JP7809557B2 - Manufacturing method of multi-core fiber preform and manufacturing method of multi-core fiber - Google Patents

Description

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

Various methods are being actively investigated as methods for manufacturing multicore fiber preforms for producing multicore fibers with multiple cores. Among these, the flame deposition method is the most attractive, as it is a low-cost technology that has been established as a conventional method for manufacturing optical fibers. However, because multicore fibers have multiple cores, when the flame deposition method is applied to multicore fibers, there are cases where the soot interferes with the cores when attempting to deposit it. In such cases, there have been many reports of problems such as soot not being deposited uniformly around specific cores, leaving voids.

As a technique for eliminating voids, Patent Document 1 discloses a technique that uses a polygonal core preform. Furthermore, Patent Document 2 discloses a technique for forming a first cladding near the central rod and a second cladding outside of that when fabricating the cladding portion of a multicore fiber.

Japanese Patent Application Publication No. 9-5541 Patent No. 5740065

However, the technology in Patent Document 1 requires the preparation of a polygonal core base material, making the manufacturing process cumbersome. On the other hand, the technology in Patent Document 2 does not require such a cumbersome manufacturing process, but there is room for improvement in terms of suppressing the formation of voids.

The present invention has been made in light of the above, and its purpose is to provide a method for manufacturing a multicore fiber preform and a method for manufacturing a multicore fiber that use simple processes to suppress the formation of voids.

In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention includes a first deposition process in which a plurality of second glass rods having a core portion and a clad portion are arranged so as not to contact each other but to contact and surround a first glass rod, and soot is deposited between the surface of the first glass rod and the second glass rod using a soot deposition burner; a second deposition process in which further soot is deposited using the soot deposition burner on the soot deposited in the first deposition process to embed the second glass rod; and after the second deposition process, a second deposition process in which further soot is deposited using the soot deposition burner on the plurality of second glass rods. and a third deposition step of depositing soot so that the soot surrounds the second glass rod, wherein in the first deposition step, the soot deposition burner oscillates relatively around the first glass rod while gradually decreasing the oscillation angle depending on the width between adjacent second glass rods, or does not oscillate relatively, and in the second deposition step, the soot deposition burner oscillates relatively around the first glass rod while gradually increasing the oscillation angle depending on the width between adjacent second glass rods.

Furthermore, the soot deposition burner may be swung so that its orientation deviates from the central axis of the first glass rod, and soot may be deposited between the first glass rod and the second glass rod.

In the first deposition process and the second deposition process, the soot deposition burners may be arranged in the same number as the plurality of second glass rods.

One aspect of the present invention is a method for manufacturing a multicore fiber, which includes a drawing step of drawing a multicore fiber preform manufactured by the above-described manufacturing method.

This invention has the effect of suppressing the formation of voids through a simple process.

FIG. 1 is a schematic cross-sectional view of a multi-core fiber to be manufactured, taken along a plane perpendicular to the longitudinal direction thereof. FIG. 2 is a flow diagram of a method for manufacturing a multicore fiber. FIG. 3 is a diagram showing a method for manufacturing a multi-core fiber preform. FIG. 4 is a diagram showing a method for manufacturing a multi-core fiber preform. FIG. 5 is a diagram showing a method for manufacturing a multi-core fiber preform. FIG. 6 is a diagram showing a method for manufacturing a multi-core fiber preform. FIG. 7 is a diagram illustrating the swing of the burner. FIG. 8 is a diagram illustrating the initial stage of soot deposition. FIG. 9 is a diagram showing an example of the relationship between the thickness of the deposited soot and the oscillation angles θ1 and θ2.

Embodiments of the present invention will be described in detail below with reference to the drawings. Note that the present invention is not limited to the embodiments described below. Furthermore, in each drawing, identical or corresponding components will be appropriately designated by the same reference numerals, and duplicate descriptions will be omitted where appropriate.

(Embodiment)
1 is a schematic cross-sectional view of a multicore fiber 10 in a plane perpendicular to the longitudinal direction thereof, the multicore fiber 10 being made of silica-based glass and including a first glass body 1, four second glass bodies 2, four inner cladding portions 3, and an outer cladding portion 4.

