Optical fiber

Abstract

An optical fiber comprising: a core (10) made of silica glass; and a cladding (20) that is made of silica glass and that surrounds the core (10), wherein Δ1-Δ2≥0.02 is satisfied, where Δ1 represents the greatest value of the specific refractive index difference of the core (10) in 0≤r / a≤0.3, and Δ2 represents the smallest value of the specific refractive index difference of the core (10) in 0≤r / a≤0.9, where r represents the radial distance from the center axis of the core (10), and a represents the radius of the core (10).

Description

optical fiber

[0001] This application claims priority to Japanese Patent Application No. 2022-086237, filed May 26, 2022, and incorporates by reference all of the contents of said Japanese application.

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

[0003] One method for producing an optical fiber with low transmission loss is to reduce Rayleigh scattering by reducing the concentration of added halogen elements. It is known that Rayleigh scattering is caused by uneven concentration distribution localized on an atomic scale and tends 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 due to uneven concentration tends to increase in proportion to the concentration.

[0004] US Patent No. 7,689,085 US Patent Application Publication No. 2014 / 0294355 US Patent Application Publication No. 2017 / 0017032

[0005] 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

[0006] An optical fiber according to one aspect of the present disclosure comprises a core made of silica-based glass and a cladding surrounding the core and made of silica-based glass, wherein, where r is the radial distance from the central axis of the core and a is the radius of the core, the maximum value Δ1 of the relative refractive index difference in the range 0≦r / a≦0.3 and the minimum value Δ2 of the relative refractive index difference in the range 0≦r / a≦0.9 satisfy Δ1−Δ2≧0.02.

[0007] Fig. 1 is a cross-sectional view showing an optical fiber according to an embodiment. Fig. 2 is a graph showing the refractive index profile of the optical fiber according to an embodiment. Fig. 3 is a graph showing the results of summarizing the relationship between the refractive index profile and the transmission loss. Fig. 4 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. Fig. 5 is a graph showing the refractive index profile of the optical fiber according to Experimental Example 3.

[0008] [Problem to be Solved by the Present Disclosure] The halogen element added to the optical fiber preform not only contributes to adjusting the increase or decrease in the refractive index but also has the effect of removing impurities present inside the glass at the time of preform formation. Therefore, if the amount of halogen element added is reduced to zero, the transmission loss will actually worsen.

[0009] In the optical fibers described in Patent Documents 1 to 3, the refractive index is highest at the core center and tends to decrease radially outward due to the detachment of the halogen element at the core center. This results in a large variation in refractive index within the core, which is affected by structural imperfection scattering and worsens transmission loss.

[0010] An object of the present disclosure is to provide an optical fiber capable of reducing transmission loss.

[0011] Effect of the Present Disclosure According to the present disclosure, an optical fiber capable of reducing transmission loss can be provided.

[0012] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. (1) An optical fiber according to one aspect of the present disclosure includes a core made of silica-based glass and a cladding surrounding the core and made of silica-based glass, wherein, where r is the radial distance from the central axis of the core and a is the radius of the core, the maximum value Δ1 of the relative refractive index difference in the range 0≦r / a≦0.3 and the minimum value Δ2 of the relative refractive index difference in the range 0≦r / a≦0.9 satisfy Δ1−Δ2≧0.02. This optical fiber can reduce transmission loss.

[0013] (2) In the above (1), the maximum value Δ1, the minimum value Δ2, and the maximum value Δ3 of the relative refractive index difference in the range of 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.

[0014] (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, 0≦r1 / a≦0.1 may be satisfied. In this case, the transmission loss can be reduced.

[0015] (4) In any of the above (1) to (3), if the radial distance at which the relative refractive index difference becomes the minimum value Δ2 is r2, 0≦r2 / a≦0.5 may be satisfied. In this case, the transmission loss can be further reduced.

[0016] (5) In any of the above (1) to (3), when the radial distance at which the relative refractive index difference becomes the minimum value Δ2 is r2, 0≦r2 / a≦0.4 may be satisfied. In this case, the transmission loss can be further reduced.

