Multi-mode multicore optical fiber and optical transmission system

Abstract

To provide: a few-mode multi-core optical fiber capable of suppressing microbend loss and maintaining a low loss property; and an optical transmission system.SOLUTION: A few-mode multi-core optical fiber comprises: at least two or more core regions through which lightwave modes propagate; a depressed region having a refractive index less than refractive indexes of the core regions and disposed to include all the core regions; and a cladding region that has a refractive index equal to or more than the refractive index of the depressed region and less than the refractive indexes of the core regions, and is disposed to surround the depressed region. At a wavelength of 1625 nm, a difference between an effective refractive index in a highest order mode out of the lightwave modes and an effective refractive index in a propagation mode of the cladding region is 0.0021 or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This disclosure relates to multi-mode multicore optical fibers and optical transmission systems. [Background technology]

[0002] Research into spatial division multiplexing technology is actively underway to realize future high-capacity optical networks. Spatial division multiplexing technology is a technique that houses multiple spatial channels (cores, modes) in a single optical fiber, and is expected to dramatically improve the transmission capacity per optical fiber compared to conventional optical fibers.

[0003] Non-patent document 1 discloses a multicore optical fiber structure having multiple cores, in which, from the viewpoint of manufacturability and compatibility with existing equipment, the same type of core capable of single-mode propagation is arranged in a cladding (standard cladding) with a diameter of 125 μm, equivalent to that of a standard optical fiber.

[0004] Non-patent document 2 discloses a multi-mode multicore optical fiber in which a cladding with a diameter of 125 μm is arranged with identical cores capable of propagating three modes, in order to further increase transmission capacity.

[0005] Non-patent document 3 discloses that, compared to general-purpose single-mode core optical fibers, multi-mode core optical fibers tend to exhibit increased microbend loss. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] T. Matsui et al., "Design of 125μm cladding multi-core fiber with full-band compatibility to conventional single-mode fiber," 2015 European Conference on Optical Communication (ECOC), Valencia, Spain, 2015, pp. 1-3, doi: 10.1109 / ECOC.2015.7341966. [Non-Patent Document 2] Y. Sagae et al., “A 125-μm Cladding Diameter Uncoupled 3-mode 4-core Fibre with the Highest Core Multiplicity Factor," 2022 European Conference on Optical Communication (ECOC), 2022, Tu3A.2 [Non-Patent Document 3] P. Sillard et al., “Micro-bend-Resistant Low-Differential-Mode-Group-Delay Few-Mode Fibers," Journal of lightwave technology, vol. 35, no. 4, pp. 734-740, 2017 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] According to the technology described in Non-Patent Document 2, four 3-mode cores are arranged with a standard cladding diameter by using a common depressed structure. However, the structure capable of suppressing microbend loss is not clear, and there is still a problem that the microbend loss tends to increase in multi-mode core optical fibers.

[0008] The present disclosure has been made in view of the above problems. The object is to provide a few-mode multi-core optical fiber and an optical transmission system that can suppress microbend loss and maintain low loss characteristics.

Means for Solving the Problems

[0009] In order to solve the above problems, a few-mode multi-core optical fiber according to one aspect of the present disclosure includes at least two or more core regions through which optical wave modes propagate, a depressed region having a refractive index less than that of the core regions and arranged to enclose all the core regions, and a cladding region having a refractive index not less than that of the depressed region and less than that of the core regions and arranged to surround the depressed region. And at a wavelength of 1625 nm, the difference between the effective refractive index of the highest-order mode among the optical wave modes and the effective refractive index of the propagation mode in the cladding region is 0.0021 or more.

[0010] Further, an optical transmission system according to one aspect of the present disclosure includes the few-mode multi-core optical fiber of the present disclosure, a transmitter that generates signal light, a mode multiplexer that converts the signal light into optical wave modes, an optical coupler arranged on one end side of the few-mode multi-core optical fiber for inputting input light including optical wave modes into the core regions, an optical extraction unit arranged on the other end side of the few-mode multi-core optical fiber for extracting output light from the core regions, a mode separator that separates optical wave modes from the output light and extracts signal light, and a receiver that receives the signal light from the mode separator.