The first glass body 1 has a substantially circular cross section perpendicular to the longitudinal direction and is made of silica-based glass with a predetermined refractive index. The outer diameter of the first glass body 1 is, for example, 26 μm.

The four second glass bodies 2 are arranged so as to surround the first glass body 1 and to be in contact with the first glass body 1. The four second glass bodies 2 are also arranged so as not to be in contact with each other.

Each second glass body 2 has a substantially circular cross section perpendicular to the longitudinal direction, and includes a core portion 2a and a cladding portion 2b that has a refractive index lower than the maximum refractive index of the core portion 2a and surrounds the outer periphery of the core portion 2a. The core diameter of the core portion 2a is, for example, 9 μm, and the cladding diameter of the cladding portion 2b is, for example, 32 μm.

The core pitch P, which is the distance between the centers of the core portions 2a in adjacent second glass bodies 2, is, for example, 40 μm.

The refractive index of the first glass body 1 is equal to or greater than the refractive index of the cladding portion 2b. In other words, the refractive index of the portion of the first glass body 1 that is in contact with the cladding portion 2b is equal to or greater than the refractive index of the cladding portion 2b.

The four inner cladding portions 3 are arranged to fill the spaces between adjacent second glass bodies 2. Each inner cladding portion 3 is interposed between the core portions 2a of adjacent second glass bodies 2, and its cross section perpendicular to the longitudinal direction has a shape surrounded by four arcs. The inner cladding portions 3 are made of, for example, silica-based glass with the same refractive index as the first glass body 1. The inner cladding portions 3 may also be made of silica-based glass with a lower refractive index than the cladding portions 2b in the second glass body 2.

If the width of the inner cladding portion 3 is defined as the length of the inner cladding portion 3 in a direction perpendicular to the radial direction of the multicore fiber 10, the width of the inner cladding portion 3 first gradually narrows as one moves away from the center in the radial direction of the multicore fiber 10, and then widens as one moves further away. The minimum width W of the inner cladding portion 3 is the width of the inner cladding portion 3 on a line connecting the centers of the core portions 2a of adjacent second glass bodies 2. The minimum width W is, for example, approximately 14 μm.

The outer cladding portion 4 surrounds the four second glass bodies 2 and the four inner cladding portions 3. The outer cladding portion 4 also contacts the four second glass bodies 2 and the four inner cladding portions 3. The outer cladding portion 4 is made of silica-based glass whose refractive index is equal to or greater than that of the cladding portion 2b.

Examples of materials for the first glass body 1, second glass body 2, inner cladding 3, and outer cladding 4 are given below. The materials for the first glass body 1, second glass body 2, inner cladding 3, and outer cladding 4 are not particularly limited as long as they satisfy the refractive index relationship described above. For example, the first glass body 1 may be made of pure silica glass or silica-based glass that contains substantially no dopants other than chlorine. Here, chlorine is a dopant added during the manufacturing process of the multi-core fiber 10 and is not intentionally added. Furthermore, pure silica glass is extremely high-purity silica glass with a refractive index of approximately 1.444 at a wavelength of 1550 nm. The first glass body 1 may also be made of silica-based glass containing fluorine.

Furthermore, for example, the core portion 2a of the second glass body 2 is made of silica-based glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium. Germanium is a dopant that increases the refractive index of silica-based glass. Fluorine is a dopant that decreases the refractive index of silica-based glass. Chlorine, potassium, and sodium are dopants that increase the refractive index and decrease the viscosity of silica-based glass. Furthermore, for example, the cladding portion 2b and outer cladding portion 4 of the second glass body 2 are made of pure silica glass, silica-based glass that contains substantially no dopants other than chlorine, or silica-based glass that contains fluorine.

(Manufacturing method)
Next, a method for manufacturing the multicore fiber according to the embodiment will be described. As shown in the flow chart of Fig. 2, the method for manufacturing the multicore fiber 10 includes a first deposition step in step S101, a second deposition step in step S102, a third deposition step in step S103, and a drawing step in step S104.

