Optical fiber

Abstract

An optical fiber is provided with: a core (10) comprising silica-based glass; and a cladding layer (20) that wraps the core (10) and is made of silica-based glass, the maximum value [Delta] 1 of the relative refractive index difference at 0 < = r / a < = 0.3 and the minimum value [Delta] 2 of the relative refractive index difference at 0 < = r / a < = 0.9 satisfying [Delta] 1-[Delta] 2 > = 0.02, where r is the distance in the radial direction from the central axis of the core (10) and a is the radius of the core (10).

Description

Technical Field

[0001] The present disclosure relates to an optical fiber. This application claims priority based on Japanese Application No. 2022-086237 filed on May 26, 2022, and all the contents described in the Japanese Application are incorporated herein by reference. Background Art

[0002] The transmission loss value at the wavelength in the near-infrared region is formed by the sum of multiple scattering and absorption factors such as Rayleigh scattering, infrared absorption, OH absorption, and structural irregular scattering. Rayleigh scattering, infrared absorption, and OH absorption are scattering and absorption effects on the atomic scale of glass. Structural irregular scattering is an effect caused by the deviation of the refractive index distribution on a scale slightly larger than the atomic scale affecting the scattering of light.

[0003] As one of the solutions for making optical fibers with low transmission loss, there is a method of reducing Rayleigh scattering by reducing the concentration of added halogen elements. It is known that Rayleigh scattering is caused by the uneven concentration distribution that exists locally at the atomic scale, and has a tendency to increase in proportion to the concentration. Patent documents 1 to 3 describe optical fibers in which the concentration of added halogen elements is reduced. Non-patent document 1 describes that Rayleigh scattering caused by uneven concentration has a tendency to increase in proportion to the concentration.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: U.S. Patent No. 7,689,085

[0007] Patent Document 2: U.S. Patent Application Publication No. 2014 / 0294355

[0008] Patent Document 3: U.S. Patent Application Publication No. 2017 / 0017032

[0009] Non-patent literature

[0010] Non-patent document 1: H. Kakiuchida et al. "Rayleigh Scattering in Fluorine-Doped Silica Glass" Journal of Japan Applied Physics Vol. 42 (2003), pp. 6516-6517, Part 1, No. 10, October 2003 Summary of the invention

[0011] An optical fiber involved in one embodiment of the present invention comprises: a core composed of silica-based glass; and a cladding that wraps the core and is composed of silica-based glass. If the radial distance from the central axis of the core is set to r and the radius of the core is set to a, the maximum value Δ1 of the relative refractive index difference at 0≤r / a≤0.3 and the minimum value Δ2 of the relative refractive index difference at 0≤r / a≤0.9 satisfy Δ1-Δ2≥0.02. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a cross-sectional view showing an optical fiber according to an embodiment.

[0013] Figure 2 : is a graph showing the refractive index distribution of the optical fiber according to the embodiment.

[0014] Figure 3 This is a graph showing the results of summarizing the relationship between the refractive index distribution and the transmission loss.

[0015] Figure 4 It is a graph showing the relationship between (Δ3-Δ2) / (Δ1-Δ2) and the root mean square of the relative refractive index difference of the core.

[0016] Figure 5 : is a graph showing the refractive index distribution of the optical fiber involved in Experimental Example 3. DETAILED DESCRIPTION

[0017] [Technical Problems to be Solved by the Present Disclosure]

[0018] The halogen element added to the optical fiber base material not only helps to adjust the increase or decrease of the refractive index, but also has the effect of removing impurities existing in the glass at the time of the base material. Therefore, if the amount of halogen element added is suppressed to zero, the transmission loss will deteriorate instead.

[0019] In the optical fibers described in Patent Documents 1 to 3, the refractive index is highest at the core center due to the halogen element being released, and tends to decrease radially outward. Therefore, the deviation of the refractive index in the core becomes large, and as a result, the transmission loss deteriorates due to the influence of structural irregular scattering.

[0020] An object of the present disclosure is to provide an optical fiber capable of reducing transmission loss.

[0021] [Effects of the present disclosure]

[0022] According to the present disclosure, it is possible to provide an optical fiber capable of reducing transmission loss.

