Optical fiber device

Abstract

This optical fiber device comprises: a first optical fiber that is a coupled multi-core optical fiber having a plurality of first cores that extend along a first fiber axis direction and are mode-coupled with each other, and a first cladding that surrounds the plurality of first cores; and a second optical fiber having one or more second cores extending along a second fiber axis direction, and a second cladding that surrounds the one or more second cores. The number of cores of the second optical fiber is less than the number of cores of the first optical fiber, and the second optical fiber and the first optical fiber are connected such that the second core is coaxial with any one of the plurality of first cores.

Description

Optical Fiber Devices

[0001] This application claims priority to Japanese Patent Application No. 2024-020562, filed February 14, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] A multi-core optical fiber (hereinafter also referred to as "MCF") is an optical fiber having multiple cores made of glass, a cladding also made of glass surrounding the multiple cores, and a resin layer coating the cladding. Among MCFs, a coupled multi-core optical fiber (hereinafter also referred to as "CMCF") in which guided modes are coupled between multiple cores has the advantage of enabling large-capacity transmission and a thinner optical fiber cable because of its high core spatial density and high spatial density of information transmission. The degree of coupling of guided modes between cores is expressed by the inter-core mode coupling coefficient or the inter-core power coupling coefficient. In the present disclosure, an MCF having an inter-core mode coupling coefficient of 1 [1 / m] or more or an inter-core power coupling coefficient of 10 [1 / km] or more is defined as a CMCF.

[0003] At the input end of the CMCF, optical signals input to each of the multiple cores are partially coupled to other cores due to mode coupling while propagating. As a result, at the output end of the CMCF, multiple coupled optical signals are output from the respective cores. At the receiving end, each of the multiple coupled optical signals is converted into a digital signal by a digital coherent receiver. These digital signals are then subjected to multiple-input multiple-output (MIMO) processing to restore a digital signal equivalent to the original input signal.

[0004] In this case, the greater the mode-dependent loss (hereinafter also referred to as "MDL"), which is the difference in loss between the propagation modes of the CMCF, the less effective the MIMO processing becomes, and the more difficult it becomes to restore the signal. Therefore, when constructing a transmission path using CMCF, it is necessary to measure the MDL of the CMCF that constitutes the transmission path and manage the MDL of the entire transmission path to be low.

[0005] Patent Document 1 discloses an apparatus and method for measuring MDL by injecting measurement light into a CMCF under test via a pump CMCF, changing the mode state of the pump CMCF using a mode coupling state changing means, and analyzing fluctuations in the output light power from the fiber under test. This measurement method is called the scrambling method because the mode state is scrambled. Regarding the scrambling method, Non-Patent Document 1 discloses that, while the lower limit of MDL measurable by other MDL measurement methods is 0.1 dB, the scrambling method can measure MDL down to 0.04 dB. To maintain a low MDL for the entire transmission line, it is desirable to maintain the MDL of each component, such as an optical fiber or optical component, below 0.1 dB. The scrambling method is an effective method for measuring the MDL of such components.

[0006] The scrambling method requires a comprehensive change in the mode state. To this end, Non-Patent Document 1 discloses that a known MDL is generated using an MDL emulator including a fan-out and an optical attenuator, and a comprehensive change in the mode state is ensured by obtaining a measured value of MDL that matches the theoretical value with good reproducibility. The fan-out is a device that optically couples each of the multiple cores of a CMCF to each of the cores of multiple single-mode fibers (hereinafter also referred to as "SMF").

[0007] International Publication No. 2020 / 171187

[0008] T. Hasegawa and T. Hayashi, “Measurement of Mode Dependent Loss of Randomly-Coupled Multi-Core Fiber using Scrambling Method”, Proceedings of 26th Optoelectronics and Communications Conference (OECC), T2C. 2 (2021)

[0009] An optical fiber device according to one aspect of the present disclosure comprises: a first optical fiber that is a coupled multi-core optical fiber having a plurality of first cores extending along a first fiber axis direction and mode-coupled to each other, and a first cladding surrounding the plurality of first cores; and a second optical fiber having one or more second cores extending along a second fiber axis direction and a second cladding surrounding the one or more second cores, wherein the number of cores in the second optical fiber is smaller than the number of cores in the first optical fiber, and the second optical fiber and the first optical fiber are connected such that the second core is coaxial with any one of the plurality of first cores.