Advantages of the Invention

[0011] According to the present disclosure, it is possible to provide a few-mode multi-core optical fiber and an optical transmission system that can suppress microbend loss and maintain low loss characteristics.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a cross-sectional view showing the structure of a few-mode multi-core optical fiber according to an embodiment of the present disclosure. [Figure 2] Figure 2 shows an example of the refractive index distribution in a multi-mode multicore optical fiber according to the embodiment of this disclosure. [Figure 3] Figure 3 shows the structural conditions related to the difference in specific refractive index. [Figure 4] Figure 4 shows the characteristics of the first parameter that appear in the effective refractive index difference and structural conditions. [Figure 5] Figure 5 shows the characteristics of the second parameter that appear in the effective refractive index difference and structural conditions. [Figure 6] Figure 6 shows the structural conditions (first pattern) related to the cutoff wavelength and leakage loss. [Figure 7] Figure 7 shows the structural conditions (second pattern) related to the cutoff wavelength and leakage loss. [Figure 8] Figure 8 shows the structural conditions (third pattern) related to the cutoff wavelength and leakage loss. [Figure 9] Figure 9 shows the characteristics of the third parameter that appear in the shape dependence of the cutoff wavelength. [Figure 10] Figure 10 shows the characteristics of the fourth parameter that appear in the shape dependence of the cutoff wavelength. [Figure 11] Figure 11 shows the characteristics of the fifth parameter that appear in the shape dependence of leakage loss. [Figure 12] Figure 12 shows the structural conditions (fourth pattern) related to the cutoff wavelength and leakage loss. [Figure 13] Figure 13 shows the designable region in relation to the distance from the center of the core region to the outer surface of the cladding region. [Figure 14] Figure 14 shows the characteristics of the sixth parameter that appear in the shape dependence of leakage loss. [Figure 15] Figure 15 shows the characteristics of the seventh parameter that appear in the shape dependence of leakage loss. [Figure 16] Figure 16 shows the shape dependence of the effective refractive index difference. [Figure 17]Figure 17 shows the configuration of an optical transmission system using a multi-mode multicore optical fiber according to an embodiment of this disclosure. [Modes for carrying out the invention]

[0013] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, identical components are denoted by the same reference numerals, and redundant explanations will be omitted.

[0014] [1. Configuration of multi-mode multicore optical fiber] Figure 1 is a cross-sectional view showing the structure of a multi-mode multicore optical fiber according to an embodiment of the present disclosure. Figure 1 shows a cross-sectional view in a section perpendicular to the direction of extension of the multi-mode multicore optical fiber FB (direction of the central axis). The multi-mode multicore optical fiber FB comprises a core region 10 through which light wave modes propagate, a depressed region 20, and a cladding region 30.

[0015] A multi-mode multicore optical fiber FB may have at least two or more core regions 10. Figure 1 shows an example in which four core regions 10 are arranged. The number and arrangement of core regions 10 in a multi-mode multicore optical fiber FB are not limited to the example shown in Figure 1. For example, a multi-mode multicore optical fiber FB may have any number of core regions 10, two or more. In addition, in a multi-mode multicore optical fiber FB, the multiple core regions 10 may be arranged in a ring shape or in a hexagonal close-packed arrangement.

[0016] Light rays propagating within a multi-mode multicore optical fiber FB propagate through repeated total internal reflection in the core region 10. Here, the slope of the light ray cannot be arbitrary; only light rays with a specific angle can propagate. Such ray forms are called optical wave modes. Optical wave modes capable of propagating in the core region 10 can be determined, for example, by electromagnetic field analysis.

[0017] For example, among the light wave modes, there is a linearly polarized mode. The linearly polarized mode is called "LP ml It is expressed in the form of ". Here, "m" is the mode order that indicates the angular change in the intensity distribution of the transverse electric field of the light ray propagating through the core region 10, and "l" is the mode order that indicates the radial change in the intensity distribution of the transverse electric field of the light ray propagating through the core region 10.

[0018] In this embodiment, the core region 10 has at least two or more optical wave modes. For example, the core region 10 is "LP 01 " and "LP 11 The core region 10 has the following as an optical wave mode. The optical wave modes of the core region 10 are not limited to the examples given here.

[0019] The depressed region 20 is positioned to encompass all the core regions 10 arranged in the multi-mode multicore optical fiber FB. The depressed region 20 has a refractive index lower than that of the core regions 10. For example, the cross-sectional shape of the depressed region 20 is circular.

