JP7716200B2 - Multicore fiber and method of manufacturing multicore fiber - Google Patents

Description

The present invention relates to a multicore fiber and a method for manufacturing a multicore fiber.

In recent years, there has been a trend toward a rapid increase in communication traffic in communication networks. Therefore, to meet this demand, multicore fibers, in which multiple cores are arranged within a single cladding, are sometimes used. In such multicore fibers, the outer surface of the cross section of the cladding that surrounds the multiple cores generally has a circular shape.

Another known multicore fiber is the multicore fiber described in Non-Patent Document 1 below. The multicore fiber described in Non-Patent Document 1 has a configuration in which multiple single-core fibers, each consisting of one core and a cladding surrounding that core, are bundled together and then coated collectively with resin. Such multicore fibers are sometimes called multi-element fibers.

S. Jain et al., “Multi-Element Fiber Technology for Space-Division Multiplexing Applications,” Opt. Express, vol. 22, issue 4, pp. 3787-3796, 2014.

The multi-element fiber described in Non-Patent Document 1 above is configured by bundling single-core fibers, so the combined cross-sectional area of the cladding of each single-core fiber can be smaller than the cross-sectional area of the cladding of a conventional multi-core fiber, whose cladding has a circular cross section. Therefore, it is thought that the multi-element fiber can be made lighter than a conventional multi-core fiber, whose cladding has a circular outer surface shape.

However, in the multi-element fiber described in Non-Patent Document 1, the single-core fibers are merely held together via resin. Therefore, if an external force acts on the multi-element fiber, for example, when the multi-element fiber is cut, this external force may cause the single-core fibers to shift positions, resulting in misalignment between the cores.

The present invention was made in consideration of the above circumstances, and aims to provide a multicore fiber that can be made lighter and that can suppress misalignment between cores, as well as a method for manufacturing such a multicore fiber.

To achieve the above object, the multicore fiber of the present invention comprises a plurality of cores and a cladding surrounding each of the cores, and the cladding has a joint where parts of the outer surfaces of at least two cladding sections in a plurality of single-core fibers in which each core is arranged in the center and the cladding section surrounds the core are joined together over a width smaller than the diameter of the cladding section, and the outer surface of the cladding includes other parts of the outer surfaces of the at least two cladding sections.

With the above configuration, portions of the cladding of the single-core fibers are integrated, which prevents the single-core fibers from shifting in position when an external force acts on the multi-core fiber, for example, when cutting. Therefore, it is possible to prevent misalignment between the cores.

The width of the splice portion in the cladding of this multicore fiber is smaller than the diameter of the cladding of the single-core fiber. Incidentally, since the core is disposed at the center of the cladding, the remaining portion of the outer circumferential surface of the cladding, excluding the splice portion, tends to be at a roughly constant distance from the center of the core. Therefore, the remaining portions of the outer circumferential surfaces of the pair of single-core fibers that are joined together at the splice portion are spaced apart from each other. Consequently, a constriction exists in the cladding of this multicore fiber near the splice portion. Because the cladding of this multicore fiber has such a constriction, the cross-sectional area of the outer circumferential surface in cross section can be smaller than, for example, a circular cladding that includes the cladding portions of each single-core fiber inside, and the amount of glass in the cladding can be reduced. Therefore, the multicore fiber of the present invention can be made lighter.

Furthermore, in the above-mentioned multicore fiber, it is preferable that the outer surface of the cladding at at least one pre-joint is concave in an arc shape toward the center of the joint.

With this configuration, the outer peripheral surface of the cladding at the joint is recessed in an arc shape, which can prevent peeling between the cladding and the inner coating layer when an inner coating layer is provided around the outer periphery of the cladding, thereby preventing the inner coating layer from peeling off from the cladding. This can prevent uneven stress caused by peeling of the inner coating layer, suppress microbending, and reduce transmission loss of light propagating through the core.

The multicore fiber may also have three or more single-core fibers arranged in parallel with each other.

This configuration allows it to be used as a so-called tape fiber with three or more cores, reducing the misalignment between cores while reducing weight.

Furthermore, when the multicore fiber has three or more single-core fibers, it is preferable that the cladding has a polygonal void surrounded by the outer peripheral surfaces of the cladding portions of the three or more single-core fibers.

The presence of such gaps can make it difficult for light leaking from one core to be transmitted to other cores, thereby suppressing crosstalk between cores.

Furthermore, if the voids are present in the cladding, it is preferable that the portions of the outer surface of each cladding portion that form the voids are curved so as to surround the core that is closest to those portions.

Furthermore, if the above-mentioned voids are present, it is preferable that the width of each portion of the outer surface of the cladding portion that forms the void is equal to or greater than the diameter of the core closest to that portion.

In this case, light leaking from one core may be less likely to be transmitted to other cores, and crosstalk between cores may be suppressed, compared to when the width of the portion of the outer surface of the cladding that forms the void is less than the diameter of the core.

Furthermore, it is preferable that at least one of the single-core fibers has a trench layer that has a lower refractive index than the cladding portion of the single-core fiber and surrounds the core of the single-core fiber, and that the width of the splice in the single-core fiber is equal to or less than the diameter of the trench layer.

In this case, compared to when the width of the junction exceeds the diameter of the trench layer, light leaking from one core may be less likely to be transmitted to an adjacent core, thereby suppressing crosstalk between cores.

Furthermore, the width of the splice may be equal to or less than the diameter of the core of the single-core fiber having the splice.

In this case, compared to when the width of the splice is greater than the diameter of the core, light leaking from the core may be less likely to be transmitted to the core of the spliced single-core fiber, and crosstalk between the cores may be suppressed.

Furthermore, at least one of the cores may be elliptical.

Making the core elliptical in this way can make it easier to maintain the polarization mode of light propagating through the core.

Furthermore, to achieve the above-mentioned object, the manufacturing method of the multicore fiber of the present invention is characterized by comprising: a bundling step of bundling single-core fiber rods, each of which includes a core glass body disposed at the center and a clad glass body surrounding the core glass body; and a drawing step of drawing the bundled single-core fiber rods so that, in a molten state, portions of the outer circumferential surfaces of the clad glass bodies are in contact with each other over a width smaller than the diameter of the clad glass bodies.

