Several-mode multicore optical fiber and optical transmission system

Abstract

To provide a several-mode multicore optical fiber and an optical transmission system capable of both suppressing both inter-core crosstalk (XT) and differential group delay (DMD) between modes.SOLUTION: A several-mode multicore optical fiber comprises: at least two cores that propagate a plurality of LP modes; a low refractive index region that has a refractive index lower than that of the cores and surrounds all of the cores; and a cladding region that has a refractive index equal to or higher than that of the low refractive index region and surrounds the low refractive index region. Each core has at least two different core regions that are arranged concentrically around a core center and have different refractive indices. The differential group delay between LP modes is 0.1 ns / km or less, and inter-core crosstalk of the LP modes is -24 dB or less after propagation over 1 km.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present disclosure relates to few-mode multi-core optical fibers and optical transmission systems. [Background technology]

[0002] With the widespread use of optical communication systems, there is a demand for an increase in communication capacity. Non-Patent Document 1 discloses a technology for increasing transmission capacity by using spatially multiplexed channels. Non-Patent Document 2 discloses a single-mode multi-core fiber with a standard cladding diameter.

[0003] In order to maximize the spatial multiplexing of optical fibers, the use of few-mode multi-core optical fibers has been considered. However, in few-mode multi-core optical fibers, inter-core crosstalk (XT) and differential group delay (DMD) between modes in each core occur, degrading transmission quality as the transmission distance increases. Non-Patent Documents 3 and 4 disclose structural conditions for suppressing XT in a step-index (SI) three-mode four-core fiber (3M-4CF) with a standard cladding diameter. Non-Patent Documents 5 and 6 disclose techniques for suppressing DMD in each core by changing the core structure from an SI type to a graded-index (GI) type or a step type. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] DJ Richardson, JM Fini, and LE Nelson, “Space-division multiplexing in optical fibers,” Nat. Photonics, vol. 7, no. 5, pp. 354-362, 2013. [Non-patent document 2] internet<URL:https: / / sumitomoelectric.com / jp / press / 2023 / 09 / prs115> [Non-patent document 3] Nozoe, Saki, et al. "Ultra-Low Crosstalk 125-μm-Clading Four-Hole Four-Core Fibers Fabricated by the Over-Clading Bundled Rods Method." JLT., vol. 37, no. 21, pp. 5600-5608, 2019. [Non-patent document 4] Sagae, Yuto, et al. "Weakly Coupled Homogeneous 3-Mode 4-Core Fiber With Standard Cladding Diameter." JLT., vol. 41, no. 12, pp. 3950-3956, 2023. [Non-Patent Document 5] Sato, Kiminori, et al. "Optimized graded index two-mode optical fiber with low DMD, large A eff and low bending loss." Opt. express, vol. 21, no. 14, pp. 16231-16238, 2013. [Non-patent document 6] Sakamoto, Taiji, et al. "Differential mode delay managed transmission line for WDM-MIMO system using multi-step index fiber." JLT., vol. 30, no. 17, pp. 2783-2787, 2012. Summary of the Invention [Problem to be solved by the invention]

[0005] However, the GI type and step type disclosed in Non-Patent Document 5 and Non-Patent Document 6 have different light confinement effects from the SI type. Therefore, the structural conditions disclosed in Non-Patent Document 3 and Non-Patent Document 4 cannot be applied to the GI type and step type. Therefore, there is a problem that it is not possible to realize a few-mode multi-core optical fiber and an optical transmission system that simultaneously suppress both inter-core crosstalk (XT) and differential group delay (DMD).

[0006] The present disclosure has been made in view of the above-mentioned problems, and has an object to provide a few-mode multi-core optical fiber and an optical transmission system that achieve both suppression of inter-core crosstalk (XT) and differential group delay (DMD) between modes. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, a few-mode multi-core optical fiber according to one embodiment of the present disclosure includes at least two cores that propagate multiple LP modes, low-refractive-index regions that have a refractive index lower than that of the cores and surround all of the cores, and cladding regions that have a refractive index equal to or higher than that of the low-refractive-index regions and surround the low-refractive-index regions. Each core has at least two or more different core regions that are arranged concentrically about the center of the core and have different refractive indices. The differential group delay between the LP modes is 0.1 ns / km or less, and the inter-core crosstalk of the LP modes is −24 dB or less after propagation over 1 km.

