Optical characteristic evaluation device and optical characteristic evaluation method

Abstract

The purpose of the present invention is to provide an optical characteristic evaluation device and an optical characteristic evaluation method that make it possible to evaluate mode-dependent loss from one end of an optical fiber subject to testing. An optical characteristic evaluation device 301 according to the present invention is provided with an optical pulse tester 10 and an analysis unit 20.　The optical pulse tester 10 is characterized by: introducing, at one end of a multicore optical fiber 50 that is subject to testing, a test light pulse to one of the cores of the multicore optical fiber 50, and receiving backscattered light output from the core; and acquiring the waveform of the intensity of the backscattered light in relation to the distance of the multicore optical fiber 50. The analysis unit 20 is characterized by: performing linear approximation on a desired section of the waveform; calculating the variance of the waveform in the desired section with respect to the linear approximation; and considering the variance as an evaluation value of mode-dependent loss of the multicore optical fiber 50.

Description

Optical property evaluation device and optical property evaluation method

[0001] The present disclosure relates to an apparatus and method for evaluating mode-dependent loss in a coupled multi-core optical fiber.

[0002] A multi-core optical fiber is an optical fiber that has multiple cores in a single optical fiber. Compared to conventional fibers, conventional fibers have only one core. Because the number of cores increases, a multi-core optical fiber has the characteristic of increasing the communication capacity per optical fiber.

[0003] Furthermore, multi-core optical fibers are classified into uncoupled and coupled types. The uncoupled type is a type in which the cores of a multi-core optical fiber are separated by a large distance, and each core is treated as an independent entity. In contrast, the coupled type is a type in which the cores of a multi-core optical fiber are not separated by a large distance, and part of the communication light propagating through one core is actively coupled to the adjacent core.

[0004] Therefore, a coupled multi-core optical fiber can have a larger number of cores per optical fiber than an uncoupled multi-core optical fiber. In other words, the communication capacity per optical fiber can be further increased. Therefore, a coupled multi-core optical fiber is one of the promising optical fibers as a medium for realizing large-capacity optical communications in the future.

[0005] However, in a coupled multi-core optical fiber, depending on the state of the incident light (phase, amplitude, etc.), part of the communication light randomly couples with adjacent cores, resulting in different losses depending on the optical path. This difference in loss is defined as mode-dependent loss (MDL).

[0006] Mode-dependent loss is considered a parameter that limits transmission capacity, and is known to be highly dependent on bending of coupled multi-core fibers and core misalignment during splicing. When considering a transmission system, mode-dependent loss changes due to bending of coupled multi-core fibers and poor splicing that occur during construction of the transmission line. Therefore, it is necessary to evaluate the mode-dependent loss of coupled multi-core fibers when constructing a transmission line.

[0007] 1A and 1B are diagrams illustrating a coupled multi-core optical fiber and mode-dependent loss. A coupled multi-core optical fiber 50 with two cores (core C1 and core C2) is taken as an example. As shown in Fig. 1A, the two cores are parallel, and light propagating to core C1 moves from core C1 to core C2, and then moves from core C2 to core C1. This transition of light across cores is called mode coupling, and an optical fiber in which the cores are coupled at any location is called a coupled multi-core optical fiber.

[0008] FIG. 1B is a diagram illustrating mode-dependent loss. The loss per segment is considered. For example, the loss in segment S1 of core C1 is assumed to be 1 (shown as "-1" in the diagram). Light propagating through core C1 then moves from core C1 to core C2, losing 1 in each of segments S2 and S3. Therefore, light passing through three segments (segment S1 to core C1, segment S2 to core C2, and segment S3 to core C2) loses a total of 3. Similarly, light propagating through core 2 loses a total of 7 in three segments (segment S1 to core C2, segment S2 to core C1, and segment S3 to core C1). Note that the path taken (including the transition between core C1 and core C2) is determined by the state (amplitude and phase) of the incident light on each core. In other words, mode-dependent loss can be said to be the difference in loss depending on the path the light has traveled.

[0009] For example, Non-Patent Documents 1 and 2 disclose methods for evaluating mode-dependent loss. Non-Patent Document 1 discloses a method for measuring mode-dependent loss from amplitude and phase information of propagating light output from an optical fiber under test. Non-Patent Document 2 proposes a method for evaluating mode-dependent loss from changes in the intensity of light output from the optical fiber under test by randomly changing the intensity and phase state of light input to the optical fiber under test.

