Optical fiber

Abstract

This optical fiber comprises: a core part (11, A11) to which an alkaline metal or an alkaline-earth metal is added in a distribution in a radial direction having a first peak (P1) in a center or near the center; and a cladding part (12, A12) which surrounds the outer periphery of the core part and has an average refractive index lower than the maximum refractive index of the core part, and to which an alkaline metal or alkaline-earth metal is added in a distribution in the radial direction having at least a second peak (P2) at any position in the radial direction. The maximum value of the inclination of the distribution curve of virtual temperature in the radial direction may be 4°C / μm or less.

Description

optical fiber

[0001] The present invention relates to optical fibers.

[0002] One of the most important characteristics of optical fibers for realizing next-generation undersea optical communication infrastructure is transmission loss. Many undersea optical fibers are doped with alkali metals (mainly potassium) in the core to promote structural relaxation of the glass, thereby suppressing Rayleigh scattering and structural disorder, thereby achieving extremely low transmission loss (Patent Documents 1 to 9).

[0003] Japanese Patent Publication No. 2020-204727 Japanese Patent No. 6615905 Japanese Patent Publication No. 2017-27050 Japanese Patent No. 6551109 Japanese Patent Publication No. 2020-12933 Japanese Patent No. 7119531 Japanese Patent No. 6536036 Japanese Patent No. 5706374 Japanese Patent Publication No. 2008-536190

[0004] There is a demand for further reduction in transmission loss of optical fibers, not limited to submarine optical fibers.

[0005] The present invention has been made in view of the above, and an object of the present invention is to provide an optical fiber with a further reduced transmission loss.

[0006] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is an optical fiber comprising: a core portion doped with an alkali metal or alkaline earth metal in a radial distribution having a first peak at or near the center; and a cladding portion surrounding the outer periphery of the core portion, having an average refractive index lower than the maximum refractive index of the core portion, and doped with an alkali metal or alkaline earth metal in a radial distribution having at least a second peak somewhere in the radial direction.

[0007] The maximum gradient of the distribution curve of the fictive temperature in the radial direction may be 4° C. / μm or less.

[0008] The cladding portion may be doped with an alkali metal or alkaline earth metal in a radial distribution having a third peak at a position radially spaced apart from the second peak.

[0009] The cladding portion may further be doped with an alkali metal or alkaline earth metal in a radial distribution that has a fourth peak at a position spaced apart from the third peak in the radial direction.

[0010] The distance between adjacent peaks in the radial direction may be 40 μm or less.

[0011] The interval may be 20 μm or less.

[0012] The alkali metal may be potassium.

[0013] At least one of fluorine and chlorine may be added to the core portion.

[0014] The peak value of the first peak may be greater than the peak value of the second peak.

[0015] The peak value of the first peak may be greater than the peak value of the second peak, and the peak value of the second peak may be greater than the peak value of the third peak.

[0016] The peak value of the second peak may be 95% or less of the peak value of the first peak, and the peak value of the third peak may be 90% or less of the peak value of the first peak.

[0017] The peak value of the first peak may be greater than the peak value of the second peak, the peak value of the second peak may be greater than the peak value of the third peak, and the peak value of the third peak may be greater than the peak value of the fourth peak.

[0018] The optical fiber has an effective core area of ​​70 μm at the wavelength of the input light. ２ 180 μm or more ２ The following is also acceptable.

[0019] The optical fiber has an effective core area of ​​100 μm at the wavelength of the input light. ２ 180 μm or more ２ The following is also acceptable.

[0020] The optical fiber has an effective core area of ​​140 μm at the wavelength of the input light. ２ 180 μm or more ２The following is also acceptable.

[0021] The optical fiber may have a transmission loss of 0.174 dB / km or less at a wavelength of 1550 nm.

[0022] The optical fiber may have a transmission loss of 0.164 dB / km or less at a wavelength of 1550 nm.

[0023] The optical fiber may have a transmission loss of 0.154 dB / km or less at a wavelength of 1550 nm.

[0024] The optical fiber may have a cutoff wavelength of 1530 nm or less.

