Mode dependent loss measuring device and mode dependent loss measuring method

Abstract

A mode-dependent loss measurement device according to an embodiment of the present disclosure measures the mode-dependent loss of an optical fiber under test that is made of a coupled MCF. The device includes a light source, a photodetector, a mode-coupling state changing means, and an analyzer. The light source inputs light into the input end of a pump optical fiber that is made of another coupled MCF. The photodetector detects the sum of the power of output light from multiple core end faces located on the output end of the optical fiber under test. The mode-coupling state changing means changes the mode-coupling state of the pump optical fiber. The analyzer calculates the mode-dependent loss of the optical fiber under test from the fluctuations in the optical power detected by the photodetector.

Description

[Technical field]

[0001] The present disclosure relates to a mode-dependent loss measurement apparatus and a mode-dependent loss measurement method. This application claims priority from Japanese Patent Application No. 2019-030210, filed on February 22, 2019, the contents of which are incorporated herein by reference in their entirety. [Background technology]

[0002] A multi-core optical fiber (MCF), which has multiple cores surrounded by a common cladding, can increase the spatial density of information transmission. Therefore, MCF is expected to be a transmission medium that can efficiently utilize the limited cross-sectional area of ​​communication paths such as underground pipes and submarine cables. Among MCFs, a coupled multi-core optical fiber (CMCF), in which guided modes are coupled between multiple cores, can have more cores in one optical fiber because the distance between two adjacent cores is short. Therefore, CMCF can further increase the spatial density of information transmission and is highly productive.

[0003] Light input to one of the multiple cores at the input end of the CMCF is distributed and guided to multiple cores due to mode coupling while guiding. Therefore, light input to one of the cores is output from multiple cores at the output end. By digitally coherently receiving the optical signals output from each of the multiple cores at the output end of the CMCF and performing multiple-input multiple-output (MIMO) processing on the received signals, it is possible to restore an output signal equivalent to the original input signal.

[0004] However, the effectiveness of MIMO processing decreases as the mode-dependent loss (MDL), which is the loss difference (difference on a logarithmic scale) between the propagation modes of the CMCF, increases, making signal restoration difficult. Therefore, when constructing a transmission path using CMCFs, it is necessary to measure the MDL of the CMCFs that make up the transmission path and keep the MDL of the entire transmission path low.

[0005] Patent Document 1 discloses an apparatus and method for measuring the MDL of a CMCF. In this apparatus and method, light is input to the kth spatial mode at the input end of a CMCF having N spatial modes, the power of the light output from the mth spatial mode at the output end of the CMCF is measured, and the transmittance T(k,m) is calculated as the ratio of the output optical power to the input optical power on a linear scale. By calculating the transmittance T(k,m) by setting k and m to each value in the range of 1 to N, an N-row, N-column matrix is ​​calculated in which the transmittance T(k,m) is the element of the kth row and the mth column. Then, the MDL on a linear scale is calculated as the ratio of the maximum value to the minimum value of the N eigenvalues ​​or singular values ​​of this matrix. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2017 / 149910 Summary of the Invention

