JP7508294B2 - Multicore fiber and its manufacturing method - Google Patents

Description

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

In the fields of datacom and telecom, multicore fibers are known as optical fibers that can realize high-density optical fiber cables. The multicore fibers described in Patent Documents 1 and 2 have a core portion that employs a trench structure to suppress crosstalk between cores.

JP 2012-118495 A JP 2016-075938 A

However, when trying to further suppress inter-core crosstalk in the design of a trench structure, the cutoff wavelength often increases. When the cutoff wavelength of an optical fiber increases, the single-mode characteristics decrease, and there is a risk that the optical fiber will become a multimode optical fiber.

The present invention has been made in view of the above, and its purpose is to provide a multicore fiber and a manufacturing method thereof that simultaneously suppresses inter-core crosstalk and suppresses an increase in the cutoff wavelength.

In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a method for manufacturing a semiconductor device comprising: a central core portion; an intermediate layer surrounding the outer periphery of the central core portion; and a cladding portion surrounding the outer periphery of the intermediate layer, the cladding portion being provided with a cladding layer having a cladding layer and a cladding layer, the cladding layer being provided with a cladding layer having a cladding layer and a cladding layer, the cladding layer being provided with a cladding layer having a cladding layer and a cladding layer having a cladding layer and a cladding layer having a cladding layer and a cladding layer; Δ3 holds, and in a cross section perpendicular to the longitudinal direction, the first thickness of the trench layer on a line connecting two adjacent centers of the multiple central core portions is thicker than the second thickness of the trench layer on the shortest line from one of the two centers toward the outer periphery of the cladding portion, the cutoff wavelength of each of the core portions is 1550 nm or less, and the inter-core crosstalk between two adjacent core portions over a length of 10 m or more in the wavelength band used is -10 dB or less.

The first thickness may be 1.2 times the second thickness.

The first thickness may be 1.5 times the second thickness.

The core-to-core crosstalk between adjacent core sections over a length of 1 km in the wavelength band used may be -10 dB or less.

The core-to-core crosstalk between adjacent core sections over a length of 100 km in the wavelength band used may be -10 dB or less.

The cutoff wavelength may be equal to or less than a wavelength included in the wavelength band used.

The Δ1 may be 0.20% or more and 0.60% or less, and the Δ3 may be -0.7% or more and -0.10% or less.

Each of the multiple cores may have a bending loss of 1.59 dB/m or less at a wavelength of 1550 nm when bent with a diameter of 20 mm.

The multiple core sections may include an inner core section whose outer periphery is surrounded by the multiple core sections, and the inner core section may be designed to have a shorter cutoff wavelength than the core section surrounding the inner core section.

One aspect of the present invention is a method for manufacturing the multicore fiber, which includes preparing a plurality of core preforms each including a portion that will become the plurality of core portions, stacking the plurality of core preforms inside a jacket tube, and drawing the jacket tube with the plurality of core preforms stacked inside.

The multiple core substrates may be formed by the VAD method, the OVD method, the MCVD method, or the plasma CVD method.

According to the present invention, it is possible to suppress both inter-core crosstalk and the increase in cutoff wavelength in a multicore fiber.

FIG. 1 is a schematic cross-sectional view of a multi-core fiber according to a first embodiment. FIG. 2 is a schematic diagram of the refractive index profile of the two core portions and their surroundings of the multicore fiber of FIG. FIG. 3 is an explanatory diagram of a method for manufacturing the multi-core fiber shown in FIG. FIG. 4 is a diagram illustrating an example of the relationship between the core interval and XT between two cores. FIG. 5 is a schematic cross-sectional view of a multi-core fiber according to the second embodiment. FIG. 6 is a schematic cross-sectional view of a multi-core fiber according to the third embodiment. FIG. 7 is a schematic cross-sectional view of a multi-core fiber according to the fourth embodiment. FIG. 8 is a schematic cross-sectional view of a multi-core fiber according to the fifth embodiment. FIG. 9 is a diagram illustrating a first example of a refractive index profile that can be regarded as a trench type. FIG. 10 is a diagram illustrating a second example of a refractive index profile that can be regarded as a trench type. FIG. 11 is a diagram illustrating a third example of a refractive index profile that can be regarded as a trench type. FIG. 12 is a diagram illustrating a fourth example of a refractive index profile that can be regarded as a trench type.