[0023] [Description of Embodiments of the Present Disclosure]

[0024] First, the embodiments of the present disclosure are listed for explanation. (1) An optical fiber according to one embodiment of the present disclosure comprises: a core composed of silica glass; and a cladding, which surrounds the core and is composed of silica glass, wherein, if the radial distance from the central axis of the core is r and the radius of the core is a, the maximum value Δ1 of the relative refractive index difference at 0≤r / a≤0.3 and the minimum value Δ2 of the relative refractive index difference at 0≤r / a≤0.9 satisfy Δ1-Δ2≥0.02. In this optical fiber, the transmission loss can be reduced.

[0025] (2) In the above (1), the maximum value Δ1, the minimum value Δ2, and the maximum value Δ3 of the relative refractive index difference at 0.8≤r / a≤1 may satisfy 0.5≤(Δ3-Δ2) / (Δ1-Δ2)≤1.5. In this case, the transmission loss can be further reduced.

[0026] (3) In the above (1) or (2), if the radial distance at which the relative refractive index difference reaches the maximum value Δ1 is r1, then 0≤r1 / a≤0.1. In this case, the transmission loss can be reduced.

[0027] (4) In any of (1) to (3), when the radial distance at which the relative refractive index difference reaches the minimum value Δ2 is r2, 0≤r2 / a≤0.5 may be satisfied. In this case, the transmission loss can be further reduced.

[0028] (5) In any of (1) to (3), when the radial distance at which the relative refractive index difference reaches the minimum value Δ2 is r2, 0≤r2 / a≤0.4 may be satisfied. In this case, the transmission loss can be further reduced.

[0029] (6) In any of (1) to (3), when the radial distance at which the relative refractive index difference reaches the minimum value Δ2 is r2, 0≤r2 / a≤0.35 may be satisfied. In this case, the transmission loss can be further reduced.

[0030] (7) In any one of (1) to (6) above, the minimum value Δ4 of the relative refractive index difference of the cladding may be -0.6≤Δ4≤-0.2. In this case, the transmission loss can be further reduced.

[0031] (8) In any of the above (1) to (7), the maximum value Δ1 and the minimum value Δ2 may satisfy Δ1-Δ2≥0.04. In this case, the transmission loss can be further reduced.

[0032] (9) In any of the above (1) to (7), the maximum value Δ1 and the minimum value Δ2 may satisfy Δ1-Δ2≥0.06. In this case, the transmission loss can be further reduced.

[0033] [Details of the embodiments of the present disclosure]

[0034] Hereinafter, specific examples of the optical fiber disclosed in the present invention will be described with reference to the accompanying drawings. It should be noted that the present invention is not limited to these examples, but is represented by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are marked with the same reference numerals, and repeated descriptions are omitted.

[0035] Figure 1 is a cross-sectional view showing an optical fiber according to an embodiment of the present invention. Figure 1 As shown, the optical fiber 1 includes a core 10 extending along a central axis 1a, and a cladding 20 covering the core 10. The core 10 and the cladding 20 are composed of silica-based glass having silica glass as a main component, with silica glass accounting for 90% or more by mass. The core 10 contains, for example, halogen elements such as fluorine (F) and chlorine (Cl), and alkali metal elements such as lithium (Li), sodium (Na), potassium (K), and rubidium (Rb). The cladding 20 may also contain, for example, halogen elements such as F and Cl.

[0036] The refractive index of the core 10 is higher than that of the cladding 20. The effective cross-sectional area Aeff of the optical fiber 1 at a wavelength of 1550 nm is 80 μm 2 Above and 160μm 2 The transmission loss of the optical fiber 1 at a wavelength of 1550 nm is 0.150 dB / km or less. The radius a of the core 10 is, for example, 4 μm or more and 7 μm or less.

[0037] Figure 2 : is a graph showing the refractive index distribution of the optical fiber involved in the embodiment. The vertical axis represents the relative refractive index difference Δ[%] based on the refractive index of pure silica (SiO2), and the horizontal axis represents the radial distance r[μm] from the central axis 1a of the core 10. The position where the value dΔ(r) / dr obtained by differentiating the relative refractive index difference Δ(r) with the radial distance r is the minimum value (steepest descending gradient) is defined as the interface (boundary) between the core 10 and the cladding 20. The radial distance r of the interface between the core 10 and the cladding 20 is equivalent to the radius a of the core 10.

