optical fiber

Abstract

An optical fiber according to the present application has a core (10), an inner cladding (21) surrounding the core, and an outer cladding (22) surrounding the inner cladding. The average relative refractive index difference Δ1 of the core with respect to pure silica, the average relative refractive index difference Δ2 of the inner cladding with respect to pure silica, and the average relative refractive index difference Δ3 of the outer cladding with respect to pure silica satisfy the relationship Δ1 > Δ3 ≥ Δ2. The ratio r2 / r1 of the inner cladding radius r2 with respect to the core radius r1 is 4.5 or more and 5.5 or less. The minimum value Δmin of the relative refractive index difference with respect to pure silica is -0.030% or more and -0.010% or less. The radius rmin at which the relative refractive index difference is the minimum value Δmin is r1 < rmin < r2. When the relative refractive index difference at the core radius r1 is Δ(r1), (Δmin - Δ(r1)) / (rmin - r1) is -0.002% / μm or less.

Description

Technical Field

[0001] This application claims the benefit of priority based on Japanese Application No. 2020-46648, filed on March 17, 2020, the entire contents of which are incorporated herein by reference. Background Art

[0002] ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G.657 defines standards for optical fibers with excellent bending resistance. This standard envisions the use of optical fibers in access systems such as FTTx (Fiber to the x), where stringent bending conditions are required. For example, ITU-T G.657 A1 specifies upper limits for bending losses at bend radii of 10 mm and 15 mm. ITU-T G.657 A2 specifies upper limits for bending losses at bend radii of 7.5 mm, 10 mm, and 15 mm.

[0003] To improve bending resistance, a method is known to employ a refractive index profile with a recessed structure in the cladding. This recessed structure increases the refractive index difference between the core and cladding while limiting the cutoff wavelength to below a predetermined wavelength, thereby improving bending resistance. Patent Document 1 discloses the addition of chlorine (Cl) to the outer cladding as one method for achieving a recessed structure.

[0004] Patent Document 2 discloses a method for improving bending resistance by adopting a refractive index distribution in which the refractive index gradually decreases from the core toward the cladding at the boundary between the core and the cladding.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: U.S. Patent Application Publication No. 2019 / 0119143;

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2017-26698. Summary of the Invention

[0009] An optical fiber according to one embodiment of the present invention comprises a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding. The average relative refractive index difference Δ1 of the core relative to pure silica, the average relative refractive index difference Δ2 of the inner cladding relative to pure silica, and the average relative refractive index difference Δ3 of the outer cladding relative to pure silica satisfy the relationship Δ1>Δ3≥Δ2. The ratio r2 / r1 of the inner cladding radius r2 to the core radius r1 is 4.5 or greater and 5.5 or less. The minimum value Δmin of the relative refractive index difference relative to pure silica is -0.030% or greater and -0.010% or less. The radius rmin at which the relative refractive index difference reaches the minimum value Δmin satisfies r1<rmin<r2. When the relative refractive index difference at the core radius r1 is Δ(r1), (Δmin-Δ(r1)) / (rmin-r1) is -0.002% / μm or less. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 Graphs showing the cross section and refractive index distribution of an optical fiber according to an embodiment.

[0011] Figure 2 Graph showing the relationship between the first slope of the relative refractive index difference and the bending loss.

[0012] Figure 3 Graph showing the relationship between the second slope of the relative refractive index difference and the bending loss.

[0013] Figure 4 This is a graph showing the relationship between the ratio of the inner cladding radius to the core radius and the transmission loss. DETAILED DESCRIPTION

[0014] [Problems to be Solved by the Invention]

[0015] To improve bending resistance, it's necessary to strengthen the ability to confine light waves within the core. Specifically, in a concave structure, to increase the refractive index difference between the core and cladding, a large amount of dopant that modifies the refractive index must be added to the core and inner cladding. The method disclosed in Patent Document 1 also requires the addition of a large amount of Cl to the outer cladding. Consequently, transmission loss deteriorates.

[0016] The method described in Patent Document 2 is essentially equivalent to increasing the core diameter, which increases the overlap between the power distribution of light and the dopant, thereby worsening the transmission loss.

[0017] Therefore, an object of the present invention is to provide an optical fiber that can achieve both low bending loss and low transmission loss.

[0018] [Effects of the Invention]

[0019] According to the present invention, an optical fiber capable of achieving both low bending loss and low transmission loss can be provided.