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

(Embodiment 1)
1 is a schematic cross-sectional view of a multi-core fiber according to a first embodiment, and is a cross-sectional view taken along a plane perpendicular to the longitudinal direction. The multi-core fiber 10 includes, as a plurality of core portions, four core portions 15a, 15b, 15c, and 15d, each of which includes a central core portion 11, an intermediate layer 12, and a trench layer 13, and a cladding portion 14 surrounding the outer periphery of the core portions 15a, 15b, 15c, and 15d. The four core portions 15a, 15b, 15c, and 15d are arranged in a square shape with the central axis of the cladding portion 14 as the center. Hereinafter, when the core portions 15a, 15b, 15c, and 15d are not to be distinguished from each other, they may be referred to as core portions 15.

The multicore fiber 10 may have a coating layer formed on the outer periphery of the cladding portion 14. Such a coating layer may be made of a resin or the like that can be used as a coating layer for optical fibers.

The core portion 15 and the cladding portion 14 are both made of silica glass. The cladding portion 14 has a substantially circular cross section and a refractive index lower than the maximum refractive index of the core portion 15. The outer diameter of the cladding portion 14 is the cladding diameter D.

The core portion 15 has a substantially circular cross section. In the core portion 15, the intermediate layer 12 surrounds the outer periphery of the central core portion 11, and the trench layer 13 surrounds the outer periphery of the intermediate layer 12. The central core portion 11, the intermediate layer 12, and the trench layer 13 are substantially concentric.

The length of the shortest straight line from the center of the central core portion 11 of the core portion 15 to the outer periphery of the cladding portion 14, for example the thickness T shown in Figure 1, is defined as the cladding thickness.

Figure 2 is a schematic diagram of the refractive index profile of adjacent core portions 15a, 15b and the surrounding cladding portion 14 of the multicore fiber 10. In Figure 2, profiles P11, P12, and P13 show the refractive index profile for the core portion 15a. Profiles P21, P22, and P23 show the refractive index profile for the core portion 15b. Profiles P11 and P21 show the refractive index profile of the central core portion 11, profiles P12 and P22 show the refractive index profile of the intermediate layer 12, and profiles P13 and P23 show the refractive index profile of the trench layer 13. Profile P14 shows the refractive index profile of the cladding portion 14.

The central core diameter of the central core portion 11 is 2a, and the average maximum relative refractive index difference of the central core portion 11 with respect to the cladding portion 14 is Δ1. Here, the refractive index profile of the central core portion 11 may not only be a step type with a geometrically ideal shape, but may also have a shape where the top is not flat but has irregularities due to manufacturing characteristics, or has a shape that trails from the top. In this case, the refractive index of the region that is approximately flat at the top of the refractive index profile within the range of the central core diameter 2a in the manufacturing design is the index that determines Δ1.

The outer diameter of the intermediate layer 12 is 2b, and the average relative refractive index difference of the intermediate layer 12 with respect to the cladding portion 14 is Δ2. The inner diameter of the trench layer 13 is 2b, the outer diameter is 2c, and the average relative refractive index difference of the trench layer 13 with respect to the cladding portion 14 is Δ3.

Here, in the multicore fiber 10, Δ1>Δ2>Δ3 and 0%>Δ3 are satisfied in each core portion. That is, in the multicore fiber 10, each core portion has a trench-type refractive index profile. For example, the above inequality is satisfied when the central core portion 11 is made of quartz glass doped with a dopant that increases the refractive index, such as germanium (Ge), the intermediate layer 12 contains almost no dopant that changes the refractive index, the trench layer 13 is made of quartz glass doped with a dopant that decreases the refractive index, such as fluorine (F), and the cladding portion 14 is made of pure quartz glass. Note that pure quartz glass is extremely high-purity quartz glass that does not substantially contain a dopant that changes the refractive index and has a refractive index of about 1.444 at a wavelength of 1550 nm.

As shown in Figures 1 and 2, the trench layers 13 are in contact with each other in adjacent core portions 15a and 15b. That is, the core spacing P, which is the distance between the centers of the central core portions 11 of the core portions 15a and 15b, is equal to 2c. Similarly, the trench layers 13 are in contact with each other in adjacent core portions 15b and 15c, and the trench layers 13 are in contact with each other in adjacent core portions 15c and 15d. Therefore, if the thickness of the trench layer 13 on a straight line connecting the centers of two adjacent central core portions 11 is defined as the first thickness T1, the first thickness T1 is 2(c-b).