[0017] (6) In any of the above (1) to (3), if the radial distance at which the relative refractive index difference becomes the minimum value Δ2 is r2, 0≦r2 / a≦0.35 may be satisfied. In this case, the transmission loss can be further reduced.

[0018] (7) In any of the above (1) to (6), the minimum value Δ4 of the relative refractive index difference of the cladding may be −0.6≦Δ4≦−0.2, which can further reduce the transmission loss.

[0019] (8) In any of the above (1) to (7), the maximum value Δ1 and the minimum value Δ2 may satisfy Δ1-Δ2≧0.04, which can further reduce transmission loss.

[0020] (9) In any of the above (1) to (7), the maximum value Δ1 and the minimum value Δ2 may satisfy Δ1-Δ2≧0.06, which can further reduce transmission loss.

[0021] [Details of the embodiments of the present disclosure] Specific examples of the optical fiber of the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted.

[0022] FIG. 1 is a cross-sectional view showing an optical fiber according to an embodiment. As shown in FIG. 1, the optical fiber 1 includes a core 10 extending along a central axis 1a and a cladding 20 surrounding the core 10. The core 10 and the cladding 20 are made of silica-based glass, with silica glass being the 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 halogen elements such as F and Cl.

[0023] The refractive index of the core 10 is higher than the refractive index of the cladding 20. The effective area Aeff of the optical fiber 1 at a wavelength of 1550 nm is 80 μm ２ 160 μm or more ２ The optical fiber 1 has a transmission loss of 0.150 dB / km or less at a wavelength of 1550 nm. The radius a of the core 10 is, for example, 4 μm or more and 7 μm or less.

[0024] 2 is a graph showing the refractive index profile of the optical fiber according to the embodiment. The vertical axis shows the refractive index profile of pure silica (SiO ２) as a reference, and the horizontal axis represents the radial distance r [μm] from the central axis 1a of the core 10. The position at which the value dΔ(r) / dr obtained by differentiating the relative refractive index difference Δ(r) with respect to the radial distance r is the minimum value (the steepest downward 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 corresponds to the radius a of the core 10.

[0025] 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 Δ1 [%], and the radial distance at which the relative refractive index difference is Δ1 is defined as r1. When multiple maximum values ​​exist, the smallest radial distance is defined as r1, and the maximum value of that maximum value is defined as the maximum value Δ1. Δ1 satisfies -0.1≦Δ1≦0.1. It may also be 0≦r1 / a≦0.1.

[0026] In the radial range where the radial distance r satisfies 0≦r / a≦0.9 with respect to the radius a, the minimum value (local minimum value) of the relative refractive index difference is defined as Δ2 [%], and the radial distance at which the relative refractive index difference is Δ2 is defined as r2. When multiple local minimum values ​​exist, the smallest radial distance is defined as r2, and that local minimum value is defined as the minimum value Δ2. Δ2 satisfies -0.2≦Δ2≦0, with the proviso that Δ1≧Δ2. It may be 0≦r2 / a≦0.5, 0≦r2 / a≦0.4, or 0≦r2 / a≦0.35.

[0027] 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 Δ3 [%], and the radial distance at which the relative refractive index difference is Δ3 is defined as r3. If multiple maximum values ​​exist, the largest radial distance is defined as r3, and that maximum value is defined as the maximum value Δ3. Δ3 satisfies -0.1≦Δ3≦0.1, where Δ3≧Δ2.

[0028] In the radial range satisfying r≧a, i.e., in the cladding 20, the minimum value (local minimum value) of the relative refractive index difference is defined as Δ4 [%], and the radial distance at which the relative refractive index difference is Δ4 is ​​defined as r4. When there are multiple local minimum values, the smallest radial distance is defined as r4, and the local minimum value is defined as the minimum value Δ4. Δ4 may be in the range of −0.6≦Δ4≦−0.2.

[0029] In the optical fiber 1, the maximum value Δ1 and the minimum value Δ2 satisfy Δ1-Δ2≧0.02. Furthermore, 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 showing the variation in the relative refractive index difference Δ within the core 10. The closer the maximum value Δ1 at the center of the core 10 and the maximum value Δ3 at the outer periphery are to each other, the closer (Δ3-Δ2) / (Δ1-Δ2) becomes to 1.