[0008] Moreover, an optical transmission system according to an aspect of the present disclosure includes: a few-mode multi-core optical fiber according to the present disclosure; a transmitter that generates signal light; a mode multiplexer that converts the signal light into an LP mode that propagates through the few-mode multi-core optical fiber; an optical coupling unit that is arranged on one end of the few-mode multi-core optical fiber and inputs input light including the LP mode into a core; a light extraction unit that is arranged on the other end of the few-mode multi-core optical fiber and extracts output light from the core; a mode separator that separates the LP mode from the output light to extract signal light; and a receiver that receives the signal light from the mode separator. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a few-mode multi-core optical fiber and an optical transmission system that achieve both suppression of inter-core crosstalk (XT) and differential group delay (DMD) between modes. [Brief explanation of the drawings]

[0010] [Figure 1A] 1 is a cross-sectional view showing the structure of a few-mode multi-core optical fiber according to a first embodiment. [Figure 1B] FIG. 2 is a diagram showing a refractive index profile in a few-mode multi-core optical fiber according to the first embodiment. [Figure 2] FIG. 10 is a characteristic diagram of differential group delay time between modes with respect to radius ratio and relative refractive index difference ratio. [Figure 3] FIG. 10 is a characteristic diagram of inter-core crosstalk versus inter-core distance. [Figure 4] FIG. 10 is a characteristic diagram of the cutoff wavelength, leakage loss, and differential group delay time between modes with respect to the core radius and the inter-core distance. [Figure 5A] FIG. 4 is a cross-sectional view showing the structure of a few-mode multi-core optical fiber according to a second embodiment. [Figure 5B] FIG. 10 is a diagram showing a refractive index profile in a few-mode multi-core optical fiber according to a second embodiment. [Figure 6] FIG. 10 is a characteristic diagram of the differential group delay time between modes versus parameters that define the refractive index profile. [Figure 7] FIG. 10 is a characteristic diagram of leakage loss and inter-core crosstalk with respect to parameters that define the refractive index profile and inter-core distance. [Figure 8] 1 is a diagram illustrating a configuration of an optical transmission system using a few-mode multi-core optical fiber according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and redundant description will be omitted.

[0012] (First embodiment) [1-1. Configuration of few-mode multi-core optical fiber] Fig. 1A is a cross-sectional view showing the structure of a few-mode multi-core optical fiber according to a first embodiment. Fig. 1B is a diagram showing the refractive index profile in the few-mode multi-core optical fiber according to the first embodiment. Hereinafter, the few-mode multi-core optical fiber will be referred to as an optical fiber FB.

[0013] 1A shows a cross-sectional view of an optical fiber FB at a cross section perpendicular to the direction in which the optical fiber FB extends (the direction of the central axis). The optical fiber FB comprises a core 10, a low refractive index region 20, and a cladding region 30. The optical fiber FB comprises at least two cores 10. FIG. 1B shows the refractive index profile in a cross section passing through the center of the optical fiber FB and the center of the core 10.

[0014] The core 10 propagates multiple lightwave modes (LP modes). In the example shown in Figures 1A and 1B, the core 10 has a step-index (SI) structure. That is, the core 10 is composed of at least two or more different core regions that are arranged concentrically around the center of the core 10 and have different refractive indices.

[0015] The low refractive index region 20 has a refractive index lower than that of the cores 10, and is disposed so as to surround all of the cores 10 included in the optical fiber FB.

[0016] The cladding region 30 has a refractive index equal to or higher than the refractive index of the low refractive index region 20, and is disposed so as to surround the low refractive index region 20. The diameter of the cladding region 30 may be equal to or greater than 124 μm and equal to or less than 126 μm.