For example, if the trench layer 13 on the shortest straight line from the center of the central core portion 11 of the core portion 15a to the outer periphery of the cladding portion 14 is defined as the second thickness T2, the second thickness T2 is (c-b). Also, the second thickness T2 of the other core portions 15b, 15c, and 15d is also (c-b).

Therefore, in the multi-core fiber 10, the first thickness T1 is thicker than the second thickness T2, specifically twice as thick. As a result, the trench layers 13 are substantially connected to each other between adjacent core parts. As a result, the first thickness T1, which affects the inter-core crosstalk (XT) between adjacent core parts, is twice the second thickness T2, which is the thickness of each trench layer 13. As a result, the multi-core fiber 10 can effectively suppress the inter-core crosstalk. In addition, since the second thickness T2 is the thickness of one layer of the trench layer 13 for each core part, the higher-order mode of the light propagating through each core part leaks from the region of one layer thickness, as shown by the wavy arrow in FIG. 1. Therefore, in the multi-core fiber 10, the inter-core crosstalk can be effectively suppressed without substantially increasing the cutoff wavelength, and it is possible to achieve both the suppression of the inter-core crosstalk and the suppression of the increase in the cutoff wavelength.

As a characteristic of the multicore fiber 10, for example, the cutoff wavelength of each core portion is 1550 nm or less. This ensures the single mode properties of the multicore fiber 10 at wavelengths of at least 1550 nm or more. Preferably, the cutoff wavelength is equal to or less than a wavelength included in the wavelength band used. Here, the wavelength band used is the wavelength band of light that propagates as signal light when the multicore fiber 10 is used for optical fiber communication. The wavelength band used is, for example, included in 1530 nm to 1625 nm. This ensures the single mode properties of the multicore fiber 10 in the wavelength band used. Furthermore, if the cutoff wavelength of each core portion is 1530 nm or less, the G.654 standard can be satisfied.

The characteristics of the multicore fiber 10 include, for example, inter-core crosstalk of -10 dB or less at a length of 10 m or more in the wavelength band used. Preferably, inter-core crosstalk of -10 dB or less at a length of 1 km in the wavelength band used, and more preferably, inter-core crosstalk of -10 dB or less at a length of 100 km. It is more preferable that inter-core crosstalk at these lengths is -30 dB or less. In this specification, inter-core crosstalk means inter-core crosstalk between two adjacent core portions.

As a characteristic of the multicore fiber 10, for example, the bending loss at the core 15 at a wavelength of 1550 nm when bent with a diameter of 20 mm is 1.59 dB/m or less. This value of 1.59 dB/m is expressed by converting the unit of the value of 0.1 dB/turn in the G.657 A2 standard, which is the ITU standard for optical fiber.

For example, the structural parameters of the multi-core fiber 10 are Δ1 between 0.20% and 0.60%, Δ3 between -0.7% and -0.10%, Δ2 between -0.1% and 0.1%, b/a between 1.2 and 3.0, and c/a between 1.4 and 4.4. 2a is set so that the cutoff wavelength is between 1000 nm and 1260 nm. According to the inventor's comprehensive study, it has been found that by appropriately designing the structural parameters within the above ranges, it is possible to obtain good characteristics that comply with the G.652 standard, the G.654 standard, the G.657 standard, and the like.

The manufacturing method of the multicore fiber 10 is not particularly limited, but it can be manufactured by, for example, a known stack method. FIG. 3 is an explanatory diagram of an example of a manufacturing method of the multicore fiber 10. In this example, first, four core preforms 115 are prepared as multiple core preforms. Each of the core preforms 115 includes a portion that becomes the core portion 15 of the multicore fiber 10. Specifically, the central core portion 111 is the portion that becomes the central core portion 11. The intermediate layer 12 that surrounds the outer periphery of the central core portion 111 is the portion that becomes the intermediate layer 112. The trench layer 113 that surrounds the outer periphery of the intermediate layer 112 is the portion that becomes the trench layer 13. The trench layer 113 forms the outermost periphery of the core preform 115. The core preform 115 can be formed by a known VAD (Vapor-phase Axial Deposition) method, OVD (Outside Vapor Deposition) method, MCVD (Modified Chemical Vapor Deposition) method, or plasma CVD method.