[0030] 2, it is necessary 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. In the optical fiber 1 according to this embodiment, this is achieved by the following means.

[0031] The maximum value Δ1 is achieved by locally depleting the halogen concentration, particularly the F concentration, in the center of the core 10. The local depletion of the F concentration can be achieved, for example, by locally heating a glass portion corresponding to the center of the core 10 in an optical fiber preform after impurities have been removed in a dehydration process. The heating temperature may be 1000°C or higher, or 1400°C or higher. The Cl concentration and the F concentration at a position at a radial distance r1 may be 1000 wtppm or lower, or 500 wtppm or lower, respectively.

[0032] In the manufacture of optical fiber preforms, the amount of halogen added during sintering of the glass body that will become the core is adjusted so that the halogen concentration in the outer periphery of the core is reduced. However, since the F concentration in the cladding is 10,000 wtppm or more, 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 cladding becomes large. As a result, defects that lead to increased transmission loss are likely to occur at the interface. Therefore, at a radial distance r3 corresponding to the interface between the core 10 and the cladding 20, the halogen concentration may be 100 wtppm or more, or even 500 wtppm or more.

[0033] Figure 3 is a graph showing the results of analyzing the relationship between refractive index profile and transmission loss. In Figure 3, the relationship between (Δ3 - Δ2) / (Δ1 - Δ2) and transmission loss is plotted for optical fibers with Δ1 - Δ2 values ​​of 0%, 0.02%, 0.04%, and 0.06%. The Δ1 - Δ2 values ​​for each optical fiber include an error of ±0.005%.

[0034] 3, it can be seen that, in order to reduce transmission loss regardless of (Δ3−Δ2) / (Δ1−Δ2), Δ1−Δ2≧0.02, Δ1−Δ2≧0.04, or Δ1−Δ2≧0.06 may be satisfied. When Δ1−Δ2≧0.02, a transmission loss of 0.150 dB / km or less can be realized. When Δ1−Δ2≧0.04, a transmission loss of 0.1495 dB / km or less can be realized. When Δ1−Δ2≧0.06, a transmission loss of 0.149 dB / km or less can be realized. This result suggests that Δ1−Δ2 corresponds to the reduction in halogen concentration at the center of the core 10, and that as the halogen concentration decreases, Rayleigh scattering due to concentration variations decreases, resulting in a reduction in transmission loss.

[0035] 3, it can be seen that the transmission loss can be reduced by satisfying the condition 0.5≦(Δ3−Δ2) / (Δ1−Δ2)≦1.5. In this range, for example, a transmission loss of 0.1465 dB / km or less can be achieved for an optical fiber in which Δ1−Δ2=0.06±0.005%.

[0036] 4 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 is the mean square variation (Δ(r)-Δe) of the relative refractive index difference Δ(r) with respect to the effective relative refractive index difference Δe in the range of 0≦r≦r3. ２ The square root of the average value of r in the range of 0≦r≦r3. The closer (Δ3-Δ2) / (Δ1-Δ2) is to 1, the smaller the mean square variation (Δ(r)-Δe) ２ This reduces scattering due to structural imperfections, and as a result, it can be confirmed that transmission loss is reduced.

[0037] From the above, to achieve low-loss optical fiber 1, Δ1-Δ2 may be 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. When 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 achieved. When 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 achieved. 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.

[0038] Optical fibers according to Experimental Examples 1 to 4 were used to investigate optical characteristics depending on differences in refractive index distribution. The optical fiber according to Experimental Example 1 corresponds to the optical fiber 1 according to the embodiment and has a refractive index profile in which the relative refractive index difference increases at radial distances r1 and r3. The optical fiber according to Experimental Example 2 has a refractive index profile in which the relative refractive index difference is high at radial distance r1 and low at radial distance r3. The optical fiber according to Experimental Example 3 has a typical ring-core shaped refractive index profile in which the relative refractive index difference is high at radial distance r3 and low at radial distance r1, as shown in FIG. 5 . It can be said that the optical fiber according to Experimental Example 1 also has a ring-core shaped refractive index profile. The optical fiber according to Experimental Example 4 has a refractive index profile in which the relative refractive index difference is low at both radial distances r1 and r3.