[0020] The clad region 30 is positioned to surround the depressed region 20. For example, the cross-sectional shape of the clad region 30 may be circular, and the central axis of the outer surface of the depressed region 20 and the central axis of the outer surface of the clad region 30 may be the same. In other words, the clad region 30 may be positioned coaxially with the depressed region 20.

[0021] Furthermore, the cladding region 30 has a refractive index that is greater than or equal to that of the depressed region 20, and less than that of the core region 10.

[0022] For the purposes of the following explanation, let the radius of the core region 10 be "a" (in μm). Let the distance from the center of the core region 10 to the outer surface of the depressed region 20 be "w" (in μm). Let the distance from the center of the core region 10 to the outer surface of the cladding region 30 be "τ" (in μm). Also, let the difference in relative refractive index of the core region 10 with respect to the depressed region 20 be Δ1. Let the difference in relative refractive index of the cladding region 30 with respect to the depressed region 20 be Δ2.

[0023] Figure 2 shows an example of the refractive index distribution in a multi-mode multicore optical fiber according to an embodiment of the present disclosure. In Figure 2, the vertical axis shows the relative refractive index difference with respect to the depressed region 20, and the horizontal axis shows the radial position. For example, the distribution of the relative refractive index difference in a cross-section passing through the central axis of the multi-mode multicore optical fiber FB in Figure 1 is shown as in Figure 2.

[0024] For the purpose of explanation below, in the configuration of the multi-mode multicore optical fiber FB shown in Figures 1 and 2, the difference between the effective refractive index of the highest-order mode among the light wave modes propagating in the core region 10 and the effective refractive index of the propagation mode in the cladding region 30 is given by Δn eff_cl Let's assume that.

[0025] According to Non-Patent Document 3, "Δn eff_cl It has been reported that there is a strong correlation between "Δn" and microbend loss. Focusing on this, the inventor of the present invention has stated "Δn eff_cl We found that microbend loss can be suppressed when the condition "≥0.0021" is met. The reason why microbend loss can be suppressed is explained below.

[0026] [2. Structural conditions from the perspective of the difference in effective refractive index] "Δn eff_cl To examine the structural conditions imposed on "Δ1" and "Δ2" in order to satisfy the condition "≥0.0021", structural calculations were performed for various multi-mode multicore optical fiber FBs.

[0027] FIG. 3 is a diagram showing the structural conditions regarding the difference in specific refractive index. In FIG. 3, for the case of “τ = 33.5 μm”, the relationship between “Δ1” and “Δ2 / Δ1” when “Δn eff_cl = 0.0021, 0.0024, 0.0027, 0.0030” at a wavelength of 1625 nm is shown. Note that the mode field diameter (MFD) of the “LP 01 ” of the optical wave mode at a wavelength of 1550 nm is set to 10.0 μm, which is equivalent to that of a standard single-mode optical fiber. Also, it is assumed that the diameter of the cladding region 30 is 126 μm or less.

[0028] As a result, it was found that the following Equation 1 holds between “Δ1” and “Δ2 / Δ1”.

Equation

[0029] Note that the following Equation 2 holds between “Δn eff_cl ” and the first parameter “K1” appearing in Equation 1. FIG. 4 is a diagram showing the aspect of the effective refractive index difference and the first parameter appearing in the structural conditions.

Equation

[0030] The following Equation 3 holds between “Δn eff_cl ” and the second parameter “K2” appearing in Equation 1. FIG. 5 is a diagram showing the aspect of the effective refractive index difference and the second parameter appearing in the structural conditions.

Equation

[0031] Based on the above-mentioned Equations 1 to 3, the relationship between “Δ1” and “Δ2 / Δ1” when “Δn eff_cl ≥ 0.0021” is represented by the following Equation 4.

Equation

[0032] [3. Structural conditions from the perspective of cutoff wavelength and leakage loss] Next, in order to examine the structural conditions imposed on "Δ1" and "Δ2" from the perspective of cutoff wavelength and leakage loss, structural calculations were performed for various multi-mode multicore optical fiber fiber FBs.

