Optical Transmission System

Abstract

To provide a multi-core fiber which is of a simple structure and is easily manufacturable, and an optical transmission system that uses the same.SOLUTION: The multi-core fiber comprises a first core part, a first clad part, at least one second core part, and a second clad part. The refractive index of the first clad part is lower than the maximum refractive indices of the first and second core parts, and the shape of the first clad part in a cross section enclosing the outer circumference of the first core part and perpendicular to the longitudinal direction is ring-like. The shape of the second core part in a cross section enclosing the outer circumference of the first clad part and perpendicular to the longitudinal direction is ring-like. The refractive index of the second clad part is lower than the maximum refractive index of the second core part, and the shape of the second clad part in a cross section enclosing the outer circumference of the second core part and perpendicular to the longitudinal direction is ring-like.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a multicore fiber and an optical transmission system. [Background technology]

[0002] In the field of optical fibers, techniques for mode multiplexing transmission and techniques for multicore fibers having multiple cores are being actively studied. For example, Patent Document 1 discloses a technique for a multicore fiber in which each core has a ring-shaped several-mode structure capable of mode multiplexing transmission. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 130487 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in known multi-core fibers, the cores are arranged in a lattice pattern in a cross section perpendicular to the longitudinal direction, which makes it difficult to apply the Vapor-phase Axial Deposition (VAD) method or the Modified Chemical Vapor Deposition (MCVD) method to the manufacturing process, and necessitates a special method such as a drilling method or a stacking method.

[0005] The present invention has been made in view of the above, and an object of the present invention is to provide a multi-core fiber that has a simple structure and is easy to manufacture, and an optical transmission system using the same. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a multi-core fiber comprising a first core portion, a first clad portion, at least one second core portion, and a second clad portion, wherein the first clad portion has a refractive index lower than the maximum refractive index of the first core portion and the maximum refractive index of the second core portion, surrounds the outer periphery of the first core portion, and has a ring-shaped shape in a cross section perpendicular to the longitudinal direction, the second core portion surrounds the outer periphery of the first clad portion, and has a ring-shaped shape in a cross section perpendicular to the longitudinal direction, and the second clad portion has a refractive index lower than the maximum refractive index of the second core portion, surrounds the outer periphery of the second core portion, and has a ring-shaped shape in a cross section perpendicular to the longitudinal direction.

[0007] The first core portion may propagate light in a predetermined wavelength band in a single mode.

[0008] The optical fiber may include a plurality of the second core portions, and at least one intermediate cladding portion interposed between the plurality of second core portions.

[0009] The center core of the first core portion and the center core of the second core portion may be made of silica glass containing a dopant that increases the refractive index, and the first cladding portion and the second cladding portion may be made of silica glass that contains substantially no dopants other than chlorine.

[0010] The center core of the first core portion and the center core of the second core portion may be made of silica glass containing substantially no dopants other than chlorine, and the first cladding portion and the second cladding portion may be made of silica glass containing a dopant that lowers the refractive index.

[0011] One aspect of the present invention is an optical transmission system comprising the multicore fiber, a transmitting device that outputs a first light that is a first communication signal light and at least one second light, a first optical coupling device that couples the first light to the first core section and couples the second light to any one of the second core sections, a receiving device that receives the first light transmitted through the multicore fiber and the second light transmitted through the multicore fiber, and a second optical coupling device that couples the first light transmitted through the multicore fiber and the second light transmitted through the multicore fiber to the receiving device.

[0012] The second light may include a second communication signal light.

[0013] The second communication signal light may be a mode-multiplexed communication signal light.

[0014] The second light may include non-communication light.

[0015] The non-communication light may include at least one of light for transmitting optical power and test light for optical sensing.

[0016] The non-communication light may include light for optical power transmission and test light for optical sensing, and the second core portion to which the test light for optical sensing is coupled may be located more outer than the second core portion to which the light for optical power transmission is coupled.

[0017] the second light includes a second communication signal light, a light for optical power transmission, and a test light for optical sensing; The second core section to which the optical power transmission light is coupled may be located more inward than the second core section to which the second communication signal light is coupled, and the second core section to which the optical sensing test light is coupled may be located more outward than the second core section to which the second communication signal light is coupled. Effect of the Invention

[0018] The present invention has an effect of realizing a multi-core fiber that has a simple structure and is easy to manufacture, and an optical transmission system using the same. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a schematic cross-sectional view of a multi-core fiber according to a first embodiment, taken along a plane perpendicular to the longitudinal direction thereof. [Diagram 2] FIG. 2 is an explanatory diagram of a step-type refractive index profile. [Diagram 3] FIG. 3 is an explanatory diagram of a trench-type refractive index profile. [Figure 4] FIG. 4 is a diagram showing an example of a refractive index profile and a power intensity distribution of the second core portion. [Diagram 5] FIG. 5 is a schematic configuration diagram of an optical transmission system according to the second embodiment. [Figure 6] FIG. 6 is a schematic configuration diagram of an optical transmission system according to the third embodiment. [Figure 7] FIG. 7 is a schematic configuration diagram of an optical transmission system according to the fourth embodiment. [Figure 8] FIG. 8 is a schematic configuration diagram of an optical transmission system according to the fifth embodiment. [Figure 9] FIG. 9 is a schematic configuration diagram of an optical transmission system according to the sixth embodiment. [Figure 10] FIG. 10 is an explanatory diagram of an example of the first optical coupling device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiment described below. In addition, in each drawing, the same or corresponding components are appropriately given the same reference numerals, and duplicated explanations are appropriately omitted. In addition, in this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength (λcc) defined in ITU-T G.650.1 of the International Telecommunications Union (ITU). In addition, other terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0021] (Embodiment 1) Fig. 1 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of a multi-core fiber according to embodiment 1. The multi-core fiber 10 is made of silica-based glass, and includes a first core portion 11, a first cladding portion 12, a second core portion 13, and a second cladding portion 14. The second core portion 13 is an example of at least one second core portion.

[0022] The first core portion 11 is located approximately at the center of the multicore fiber 10, has a substantially circular shape in a cross section perpendicular to the longitudinal direction, and has an outer diameter of Dc1. The first cladding portion 12 has a refractive index lower than the maximum refractive index of the first core portion 11 and the maximum refractive index of the second core portion 13, and surrounds the outer periphery of the first core portion 11. The first cladding portion 12 has a ring-shaped shape in a cross section perpendicular to the longitudinal direction, and has an inner diameter of Dc1 and an outer diameter of Dc21.