Then, the four core preforms 115 are inserted into a jacket tube 114a made of glass and stacked. The jacket tube 114a is the part that will become the cladding section 14. After that, a number of glass rods 114b are inserted to fill the gaps between the four core preforms 115 inside the jacket tube 114a. Powdered glass may also be used to fill the gaps. In this way, the optical fiber preform 100 is produced.

Then, the optical fiber preform 100 is placed in a known drawing furnace, the lower end is heated and melted, and the multi-core fiber 10 is drawn. At this time, a coating layer may be formed on the drawn multi-core fiber 10.

(Optical characteristics of known multi-core fibers)
As a comparative example, the results of simulation calculations of the optical characteristics of a known multi-core fiber adopting a trench structure will be described. Table 1 shows the optical characteristics of an optical fiber when Δ1 is 0.37%, the central core diameter 2a is a step type of 8 μm, an intermediate layer and a trench layer are provided so that Δ2 is 0%, b/a is 2, and Δ3 and c/a are changed in a multi-core fiber. The optical characteristics are the mode field diameter (MFD) at a wavelength of 1310 nm, the cutoff wavelength (λcc), the required cladding thickness, and the inter-core crosstalk (XT) at a length of 100 km. The required cladding thickness is the minimum cladding thickness required to suppress the excess loss at a wavelength of 1625 nm to 0.001 dB/km or less. The core spacing was set to 40 μm.

As shown in Table 1, it can be seen that increasing c/a and widening the trench layer width dramatically improves XT. There is also little change in MFD. In contrast, the cutoff wavelength increases as the trench layer width is widened. For example, when Δ3 is -0.2%, if c/a is increased to 4.5, the cutoff wavelength approaches 1260 nm, which is the upper limit of the cable cutoff wavelength in the G.652 and G.657 standards. Furthermore, when Δ3 is -0.25%, it exceeds 1260 nm. Thus, in known multicore fibers, there is a trade-off between inter-core XT and cutoff wavelength.

Example 1
Next, as Example 1, a multicore fiber having the structure of the first embodiment shown in Fig. 1 was manufactured. Prior to manufacture, structural parameters (Δ1, Δ2, Δ3, b/a, c/a, 2a) were designed by simulation calculation. Table 2 shows the set structural parameters and the optical characteristics calculated using them. As shown in Table 2, Δ3 was set to -0.2% (sample No. 1) or -0.25% (sample No. 2).

Next, a multicore fiber was manufactured using the stack method. First, four core preforms with a diameter of 16 mm and a length of 500 mm were manufactured using the VAD method. Next, the core preforms were stacked inside a silica glass jacket tube with an outer diameter of 62.5 mm and an inner diameter of 38.6 mm. Silica rods of an appropriate size were inserted into the gaps in the jacket tube to fill the gaps. The optical fiber preform manufactured in this way was drawn to manufacture a multicore fiber with a cladding diameter of 125 μm. The core spacing in this case was 32 μm.

The optical characteristics of the manufactured multicore fiber of Example 1 are shown in Table 3. In Table 3, λ0 is the zero dispersion wavelength, and the bending loss is the bending loss when bending with a diameter of 20 mm. For both Samples No. 1 and 2, it can be seen that the XT characteristics at 100 km are significantly lower than the calculated value, even though the cutoff wavelength is not very different from the calculated value. In addition, it was confirmed that the transmission loss was sufficiently low by appropriately designing the excess loss. It was also confirmed that the dispersion characteristics, bending loss, and other characteristics conform to the G. 652 and G. 657 standards. From this example, it was actually confirmed that the inter-core XT can be suppressed more effectively than the original design value, and that the single mode characteristics (cutoff wavelength characteristics) are not impaired.

Figure 4 is a diagram showing an example of the relationship between core spacing and inter-core XT. In Figure 4, the straight line is the result predicted using the method used in the calculations in Table 1. For example, when the core spacing is 40 μm, the inter-core XT is -37.8 dB as shown in Table 1. When the core spacing is 32 μm, the inter-core XT predicted from this straight line is approximately -13 dB, but according to Example 1, it is significantly lower at -30.2 dB, as shown by the white circle.