According to this method for manufacturing a multi-core fiber, portions of the outer circumferential surfaces of the clad glass bodies that are brought into contact in a molten state can form a joint where portions of the outer circumferential surfaces of the clad portions of the single-core fiber are joined together over a width smaller than the diameter of the clad portions. Furthermore, other portions of the outer circumferential surfaces of the clad glass bodies are portions of the outer circumferential surfaces of the clad portions of the single-core fiber that tend to be at a constant distance from the center of the core. Therefore, according to this method for manufacturing a multi-core fiber, any of the multi-core fibers described above can be manufactured.

Furthermore, in the above-mentioned method for manufacturing a multicore fiber, it is preferable that in the bundling step, multiple single-core fiber rods are bundled together so that adjacent single-core fiber rods are in contact with each other.

In this case, adjacent single-core fiber rods are drawn in contact with each other, so portions of the outer circumferential surfaces of the single-core fiber rods can be more effectively integrated.

Furthermore, in the above-mentioned method for manufacturing a multi-core fiber, it is preferable that in the bundling step, the outer surfaces of the clad glass bodies of adjacent single-core fiber rods are fused to each other.

In this case, misalignment between adjacent single-core fiber rods can be suppressed. In particular, if the fused portion includes the end portion of the single-core fiber rod where drawing begins, this is preferable because it acts as a trigger for bringing parts of the outer circumferential surfaces of the clad glass bodies into contact with each other during the drawing process.

As described above, the present invention can provide a multicore fiber that can be made lighter and that can suppress misalignment between cores, as well as a method for manufacturing such a multicore fiber.

1 is a diagram showing the structure of a cross section perpendicular to the longitudinal direction of a multi-core fiber according to a first embodiment of the present invention. FIG. 2A to 2C are diagrams illustrating steps in a method for manufacturing the multi-core fiber shown in FIG. 1. FIG. 10 is a diagram showing the first half of the bundling process. 10A and 10B are diagrams showing the second half of the bundling process and the pre-processing process. FIG. 10 is a side view of the multicore fiber preform after the pretreatment process. 7 is a view of the multi-core fiber preform shown in FIG. 6 as seen from the fused tip end side. FIG. FIG. 1. FIG. 3 is a view showing a multicore fiber according to a second embodiment of the present invention from the same perspective as FIG. 1. FIG. 4 is a view showing a multicore fiber according to a third embodiment of the present invention from the same perspective as FIG. 1. FIG. 4 is a view showing a multicore fiber according to a fourth embodiment of the present invention from the same perspective as FIG. FIG. 2 is a view showing a modified example of the present invention from the same perspective as FIG. 1 . FIG. 10 is a view showing another modified example of the present invention from the same perspective as FIG. FIG. 10 is a view showing yet another modified example of the present invention from the same perspective as FIG. 2 .

Below, embodiments for implementing a multicore fiber and a method for manufacturing a multicore fiber according to the present invention are illustrated with reference to the accompanying drawings. The embodiments illustrated below are intended to facilitate understanding of the present invention and are not intended to limit the scope of the present invention. The present invention can be modified or improved from the following embodiments without departing from the spirit of the present invention. Furthermore, in this specification, the dimensions of each component may be exaggerated to facilitate understanding.

(First embodiment)
Fig. 1 is a diagram showing the structure of a cross section perpendicular to the longitudinal direction of a multi-core fiber according to the first embodiment. Note that hatching is omitted in Fig. 1 to avoid complicating the drawing. As shown in Fig. 1, the multi-core fiber 1 of this embodiment mainly comprises a plurality of single-core fibers 10, a single inner coating layer 30 that coats each of the single-core fibers 10, and an outer coating layer 40 that coats the inner coating layer 30. Note that Fig. 1 shows an example in which there are three single-core fibers 10.

The inner coating layer 30 and the outer coating layer 40 are formed from resin. Examples of such resins include thermosetting resin and ultraviolet-curing resin. Furthermore, if the inner coating layer 30 and the outer coating layer 40 are formed from thermosetting resin, the outer coating layer 40 may be formed from a different type of thermosetting resin than the inner coating layer 30. Furthermore, if the inner coating layer 30 and the outer coating layer 40 are formed from ultraviolet-curing resin, the outer coating layer 40 may be formed from a different type of ultraviolet-curing resin than the inner coating layer 30.

Note that the inner coating layer 30 and outer coating layer 40 are not essential components of the multicore fiber 1. A multicore fiber 1 that does not have the inner coating layer 30 and outer coating layer 40 is also called a bare multicore fiber.

Each single-core fiber 10 includes a centrally located circular core 11, an inner cladding layer 13 surrounding the core 11, a trench layer 14 surrounding the inner cladding layer 13, and a cladding portion 12 surrounding the trench layer 14. This can be understood as meaning that the cores 11 of each single-core fiber 10 are spaced apart from one another. In this manner, in this embodiment, the cladding portion 12 surrounds the core 11 via the inner cladding layer 13 and trench layer 14, and has a circular outer shape. Therefore, the outer peripheral surface of the cladding portion 12 tends to be at a roughly constant distance from the center of the core 11. The core 11, inner cladding layer 13, and trench layer 14 are sometimes collectively referred to as the core element. Note that the trench layer 14 may directly surround the core 11 without providing the inner cladding layer 13.

The inner cladding layer 13 and cladding portion 12 have a lower refractive index than the core 11, and the trench layer 14 has a lower refractive index than the inner cladding layer 13 and cladding portion 12. By having such a trench layer 14 in the multicore fiber 1, crosstalk between the cores 11 of adjacent single-core fibers 10 can be suppressed compared to when the trench layer 14 is not included.