[0007] The mode-dependent loss measurement device according to the present disclosure is a device for measuring the mode-dependent loss of a measured optical fiber, and a CMCF (coupled multi-core optical fiber) having a plurality of cores extending along a fiber axis and a common cladding surrounding the plurality of cores is applied as the measured optical fiber. As one aspect, the mode-dependent loss measurement device includes a light source, an optical receiver, a mode coupling state changing means, and an analysis unit. The light source is optically coupled to an input end of an excitation optical fiber, and inputs light to the input end of the excitation optical fiber. The excitation optical fiber is a CMCF (coupled multi-core optical fiber) having a plurality of cores extending along a fiber axis and a cladding surrounding the plurality of cores. The optical receiver is optically coupled to an output end of the measured optical fiber. In addition, the optical receiver detects the sum of the power of output light from a plurality of core end faces located on the output end of the measured optical fiber during a period in which light from the light source is input to the input end of the excitation optical fiber in a state in which the output end of the excitation optical fiber and the input end of the measured optical fiber are optically coupled to each other. The mode coupling state changing means changes the mode coupling state of the excitation optical fiber. The analysis unit analyzes the fluctuation of the optical power detected by the optical receiver during the period when the mode coupling state of the excitation optical fiber is changing. Furthermore, the analysis unit obtains the mode dependent loss of the measured optical fiber based on the magnitude of the fluctuation of the insertion loss from the input end of the excitation optical fiber to the optical receiver (hereinafter simply referred to as "insertion loss") obtained from the fluctuation of the detected optical power. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating an example of a CMCF structure. [Diagram 2] FIG. 2 is a diagram for explaining mode coupling in a CMCF. [Diagram 3] FIG. 3 is a diagram showing the configuration of a mode-dependent loss measurement apparatus 30 according to the first embodiment, together with an excitation optical fiber and an optical fiber to be measured. [Figure 4] FIG. 4 is a flowchart of the mode dependent loss measurement method of the first embodiment. [Diagram 5]FIG. 5 is a diagram showing the configuration of a mode-dependent loss measurement apparatus 35 according to the second embodiment, together with an excitation optical fiber and an optical fiber to be measured. [Figure 6] FIG. 6 is a diagram showing the configuration of a mode-dependent loss measurement device 37 according to the third embodiment, together with an excitation optical fiber and an optical fiber to be measured. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Description of the embodiment of the present invention] First, the contents of the embodiments of the present disclosure will be individually listed and described.

[0010] (1) A mode-dependent loss measurement device according to the present disclosure is a device for measuring a mode-dependent loss of an optical fiber under test, and a CMCF (coupled multi-core optical fiber) having a plurality of cores extending along a fiber axis and a common cladding surrounding the plurality of cores is applied as the optical fiber under test. As one aspect of the mode-dependent loss measurement device, the device includes a light source, an optical receiver, a mode coupling state changing means, and an analyzing unit. The light source is optically coupled to an input end of an excitation optical fiber, and inputs light to the input end of the excitation optical fiber. The excitation optical fiber is a CMCF (coupled multi-core optical fiber) having a plurality of cores extending along a fiber axis and a cladding surrounding the plurality of cores. The optical receiver is optically coupled to an output end of the optical fiber under test. The optical receiver detects the sum of the power of output light from a plurality of core end faces located on the output end of the optical fiber under test during a period in which light from the light source is input to the input end of the excitation optical fiber in a state in which the output end of the excitation optical fiber and the input end of the optical fiber under test are optically coupled to each other. The mode coupling state changing means changes the mode coupling state of the pump optical fiber. The analysis unit analyzes the fluctuation of the optical power detected by the photoreceiver during the period when the mode coupling state of the pump optical fiber is changing. Furthermore, the analysis unit obtains the mode dependent loss of the measured optical fiber from the detected optical power fluctuation. Note that the CMCFs constituting the pump optical fiber and the measured optical fiber may be prepared separately, or a part of the CMCF constituting the measured optical fiber may be applied to the pump optical fiber.

[0011] In addition, when measuring the MDL of the optical fiber under test using the variation of the insertion loss from the input end of the excitation optical fiber to the photoreceiver, it is necessary to set the length of the excitation optical fiber sufficiently short with respect to the length of the optical fiber under test. Specifically, the ratio of the length of the optical fiber under test to the length of the excitation optical fiber is, for example, 10 or more, preferably 100 or more. In the present disclosure, a "CMCF (coupled multi-core optical fiber)" is an optical fiber in which optical power input to one core at one end and propagated to the other end is output from two or more cores at the other end, and the power output from any core does not exceed 67% of the total power output from the other end.

[0012] (2) As one aspect of the present disclosure, the mode coupling state changing means may include a disturbance applying unit that changes the mode coupling state of the excitation optical fiber by applying a disturbance. As one aspect of the present disclosure, the mode coupling state changing means may include a wavelength changing unit that changes the wavelength of the light output by the light source. Furthermore, the mode coupling state changing means may include phase modulation means that changes one or more phases of the light propagating through the multiple cores in the excitation optical fiber.

[0013] (3) As an aspect of the present disclosure, the mode coupling state changing means may include at least two elements of a first element, a second element, and a third element. The first element corresponds to a disturbance applying unit that changes the mode coupling state of the excitation optical fiber by applying a disturbance. The second element corresponds to a wavelength changing unit that changes the wavelength of the light output from the light source. The third element corresponds to a phase modulation means that changes one or more phases of the light propagating through the multiple cores in the excitation optical fiber. In such a configuration, the analysis unit selectively extracts a frequency component of the detected fluctuation in optical power based on a frequency synchronized with a change in the mode coupling state caused by the operation of at least one element among the elements included in the mode coupling state changing means, in order to analyze the fluctuation in the detected optical power.