[0038] In the radial range where the radial distance r satisfies 0≤r / a≤0.3 with respect to the radius a, the maximum value (maximum value) of the relative refractive index difference is set to Δ1[%], and the radial distance where the relative refractive index difference is Δ1 is set to r1. In the case where there are multiple maximum values, the minimum radial distance is defined as r1, and the maximum value is set to the maximum value Δ1. Δ1 satisfies -0.1≤Δ1≤0.1. It can also be 0≤r1 / a≤0.1.

[0039] In the radial range where the radial distance r satisfies 0≤r / a≤0.9 relative to the radius a, the minimum value (minimum) of the relative refractive index difference is set to Δ2[%], and the radial distance where the relative refractive index difference is Δ2 is set to r2. In the case where there are multiple minimum values, the minimum radial distance is defined as r2, and the minimum value is set to the minimum value Δ2. Δ2 satisfies -0.2≤Δ2≤0. Among them, Δ1≥Δ2. It can be 0≤r2 / a≤0.5, 0≤r2 / a≤0.4, or 0≤r2 / a≤0.35.

[0040] In the radial range where the radial distance r satisfies 0.8≤r / a≤1 with respect to the radius a, the maximum value (maximum value) of the relative refractive index difference is set to Δ3[%], and the radial distance at which the relative refractive index difference becomes Δ3 is set to r3. In the case where there are multiple maximum values, the maximum radial distance is defined as r3, and the maximum value is set to the maximum value Δ3. Δ3 satisfies -0.1≤Δ3≤0.1, where Δ3≥Δ2.

[0041] In the radial range satisfying r ≥ a, that is, in the cladding 20, the minimum value (local minimum) of the relative refractive index difference is set to Δ4 [%], and the radial distance at which the relative refractive index difference reaches Δ4 is ​​set to r4. When there are multiple local minimums, the minimum radial distance is defined as r4, and the local minimum is defined as the minimum value Δ4. Δ4 may also be -0.6≤Δ4≤-0.2.

[0042] In the optical fiber 1, the maximum value Δ1 and the minimum value Δ2 satisfy Δ1-Δ2≥0.02. In addition, the maximum value Δ1, the minimum value Δ2, and the maximum value Δ3 satisfy 0.5≤(Δ3-Δ2) / (Δ1-Δ2)≤1.5. Here, (Δ3-Δ2) / (Δ1-Δ2) is an index representing the deviation of the relative refractive index difference Δ in the core 10. The closer the maximum value Δ1 in the center of the core 10 and the maximum value Δ3 in the peripheral portion are to each other, the closer (Δ3-Δ2) / (Δ1-Δ2) is to 1.

[0043] In order to get Figure 2 The refractive index profile shown needs to increase the maximum value Δ1 at the center of the core 10 and the maximum value Δ3 near the interface between the core 10 and the cladding 20. This is achieved in the optical fiber 1 according to the present embodiment as follows.

[0044] The maximum value Δ1 is achieved by locally reducing the halogen concentration, especially the F concentration, in the center of the core 10. The local reduction in F concentration can be achieved, for example, by locally heating the glass portion corresponding to the center of the core 10 in the optical fiber mother material after the impurities are removed by the dehydration process. The heating temperature may be 1000° C. or more, or 1400° C. or more. The Cl concentration and the F concentration at the position of the radial distance r1 may be 1000 wtppm or less, or 500 wtppm or less, respectively.

[0045] In the manufacture of optical fiber mother material, the amount of halogen element added is adjusted when the glass body that becomes the core is sintered so that the halogen concentration in the outer periphery of the core is reduced. However, since the F concentration in the cladding is above 10000wtppm, if the halogen concentration in the outer periphery of the core is too low, the difference in glass viscosity at the interface between the core and the cladding will become larger. As a result, defects that cause increased transmission loss are likely to occur at the interface. Therefore, at a position of radial distance r3 corresponding to the interface position between the core 10 and the cladding 20, the halogen concentration can be above 100wtppm, or above 500wtppm.