As shown in FIG. 1, portions of the outer circumferential surfaces of the cladding portions 12 of a pair of adjacent single-core fibers 10 are integrated at a joint 21. Therefore, the multi-core fiber 1 of this embodiment has three joints 21. In this way, the multi-core fiber 1 of this embodiment forms a single cladding 20 consisting of the cladding portions 12 of each single-core fiber 10. Here, the cladding 20 includes the cladding portions 12 and the joint 21. Therefore, the cladding 20 surrounds each core 11 of each single-core fiber 10. Therefore, the inner coating layer 30 is a single coating layer that surrounds the outer circumferential surface of the cladding 20.

Figure 2 is an enlarged view showing the joint 21. Note that Figure 2 shows only one joint 21. The outer surface of the cladding 20 at each joint 21 is concave in an arc shape toward the center of the joint 21. In other words, the bottom 21B of the valley, which is the outer surface of the joint 21, is concave in a rounded arc shape. The width W of each joint 21 is smaller than the diameter of the cladding 12. Note that the width W of each joint 21 is the length along a direction perpendicular to the line connecting the centers of the cores 11 of the pair of single-core fibers 10. Note that in this embodiment, the widths W of each joint 21 are approximately the same size. However, at least one of these widths may be a different size. Furthermore, the number of joints 21 at which the outer surface of the cladding 20 is concave in an arc shape is not particularly limited, and may be one or more.

Furthermore, the diameter of the circumscribing circle Co of the cladding 20, shown by the dashed line in Figure 1, is not particularly limited, but may be, for example, 125 μm.

In this embodiment, the width W of each joint 21 is smaller than the diameter of the trench layer 14 of each single-core fiber 10, and is also smaller than the diameter of the core 11 of each single-core fiber 10. By making the width W of the joint 21 smaller than the diameter of the trench layer 14 and the diameter of the core 11 in this way, light leaking from a core 11 is less likely to be transmitted to an adjacent core 11 via the joint 21, and crosstalk between cores 11 can be more effectively suppressed. However, the width W of the joint 21 may be greater than or equal to the diameter of the core 11 or the diameter of the trench layer 14.

As described above, a portion of the cladding 12 of each single-core fiber 10 is formed as a splice 21. In this embodiment, another portion of the outer circumferential surface of each cladding 12 is divided by two splices 21 into a surface 12F1 located between the two splices 21 and a surface 12F2 other than surface 12F1. Surfaces 12F1 and 12F2 generally maintain the shape of the outer circumferential surface of the single-core fiber 10 and are surfaces that are at a constant distance from the center of the core 11. Note that the constant distance from the core includes deviations due to manufacturing errors, etc. The surfaces 12F2 of each pair of adjacent single-core fibers 10 are spaced apart from each other while drawing an arc from the splice 21. Therefore, a constriction is formed in the cladding 20. Here, the splice 21 is provided throughout the entire longitudinal direction of the multi-core fiber 1.

In this embodiment, a polygonal gap G is formed in the center of the multi-core fiber 1, surrounded by the surfaces 12F1 of the cladding portions 12 of the single-core fibers 10. This gap G is not filled with resin, glass, or the like. In this embodiment, since there are three single-core fibers, the outer shape of the gap G is an equilateral triangle.

In this embodiment, surface 12F1, which is the portion of the outer surface of the cladding portion 12 of each single-core fiber 10 that forms the gap G, is curved so as to surround the core 11 closest to surface 12F1. With this configuration, it is believed that light leaking from the core 11 is more likely to be blocked by the gap G, thereby suppressing crosstalk between cores.

Here, in Figure 1, a line segment L connecting the two joints 21 at the shortest distance is shown by a dashed line. If the length of this line segment L is the width of surface 12F1, in this embodiment, the width of surface 12F1 is larger than the diameter of the core 11 of the single-core fiber 10 closest to surface 12F1 and the diameter of the trench layer 14. With this configuration, light leaking from a core 11 can be less likely to be transmitted to other cores 11 than when the width of surface 12F1, which forms gap G, is equal to or smaller than the diameter of the core 11, and crosstalk between cores can be suppressed. However, the width of surface 12F1 may be equal to or smaller than the diameter of the core 11 or the diameter of the trench layer 14.

As described above, the multi-core fiber 1 of this embodiment comprises a plurality of cores 11 and claddings 20 surrounding each of the cores 11. The cladding 20 has a joint 21 where portions of the outer surfaces of the claddings 12 of a plurality of single-core fibers 10, each of which has a core 11 arranged at its center and a cladding 12 surrounding the core 11, are joined together over a width smaller than the diameter of the cladding 12. The outer surface of the cladding 20 includes surfaces 12F1 and 12F2, which are other portions of the outer surfaces of the claddings 12.

With this configuration, the cladding portions 12 of the single-core fibers 10 are integrated, so that when an external force acts on the multi-core fiber 1, for example, during cutting, misalignment between the single-core fibers is suppressed. Therefore, misalignment between the cores can be suppressed.

The width of the splice 21 in the cladding 20 of this multi-core fiber 1 is smaller than the diameter of the cladding 12 of the single-core fiber 10. Since the core 11 is disposed at the center of the cladding 12, the surface 12F2, which is the remaining part of the outer surface of the cladding 12 excluding the splice 21, tends to be at a constant distance from the center of the core 11. Therefore, the surfaces 12F2 of each of the pair of single-core fibers 10 that are joined together at the splice 21 are spaced apart from each other while drawing an arc from the splice 21, as described above. Therefore, a constriction consisting of the pair of surfaces 12F2 exists in the cladding 20 of this multi-core fiber 1 near the splice 21. Because the cladding 20 of this multi-core fiber 1 has such a constriction, the cross-sectional area of the outer surface in cross section may be smaller than that of a cladding whose outer surface is, for example, a circumscribed circle Co shown in FIG. 1 that includes the cladding 12 of each single-core fiber 10. Therefore, the amount of glass in the cladding 20 may be reduced. Note that the glass that forms the core 11 and the cladding 20 is generally covered with a coating, but the resin that forms the coating is generally lighter than glass. Therefore, the multicore fiber 1 of this embodiment can achieve weight reduction. Furthermore, in the multicore fiber 1 of this embodiment, the outer peripheral surface of the cladding 20 includes other portions of the outer peripheral surfaces of at least two cladding portions 12. Therefore, compared to a case in which the outer peripheral surface of the cladding 20 does not include other portions of the outer peripheral surfaces of at least two cladding portions 12, it is possible to make the thickness of the cladding 20 the minimum necessary in these other portions, and it is possible to realize a multicore fiber 1 that can more effectively contribute to weight reduction.