[0025] The cladding portion may be doped with at least one of fluorine and chlorine.

[0026] According to the present invention, an optical fiber with a further reduced transmission loss can be realized.

[0027] FIG. 1 is a schematic cross-sectional view of an optical fiber according to a first embodiment. FIG. 2A is a diagram showing an example of a refractive index profile of the optical fiber shown in FIG. 1. FIG. 2B is a diagram showing an example of a refractive index profile of the optical fiber shown in FIG. 1. FIG. 2C is a diagram showing an example of a refractive index profile of the optical fiber shown in FIG. 1. FIG. 3A is a diagram showing the potassium doping amount in the radial direction of the optical fiber shown in FIG. 1. FIG. 3B is a diagram showing the fictive temperature in the radial direction of the optical fiber shown in FIG. 1. FIG. 4 is a schematic cross-sectional view of an optical fiber according to a second embodiment. FIG. 5A is a diagram showing the potassium doping amount in the radial direction of the optical fiber shown in FIG. 4. FIG. 5B is a diagram showing the fictive temperature in the radial direction of the optical fiber shown in FIG. 4. FIG. 6 is a schematic cross-sectional view of an optical fiber according to a third embodiment. FIG. 7A is a diagram showing the potassium doping amount in the radial direction of the optical fiber shown in FIG. 6. FIG. 7B is a diagram showing the fictive temperature in the radial direction of the optical fiber shown in FIG. 6.

[0028] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments described below. Furthermore, in each drawing, identical or corresponding components are appropriately designated by the same reference numerals, and duplicate explanations are appropriately omitted. Furthermore, in this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength (λcc) defined in ITU-T G.650.1 of the International Telecommunications Union (ITU). Furthermore, other terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0029] 1 is a schematic cross-sectional view of an optical fiber according to embodiment 1. The optical fiber 10 is made of silica glass and includes a core 11 and a cladding 12.

[0030] The core 11 has a substantially circular cross section and is disposed approximately at the center of the cladding 12, which also has a substantially circular cross section. The cladding 12 surrounds the outer periphery of the core 11. The maximum refractive index of the core 11 is higher than the average refractive index of the cladding 12. The average refractive index of the cladding 12 is the average refractive index of the cladding 12 in the radial direction.

[0031] The optical fiber 10 has a refractive index profile as shown in Figures 2A to 2C, for example. Figures 2A to 2C all show the refractive index profile in the radial direction from the central axis of the core 11 of the optical fiber 10. The refractive index profile is shown in terms of the relative refractive index difference with respect to pure silica glass. Here, pure silica glass is extremely high-purity silica glass that does not substantially contain dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm.

[0032] 2A shows a step-type refractive index profile. In FIG. 2A, profile P11 shows the refractive index profile of the core region 11, and profile P12 shows the refractive index profile of the cladding region 12. In the step-type refractive index profile, the diameter of the core region 11 (core diameter) is 2a, and the relative refractive index difference (maximum relative refractive index difference) of the maximum refractive index of the core region 11 with respect to the refractive index of pure silica glass is Δ1. Furthermore, the relative refractive index difference of the average refractive index of the cladding region 12 with respect to the refractive index of pure silica glass is Δclad.

[0033] 2B shows a so-called W-shaped refractive index profile. In FIG. 2B, profile P21 shows the refractive index profile of the core region 11, and profile P22 shows the refractive index profile of the cladding region 12. In the W-shaped refractive index profile, the core region 11 is composed of a center core with a diameter of 2a and a depressed layer formed to surround the outer periphery of the center core and having a refractive index smaller than that of the cladding region, an inner diameter of 2a, and an outer diameter of 2b. The center core is the portion of the core region 11 with the largest average refractive index. The maximum relative refractive index difference of the center core with respect to the refractive index of pure silica glass is Δ1. The relative refractive index difference of the average refractive index of the depressed layer with respect to the refractive index of pure silica glass is Δ2. The relative refractive index difference of the average refractive index of the cladding region 12 with respect to the refractive index of pure silica glass is Δclad.