[0020] [Description of Embodiments of the Invention]

[0021] First, the embodiments of the present invention are listed for explanation. An optical fiber of one embodiment has a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding. The average relative refractive index difference Δ1 of the core relative to pure silica, the average relative refractive index difference Δ2 of the inner cladding relative to pure silica, and the average relative refractive index difference Δ3 of the outer cladding relative to pure silica satisfy the relationship Δ1>Δ3≥Δ2. The ratio r2 / r1 of the inner cladding radius r2 to the core radius r1 is greater than or equal to 4.5 and less than or equal to 5.5. The minimum value Δmin of the relative refractive index difference relative to pure silica is greater than or equal to -0.030% and less than or equal to -0.010%. The radius rmin at which the relative refractive index difference is the minimum value Δmin satisfies r1<rmin<r2. When the relative refractive index difference at the core radius r1 is set to Δ(r1), (Δmin-Δ(r1)) / (rmin-r1) is less than or equal to -0.002% / μm. In addition, as the refractive index described in the present invention, a value obtained by moving average of the measured values ​​of the refractive index at each radius in, for example, 0.5 μm intervals can be used.

[0022] In the optical fiber of the above embodiment, since Δ1>Δ3≥Δ2, a relative refractive index difference between the core and the inner cladding can be adjusted. Since the ratio r2 / r1 is 4.5 or greater, transmission loss can be suppressed. If the ratio r2 / r1 is 5.5 or less, bending loss can be suppressed. Since the slope of the refractive index profile (hereinafter referred to as the "first slope") (Δmin - Δ(r1)) / (rmin - r1) is -0.002% / μm or less, bending loss can be suppressed. Therefore, both low bending loss and low transmission loss can be achieved.

[0023] When the radius at which the relative refractive index difference with respect to pure silica is zero is defined as r0, in the case of r0 ≥ r1, the slope (hereinafter referred to as "second slope") Δmin / (rmin-r0) may be -0.002% / μm or less.

[0024] The average relative refractive index difference Δ2 may be not less than −0.025% and not more than −0.010%. In this case, it is easy to increase the relative refractive index difference between the core and the inner cladding.

[0025] The average concentration of chlorine in the outer cladding layer may be 500 wtppm or less. In this case, blistering due to Cl can be suppressed.

[0026] The average relative refractive index difference Δ1 may be 0.35% or more and 0.45% or less. In this case, it is easy to increase the relative refractive index difference between the core and the inner cladding.

[0027] The radius rmin may be not less than 7 μm and not more than 15 μm. In this case, it is easy to achieve both low bending loss and low transmission loss.

[0028] The core radius r1 may be 4 μm or more and 5 μm or less. In this case, the first slope (Δmin−Δ(r1)) / (rmin−r1) or the second slope Δmin / (rmin−r0) is likely to be −0.002% / μm or less.

[0029] [Details of the embodiments of the present invention]

[0030] The following is a reference to the attached Figure 1 Specific examples of the optical fiber of the present invention will be described below. The present invention is not limited to these examples but is defined by the scope of the claims, which are intended to encompass all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, identical elements are denoted by the same reference numerals, and duplicate descriptions are omitted.

[0031] As mentioned above, the depressed structure has the problem of worsening transmission loss due to the large amount of dopants added to the core and inner cladding. Furthermore, according to the inventors' experience and research, the method disclosed in Patent Document 1 sometimes generates bubbles due to Cl. This also significantly reduces productivity.

[0032] Even in the method disclosed in Patent Document 2, when Ge or the like is added to the core, the ability to confine light in the core is enhanced, but the overlap between the Ge distribution and the optical power distribution increases accordingly, thereby deteriorating the transmission loss.

[0033] Figure 1 : is a diagram showing the cross section and refractive index distribution of an optical fiber according to an embodiment. Figure 1 As shown, the optical fiber 1 of this embodiment includes a core 10 and a cladding 20 surrounding the core 10 . Figure 1 The cross-sectional view of shows a cross section perpendicular to the central axis C of the optical fiber 1. Figure 1 In the refractive index distribution diagram, the horizontal axis is the radial position of the optical fiber 1, and the vertical axis is the relative refractive index difference of the optical fiber 1 relative to pure silica (SiO2).

