Multicore optical fiber

Abstract

An MCF according to the present disclosure comprises a glass fiber provided with a plurality of cores, a first cladding, and a second cladding, the main component of the glass fiber being silica glass. Each of the plurality of cores extends along a central axis and is doped with rare earth ions. The first cladding surrounds each of the plurality of cores and has a refractive index lower than the refractive index of each of the plurality of cores. The second cladding surrounds the first cladding and has a refractive index lower than the refractive index of the first cladding. In a cross section of the MCF orthogonal to the central axis, the ratio of the outside diameter of the first cladding to the outside diameter of the second cladding is 0.56 or less. The residual stress in the first cladding is a tensile stress of 250 MPa or less.

Description

Multicore optical fiber

[0001] This disclosure relates to multicore optical fibers (hereinafter referred to as "MCF"). This application claims priority under Japanese application No. 2024-155524, filed on 10 September 2024, and incorporates all the provisions contained herein.

[0002] Multi-core fiber (MCF), which has multiple cores arranged within a single cladding, is expected to be a technology that increases the spatial density of information transmission and efficiently utilizes the limited cross-sectional area of ​​optical fibers, which are the communication medium. Optical amplifiers are necessary to transmit optical signals over long distances, and Patent Document 1 discloses an example in which MCF is applied as an optical amplification medium for an optical amplifier. In MCF as an optical amplification medium, rare earth elements, particularly erbium (Er), are added to each of the multiple cores through which the signal light to be amplified propagates. Optical fibers with Er added to the core are known as erbium-doped optical fibers (hereinafter referred to as "EDF"). In this specification, an MCF having multiple Er-doped cores, such as the MCF in Patent Document 1, will be referred to as a multi-core EDF (hereinafter referred to as "MC-EDF").

[0003] Patent documents 2 and 3 disclose clad-excited MC-EDFs. Supplying excitation light is necessary to make Er exhibit optical amplification characteristics. The optical amplifier in Patent Document 1 has a structure in which excitation light is supplied to each Er-doped core of the MC-EDF. The clad-excited MC-EDFs disclosed in Patent Documents 2 and 3 have a structure in which excitation light is supplied into the cladding, and the Er in multiple Er-doped cores is excited simultaneously. Generally, the cladding of a clad-excited MC-EDF consists of an inner cladding that propagates the excitation light and an outer cladding that confines the excitation light within the inner cladding. In order to propagate the excitation light within the inner cladding, the outer cladding has a refractive index lower than that of the inner cladding. In the example in Patent Document 2, a low refractive index resin is used as the outer cladding, and in the example in Patent Document 3, a low refractive index glass is used as the outer cladding.

[0004] International Publication No. WO2018 / 047867, Japanese Unexamined Patent Application Publication No. 2021-153166, Japanese Unexamined Patent Application Publication No. 2018-198287

[0005] The MCF of the present disclosure includes a glass fiber having a plurality of cores, a first cladding, and a second cladding, and the main component is silica glass. Each of the plurality of cores extends along a central axis and rare earth ions are added. The first cladding surrounds each of the plurality of cores and has a refractive index lower than that of each of the plurality of cores. The second cladding surrounds the first cladding and has a refractive index lower than that of the first cladding. In a cross-section of the glass fiber orthogonal to the central axis, the ratio of the outer diameter of the first cladding to the outer diameter of the second cladding is 0.56 or less. The residual stress in the first cladding is a tensile stress of 250 MPa or less.

[0006] FIG. 1 is a diagram showing a schematic structure of an example of an optical amplifier of the present disclosure and an example of the MCF of the present disclosure. FIG. 2 is an example of a refractive index profile of the MCF of the present disclosure and a residual stress distribution corresponding to this example. FIG. 3 is a diagram showing a schematic structure of a wire drawing device for manufacturing the MCF of the present disclosure. FIG. 4 is a table summarizing the specifications of Samples 1 to 6 of the MCF prepared to confirm the relationship between the tensile stress remaining in the first cladding and the wire breakage frequency during wire drawing. FIG. 5 is a graph showing the relationship between the tensile stress remaining in the first cladding and the wire breakage frequency during wire drawing for Samples 1 to 6 of the MCF whose specifications are shown in FIG. 4.

[0007] As a result of studying the above-mentioned prior art, the inventors have found the following problems: That is, in the MC-EDF disclosed in Patent Document 1, since it is necessary to individually supply excitation light to a plurality of Er-added cores, there is a problem that the power consumption for supplying excitation light increases as the number of cores increases.