Furthermore, with this multi-core fiber 1, the cladding portions 12 are integrated together, so when cutting the multi-core fiber 1, the cleavage force is easily propagated from the point where the cutting blade is inserted to each single-core fiber 10 via the joint 21, making it possible to make the cut surface closer to flat.

In addition, in the multicore fiber 1 of this embodiment, the outer peripheral surface of the cladding 20 at the joint 21 is recessed in an arc shape toward the center of the joint 21. In this way, when an inner coating layer 30 is provided on the outer periphery of the cladding 20, the outer peripheral surface of the cladding 20 at the joint 21 is recessed in an arc shape, which can suppress the initiation of peeling between the cladding 20 and the inner coating layer 30 and prevent peeling of the inner coating layer 30 from the cladding 20. This can suppress non-uniform stress caused by peeling of the inner coating layer 30, suppress microbending, and reduce transmission loss of light propagating through the cores 11.

Furthermore, according to the multi-core fiber 1 of this embodiment, the cladding 20 has polygonal gaps G surrounded by the outer peripheral surfaces of the cladding portions 12 of the single-core fibers 10. With this configuration, the weight of the multi-core fiber can be reduced compared to when the gaps G do not exist and the gaps G are filled with, for example, glass. Furthermore, the presence of such gaps G can suppress crosstalk between cores.

Furthermore, the multi-core fiber 1 of this embodiment has a configuration in which the centers of the cores 11 of the three single-core fibers 10 are arranged on the vertices of an equilateral triangle, as described above. Therefore, when fusion-splicing the multi-core fibers 1 together, for example, by arranging the ends of a pair of multi-core fibers 1 facing each other in a V-shaped groove having two non-parallel surfaces, one surface contacting both of a pair of single-core fibers 10 and the other surface contacting both of the other pair of single-core fibers 10, the positions of the cores 11 of the multi-core fiber 1 on one side can be made to roughly coincide with the positions of the cores 11 of the multi-core fiber 1 on the other side. Therefore, with this multi-core fiber 1, the process of aligning the multi-core fiber 1 on one side with the multi-core fiber 1 on the other side can be simplified when fusion-splicing the multi-core fibers 1 together.

Next, we will explain the manufacturing method of the multi-core fiber 1.

Figure 3 shows the steps of the method for manufacturing the multicore fiber 1. As shown in Figure 3, the method for manufacturing the multicore fiber 1 includes a bundling process SP1, a pre-processing process SP2, and a drawing process SP3.

(Bundling process SP1)
[0033] Fig. 4 is a diagram showing roughly the first half of this step. In this step, first, as shown in Fig. 4, a plurality of single-core fiber rods 10R are prepared, each including a core glass body 11R arranged at the center and serving as a core, and a clad glass body 12R surrounding the core glass body 11R and serving as a clad portion 12. In this embodiment, as shown in Fig. 1, the number of cores 11 is three, and therefore three single-core fiber rods 10R are prepared. In addition, in this embodiment, each single-core fiber rod 10R has the same size and configuration. Note that in this embodiment, each single-core fiber 10 has an inner clad layer 13 and a trench layer 14, and therefore each single-core fiber rod 10R has an inner clad glass body 13R surrounding the core glass body 11R and serving as the inner clad layer 13, and a trench glass body 14R surrounding the inner clad glass body 13R and serving as the trench layer 14.

Next, as shown in FIG. 4, multiple single-core fiber rods 10R are arranged in a position where they will be bundled. In this embodiment, the three single-core fiber rods 10R are arranged so that the centers of each are located on the vertices of an equilateral triangle. Next, the arranged single-core fiber rods 10R are bundled together, for example, with a cable tie 51. In this way, the single-core fiber rods 10R are bundled together in a bundled state. In this embodiment, the three single-core fiber rods 10R are bundled together so that adjacent single-core fiber rods 10R are in contact with each other. Note that, unlike the example in FIG. 4, the single-core fiber rods 10R do not necessarily have to be in contact with each other when they are bundled together.

Figure 5 shows the process roughly in the latter half of this process and the pretreatment step SP2. As shown in Figure 5, dummy glass rods 52 are fixed to both ends of the bundled single-core fiber rods 10R to form a multi-core fiber preform 1P. By fixing such dummy glass rods 52, the bundled state of the single-core fiber rods 10R is maintained even when a fixing jig such as a cable tie 51 is removed, and misalignment between the single-core fiber rods 10R is suppressed. Note that the dummy glass rods 52 are preferably fixed by welding. Fixation by welding can effectively suppress the adhesion of impurities to the single-core fiber rods 10R. Alternatively, the dummy glass rod 52 may be fixed to only one end of the bundled single-core fiber rods 10R. Alternatively, multiple single-core fiber rods 10R may be bundled without using a cable tie 51. In this case, for example, the single-core fiber rods 10R may be fixed one by one to the dummy glass rod 52, and multiple single-core fiber rods 10R may be fixed to the dummy glass rods 52, resulting in multiple single-core fiber rods 10R being bundled together.

(Pretreatment step SP2)
Next, this step is performed. Fig. 6 is a side view of the multicore fiber preform after this step. In this step, after removing the binding band 51, one dummy glass rod 52 is pulled in the opposite direction from the other dummy glass rod 52 while heating a portion of the side of the bundled single-core fiber rods 10R, as shown in Fig. 5. As a result, as shown in Fig. 6, one dummy glass rod 52 and a portion of the single-core fiber rod 10R on the dummy glass rod 52 side are fused and removed, and the vicinity of the fused portion of the multicore fiber preform 1P has a tapered shape. Fig. 7 is a view of the multicore fiber preform 1P shown in Fig. 6 as seen from the fused tip side. As shown in Fig. 7, by being fused as described above, at the tip portion 1PF of the multicore fiber preform 1P, the outer peripheral surfaces of the clad glass bodies 12R of adjacent single-core fiber rods 10R are fused to each other, forming three joints 21R. The multi-core fiber preform 1P is drawn from this tapered side. Therefore, in this embodiment, the outer peripheral surfaces of the clad glass bodies 12R are fused together, including the end portion where drawing of the single-core fiber rod 10R starts.