[0033] Figure 6 shows the structural conditions (first pattern) related to the cutoff wavelength and leakage loss. In Figure 6, when "τ = 33.5 μm" and "w / a = 2.5", the optical mode "LP 21 The cutoff wavelength of "λ" c The wavelength is 1530nm or less, and the leakage loss at a wavelength of 1625nm is "α c This shows the relationship between "Δ1" and "Δ2 / Δ1" such that the noise level is 0.01 dB / km or less. Note that at a wavelength of 1550 nm, the optical mode is "LP 01 The mode field diameter (MFD) of this product is set to 10.0 μm, which is equivalent to that of a standard single-mode optical fiber.

[0034] In Figure 6, the solid line represents "λ c The boundary line is determined by the conditions related to "α", and the dashed line is "α c This boundary is determined by the conditions related to "[ ]". The shaded area in Figure 6 indicates the available conditions. The solid and dashed lines in Figure 6 are represented by the following equation 5.

number

[0035] Figure 7 shows the structural conditions (second pattern) related to the cutoff wavelength and leakage loss. Unlike the case shown in Figure 6, Figure 7 shows the relationship between "Δ1" and "Δ2 / Δ1" when "w / a = 3.0".

[0036] In Figure 7, the solid line represents "λ c The boundary line is determined by the conditions related to "α", and the dashed line is "α cThis boundary is determined by the conditions related to "[ ]". The shaded area in Figure 7 indicates the usable conditions. The solid and dashed lines in Figure 7 are represented by the following equation 6.

number

[0037] Figure 8 shows the structural conditions (third pattern) related to the cutoff wavelength and leakage loss. Unlike the case shown in Figure 6, Figure 8 shows the relationship between "Δ1" and "Δ2 / Δ1" when "w / a = 3.5".

[0038] In Figure 8, the solid line represents "λ c The boundary line is determined by the conditions related to "α", and the dashed line is "α c This boundary is determined by the conditions related to "[ ]". The shaded area in Figure 8 indicates the usable conditions. The solid and dashed lines in Figure 7 are represented by the following equation 7.

number

[0039] Therefore, from the results in Figures 6-8, when "w / a" is in the range of "2.5" to "3.5", "λ c The relationship between " and "w / a" was found to be expressed by the following equation 8.

number

[0040] Here, the following equation 9 holds between "w / a" and the third parameter "K3" that appears in equation 8. Figure 9 shows the characteristics of the third parameter that appear in the shape dependence of the cutoff wavelength.

number

[0041] Furthermore, the following equation 10 holds between "w / a" and the fourth parameter "K4" that appears in equation 8. Figure 10 shows the characteristics of the fourth parameter that appear in the shape dependence of the cutoff wavelength.

number

[0042] Therefore, the "LP" in light wave mode 21 The cutoff wavelength of "λ" c The structural condition for which the wavelength is 1530 nm or less is expressed by the following equation 11, using "Δ1", "Δ2 / Δ1", and "w / a".

number

[0043] Furthermore, from the results in Figures 6-8, when "w / a" is in the range of "2.5" to "3.5", "α c It was found that the relationship between " and "w / a" can be expressed by the following equation 12.

number

[0044] Here, the following equation 13 holds between "w / a" and the fifth parameter "K5" that appears in equation 12. Figure 11 shows the characteristics of the fifth parameter that appear in the shape dependence of leakage loss.

number

[0045] Therefore, at "τ = 33.5 μm", "λ c " is 1530nm or less, and "α c The structural conditions for "Δ1", "Δ2 / Δ1", and "w / a" such that the noise level is 0.01 dB / km or less are expressed by the following equation 14.

number

[0046] [4. Structural conditions from the perspective of the distance to the outer surface of the cladding region] Next, in order to examine the structural conditions imposed on "Δ1" and "Δ2" from the perspective of the distance to the outer surface of the cladding region, structural calculations were performed for various multi-mode multicore optical fiber FBs.

[0047] Figure 12 shows the structural conditions (fourth pattern) related to the cutoff wavelength and leakage loss. In Figure 12, when "τ = 30.7 μm" and "w / a = 3.5", the optical mode "LP 21 The cutoff wavelength of "λ" c The wavelength is 1530nm or less, and the leakage loss at a wavelength of 1625nm is "α c This shows the relationship between "Δ1" and "Δ2 / Δ1" such that the noise level is 0.01 dB / km or less. Note that at a wavelength of 1550 nm, the optical mode is "LP 01 The mode field diameter (MFD) of this product is set to 10.0 μm, which is equivalent to that of a standard single-mode optical fiber.