[0046] Figure 3 This is a graph showing the relationship between the refractive index distribution and the transmission loss. Figure 3 In the figure, the relationship between (Δ3-Δ2) / (Δ1-Δ2) and transmission loss is plotted for each optical fiber with Δ1-Δ2 of 0%, 0.02%, 0.04% and 0.06%. It should be noted that the value of Δ1-Δ2 for each optical fiber includes an error of ±0.005%.

[0047] according to Figure 3 , it can be confirmed that in order to reduce the transmission loss regardless of (Δ3-Δ2) / (Δ1-Δ2), Δ1-Δ2≥0.02, Δ1-Δ2≥0.04, or Δ1-Δ2≥0.06. In the case of Δ1-Δ2≥0.02, a transmission loss of less than 0.150dB / km can be achieved. In the case of Δ1-Δ2≥0.04, a transmission loss of less than 0.1495dB / km can be achieved. In the case of Δ1-Δ2≥0.06, a transmission loss of less than 0.149dB / km can be achieved. This result shows that Δ1-Δ2 corresponds to the reduction in the halogen concentration in the center of the core 10. As the halogen concentration decreases, the Rayleigh scattering caused by concentration unevenness decreases, and as a result, the transmission loss decreases.

[0048] In addition, according to Figure 3It can be confirmed that in order to reduce the transmission loss, 0.5≤(Δ3-Δ2) / (Δ1-Δ2)≤1.5 is also possible. Within this range, for example, for an optical fiber with Δ1-Δ2=0.06±0.005%, a transmission loss of 0.1465 dB / km or less can be achieved.

[0049] Figure 4 This is a graph showing the relationship between (Δ3-Δ2) / (Δ1-Δ2) and the root mean square (RMS) of the relative refractive index difference of the core. The RMS of the relative refractive index difference of the core refers to the mean square deviation (Δ(r)-Δe) of the relative refractive index difference Δ(r) in the range of 0≤r≤r3 from the effective relative refractive index difference Δe. 2 The square root of the average value in the range of 0≤r≤r3. The closer (Δ3-Δ2) / (Δ1-Δ2) is to 1, the greater the mean square deviation (Δ(r)-Δe) 2 As a result, it can be confirmed that the transmission loss is reduced.

[0050] According to the above, in order to realize a low-loss optical fiber 1, Δ1-Δ2 ≥ 0.02 and 0.5 ≤ (Δ3-Δ2) / (Δ1-Δ2) ≤ 1.5, Δ1-Δ2 ≥ 0.04 and 0.5 ≤ (Δ3-Δ2) / (Δ1-Δ2) ≤ 1.5, or Δ1-Δ2 ≥ 0.06 and 0.5 ≤ (Δ3-Δ2) / (Δ1-Δ2) ≤ 1.5. In the case where Δ1-Δ2 ≥ 0.02 and 0.5 ≤ (Δ3-Δ2) / (Δ1-Δ2) ≤ 1.5, a transmission loss of 0.1485 dB / km or less can be realized. In the case where Δ1-Δ2 ≥ 0.04 and 0.5 ≤ (Δ3-Δ2) / (Δ1-Δ2) ≤ 1.5, a transmission loss of 0.1475 dB / km or less can be realized. When Δ1-Δ2≥0.06 and 0.5≤(Δ3-Δ2) / (Δ1-Δ2)≤1.5, a transmission loss of 0.1465 dB / km or less can be achieved.

[0051] The optical fibers of Experimental Examples 1 to 4 were used to study the optical characteristics caused by the differences in refractive index distribution. The optical fiber of Experimental Example 1 corresponds to the optical fiber 1 involved in the embodiment, and has a refractive index curve in which the relative refractive index difference becomes high at radial distances r1 and r3. The optical fiber involved in Experimental Example 2 has a refractive index curve in which the relative refractive index difference is high at radial distance r1 and low at radial distance r3. Figure 5As shown, the optical fiber involved in Experimental Example 3 has a typical toroidal-shaped refractive index curve with a high relative refractive index difference at radial distance r3 and a low relative refractive index difference at radial distance r1. It can be said that the optical fiber involved in Experimental Example 1 also has a toroidal-shaped refractive index curve. The optical fiber involved in Experimental Example 4 has a refractive index curve with low relative refractive index differences at radial distances r1 and r3.