[0010] Fontaine, N. K. , et al. “Characterization of Space-Division Multiplexing Systems using a Swept-Wavelength Interferometer”. Optical Fiber Communication Conference / National Fiber Optic Engineers Conference 2013. OW1K. 2, 2013. Hasegawa, T. and T. Hayashi. “Measurement of Mode Dependent Loss of Randomly-Coupled Multi-Core Fiber using Scrambling Method”, T2C, in 26th Optoelectronics and Communications Conference. 2021.

[0011] As shown in Figure 2, when constructing a transmission line, both ends of the optical fiber under test are located at separate locations. To evaluate mode-dependent loss using the two-end measurement methods of Non-Patent Documents 1 and 2, it is necessary to place workers at both ends of the optical fiber under test. In other words, the methods of Non-Patent Documents 1 and 2 cannot evaluate mode-dependent loss from one end of the optical fiber under test, which makes it difficult to improve work efficiency.

[0012] SUMMARY OF THE INVENTION In order to solve the above problems, an object of the present invention is to provide an optical characteristic evaluation apparatus and an optical characteristic evaluation method that can evaluate mode-dependent loss from one end of an optical fiber under test.

[0013] In order to achieve the above object, the optical characteristic evaluation apparatus according to the present invention evaluates mode-dependent loss by utilizing backscattered light of an optical pulse input from one end of an optical fiber under test.

[0014] Specifically, the optical characteristic evaluation device according to the present invention is an optical characteristic evaluation device including an optical pulse tester and an analysis unit, wherein the optical pulse tester is configured to: input a test optical pulse into one of the cores of a multi-core optical fiber, which is an optical fiber under test, at one end of the multi-core optical fiber, and receive backscattered light output from the core; and acquire a waveform of the optical intensity of the backscattered light versus the distance of the multi-core optical fiber; and the analysis unit is configured to: linearly approximate a desired section of the waveform; calculate a dispersion for the linear approximation of the waveform in the desired section; and use the dispersion as an evaluation value for the mode-dependent loss of the multi-core optical fiber.

[0015] Moreover, an optical characteristic evaluation method according to the present invention is an evaluation method for evaluating a mode-dependent loss of a multi-core optical fiber that is an optical fiber under test, the method comprising: connecting an optical pulse tester to one end of the multi-core optical fiber; injecting a test optical pulse into one of the cores of the multi-core optical fiber; receiving backscattered light output from the core; acquiring a waveform of the light intensity of the backscattered light versus the distance of the multi-core optical fiber; performing a linear approximation for a desired section of the waveform; calculating a dispersion for the linear approximation of the waveform in the desired section; and using the dispersion as an evaluation value for the mode-dependent loss of the multi-core optical fiber.

[0016] The backscattered light of an optical pulse incident on an optical fiber has various states (phase / amplitude). The backscattered light returns to the input end of the optical pulse. Using this backscattered light, it is possible to evaluate the mode-dependent loss at one end of a coupled multi-core optical fiber.

[0017] The intensity of backscattered light that passes through a segment in which mode-dependent loss (difference in loss between cores) occurs varies depending on the core through which it passes within that segment, resulting in a large difference (variation) in the intensity of the backscattered light. In other words, if the difference (variation) in the intensity of the backscattered light is small, it is backscattered light that occurred when the test optical pulse passed through a segment with no (or small) mode-dependent loss, while if the difference (variation) in the intensity of the backscattered light is large, it is backscattered light that occurred when the test optical pulse passed through a segment with (or large) mode-dependent loss. Therefore, mode-dependent loss can be evaluated by linearly approximating the waveform of the backscattered light and calculating the variance of the waveform relative to that approximate straight line.

[0018] Therefore, the present invention can provide an optical characteristic evaluation apparatus and an optical characteristic evaluation method that can evaluate mode-dependent loss from one end of an optical fiber under test.

[0019] Furthermore, the analyzing unit determines the position where the dispersion changes as the position where mode-dependent loss occurs.

[0020] The above inventions can be combined as much as possible.

[0021] The present invention can provide an optical characteristic evaluation apparatus and an optical characteristic evaluation method that can evaluate mode-dependent loss from one end of an optical fiber under test.