[0017] Light rays propagate through the optical fiber FB by repeated total reflection in the core 10. However, the inclination of the light rays is not allowed to be any arbitrary value; only light rays with a specific angle can propagate. Such a light ray configuration is called a light wave mode. The light wave modes that can propagate through the core 10 can be determined, for example, by electromagnetic field analysis.

[0018] For example, there is a linearly polarized mode in the light wave mode. ml Here, "m" is the mode order indicating how the intensity distribution of the transverse electric field of a light ray propagating through the core 10 changes in the angular direction, and "l" is the mode order indicating how the intensity distribution of the transverse electric field of a light ray propagating through the core 10 changes in the radial direction.

[0019] In this embodiment, the core 10 has at least two or more lightwave modes. For example, the core 10 has an "LP 01 " and "LP 11 The lightwave modes of the core 10 are not limited to the examples given here.

[0020] Here, the radii and refractive indices of the multiple core regions constituting the core 10 are set so that the differential group delay (DMD) between LP modes is 0.1 ns / km or less. Also, to achieve a sufficient signal-to-crosstalk-to-noise ratio, it is desirable that the inter-core crosstalk of the LP mode be -24 dB or less after propagation over 1 km. Therefore, the distance Λ between the cores 10 is set so that the inter-core crosstalk of the LP mode is -24 dB or less after propagation over 1 km.

[0021] As an example, a case will be described in which the core 10 is composed of an inner core region 11 and an outer core region 12, and the optical fiber FB has four cores 10.

[0022] For example, the radius and refractive index of the inner core region 11 are a1 and n1, respectively. The radius and refractive index of the outer core region 12 are a2 and n2, respectively. The refractive index of the low refractive index region 20 is nd In this case, the relative refractive index difference Δ1 of the inner core region 11 with respect to the low refractive index region 20 is expressed as follows: Δ1=(n1 2 -n d 2 ) / 2n d 2 ". The relative refractive index difference Δ2 of the outer core region 12 with respect to the low refractive index region 20 can be expressed as "Δ2 = (n2 2 -n d 2 ) / 2n d 2 " can be expressed as:

[0023] The diameter of the cladding region 30 is D, the distance from the center of the core 10 to the outer surface of the cladding region 30 is t, the width of the cladding region 30 is w, and the distance between the cores 10 is Λ.

[0024] Here, the relationship "t=D / 2-Λ / √2" holds. From the viewpoint of providing the cladding region 30 during the manufacturing of the optical fiber FB, the relationship "tw≦3a k (where k=1, 2, . . . , N, where N is the number of core regions) is desirable. k If "," the width w of the cladding region 30 becomes small, which is not practical in terms of manufacturing the optical fiber FB.

[0025] In FIG. 1B, the relative refractive index difference Δ C is the refractive index n of the low refractive index region 20 C Using "Δ C =(n C 2 -n d 2 ) / 2n d 2 " is expressed as:

[0026] Here, the radius ratio and the relative refractive index difference ratio of the inner core region 11 and the outer core region 12 are R a , R Δ More specifically, "R a =a1 / a2", "R Δ =Δ1 / Δ2. Radius ratio Ra and the relative refractive index difference ratio R Δ By setting the appropriate 01 LP for Mode 11 The distance Λ between the cores 10 is set so that the crosstalk between the cores of the LP mode is -24 dB or less after 1 km propagation. 21 Mode and LP 02 The relative refractive index difference Δ C is set.

[0027] [1-2. Conditions for radius ratio and relative refractive index difference ratio] Figure 2 shows the differential group delay between modes as a function of the radius ratio and the relative refractive index difference ratio. a and the relative refractive index difference ratio R Δ LP 01 Mode and LP 11 The figure shows the differential group delay between modes of a DMD, where the wavelength is 1550 nm.