[0048] In Figure 12, the solid line represents "λ c The boundary line is determined by the conditions related to "α", and the dashed line is "α c This boundary is determined by the conditions related to "[ ]". The shaded area in Figure 12 indicates the available conditions. The solid and dashed lines in Figure 12 are represented by the following equation 15.

number

[0049] Therefore, from the results in Figures 8 and 12, the designable area can be determined according to the distance from the center of the core area 10 to the outer surface of the cladding area 30. Figure 13 is a diagram showing the designable area with respect to the distance from the center of the core area to the outer surface of the cladding area.

[0050] In Figure 13, "Δ1 = 0.78%", and the "LP" in the light wave mode. 21 The cutoff wavelength of "λ"c The relationship between "Δ2 / Δ1" and "τ" when "α" is 1530 nm is shown by the solid line. Also, the leakage loss "α" c The relationship between "Δ2 / Δ1" and "τ" when the noise level is 0.01 dB / km is shown by the dashed line. The shaded area in Figure 13 indicates the usable conditions.

[0051] The solid line in Figure 13 indicates the cutoff wavelength "λ". c It can be seen that the "Δ2 / Δ1" related to "" does not show a significant dependence on "τ". On the other hand, the leakage loss "α c The "Δ2 / Δ1" related to "τ" decreases as "τ" decreases. And the designable region disappears in the range of "τ < 29.3 μm".

[0052] As "τ" increases, the designable region for "Δ2 / Δ1" expands. However, as "τ" increases, the distance between core regions 10 decreases, increasing inter-core crosstalk. Generally, it is desirable that inter-core crosstalk be suppressed to -17 dB or less in the receiver (see the following reference: PJ Winzer et al., “Penalties from In-Band Crosstalk for Advanced Optical Modulation Formants,” ECOC2011 Tu5B7 (2011)).

[0053] Therefore, it is desirable to select "τ" such that, in the region where "τ ≥ 29.3 μm", the inter-core crosstalk is below a threshold determined based on the required level of the optical transmission system.

[0054] The above-mentioned leakage loss "α c The relationship between "Δ2 / Δ1" and "τ" when "" is 0.01 dB / km is expressed by the following equation 16.

number

[0055] Here, the following equation 17 holds between "τ" and the sixth parameter "K6" that appears in equation 16. Figure 14 shows the characteristics of the sixth parameter that appear in the shape dependence of leakage loss.

number

[0056] Furthermore, the following equation 18 holds between "τ" and the seventh parameter "K7" that appears in equation 16. Figure 15 shows the characteristics of the seventh parameter that appear in the shape dependence of leakage loss.

number

[0057] Based on the above results, in the range of "2.5≦w / a≦3.5" and "τ≧29.3μm", "λ c " is 1530nm or less, and "α c The structural conditions for "Δ1", "Δ2 / Δ1", and "w / a" such that the noise level is 0.01 dB / km or less are expressed by the following equation 19.

number

[0058] [5. Supplement] Note that the difference in effective refractive index "Δn eff_cl Regarding this, we also investigated its dependence on the distance "τ" from the center of the core region 10 to the outer surface of the cladding region 30. Figure 16 shows the shape dependence of the effective refractive index difference.

[0059] Figure 16 shows the values ​​for "Δ1 = 0.78%" and "Δ2 / Δ1 = 0.29" for "Δn eff_cl This shows the relationship between "Δn" and "τ". At least in the range of "τ≧29.3μm", eff_cl The above-mentioned "Δn" does not show a significant dependence on "τ". eff_cl It can be seen that the relationships between "Δ1" and "Δ2 / Δ1" hold independently of "τ".

[0060] [6. Configuration of the Optical Transmission System] Figure 17 shows the configuration of an optical transmission system using a multi-mode multicore optical fiber according to an embodiment of the present disclosure. The optical transmission system 100 comprises a multi-mode multicore optical fiber FB according to an embodiment of the present disclosure, a transmitter ST, a mode multiplexer MT, an optical coupler FI, an optical extractor FO, a mode separator MR, and a receiver SR.

[0061] The transmitter ST generates a signal light. There may be multiple transmitters ST. In particular, there may be four or more transmitters ST.