[0022] FIG. 1 is a diagram explaining a coupled multi-core optical fiber and mode-dependent loss. FIG. 2 is a diagram explaining a conventional coupled multi-core optical fiber and a method for evaluating mode-dependent loss. FIG. 3 is a diagram explaining an optical property evaluation apparatus according to the present invention. FIG. 4 is a diagram explaining an optical property evaluation method according to the present invention. FIG. 5 is a diagram explaining an optical property evaluation method according to the present invention. FIG. 6 is a diagram explaining a method for evaluating mode-dependent loss. FIG. 7 is a diagram explaining a difference between a conventional method and the method of the present invention for evaluating mode-dependent loss. FIG. 8 is a diagram explaining a waveform of backscattered light obtained in evaluating mode-dependent loss. FIG. 9 is a diagram explaining information displayed on a display unit of the optical property evaluation apparatus according to the present invention.

[0023] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components.

[0024] (Embodiment 1) Fig. 3 is a diagram illustrating an optical characteristic evaluation apparatus 301 of this embodiment. The optical characteristic evaluation apparatus 301 includes an optical pulse tester 10 and an analysis unit 20. The optical pulse tester 10 is characterized by: injecting a test optical pulse into one of the cores of a multi-core optical fiber 50, which is an optical fiber under test, at one end of the multi-core optical fiber 50, and receiving backscattered light output from the core; and acquiring a waveform of the optical intensity of the backscattered light with respect to the distance through the multi-core optical fiber 50. The analysis unit 20 is characterized by: performing a linear approximation for a desired section of the waveform; calculating a dispersion for the linear approximation of the waveform in the desired section; and using the dispersion as an evaluation value for the mode-dependent loss of the multi-core optical fiber 50.

[0025] As shown in Fig. 3, the optical pulse tester 10 includes a pulse light source unit 11, an input / output unit 12, and a receiving unit 13. The pulse light source unit 11 outputs a test optical pulse to be incident on the optical fiber under test. The input / output unit 12 includes, for example, an optical circulator 12a and an input / output device 12b. The receiving unit 13 includes, for example, an opto-electrical converter 13a and an AD converter 13b. As shown in Fig. 3, the analyzing unit 20 includes a waveform analyzing unit 21, a mode-dependent loss evaluating unit 22, and a display unit 23.

[0026] 4 is a diagram illustrating the operation of the optical characteristic evaluation device 301. In order to evaluate the mode-dependent loss of the multi-core optical fiber 50, the optical characteristic evaluation device 301 is characterized by the following steps: connecting the optical pulse tester 10 to one end of the multi-core optical fiber 50; injecting a test optical pulse from the optical pulse tester 10 into one of the cores of the multi-core optical fiber 50 (step St01); receiving backscattered light output from the core by the optical pulse tester 10 (step St02); acquiring a waveform of the light intensity of the backscattered light versus the distance along the multi-core optical fiber 50 by the analysis unit 20 (step St03); performing a linear approximation of the waveform for a desired section by the analysis unit 20; calculating dispersion for the linear approximation of the waveform in the desired section by the analysis unit 20; determining the dispersion as an evaluation value of the mode-dependent loss of the multi-core optical fiber by the analysis unit 20 (step St04); and displaying the evaluation value on the display unit 23 (step St05).

[0027] This will be explained in more detail. First, in step St00, the input / output unit 12 of the optical pulse tester 10 is connected to one end of the multi-core optical fiber 50. Next, in step St01, a test optical pulse is generated by the pulse light source 11, and the test optical pulse is incident on an arbitrary core at one end of the multi-core optical fiber 50 via the input / output unit 12, thereby generating backscattered light within the multi-core optical fiber 50. Then, in step St02, the backscattered light output from an arbitrary core at one end of the multi-core optical fiber 50 is sent to the receiving unit 13 via the input / output unit 12. The photoelectric converter 13a of the receiving unit 13 converts the backscattered light intensity into an electric signal, and the AD converter 13b converts it into a digital signal and sends it to the analyzing unit 20. In step St03, the waveform analyzing unit 21 of the analyzing unit 20 acquires a backscattered light intensity waveform versus distance from one end of the multi-core optical fiber 50 from the received light signal. In step St04, the mode-dependent loss evaluation unit 22 of the analysis unit 20 analyzes the backscattered light intensity waveform and performs evaluation. Details of the evaluation method will be described later. Finally, in step St05, the evaluation value obtained by the mode-dependent loss evaluation unit 22 is displayed on the display unit 23.

[0028] 5 is a diagram for explaining this optical characteristic evaluation method. The explanation will be given assuming that the optical fiber under test is a coupled multi-core fiber 50 with two cores, but the same applies to a multi-core fiber 50 with three or more cores.

[0029] First, we will explain how a test optical pulse is input into the multi-core fiber 50. The test optical pulse generated by the pulse light source 11 is input into any one core of the multi-core fiber 50 via the optical circulator 12a and the input / output device 12b. For example, it is assumed that the test optical pulse is input into core C1.