[0034] FIG. 2C shows a so-called trench-type refractive index profile. In FIG. 2C, profile P31 shows the refractive index profile of the core region 11, and profile P32 shows the refractive index profile of the cladding region 12. In the trench-type refractive index profile, the core region 11 is composed of a center core with a diameter of 2a, an intermediate layer formed to surround the outer periphery of the center core and having an inner diameter of 2a and an outer diameter of 2b, and a trench layer formed to surround the outer periphery of the intermediate layer and having an inner diameter of 2b and an outer diameter of 2c, and having a refractive index smaller than the refractive index of the cladding region 12. The center core is the portion of the core region 11 with the largest average refractive index. The maximum relative refractive index difference of the center core with respect to the refractive index of pure silica glass is Δ1. The relative refractive index difference of the intermediate layer with respect to the refractive index of pure silica glass is Δ2. The relative refractive index difference of the trench layer with respect to the refractive index of pure silica glass is Δ3. The relative refractive index difference of the average refractive index of the cladding portion 12 with respect to the refractive index of pure silica glass is Δclad. Note that Δ2 is usually set to the same value as Δclad or to a value close to it.

[0035] Next, the constituent materials of the core 11 and cladding 12 of the optical fiber 10 will be described. The core 11 when the refractive index profile is step-type, or the center core in the core 11 when the refractive index profile is W-type, trench-type, or the like, is made of silica glass doped with chlorine (Cl) and an alkali metal or alkaline earth metal. Examples of alkali metals include potassium (K) and sodium (Na), but K is most preferable from a manufacturing perspective, considering dopant control based on the diffusion coefficient and cost. Cl, alkali metals, and alkaline earth metals are dopants that increase the refractive index of silica glass. The core 11 and center core may also be doped with a dopant that lowers the refractive index. An example of a dopant that lowers the refractive index is fluorine (F).

[0036] Furthermore, when the refractive index profile is W-shaped or trench-shaped, the depressed layer and trench layer in the core region 11 are made of silica glass doped with an alkali metal or alkaline earth metal and a dopant that reduces the refractive index. Furthermore, these depressed layer and trench layer may be doped with Cl. That is, the core region 11 may be doped with at least one of F and Cl.

[0037] The cladding 12 is also made of silica glass doped with an alkali metal or alkaline earth metal and a dopant that reduces the refractive index. The intermediate layer is made of silica glass with the same or similar composition as the cladding 12. The cladding 12 may also be doped with at least one of F and Cl.

[0038] The above dopants are added in amounts that will achieve a desired refractive index profile.

[0039] Next, the distribution of the doping amount of an alkali metal or alkaline earth metal in the optical fiber 10 will be described using an example in which the alkali metal or alkaline earth metal is K. Figures 3A and 3B are diagrams showing the potassium doping amount (K doping amount) or fictive temperature in the radial direction of the optical fiber 10 shown in Figure 1. Figure 3A shows the relationship between the radial position with respect to the center of the optical fiber 10 and the K doping amount, and Figure 3B shows the relationship between the radial position with respect to the center of the optical fiber 10 and the fictive temperature. In Figures 3A and 3B, region A11 is the region corresponding to the core portion 11, and region A12 is the region corresponding to the cladding portion 12.

[0040] 3A, the core 11 is doped with K in a radial distribution having a first peak P1 at or near the center. The cladding 12 is doped with K in a radial distribution having a second peak P2 at the radial position indicated by the dashed line. The peak value of the first peak P1 is greater than the peak value of the second peak P2.

[0041] In the optical fiber 10, the core 11 is doped with K in a radial distribution having a first peak P1, and the cladding 12 is also doped with K in a radial distribution having a second peak P2. As a result, compared to a case where K is doped in a radial distribution having a peak only in the core, the distribution of the fictive temperature in the entire radial direction is smoothed as shown in FIG. 3B. As a result, stress differences inside the optical fiber 10 are alleviated, and a decrease in transmission loss due to viscosity mismatch is suppressed. This allows for a further reduction in the transmission loss of the optical fiber 10.