In the above, an example has been described in which the joint 21R is formed only at the tip 1PF of the multicore fiber preform 1P. However, the joint 21R may be formed over the entire longitudinal length of the multicore fiber preform 1P by heating the multicore fiber preform 1P over the entire longitudinal length.

In addition, in this embodiment, an example in which one of the dummy glass rods 52 is removed as described above has been described, but it is also possible to heat only that one of the dummy glass rods 52 and make it conical. In this way, a multicore fiber preform 1P may be formed in which a conical dummy glass rod 52 is fixed on one side and a cylindrical dummy glass rod 52 is fixed on the other side.

(Drawing process SP3)
Next, the main step is performed. FIG. 8 is a diagram showing the state of the main step. As shown in FIG. 8, first, as a preparatory stage for performing the main step, the multicore fiber preform 1P that has undergone the pretreatment step SP2 is placed in the spinning furnace 110 so that the tip portion 1PF of the multicore fiber preform 1P is vertically downward. Then, the heating unit 111 of the spinning furnace 110 is heated, and the multicore fiber preform 1P is inserted into the heating unit 111 from the tip portion 1PF side and heated. At this time, the portion of the multicore fiber preform 1P located in the heating unit 111 is heated to, for example, 1900°C to 2300°C and becomes a molten state. In the main step, the multicore fiber preform 1P is drawn in this molten state so that parts of the outer circumferential surfaces of the clad glass body 12R are in contact with each other over a width smaller than the diameter of the clad glass body 12R. By drawing at such a temperature, adjacent clad glass bodies 12R are melted and integrated together through their surface tension, even in areas other than the tip portion 1PF, so that three joints 21R are formed along the longitudinal direction of the multicore fiber preform 1P.

The portion of the multicore fiber preform 1P that emerges from the spinning furnace 110 immediately solidifies. Thus, in each single-core fiber rod 10R, the core glass body 11R becomes the core element described above, and the clad glass body 12R becomes the clad portion 12. As a result, each single-core fiber rod 10R becomes three single-core fibers 10 having outer surfaces that tend to be at a constant distance from the center of the core 11. The molten joint portion 21R also immediately solidifies, becoming three joint portions 21. In this way, the bare multicore fiber wire shown in FIG. 1 is formed. Then, as shown in FIG. 8, this bare multicore fiber wire 1N passes through a cooling device 120 and is cooled to an appropriate temperature. For example, it is cooled to 40°C to 50°C.

Next, the bare multicore fiber wire 1N passes through a first coating device 131 containing a first thermosetting resin that will become the inner coating layer 30, and the cladding 20 is coated with the first thermosetting resin. The bare multicore fiber wire 1N coated with the first thermosetting resin passes through a first heating furnace 132 and is heated within the first heating furnace 132. This heating causes crosslinking of the material that forms the first thermosetting resin, hardening the first thermosetting resin and forming the inner coating layer 30.

When the inner coating layer 30 is formed from an ultraviolet-curable resin, the cladding 20 is coated using a first coating device 131 containing a first ultraviolet-curable resin, and then the resin is cured.

Next, the bare multicore fiber wire 1N coated with the inner coating layer 30 passes through a second coating device 133 containing a second thermosetting resin that will become the outer coating layer 40, and the inner coating layer 30 is coated with the second thermosetting resin. The bare multicore fiber wire 1N coated with the second thermosetting resin passes through a second heating furnace 134 and is heated within the second heating furnace 134. This heating causes crosslinking of the material that forms the second thermosetting resin, hardening the second thermosetting resin and forming the outer coating layer 40.

If the inner coating layer 30 is formed from an ultraviolet-curable resin, the cladding 20 is coated using a second coating device 133 containing a second ultraviolet-curable resin, and then the resin is cured.

In this way, the multi-core fiber 1 shown in Figure 1 is formed.

The multi-core fiber 1 is then changed in direction by the turn pulley 141 and wound up by the reel 142.

According to the manufacturing method of the multi-core fiber 1 of this embodiment, portions of the outer circumferential surfaces of the clad glass bodies 12R that are in contact in a molten state can form a joint where portions of the outer circumferential surfaces of the clad portions 12 in each single-core fiber 10 are joined together over a width smaller than the diameter of the clad portions 12. Furthermore, other portions of the outer circumferential surfaces of the clad glass bodies 12R are portions of the outer circumferential surfaces of the clad portions 12 in each single-core fiber 10 that tend to be at a constant distance from the center of the core 11. Therefore, according to this manufacturing method of a multi-core fiber, the above-mentioned multi-core fiber 1 can be manufactured.

Furthermore, with this method for manufacturing a multi-core fiber, a multi-core fiber can be manufactured simply by drawing a plurality of single-core fiber rods 10R while they are bundled together, which eliminates the need for processes such as drilling holes in the glass member that will become the cladding and inserting the glass member that will become the core into the hole. This makes it possible to reduce the number of processes and manufacturing costs.

In addition, in this multi-core fiber manufacturing method, the single-core fiber rods are assembled during the bundling process SP1 so that adjacent single-core fiber rods are in contact with each other. This makes it easier for the clad glass bodies to be fused together than when the single-core fiber rods are close to each other but not in contact with each other. This makes it possible to form joints more effectively.

Furthermore, in this embodiment, in the bundling step SP1, the outer peripheral surfaces of the clad glass bodies 12R of adjacent single-core fiber rods 10R are fused together. This makes it possible to prevent misalignment between adjacent single-core fiber rods 10R. In particular, in this embodiment, the outer peripheral surfaces of the clad glass bodies 12R are fused together on the side of the single-core fiber rod 10R where drawing begins, which can serve as a trigger for bringing portions of the outer peripheral surfaces of the clad glass bodies 12R into contact with each other in the drawing step SP3.