[0030] The test optical pulse propagates through the multi-core optical fiber 50, and is coupled to another core C2 during the propagation process. Assume that there is a region 50X in which the difference in loss between cores is large (the mode-dependent loss is large) at an arbitrary location in the multi-core optical fiber 50. In the region 50X, a difference in loss occurs between the core C1 and the core C2 (the loss of the core C1 is large, and the loss of the core C2 is small) (see FIG. 5A ).

[0031] The test light pulse is scattered as it propagates through the optical fiber under test, and a portion of it returns to the side where the test light pulse is incident. This returning light is called backscattered light. The measurement target is the backscattered light intensity of one arbitrary core (core C1). The backscattered light is converted into a digital signal by the photoelectric converter 13a and AD converter 13b via the input / output device 12b and the optical circulator 12a.

[0032] The waveform analyzer 21 acquires the relationship between the distance and the backscattered light intensity of the optical fiber under test (OTDR waveform). Figure 5(B) shows an OTDR waveform with the horizontal axis representing distance and the vertical axis representing backscattered light intensity. This OTDR waveform has two sections: one with small variation and one with large variation. It is believed that differences in mode-dependent loss occur where the variation in the OTDR waveform changes (including steps). In other words, the magnitude of the difference in mode-dependent loss is evaluated based on the magnitude of the change in variation in the OTDR waveform. Specifically, as shown by I-1 in Figure 5(B), if the change in the amount of variation is large, the MDL is large, and if the change in the amount of variation is small, the MDL is small. Furthermore, the location can be identified from the horizontal axis. Specifically, as shown by I-2 in Figure 5(B), the location where the variation changes is defined as the MDL occurrence point (the location of area 50X).

[0033] FIG. 6 is a diagram explaining a method for evaluating mode-dependent loss. By randomly changing the state of incident light, all manner of mode-coupled states can be generated, and the magnitude of mode-dependent loss can be evaluated from the variation in intensity of light (output light) that passes through the core. FIG. 6(A) is a diagram explaining a case where light is concentrated on a path Rs with low loss. FIG. 6(B) is a diagram explaining a case where light is concentrated on a path Rl with high loss. FIG. 6(C) is a diagram explaining the change in light intensity of output light when the difference in mode-dependent loss is large. FIG. 6(D) is a diagram explaining the change in light intensity of output light when the difference in mode-dependent loss is small.

[0034] When light is concentrated on the low-loss path Rs as in Figure 6(A), the light intensity is strong in Figures 6(C) and 6(D) (I-3). On the other hand, when light is concentrated on the high-loss path Rl as in Figure 6(B), the light intensity is weak in Figures 6(C) and 6(D) (I-4). In this way, when the difference in mode-dependent loss is large, the variation in light intensity is large as in Figure 6(C), and when the difference in mode-dependent loss is small, the variation in light intensity is small as in Figure 6(D). Therefore, the magnitude of the difference in mode-dependent loss can be evaluated from the waveform.

[0035] 7A and 7B are diagrams illustrating the difference between the conventional method and the method of the present invention for evaluating mode-dependent loss. Fig. 7A is a diagram illustrating the conventional method. Conventionally, test light is generated by giving random phase and amplitude to light from a light source 31 using an optical modulator 32. The test light is then incident on one end of a multi-core optical fiber 50, passes through the multi-core optical fiber 50, and is received by a light-receiving device 33 located at the other end, and a waveform such as that described in Fig. 6 is obtained. For this reason, it is necessary to position operators at both ends of the multi-core optical fiber 50.

[0036] 7(B) is a diagram illustrating the technique of the present invention. The test light pulse incident on the multi-core optical fiber 50 is scattered at each point of the multi-core optical fiber 50, and a part of it returns as backscattered light. The backscattered light has a random phase and amplitude. Therefore, it is possible to evaluate the mode-dependent loss without modulating the light on the side where the test light is incident, and since the backscattered light returning to the side where the test light is incident is used, the mode-dependent loss can be evaluated at the incident end of the test light.