In the multicore fiber in Table 1, if Δ3 is -0.25%, c/a is 4.0, and the cladding diameter is [(core spacing) + (required cladding thickness)] × 2, the cladding diameter will be approximately 150 μm to 160 μm, which is larger than the standard 125 μm for many optical fibers. However, to make the cladding diameter 125 μm, the core spacing must be 33.5 μm. In this case, according to FIG. 4, the inter-core XT is expected to be approximately -13.1 dB, but according to Example 1, the inter-core XT can be -30.2 dB even with a cladding diameter of 125 μm. In other words, according to Example 1 of the present invention, the restrictions on the cladding diameter are relaxed.

(Embodiment 2)
5 is a schematic cross-sectional view of a multi-core fiber according to embodiment 2. The multi-core fiber 10A has a configuration in which the cladding portion 14 in the multi-core fiber 10 according to embodiment 1 is replaced with a cladding portion 14A.

The cladding portion 14A has a configuration in which holes 14Aa are provided in the cladding portion 14. In the multicore fiber 10A, five holes 14Aa are provided at the central axis and four positions around it.

In the multicore fiber 10A, like the multicore fiber 10, it is possible to suppress both inter-core crosstalk and an increase in the cutoff wavelength. In addition, the air holes 14Aa can effectively suppress inter-core crosstalk in areas other than the straight line connecting the two adjacent centers of the central core portion 11. In particular, the air holes 14Aa located on the central axis can suppress inter-core crosstalk between core portions located diagonally apart from the most adjacent core portions. According to the study by the inventors, even if the air holes 14Aa are provided as in the multicore fiber 10A, the increase in the cutoff wavelength relative to the multicore fiber 10 is less than 10%, and it has been confirmed that the increase in the cutoff wavelength due to the air holes is small. The air holes 14Aa have the same diameter except for the one located on the central axis, but may be the same or different from each other.

(Embodiment 3)
6 is a schematic cross-sectional view of a multi-core fiber according to embodiment 3. The multi-core fiber 10B has a configuration in which the core portion 15 in the multi-core fiber 10 according to embodiment 1 is replaced with a core portion 15B.

The core portion 15B has a configuration in which the trench layer 13 of the core portion 15 is replaced with a trench layer 13B. As shown in FIG. 6, the trench layers 13B of adjacent core portions 15B partially overlap each other. Even if the trench layers 13B partially overlap each other in this way, it is possible to simultaneously suppress inter-core crosstalk and suppress an increase in the cutoff wavelength, as in the multi-core fiber 10. According to the inventor's careful study, if the first thickness is 1.2 times the second thickness, it is possible to simultaneously suppress inter-core crosstalk and suppress an increase in the cutoff wavelength, and if it is 1.5 times, an even better balance is achieved.

(Embodiment 4)
7 is a schematic cross-sectional view of a multi-core fiber according to embodiment 4. The multi-core fiber 10C includes six core portions 15 as a plurality of core portions, a core portion 15C that is an inner core portion whose outer periphery is surrounded by the six core portions 15, and a cladding portion 14. The six core portions 15 may be referred to as outer core portions.

The core portion 15C has a configuration in which the central core portion 11 of the core portion 15 is replaced with a central core portion 11C. The core portion 15C is designed to have a shorter cutoff wavelength than the six core portions 15 that are the outer core portions. In this embodiment, the central core portion 11C is designed by adjusting Δ1 and the central core diameter 2a to satisfy the cutoff wavelength conditions.

In the multicore fiber 10C, the trench layers 13 of the six cores 15 are in contact with each other. As a result, the first thickness is thicker than the second thickness, specifically twice as thick, so that it is possible to suppress both inter-core crosstalk and an increase in the cutoff wavelength.

In addition, since core portion 15C is surrounded by core portion 15 and the effective trench layer thickness is thick, the higher-order mode of light propagating through core portion 15C cannot leak out. Therefore, in multicore fiber 10C, the cutoff wavelength is designed to be shorter than that of core portion 15, which is the outer core portion. In addition, since core portion 15C is of a different type from core portion 15, the increase in inter-core crosstalk is relatively suppressed compared to when core portions of the same type are adjacent. As a result, it is possible to achieve both suppression of inter-core crosstalk and suppression of increase in cutoff wavelength in all of core portions 15C and 15.

(Embodiment 5)
Fig. 8 is a schematic cross-sectional view of a multi-core fiber according to embodiment 5. The multi-core fiber 10D has a configuration in which the core portion 15C in the multi-core fiber 10C according to embodiment 4 shown in Fig. 7 is replaced with a core portion 15D as an inner core portion.