[0008] The clad-excited MC-EDF described in Patent Documents 2 and 3 was expected to reduce power consumption because the excitation light supplied to the clad simultaneously excites Er in multiple Er-doped cores. However, due to its structure, the overlap between the signal light propagating through the multiple Er-doped cores and the excitation light propagating through the clad is small in clad-excited MC-EDFs. To avoid a decrease in the excitation efficiency of the Er added in each of the multiple Er-doped cores, it is necessary to increase the intensity of the supplied excitation light. As a result, there was a problem in that power saving could not be achieved even with clad-excited MC-EDFs. This means that in order to save power, it is necessary to efficiently couple the excitation light to the multiple Er-doped cores, i.e., to improve the excitation efficiency, even in clad-excited MC-EDFs.

[0009] One way to improve the excitation efficiency in clad-excited MC-EDFs is to reduce the cross-sectional area of ​​the region that contributes to excitation light propagation, thereby increasing the excitation light density (excitation light intensity per unit cross-sectional area). For example, if the cladding consists of an inner cladding and an outer cladding, the inner cladding corresponds to the region that contributes to excitation light propagation. For example, as described in Patent Document 3, when low refractive index glass is applied to the outer cladding to reduce the cross-sectional area of ​​the inner cladding, frequent breakage of the MC-EDF occurred during wire drawing when manufacturing the MC-EDF from the base material and during screening tests where a certain tension was applied to the MC-EDF, making it difficult to obtain an MC-EDF of a certain length. When the MC-EDF was cut, cracks were prone to occur on the end face, making fusion splicing of the MC-EDF with other MCFs difficult.

[0010] According to the inventors' research, the occurrence of wire breakage and cracking is thought to be due to the difference in viscosity between the materials of the multiple Er-added cores, inner cladding, and outer cladding. Due to this difference in viscosity, tensile stress concentrates in the inner cladding during the heating and rapid cooling process when drawing the wire. Therefore, it is thought that wire breakage and cracking are more likely to occur starting from the inner cladding.

[0011] This disclosure is made to solve the problems described above and aims to provide an MCF with a structure that achieves both improved excitation efficiency and reduced risk of disconnection.

[0012] According to the MCF of this disclosure, it is possible to achieve both improved excitation efficiency and reduced risk of disconnection.

[0013] The contents of the embodiments disclosed here will be described individually.

[0014] (1) The MCF of this disclosure includes a glass fiber whose main component is silica glass, comprising a plurality of cores, a first cladding, and a second cladding. Each of the plurality of cores extends along a central axis and is doped with rare earth ions. The first cladding surrounds each of the plurality of cores and has a refractive index lower than that of each of the plurality of cores. The second cladding surrounds the first cladding and has a refractive index lower than that of the first cladding. In a cross-section of the glass fiber perpendicular to the central axis (hereinafter simply referred to as "cross-section of the MCF"), the ratio of the outer diameter of the first cladding to the outer diameter of the second cladding is 0.56 or less. The residual stress in the first cladding is a tensile stress of 250 MPa or less.

[0015] Generally, if large tensile stresses remain in the first cladding, frequent breakage of the MCF occurs during wire drawing when manufacturing MCF from the base material and during screening tests where a certain tension is applied to the MCF, making it difficult to obtain MCF of a consistent fiber length. Furthermore, if large tensile stresses remain in the first cladding, cracks are more likely to occur on the end faces of the cut MCF, which can make fusion splicing of MCFs difficult.

[0016] In contrast, with the MCF of this disclosure, tensile stress remains in the first cladding, but this tensile stress is 250 MPa or less. This reduces the frequency of breakage during drawing and screening tests, making it easier to obtain MCFs of a constant fiber length. The flatness of the cross-section of the cut MCF is improved, thus improving the connectivity between MCFs. With the MCF of this disclosure, the ratio of the outer diameter of the first cladding to the outer diameter of the second cladding in the cross-section of the MCF is 0.56 or less. By reducing the cross-sectional area of ​​the first cladding in this way, the excitation light density of the first cladding is increased.

[0017] (2) In (1) above, the tensile stress in the first cladding may be 10 MPa or more. If a certain level of tensile stress remains in the first cladding, the tensile stress is released when the MCFs fuse together, and the mode field diameter (hereinafter referred to as "MFD") expands. This reduces the fusion loss caused by the misalignment of the cores of the two MCFs being fused.