[0014] (4) A mode-dependent loss measurement method according to the present disclosure measures the mode-dependent loss of a test optical fiber made of a CMCF having multiple cores extending along a fiber axis and a common cladding surrounding the multiple cores. As an embodiment of the mode-dependent loss measurement method, the method includes a light input step, a light detection step, a mode coupling state change step, and an analysis step. In the light input step, light from a light source optically coupled to an input end of an excitation optical fiber made of a CMCF having the above-mentioned structure is input to the input end of the excitation optical fiber. In the light detection step, during a period in which light from the light source is input to the input end of the excitation optical fiber in a state in which the output end of the excitation optical fiber and the input end of the test optical fiber are optically coupled to each other, a photoreceiver optically coupled to the output end of the test optical fiber detects the sum of the power of output light from multiple core end faces located on the output end of the test optical fiber. In the mode coupling state change step, the mode coupling state of the excitation optical fiber is changed. In the analysis step, the fluctuation in the optical power detected by the photoreceiver during a period in which the mode coupling state of the excitation optical fiber is changing is analyzed. Furthermore, the mode dependent loss of the measured optical fiber is obtained from the fluctuation of the detected optical power. The CMCFs constituting the pump optical fiber and the measured optical fiber may be prepared separately, or a part of the CMCF constituting the measured optical fiber may be applied to the pump optical fiber.

[0015] (5) As an aspect of the present disclosure, the mode coupling state changing step may include a disturbance applying step of changing the mode coupling state of the excitation optical fiber by applying a disturbance. As an aspect of the present disclosure, the mode coupling state changing step may include a wavelength changing step of changing the wavelength of light output by the light source. Furthermore, as an aspect of the present disclosure, the mode coupling state changing step may include a phase modulation step of changing one or more phases of the light propagating through the multiple cores of the excitation optical fiber.

[0016] (6) As an aspect of the present disclosure, the mode coupling state changing step may include at least two sub-steps of a first sub-step, a second sub-step, and a third sub-step. The first sub-step corresponds to a disturbance applying step of changing the mode coupling state of the excitation optical fiber by applying a disturbance. The second sub-step corresponds to a wavelength changing step of changing the wavelength of light output from the light source. The third sub-step corresponds to a phase modulation step of changing one or more phases of light propagating through a plurality of cores in the excitation optical fiber. In such a configuration, in the analysis step, in order to analyze the fluctuation in the detected optical power, a frequency component of the fluctuation in the detected optical power is selectively extracted based on a frequency synchronized with the change in the mode coupling state caused by the operation of at least one sub-step included in the mode coupling state changing step.

[0017] (7) As an aspect of the present disclosure, the length of the optical fiber to be measured is preferably 10 times or more the length of the excitation optical fiber. Also, as an aspect of the present disclosure, one end of the excitation optical fiber may be optically coupled to a branching device (fan-out device). The branching device is an optical component for optically coupling multiple cores in the excitation optical fiber to multiple single-core optical fibers, respectively. With such a configuration, the light source and the excitation optical fiber are optically coupled via the single-core optical fiber.

[0018] As described above, each aspect listed in the [Description of the embodiments of the present disclosure] section is applicable to each of the remaining aspects or to all combinations of these remaining aspects.

[0019] In the device and method disclosed in the above-mentioned Patent Document 1, since it is necessary to measure the transmittance for each spatial mode, there is a problem that the MDL when branching the mode becomes an error factor. 2 ), the number of measurements increases in proportion to the number of modes, which results in high costs for the measurement device. According to the present disclosure, it is possible to measure the MDL of a CMCF inexpensively by suppressing errors due to the MDL associated with mode branching.