[0010] Fig. 1 is a diagram showing the configuration of a system to which a verification device according to an embodiment is applied. Fig. 2 is a diagram showing the structure of a CMCF. Fig. 3 is a diagram showing the structure of an SMF. Fig. 4 is a graph showing theoretical calculation results and actual measurement results of the probability distribution of the relative deviation of detected optical power. Fig. 5 is a diagram showing the configuration for measuring the MDL of a CMCF using a verified system.

[0011] [Problem to be Solved by the Present Disclosure] In the MDL emulator disclosed in Non-Patent Document 1, errors occur due to the MDL of the components, which causes a problem that it is not possible to accurately verify that the CMCF mode is scrambled.

[0012] An object of the present disclosure is to provide an optical fiber device that can accurately verify that the mode of a CMCF is scrambled.

[0013] Effect of the Present Disclosure According to the present disclosure, an optical fiber device is provided that can accurately verify that the mode of a CMCF is scrambled.

[0014] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be listed and described. (1) An optical fiber device according to one aspect of the present disclosure includes: a first optical fiber that is a coupled multi-core optical fiber having a plurality of first cores extending along a first fiber axis direction and mode-coupled to each other, and a first cladding surrounding the plurality of first cores; and a second optical fiber having one or more second cores extending along a second fiber axis direction and a second cladding surrounding the one or more second cores, wherein the number of cores in the second optical fiber is smaller than the number of cores in the first optical fiber, and the second optical fiber and the first optical fiber are connected such that the second core is coaxial with any one of the plurality of first cores. With this optical fiber device, it is possible to accurately verify that the modes of a CMCF are scrambled.

[0015] (2) In the above (1), the number of cores in the second optical fiber may be 1. In this case, the optical fiber device can have a simple configuration.

[0016] (3) In the above (1) or (2), a mode-dependent loss may be generated between the first optical fiber and the second optical fiber by propagating light from the first optical fiber to the second optical fiber.

[0017] (4) In any of the above (1) to (3), the length of the first optical fiber and the length of the second optical fiber may each be 22 m or more. In this case, the higher-order modes are sufficiently attenuated, and the error in the MDL measurement can be sufficiently reduced.

[0018] (5) In any of (1) to (4) above, the optical fiber device may further include a third optical fiber that is a coupled multi-core optical fiber having a plurality of third cores extending along a third fiber axis and mode-coupled to one another, and a third cladding surrounding the plurality of third cores, wherein the number of cores in the third optical fiber is equal to the number of cores in the first optical fiber, and the second core is connected to any one of the plurality of third cores so as to be coaxial with each other, and a mode-dependent loss is generated between the first optical fiber and the second optical fiber by propagating light from the first optical fiber to the third optical fiber. In this case, the configuration of the optical fiber device can be further simplified.

[0019] (6) In the above (5), a mode-dependent loss may be generated between the first optical fiber and the second optical fiber by propagating light from the first optical fiber to the third optical fiber.

[0020] [Details of the Embodiments of the Present Disclosure] Specific examples of optical fiber devices according to the present disclosure will be described with reference to the drawings as necessary. The present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, the same elements in the drawings will be designated by the same reference numerals, and duplicate explanations will be omitted.

[0021] FIG. 1 is a diagram showing the configuration of a system to which a verification device according to an embodiment is applied. The system 10 is a system for measuring the MDL of a CMCF 6 (see FIG. 5 ) under test. The verification device 1 according to the embodiment is used to verify that the mode of a pump CMCF 2 is scrambled and to identify ideal MDL measurement conditions for the system 10. The system 10 includes the pump CMCF 2, a light source 3, a frequency control unit 4, and a photodetector 5. The verification device 1 is an optical fiber device including a CMCF 11 (first optical fiber), an SMF 12 (second optical fiber), and a CMCF 13 (third optical fiber). The verification device 1 is configured such that the SMF 12 is connected between the CMCF 11 and the CMCF 13.