[0023] The second core portion 13 surrounds the outer periphery of the first cladding portion 12, has a ring-shaped cross section perpendicular to the longitudinal direction, and has an inner diameter Dc21 and an outer diameter Dc22. The second cladding portion 14 has a refractive index lower than the maximum refractive index of the second core portion 13, and surrounds the outer periphery of the second core portion 13. The second cladding portion 14 has a ring-shaped cross section perpendicular to the longitudinal direction, and has an inner diameter Dc22 and an outer diameter Dcl.

[0024] The refractive index profile of the first core portion 11 is not particularly limited, but is, for example, any one of a step type, a staircase type, a W type, and a trench type.

[0025] Fig. 2 is an explanatory diagram of a step-type refractive index profile. In Fig. 2, profile P11 indicates the refractive index profile of the first core portion 11, and profile P12 indicates the refractive index profile of the first cladding portion 12. The step-type refractive index profile can be expressed by a core diameter 2a and a relative refractive index difference Δ1 of the maximum refractive index of the first core portion 11 with respect to the refractive index of the first cladding portion 12. In the step-type refractive index profile, the entire first core portion 11 can be defined as a center core.

[0026] 3 is an explanatory diagram of a trench-type refractive index profile. In FIG. 3, profile P31 shows the refractive index profile of the first core portion 11, and profile P32 shows the refractive index profile of the first cladding portion 12. The trench-type refractive index profile can be expressed by a core diameter 2a of the center core, a relative refractive index difference Δ1 of the maximum refractive index of the center core with respect to the refractive index of the first cladding portion 12, a relative refractive index difference Δ2 of the refractive index of the intermediate layer with respect to the refractive index of the first cladding portion 12, an inner diameter 2b of the trench layer, an outer diameter 2c of the trench layer, and a relative refractive index difference Δ3 of the refractive index of the trench layer with respect to the refractive index of the first cladding portion 12.

[0027] The refractive index profile of the second core portion 13 is not particularly limited either, and is, for example, any one of a step type, a staircase type, a W type, and a trench type.

[0028] For example, the center core of the first core portion 11 and the center core of the second core portion 13 are made of quartz glass containing a dopant that increases the refractive index. The dopant that increases the refractive index is, for example, germanium. The first cladding portion 12 and the second cladding portion 14 are made of quartz glass that does not substantially contain any dopant other than chlorine. Here, chlorine is a dopant that is included in the manufacturing process of the multi-core fiber 10 and is not intentionally added. With this configuration, a dopant that lowers the refractive index but is relatively expensive, such as fluorine, is not used or is used in a relatively small amount, so that cost reduction can be expected.

[0029] The center core of the first core region 11 and the center core of the second core region 13 may be made of quartz glass that does not substantially contain dopants other than chlorine. The first cladding region 12 and the second cladding region 14 may be made of quartz glass that contains a dopant that lowers the refractive index. The dopant that lowers the refractive index is, for example, fluorine. With this configuration, a dopant that increases the refractive index but increases the transmission loss, such as germanium, is not used or is used in a relatively small amount, so that low transmission loss can be expected.

[0030] The multi-core fiber 10 configured in this manner has a first core portion 11, a first cladding portion 12, a second core portion 13, and a second cladding portion 14 that are concentrically configured. As a result, the multi-core fiber 10 is a multi-core fiber that is easy to manufacture with a simple structure because the VAD method or MCVD method can be easily applied in the manufacturing process.

[0031] For example, in a known multi-core fiber in which the core parts are arranged in a lattice pattern, the manufacturing process requires complex steps such as drilling holes in a glass rod in a drilling method and stacking glass rods in a stacking method as main steps other than a preform synthesis step by a VAD method, an MCVD method, or the like and an optical fiber drawing step. In contrast, the manufacturing process of the multi-core fiber 10 may be a manufacturing process in which the main steps are a preform synthesis step and an optical fiber drawing step.

[0032] In the multi-core fiber 10, the first core portion 11 and the second core portion 13 can be used to transmit communication signal light and non-communication light. The non-communication light is light other than communication signal light, such as light for transmitting optical power and test light for optical sensing. The light for transmitting optical power is also called optical power supply light.

[0033] For example, the first core portion 11 and the second core portion 13 are heterogeneous cores having different propagation refractive indices (also called effective refractive indices). As a result, when used for transmitting communication signal light, inter-core crosstalk is effectively suppressed between the first core portion 11 and the second core portion 13. Furthermore, in the second core portion 13, the light field is distributed over a wide range, making it possible to reduce nonlinearity.

[0034] Furthermore, for example, when the first core portion 11 is used for transmitting communication signal light, it is preferable that the first core portion 11 has SM transmission characteristics that propagate light in a predetermined wavelength band (transmission band) in a single mode (SM). The predetermined wavelength band is a wavelength band used for optical communication, such as O-band (1260 nm to 1360 nm), S-band (1460 nm to 1530 nm), C-band (1530 nm to 1565 nm), L-band (1565 nm to 1625 nm), etc. If the first core portion 11 has SM transmission characteristics, the influence of inter-core mode interference with the second core portion 13 can be suppressed, so that a complicated system configuration such as MIMO (Multi Input Multi Output) processing is not required, or the load of the MIMO processing is reduced.

[0035] Also, for example, first core unit 11 may satisfy the ITU-T G.652A standard, G.654 standard, or G.657 standard, which are examples of SM transmission characteristics.

[0036] Furthermore, for example, the second core portion 13 may have a characteristic capable of mode multiplexing transmission in the above-mentioned predetermined wavelength band when used for transmitting communication signal light. In mode multiplexing transmission, among the propagation modes of the second core portion 13, a fundamental mode and a higher mode, such as an LP01 mode, an LP11 mode, or an LP21 mode, are used for transmission. Furthermore, when used for transmitting communication signal light, the second core portion 13 preferably has low bending loss.

[0037] Fig. 4 is a diagram showing an example of the refractive index profile and power intensity distribution of the second core portion. In Fig. 4, the refractive index profile is step type, and Δ1 is 0.4%. The inner diameter Dc21 of the second core portion 13 is 70 μm, and the outer diameter Dc22 is 77 μm. As for the power intensity distribution, the power intensity distributions of the LP01 mode, the LP11 mode, and the LP21 mode are shown, which are substantially overlapped.