[0020] [Details of the embodiment of the present disclosure] Hereinafter, the specific structures of the mode-dependent loss measurement device and the mode-dependent loss measurement method according to the embodiment of the present disclosure will be described in detail with reference to the attached drawings. Note that the present disclosure is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In addition, in the description of the drawings, the same elements are given the same reference numerals, and duplicated descriptions are omitted. FIG. 1 is a diagram showing an example of the structure of a CMCF (coupled multi-core optical fiber). FIG. 1 shows a transverse section and a longitudinal section of a CMCF1. The CMCF1 includes a plurality of (four in FIG. 1) cores 10 extending along a fiber axis (corresponding to a central axis of the CMCF1 extending along the longitudinal direction) and a common clad 11 surrounding the plurality of cores 10. The refractive index of each core 10 is higher than that of the common clad 11. This allows the CMCF1 to have an optical guided mode. Each core 10 and the common clad 11 are typically made of silica glass, and the refractive index is adjusted by adding an additive such as Ge, F, Cl, or P. The common clad 11 is protected from external damage by being covered with a resin (not shown).

[0021] The guided mode of each core 10 is significantly coupled with the guided mode of another adjacent core 10 (causing significantly large mode coupling). For example, the mode coupling coefficient is 0.1 [1 / m] or more. To achieve such significantly large mode coupling, it is preferable that each core 10 has substantially the same composition and substantially equal propagation constants. To have substantially equal propagation constants, each core 10 is preferably arranged at an interval wide enough that supermodes are not generated between adjacent cores 10. FIG. 2 is a diagram for explaining mode coupling in a CMCF. In addition to a longitudinal section of a CMCF1, FIG. 2 shows an optical power distribution 21 of multiple cores 10 at an input end 1a of the CMCF1 (region indicated by a dashed line in the figure) and an optical power distribution 22 of multiple cores 10 at an output end 1b of the CMCF1 (region indicated by a dashed line in the figure). The optical power distributions 21 and 22 are indicated by shading to indicate the magnitude of the optical power. When light is input to any one of the multiple cores 10 located at the input end 1a of the CMCF1, the light guided through that core 10 couples with the other cores 10 due to mode coupling 20 that occurs during propagation. As a result, the optical power distribution 21 at the input end 1a is localized to any one of the cores 10, whereas the optical power distribution 22 at the output end 1b is distributed to the multiple cores 10.

[0022] The position and frequency at which mode coupling occurs in a CMCF depend not only on the structure of the CMCF but also on the bending of the CMCF, twisting of the CMCF, and refractive index fluctuation due to temperature or strain of the CMCF. Therefore, the position and frequency at which mode coupling occurs in a CMCF vary randomly. In addition, the ratio of optical power coupled from one core to another also varies randomly. The expected value of the inverse of the frequency at which mode coupling occurs per unit length of the CMCF is called the mode coupling length. In a typical CMCF, the mode coupling length is 10 m or less. Therefore, in a CMCF with a length of 100 m or more, mode coupling occurs cumulatively a sufficiently large number of times during propagation, so that the power of light output from each of the multiple cores that cause mode coupling is approximately equal.

[0023] Since the action of mode coupling is the exchange of optical power between modes, without an MDL, the sum of optical power is preserved even if mode coupling occurs randomly. In other words, without an MDL, the sum of the optical power output from all the cores 10 at the output end 1b does not vary. However, with an MDL, the position or frequency of mode coupling changes randomly, and the sum of the optical power output from all the cores 10 at the output end 1b also varies randomly. The inventors have found that the MDL of a CMCF can be measured by sufficiently generating random fluctuations in mode coupling and measuring the fluctuations in the loss of the CMCF at that time. 3 is a diagram showing the configuration of a mode dependent loss measurement apparatus (MDL measurement apparatus) 30 of the first embodiment together with an excitation optical fiber and an optical fiber under test. The MDL measurement apparatus 30 includes a light source 31, a photoreceiver 32, a disturbance applying unit 33 as a mode coupling state changing means, and an analyzer 34. A coupling optical fiber 2, an excitation optical fiber 3, and an optical fiber under test 4 are arranged in this order from the light source 31 toward the photoreceiver 32. The coupling optical fiber 2 is not necessarily provided.

[0024] The light source 31 is optically coupled to the input end 2a of the coupling optical fiber 2, and emits output light at the input end 2a into the core of the coupling optical fiber 2. The coupling optical fiber 2 may be a single-core optical fiber.