[0022] FIG. 2 is a diagram illustrating the structure of a CMCF. The CMCF 11 has multiple cores 111 (first cores) and a cladding 112 (first cladding). The multiple cores 111 extend along a first fiber axis direction AX1 of the CMCF 11. The multiple cores 111 are mode-coupled to each other, and have an inter-core mode coupling coefficient of 1 [1 / m] or greater at a wavelength of 1550 nm. The first fiber axis direction AX1 is parallel to the first fiber axis, which is the central axis of the CMCF 11. The cladding 112 is a common cladding surrounding the multiple cores 111. The CMCF 11 may further include a resin layer (not shown) that coats the cladding 112 for protection. The cores 111 are not necessarily located at the center of the cladding 112. In this embodiment, the CMCF 11 has four cores. The cores 111 and the cladding 112 are primarily composed of silica glass, for example, and contain an additive element for refractive index adjustment. The number of cores in the CMCF 11 is not limited to 4, and may be 8, 12, or 16. The greater the number of cores, the greater the cost of separating and detecting the optical power of each mode in each core, and therefore the greater the benefit of improved efficiency in measurements using the scrambling method that is the premise of the device of the present disclosure.

[0023] The CMCF 13 has the same structure as the CMCF 11. That is, the CMCF 13 has multiple cores 111 (third cores) that extend along a third fiber axis direction AX3 of the CMCF 13 and are mode-coupled with each other, and a common cladding 112 (third cladding) that surrounds the multiple cores 111. The third fiber axis direction AX3 is a direction parallel to the third fiber axis, which is the central axis of the CMCF 13. The CMCF 13 may further have a resin layer (not shown) that covers the cladding 112 for protection. The number of cores in the CMCF 13 is equal to the number of cores in the CMCF 11. In this embodiment, the number of cores in the CMCF 13 is four.

[0024] FIG. 3 is a diagram showing the configuration of an SMF. The SMF 12 has a core 121 (second core) and a clad 122 (second clad). The core 121 extends along a second fiber axis direction AX2 of the SMF 12. The second fiber axis direction AX2 is a direction parallel to the second fiber axis, which is the central axis of the SMF 12. The clad 122 surrounds the core 121. The SMF 12 may further include a resin layer (not shown) that covers the clad 122 for protection. The number of cores in the SMF 12 is smaller than the number of cores in the CMCF 11. In this embodiment, the number of cores in the SMF 12 is one. The core 121 is located at the center of the clad 122. The core 121 and the clad 122 are made of, for example, silica glass as a main component and contain an additive element for refractive index adjustment.

[0025] The SMF 12 and the CMCF 11 are connected such that the core 121 is coaxial with one of the cores 111 of the CMCF 11. That is, the SMF 12 and the CMCF 11 are connected such that the center of the core 121 coincides with the center of the core 111. The diameter of the core 121 and the diameter of the core 111 may be the same or different from each other. As described above, in this embodiment, the core 121 is located at the center of the cladding 122, whereas the core 111 is not located at the center of the cladding 112. Therefore, the SMF 12 and the CMCF 11 may be connected such that there is an offset between the center of the cladding 122 and the center of the cladding 112.

[0026] The SMF 12 and the CMCF 13 are connected in the same manner as the SMF 12 and the CMCF 11. That is, the SMF 12 and the CMCF 13 are connected so that the core 121 is coaxial with one of the cores 111. The SMF 12 and the CMCF 13 may also be connected so that there is an offset between the center of the cladding 122 and the center of the cladding 112. In the verification device 1, a mode-dependent loss is generated between the CMCF 11 and the SMF 12 by propagating light from the CMCF 11 to the CMCF 13 via the SMF 12.