[0028] Here, the radius ratio R a and the relative refractive index difference ratio R Δ For each combination, LP 01 The radius a2 and the relative refractive index difference Δ2 of the outer core region 12 are optimized so that the mode field diameter (MFD) at the mode wavelength of 1550 nm is 10 μm. a ≦0.7 and 0.1≦R Δ Radius ratio R in the range of ≦0.9 a and the relative refractive index difference ratio R Δ We confirmed the conditions for this.

[0029] In Figure 2, in region RG2 (shaded region), by appropriately selecting radius a2 and relative refractive index difference Δ2, it is possible to achieve "|DMD|≦0.1 ns / km." On the other hand, in region RG1 (white region), it is not possible to achieve "|DMD|≦0.1 ns / km" by appropriately selecting radius a2 and relative refractive index difference Δ2.

[0030] The boundary between region RG1 and region RG2 is "R a R ≦ 0.5 Δ = 0.75" and R a R > 0.5 Δ =-6.5R a +4.0". Therefore, "R a R ≦ 0.5 Δ ≧0.75, and R a R > 0.5 Δ ≧-6.5R a +4.0" The radius ratio R is set to satisfy a and the relative refractive index difference ratio R Δ By setting 01 Mode and LP 11 The differential group delay (DMD) between modes can be reduced to 0.1 ns / km or less.

[0031] The number of cores 10 included in the optical fiber FB shown in FIG. 1A is four, but the radius ratio R a and the relative refractive index difference ratio R Δ The condition for can be applied regardless of the number of cores 10 that the optical fiber FB has.

[0032] [1-3. Conditions for core distance] Figure 3 shows the inter-core crosstalk characteristics versus the inter-core distance. 11 The figure shows the characteristics of the inter-core crosstalk after 1 km propagation of the mode. The wavelength is 1625 nm. As an example, R a =0.4, R Δ =0.75.

[0033] As the inter-core distance Λ increases, inter-core crosstalk decreases. Here, the relationship between inter-core crosstalk XT and distance Λ can be expressed as "XT = -4.2Λ + 122.0". Therefore, by setting "Λ ≧ 34.7 μm", inter-core crosstalk after 1 km propagation can be reduced to -24 dB or less.

[0034] Incidentally, to enable 16-quadrature amplitude modulation (16QAM) over a transmission distance of 100 km, the crosstalk at the receiver must be -24 dB or less.

[0035] In a bidirectional transmission method in which the signal propagation direction is swapped between adjacent cores in a multi-core optical fiber, crosstalk during reception can be reduced by about 20 dB compared to codirectional transmission in which the propagation direction is the same.

[0036] In a multi-core optical fiber with a core-to-core crosstalk of -24 dB after 1 km propagation, the crosstalk at the time of reception of a 100 km codirectional transmission is -24 dB + 10 log 10 (100 km) = -4 dB. By applying a bidirectional transmission method, the crosstalk during reception can be expected to be -24 dB. Therefore, it can be seen that an optical fiber FB with "Λ≧34.7 μm" can realize a 16QAM transmission link of 100 km or more.

[0037] Although the optical fiber FB shown in Figure 1A has four cores 10, the condition for the distance Λ shown in Figure 3 can be applied regardless of the number of cores 10 included in the optical fiber FB. Note that in the region of a mode field diameter larger than 10 µm, the required distance Λ is limited as the mode field diameter increases. Therefore, the condition in the region of a mode field diameter larger than 10 µm is included in the condition "Λ ≥ 34.7 µm."

[0038] [1-4. Conditions for core distance] Figure 4 shows the characteristics of the cutoff wavelength, leakage loss, and differential group delay time between modes with respect to the core radius and the core-to-core distance. 21 Mode and LP 02 Mode cutoff wavelength λ C , LP 11 Leakage loss α at mode wavelength 1625 nm C, and a characteristic diagram of the differential group delay time between modes at a wavelength of 1550 nm. The diameter D of the cladding region 30 is 125 μm, and "tw=3a2". As an example, R a =0.4, R Δ =0.75.