Second Embodiment
Next, a second embodiment will be described. Components that are the same as or equivalent to those in the first embodiment will be given the same reference numerals and will not be described again unless otherwise specified.

Figure 9 is a view showing the multi-core fiber 1 of this embodiment from the same perspective as Figure 1. As shown in Figure 9, the multi-core fiber 1 of this embodiment differs mainly from the multi-core fiber 1 of the first embodiment in that it has four single-core fibers 10. In this embodiment, the four single-core fibers 10 are arranged so that the centers of the respective cores 11 are located on the vertices of a square.

The multicore fiber 1 of this embodiment has four joints 21. At each joint 21, parts of the outer circumferential surfaces of the cladding portions 12 of a pair of adjacent single-core fibers 10 are integrated over a width smaller than the diameter of the cladding portions 12. Therefore, the cladding 20 of the multicore fiber 1 of this embodiment is made up of the cladding portions 12 of each single-core fiber 10. The outer circumferential surface of the cladding 20, excluding the joints 21 on the outer circumferential surface of each cladding portion 12, is a surface at a constant distance from the center of the core 11.

In this embodiment, the other portions of a pair of adjacent single-core fibers 10 are also spaced apart from each other while drawing an arc from the joint 21, forming a constriction in the clad 20. Furthermore, the clad 20 contains a polygonal gap G surrounded by the outer peripheral surfaces of the four single-core fibers 10, and in this embodiment, a square-shaped gap G exists.

With this configuration, as in the first embodiment, the cladding portions 12 of the single-core fibers 10 are integrated, thereby suppressing misalignment between the cores 11. Furthermore, since the cladding 20 of this multi-core fiber 1 has a constriction similar to that of the first embodiment, the amount of glass used as the cladding can be reduced, resulting in a lighter weight. Furthermore, the presence of the voids G as described above allows for even lighter weight.

(Third embodiment)
Next, a third embodiment will be described. Components that are the same as or equivalent to those in the first embodiment will be given the same reference numerals, and duplicated descriptions will be omitted unless otherwise specified.

Figure 10 is a view showing the multi-core fiber 1 of this embodiment from the same perspective as Figure 1. As shown in Figure 10, the multi-core fiber 1 of this embodiment differs from the multi-core fiber 1 of the first embodiment mainly in that it has six single-core fibers 10. In this embodiment, the six single-core fibers 10 are arranged so that the centers of their respective cores 11 are located on the vertices of a regular hexagon. Furthermore, in the multi-core fiber 1 of this embodiment, one dummy fiber 90 is arranged in the space surrounded by the six single-core fibers 10. This dummy fiber 90 does not have a core or a trench layer, and is made of the same material as the cladding portion 12. Furthermore, this dummy fiber 90 may have a portion with a lower refractive index than the cladding portion 12. In this case, crosstalk to cores located diagonally opposite the dummy fiber 90 can be suppressed. In having such a dummy fiber 90, the multi-core fiber 1 of this embodiment differs from the multi-core fiber 1 of the first embodiment, which does not have a dummy fiber.

The multi-core fiber 1 of this embodiment has six joints 21 and six joints 22. At each joint 21, parts of the outer circumferential surfaces of the cladding portions 12 of a pair of adjacent single-core fibers 10 are integrated over a width smaller than the diameter of the cladding portions 12. At each joint 22, parts of the outer circumferential surfaces of the single-core fibers 10 and the dummy fibers 90 are integrated over a width smaller than the diameter of the cladding portions 12 and the diameter of the dummy fibers. Therefore, the cladding 20 of the multi-core fiber 1 of this embodiment is made up of the cladding portions 12 of the single-core fibers 10 and the dummy fibers 90.

The outer peripheral surface of the cladding 20 of the multicore fiber 1 includes a portion of the outer peripheral surface of each cladding portion 12 excluding the joints 21, 22, and this other portion is a surface at a constant distance from the core 11. In this embodiment, the other portions of each of a pair of adjacent single-core fibers 10 are also spaced apart from each other while drawing an arc from the joint 21, forming a constriction in the cladding 20. Furthermore, six gaps G exist in the cladding 20 of this embodiment. Each gap G is surrounded by the outer peripheral surface of each of the pair of adjacent single-core fibers 10 and the outer peripheral surface of the dummy fiber 90.

The multi-core fiber 1 of this embodiment, which has such a dummy fiber 90, can be manufactured by arranging six single-core fiber rods 10R in a regular hexagonal shape around the dummy fiber rod that will become the dummy fiber 90 in the bundling process SP1 and bundling them in the same manner as in the first embodiment. Then, each of the bundled single-core fiber rods can be drawn together with the dummy fiber rod in the same manner as in the drawing process SP3.

According to the configuration of the multicore fiber 1 of this embodiment, as in the first embodiment, the cladding portions 12 of the single-core fibers 10 are integrated, thereby suppressing misalignment between the cores 11. Furthermore, because the cladding 20 of this multicore fiber 1 has a constriction similar to that of the first embodiment, the amount of glass used as the cladding can be reduced, resulting in a lighter weight. Furthermore, the presence of the voids G as described above allows for even lighter weight.

Furthermore, in the multi-core fiber 1 of this embodiment, a dummy fiber 90 is placed in a space surrounded by six single-core fibers 10, and a joint 22 with this dummy fiber 90 is present, resulting in a greater number of joints than when the dummy fiber 90 is not present. Therefore, when cutting the multi-core fiber 1, the cleavage force is more likely to propagate to the multi-core fiber 1 starting from the part where the cutting blade is inserted, making it possible to make the cut surface flatter. Furthermore, this multi-core fiber 1 is stronger and less likely to break than when no dummy fiber is present.

(Fourth embodiment)
Next, a fourth embodiment will be described. Components that are the same as or equivalent to those in the first embodiment will be given the same reference numerals, and duplicated descriptions will be omitted unless otherwise specified.

Figure 11 is a view showing the multi-core fiber 1 of this embodiment from the same perspective as Figure 1. The multi-core fiber 1 of this embodiment has one dummy fiber 91 and three dummy fibers 92. The dummy fibers 91 and 92 do not have a core or trench layer, are made of the same material as the cladding portion 12, and have a diameter smaller than the diameter of each single-core fiber 10.