[0042] Furthermore, in the optical fiber 10, the peak value of the first peak P1 is greater than the peak value of the second peak P2, so viscosity mismatch is more suppressed in the core 11 than in the cladding 12. As a result, the transmission loss of the optical fiber 10 can be more effectively reduced. For example, the peak value of the second peak P2 may be 95% or less of the peak value of the first peak P1.

[0043] The smoothness of the distribution of the fictive temperature in the radial direction can be expressed by the gradient of the distribution of the fictive temperature in the radial direction. For example, Fig. 3B shows the change in the fictive temperature change amount ΔTf when the radial position is moved by Δr, and the gradient of the distribution curve of the fictive temperature is expressed as (ΔTf / Δr). Furthermore, the gradient of the distribution curve of the fictive temperature may be expressed as a differential coefficient such as dTf / dr, where the fictive temperature Tf is a function of the radial position r.

[0044] According to the inventors' intensive studies, if the maximum value of the gradient of the distribution of the fictive temperature in the radial direction is 4°C / μm or less, the distribution of the fictive temperature in the radial direction is smooth, and it is possible to further reduce the transmission loss of the optical fiber 10. Note that, since the maximum value of the gradient of the distribution of the fictive temperature in the radial direction often occurs at the boundary between the core 11 and the cladding 12, it is preferable that the gradient of the distribution of the fictive temperature at the boundary between the core 11 and the cladding 12 is 4°C / μm or less.

[0045] The fictive temperature of silica glass can be determined using Raman scattering spectra. The ratio D2 / ω3, between the Raman peak (D2 peak) due to the three-membered ring structure in silica glass and the Raman peak (ω3 peak) due to the Si—O—Si bending vibration, is known to correspond to the fictive temperature of the silica glass. Therefore, a calibration curve for D2 / ω3 can be created using the Raman scattering spectrum of silica glass whose fictive temperature is known, and the fictive temperature can be determined using this calibration curve (see, for example, Takemi Hasegawa et al., SEI Technical Review, January 2018 (No. 192), pp. 14-19).

[0046] In known techniques, alkali metals or alkaline earth metals are intentionally added only to the core of an optical fiber. In contrast, the present inventors have conceived the idea that the gradient of the fictive temperature distribution can be reduced to 4°C / µm or less by intentionally adding alkali metals or alkaline earth metals not only to the core but also to the cladding.

[0047] As described above, the optical fiber 10 according to the first embodiment has a further reduced transmission loss.

[0048] The optical fiber 10 according to the first embodiment can be manufactured, for example, as follows: First, a core preform that will be the base material for the core region 11 is manufactured using a known glass preform manufacturing method such as a vapor phase axial deposition (VAD) method or a modified chemical vapor deposition (MCVD) method. At this time, an alkali metal or alkaline earth metal, and various dopants for realizing a desired refractive index profile are added to the core preform by a known method such as a vapor phase method.

[0049] Next, a jacket tube is manufactured as the base material for the cladding portion 12. The jacket tube can be manufactured by adding an alkali metal or alkaline earth metal to a glass tube made of silica glass to which a desired dopant has been added by a known method, and then expanding the diameter of the hole in the glass tube with a drill or the like until the hole is large enough to insert the core preform. The core preform is then inserted into the jacket tube, and the two are heated and integrated to form an optical fiber preform. The optical fiber 10 according to the first embodiment can be manufactured by applying a drawing method using a known drawing furnace to this optical fiber preform.

[0050] The distribution of the doping amount and the distribution of the fictive temperature as shown in Fig. 3A can be formed by adjusting the doping amount and distribution in the core preform and the jacket tube, adjusting the heat treatment temperature and time during heating and integration, and adjusting the drawing conditions. In particular, in order to make the maximum value of the gradient of the distribution curve of the fictive temperature in the radial direction of the optical fiber 4°C / µm or less, it is preferable to optimize the cooling gas conditions and perform an annealing treatment to promote relaxation of the glass structure in order to suppress rapid cooling of the optical fiber coming out of the drawing furnace.