[0062] A mode multiplexer MT converts signal light into optical wave modes. There may be multiple mode multiplexers MT. Each mode multiplexer MT may convert at least two or more signal lights into optical wave modes.

[0063] The optical coupling unit FI is located at one end of the multi-mode multicore optical fiber FB and inputs the input light, including the optical wave mode, into the core region 10. The optical coupling unit FI may also be connected to a mode multiplexer MT and introduce the output of each mode multiplexer MT into each core region 10 of the multi-mode multicore optical fiber FB. Alternatively, the optical coupling unit FI may be a MUX (Multiplexer). A MUX is a circuit that converts multiple input signals into a single output signal. The MUX selects only one of multiple input signals to be used as the output signal, and the signal to be selected is determined by a control signal called a selection signal.

[0064] The optical extraction unit FO is located at the other end of the multi-mode multicore optical fiber FB and extracts the output light from the core region 10. The optical extraction unit FO may be connected to the mode separator MR and introduce the output of each core region 10 of the multi-mode multicore optical fiber FB to each mode multiplexer MT. Alternatively, the optical extraction unit FO may be a DEMUX (Demultiplexer). A DEMUX has the opposite function of a MUX and is a circuit that distributes one input signal to multiple output signals. A DEMUX distributes the input signal to multiple outputs according to the selected signal.

[0065] A mode separator (MR) separates the light wave modes from the output light to extract the signal light. There may be multiple mode separators (MRs). Each mode separator (MR) may convert at least two or more light wave modes into signal light.

[0066] The receiver SR receives the signal light from the mode separator MR. There may be multiple receiver SRs. In particular, there may be four or more receiver SRs.

[0067] [Effects of the Embodiment] As described in detail above, the multi-mode multicore optical fiber according to this embodiment comprises at least two core regions through which light wave modes propagate, a depressed region having a refractive index less than that of the core regions and arranged to encompass all core regions, and a cladding region having a refractive index greater than or equal to that of the depressed regions and less than that of the core regions and arranged to surround the depressed regions. At a wavelength of 1625 nm, the difference between the effective refractive index of the highest-order light wave mode and the effective refractive index of the propagation mode in the cladding region is 0.0021 or greater.

[0068] This makes it possible to suppress microbend loss and provide a multi-mode multicore optical fiber that maintains low loss. In particular, even if microbends occur after a long period of time has passed since the multi-mode multicore optical fiber was wired, leakage of communication light from the microbends is suppressed. As a result, it is possible to provide a multi-mode multicore optical fiber that maintains low loss without significant microbend loss even after a long period of time has passed since wiring.

[0069] Furthermore, in the multi-mode multicore optical fiber according to this embodiment, the diameter of the cladding region is 126 μm or less, the relative refractive index difference of the core region with respect to the depressed region is Δ1, the relative refractive index difference of the cladding region with respect to the depressed region is Δ2, and the difference is Δn eff_cl as,

number

[0070] As a result, the difference between the effective refractive index of the highest-order optical wave mode and the effective refractive index of the propagation mode in the cladding region becomes 0.0021 or greater. Consequently, microbend loss in multi-mode multicore optical fibers can be suppressed, maintaining low loss performance. In particular, coupling between the optical wave modes propagating in the core region and the propagation modes in the cladding region is suppressed, preventing light rays propagating in the core region from leaking through the cladding region.

[0071] Furthermore, in the multi-mode multicore optical fiber according to this embodiment, where the radius of the core region is a (in μm), the distance from the center of the core region to the outer surface of the depressed region is w (in μm), and the distance from the center of the core region to the outer surface of the cladding region is τ (in μm), the following conditions apply: 2.5 ≤ w / a ≤ 3.5 and τ ≥ 29.3 μm.

number

[0072] This enables the "LP" in light wave mode. 21 The cutoff wavelength of "λ" c The wavelength is 1530nm or less, and the leakage loss at a wavelength of 1625nm is "α c The noise level becomes 0.01 dB / km or less. As a result, microbend loss in multi-mode multicore optical fibers can be suppressed, and low loss performance can be maintained. In particular, coupling between the light wave modes propagating in the core region and the propagating modes in the cladding region is suppressed, and leakage of light rays propagating in the core region through the cladding region is suppressed.