In the first deposition process of step S101, as shown in FIG. 3, four second glass rods 12, which are the base material of the second glass body 2, are placed in contact with and surrounding the first glass rod 11, which is the base material of the first glass body 1. Both ends of these first and second glass rods 11 and 12 are supported to fix their relative positions. The second glass rod 12 has a core portion 12a, which is the base material of the core portion 2a, and a clad portion 12b, which is the base material of the clad portion 2b. The second glass rods 12 are fabricated, for example, by the VAD (Vapor-Phase Axial Deposition) method.

As shown in Fig. 3, four burners B, which are soot deposition burners, are arranged between adjacent second glass rods 12 so as to face the central axis O1 of the first glass rod 11. The number of these four burners B corresponds to the number of second glass rods 12. The burners B are arranged so that their tubes are aligned along an axis X1. The axis X1 extends radially from the central axis O1 of the first glass rod 11 through the center of the gap between adjacent second glass rods 12. Then, glass raw material gas, combustible gas _H2 gas, and combustion supporting gas _O2 gas are supplied to the burners B and ejected. For example, _SiCl4 gas or the like can be used as the glass raw material gas. As shown in Fig. 4, these burners B flame hydrolyze glass raw material gases in a flame F formed by _H2 gas and _O2 gas to synthesize inner soot 13 consisting of glass particles, and deposit the inner soot 13 on the surface of the first glass rod 11 and between the adjacent second glass rods 12 (flame deposition). The inner soot 13 is the source of the base material for the inner cladding portion 3. Then, by depositing the inner soot 13 while retracting the burners B in the radial direction of the first glass rod 11, the thickness of the deposited inner soot 13 gradually increases, filling the gaps between the second glass rods 12.

As shown in FIG. 3 , in the first deposition step, burner B oscillates around the first glass rod 11 while gradually decreasing the oscillation angle depending on the width between adjacent second glass rods 12, or does not oscillate relatively. The oscillation angle refers to the angle θ1 at which the tube of burner B oscillates around the central axis O1 of the first glass rod 11, centered on axis X1. At the beginning of the deposition of the inner soot 13, the width between adjacent second glass rods 12 is wide, so the oscillation angle θ1 is increased. However, as the inner soot 13 thickens, the width between adjacent second glass rods 12 on the surface of the inner soot 13 gradually narrows, so the oscillation angle θ1 is gradually decreased. This prevents voids from forming in the inner soot 13. Note that if the width between adjacent second glass rods 12 is relatively narrow, burner B does not need to oscillate.

Burner B and the first and second glass rods 11 and 12 can move back and forth relative to each other in the longitudinal direction of the first and second glass rods 11 and 12 (depth direction in the drawing), allowing the inner soot 13 to be deposited uniformly in the longitudinal direction of the first and second glass rods 11 and 12. In this manufacturing example, burner B is moved to deposit the inner soot 13, but burner B may be fixed and the first and second glass rods 11 and 12 may be moved.

When the thickness of the inner soot 13 reaches approximately the radius of the second glass rod 12 and the width between adjacent second glass rods 12 on the surface of the inner soot 13 reaches approximately the minimum width, the second deposition process of step S102 begins. In the second deposition process, further soot is deposited on the inner soot 13 deposited in the first deposition process to embed the second glass rod 12.

In the second deposition process, the burner B remains oriented toward the central axis O1 of the first glass rod 11 and oscillates relatively around the first glass rod 11 while gradually increasing the oscillation angle θ1 according to the width between adjacent second glass rods 12. This prevents voids from forming in the inner soot 13.

The second deposition process is terminated when the inner soot 13 has embedded the second glass rod 12 to a predetermined extent or completely embedded it. The timing of the end of the second deposition process can be determined by a sensor that measures the deposition thickness of the inner soot 13 or by visual inspection. Then, the third deposition process of step S103 is initiated (Figure 5). In the third deposition process, as shown in Figure 5, for example, burner B is rotated in the circumferential direction of the first glass rod 11 to deposit outer soot 14, which will serve as the base material for the outer cladding portion 4 (Figure 6). Alternatively, burner B may be fixed while the first glass rod 11, second glass rod 12, and inner soot 13 are rotated.