[0018] (3) In (1) or (2) above, the residual stress in the second cladding may be compressive stress. In this way, by utilizing the viscosity difference of each cladding, it becomes easier to control the tensile stress remaining in the first cladding.

[0019] (4) In any of (1) to (3) above, the first cladding may contain chlorine (Cl). In this case, the concentration of Cl may be 1,000 wt / ppm or more and 10,000 wt / ppm or less. Adding Cl at a certain concentration or higher to the first cladding reduces the glass viscosity, thereby reducing the increase in tensile stress.

[0020] (5) In any of (1) to (4) above, the shortest distance among the distances between the centers of multiple cores in the cross-section of the MCF (hereinafter referred to as "shortest distance between core centers") may be 12 μm or more and 35 μm or less. By shortening the shortest distance between core centers in this way, it is possible to increase the core density in the cross-section of the MCF.

[0021] (6) In any of (1) to (5) above, the outer diameter of the first cladding in the cross-section of the MCF may be 30 μm or more and 70 μm or less. The standard outer diameter of the second cladding is 125 μm, but by making the cross-sectional area of ​​the first cladding smaller than the cross-sectional area of ​​the second cladding, it is possible to increase the excitation light density of the first cladding.

[0022] (7) In any of (1) to (6) above, the MCF of the present disclosure may include a resin coating surrounding the second cladding and having a refractive index lower than that of the second cladding. By providing a low refractive index resin coating in addition to the second cladding, it is possible to improve the confinement efficiency of the excitation light in the first cladding.

[0023] (8) The optical amplifier of the present disclosure includes any MCF of (1) to (7) above, an excitation light source, and an optical coupler. The excitation light source outputs excitation light of a wavelength that excites rare earth ions added to each of the multiple cores of the MCF. The optical coupler introduces the excitation light output from the excitation light source into the first cladding of the MCF. By applying the MCF of the present disclosure, the excitation light density of the first cladding is increased, making it possible to efficiently excite the rare earth elements added to each core without increasing power consumption.

[0024] Each of the above embodiments is applicable to each of the remaining embodiments, or to all combinations of these remaining embodiments.

[0025] [Details of Embodiments of the Disclosure] Specific examples of MCF (multicore optical fiber) according to the Disclosure will be described in detail below with reference to the accompanying drawings. The present invention is not limited to these examples, but is indicated by the claims, and all modifications within the meaning and scope of the claims are intended to be included. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0026] Figure 1 shows the schematic structures of an example of an optical amplifier and an example of an MCF of the present disclosure (labeled "Structure" in Figure 1). The upper part of Figure 1 (labeled "Optical Amplifier" in Figure 1) shows the structure of an example of an optical amplifier of the present disclosure. The lower part of Figure 1 (labeled "MCF" in Figure 1) shows the cross-sectional structure of MCF 100 of the present disclosure, which is a clad-excited MCF. Figure 2 shows an example of the refractive index profile and residual stress distribution of an MCF of the present disclosure (labeled "Refractive Index and Residual Stress" in Figure 2). The upper part of Figure 2 (labeled "Refractive Index Profile" in Figure 2) shows the refractive index profile 150 of MCF 100 of the present disclosure as an example. The lower part of Figure 2 (labeled "Residual Stress Distribution" in Figure 2) shows the residual stress distribution 160 corresponding to the refractive index profile 150 shown in the upper part of Figure 2.

[0027] The optical amplifier shown in the upper part of Figure 1 includes an MCF 100 of this disclosure that takes a signal light SL as input and outputs an amplified light AL, an optical coupler 200 that introduces the signal light SL and the excitation light PL into the MCF 100, a signal light input fiber 300 that leads the signal light SL to the optical coupler 200, an excitation light input fiber 400 that leads the excitation light PL to the optical coupler 200, and an excitation light source 500 that outputs the excitation light PL.