[0038] Furthermore, for example, when the second core portion 13 is used for transmitting light for optical power transmission, it is preferable that the second core portion 13 has both low nonlinearity and low bending loss. If both low nonlinearity and low bending loss are achieved, optical power can be transmitted efficiently. The wavelength band of the light for optical power transmission is, for example, the 900 nm band, the 1000 nm band, the S band, the C band, the L band, etc.

[0039] Furthermore, for example, when the second core portion 13 is used for transmitting test light for optical sensing, it is preferable that the bending characteristic is weak from the viewpoint of realizing sensitivity to the environment for sensing disturbances. The wavelength band of the test light for optical sensing is, for example, U band (1625 nm to 1675 nm).

[0040] Table 1 shows an example of the usage of each core part and an example of the properties required for that usage. Note that the "center core part" means the first core part, and the "ring core part" means the second core part.

[0041] [Table 1]

[0042] (Embodiment 2) Next, a description will be given of embodiment 2. Fig. 5 is a schematic configuration diagram of an optical transmission system according to embodiment 2. The optical transmission system 100 includes the multi-core fiber 10 of embodiment 1, a transmitting device 20, a first optical coupling device 30, a second optical coupling device 40, and a receiving device 50.

[0043] The transmitting device 20 outputs a first light S1 which is a first communication signal light and a second light S2 which is a second communication signal light and is also a mode multiplexed communication signal light. The first light S1 and the second light S2 are light in a wavelength band used for optical communications. The mode multiplexed communication signal light includes, for example, a communication signal light for LP01 mode transmission, a communication signal light for LP11 mode transmission, and a communication signal light for LP21 mode transmission.

[0044] The first optical coupling device 30 couples the first light S1 to the first core portion 11 of the multi-core fiber 10, and couples the second light S2 to the second core portion 13 of the multi-core fiber 10. The first optical coupling device can be configured by appropriately combining, for example, a spatial optical system, a spatial light modulator, and an optical fiber optical system.

[0045] The multi-core fiber 10 transmits the first light S1 in a single mode through the first core portion 11, and transmits the second light S2 in a mode multiplexing manner through the second core portion 13.

[0046] The second optical coupling device 40 couples the first light S1 and the second light S2 transmitted through the multi-core fiber 10 to the receiving device 50. The second optical coupling device can be configured by appropriately combining, for example, a spatial optical system, a spatial light modulator, and an optical fiber optical system.

[0047] The receiving device 50 receives the first light S1 and the second light S2 transmitted through the multi-core fiber 10 and coupled by the second optical coupling device 40.

[0048] The optical transmission system 100 configured as above is a suitable optical transmission system using the multi-core fiber 10 which has a simple structure and is easy to manufacture. In particular, in the optical transmission system 100, the influence of inter-core mode interference between the first core portion 11 and the second core portion 13 is suppressed, so that a complicated system configuration such as MIMO processing is not required or the load of the MIMO processing is reduced.

[0049] Next, a first example of a multi-core fiber suitable as the multi-core fiber 10 of the optical transmission system 100 will be described. In the first example of the multi-core fiber, the refractive index profile of the first core portion is set to a step type as shown in FIG. 2, the core diameter 2a is set to 8.7 μm, and Δ1 is set to 0.37%. Table 2 shows the simulation calculation results of the optical characteristics of the first core portion in this case. Note that "MFD" is the mode field diameter, and "neff" is the effective refractive index. Note that Table 1 also shows the G.652A standard. As shown in Table 2, the first core portion has a cutoff wavelength of 1209 nm and has SM transmission characteristics for the first light having a wavelength longer than at least the O band. In addition, the other listed characteristics also satisfy the G.652A standard.

[0050] [Table 2]

[0051] In the multicore fiber of the first example, the refractive index profile of the second core portion was set to a step type, and Δ1, the inner diameter Dc21, and the outer diameter Dc22 were set as shown in No. 1 to No. 12 in Table 3. Dcl was set to 125 μm. Simulation calculation results of the optical characteristics of the second core portion in this case are shown in Table 3. Note that "Aeff" is the effective core area, and Aeff(LP01 / LP11 / LP21) indicates the Aeff of the LP01 mode, the Aeff of the LP11 mode, and the Aeff of the LP21 mode. The same is true for neff(LP01 / LP11 / LP21). "XT of LP01 mode" is the inter-core crosstalk between the LP01 mode of the first core portion and the LP01 mode of the second core portion. As shown in Table 3, when performing mode multiplexing transmission, it is preferable to use the LP01 mode, LP11 mode, and LP21 mode, which have different neffs, in order to avoid system complexity. In addition, when using two modes, it is preferable to use the LP01 mode and the LP11 mode. However, other propagation modes such as the LP02 mode may be used in mode multiplexing transmission. Regarding the inter-core crosstalk, it was confirmed that the inter-core crosstalk between the LP01 mode of the first core part and other propagation modes of the second core part is smaller than the "XT of the LP01 mode." In addition, the "XT of the LP01 mode" increases as the inner diameter Dc21 decreases, but since the first core part and the second core part are heterogeneous cores, the value is sufficiently small even though the inter-core distance is relatively close.

[0052] [Table 3]

[0053] Here, as Examples 1-1 and 1-2, multi-core fibers were fabricated that had a first core portion having a core diameter 2a and Δ1 shown in Table 2, and a second core portion having Δ1, an inner diameter Dc21, and an outer diameter Dc22 shown in No. 11 of Table 3. In Example 1-1, Δ1 was realized by doping the first core portion and the second core portion with germanium. In Example 1-2, Δ1 was realized by doping the first cladding portion and the second cladding portion with substantially no dopant other than chlorine.

[0054] The transmission losses of the first core parts of the multicore fibers of Examples 1-1 and 1-2 were measured and were 0.184 dB / km and 0.161 dB / km, respectively. The characteristics of the second core parts of the multicore fibers of Examples 1-1 and 1-2 are shown in Table 4. The transmission losses are those when LP01 mode light, LP11 mode light, and LP21 mode light are simultaneously input, and the "Ge core system" shows the transmission loss of Example 1-1, and the "silica core system" shows the transmission loss of Example 1-2. The results shown in Table 4 are highly consistent with the simulation calculation results shown in Table 3, and the transmission losses were also low.