[0039] Table 1 shows the results for each optical fiber: Δ1-Δ2 [%], Δ3-Δ2 [%], transmission loss at a wavelength of 1550 nm [dB / km], effective cross-sectional area Aeff [μm ２ ], chromatic dispersion at a wavelength of 1550 nm [ps / nm / km], and fiber cutoff wavelength λc [nm].

[0040]

[0041] The results in Table 1 confirm that the optical fiber of Experimental Example 1 exhibits the lowest transmission loss. This is believed to be due to the fact that the halogen concentration in the core is reduced by having a protruding refractive index profile at the center of the core resulting from halogen concentration loss. It is also 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 area Aeff, chromatic dispersion, and fiber cutoff wavelength λc. This is because the optical fiber of Experimental Example 1 has a ring-core shaped refractive index profile in which the relative refractive index difference is high at the outer periphery of the core, which makes it possible to increase the effective area Aeff relative to the mode field diameter, thereby enabling stronger light confinement. From the above, it can be confirmed that the optical fiber 1 of this embodiment suppresses deterioration in transmission loss while maintaining bending loss resistance.

[0042] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the spirit of the present disclosure.

[0043] REFERENCE SIGNS LIST 1...optical fiber 1a...central axis 10...core 20...cladding

Claims

1. A core made of silica-based glass; a cladding surrounding the core and made of silica-based glass; When 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 in the range of 0≦r / a≦0.3 and the minimum value Δ2 of the relative refractive index difference in the range of 0≦r / a≦0.9 satisfy Δ1−Δ2≧0.

02. Optical fiber.

2. the maximum value Δ1, the minimum value Δ2, and the maximum value Δ3 of the relative refractive index difference in the range of 0.8≦r / a≦1 satisfy 0.5≦(Δ3−Δ2) / (Δ1−Δ2)≦1.5, 2. The optical fiber of claim 1.

3. When the radial distance at which the relative refractive index difference becomes the maximum value Δ1 is r1, 0≦r1 / a≦0.

1. The optical fiber according to claim 1 or 2.

4. When the radial distance at which the relative refractive index difference becomes the minimum value Δ2 is r2, 0≦r2 / a≦0.

5. The optical fiber according to claim 1 or 2.

5. When the radial distance at which the relative refractive index difference becomes the minimum value Δ2 is r2, 0≦r2 / a≦0.

4. The optical fiber according to claim 1 or 2.

6. When the radial distance at which the relative refractive index difference becomes the minimum value Δ2 is r2, 0≦r2 / a≦0.

35. The optical fiber according to claim 1 or 2.

7. The minimum value Δ4 of the relative refractive index difference of the cladding is −0.6≦Δ4≦−0.

2. The optical fiber according to claim 1 or 2.

8. The maximum value Δ1 and the minimum value Δ2 satisfy Δ1-Δ2≧0.

04. The optical fiber according to claim 1 or 2.

9. The maximum value Δ1 and the minimum value Δ2 satisfy Δ1-Δ2≧0.

06. The optical fiber according to claim 1 or 2.

10. The maximum value Δ1 satisfies −0.1≦Δ1≦0.1, The optical fiber according to claim 1 or 2.

11. The minimum value Δ2 satisfies −0.2≦Δ2≦0. The optical fiber according to claim 1 or 2.

12. The maximum value Δ3 satisfies −0.1≦Δ3≦0.1, The optical fiber of claim 2.

13. The minimum value Δ4 satisfies −0.6≦Δ4≦−0.

2. The optical fiber of claim 7.

14. The core is made of silica-based glass having a mass ratio of 90% or more of silica glass. The optical fiber according to claim 1 or 2.

15. The cladding is made of silica-based glass having a mass ratio of 90% or more of silica glass. The optical fiber according to claim 1 or 2.