The dummy fiber 91 is disposed in a space surrounded by three single-core fibers 10. A portion of the outer surface of the dummy fiber 91 is integrated with a portion of the outer surface of the cladding portion 12 of each single-core fiber 10 at each joint 22, over a width smaller than the diameter of the cladding portion 12 and the diameter of the dummy fiber 91. In this way, three gaps G1 are formed in the cladding 20 of the multi-core fiber 1, surrounded by a portion of the outer surface of the cladding portion 12 of each single-core fiber 10 and a portion of the outer surface of the dummy fiber 91. The gaps G1 are not filled with resin, glass, or the like.

Each dummy fiber 92 is disposed in a space surrounded by the outer circumferential surface of each single-core fiber 10 and a circumscribed triangle LA, indicated by a dashed line, that tangents to the outer circumferential surfaces of the three single-core fibers 10. At each joint 23, a portion of the outer circumferential surface of the dummy fiber 92 is integrated with a portion of the outer circumferential surface of each cladding portion 12 of a pair of adjacent single-core fibers 10 over a width smaller than the diameter of the cladding portion 12 and the diameter of the dummy fiber 92. In this way, three gaps G2 are formed in the cladding 20 of the multi-core fiber 1, surrounded by a portion of the outer circumferential surface of each pair of adjacent single-core fibers 10 and a portion of the outer circumferential surface of the dummy fiber 92. The gaps G2 are not filled with resin, glass, or the like.

The cladding 20 of the multicore fiber 1 of this embodiment consists of the cladding portions 12 of each single-core fiber 10 and the dummy fibers 91, 92. The outer peripheral surface of this cladding 20 includes a portion of the outer peripheral surface of each cladding portion 12 excluding the joints 21, 22, 23. This other portion is a surface at a constant distance from the core 11. Furthermore, the other portions of each of a pair of adjacent single-core fibers 10 are spaced apart from each other while drawing an arc from the joint 23. Therefore, a constriction is formed in the cladding 20.

The multi-core fiber 1 of this embodiment, which has such dummy fibers 91, 92, can be manufactured by placing dummy fiber rods that will become the dummy fibers 91, 92 between the single-core fiber rods 10R bundled into an equilateral triangle in the bundling process SP1, bundling each single-core fiber rod with each dummy fiber rod, and drawing each single-core fiber rod together with each dummy fiber rod as in the drawing process SP3.

With this configuration, as in the first embodiment, the cladding portions 12 of the single-core fibers are integrated, thereby suppressing misalignment between the cores 11. Furthermore, because the cladding 20 of this multi-core fiber 1 has a constriction similar to that in the first embodiment, the amount of glass used as the cladding can be reduced, resulting in a lighter weight.

Furthermore, in the multi-core fiber 1 of this embodiment, dummy fibers 91, 92 are arranged, and splices 22, 23 with the dummy fibers 91, 92 are present, resulting in a greater number of splices than when the dummy fiber 90 is not present. Therefore, when cutting the multi-core fiber 1, the cleavage force is more likely to propagate to the multi-core fiber 1 starting from the portion where the cutting blade is inserted, making it possible to make the cut surface flatter. Furthermore, this multi-core fiber 1 is stronger and less likely to break than when no dummy fibers are present.

The present invention has been described above using the above embodiment as an example, but the present invention is not limited to this.

For example, in the above embodiment, an example was described in which the cladding of the multicore fiber has a gap surrounded by the claddings of three or more single-core fibers or dummy fibers. However, the presence of such a gap is not essential. Figure 12 is a view showing a modified example of a multicore fiber from the same perspective as Figure 1. In this modified example, components that are the same as or equivalent to those in the first embodiment are assigned the same reference numerals and redundant explanations will be omitted unless otherwise specified.

As shown in FIG. 12 , the multicore fiber 1 according to this modified example has three single-core fibers 10 arranged in parallel. In this modified example, at each joint 21, parts of the outer circumferential surfaces of the cladding portions 12 of a pair of adjacent single-core fibers 10 are integrated over a width smaller than the diameter of the cladding portions 12, forming the joint 21. Therefore, the cladding 20 of this multicore fiber 1 is made up of the cladding portions 12 of multiple single-core fibers 10. The outer circumferential surface of the cladding 20 of the multicore fiber 1 includes other parts of the outer circumferential surface of each cladding portion 12 excluding the joint 21. These other parts are surfaces at a constant distance from the center of the core 11. The other parts of the pair of adjacent single-core fibers 10 are spaced apart from each other while drawing an arc from the joint 21, forming a constriction in the cladding 20.

With this configuration, as in the first embodiment, the cladding portions of the single-core fibers are integrated, thereby suppressing misalignment between cores. Furthermore, since the cladding 20 of this multi-core fiber 1 has a constriction similar to that of the first embodiment, the amount of glass used as the cladding can be reduced, resulting in a lighter weight. Furthermore, according to this modified example, three single-core fibers 10 are arranged in parallel, so the fiber can be used as a so-called tape fiber. Note that, although this modified example describes an example in which the multi-core fiber 1 is constructed from three single-core fibers, a multi-core fiber may also be constructed by arranging two single-core fibers in parallel, or a multi-core fiber may also be constructed by arranging four or more single-core fibers in parallel.

In addition, in the above embodiment, an example in which the core 11 is circular has been described, but the core 11 may also be elliptical, as shown in FIG. 13, for example. FIG. 13 is a view showing another modified example of such a multicore fiber from the same perspective as FIG. 1. Note that the inner cladding layer 13 and trench layer 14 are not shown in FIG. 13. The modified example shown in FIG. 13 has the same configuration as the multicore fiber 1 of the first embodiment, except that the core 11 is elliptical.