[0052] Table 1 shows, for each optical fiber, Δ1-Δ2[%], Δ3-Δ2[%], transmission loss [dB / km] at a wavelength of 1550 nm, effective cross-sectional area Aeff [μm 2 ], wavelength dispersion [ps / nm / km] at a wavelength of 1550nm and fiber cut-off wavelength λc [nm].

[0053] [Table 1]

[0054]

[0055] According to the results in Table 1, it can be confirmed that the transmission loss in the optical fiber of Experimental Example 1 becomes the lowest. It can be considered that this is because the halogen concentration of the core is reduced by having a protruding refractive index curve at the center of the core due to the lack of halogen concentration. In addition, it can be confirmed that the optical fiber of Experimental Example 1 has almost the same characteristics as the optical fiber of Experimental Example 3 in terms of effective cross-sectional area Aeff, wavelength dispersion and fiber cutoff wavelength λc. The reason is that by having a refractive index curve in a ring core shape in which the relative refractive index difference becomes high at the periphery of the core, the effective cross-sectional area Aeff relative to the mode field diameter can be increased, thereby strengthening the confinement of light. From the above, it can be confirmed that according to the optical fiber 1 of this embodiment, the degradation of transmission loss is suppressed while maintaining the bending loss resistance.

[0056] As mentioned above, although embodiment was described, this disclosure is not necessarily limited to the said embodiment and modification, Various changes are possible within the range which does not deviate from the summary.

[0057] Description of Reference Numerals

[0058] 1. Fiber Optic

[0059] 1a Central axis

[0060] 10 Core

[0061] 20 cladding layers.

Claims

1. An optical fiber having: A core composed of silica-based glass; and The cladding, which surrounds the core, is made of silica-based glass. If the radial distance from the central axis of the core is set to r and the radius of the core is set to a, the maximum value Δ1 of the relative refractive index difference at 0≤r / a≤0.3 and the minimum value Δ2 of the relative refractive index difference at 0≤r / a≤0.9 satisfy Δ1-Δ2≥0.

02.

2. The optical fiber according to claim 1, The maximum value Δ1, the minimum value Δ2, and the maximum value Δ3 of the relative refractive index difference at 0.8≤r / a≤1 satisfy 0.5≤(Δ3-Δ2) / (Δ1-Δ2)≤1.

5.

3. The optical fiber according to claim 1 or 2, If the radial distance at which the relative refractive index difference reaches the maximum value Δ1 is defined as r1, then 0≤r1 / a≤0.

1.

4. The optical fiber according to any one of claims 1 to 3, If the radial distance at which the relative refractive index difference reaches the minimum value Δ2 is defined as r2, then 0≤r2 / a≤0.

5.

5. The optical fiber according to any one of claims 1 to 3, If the radial distance at which the relative refractive index difference reaches the minimum value Δ2 is defined as r2, then 0≤r2 / a≤0.

4.

6. The optical fiber according to any one of claims 1 to 3, If the radial distance at which the relative refractive index difference reaches the minimum value Δ2 is defined as r2, then 0≤r2 / a≤0.

35.

7. The optical fiber according to any one of claims 1 to 6, The minimum value Δ4 of the relative refractive index difference of the cladding is -0.6≤Δ4≤-0.

2.

8. The optical fiber according to any one of claims 1 to 7, The maximum value Δ1 and the minimum value Δ2 satisfy Δ1-Δ2≥0.

04.

9. The optical fiber according to any one of claims 1 to 7, The maximum value Δ1 and the minimum value Δ2 satisfy Δ1-Δ2≥0.

06.

10. The optical fiber according to any one of claims 1 to 9, The maximum value Δ1 satisfies -0.1≤Δ1≤0.

1.

11. The optical fiber according to any one of claims 1 to 10, The minimum value Δ2 satisfies -0.2≤Δ2≤0.

12. The optical fiber according to claim 2, The maximum value Δ3 satisfies -0.1≤Δ3≤0.

1.

13. The optical fiber according to claim 7, The minimum value Δ4 satisfies -0.6≤Δ4≤-0.

2.

14. The optical fiber according to any one of claims 1 to 13, The core is composed of silica-based glass containing 90% or more of silica glass in terms of mass ratio.

15. The optical fiber according to any one of claims 1 to 14, The cladding layer is composed of silica-based glass containing 90% or more of silica glass in terms of mass ratio.

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