[0027] The lengths (fiber lengths) of the CMCFs 11 and 13 and the SMF 12 are each 22 m or more, and may be 44 m or more. As described above, the CMCFs 11 and 13 and the SMF 12 may be connected so that there is an offset between the cladding centers. In such connections, variations in the offset often result in excitation of higher-order modes. When the excited higher-order modes recombine with the fundamental mode, fluctuations in the received optical power occur. As described below, since the MDL measurement quantifies fluctuations in the received optical power, fluctuations due to higher-order modes result in measurement errors. By propagating the light through the above-mentioned fiber length, the higher-order modes are sufficiently attenuated. This allows the MDL measurement error to be sufficiently small (typically 0.05 dB or less). The fiber length may be 1 km or less. This allows the MDL measurement error due to the transmission loss and wavelength-dependent loss of the optical fiber itself to be sufficiently small.

[0028] The CMCFs 11 and 13 and the SMF 12 are each in the form of a reel and coil with an inner diameter of 280 mm or less. The inner diameter of the reel and coil may be 180 mm or less, or may be 140 mm or less. Bending the verification device 1 at such an inner diameter also sufficiently attenuates higher modes. This allows the error in the MDL measurement to be sufficiently small (typically 0.05 dB or less). The inner diameter of the reel and coil may be 30 mm or more. This allows the error in the MDL measurement due to bending loss to be sufficiently small.

[0029] 1 has a plurality of cores 111 and a common cladding 112, similar to the CMCF 11. The input end of the pump CMCF 2 is optically coupled to the light source 3. The output end of the pump CMCF 2 is connected to the input end of the verification device 1, i.e., the input end of the CMCF 11, so that the cores 111 and the claddings 112 are coaxial with each other. Measurement light is incident on one core 111 of the pump CMCF 2.

[0030] The light source 3 generates measurement light and outputs the generated measurement light toward the input end of the pump CMCF 2. The frequency control unit 4 controls the driving conditions of the light source 3 to change the frequency of the measurement light output from the light source 3. The frequency control unit 4, for example, continuously changes the frequency of the measurement light. The range (change width) over which the frequency of the measurement light is changed may be large enough to sufficiently change the mode state at the output end of the pump CMCF, while keeping the wavelength-dependent loss sufficiently small. Typically, the change width is 0.1 THz or more. The wavelength of the measurement light may be changed from 1550.4 nm (193.365 THz) to 1549.6 nm (193.465 THz) or more. The photodetector 5 is optically coupled to the output end of the verification device 1, i.e., the output end of the CMCF 13. The photodetector 5 detects the measurement light output from the output end of the CMCF 13.

[0031] The system 10 quantifies the MDL due to the scrambling method of the verification device 1 by quantifying the fluctuations in the optical power output from the verification device 1. The fluctuations in the optical power output from the verification device 1 are generated by scrambling the mode state of the light output from the pump CMCF 2, i.e., by randomly and comprehensively fluctuating it over time.

[0032] A CMCF has multiple (N) cores, each of which has two fundamental modes. Therefore, a CMCF has 2N guided modes. The mode state of a CMCF is expressed by a 2N-dimensional vector whose elements are the amplitudes of these guided modes. The square norm of this vector is normalized to 1. Therefore, scrambling the mode state is equivalent to the vector representing the mode state comprehensively and uniformly scanning the 2N-dimensional sphere.

[0033] In this embodiment, the measurement light is incident on one core 111 of the pump CMCF 2, but the measurement light may be split into multiple optical paths by an optical splitter and then input to multiple cores 111 of the pump CMCF 2. This can further randomize the mode state. In the pump CMCF 2, the product of the inter-core mode coupling coefficient and the length may be 10 or more, or may be 100 or more. This can further randomize the mode state.

[0034] In order to scramble the mode state of light at the output end of the pump CMCF 2, the optical frequency of the measurement light is continuously changed by the frequency control unit 4. At this time, the power of the measurement light output from the light source 3 is kept constant. Alternatively, among the power fluctuations in the output from the verification device 1, the power fluctuations at the light source 3 are corrected. The power fluctuations at the light source 3 can be measured, for example, by branching off a portion of the measurement light and detecting it with the photodetector 5.