The core portion 15D comprises a central core portion 11D and an intermediate layer 16D surrounding the outer periphery of the central core portion 11D. That is, the core portion 15D has a step-type refractive index profile rather than a trench-type. The core portion 15D is designed to have a shorter cutoff wavelength than the six core portions 15 surrounding the core portion 15. The Δ1 and central core diameter 2a of the central core portion 11D are, for example, the same as the Δ1 and central core diameter 2a of the central core portion 11C. The Δ2 of the intermediate layer 16D is, for example, the same as the Δ2 of the intermediate layer 12. The outer diameter of the intermediate layer 16D is, for example, the same as the outer diameter of the trench layer 13 of the core portion 15.

In the multicore fiber 10D, similar to the multicore fiber 10C, it is possible to simultaneously suppress inter-core crosstalk and suppress an increase in the cutoff wavelength in all of the core portions 15D and 15.

Example 2
Next, as Example 2, a multicore fiber having the structure of the embodiment 4 shown in FIG. 7 was manufactured. Before manufacturing, the structural parameters (Δ1, Δ2, Δ3, b/a, c/a, 2a) were designed by simulation calculation. Table 4 shows the structural parameters set assuming the G.654 standard and the optical characteristics obtained by calculation using the structural parameters. In Table 4, Sample No. 3 (outside) shows the structural parameters and optical characteristics of the outer core portion, and Sample No. 3 (center) shows the structural parameters and optical characteristics of the inner core portion. The core spacing was designed to be 27.9 μm.

Next, a multicore fiber was manufactured using the stack method. First, seven core preforms with a diameter of 13.95 mm and a length of 500 mm were manufactured using the MCVD method. Next, the core preforms were stacked inside a silica glass jacket tube with an outer diameter of 62.5 mm and an inner diameter of 41.85 mm. Silica rods of an appropriate size were inserted into the gaps in the jacket tube to fill the gaps. The optical fiber preform manufactured in this way was drawn to manufacture a multicore fiber with a cladding diameter of 125 μm.

The optical characteristics of the multicore fiber manufactured in Example 2 are shown in Table 5. For both Sample No. 3 (outer) and Sample No. 3 (center), the cutoff wavelength is not significantly different from the calculated value, but the 100 km XT characteristics are significantly lower than the calculated value. In addition, it was confirmed that the transmission loss was sufficiently low due to the appropriate design of the excess loss. It was also confirmed that the bending loss and other characteristics conform to the G.654 standard. From this example, it was actually confirmed that the inter-core XT can be suppressed more effectively than the original design value, and that the single mode characteristics (cutoff wavelength characteristics) are not impaired.

In this embodiment, the relationship between the core spacing and the inter-core XT described in Figure 4 was also considered. Figure 4 is a diagram showing an example of the relationship between the core spacing and the inter-core XT. In this embodiment, the core spacing is set to 27.9 μm, so the inter-core XT is predicted to be much larger than -30 dB as shown in Table 4, but in reality it is significantly lower at about -30 dB. Furthermore, in this embodiment, despite the number of core parts being 7, the cladding diameter was able to be made relatively small at 125 μm.

(Refractive Index Profile)
In Fig. 2, the refractive index profile is composed of straight lines, but the refractive index profile of an actually manufactured multicore fiber is often not composed of straight lines. Even in such cases, the design parameters a, b, c, Δ1, Δ2, Δ3, etc. can be confirmed from the refractive index profile measured by a profile analyzer or the like.

Figures 9 to 12 are explanatory diagrams of first to fourth examples of refractive index profiles that can be considered to be trench-type. In Figure 9, profile P32, shown by a dashed line, can be identified for profile P31, shown by a measured solid line. In Figure 10, profile P42 can be identified for measured profile P41. In Figure 11, profile P52 can be identified for measured profile P51. In Figure 12, profile P62 can be identified for measured profile P61.

Although the measured refractive index profiles shown in Figures 9 to 12 are not composed of at least a portion of straight lines, a person skilled in the art would recognize them as trench-type refractive index profiles, identify their design parameters, and identify a refractive index profile that can be expressed by those design parameters.

In the above embodiment, adjacent trench layers are in contact with or overlap each other, but may be separated by a certain amount of gap. For example, if the gap is such that the light propagating through the core is unlikely or not perceptible, the light can benefit from the effect of suppressing inter-core crosstalk by the adjacent trench layers. The size of the gap is, for example, equal to or less than the wavelength of the light, for example, about 2 μm or less.