[0028] The MCF100, as will be described in detail later, includes a plurality of cores 110, each doped with a rare earth ion, and a first cladding 120 that surrounds each of the plurality of cores 110 and propagates excitation light PL. The MCF100 is a cladding-excited MC-EDF that excites the plurality of rare earth ions doped in each of the plurality of cores 110 simultaneously by propagating excitation light PL within the first cladding 120. The excitation light source 500 outputs excitation light PL in a wavelength band that excites the plurality of cores 110 in the MCF100. For example, if the rare earth ion is an Er ion, the excitation light PL may be light in the 1.48 μm band or the 0.98 μm band. The optical coupler 200 outputs signal light SL to the plurality of cores 110 of the MCF100, and at the same time outputs the excitation light PL output from the excitation light source 500 to the first cladding 120 of the MCF100. The signal light SL, propagated sequentially through the signal light input fiber 300 and the optical coupler 200, is input to multiple cores 110 of the MC-EDF, which is the MCF 100. At this time, the first cladding 120 of the MCF 100 is input to the excitation light PL, which propagated sequentially through the excitation light input fiber 400 and the optical coupler 200, and the rare earth ions added to each of the multiple cores are excited. The signal light SL input to the multiple cores 110 of the MCF 100 is amplified as it propagates through the multiple cores 110 within the MCF 100.

[0029] The MCF 100 shown in the lower part of Figure 1 includes a glass fiber 100A whose main component is silica glass, and a resin coating 140. The glass fiber 100A includes a plurality of cores 110 each extending along the central axis AX, a first cladding 120 surrounding each of the plurality of cores 110, and a second cladding 130 surrounding the first cladding 120.

[0030] Each of the multiple cores 110 is doped with rare earth ions. Elements such as aluminum (Al) and germanium (Ge) may also be doped into the multiple cores 110 to optimize amplification characteristics and refractive index. Cl is doped into the first cladding 120. The concentration of Cl in the first cladding 120 is adjusted to between 1000 wt / ppm and 10000 wt / ppm. As an example, the average concentration of Cl is 3000 wt / ppm. Fluorine (F) is doped into the second cladding 130. As an example, the average concentration of F in the second cladding 130 is 22000 wt / ppm.

[0031] As an example, the diameter 2a of multiple cores 110 is 10 μm. The shortest distance D1 between the core centers, shown in the lower part of Figure 1, is 20 μm. The outer diameter 2b1 of the first cladding 120 is 50 μm. The outer diameter 2b2 of the second cladding 130 is 125 μm. The shortest distance D1 between the core centers should be between 12 μm and 35 μm. The outer diameter 2b1 of the first cladding 120 should be between 30 μm and 70 μm. In this case, the ratio (b1 / b2) of the outer diameter 2b1 of the first cladding 120 to the outer diameter 2b2 of the second cladding 130 is 0.56 or less. As a comparison target for the MCF 100 of this disclosure, consider an MCF that includes a common cladding with an outer diameter of 125 μm corresponding to the first cladding 120, and a resin coating corresponding to the second cladding 130. In this case, the cross-sectional area of ​​the first cladding 120 of the MCF 100 of this disclosure is 0.16 times the cross-sectional area of ​​the common cladding of the comparative example MCF, and the excitation light density is 6.25 times greater. Here, the cross-sectional area of ​​the first cladding 120 is the cross-sectional area of ​​the region surrounded by the outer circumference of the first cladding 120, including the multiple cores 110, in the cross-section of the MCF 100 perpendicular to the central axis AX. The cross-sectional area of ​​the common cladding is calculated similarly. That is, the cross-sectional area of ​​the common cladding is the area of ​​a circle with an outer diameter, and includes the region of the multiple cores 110 and the region of the first cladding.

[0032] The refractive index of each of the multiple cores 110 is n1, the refractive index of the first cladding 120 is n2, the refractive index of the second cladding 130 is n3, and the refractive index of the resin coating 140 is n4. The refractive index n2 is lower than the refractive index n1. The refractive index n3 is lower than the refractive index n2. The refractive index n4 is lower than the refractive index n3. As an example, if the resin coating 140 is made of fluororesin, the refractive index n4 of the resin coating 140 at a wavelength of 589 nm is 1.4.

[0033] The MCF100 having the structure described above has a refractive index profile 150 shown in the upper part of Figure 2. The refractive index profile 150 shows the refractive index of each part of the MCF100 along the line L shown in the lower part of Figure 1. The MCF100 has a residual stress distribution 160 shown in the lower part of Figure 2.

[0034] In the refractive index profile 150, the first region 151 corresponds to the region of each core 110 with a diameter 2a and refractive index n1. The second region 152 corresponds to the region of the first cladding 120 with an outer diameter 2b1 and refractive index n2. The second region 152 does not include the first region 151. The third region 153 corresponds to the region of the second cladding 130 with an outer diameter 2b2 and refractive index n3. The fourth region 154 corresponds to the region of the resin coating 140 with refractive index n4.