[0073] The optical transmission system according to this embodiment comprises: a multi-mode multicore optical fiber; a transmitter that generates signal light; a mode multiplexer that converts the signal light into optical wave modes; an optical coupling unit disposed at one end of the multi-mode multicore optical fiber and inputting input light including optical wave modes into the core region; an optical extraction unit disposed at the other end of the multi-mode multicore optical fiber and extracting output light from the core region; a mode separator that separates the optical wave modes from the output light and extracts signal light; and a receiver that receives signal light from the mode separator.

[0074] This allows for the suppression of microbend losses, enabling the use of multi-mode multicore optical fibers that maintain low loss and provide stable optical transmission services over long periods. Furthermore, it reduces the frequency of multi-mode multicore optical fiber replacements, thereby lowering the costs associated with the maintenance and upkeep of optical transmission systems.

[0075] Furthermore, the optical transmission system according to this embodiment may include at least four transmitters and at least two mode multiplexers, each mode multiplexer converting at least two or more signal lights into the respective optical wave modes.

[0076] This allows signals transmitted from multiple transmitters to be transmitted through multiple spatial channels (cores, modes) within a single optical fiber. As a result, the transmission capacity per optical fiber can be increased.

[0077] Furthermore, the optical transmission system according to this embodiment may include at least four receivers and at least two mode separators, each mode separator converting at least two or more optical wave modes into signal light.

[0078] This allows signals transmitted through multiple spatial channels (cores and modes) of a single optical fiber to be reliably received by multiple receivers. As a result, the transmission capacity per optical fiber can be increased.

[0079] While the contents of this disclosure have been described above in accordance with the embodiments, it will be obvious to those skilled in the art that this disclosure is not limited to these descriptions and that various modifications and improvements are possible. The discussions and drawings that constitute part of this disclosure should not be understood as limiting this disclosure. Various alternative embodiments, examples and operational techniques will become apparent to those skilled in the art from this disclosure.

[0080] This disclosure naturally includes various embodiments and other features not described herein. Therefore, the technical scope of this disclosure is determined solely by the inventive features relating to the claims that are reasonable given the above description. [Explanation of Symbols]

[0081] 10 Core Areas 20 Depressed Areas 30 Clad regions 100 Optical Transmission Systems FB multimode multicore 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 core regions through which the light wave mode propagates, A depressed region having a refractive index less than that of the core region and arranged to encompass all of the core region, A cladding region having a refractive index greater than or equal to that of the depressed region and less than that of the core region, and arranged to surround the depressed region, Equipped with, The diameter of the cladding region is 126 μm or less, Let Δ1 be the difference in relative refractive index between the core region and the depressed region. Let the difference in relative refractive index between the depressed region and the cladding region be Δ2. At a wavelength of 1625 nm, the difference between the effective refractive index in the highest-order mode among the light wave modes and the effective refractive index in the propagation mode of the cladding region is denoted as Δn eff_cl. Δn eff_cl ≥ 0.0021, [Math 1] A multi-mode multicore optical fiber that satisfies the requirements.

2. Let the radius of the core region be a (in μm), Let w (in μm) be the distance from the center of the core region to the outer surface of the depressed region. Let τ (in μm) be the distance from the center of the core region to the outer surface of the cladding region. 2.5 ≤ w / a ≤ 3.5 and τ ≥ 29.3 μm, [Math 2] A multi-mode multicore optical fiber according to claim 1, satisfying the requirements.

3. A multi-mode multicore optical fiber according to claim 1 or 2, A transmitter that generates signal light, A mode multiplexer that converts the signal light into the optical wave mode, An optical coupling unit is provided at one end of the multi-mode multicore optical fiber and inputs the input light, including the optical wave mode, into the core region. An optical extraction unit is positioned on the other end of the aforementioned multi-mode multicore optical fiber and extracts output light from the core region, A mode separator that separates the light wave modes from the output light and extracts the signal light, A receiver that receives the signal light from the mode separator, An optical transmission system equipped with [the following features].

4. At least four of the aforementioned transmitters, At least two of the aforementioned mode multiplexers, Equipped with, The optical transmission system according to claim 3, wherein each mode multiplexer converts at least two or more of the signal light into the respective optical wave modes.

5. At least four of the aforementioned receivers, At least two of the mode separators, Equipped with, The optical transmission system according to claim 3, wherein each mode separator converts at least two or more of the light wave modes into the signal light.

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