After the deposition of the outer soot 14 is complete, the third deposition process is terminated. The inner soot 13 and outer soot 14 are then sintered to form the entire multi-core fiber preform. Note that the inner soot 13 may be sintered after the deposition of the inner soot 13 is complete and before the deposition of the outer soot 14.

Then, the multi-core fiber preform is heated and melted using a known drawing device, and drawing is performed in step S104, thereby producing the multi-core fiber 10.

The manufacturing method described above makes it possible to easily manufacture a multicore fiber preform and a multicore fiber 10. Furthermore, because the formation of voids in the inner soot 13 is suppressed, voids are also suppressed in the multicore fiber 10.

In the multicore fiber 10, if the inner cladding portion 3 is made of silica-based glass with a refractive index lower than that of the cladding portion 2b, a trench-type refractive index profile is achieved in which the inner cladding portion 3 serves as a trench portion. In this case, a trench portion with a low refractive index is interposed between adjacent core portions 2a, thereby suppressing inter-core crosstalk. Furthermore, higher-order mode light propagating through the core portions 2a easily leaks from the first glass body 1 and outer cladding portion 4, which have a refractive index equal to or higher than that of the cladding portion 2b and are in contact with the cladding portion 2b. Therefore, compared to a case in which a trench layer is provided to surround the core portions, the multicore fiber 10 can achieve single-mode characteristics as a single-mode optical fiber at the operating wavelength (e.g., 1550 nm) without precise control of the width or relative refractive index difference of the inner cladding portion 3. Here, the operating wavelength refers to the wavelength of the signal light when the multicore fiber 10 is used as a transmission line.

When the inner cladding portion 3 contains fluorine, fluorine gas may be supplied together with the glass raw material gas during deposition of the inner soot 13, or fluorine may be doped in the gas phase during sintering of the inner soot 13.

(Modification of manufacturing method)
In the above embodiment, the burner B oscillates at an oscillation angle θ1 while facing the central axis O1 of the first glass rod 11, but the burner B may also be oscillated so that its orientation deviates from the central axis O1 of the first glass rod 11.

Figure 7 is a diagram illustrating the oscillation of a burner. In Figure 7, burner B oscillates at an oscillation angle θ2 with point O2 on axis X1 (see also Figure 3) as the fulcrum.

Figure 8 is a diagram illustrating the initial stage of soot deposition in the first deposition step. First, in Figure 8(a), both the oscillation angles θ1 and θ2 are set to zero. In this case, the inner soot 13 is deposited mainly on the surface of the first glass rod 11. Next, from the state shown in Figure 8(a), the burner B is oscillated clockwise as indicated by arrow Ar1 to increase the oscillation angle θ1, and then oscillated clockwise as indicated by arrow Ar2 to increase the oscillation angle θ2, resulting in the state shown in Figure 8(b). Next, by further increasing the oscillation angles θ1 and θ2 from the state shown in Figure 8(b) to the state shown in Figure 8(c), the flame F penetrates into the narrow gap between the first glass rod 11 and the second glass rod 12, and the inner soot 13 is deposited in the gap.

Next, as shown by arrow Ar1 in Figure 8(d), burner B is swung counterclockwise to reduce the swing angle θ1 to zero, and then swung counterclockwise as shown by arrow Ar2 to reduce the swing angle θ2 to zero, resulting in the state shown in Figure 8(e).

Next, as shown by arrow Ar1 in Figure 8(e), burner B is oscillated counterclockwise to increase the oscillation angle θ1, and then oscillated counterclockwise as shown by arrow Ar2 to increase the oscillation angle θ2, thereby reaching the state shown in Figure 8(f) and then the state shown in Figure 8(g). This allows flame F to penetrate into the narrow gap between the first glass rod 11 and the second glass rod 12, depositing inner soot 13 in the gap.

Next, in Figure 8(h), the oscillation angle θ1 is reduced by swinging it clockwise as indicated by arrow Ar1, and the oscillation angle θ2 is reduced by swinging it clockwise as indicated by arrow Ar2, returning both oscillation angles θ1 and θ2 to zero.