[0025] The excitation optical fiber 3 is a CMCF. An input end 3a of the excitation optical fiber 3 is optically coupled to an output end 2b of the coupling optical fiber 2. Light output from a core of the coupling optical fiber 2 at the output end 2b is input to one or more cores of the excitation optical fiber 3 at the input end 3a.

[0026] The measured optical fiber 4 is a CMCF whose MDL is to be measured by the MDL measurement device 30. The input end 4a of the measured optical fiber 4 is optically coupled to the output end 3b of the excitation optical fiber 3. Light output from the core of the excitation optical fiber 3 at the output end 3b is input to the core of the measured optical fiber 4 at the input end 4a.

[0027] The excitation optical fiber 3 and the measured optical fiber 4 may be the input end side portion of a continuous length of CMCF and the portion following this input end side portion. The excitation optical fiber 3 and the measured optical fiber 4 may be identical to each other in terms of the core arrangement in the cross section and the refractive index profile, or may differ from each other in any of these respects. It is sufficient that light output from any one or more cores of the excitation optical fiber 3 at the output end 3b is input to any one or more cores of the measured optical fiber 4 at the input end 4a.

[0028] The photoreceiver 32 is optically coupled to the output end 4b of the test optical fiber 4, and detects the power of the light output from all the cores of the test optical fiber 4 located on the output end 4b.

[0029] The light output from the light source 31 propagates through the coupling optical fiber 2 and is then input to the core of the excitation optical fiber 3 at the input end 3a. At this time, it is preferable that the coupling optical fiber 2 has a section bent with a predetermined diameter, so that the higher-order mode excited at the input end 2a is attenuated. As a result, the instability of the insertion loss caused by the higher-order mode of the coupling optical fiber 2 being coupled to the excitation optical fiber 3 can be suppressed.

[0030] The optical power distribution when coupled from the output end 2b of the coupling optical fiber 2 to the input end 3a of the excitation optical fiber 3 differs depending on mode coupling in the excitation optical fiber 3, as compared with the optical power distribution when coupled from the output end 3b of the excitation optical fiber 3 to the input end 4a of the measured optical fiber 4. Also, the optical power distribution when output from the output end 4b of the measured optical fiber 4 differs depending on mode coupling in the measured optical fiber 4, as compared with the optical power distribution when coupled from the output end 3b of the excitation optical fiber 3 to the input end 4a of the measured optical fiber 4.

[0031] The light output from all the core end faces located on the output end 4b of the test optical fiber 4 is received by the photoreceiver 32, which detects the sum of the optical powers.

[0032] The mode coupling state in the excitation optical fiber 3 varies due to disturbances. Disturbances applied to the excitation optical fiber 3 to change the mode coupling state include temperature, distortion, bending, twisting, and the like. This disturbance may be applied naturally or may be applied intentionally by the disturbance applying unit 33. The disturbance applying unit 33 is, for example, a "heater" or a "piezoelectric element."

[0033] Fluctuations in the mode coupling state in the excitation optical fiber 3 randomly fluctuate the optical power distribution when the optical fiber is coupled from the output end 3b of the excitation optical fiber 3 to the input end 4a of the measured optical fiber 4. If the measured optical fiber 4 has an MDL, the optical power detected by the photodetector 32 also randomly fluctuates.

[0034] The analysis unit 34 includes, for example, a CPU and a memory in which an analysis program is stored. The analysis unit 34 analyzes the fluctuation of the optical power detected by the photoreceiver 32 during a period in which the mode coupling state in the excitation optical fiber 3 changes due to the application of a disturbance, and obtains the MDL of the measured optical fiber 4 based on the magnitude of the fluctuation of the insertion loss (the loss from the input end of the excitation optical fiber 3 to the photoreceiver 32) obtained from the optical power fluctuation. Specifically, the analysis unit 34 performs the following steps.

[0035] The optical receiver 32 detects the optical power P repeatedly, for example, at regular time intervals, to obtain the respective detection values ​​P1, P2, . . . , P M (M is an integer equal to or greater than 2). The analysis unit 34 obtains each detection value P m (m is an integer between 1 and M) and the input optical power P0 to obtain the insertion loss A m (=P m Further, the analysis unit 34 calculates the insertion losses A1, A2, . . . , A M The maximum value A max and the minimum value A minIdentify the maximum and minimum values, and calculate the ratio (A max / A min ) is calculated as the MDL of the optical fiber 4 to be measured. If the input optical power P0 is constant (if the fluctuations in the output power of the light source 31 and the loss of the coupling optical fiber 2 are small), the analysis unit 34 calculates the detected values ​​P1, P2, . . . , P M The maximum value of P max and the minimum value P min The ratio of these maximum and minimum values ​​(P max / P min ) can be obtained as the MDL of the optical fiber 4 to be measured.