[0034] The core 10 is made of quartz glass containing Ge. The cladding 20 includes an inner cladding 21 surrounding the core 10 and an outer cladding 22 surrounding the inner cladding 21. The inner cladding 21 is made of quartz glass containing fluorine (F). The outer cladding 22 is made of quartz glass substantially free of dopants. The average chlorine (Cl) concentration in the outer cladding 22 is 500 wtppm or less by mass.

[0035] The outer diameter (diameter) of the cladding 20 is equal to the outer diameter (diameter) of the outer cladding 22, and is, for example, not less than 124 μm and not more than 126 μm. The radius of the core 10, i.e., the core radius r1, is, for example, not less than 4 μm and not more than 5 μm. The radius of the inner cladding 21, i.e., the inner cladding radius r2, is, for example, not less than 18 μm and not more than 28 μm.

[0036] The average relative refractive index difference Δ1 of the core 10 relative to pure silica, the average relative refractive index difference Δ2 of the inner cladding 21 relative to pure silica, and the average relative refractive index difference Δ3 of the outer cladding 22 relative to pure silica satisfy the relationship Δ1>Δ3≥Δ2. Alternatively, the relationship Δ1>Δ3>Δ2 may be satisfied. The average relative refractive index difference Δ1 is, for example, greater than or equal to 0.35% and less than or equal to 0.45%. The average relative refractive index difference Δ2 is, for example, greater than or equal to -0.025% and less than or equal to -0.010%. The average relative refractive index difference Δ3 is, for example, greater than or equal to -0.015% and less than or equal to 0.035%, preferably greater than or equal to -0.010% and less than or equal to 0.010%, and more preferably greater than or equal to -0.005% and less than or equal to 0.005%. In this manner, by providing the inner cladding 21 as a low-refractive-index region between the core 10 and the outer cladding 22, the desired cutoff wavelength can be maintained while enhancing the ability to confine fundamental mode light waves. The boundary between the core 10 and the inner cladding 21 and the boundary between the inner cladding 21 and the outer cladding 22 can be detected by measuring the refractive index distribution. For example, the radial refractive index measurement result can be differentiated in the radial direction, and the position where the differential value is equal to or less than a fixed value can be defined as the boundary between the core 10 and the inner cladding 21, while the position where the differential value is equal to or greater than the fixed value can be defined as the boundary between the inner cladding 21 and the outer cladding 22.

[0037] The relative refractive index difference of optical fiber 1 relative to pure silica reaches a minimum value Δmin in inner cladding 21. This minimum value Δmin is between -0.030% and -0.010%. The radius rmin at which the relative refractive index difference of optical fiber 1 relative to pure silica reaches the minimum value Δmin is, for example, between 7 μm and 15 μm. Furthermore, r1<rmin<r2.

[0038] To enhance the ability to confine light waves, the difference between Δ1 and Δ2 needs to be increased. In other words, Δ1 needs to be increased or Δ2 needs to be decreased. Typically, GeO2 is used as a dopant to increase Δ1, while F is used as a dopant to decrease Δ2. However, in either case, transmission loss increases due to increased Rayleigh scattering.

[0039] Therefore, in the optical fiber 1, the F concentration is reduced in the inner portion (near the core 10) of the depression (inner cladding 21). This suppresses Rayleigh scattering, thereby suppressing an increase in transmission loss. Figure 1 As shown, the refractive index profile has a slope by reducing the F concentration near the core 10 of the inner cladding 21.

[0040] Figure 2 is a graph showing the relationship between the first slope of the relative refractive index difference and the bending loss. Figure 2 In this graph, the horizontal axis represents the first slope (Slope1), and the vertical axis represents the bending loss. Here, the first slope refers to the slope of the refractive index profile near the core 10 of the inner cladding 21. Specifically, it is defined as the slope of the refractive index profile between the radii (radial positions) r1 and rmin. That is, when the relative refractive index difference of the optical fiber 1 at the core radius r1 relative to pure silica is Δ(r1), the first slope is expressed as (Δmin - Δ(r1)) / (rmin - r1). Furthermore, the bending loss here represents the increase in loss at a wavelength of 1550 nm when the optical fiber is wound 10 times around a core with a diameter of 15 mm.

[0041] Figure 3 is a graph showing the relationship between the second slope of the relative refractive index difference and the bending loss. Figure 3 In the figure, the horizontal axis represents the second slope (Slope2), and the vertical axis represents the bending loss. Here, the second slope is also the slope of the refractive index profile near the core 10 of the inner cladding 21. Specifically, it is defined as the slope of the refractive index profile between the radius r0 (radial position) where the relative refractive index difference with pure silica is zero, and rmin. In other words, the second slope is expressed as Δmin / (rmin-r0).