[0055] [Table 4]

[0056] (Embodiment 3) 6 is a schematic configuration diagram of an optical transmission system according to embodiment 3. The optical transmission system 100A includes a multi-core fiber 10A which is a modified example of embodiment 1, a transmitting device 20A, a first optical coupling device 30A, a second optical coupling device 40A, and a receiving device 50A.

[0057] The multi-core fiber 10A is made of silica-based glass, and includes a first core portion 11A, a first cladding portion 12A, a second core portion 13A, and a second cladding portion 14A.

[0058] The first core portion 11A is located approximately at the center of the multi-core fiber 10A, has a substantially circular shape in a cross section perpendicular to the longitudinal direction, and has an outer diameter of Dc1A. The first cladding portion 12A has a refractive index lower than the maximum refractive index of the first core portion 11A and the maximum refractive index of the second core portion 13A, and surrounds the outer periphery of the first core portion 11A. The first cladding portion 12A has a ring-shaped shape in a cross section perpendicular to the longitudinal direction, and has an inner diameter of Dc1A and an outer diameter of Dc21A.

[0059] The second core portion 13A surrounds the outer periphery of the first cladding portion 12A, has a ring-shaped cross section perpendicular to the longitudinal direction, has an inner diameter Dc21A, and has an outer diameter Dc22A. The second cladding portion 14A has a refractive index lower than the maximum refractive index of the second core portion 13A, and surrounds the outer periphery of the second core portion 13A. The second cladding portion 14A has a ring-shaped cross section perpendicular to the longitudinal direction, has an inner diameter Dc22A, and has an outer diameter Dc1A.

[0060] The refractive index profile of the first core portion 11A and the second core portion 13A is not particularly limited, but is, for example, any one of a step type, a staircase type, a W type, and a trench type.

[0061] For example, the center core of the first core portion 11A and the center core of the second core portion 13A are made of silica glass containing a dopant that increases the refractive index, and the first cladding portion 12A and the second cladding portion 14A are made of silica glass that does not substantially contain any dopant other than chlorine.

[0062] The center core of the first core portion 11A and the center core of the second core portion 13A may be made of silica glass that does not substantially contain dopants other than chlorine, and the first cladding portion 12A and the second cladding portion 14A may be made of silica glass that contains a dopant that lowers the refractive index.

[0063] The transmitting device 20A outputs a first light S1A which is a first communication signal light, and a second light S2A which is a non-communication light and light for optical power transmission. The first light S1A is light in a wavelength band used for optical communication. The second light S2A is light in a wavelength band used for optical power transmission. From the viewpoint of crosstalk suppression, it is preferable that the wavelength band to which the first light S1A belongs and the wavelength band to which the second light S2A belongs are different.

[0064] The first optical coupling device 30A couples the first light S1A to the first core portion 11A of the multi-core fiber 10A, and couples the second light S2A to the second core portion 13A of the multi-core fiber 10A.

[0065] The multi-core fiber 10A transmits the first light S1A in a single mode through the first core portion 11A, and transmits the second light S2A in a multimode through the second core portion 13A.

[0066] The second optical coupling device 40A couples the first light S1A and the second light S2A transmitted through the multi-core fiber 10A to the receiving device 50A.

[0067] The receiving device 50A receives the first light S1A and the second light S2A transmitted through the multi-core fiber 10A and coupled by the second optical coupling device 40A.

[0068] The optical transmission system 100A configured as above is a suitable optical transmission system that uses the multi-core fiber 10A, which has a simple structure and is easy to manufacture.

[0069] Next, a multi-core fiber of a second example suitable as the multi-core fiber 10A of the optical transmission system 100A will be described. In the multi-core fiber of the second example, the refractive index profile of the first core section is a step type as shown in FIG. 2, and the core diameter 2a is set to 8.7 μm and Δ1 is set to 0.37%, that is, each is set to the same as in the first example. Therefore, as shown in Table 2, the simulation calculation result of the optical characteristics of the first core section in this case shows that the cutoff wavelength is 1209 nm, and the fiber has SM transmission characteristics for the first light having a wavelength longer than at least the O band. In addition, the other characteristics described above also satisfy the G.652A standard.

[0070] In the multicore fiber of the second example, the refractive index profile of the second core portion was step type, the inner diameter Dc21A was set to 25 μm, the outer diameter Dc22A was set to 40 μm, and Δ1 was set to 0.5% or 1.0% as shown in No. 21 and No. 22 of Table 5. Also, Dcl was set to 125 μm. Then, as Examples 2-1 and 2-2, multicore fibers including a first core portion having a core diameter 2a and Δ1 shown in Table 2 and a second core portion having Δ1, an inner diameter Dc21A, and an outer diameter Dc22A shown in No. 21 of Table 5 were fabricated. Also, as Examples 2-3 and 2-4, multicore fibers including a first core portion having a core diameter 2a and Δ1 shown in Table 2 and a second core portion having Δ1, an inner diameter Dc21A, and an outer diameter Dc22A shown in No. 22 of Table 5 were fabricated. In Examples 2-1 and 2-3, Δ1 was achieved by adding germanium to the first and second core parts. In Examples 2-2 and 2-4, Δ1 was achieved by adding fluorine to the first and second clad parts while the first and second core parts did not substantially contain any dopants other than chlorine.

[0071] The results of simulation calculations or measurements of the optical properties of the second core section in this case are shown in Table 5. Note that "neff" is the calculation result of the effective refractive index of the LP01 mode, and Aeff and transmission loss are average measured values ​​for all modes. As shown in Table 5, the properties at a wavelength of 1000 nm are as follows: neff is high and optical confinement is strong, but 2Since a large effective core area as described above is obtained, low nonlinearity is realized. Furthermore, when light with a wavelength of 1550 nm was input into the first core section and light with a wavelength of 980 nm was input into the second core section, the inter-core crosstalk between the first and second core sections was measured, and it was confirmed that a reasonably low crosstalk value of -40 dB or less per 100 km could be obtained.

[0072] [Table 5]

[0073] (Embodiment 4) 7 is a schematic configuration diagram of an optical transmission system according to embodiment 4. The optical transmission system 100B includes a multi-core fiber 10B which is a modified example of embodiment 1, a transmitting device 20B, a first optical coupling device 30B, a second optical coupling device 40B, and a receiving device 50B.

[0074] The multi-core fiber 10B is made of silica-based glass, and includes a first core portion 11B, a first cladding portion 12B, a second core portion 13B, and a second cladding portion 14B.