[0036] To be precise, the MDL obtained here is not the MDL of only the measured optical fiber 4, but a composite of the MDLs of the measured optical fiber 4 and the excitation optical fiber 3. Therefore, in order to obtain the MDL of the measured optical fiber 4, it is necessary to make the length L4 of the measured optical fiber 4 sufficiently longer than the length L3 of the excitation optical fiber 3. Specifically, L4 / L3 is preferably 10 or more, and more preferably 100 or more.

[0037] It is necessary to generate sufficient mode coupling in the excitation optical fiber 3 to distribute power to all coupled cores. For this purpose, the length L3 of the excitation optical fiber 3 is preferably 10 times or more, more preferably 100 times or more, the mode coupling length of the excitation optical fiber 3. Since the mode coupling length is usually 10 m or less, the length L3 of the excitation optical fiber 3 is preferably 100 m or more, more preferably 1 km or more. Moreover, the length L4 of the measured optical fiber 4 is preferably 1 km or more, more preferably 100 km or more.

[0038] The mode coupling length of the excitation optical fiber 3 is often unknown in advance. In such a case, it is desirable to confirm that the optical power is output from each core at a substantially equal ratio by photographing the mode distribution of the light output from the excitation optical fiber 3 with a camera or the like before or after the MDL measurement, or by measuring the average value or the ratio of the average value and the variation width of the optical power detected by selectively optically coupling each core of the excitation optical fiber to a photodetector. At this time, if the optical power output from each core is not equal, it is desirable to lengthen the excitation optical fiber 3 or increase the strength of the disturbance applied to the excitation optical fiber 3. FIG. 4 is a flowchart of the mode dependent loss measurement method (MDL measurement method) of the first embodiment.

[0039] In step S1, an excitation optical fiber and a coupling optical fiber are prepared in addition to the optical fiber under test, and these are connected. The coupling optical fiber does not have to be connected. The excitation optical fiber may be connected to the optical fiber under test, or the input end side portion of the optical fiber under test may be used as the excitation optical fiber.

[0040] In step S2, light having a predetermined wavelength and a predetermined power output from a light source is input to an excitation optical fiber via a coupling optical fiber. The light output after propagating through the excitation optical fiber is coupled to the measured optical fiber. That is, the output light from the excitation optical fiber is output from the output end after propagating through the measured optical fiber. At this time, a disturbance is applied to the excitation optical fiber.

[0041] In step S3, the sum of the optical powers output from the optical fibers under test P m In step S4, the input optical power P0 and the output optical power P m Insertion loss A m The insertion loss is calculated as a function of time. In step S5, the insertion loss A m The maximum value of the series of fluctuations in A max and the minimum value A minFrom the ratio (A max / A min ) to obtain the MDL.

[0042] In step S6, the convergence of the calculated MDL value is judged from the series of calculated MDL values ​​up to that point. In many cases, convergence with an accuracy of three significant digits is sufficient. If convergence has not yet been achieved, in step S7, the wavelength of the light or the disturbance to the excitation optical fiber is changed, and steps S3 and after are repeated. If convergence is achieved, the measured MDL value is determined. Since the wavelength of the light and the power distribution of the excitation optical fiber are closed sets, it is possible to reach convergence within a finite time.

[0043] 5 is a diagram showing the configuration of a mode dependent loss measurement device (MDL measurement device) 35 of the second embodiment together with an excitation optical fiber and an optical fiber under test. The MDL measurement device 35 includes a light source 31, a photoreceiver 32, a wavelength changer 36 as a mode coupling state changing means, and an analyzer 34. The coupling optical fiber 2, the excitation optical fiber 3, and the optical fiber under test 4 are arranged in this order from the light source 31 toward the photoreceiver 32. The functions and operations of the light source 31, the photoreceiver 32, and the analyzer 34 are the same as those of the first embodiment.