[0051] 4 is a schematic cross-sectional view of an optical fiber according to embodiment 2. An optical fiber 10A is made of silica glass and includes a core 11 and a cladding 12A.

[0052] The core 11 is the same as the core 11 of the optical fiber 10 according to the first embodiment, and therefore a description thereof will be omitted.

[0053] The cladding portion 12A includes an inner layer 12A1 surrounding the outer periphery of the core portion 11 and an outer layer 12A2 surrounding the outer periphery of the inner layer 12A1. The maximum refractive index of the core portion 11 is higher than the average refractive index of the cladding portion 12A. The average refractive index of the cladding portion 12A is the average refractive index in the radial direction of the cladding portion 12A. The average refractive index of the inner layer 12A1 and the average refractive index of the outer layer 12A2 are approximately the same. The cladding portion 12A is also made of silica glass doped with an alkali metal or alkaline earth metal and a dopant that reduces the refractive index. The cladding portion 12A may also be doped with at least one of F and Cl.

[0054] Next, the distribution of the doping amount of alkali metal or alkaline earth metal in the optical fiber 10A will be described using an example in which the alkali metal or alkaline earth metal is K. Figures 5A and 5B are diagrams showing the K doping amount or fictive temperature in the radial direction of the optical fiber 10A shown in Figure 4. Figure 5A shows the relationship between the K doping amount and the radial position relative to the center of the optical fiber 10A, and Figure 5B shows the relationship between the K doping amount and the radial position relative to the center of the optical fiber 10A. In Figures 5A and 5B, region A11 corresponds to the core 11, region A12A1 corresponds to the inner layer 12A1 of the cladding 12A, and region A12A2 corresponds to the outer layer 12A2 of the cladding 12A.

[0055] As shown in FIG. 5A , the core 11 is doped with potassium in a radial distribution having a first peak P1 at or near the center. The inner layer 12A1 of the cladding 12A is doped with potassium in a radial distribution having a second peak P2A at a radial position indicated by a dashed line. The outer layer 12A2 of the cladding 12A is doped with potassium in a radial distribution having a third peak P3A at a position radially spaced from the second peak P2A and indicated by a dashed line. The peak value of the first peak P1 is greater than the peak value of the second peak P2A, which is greater than the peak value of the third peak P3A. For example, the peak value of the second peak P2A may be 95% or less of the peak value of the first peak P1. For example, the peak value of the third peak P3A may be 90% or less of the peak value of the first peak P1.

[0056] In the optical fiber 10A, K is doped in a radial distribution such that, in addition to a first peak P1 in the core 11 and a second peak P2A in the inner layer 12A1 of the cladding 12A, a third peak P3A is also present in the outer layer 12A2 of the cladding 12A. This makes the radial distribution of the fictive temperature even smoother, as shown in FIG. 5B. As a result, the transmission loss of the optical fiber 10A can be further reduced.

[0057] Furthermore, in the optical fiber 10A, because the peak value of the first peak P1 is greater than the peak value of the second peak P2A, viscosity mismatch is more suppressed in the core 11 than in the cladding 12A. As a result, the transmission loss of the optical fiber 10A can be more effectively reduced. Furthermore, because the third peak P3A, which has a smaller peak value, is located radially away from the second peak P2A, viscosity mismatch throughout the entire radial direction of the cladding 12A is more suitably alleviated.

[0058] As described above, the optical fiber 10A according to the second embodiment has a further reduced transmission loss.

[0059] 6 is a schematic cross-sectional view of an optical fiber according to embodiment 3. The optical fiber 10B is made of silica glass and includes a core 11 and a cladding 12B.

[0060] The core 11 is the same as the core 11 of the optical fiber 10 according to the first embodiment, and therefore a description thereof will be omitted.