[0035] In the residual stress distribution 160, the first region 161 corresponds to each core 110. The second region 162 corresponds to the first cladding 120. The third region 163 corresponds to the second cladding 130. In the example shown in the lower part of Figure 2, no stress remains in the first region 161. The residual stress in the second region 162 is tensile stress. The residual stress in the third region 163 is compressive stress. The second region 162 is a region that does not include the first region 161. In this specification, "residual stress" refers to the component of the stress remaining inside the optical fiber after drawing and cooling to room temperature, acting on the cross-section of the optical fiber perpendicular to the central axis and along the central axis. As shown in the lower part of Figure 2, residual stress is expressed as a positive value when the residual stress is tensile stress, and as a negative value when the residual stress is compressive stress. Residual stress can be measured using an optical system combining a microscope and an interferometer.

[0036] Figure 3 shows a schematic structure of a wire drawing apparatus for manufacturing the MCF 100 of the present disclosure. The wire drawing apparatus shown in Figure 3 includes a heater 610 for heating one end of an optical fiber base material 100B for obtaining the MCF 100, a resin coating apparatus 620 for applying a resin coating 140 to the outer circumference of the drawn glass fiber 100A, a roller 630 for changing the direction of travel of the MCF 100 obtained by passing through the resin coating apparatus 620, a winding roller 640 for winding the MCF 100 by rotating in the direction indicated by arrow S1, and a control unit 650 for adjusting the wire tensile force applied to the molten portion of the optical fiber base material 100B by controlling the rotation speed of the winding roller 640, i.e., the wire drawing speed.

[0037] In the drawing process of the optical fiber preform 100B, one end of the optical fiber preform 100B is melted at a high temperature of approximately 2000°C by the heater 610, and the molten portion is stretched into the shape of MCF 100 with a tension of 10g to 300g. At this time, in the MCF 100 obtained by stretching, due to the non-uniformity of the composition and temperature distribution within the cross-section, tensile stress is likely to occur in regions with high glass viscosity, and compressive stress is likely to occur in regions with low glass viscosity. In addition, even in the MCF 100 when it is cooled to room temperature after stretching, due to the non-uniformity of the coefficient of linear expansion within the cross-section, compressive stress is likely to occur in regions with a small coefficient of linear expansion, and tensile stress is likely to occur in regions with a large coefficient of linear expansion.

[0038] In the manufacturing of the MCF100 of this disclosure, the additive concentration and drawing conditions are adjusted so that tensile stress remains in the first cladding 120 and compressive stress remains in the second cladding 130, as shown in the residual stress distribution 160 in the lower part of Figure 2. In the MCF100 of this disclosure, the tensile stress in the first cladding 120 is between 10 MPa and 250 MPa.

[0039] Figure 4 is a table summarizing the specifications of MCF samples 1 to 6, which were prepared to confirm the relationship between the tensile stress remaining in the first cladding and the frequency of wire breakage during drawing. Figure 5 is a graph showing the relationship between the tensile stress remaining in the first cladding and the frequency of wire breakage during drawing for samples 1 to 6, whose specifications are shown in Figure 4. All of the prepared samples 1 to 6 are MC-EDF in which Er is added to each of the multiple cores.

[0040] In the prepared samples 1 through 6, the "core diameter 2a", "core Er concentration", "shortest distance between core centers D1", "outer diameter of the first cladding 2b1", "outer diameter of the second cladding 2b2", and "first cladding Cl concentration" are consistent. That is, the "core diameter 2a" is 10 μm, the "core Er concentration" is 800 wt ppm, the "shortest distance between core centers D1" is 20 μm, the "outer diameter of the first cladding 2b1" is 50 μm, the "outer diameter of the second cladding 2b2" is 125 μm, and the "first cladding Cl concentration" is 3000 wt ppm.

[0041] The "residual stress in the first cladding" for Samples 1 through 6 is tensile stress, and is 80 MPa, 100 MPa, 150 MPa, 250 MPa, 300 MPa, and 500 MPa, respectively. The tensile stress remaining in the first cladding 120 of each of Samples 1 through 6 is obtained by adjusting the wire tension in the wire drawing device shown in Figure 3. At this time, the "frequency of wire breakage during wire drawing" for Samples 1 through 6 was 0.0 times / km, 0.3 times / km, 0.5 times / km, 2.0 times / km, 16.7 times / km, and 44.4 times / km, respectively. Figure 5 plots the relationship between the "residual stress in the first cladding" and the "frequency of wire breakage during wire drawing" for Samples 1 through 6.