[0039] In Figure 4, the dashed line indicates "λ c 1530 nm”. When “a2≦7.2 μm”, the relative refractive index difference Δ C Optimize "λ C Therefore, if "a2 ≦ 7.2 μm" is satisfied, noise caused by unnecessary modes can be suppressed.

[0040] Next, the dashed line indicates the boundary of the structure where "DMD≧-0.1ns / km." By making "a2≧6.9μm," the differential group delay between modes can be reduced.

[0041] The solid line indicates the LP 11 At the wavelength of 1625 nm, C = 0.01 dB / km. In the area where the distance Λ is smaller than the solid line, C ≦0.01 dB / km”, achieving low loss.

[0042] The solid line is calculated using radius a2 and distance Λ. "Λ=-5.0a2 2 +73.5a2-227.6" It can be expressed as:

[0043] Therefore, we set it to "6.9≦a2≦7.2μm" and further set it to "Λ≦-5.0a2 2 By setting it to "+73.5a2-227.6", unnecessary LP modes can be blocked and low loss can be achieved.

[0044] (Second embodiment) [2-1. Configuration of few-mode multi-core optical fiber (variation)] Fig. 5A is a cross-sectional view showing the structure of a few-mode multi-core optical fiber according to the second embodiment, and Fig. 5B is a diagram showing the refractive index profile in the few-mode multi-core optical fiber according to the second embodiment.

[0045] 5A shows a cross-sectional view of a cross section perpendicular to the direction in which the optical fiber FB extends (the direction of the central axis), and FIG. 5B shows the refractive index profile of a cross section passing through the center of the optical fiber FB and the center of the core 10.

[0046] 5A and 5B, the core 10 has a graded-index (GI) structure. This corresponds to the step-index (SI) structure shown in FIGS. 1A and 1B, where the number of core regions N is infinite. Furthermore, the diameter of the cladding region 30 is D, the distance from the center of the core 10 to the outer surface of the cladding region 30 is t, the width of the cladding region 30 is w, and the distance between the cores 10 is Λ.

[0047] The radius of the core 10 is a, and the refractive index at the center of the core 10 is n1. The relative refractive index difference Δ1 at the center of the core 10 with respect to the low refractive index region 20 is expressed as follows: Δ1=(n1 2 -n d 2 ) / 2n d 2 " can be expressed as:

[0048] The refractive index profile of the graded index type is determined by the following formula using a refractive index shape factor α and a radial coordinate r (0≦r≦a) with the center of the core 10 as the origin.

number

[0049] By appropriately setting the refractive index shape factor α, LP 01 LP for Mode 11 The differential group delay of the modes can be reduced.

[0050] In addition, similarly to the first embodiment, the distance Λ between the cores 10 is set so that the inter-core crosstalk of the LP mode is −24 dB or less after propagation of 1 km. 21 Mode and LP 02 The relative refractive index difference Δ C is set.

[0051] [2-2. Conditions for parameters that determine the refractive index profile] Figure 6 is a characteristic diagram of the differential group delay time between modes versus the parameters that determine the refractive index shape. Figure 6 shows a characteristic diagram showing the relationship between the radius a and the refractive index shape factor α when the differential group delay time between modes is ±0.1 ns / km. At a wavelength of 1550 nm, 01 The relative refractive index difference Δ1 is optimized so that the mode field diameter of the mode is 10 μm.

[0052] The solid line indicates the boundary between the radius a and refractive index shape factor α structures for which the differential group delay between modes is 0.1 ns / km. The dashed line indicates the boundary between the radius a and refractive index shape factor α structures for which the differential group delay between modes is -0.1 ns / km. Core structures with combinations of radius a and refractive index shape factor α in the shaded area surrounded by the solid and dashed lines can achieve sufficiently small differential group delay.

[0053] The solid line is calculated by using the radius a and the refractive index shape factor α. "a=3.5α 2 -19.5α+34.9" It can be expressed as:

[0054] The dashed line is expressed by using the radius a and the refractive index shape factor α. "a=2.0α 2 -11.8α+25.1" It can be expressed as:

[0055] Therefore, the radius a and the refractive index shape factor α are "2.0α 2 -11.8α+25.1≦a≦3.5α2 -19.5α+34.9" By satisfying the following relationship, a 16QAM transmission link of 100 km or more can be realized.