[0061] The cladding portion 12B includes an inner layer 12B1 surrounding the outer periphery of the core portion 11, a middle layer 12B2 surrounding the outer periphery of the inner layer 12B1, and an outer layer 12B3 surrounding the outer periphery of the middle layer 12B2. The maximum refractive index of the core portion 11 is higher than the average refractive index of the cladding portion 12B. The average refractive index of the cladding portion 12B is the average refractive index in the radial direction of the cladding portion 12B. The average refractive index of the inner layer 12B1, the average refractive index of the middle layer 12B2, and the average refractive index of the outer layer 12B3 are approximately the same. The cladding portion 12B is also made of silica glass doped with an alkali metal or alkaline earth metal and a dopant that reduces the refractive index. The cladding portion 12B may also be doped with at least one of F and Cl.

[0062] Next, the distribution of the doping amount of alkali metal or alkaline earth metal in the optical fiber 10B will be described using an example in which the alkali metal or alkaline earth metal is K. Figures 7A and 7B are diagrams showing the K doping amount or fictive temperature in the radial direction of the optical fiber 10B shown in Figure 6. Figure 7A shows the relationship between the K doping amount and the radial position relative to the center of the optical fiber 10B, and Figure 7B shows the relationship between the K doping amount and the radial position relative to the center of the optical fiber 10B. In Figures 7A and 7B, region A11 corresponds to the core 11, region A12B1 corresponds to the inner layer 12B1 of the cladding 12B, region A12B2 corresponds to the middle layer 12B2 of the cladding 12B, and region A12B3 corresponds to the outer layer 12B3 of the cladding 12B.

[0063] 7A , the core 11 is doped with K in a radial distribution having a first peak P1 at or near the center. The inner layer 12B1 of the cladding 12B is doped with K in a radial distribution having a second peak P2B at a radial position indicated by a dashed line. The middle layer 12B2 of the cladding 12B is doped with K in a radial distribution having a third peak P3B at a position radially spaced apart from the second peak P2B and indicated by a dashed line. The outer layer 12B3 of the cladding 12B is doped with K in a radial distribution having a fourth peak P4B at a position radially spaced apart from the third peak P3B and indicated by a dashed line. The peak value of the first peak P1 is greater than the peak value of the second peak P2B, the peak value of the second peak P2B is greater than the peak value of the third peak P3B, and the peak value of the third peak P3B is greater than the peak value of the fourth peak P4B.

[0064] In the optical fiber 10B, K is doped with a radial distribution having a first peak P1 in the core 11, a second peak P2A in the inner layer 12B1 of the cladding 12B, a third peak P3B in the middle layer 12B2 of the cladding 12B, and a fourth peak P4B in the outer layer 12B3 of the cladding 12B. This makes the radial distribution of the fictive temperature even smoother, as shown in FIG. 7B . As a result, the transmission loss of the optical fiber 10B can be further reduced.

[0065] Furthermore, in the optical fiber 10B, the peak value of the first peak P1 is greater than the peak value of the second peak P2B, so viscosity mismatch is more suppressed in the core 11 than in the cladding 12B. As a result, the transmission loss of the optical fiber 10B can be more effectively reduced. Furthermore, the presence of the third peak P3B, which has a smaller peak value, at a position radially spaced apart from the second peak P2B, and the presence of the fourth peak P4B, which also has an even smaller peak value, more suitably alleviates the viscosity mismatch throughout the entire radial direction of the cladding 12B.

[0066] As described above, the optical fiber 10B according to the third embodiment has a further reduced transmission loss.

[0067] The optical fibers 10A and 10B according to the second and third embodiments can be manufactured by the same method as the optical fiber 10 according to the first embodiment. For example, when manufacturing the optical fiber 10A, a first jacket tube that serves as the base material for the inner layer 12A1 of the cladding 12A and a second jacket tube that serves as the base material for the outer layer 12A2 are manufactured. Then, a core preform is inserted into the first jacket tube, and the core preform and the first jacket tube are inserted into the second jacket tube, and these are heated and integrated to form an optical fiber preform.

[0068] Comparative Example 1 and Examples 1 to 6 As the optical fibers of Comparative Example 1 and Examples 1 to 6 of the present invention, the optical fibers shown below were examined using simulation calculations.

[0069] The optical fiber of Comparative Example 1 has a W-shaped refractive index profile, and is doped with K in a radial distribution that has a peak only in the core portion.