[0044] The mode coupling state of the excitation optical fiber 3 varies depending on the wavelength of the propagating light. In order to change the mode coupling state in step S2 of Fig. 4, the wavelength of the light output from the light source 31 can be changed by changing the driving conditions of the light source 31 using the wavelength changing unit 36.

[0045] In the first and second embodiments, it is not necessary to measure the transmittance for each spatial mode, so the problem of the MDL being an error factor when splitting a mode can be solved. In addition, the configuration of the measurement device is simple. Therefore, the error due to the MDL associated with mode splitting can be suppressed and the MDL of the CMCF can be measured inexpensively.

[0046] 6 is a diagram showing the configuration of a mode dependent loss measurement device (MDL measurement device) 37 of the third embodiment together with an excitation optical fiber and an optical fiber under test. The MDL measurement device 37 includes a light source 31, a photoreceiver 32, optical phase change units 39a and 39b (phase modulation means) as mode coupling state changing means, and an analyzer 34. An optical branching device 38, optical phase change units 39a and 39b, a multi-core fan-out device 40, an excitation optical fiber 3, and an optical fiber under test 4 are arranged in this order from the light source 31 toward the photoreceiver 32. The functions and operations of the light source 31, the photoreceiver 32, and the analyzer 34 are the same as those of the first embodiment.

[0047] The light output from the light source 31 is branched into two or more optical paths by the optical brancher 38, and then the optical phase is changed by two optical phase change units 39a and 39b, and then the multiple optical paths are respectively coupled to the multiple cores of the excitation optical fiber 3 by the multi-core fan-out unit 40. The mode coupling state in the excitation optical fiber 3 varies depending on the phase difference of the light propagating through the multiple cores. Therefore, changing the phases of the two lights by the optical phase change units 39a and 39b corresponds to changing the mode coupling state in step S2 in Fig. 4 (phase modulation step).

[0048] It is preferable that the optical phase shifters 39a and 39b apply a periodic phase change of a predetermined frequency, and the optical receiver 32 detects the optical power that changes at the predetermined frequency. It is even more preferable that the two optical phase shifters 39a and 39b apply periodic phase changes of different predetermined frequencies, and the optical receiver 32 detects the optical power that changes at a frequency that is the difference between the two predetermined frequencies. This frequency-selective detection makes it possible to distinguish, by frequency, between errors due to optical power changes caused by factors other than mode-dependent losses, such as power fluctuations of the light source, errors due to optical power changes parasitically occurring in the phase shifters, and optical power changes caused by mode-dependent losses, and to suppress errors.

[0049] Although the first, second, and third embodiments have different mode coupling state changing means, the mode coupling state changing means may be a combination of two or more of the mode coupling state changing means of these embodiments. In this case, at least one mode coupling state may be periodically changed at a predetermined frequency as described above, and the optical power that changes at the predetermined frequency or at a frequency that is the difference between the two predetermined frequencies may be detected, thereby suppressing errors. [Explanation of symbols]

[0050] 1...CMCF (coupled multi-core optical fiber), 2...coupling optical fiber, 3...excitation optical fiber, 4...measured optical fiber, 10...core, 11...common clad, 30, 35, 37...mode dependent loss measurement device (MDL measurement device), 31...light source, 32...photoreceiver, 33...disturbance injection unit, 34...analysis unit, 36...wavelength change unit, 38...optical branching unit, 39a, 39b...optical phase change unit, 40...multi-core fan-out device.

Claims

1. A mode-dependent loss measurement device for measuring a mode-dependent loss of an optical fiber under test that is a coupled multi-core optical fiber having a plurality of cores extending along a fiber axis and a common cladding surrounding the plurality of cores, comprising: a light source optically coupled to an input end of an excitation optical fiber that is a coupled-type multi-core optical fiber having a plurality of cores extending along a fiber axis and a cladding surrounding the plurality of cores, the light source inputting light to the input end of the excitation optical fiber; a photoreceiver optically coupled to an output end of the test optical fiber, the photoreceiver detecting a sum of powers of output lights from a plurality of core end faces located on the output end of the test optical fiber during a period in which the output end of the excitation optical fiber and the input end of the test optical fiber are optically coupled to each other and the light from the light source is input to the input end of the excitation optical fiber; a mode coupling state changing means for changing a mode coupling state of the excitation optical fiber; an analysis unit that analyzes a fluctuation in optical power detected by the optical receiver during a period in which the mode coupling state of the excitation optical fiber is changing, and obtains the mode dependent loss of the test optical fiber from the detected fluctuation in optical power; A mode dependent loss measurement apparatus comprising:

2. the mode coupling state changing means includes a disturbance applying unit that applies a disturbance to change the mode coupling state of the excitation optical fiber, 2. The modal dependent loss measurement apparatus of claim 1.