[0056] [2-3. Conditions for core distance] Fig. 7 is a characteristic diagram of leakage loss and inter-core crosstalk versus the parameters that determine the refractive index profile and the inter-core distance. In Fig. 7, LP versus the refractive index profile factor α and the distance Λ 11 Inter-core crosstalk and leakage loss α in the mode C The diameter D of the cladding region 30 is 125 μm, and "tw=3a2" is set. The wavelength is 1625 nm.

[0057] In Figure 7, the solid line indicates "α C = 0.01 dB / km. The dashed line indicates the boundary of the structure where the inter-core crosstalk XT is -24 dB / km.

[0058] In the region where the distance Λ is smaller than the solid line and larger than the dashed line, the inter-core crosstalk is reduced and the leakage loss α C A reduction in the amount of heat can be achieved.

[0059] The solid line is calculated by using the refractive index shape factor α and the distance Λ as follows: "Λ=-30.0α 2 +151.0α-149.1」 It can be expressed as:

[0060] The dashed line is expressed by using the refractive index shape factor α and the distance Λ as follows: "Λ=-20.0α 2 +103.0α-94.5」 It can be expressed as:

[0061] Therefore, the refractive index shape factor α and the distance Λ are -20.0α 2 +103.0α-94.5≦Λ≦ -30.0α 2 +151.0α-149.1」 By satisfying the following relationship, a 16QAM transmission link of 100 km or more can be realized.

[0062] [3. Optical Transmission System Configuration] 8 is a diagram illustrating the configuration of an optical transmission system using a few-mode multi-core optical fiber according to an embodiment of the present disclosure. The optical transmission system 100 includes an optical fiber FB according to an embodiment of the present disclosure, a transmitter ST, a mode multiplexer MT, an optical coupler FI, a light extractor FO, a mode separator MR, and a receiver SR.

[0063] The transmitter ST generates a signal light. The number of transmitters ST may be one or more.

[0064] The mode multiplexer MT converts the signal light into a lightwave mode. The number of mode multiplexers MT may be one or more.

[0065] The optical coupling unit FI is disposed on one end side of the optical fiber FB, and inputs input light including a lightwave mode into the core 10. The optical coupling unit FI may be connected to the mode multiplexers MT, and introduce the output of each mode multiplexer MT into the core 10 of the optical fiber FB.

[0066] Alternatively, the optical coupling unit FI may be a MUX (Multiplexer). A MUX is a circuit that converts multiple input signals into one output signal. A MUX selects only one of the multiple input signals to be the output signal. The selected signal is determined by a control signal called a selection signal.

[0067] The light extraction unit FO is disposed on the other end side of the optical fiber FB and extracts the output light from the core 10. The light extraction unit FO may be connected to the mode separator MR and introduce the output of the core 10 of the optical fiber FB into each mode multiplexer MT.

[0068] Alternatively, the optical extraction unit FO may be a DEMUX (Demultiplexer). A DEMUX has the opposite function to a MUX and is a circuit that distributes one input signal to multiple output signals. A DEMUX distributes an input signal to multiple outputs according to a selection signal.

[0069] The mode separator MR separates the optical wave mode from the output light to extract the signal light. The number of mode separators MR may be one or more.

[0070] The receiver SR receives the signal light from the mode separator MR. There may be a plurality of receivers SR. The number of receivers SR may be any number equal to or greater than one.

[0071] [Effects of the embodiment] As described above in detail, the few-mode multi-core optical fiber according to this embodiment comprises at least two cores that propagate a plurality of LP modes, low-refractive-index regions that have a refractive index lower than that of the cores and surround all of the cores, and cladding regions that have a refractive index equal to or higher than that of the low-refractive-index regions and surround the low-refractive-index regions. Each core has at least two or more different core regions that are arranged concentrically about the center of the core and have different refractive indices. The differential group delay between the LP modes is 0.1 ns / km or less, and the inter-core crosstalk of the LP modes is −24 dB or less after propagation over 1 km.