The present invention is not limited to the above-mentioned embodiments. The present invention also includes configurations in which the above-mentioned components are appropriately combined. For example, the number and arrangement of core parts are not limited to the above-mentioned embodiments, and it is possible to delete core part 15C from the configuration of embodiment 4 shown in FIG. 7 to make the number of core parts six. Further effects and modifications can be easily derived by those skilled in the art. Thus, the broader aspects of the present invention are not limited to the above-mentioned embodiments, and various modifications are possible.

Reference Signs List 10, 10A, 10B, 10C, 10D: Multi-core fiber 11, 11C, 11D, 111: Central core portion 12, 16D, 112: Intermediate layer 13, 13B, 113: Trench layer 14, 14A: Cladding portion 14Aa: Air holes 15, 15a, 15b, 15c, 15d, 15B, 15C, 15D: Core portion 100: Optical fiber preform 114a: Jacket tube 114b: Glass rod 115: Core preform P: Core spacing T1: First thickness T2: Second thickness

Claims (10)

a plurality of adjacent core portions each having a central core portion, an intermediate layer surrounding the outer periphery of the central core portion, and a trench layer surrounding the outer periphery of the intermediate layer, the trench layer being spaced apart by an interval of 2 μm or less ;
A clad portion surrounding the outer periphery of the plurality of core portions;
Equipped with
In each of the plurality of core portions, when the average maximum relative refractive index difference of the central core portion with respect to the cladding portion is Δ1, the average relative refractive index difference of the intermediate layer is Δ2, and the average relative refractive index difference of the trench layer is Δ3, Δ1>Δ2>Δ3 and 0%>Δ3 are satisfied,
In a cross section perpendicular to the longitudinal direction, a first thickness including a thickness of the trench layer on a straight line connecting two adjacent centers of the plurality of central core portions and a thickness of the trench layer on the straight line of the adjacent central core portions is thicker than a second thickness of the trench layer on a shortest straight line extending from one of the two centers toward an outer periphery of the cladding portion,
The cutoff wavelength of each of the core portions is 1550 nm or less,
a cutoff wavelength is 1000 nm or more and 1249 nm or less, Δ1 is 0.20% or more and 0.60% or less, Δ2 is -0.1% or more and 0.1% or less, and Δ3 is -0.7% or more and -0.10% or less, and when an outer diameter of the central core portion is a, an outer diameter of the intermediate layer is b, and an outer diameter of the trench layer is c, b/a is 1.2 or more and 3.0 or less, and c/a is 1.4 or more and 4.4 or less. The multicore fiber according to claim 1 , wherein the first thickness is 1.2 times the second thickness. The multicore fiber according to claim 1 , wherein the first thickness is 1.5 times the second thickness. The multicore fiber according to any one of claims 1 to 3, wherein inter-core crosstalk between adjacent core portions over a length of 1 km in the used wavelength band is -10 dB or less. The multicore fiber according to any one of claims 1 to 3, wherein inter-core crosstalk between adjacent core portions over a length of 100 km in the used wavelength band is -10 dB or less. The multicore fiber according to any one of claims 1 to 5, wherein the cutoff wavelength is equal to or shorter than a wavelength included in the operating wavelength band. The multicore fiber according to claim 1 , wherein in each of the plurality of core portions, a bending loss at a wavelength of 1550 nm when bent with a diameter of 20 mm is 1.59 dB/m or less. an inner core portion whose outer periphery is surrounded by the plurality of core portions;
The multicore fiber according to claim 1 , wherein the inner core portion is designed to have a cutoff wavelength shorter than that of the core portion surrounding the inner core portion. A method for producing a multi-core fiber according to any one of claims 1 to 8 , comprising the steps of:
preparing a plurality of core preforms each including a portion that will become the plurality of core portions;
Stacking the plurality of core preforms inside a jacket tube;
A jacket tube having the plurality of core preforms stacked therein is drawn.
The method for manufacturing a multicore fiber comprises: The method for manufacturing a multi-core fiber according to claim 9 , wherein the plurality of core preforms are formed by a vapor-phase axial deposition (VAD) method, an outside vapor deposition (OVD) method, a modified chemical vapor deposition (MCVD) method, or a plasma CVD method.

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