3. The mode coupling state changing means includes a wavelength changing unit that changes the wavelength of the light output from the light source.

2. The modal dependent loss measurement apparatus of claim 1.

4. the mode coupling state changing means includes a phase modulation means for changing one or more phases of light propagating through the multiple cores in the excitation optical fiber, 2. The modal dependent loss measurement apparatus of claim 1.

5. the mode coupling state changing means includes at least two elements selected from a first element that changes the mode coupling state of the excitation optical fiber by applying a disturbance, a second element that changes a wavelength of the light output from the light source, and a third element that changes one or more phases of the light propagating through the multiple cores in the excitation optical fiber; the analysis unit selectively extracts a frequency component of the detected fluctuation in optical power based on a frequency synchronized with a change in mode coupling state caused by an operation of at least one element among elements included in the mode coupling state changing means, in order to analyze the fluctuation in the detected optical power.

2. The modal dependent loss measurement apparatus of claim 1.

6. The excitation optical fiber is a part of the coupled multi-core optical fiber which is the optical fiber to be measured. A mode dependent loss measurement apparatus according to any one of claims 1 to 5.

7. A mode-dependent loss measurement method for measuring a mode-dependent loss of an optical fiber under test that is a coupled multi-core optical fiber having a plurality of cores extending along a fiber axis and a common cladding surrounding the plurality of cores, comprising: an optical input step of inputting light from a light source optically coupled to an input end of an excitation optical fiber, the input end being made of a coupled-type multi-core optical fiber having a plurality of cores extending along a fiber axis and a common cladding surrounding the plurality of cores; a light detection step of detecting a sum of powers of output lights from a plurality of core end faces located on the output end of the test optical fiber by a light receiver optically coupled to the output end of the test optical fiber during a period in which the light from the light source is input to the input end of the test optical fiber in a state in which the output end of the test optical fiber and the input end of the test optical fiber are optically coupled to each other; a mode coupling state changing step of changing a mode coupling state of the excitation optical fiber; an analysis step of analyzing a fluctuation in optical power detected by the optical receiver during a period in which the mode coupling state of the excitation optical fiber is changing, and determining a mode dependent loss of the test optical fiber from the detected fluctuation in optical power; A mode dependent loss measurement method comprising:

8. the mode coupling state changing step includes a disturbance applying step of changing the mode coupling state of the excitation optical fiber by applying a disturbance. The mode dependent loss measurement method according to claim 7.

9. The mode coupling state changing step includes a wavelength changing step of changing the wavelength of the light output from the light source. The mode dependent loss measurement method according to claim 7.

10. the mode coupling state changing step includes a phase modulation step of changing one or more phases of light propagating through the multiple cores of the excitation optical fiber, The mode dependent loss measurement method of claim 7.

11. the mode coupling state changing step includes at least two substeps of a first substep of changing the mode coupling state of the excitation optical fiber by applying a disturbance, a second substep of changing a wavelength of the light output from the light source, and a third substep of changing one or more phases of the light propagating through the multiple cores in the excitation optical fiber, the analyzing step selectively extracts a frequency component of the detected optical power fluctuation based on a frequency synchronized with a change in mode coupling state caused by an operation of at least one sub-step among the sub-steps included in the mode coupling state changing step, in order to analyze the detected optical power fluctuation; The mode dependent loss measurement method according to claim 7.

12. The excitation optical fiber is a part of the coupled multi-core optical fiber which is the optical fiber to be measured. The mode dependent loss measurement method according to any one of claims 7 to 11.

13. The length of the optical fiber to be measured is 10 times or more the length of the excitation optical fiber. A mode dependent loss measurement method according to any one of claims 7 to 12.

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