[0075] The first core portion 11B is located approximately at the center of the multi-core fiber 10B, has a substantially circular shape in a cross section perpendicular to the longitudinal direction, and has an outer diameter of Dc1B. The first cladding portion 12B has a refractive index lower than the maximum refractive index of the first core portion 11B and the maximum refractive index of the second core portion 13B, and surrounds the outer periphery of the first core portion 11B. The first cladding portion 12B has a ring-shaped shape in a cross section perpendicular to the longitudinal direction, and has an inner diameter of Dc1B and an outer diameter of Dc21B.

[0076] The second core portion 13B surrounds the outer periphery of the first cladding portion 12B and has a ring-shaped cross section perpendicular to the longitudinal direction, with an inner diameter of Dc21B and an outer diameter of Dc22B. The second cladding portion 14B has a refractive index lower than the maximum refractive index of the second core portion 13B and surrounds the outer periphery of the second core portion 13B. The second cladding portion 14B has a ring-shaped cross section perpendicular to the longitudinal direction, with an inner diameter of Dc22B and an outer diameter of Dc1B.

[0077] The refractive index profile of the first core portion 11B and the second core portion 13B is not particularly limited, but is, for example, any one of a step type, a staircase type, a W type, and a trench type.

[0078] For example, the center core of the first core portion 11B and the center core of the second core portion 13B are made of silica glass containing a dopant that increases the refractive index, and the first cladding portion 12B and the second cladding portion 14B are made of silica glass that does not substantially contain a dopant other than chlorine.

[0079] The center core of the first core portion 11B and the center core of the second core portion 13B may be made of silica glass that does not substantially contain dopants other than chlorine. The first cladding portion 12B and the second cladding portion 14B may be made of silica glass that contains a dopant that lowers the refractive index.

[0080] The transmitting device 20B outputs a first light S1B which is a first communication signal light, and a second light S2B which is a non-communication light and a test light for optical sensing. The first light S1B is light in a wavelength band used for optical communication. The second light S2B is light in a wavelength band used for optical sensing. From the viewpoint of crosstalk suppression, it is preferable that the wavelength band to which the first light S1B belongs is different from the wavelength band to which the second light S2B belongs.

[0081] The first optical coupling device 30B couples the first light S1B to the first core portion 11B of the multi-core fiber 10B, and couples the second light S2B to the second core portion 13B of the multi-core fiber 10B.

[0082] The multi-core fiber 10B transmits the first light S1B in a single mode through the first core portion 11B, and transmits the second light S2B in a single mode through the second core portion 13B.

[0083] The second optical coupling device 40B couples the first light S1B and the second light S2B transmitted through the multi-core fiber 10B to the receiving device 50B.

[0084] The receiving device 50B receives the first light S1B and the second light S2B transmitted through the multi-core fiber 10B and coupled by the second optical coupling device 40B.

[0085] The optical transmission system 100B configured as above is a suitable optical transmission system that uses a multi-core fiber 10B that has a simple structure and is easy to manufacture.

[0086] Next, a third example of a multi-core fiber suitable as the multi-core fiber 10B of the optical transmission system 100B will be described. In the third example of the multi-core fiber, the refractive index profile of the first core portion is a trench type as shown in FIG. 3, and the core diameter 2a is set to 8 μm, Δ1 to 0.3%, Δ2 to 0%, Δ3 to -0.6%, b / a to 3.0, and c / a to 4.0. As a result, the simulation calculation result of the optical characteristics of the first core portion is, as shown in Table 6, a cutoff wavelength of 1241 nm, and at least SM transmission characteristics are provided for the first light having a wavelength longer than the O band. In addition, the other characteristics described above also satisfy the G.657A2 standard, which is a standard that has higher bending resistance than the G.652 standard.

[0087] [Table 6]

[0088] In the multi-core fiber of the third example, the refractive index profile of the second core portion was step-type, the inner diameter Dc21B was set to 90 μm, the outer diameter Dc22B was set to 99 μm, and Δ1 was set to 0.4% or 0.6% as shown in No. 31 and No. 32 of Table 7. DclB was set to 125 μm. As Examples 3-1 and 3-2, multi-core fibers including a first core portion shown in Table 6 and a second core portion having Δ1 shown in No. 31 of Table 7 were fabricated. As Examples 3-3 and 3-4, multi-core fibers including a first core portion shown in Table 6 and a second core portion having Δ1 shown in No. 32 of Table 7 were fabricated. In Examples 3-1 and 3-3, Δ1 was realized by adding germanium to the first core portion and the second core portion. In addition, in Examples 3-2 and 3-4, the first and second core regions did not substantially contain any dopant other than chlorine, and Δ1 was realized by adding fluorine to the first and second cladding regions.

[0089] The results of simulation calculations or measurements of the optical properties of the second core section in this case are shown in Table 7. Note that "neff" is the calculation result of the effective refractive index of the LP01 mode, and Aeff and transmission loss are average measured values ​​of all modes. As shown in Table 7, neff is 1.446 or more when Δ1 is 0.4%, and 1.448 or more when Δ1 is 0.6%, and light can propagate without any problems. However, since Aeff is large and the position of the second core section is close to the outer periphery of the multicore fiber, it is extremely sensitive to disturbances, and the transmission loss increases significantly, for example, in a bent portion. Therefore, the second core section can be used for optical sensing of disturbances and bending, and optical sensing of shape. Note that the sensitivity to disturbances can be increased by lowering Δ1 or by moving the position of the second core section closer to the outer periphery of the multicore fiber. In addition, when light with a wavelength of 1550 nm was input into the first core section and light with a wavelength of 1650 nm was input into the second core section, the inter-core crosstalk between the first and second core sections was measured. It was confirmed that a reasonably low crosstalk value of -30 dB or less per 100 km could be obtained.

[0090] [Table 7]

[0091] (Embodiment 5) 8 is a schematic configuration diagram of an optical transmission system according to embodiment 5. The optical transmission system 100C includes a multi-core fiber 10C which is a modified example of embodiment 1, a transmitting device 20C, a first optical coupling device 30C, a second optical coupling device 40C, and a receiving device 50C.

[0092] The multi-core fiber 10C is made of silica-based glass, and includes a first core portion 11C, a first cladding portion 12C, two second core portions 13C and 15C, an intermediate cladding portion 14C, and a second cladding portion 16C.