[0037] FIG. 8 is a diagram illustrating the waveform of backscattered light obtained in the evaluation of mode-dependent loss. The backscattered light at each point in a coupled multi-core optical fiber has a random optical state (phase and amplitude). The mode coupling state in a coupled multi-core optical fiber varies depending on the phase difference between the light propagating through the multiple cores. Therefore, the backscattered light generated at each point (generated beyond the target position of the multi-core optical fiber 50) randomly changes the mode coupling state at the target position. FIG. 8 shows the backscattered light intensity waveform that passes through the target position and returns to the input end of the test light. This is backscattered light intensity data of light incident on the target position in various mode coupling states, and light that has passed through a path with low loss (data I-5 with high backscattered light intensity) and light that has passed through a path with high loss (data I-6 with low backscattered light intensity) are randomly distributed.

[0038] 8 is linearly approximated (linear approximation line I-7), and the variance of the backscattered light intensity waveform relative to the linear approximation line I-7 is used as the evaluation value for the mode-dependent loss. If the variance is large, it can be evaluated that the difference in mode-dependent loss at the point of interest is large.

[0039] 9 is a diagram illustrating information displayed on the display unit 23. The display unit 23 displays the acquired backscattered light intensity waveform. The operator sets a range I-9 for calculating the mode-dependent loss evaluation value. The mode-dependent loss evaluation unit 22 performs linear approximation of the range I-9, calculates the variance within the same range, and displays the variance on the display unit 23 (I-10).

[0040] Second Embodiment In the present embodiment, a description will be given of a case in which a coupled multi-core fiber is cabled and the optical characteristics of the cable are evaluated. After the cable is laid, the measurements described in the first embodiment are performed.

[0041] Mode-dependent loss is evaluated when the cable is laid (measurements explained in Figure 9 are performed). By obtaining a waveform like that shown in Figure 5, I-1 (presence or absence of mode-dependent loss) and I-2 (location where mode-dependent loss occurs) can be determined. If the evaluation value (dispersion) of mode-dependent loss is large, it can be determined that there was an abnormality during installation. For example, if an optical cable is bent more than expected and this condition is discovered through measurements, the cable can be re-laid, or if re-installation is not possible, the cable can be replaced. Furthermore, if the quality of the optical fiber connection is worse than expected and this condition is discovered through measurements, a decision can be made to re-connect the cable.

[0042] (Effects) The present invention makes it possible to easily evaluate the magnitude of the mode-dependent loss of a coupled multi-core optical fiber by measuring it at one end of the optical fiber. By repeating such measurements, it is possible to build a high-quality network and provide high-quality services to users.

[0043] (Definitions, etc.) Mode-dependent loss: The loss ratio between the core with the largest loss and the core with the smallest loss among the cores of a multi-core optical fiber. PM in the optical modulation unit 32 in Fig. 7: Phase modulator PM connected to the far end of the multi-core optical fiber 50 in Fig. 7: Power meter

[0044] 10: Optical pulse tester 11: Pulse light source 12: Input / output section 12a: Optical circulator 12b: Input / output device 13: Receiver 13a: Photoelectric converter 13b: AD converter 20: Analyzer 21: Waveform analyzer 22: Mode-dependent loss evaluation section 23: Display 31: Light source (tunable light source) 32: Modulator 33: Light-receiving device (power meter) 50: Multi-core optical fiber 50X: Area 301: Optical characteristic evaluation device

Claims

1. An optical characteristic evaluation device comprising an optical pulse tester and an analysis unit, wherein the optical pulse tester: at one end of a multi-core optical fiber that is an optical fiber under test, inputs a test optical pulse into one of the cores of the multi-core optical fiber and receives backscattered light output from the core; and acquires a waveform of the optical intensity of the backscattered light versus the distance of the multi-core optical fiber; and the analysis unit: performs a linear approximation for a desired section of the waveform; calculates dispersion for the linear approximation of the waveform in the desired section; and uses the dispersion as an evaluation value for the mode-dependent loss of the multi-core optical fiber.

2. The optical characteristic evaluation device according to claim 1, wherein the analysis unit determines the position where the dispersion changes as the position where mode-dependent loss occurs.

3. An evaluation method for evaluating mode-dependent loss of a multi-core optical fiber that is an optical fiber under test, comprising: connecting an optical pulse tester to one end of the multi-core optical fiber; injecting a test optical pulse into one of the cores of the multi-core optical fiber; receiving backscattered light output from the core; obtaining a waveform of the optical intensity of the backscattered light versus the distance of the multi-core optical fiber; performing a linear approximation of a desired section of the waveform; calculating dispersion for the linear approximation of the waveform in the desired section; and using the dispersion as an evaluation value for the mode-dependent loss of the multi-core optical fiber.

4. The evaluation method according to claim 3, further characterized in that the position where the dispersion changes is determined to be the position where mode dependent loss occurs.

Images