As described above, by oscillating burner B at oscillation angles θ1 and θ2, the inner soot 13 can be more reliably deposited in the narrow gap between the first glass rod 11 and the second glass rod 12. This more reliably suppresses the formation of voids in the inner soot 13, and therefore more reliably suppresses voids in the multicore fiber preform and the multicore fiber 10.

Note that while Figure 8 illustrates the case where burner B is oscillated at an oscillation angle θ1, the first glass rod 11 and the second glass rod 12 may also be oscillated at an oscillation angle θ1 relative to burner B. Furthermore, oscillation at an oscillation angle θ1 may not be performed, and only oscillation at an oscillation angle θ2 may be performed. Furthermore, oscillation at an oscillation angle θ2 may also be performed in the second deposition step.

(Example)
As an example, a multicore fiber preform was produced by the manufacturing method described with reference to Figures 3 to 8. At this time, the first glass rod, the cladding portion of the second glass rod, the inner soot, and the outer soot were made of silica-based glass having the same refractive index. The outer diameter of the first glass rod was 32 mm. The cladding diameter of the second glass rod was 26 mm. The inner soot was deposited to a total thickness of 26 mm. The outer soot was deposited to a total thickness of 21.5 mm.

Figure 9 shows an example of the relationship between the soot thickness after deposition and the oscillation angles θ1 and θ2. The horizontal axis shows the total thickness of the inner and outer soot. First, at the start of deposition, θ1 was set to approximately 12° and θ2 to approximately 60°. Thereafter, θ1 and θ2 were decreased as the thickness increased. After that, when the thickness of the inner soot reached 13 mm, as shown by line L1, i.e., half the cladding diameter of the second glass rod, and half of the second glass rod was filled with inner soot, θ1 was then increased. When the thickness reached 26 mm, as shown by line L2, the burner was rotated to produce outer soot (i.e., θ1 = 360°).

After the deposition of the outer soot was completed, sintering was performed to produce a multi-core fiber preform. This confirmed that the formation of voids in the multi-core fiber preform was suppressed.

In the above embodiment or example, the number of second glass bodies is four, but it may be two, three, five or more.

Furthermore, the present invention is not limited to the above-described embodiments. The present invention also includes configurations that appropriately combine the above-described components. Furthermore, 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-described embodiments, and various modifications are possible.

1: First glass body 2: Second glass body 2a, 12a: Core portion 2b, 12b: Cladding portion 3: Inner cladding portion 4: Outer cladding portion 10: Multicore fiber 11: First glass rod 12: Second glass rod 13: Inner soot 14: Outer soot Ar1, Ar2: Arrow B: Burner F: Flame O1: Central axis O2: Point P: Core pitch W: Minimum width X1: Axis

Claims (4)

a first deposition step of depositing soot between a surface of the first glass rod and the second glass rod by a soot deposition burner, in a state in which a plurality of second glass rods each having a core portion and a clad portion are arranged so as not to contact each other but to contact and surround the first glass rod;
a second deposition step of further depositing soot on the soot deposited in the first deposition step using a soot deposition burner to embed the second glass rod;
a third deposition step of depositing soot by a soot deposition burner so as to surround the plurality of second glass rods after the second deposition step;
Equipped with
In the first deposition step, the soot deposition burner is oscillated around the first glass rod with a oscillating angle gradually decreasing as the width between the adjacent second glass rods decreases , or is not oscillated relatively;
In the second deposition step, the soot deposition burner is oscillated relatively around the first glass rod while the oscillation angle gradually increases as the width between adjacent second glass rods increases . The method for manufacturing a multicore fiber preform according to claim 1 , further comprising: swinging the soot deposition burner so that the direction of the soot deposition burner deviates from the central axis of the first glass rod, and depositing soot between the first glass rod and the second glass rod. The method for manufacturing a multi-core fiber preform according to claim 1 or 2, wherein in the first deposition step and the second deposition step, the number of soot deposition burners arranged is equal to the number of the second glass rods. A method for manufacturing a multicore fiber, comprising: a drawing step of drawing a multicore fiber preform manufactured by the method for manufacturing a multicore fiber preform according to claim 1 or 2.