[0093] The first core portion 11C is located approximately at the center of the multi-core fiber 10C, has a substantially circular shape in a cross section perpendicular to the longitudinal direction, and has an outer diameter of Dc1C. The first cladding portion 12C has a refractive index lower than the maximum refractive index of the first core portion 11C and the maximum refractive indexes of the second core portions 13C and 15C, and surrounds the outer periphery of the first core portion 11C. The first cladding portion 12C has a ring-shaped shape in a cross section perpendicular to the longitudinal direction, and has an inner diameter of Dc1C and an outer diameter of Dc21C.

[0094] The two second core portions 13C, 15C surround the outer periphery of the first cladding portion 12C, and have a ring-shaped cross section perpendicular to the longitudinal direction. The two second core portions 13C, 15C are an example of a plurality of second core portions. The intermediate cladding portion 14C is interposed between the two second core portions 13C, 15C, and has a refractive index lower than the maximum refractive index of the second core portions 13C, 15C. The intermediate cladding portion 14C has a ring-shaped cross section perpendicular to the longitudinal direction. The intermediate cladding portion 14C is an example of at least one intermediate cladding portion interposed between a plurality of second core portions.

[0095] The second core portion 13C has an inner diameter Dc21C and an outer diameter Dc22C. The intermediate cladding portion 14C has an inner diameter Dc22C and an outer diameter Dc31C. The second core portion 15C has an inner diameter Dc31C and an outer diameter Dc32C. The second core portion 15C is located on the outer periphery side of the second core portion 13C.

[0096] The second cladding portion 16C has a refractive index lower than the maximum refractive index of the second core portions 13C and 15C, and surrounds the outer periphery of the second core portions 13C and 15C. The second cladding portion 16C has a ring-shaped cross section perpendicular to the longitudinal direction, an inner diameter of Dc32C, and an outer diameter of Dc1C.

[0097] The refractive index profile of the first core portion 11C, the second core portion 13C, and the second core portion 15C is not particularly limited, and is, for example, any one of a step type, a staircase type, a W type, and a trench type.

[0098] For example, the center core of the first core portion 11C and the center cores of the second core portions 13C and 15C are made of silica glass containing a dopant that increases the refractive index, and the first cladding portion 12C, the intermediate cladding portion 14C, and the second cladding portion 16C are made of silica glass that does not substantially contain a dopant other than chlorine.

[0099] The center core of the first core portion 11C and the center cores of the second core portions 13C and 15C may be made of silica glass that does not substantially contain dopants other than chlorine. The first cladding portion 12C, the intermediate cladding portion 14C, and the second cladding portion 16C may be made of silica glass that contains a dopant that lowers the refractive index.

[0100] The transmitting device 20C outputs a first light S1C which is a first communication signal light, a second light S2C which is an optical power transmission light, and a second light S3C which is an optical sensing test light. From the viewpoint of crosstalk suppression, it is preferable that the wavelength band to which the first light S1C belongs, the wavelength band to which the second light S2C belongs, and the wavelength band to which the second light S3C belongs are different.

[0101] The first optical coupling device 30C couples the first light S1C to the first core portion 11B of the multi-core fiber 10C, couples the second light S2C to the second core portion 13C of the multi-core fiber 10C, and couples the second light S3C to the second core portion 15C of the multi-core fiber 10C. That is, the second core portion 15C to which the second light S3C, which is the test light for optical sensing, is coupled is located on the outer circumferential side of the second core portion 13C to which the second light S2C, which is the light for optical power transmission, is coupled.

[0102] The multi-core fiber 10C transmits the first light S1C in a single mode through the first core portion 11C, transmits the second light S2C in a multimode through the second core portion 13C, and transmits the second light S3C in a single mode through the second core portion 15C.

[0103] The second optical coupling device 40C couples the first light S1C and the second lights S2C and S3C transmitted through the multi-core fiber 10C to the receiving device 50C.

[0104] The receiving device 50C receives the first light S1C and the second lights S2C and S3C that are transmitted through the multi-core fiber 10C and coupled by the second optical coupling device 40C.

[0105] The optical transmission system 100C configured as above is a suitable optical transmission system using the multi-core fiber 10C that has a simple structure and is easy to manufacture. In particular, the first core portion 11C located at the center transmits the first light S1C that is a communication signal light, the second core portion 15C located at the outermost periphery that is susceptible to disturbances transmits the second light S3C that is a test light for optical sensing, and the second core portion 13C located in the middle transmits the second light S2C that is a light for optical power transmission, so that optimal transmission for each light can be realized.

[0106] Next, a fourth example of a multi-core fiber suitable as the multi-core fiber 10C of the optical transmission system 100C will be described. In the fourth example of the multi-core fiber, the refractive index profile of the first core portion is set to a step type as shown in FIG. 2, the core diameter 2a is set to 10.0 μm, and Δ1 is set to 0.40%. As a result, as shown in Table 8, the simulation calculation result of the optical characteristics of the first core portion has a cutoff wavelength of 1436 nm, and has SM transmission characteristics for the first light having a wavelength longer than at least the S band. In addition, the other characteristics described above also satisfy the G.654A2 standard, which is a standard that has higher nonlinear resistance than the G.652 standard.

[0107] [Table 8]

[0108] In the multi-core fiber of the third example, the refractive index profile of the second core portion was set to a step type, the inner diameter Dc21C was set to 30 μm, the outer diameter Dc22C was set to 45 μm, Dc31C was set to 100 μm, and Dc32C was set to 105 μm, and Δ1 was set to 0.5% for the second core portion on the inner circumference side and 0.65% for the second core portion on the outermost circumference as shown in No. 41 and No. 42 in Table 9. Also, DclC was set to 125 μm. Then, as Examples 4-1 and 4-2, multi-core fibers were fabricated that include a first core portion having a core diameter 2a and Δ1 shown in Table 8, and a second core portion having Δ1, an inner diameter (Dc21C or Dc31C), and an outer diameter (Dc22C or Dc32C) shown in Table 9. In Example 4-1, germanium was added to the first core portion and the second core portion to realize Δ1. In Example 4-2, the first and second core regions did not substantially contain any dopants other than chlorine, and fluorine was added to the first cladding region, intermediate cladding region, and second cladding region to achieve Δ1.