[0042] By appropriately designing the slope of the refractive index profile, it is possible to suppress the expansion of the electromagnetic field distribution when the optical fiber is bent, thereby maintaining the bending resistance. Figure 2 and Figure 3 As shown, when the first slope and the second slope are -0.002% / μm or less, the bending loss is maintained at 0.03 dB / 10 turns or less. Therefore, in the optical fiber 1, the slope is set to -0.002% / μm or less. This can suppress the bending loss.

[0043] As described above, the inner cladding 21 is composed of silica glass containing F, while the outer cladding 22 is composed of silica glass that is substantially free of dopants. Consequently, a large stress difference occurs at the boundary between the inner cladding 21 and the outer cladding 22. This deteriorates transmission loss. However, by designing the boundary between the inner cladding 21 and the outer cladding 22 away from the central axis C, the deterioration of transmission loss can be suppressed.

[0044] Figure 4 This is a graph showing the relationship between the ratio of the inner cladding radius to the core radius and the transmission loss. Figure 4 In the figure, the horizontal axis is the ratio of the inner cladding radius to the core radius, r2 / r1, and the vertical axis is the transmission loss at a wavelength of 1550nm. In other words, Figure 4 This is a graph showing the correlation between the transmission loss at a wavelength of 1550nm and the ratio r2 / r1. Figure 4 As shown, by setting the ratio r2 / r1 to 4.5 or more, the transmission loss can be reduced to 0.184 dB / km or less. Therefore, in the optical fiber 1, the ratio r2 / r1 is set to 4.5 or more and 5.5 or less. This suppresses the transmission loss.

[0045] As can be seen from the above, optical fiber 1 achieves both low bending loss and low transmission loss. Furthermore, the average Cl concentration in outer cladding 22 is 500 wtppm or less. Therefore, Cl-induced blistering in optical fiber 1 is suppressed, thereby improving productivity.

[0046] [Example]

[0047] The following describes the results of evaluation tests using examples and comparative examples of the present invention, and further details the present invention.

[0048] Table 1 shows the refractive index parameters, cutoff wavelength λcc, and mode field diameter (MFD1.31) at 1310 nm of the optical fibers of Examples and Comparative Examples.

[0049] [Table 1]

[0050]

[0051] Table 2 shows the transmission loss (α1.55) and bending loss at a wavelength of 1550 nm for the optical fibers of Examples and Comparative Examples. Here, bending loss refers to the increase in loss at wavelengths of 1550 nm and 1625 nm when the optical fiber is wound once around a 7.5 mm diameter mandrel, once around a 10 mm diameter mandrel, and ten times around a 15 mm diameter mandrel.

[0052] [Table 2]

[0053]

[0054] Experimental Examples 1 to 16 and 24 are examples. Experimental Examples 17 to 23 are comparative examples. In Experimental Examples 1 to 8 of the example and Experimental Examples 17 to 21 of the comparative examples, the MFD at a wavelength of 1310 nm was within the range of 8.5 μm to 8.8 μm. In Experimental Examples 9 to 16 and 24 of the example and Experimental Examples 22 and 23 of the comparative examples, the MFD at a wavelength of 1310 nm was within the range of 9 μm to 9.2 μm.

[0055] In the optical fibers of Experimental Examples 1 to 16 and 24, the ratio r2 / r1 was within the range of 4.5 to 5.5. In contrast, in the optical fiber of Experimental Example 19, the ratio r2 / r1 was less than 4.5, and the transmission loss at a wavelength of 1550 nm was as high as 0.185 dB / km. Furthermore, in the optical fibers of Experimental Examples 20 to 22, the ratio r2 / r1 was greater than 5.5, and the bending loss values ​​were high. Generally, bending loss increases with increasing MFD, so here, bending loss was compared between optical fibers with similar MFD at a wavelength of 1310 nm. Specifically, the bending loss values ​​of the optical fibers of Experimental Examples 20 and 21 were generally higher than those of the optical fibers of Experimental Examples 1 to 8. The bending loss values ​​of the optical fiber of Experimental Example 22 were all higher than those of the optical fibers of Experimental Examples 9 to 16 and 24.

