a delay time has been imparted by being input into a delay optical fiber having a length corresponding to a group delay time when the local light is propagated through the entire section of the test optical fiber in a fundamental mode, with the probe light output from the test optical fiber; an interference waveform measurement unit that measures an interference waveform between the probe light and the local light using the signal intensity obtained from the light receiving unit; a fundamental mode time waveform analysis unit for acquiring a fundamental mode time waveform of the probe light propagated in the entire section of the test optical fiber as a fundamental mode based on the interference waveform; a control and calculation unit for calculating a loss and a crosstalk at a desired point of the optical fiber under test based on the fundamental mode time waveform; An apparatus comprising:

2. The control calculation unit is Obtaining the Brillouin gain coefficient of the test optical fiber for all combinations of pump light and probe light in modes that can propagate through the test optical fiber; acquiring a Brillouin amplified component generated by the probe light as a distribution in the longitudinal direction of the optical fiber under test using the fundamental mode time waveform; obtaining a ratio of the amount of amplification due to Brillouin amplification at a point where loss and crosstalk occur, using the distribution of the Brillouin amplification component in the longitudinal direction of the test optical fiber; Calculating the loss and crosstalk at an arbitrary point of the test optical fiber using the ratio of the amplification amounts from the input point of the pump light to the desired point and the Brillouin gain coefficient; The apparatus of claim 1 , further comprising:

3. The control calculation unit is a pump light and a probe light having an arbitrary frequency difference with respect to the probe light are inputted to the light input unit in combination with a probe light of any one of the modes capable of propagation of the test optical fiber into a reference optical fiber having the same propagating mode as the test optical fiber and free of mode coupling; acquiring a Brillouin gain coefficient of the test optical fiber for each combination of modes that can propagate through the test optical fiber from the signal intensity measured by the light receiving unit; changing the arbitrary frequency difference with respect to the light input unit, and inputting the arbitrary frequency difference into the reference optical fiber in combination with a probe light of any one of the modes capable of propagation through the test optical fiber; obtaining the Brillouin gain coefficient when the arbitrary frequency difference is changed from the signal intensity measured by the light receiving unit; Calculating a loss and a crosstalk at a desired point of the test optical fiber based on the Brillouin gain coefficient of the test optical fiber corresponding to the frequency difference and the fundamental mode time waveform; The apparatus of claim 2 , further comprising:

4. The control calculation unit is Calculating the loss and crosstalk at the first loss and crosstalk occurrence point from the incident point of the pump light using the ratio of the amplification amounts and the Brillouin gain coefficient at the first loss and crosstalk occurrence point from the incident point of the pump light; Calculating the loss and crosstalk at the desired point using the loss and crosstalk from the incident point of the pump light to the desired point, the ratio of the amplification amounts at the desired point, and the Brillouin gain coefficient; 4. The apparatus according to claim 2 or 3, further comprising:

5. The fundamental mode time waveform analysis unit The interference waveform measured by the interference waveform measuring unit is subjected to a Fourier transform to obtain a probe light intensity of a beat frequency of the probe light and the local light; extracting a frequency component of a minimum peak among peaks of the probe light intensity of the beat frequency; The fundamental mode time waveform is obtained by performing an inverse Fourier transform on the extracted frequency components.

5. Apparatus according to any one of claims 1 to 4.

6. 1. A method for measuring loss and crosstalk of a test optical fiber, comprising the steps of: a light input unit inputs a frequency-swept probe light into one end of the optical fiber under test, and inputs a pump light having a frequency difference corresponding to the Brillouin frequency shift amount with respect to the probe light into the other end of the optical fiber under test, thereby amplifying the probe light in the optical fiber under test; a light receiving unit receives a multiplexed light obtained by multiplexing the local light, to which a delay time has been imparted by being input into a delay optical fiber having a length corresponding to a group delay time when the local light propagates through the entire section of the measured optical fiber in a fundamental mode, with the probe light output from the measured optical fiber; an interference waveform measuring unit measures an interference waveform between the probe light and the local light using the signal intensity obtained from the light receiving unit; a fundamental mode time waveform analysis unit acquires a fundamental mode time waveform of the probe light propagated in the entire section of the measured optical fiber as a fundamental mode based on the interference waveform; a control and calculation unit for calculating a loss and a crosstalk at a desired point of the optical fiber to be measured based on the fundamental mode time waveform; method.

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