[0109] Table 9 shows the results of simulation calculations or measurements of the optical properties of the second core section in this case. Note that "neff" is the calculation result of the effective refractive index of the LP01 mode, and Aeff and transmission loss are average measured values ​​for all modes. As shown in Table 9, neff is 1.449 or more in the second core section used for power transmission on the inner circumference side, realizing strong optical confinement. On the other hand, in the second core section used for sensing on the outermost circumference, neff is 1.446, which is a sufficient value for optical propagation, and Aeff is 2000 μm 2 Since it is so large and located at the outermost periphery, it is extremely sensitive to disturbances and can be used for optical sensing of disturbances and bending, as well as optical sensing of shape. Furthermore, when light with a wavelength of 1000 nm was input into the second core section on the inner periphery and light with a wavelength of 1650 nm was input into the second core section on the outermost periphery, the inter-core crosstalk between the second core sections was measured, and it was confirmed that a reasonably low crosstalk value of -30 dB or less per 100 km could be obtained.

[0110] [Table 9]

[0111] As a modified example of the optical transmission system 100C, the optical transmission system may be configured by selecting any two of communication signal light and non-communication light such as test light for optical sensing or light for optical power transmission as the second light transmitted by the second core portions 13C, 15C of the multi-core fiber 10C.

[0112] For example, in the optical transmission system 100C, the second core portion 15C of the multicore fiber 10C may be configured to transmit the second light, which is a mode multiplexed communication signal light, instead of the second light S3C, which is a test light for optical sensing. In this case, the first core portion 11C transmitting the first light S1, which is the first communication signal light, and the second core portion 15C are separated via the second core portion 13C, so that even if the first light and the second light are lights in the same wavelength band for communication, inter-core interference is suppressed, which is preferable.

[0113] (Embodiment 6) 9 is a schematic configuration diagram of an optical transmission system according to embodiment 6. The optical transmission system 100D includes a multi-core fiber 10D which is a modified example of embodiment 1, a transmitting device 20D, a first optical coupling device 30D, a second optical coupling device 40D, and a receiving device 50D.

[0114] The multi-core fiber 10D is made of silica-based glass, and includes a first core portion 11D, a first cladding portion 12D, three second core portions 13D, 15D, and 17D, intermediate cladding portions 14D and 16D, and a second cladding portion 18D.

[0115] The first core portion 11D is located approximately at the center of the multi-core fiber 10D, has a substantially circular shape in a cross section perpendicular to the longitudinal direction, and has an outer diameter of Dc1D. The first cladding portion 12D has a refractive index lower than the maximum refractive index of the first core portion 11D and the maximum refractive index of the second core portions 13D, 15D, and 17D, and surrounds the outer periphery of the first core portion 11D. The first cladding portion 12D has a ring-shaped shape in a cross section perpendicular to the longitudinal direction, and has an inner diameter of Dc1D and an outer diameter of Dc21D.

[0116] The three second core portions 13D, 15D, and 17D surround the outer periphery of the first cladding portion 12D, and have a ring-shaped cross section perpendicular to the longitudinal direction. The three second core portions 13D, 15D, and 17D are an example of a plurality of second core portions. The intermediate cladding portions 14D and 16D are interposed between the three second core portions 13D and 15D, or between the second core portions 15D and 17D, respectively, and have a refractive index lower than the maximum refractive index of the second core portions 13D, 15D, and 17D. The intermediate cladding portions 14D and 16D have a ring-shaped cross section perpendicular to the longitudinal direction. The intermediate cladding portions 14D and 16D are an example of at least one intermediate cladding portion interposed between a plurality of second core portions.

[0117] The second core portion 13D has an inner diameter Dc21D and an outer diameter Dc22D. The intermediate cladding portion 14C has an inner diameter Dc22D and an outer diameter Dc31D. The second core portion 15D has an inner diameter Dc31D and an outer diameter Dc32D. The intermediate cladding portion 16D has an inner diameter Dc32D and an outer diameter Dc41D. The second core portion 17D has an inner diameter Dc41D and an outer diameter Dc42D. The second core portion 15D is located on the outer periphery side of the second core portion 13D. The second core portion 17D is located on the outer periphery side of the second core portion 15D.

[0118] The second cladding portion 18D has a refractive index lower than the maximum refractive index of the second core portions 13D, 15D, and 17D, and surrounds the outer periphery of the second core portions 13D, 15D, and 17D. The second cladding portion 18D has a ring-shaped cross section perpendicular to the longitudinal direction, and has an inner diameter of Dc42D and an outer diameter of Dc1D.

[0119] The refractive index profile of the first core portion 11D, the second core portions 13D, 15D, and 17D is not particularly limited, and is, for example, any one of a step type, a staircase type, a W type, and a trench type.

[0120] For example, the center core of the first core portion 11D and the center cores of the second core portions 13D, 15D, and 17D are made of silica glass containing a dopant that increases the refractive index, and the first cladding portion 12D, the intermediate cladding portions 14D and 16D, and the second cladding portion 18D are made of silica glass that does not substantially contain dopants other than chlorine.

[0121] The center core of the first core portion 11D and the center cores of the second core portions 13D, 15D, and 17D may be made of silica glass that does not substantially contain dopants other than chlorine. The first cladding portion 12D, the intermediate cladding portions 14D and 16D, and the second cladding portion 18D may be made of silica glass that contains a dopant that lowers the refractive index.

[0122] The transmitting device 20D outputs a first light S1D which is a first communication signal light, a second light S2D which is optical power transmission light, a second light S3D which is a mode multiplexed communication signal light, and a second light S4D which is test light for optical sensing.

[0123] The first optical coupling device 30D couples the first light S1D to the first core portion 11D of the multi-core fiber 10C, couples the second light S2D to the second core portion 13D of the multi-core fiber 10D, couples the second light S3D to the second core portion 15D of the multi-core fiber 10D, and couples the second light S4D to the second core portion 17D of the multi-core fiber 10D. That is, the second core portion 13D to which the optical power transmission light is coupled is located on the inner circumferential side of the second core portion 15D to which the mode multiplexed communication signal light is coupled, and the second core portion 17D to which the optical sensing test light is coupled is located on the outer circumferential side of the second core portion 15D to which the mode multiplexed communication signal light is coupled.

[0124] The multi-core fiber 10C transmits the first light S1D in single mode through the first core portion 11D, transmits the second light S2D in multimode through the second core portion 13D, transmits the second light S3D in mode multiplexing through the second core portion 15D, and transmits the second light S4D in single mode through the second core portion 17D.

[0125] The second optical coupling device 40D couples the first light S1D and the second light S2D, S3D, and S4D transmitted through the multi-core fiber 10D to the receiving device 50D.