[0056] In the optical fibers of Experimental Examples 1 to 16 and 24, the first slope (Δmin-Δ(r1)) / (rmin-r1) and the second slope Δmin / (rmin-r0) were both -0.002% / μm or less. In the optical fibers of Experimental Examples 17, 18, and 23, the first slope (Δmin-Δ(r1)) / (rmin-r1) was -0.001% / μm. Furthermore, in the optical fibers of Experimental Examples 17 to 21, the second slope Δmin / (rmin-r0) was -0.001% / μm. Here, too, bending losses were compared using optical fibers having the same MFD. The various bending loss values ​​of the optical fibers of Experimental Examples 17 and 18 were generally higher than those of the optical fibers of Experimental Examples 1 to 8. Furthermore, the various bending loss values ​​of the optical fiber of Experimental Example 23 were all higher than those of the optical fibers of Experimental Examples 9 to 16 and 24.

[0057] The optical fibers of Experimental Examples 1 to 16 and 24 satisfy the bending loss specified in ITU-T G.657A1 or A2, while keeping the transmission loss at a wavelength of 1550 nm to 0.184 dB / km or less (α1.55≤0.184 dB / km), achieving both low bending loss and low transmission loss.

[0058] Description of Reference Numerals

[0059] 1: Fiber optic

[0060] 10: Core

[0061] 20: Cladding

[0062] 21: Inner cladding

[0063] 22: Outer layer

[0064] C: Center axis

Claims

1. An optical fiber comprising a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding, The average relative refractive index difference Δ1 of the core relative to pure silica, the average relative refractive index difference Δ2 of the inner cladding relative to pure silica, and the average relative refractive index difference Δ3 of the outer cladding relative to pure silica satisfy the relationship Δ1>Δ3≥Δ2, The ratio r2 / r1 of the inner cladding radius r2 to the core radius r1 is 4.5 or more and 5.5 or less, The minimum value Δmin of the relative refractive index difference with respect to pure silica is not less than -0.030% and not more than -0.010%, The radius rmin where the relative refractive index difference is the minimum value Δmin is r1<rmin<r2, When the relative refractive index difference at the core radius r1 is defined as Δ(r1), (Δmin-Δ(r1)) / (rmin-r1) is equal to or less than -0.002% / μm.

2. The optical fiber according to claim 1, wherein When the radius at which the relative refractive index difference with respect to pure silica is zero is defined as r0, r0 ≥ r1, and Δmin / (rmin-r0) is equal to or less than -0.002% / μm.

3. The optical fiber according to claim 1 or 2, wherein: The average relative refractive index difference Δ2 is not less than -0.025% and not more than -0.010%.

4. The optical fiber according to claim 1 or 2, wherein: The average concentration of chlorine in the outer cladding is 500 wtppm or less.

5. The optical fiber according to claim 1 or 2, wherein: The average relative refractive index difference Δ1 is greater than or equal to 0.35% and less than or equal to 0.45%.

6. The optical fiber according to claim 1 or 2, wherein: The radius rmin is greater than or equal to 7 μm and less than or equal to 15 μm.

7. The optical fiber according to claim 1 or 2, wherein: The core radius r1 is greater than or equal to 4 μm and less than or equal to 5 μm.

8. The optical fiber according to claim 1 or 2, wherein: The average relative refractive index difference Δ1, the average relative refractive index difference Δ2, and the average relative refractive index difference Δ3 satisfy the relationship of Δ1>Δ3>Δ2.

9. The optical fiber according to claim 1 or 2, wherein: The average relative refractive index difference Δ3 is greater than or equal to −0.015% and less than or equal to 0.035%.

10. The optical fiber according to claim 1 or 2, wherein: When the film is wound 10 times around a core having a diameter of 15 mm, the increase in loss at a wavelength of 1550 nm is 0.03 dB / 10 turns or less.

11. The optical fiber according to claim 1 or 2, wherein: The transmission loss at a wavelength of 1550nm is less than 0.184dB / km.

12. The optical fiber according to claim 1 or 2, wherein: The inner cladding radius r2 is greater than or equal to 18 μm and less than or equal to 28 μm.

13. The optical fiber according to claim 1 or 2, wherein: The inner cladding is composed of quartz glass containing fluorine.

14. The optical fiber according to claim 1 or 2, wherein: The mode field diameter at 1310 nm is 8.5 μm or more and 8.8 μm or less.

15. The optical fiber according to claim 1 or 2, wherein: The mode field diameter at 1310 nm is greater than or equal to 9 μm and less than or equal to 9.2 μm.

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