[0042] As can be seen from the specifications in Figure 4 and the graph in Figure 5, the tensile stress on the first cladding 120 affects the frequency of wire breakage during drawing. For each MC-EDF from Sample 1 to Sample 4, where the tensile stress of the first cladding 120 is small, the frequency of wire breakage is effectively reduced. For each MC-EDF in Sample 5 and Sample 6, where the tensile stress of the first cladding 120 exceeds 250 MPa, the frequency of wire breakage increases significantly, making it difficult to obtain an MC-EDF with a constant fiber length.

[0043] For MC-EDF samples 1 to 4, it was confirmed that the lower the residual tensile stress in the first cladding 120, the lower the frequency of crack occurrence at the end face when the obtained MC-EDF was cut, and the improved connectability during fusion. If a certain level of tensile stress remains in the first cladding 120, the tensile stress will be released during fusion. This expands the MFD, and when fusion bonding MC-EDF samples 1 to 4 to other MCFs, a reduction in fusion loss due to core misalignment can be expected. For all samples 1 to 6, the residual tensile stress in the first cladding 120 was adjusted by changing the linear tensile force. The residual tensile stress in the first cladding 120 can also be adjusted by adjusting the viscosity difference of each glass layer constituting the MC-EDF. For example, increasing the concentration of Cl added to the first cladding 120 lowers the glass viscosity, which can be expected to reduce the increase in tensile stress in the first cladding 120. To reduce the increase in tensile stress in the first cladding, the outer diameter 2b1 of the first cladding 120 may be increased. That is, the area of ​​the inner region surrounded by the outer circumference of the first cladding 120 may be increased. However, it should be noted that such an enlargement of the first cladding 120 will decrease the excitation light density.

[0044] 100...MCF 100A...Glass fiber 100B...Optical fiber base material 110...Core 120...First cladding 130...Second cladding 140...Resin coating 150...Refractive index profile 160...Residual stress distribution 151, 161...First region 152, 162...Second region 153, 163...Third region 154...Fourth region 200...Optical coupler 300...Fiber for signal light input 400...Fiber for excitation light input 500...Excitation light source 610...Heater 620...Resin coating device 630...Roller 640...Winding roller 650...Control unit D1...Shortest distance S1...Arrow SL...Signal light AL...Amplified light PL...Excitation light AX...Central axis

Claims

1. A multicore optical fiber comprising: a plurality of cores extending along a central axis, each doped with rare earth ions; a first cladding surrounding each of the plurality of cores and having a refractive index lower than that of each of the plurality of cores; and a second cladding surrounding the first cladding and having a refractive index lower than that of the first cladding, wherein the main component of the glass fiber is silica glass, and in a cross-section of the glass fiber perpendicular to the central axis, the ratio of the outer diameter of the first cladding to the outer diameter of the second cladding is 0.56 or less, and the residual stress in the first cladding is a tensile stress of 250 MPa or less.

2. The multicore optical fiber according to claim 1, wherein the tensile stress in the first cladding is 10 MPa or more.

3. The multicore optical fiber according to claim 1 or claim 2, wherein the residual stress in the second cladding is compressive stress.

4. The multicore optical fiber according to any one of claims 1 to 3, wherein the first cladding contains chlorine, and the concentration of chlorine is 1,000 wt / ppm or more and 10,000 wt / ppm or less.

5. The multicore optical fiber according to any one of claims 1 to 4, wherein in the cross-section of the glass fiber, the shortest distance among the distances between the centers of the plurality of cores is 12 μm or more and 35 μm or less.

6. The multicore optical fiber according to any one of claims 1 to 5, wherein in the cross-section of the glass fiber, the outer diameter of the first cladding is 30 μm or more and 70 μm or less.

7. A multicore optical fiber according to any one of claims 1 to 6, comprising a resin coating surrounding the second cladding and having a refractive index lower than that of the second cladding.

8. An optical amplifier comprising: a multicore optical fiber according to any one of claims 1 to 7; an excitation light source that outputs excitation light of a wavelength that excites the rare earth ions added to each of the plurality of cores of the multicore optical fiber; and an optical coupler that introduces the excitation light output from the excitation light source into the multicore optical fiber.

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