[0126] The receiving device 50D receives the first light S1D and the second lights S2D, S3D, and S4D that are transmitted through the multi-core fiber 10D and coupled by the second optical coupling device 40D.

[0127] The optical transmission system 100D configured as above is a suitable optical transmission system using the multi-core fiber 10D that has a simple structure and is easy to manufacture. In particular, the first core portion 11D located at the center transmits the first light S1D, which is the first communication signal light, the second core portion 17D located at the outermost periphery that is susceptible to disturbances transmits the second light S4D, which is the test light for optical sensing, the second core portion 13D located in the middle transmits the second light S2D, which is the light for optical power transmission, and the second core portion 15D located across the second core portion 13D from the first core portion 11D transmits the second light S3D, which is the mode multiplexed communication signal light, so that optimal transmission for each light can be realized.

[0128] (An example of a first optical coupling device) 10 is an explanatory diagram of an example of a first optical coupling device. The first optical coupling device 30E is a fiber bundle having a ribbon structure, and includes core portions 31E, 32E, 33E, and 34E. The first optical coupling device 30E is connected to the multi-core fiber 10E by thermal fusion, adhesive, or the like. In the connected state, the core portion 31E of the first optical coupling device 30E is optically coupled to the first core portion 11D of the multi-core fiber 10D. Similarly, the core portion 32E is optically coupled to the second core portion 13D, the core portion 33E is optically coupled to the second core portion 15D, and the core portion 34E is optically coupled to the second core portion 17D.

[0129] Core portion 31E couples the inputted first light to first core portion 11D. Core portion 32E couples the inputted second light to second core portion 13D. Core portion 33E couples the inputted second light to second core portion 15D. Core portion 34E couples the inputted second light to second core portion 17D.

[0130] The positional relationship, refractive index profile, size, and the like of the core portions 31E, 32E, 33E, and 34E are set so as to optimize the coupling efficiency with the core portion of the multi-core fiber 10D to be coupled.

[0131] With a fiber bundle having such a ribbon structure, the first optical coupling device 30E can be realized with a simple configuration.

[0132] The present invention is not limited to the above-mentioned embodiment. The present invention also includes a configuration in which the above-mentioned components are appropriately combined. Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-mentioned embodiment, and various modifications are possible. [Explanation of symbols]

[0133] 10, 10A, 10B, 10C, 10D, 10E: Multi-core fiber 11, 11A, 11B, 11C, 11D: First core part 12, 12A, 12B, 12C, 12D: First clad section 13, 13A, 13B, 13C, 13D, 15C, 15D, 17D: Second core part 14C, 14D, 16D: Intermediate clad section 14, 14A, 14B, 16C, 18D: Second clad section 20, 20A, 20B, 20C, 20D: Transmitting device 30, 30A, 30B, 30C, 30D, 30E: 1st optical coupling device 31E, 32E, 33E, 34E: Core section 40, 40A, 40B, 40C, 40D: 2nd optical coupler 50, 50A, 50B, 50C, 50D: Receiving device 100, 100A, 100B, 100C, 100D: Optical transmission systems P11, P12, P31, P32: Profiles S1, S1A, S1B, S1C, S1D: First light S2, S2A, S2B, S2C, S2D, S3C, S3D, S4D: Second light

Claims

1. A first core portion; A first cladding portion; At least one second core portion; A second cladding portion; Equipped with the first cladding portion has a refractive index lower than the maximum refractive index of the first core portion and the maximum refractive index of the second core portion, surrounds an outer periphery of the first core portion, and has a ring-shaped cross section perpendicular to a longitudinal direction; the second core portion surrounds an outer periphery of the first cladding portion and has a ring-like shape in a cross section perpendicular to a longitudinal direction, the second cladding portion has a refractive index lower than the maximum refractive index of the second core portion, surrounds an outer periphery of the second core portion, and has a ring-shaped cross section perpendicular to the longitudinal direction, A relative refractive index difference Δ1 of at least one maximum refractive index of the second core portion with respect to the first cladding portion and the second cladding portion is 0.30% or more and 0.50% or less, an inner diameter of the second core portion is 22.0 μm or more, and an outer diameter of the second core portion is 88 μm or less. A multicore fiber; a transmitting device that outputs a first light, which is a first communication signal light, and at least one second light; a first optical coupling device that couples the first light to one of the first core portions and couples the second light to one of the second core portions; a receiving device that receives the first light transmitted through the multi-core fiber and the second light transmitted through the multi-core fiber; a second optical coupling device that couples the first light transmitted through the multi-core fiber and the second light transmitted through the multi-core fiber to the receiving device; Equipped with the first core portion of the multicore fiber propagates light having a wavelength longer than 1260 nm in a single mode; The second light includes non-communication light. Optical transmission system.

2. The multicore fiber, A plurality of the second core portions are provided, At least one intermediate clad portion is provided between the second core portions.

2. The optical transmission system according to claim 1.

3. The center core of the first core portion and the center core of the second core portion of the multicore fiber are made of quartz glass containing a dopant that increases the refractive index, and the first cladding portion and the second cladding portion are made of quartz glass that does not substantially contain dopants other than chlorine.

3. The optical transmission system according to claim 1 or 2.

4. The center core of the first core portion and the center core of the second core portion of the multicore fiber are made of quartz glass containing substantially no dopants other than chlorine, and the first cladding portion and the second cladding portion are made of quartz glass containing a dopant that lowers the refractive index.

3. The optical transmission system according to claim 1 or 2.

5. The second light includes a second communication signal light.

2. The optical transmission system according to claim 1.

6. The second communication signal light is a mode-multiplexed communication signal light.

6. The optical transmission system according to claim 5.

7. The non-communication light includes at least one of light for optical power transmission and test light for optical sensing.

2. The optical transmission system according to claim 1.

8. the non-communication light includes light for optical power transmission and test light for optical sensing, The second core portion to which the optical sensing test light is coupled is located on the outer circumferential side of the second core portion to which the optical power transmission light is coupled.

2. The optical transmission system according to claim 1.

9. the second light includes a second communication signal light, a light for optical power transmission, and a test light for optical sensing; the second core portion to which the optical power transmission light is coupled is located on an inner peripheral side than the second core portion to which the second communication signal light is coupled, The second core portion to which the optical sensing test light is coupled is located on the outer periphery side of the second core portion to which the second communication signal light is coupled.

2. The optical transmission system according to claim 1.

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