[0072] This makes it possible to provide a few-mode multi-core optical fiber that achieves both suppression of inter-core crosstalk (XT) and differential group delay (DMD) between modes. Furthermore, the reduced differential group delay reduces the load on devices connected to the few-mode multi-core optical fiber. The reduced signal processing in devices connected to both ends of the optical fiber also reduces the cost of the transmission equipment. Furthermore, the reduced propagation loss allows signals to propagate further. As a result, the cost of the transmission equipment, especially the equipment that transmits the signals, can be reduced. In addition, this also leads to a reduction in power consumption in devices connected to both ends of the optical fiber.

[0073] In the few-mode multi-core optical fiber according to this embodiment, the core may have an inner core region and an outer core region as core regions, where the radius a1 and refractive index n1 of the inner core region, the radius a2 and refractive index n2 of the outer core region, and the refractive index n of the low-refractive-index region are d , relative refractive index difference Δ1=(n1 2 -n d 2 ) / 2n d 2 , relative refractive index difference Δ2=(n2 2 -n d 2 ) / 2n d 2 , R a = a1 / a2 and R Δ = Δ1 / Δ2, then "0.3 ≦ R a ≦0.7 and 0.1≦R Δ ≦0.9" and "R a R ≦ 0.5 Δ ≥ 0.75" and "R a R > 0.5 Δ ≧-6.5R a +4.0". The distance Λ between the cores may be 34.7 μm or more.

[0074] This allows for a sufficiently small differential group delay time to be achieved. In addition, the core-to-core crosstalk and leakage loss α C Furthermore, 16QAM transmission links of 100 km or more can be realized.

[0075] Furthermore, in the few-mode multi-core optical fiber according to this embodiment, the diameter of the cladding region may be 124 μm or more and 126 μm or less. 02 Mode and LP 21 The cutoff wavelength of the mode may be 1530 nm. The units of a2 and Λ are μm, and "a2≦7.2" and "Λ≦−5.0a2 2 +73.5a2-227.6" is also acceptable.

[0076] This allows blocking of unnecessary LP modes and low loss, and also enables noise suppression due to unnecessary modes.

[0077] In the few-mode multi-core optical fiber according to this embodiment, the cores may have a graded-index refractive index profile. When the radius of the core is a, the refractive index shape factor of the refractive index profile is α, and the distance between the cores is Λ, the LP mode is 01 Under the design conditions where the mode field diameter of the mode is 10 μm, the unit of a is μm, and "2.0α 2 -11.8α+25.1≦a≦3.5α 2 Under the condition that the diameter of the cladding region is 125 μm and the wavelength is 1.625 μm, the unit of Λ is μm, and "-20.0α 2 +103.0α-94.5≦Λ≦-30.0α 2 +151.0α-149.1" is also acceptable.

[0078] A core structure with a combination of radius a and refractive index shape factor α that satisfies the conditions can realize a sufficiently small differential group delay time. In addition, the core-to-core crosstalk can be reduced and the leakage loss α C Furthermore, 16QAM transmission links of 100 km or more can be realized.

[0079] Furthermore, the optical transmission system according to this embodiment includes: a few-mode multi-core optical fiber according to the present disclosure; a transmitter that generates signal light; a mode multiplexer that converts the signal light into an LP mode that propagates through the few-mode multi-core optical fiber; an optical coupling unit that is arranged on one end of the few-mode multi-core optical fiber and inputs input light including the LP mode into the core; a light extraction unit that is arranged on the other end of the few-mode multi-core optical fiber and extracts output light from the core; a mode separator that separates the LP mode from the output light to extract the signal light; and a receiver that receives the signal light from the mode separator.