[0035] As described above, the mode state is expressed by a 2N-dimensional vector whose square norm is normalized to 1. In principle, the mode state can be measured by separating the output light from the CMCF into each core and each polarization mode and measuring the optical power of each separated signal (i.e., by measuring the real and imaginary parts of each component of the 2N-dimensional vector). However, the method of detecting the separated optical power in a spatially parallel manner increases the cost of the detector. Because the power fluctuates randomly over time, it is difficult to detect it with a single detector at different times. Therefore, it is generally not easy to confirm whether the mode state at the output end of the pump CMCF 2 is ideally scrambled.

[0036] The optical power P [mW] detected by the photodetector 5 is expressed by the following equation. Here, P0 [mW] is the power input to the excitation CMCF2, A [dimensionless] is the loss that occurs commonly to all modes from the input end of the excitation CMCF2 to the output end of the verification device 1 (such as transmission loss due to the material of the optical fiber), m is the mode number, M is the number of modes, Tm [dimensionless] is the relative transmittance of mode m from the input end of the excitation CMCF2 to the output end of the verification device 1, and cm [dimensionless] is the complex amplitude of mode m.

[0037] The basis of the mode is the fundamental mode of each core 111 at the output end of the CMCF 11 in the verification device 1. In both the excitation CMCF 2 and the optical fibers constituting the verification device 1, the MDL of the optical fiber itself is assumed to be negligible. The relative transmittance is the transmittance expressed as a ratio to the transmittance of the mode with the maximum transmittance. The complex amplitude cm of mode m is normalized as follows:

[0038] In the verification device 1, the transmittance of the mode coupled to the core 121 of the SMF 12 is 1, whereas the transmittance of the other modes is 0. Therefore, the relative transmittance Tm is expressed by the following equation.

[0039] Therefore, the optical power P [mW] output from the verification device 1 is expressed by the following equation.

[0040] Fluctuations in the mode state cause fluctuations in the complex amplitude cm, which in turn causes fluctuations in the optical power P. The instantaneous magnitude of the fluctuations in the optical power P is quantified as the relative deviation Δp [dB] based on the average value using the following equation: Here, the coefficient (10 / ln10) is a coefficient for expressing the fluctuation of the optical power P expressed in decibel scale.

[0041] The statistical magnitude of the fluctuation of the optical power P is expressed by the following equation as the RMS relative deviation Δprms [dB] obtained by RMS averaging the relative deviation Δp.

[0042] When the mode state is ideally scrambled, the complex amplitude array {cm} is a vector whose square norm is 1 and whose direction is uniformly random. Numerically, the array {cm} is expressed by the following equation as a vector whose elements are complex normally distributed random numbers {rm} normalized so that the square norm becomes 1. By substituting equation (7) into equations (4) to (6), the probability distribution of the relative deviation Δp and the theoretical value of the RMS relative deviation Δprms in the case of ideal scrambling can be obtained.

[0043] 1, if the probability distribution and RMS relative deviation of the relative deviation of the optical power detected by the photodetector 5 are found to coincide with the theoretical probability distribution and RMS relative deviation calculated from Equation (7) with good reproducibility, it can be verified that the scrambling of the mode state at the output end of the pump CMCF 2 is ideal. Specifically, the RMS relative deviation may coincide within ±10%.

[0044] Figure 4 is a graph showing theoretical calculation results and actual measurement results of the probability distribution of the relative deviation of the detected optical power. The horizontal axis of Figure 4 represents the relative deviation Δp [dB]. The vertical axis of Figure 4 represents the relative frequency. The relative frequency is a number obtained by repeatedly measuring the randomly changing relative deviation Δp so that the sum of the numbers is 1. The theoretical calculation result of the RMS relative deviation of the detected optical power was 2.5 dB, and the actual measurement result was 2.4 dB. By selecting the range and rate of change of the frequency of the measurement light changed by the frequency control unit 4, and by selecting the length of the pump CMCF 2 and the number of cores to be pumped, so that the difference from the theoretical value of the RMS relative deviation is 0.2 dB or less, or 0.1 dB or less, measurement conditions that enable ideal mode state scrambling can be identified.