Such an elliptical core 11 can be formed, for example, during the drawing step SP3. For example, the multicore fiber 1 of the first embodiment is manufactured by drawing three single-core fiber rods 10R in a bundle with their centers positioned at the vertices of an equilateral triangle, as shown in FIG. 3 . During this drawing, the single-core fiber rod 10R is subjected to surface tension from both of the other two single-core fiber rods 10R that are fused together. This surface tension is a force that pulls the core glass body 11R toward the joint 21R. In this example, surface tension acts on one core glass body 11R in the directions of the two joints 21R. As a result of this surface tension being combined, at least one of the multiple cores 11 may be deformed into an elliptical shape, as shown in FIG. 13 . Deforming the core 11 into an elliptical shape in this manner can be achieved by adjusting the temperature of the spinning furnace 110 in the drawing step SP3 to adjust the magnitude of the surface tension. If the core 11 has such an elliptical shape, the polarization mode of the light propagating through the core 11 can be maintained.

In the above embodiment, an example was described in which the outer peripheral surface of the cladding 20 at the joint 21 is concave in an arc shape. However, the outer peripheral surface of the cladding 20 at the joint 21 does not have to be concave in an arc shape. Figure 14 is a view showing yet another modified example of such a multicore fiber 1, taken from the same perspective as Figure 2. As shown in Figure 14, in this example, there is a portion where the width W of the joint 21 is constant. In other words, in this example, the width of the joint 21 is constant when the joint 21 is moved along the line connecting adjacent cores 11. Therefore, at least a portion of the outer peripheral surface of the cladding 20 at the joint 21 is flat. Having such a shape for the outer peripheral surface of the cladding 20 at the joint 21 can make it easier to peel off the inner coating layer 30.

In the above embodiment, an example has been described in which the joint 21 is provided over the entire longitudinal direction of the multicore fiber 1. However, the joint 21 may be provided only at a portion of the longitudinal direction of the multicore fiber 1. In this case, the weight of the multicore fiber 1 can be reduced and misalignment between cores can be suppressed at that portion.

In the above embodiment, the joint 21 is described as an example in which at least two cladding portions 12 are joined together over a width smaller than the diameter of the cladding portions 12. However, the joint 21 may be configured as at least two cladding portions 12 are joined together over a width larger than the diameter of the cladding portions 12. In this case, misalignment between cores in the multicore fiber 1 can be suppressed at the location where the joint 21 is directed. Furthermore, even if an external force is applied to the cladding 20, the joint 21, which is joined together over a width larger than the diameter of the cladding portions 12, can protect the cladding 20, thereby reducing damage to the cladding portions 12. Furthermore, because the cross-sectional area of the joint 21 is large, when the multicore fiber 1 is cut, the cleavage force can be more easily propagated from the portion where the cutting blade is inserted to each single-core fiber 10 via the joint 21, which has a large cross-sectional area, and the cut surface can be made even flatter.

Furthermore, in the above embodiment, when three or more single-core fibers 10 are provided, the joint 21 is not limited to being formed by integrating portions of the outer circumferential surfaces of the respective cladding portions 12, but may be formed by integrating portions of the outer circumferential surfaces of at least two cladding portions 12. In this case, the outer circumferential surface of the cladding 20 includes other portions of the outer circumferential surfaces of the at least two cladding portions 12.

Furthermore, in the above embodiment, the light propagating through at least two cores 11 of the multicore fiber 1 may have a region in which the modes are not coupled to each other, or the light propagating through at least two cores 11 of the multicore fiber 1 may have a region in which the modes are coupled to each other.

Furthermore, in the above embodiment, an example was described in which a trench layer was included in the cladding portion of the single-core fiber, but such a trench layer is not a required configuration.

The present invention provides a multi-core fiber that can be lightweight and suppress misalignment between cores, as well as a method for manufacturing such a multi-core fiber, which can be used in fields such as communications.

1... multi-core fiber 10... single-core fiber 10R... single-core fiber rod 11... core 12... cladding portion 14... trench layer 20... cladding 21... joint portion G... gap

Claims (7)

Multiple cores and
a cladding surrounding each of the cores;
Equipped with
the clad has a joint portion where parts of outer circumferential surfaces of at least two of the clad portions in a plurality of single-core fibers, each of which has the core disposed at the center and surrounded by a clad portion, are joined together over a width smaller than the diameter of the clad portion;
the outer peripheral surface of the clad includes other portions of the outer peripheral surfaces of the at least two clad portions,
the plurality of single-core fibers is three or more;
the cladding has polygonal gaps surrounded by outer peripheral surfaces of the cladding portions of three or more of the single-core fibers;
the width of at least one portion of the outer peripheral surface of each of the cladding portions that forms the void is equal to or greater than the diameter of the core that is closest to that portion;
The width of the splice is smaller than the diameter of the core of the single-core fiber having the splice.
A multicore fiber characterized by: The multi-core fiber according to claim 1 , wherein an outer peripheral surface of the cladding at at least one of the joints is recessed in an arc shape toward the center of the joint. 3. The multi-core fiber according to claim 1, wherein the multi-core fiber comprises three or more single-core fibers arranged in parallel with each other. 2. The multi-core fiber according to claim 1, wherein at least one of the portions of the outer peripheral surface of each of the cladding portions that form the void is curved so as to surround the core that is closest to that portion. At least one of the single-core fibers has a trench layer that has a refractive index lower than that of the cladding portion of the single-core fiber and surrounds the core of the single-core fiber;
5. The multi-core fiber according to claim 1, wherein the width of the joint in the single-core fiber is smaller than the diameter of the trench layer. 6. The multicore fiber according to claim 1, wherein at least one of the cores is elliptical. a bundling step of bundling rods for single-core fibers, each rod including a core glass body disposed at the center and a clad glass body surrounding the core glass body;
a pretreatment step of welding the outer peripheral surfaces of adjacent single-core fiber rods together in a place other than a spinning furnace;
a drawing step of drawing the bundled rods for single-core fiber so that parts of the outer peripheral surfaces of the clad glass bodies in a molten state are in contact with each other over a width smaller than the diameter of the clad glass bodies;
Equipped with
In a preparation stage for performing the drawing step, a multi-core fiber preform in which the outer peripheral surfaces of adjacent single-core fiber rods are welded to each other is placed in the spinning furnace.
A method for manufacturing a multicore fiber.