[0080] This makes it possible to provide an optical transmission system that suppresses both inter-core crosstalk (XT) and differential mode delay (DMD). In addition, the differential group delay is reduced, which reduces the load on the signal processing side. The cost of transmission equipment can also be reduced because the signal processing required by devices connected to both ends of the optical fiber is reduced. In addition, the propagation loss is reduced, allowing signals to propagate further. As a result, it is possible to reduce the cost of transmission equipment, especially the equipment that transmits signals. In addition, this also leads to reduced power consumption in devices connected to both ends of the optical fiber.

[0081] Although the contents of the present disclosure have been described above based on the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which will be apparent to those skilled in the art. The descriptions and drawings that form part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0082] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description. [Explanation of symbols]

[0083] 10 cores 11. Inner Core Region 12 outer core region 20 Low refractive index region 30 Cladding region 100 Optical Transmission System FB optical fiber (few-mode multi-core optical fiber) FI optical coupling section FO light extraction section MR mode separator MT mode multiplexer SR receiver ST transmitter

Claims

1. At least two or more cores that propagate a plurality of LP modes; a low refractive index region having a refractive index lower than that of the core and surrounding the entire core; a cladding region having a refractive index equal to or greater than the refractive index of the low refractive index region and surrounding the low refractive index region; A few-mode multi-core optical fiber comprising: Each of the cores has at least two or more different core regions arranged concentrically about the center of the core and having different refractive indices; The differential group delay between the LP modes is 0.1 ns / km or less, and The LP mode core-to-core crosstalk is -24 dB or less after 1 km propagation. A few-mode multicore optical fiber.

2. The core has an inner core region and an outer core region as the core region, The radius a of the inner core region １ , refractive index n １ , The radius a of the outer core region ２ , refractive index n ２ , The refractive index n of the low refractive index region ｄ , Differential than the refractive index Δ １ = (n １ ２ -n ｄ ２ ) / 2n ｄ ２ Differential than the refractive index Δ ２ = (n ２ ２ -n ｄ ２ ) / 2n ｄ ２ , R ａ = a １ / a ２ , and R Δ =Δ １ / Δ ２ When 0.3≦R ａ ≦0.7, and 0.1≦R Δ ≦0.9, and R ａ R≦0.5 Δ ≧0.75, and R ａ >0.5 Δ ≧-6.5R ａ +4.0 Fulfilling The distance Λ between the cores is 34.7 μm or more The few-mode multi-core optical fiber according to claim 1 ,

3. the diameter of the cladding region is 124 μm or more and 126 μm or less; Among the LP modes, LP ０２ Mode and LP ２１ The cutoff wavelength of the mode is 1530 nm, a ２ and Λ is in μm, a ２ ≦7.2, and Λ≦-5.0a ２ ２ +73.5a ２ -227.6 The few-mode multi-core optical fiber according to claim 2 ,

4. the core has a graded-index refractive index profile; When the radius of the core is a, the refractive index shape factor of the refractive index profile is α, and the distance between the cores is Λ, Among the LP modes, LP ０１ Under the design condition that the mode field diameter of the mode is 10 μm, the unit of a is μm, 2.0a ２ -11.88a+25.1≦a≦3.5a ２ -19.5a+34.9 and Under the condition that the diameter of the cladding region is 125 μm and the wavelength is 1.625 μm, and Λ is in μm, -20.0a ２ +103.0a-94.5≦Λ≦ －３０．０α ２ ＋１５１．０α－１４９．１ The few-mode multi-core optical fiber according to claim 1 ,

5. A few-mode multi-core optical fiber according to any one of claims 1 to 4, a transmitter that generates a signal light; a mode combiner that converts the signal light into the LP mode propagating through the few-mode multi-core optical fiber; an optical coupling unit arranged on one end side of the few-mode multi-core optical fiber, which inputs input light including the LP mode into the core; a light extraction unit disposed on the other end side of the few-mode multi-core optical fiber and configured to extract output light from the core; a mode separator that separates the LP mode from the output light to extract the signal light; a receiver for receiving the signal light from the mode separator; An optical transmission system comprising:

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