[0045] 5 shows a configuration for measuring the MDL of a CMCF under test using a verified system. The system 10 has been verified using a verification device 1, and measurement conditions that enable ideal mode state scrambling are applied. In ideal mode state scrambling, the average mode amplitude, which is the square of the complex amplitude cm of mode m, is expressed by the following equation:

[0046] Since the optical power detected by the photodetector 5 is expressed by equation (1), the average value of the optical power is expressed by the following equation. where the average of T for mode m is Therefore, the relative deviation Δp is expressed by the following equation. However, ΔTm is expressed by the following formula.

[0047] Therefore, the RMS relative deviation Δprms of the detected optical power is the RMS average of the normalized deviation of the modal transmittance for the mode as follows: However, the constant term C is expressed by the following equation.

[0048] Assuming that the MDL of the CMCF6 to be measured is small, and therefore the deviation of Tm from the average value is sufficiently small, the following formula is obtained: When the transmittance of mode m is expressed on a logarithmic scale as in equation (17), equations (18) and (19) are obtained.

[0049] Therefore, the RMS relative deviation Δprms of the detected power expressed by equation (14) is equal to the right side of equation (20). This coincides with the definition of the RMS mode-dependent loss except for the constant term C. The constant term C can be obtained by numerical calculation from equation (15). Therefore, in the configuration of FIG. 5, the MDL of the CMCF 6 under test can be measured.

[0050] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the spirit of the present disclosure.

[0051] The verification device 1 may not include the CMCF 13, and the optical power output from the SMF 12 may be detected by the photodetector 5. In this case, it is necessary to ensure that the coupling efficiencies of the CMCF 13 and the SMF 12 to the photodetector 5 are consistent, but this simplifies the configuration of the verification device 1.

[0052] The verification device 1 may include an MCF instead of the SMF 12 as the second optical fiber. That is, the second optical fiber may have multiple cores 121 extending along the second fiber axis direction. In this case, the number of cores in the second optical fiber is smaller than the number of cores in the CMCF 11. The cladding 122 is a common cladding that surrounds the multiple cores 121. In this way, by using an MCF with a number of cores smaller than the number of cores in the CMCFs 11 and 13 instead of the SMF 12 in the verification device 1, the level of optical power reaching the photodetector 5 can be increased and the influence of noise can be relatively reduced.

[0053] REFERENCE SIGNS LIST 1... Verification device 3... Light source 4... Frequency control section 5... Photodetector 6... CMCF to be measured 10... System 11... CMCF 12... SMF 13... CMCF 111... Core 112... Cladding 121... Core 122... Cladding AX1... First fiber axial direction AX2... Second fiber axial direction AX3... Third fiber axial direction

Claims

1. An optical fiber device comprising: a first optical fiber that is a coupled multi-core optical fiber having a plurality of first cores extending along a first fiber axis direction and mode-coupled to each other, and a first cladding surrounding the plurality of first cores; and a second optical fiber having one or more second cores extending along a second fiber axis direction and a second cladding surrounding the one or more second cores, wherein the number of cores in the second optical fiber is smaller than the number of cores in the first optical fiber, and the second optical fiber and the first optical fiber are connected so that the second core is coaxial with any one of the plurality of first cores.

2. The optical fiber device according to claim 1, wherein the second optical fiber has one core.

3. An optical fiber device according to claim 1 or 2, wherein a mode-dependent loss is generated between the first optical fiber and the second optical fiber by propagating light from the first optical fiber to the second optical fiber.

4. An optical fiber device according to any one of claims 1 to 3, wherein the length of the first optical fiber and the length of the second optical fiber are each 22 m or more.

5. An optical fiber device according to any one of claims 1 to 4, further comprising a third optical fiber that is a coupled multi-core optical fiber having a plurality of third cores that extend along the third fiber axial direction and are mode-coupled to each other, and a third cladding that surrounds the plurality of third cores, wherein the number of cores in the third optical fiber is equal to the number of cores in the first optical fiber, and the second core is connected to one of the plurality of third cores so that the cores are coaxial with each other.

6. The optical fiber device according to claim 5, wherein a mode-dependent loss is generated between the first optical fiber and the second optical fiber by propagating light from the first optical fiber to the third optical fiber.

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