[0070] The optical fibers of Examples 1 and 2 are doped with K in a radial distribution such that the core has a first peak and the cladding has a second peak, as in Embodiment 1. The optical fibers of Examples 1 and 2 also have a trench-type refractive index profile. The value of the second peak was 500 ppm or less.

[0071] The optical fibers of Examples 3 and 4 are doped with K in a radial distribution such that the core has a first peak and the cladding has a second and third peak, as in Example 2. The optical fibers of Examples 3 and 4 also have a W-shaped refractive index profile. The values ​​of the second and third peaks were 500 ppm or less.

[0072] The optical fibers of Examples 5 and 6 were doped with potassium in a radial distribution such that the core had a first peak and the cladding had a second, third, and fourth peaks, as in Example 3. The optical fibers of Examples 5 and 6 also had a W-shaped refractive index profile. The values ​​of the second, third, and fourth peaks were 500 ppm or less.

[0073] The structural parameters (Δ1 [%], Δ2 [%], Δ3 [%], 2a [μm]) of the optical fibers of Comparative Example 1 and Examples 1 to 6, the maximum value [°C / μm] of the slope of the distribution curve of the fictive temperature (dTf / dr), and the effective core area (Aeff) [μm] at a wavelength of 1.55 μm ２ The optical fibers of Examples 1 to 6 had a maximum dTf / dr of 4°C / μm or less and a relatively small transmission loss of 0.174 dB / km or less. In particular, the optical fibers of Examples 3 to 6, which had the third peak, had a transmission loss of 0.164 dB / km or less, and the optical fibers of Examples 5 and 6, which had the fourth peak, had a transmission loss of 0.154 dB / km or less.

[0074] The optical fibers of Examples 1 to 6 have an effective core area of ​​70 μm ２ 180 μm or more２ The effective core area at the wavelength of the input light was 70 μm or less. ２ 180 μm or more ２ In particular, from the viewpoint of suppressing nonlinear optical effects, it is preferable that the effective core area is 100 μm or less. ２ 180 μm or more ２ More preferably, 140 μm or less ２ 180 μm or more ２ The following is even more preferred:

[0075] The optical fibers of Examples 1 to 6 have a cutoff wavelength of 1530 nm or less and can transmit C-band (for example, wavelengths of 1530 nm to 1565 nm) light and L-band (for example, wavelengths of 1565 nm to 1625 nm) light in single mode.

[0076] In the above-described embodiment, an example was shown in which the optical fiber has one peak in the core and a maximum of three peaks in the cladding, but this is not limitative and more peaks may be provided. In this case, the value of the nth peak (n is a natural number) from the inside may be greater than the value of the (n+1)th peak.

[0077] Furthermore, although there are no particular restrictions on the spacing between each peak, it is preferable to configure the spacing between each peak in the radial direction to be 40 μm or less, more preferably 20 μm or less, as this will result in a more gradual change in the fictive temperature of the cladding region.

[0078] It should be noted that the present invention is not limited to the above-described embodiments. The present invention also includes configurations in which the above-described components are appropriately combined. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible.

[0079] 10, 10A, 10B: Optical fiber 11: Core portion 12, 12A, 12B: Cladding portion 12A1, 12B1: Inner layer 12A2, 12B3: Outer layer 12B2: Middle layer A11, A12, A12A1, A12A2, A12B1, A12B2, A12B3: Region P1: First peak P11, P12, P21, P22, P31, P32: Profile P2: Second peak P2A, P2B: Second peak P3A, P3B: Third peak P4B: Fourth peak

Claims

1. An optical fiber comprising: a core portion doped with an alkali metal or alkaline earth metal in a radial distribution having a first peak at or near the center; and a cladding portion surrounding the outer periphery of the core portion, having an average refractive index lower than the maximum refractive index of the core portion, and doped with an alkali metal or alkaline earth metal in a radial distribution having at least a second peak somewhere in the radial direction.

2. The optical fiber according to claim 1, wherein the maximum gradient of the distribution curve of the fictive temperature in the radial direction is 4°C / μm or less.

3. The optical fiber according to claim 1, wherein the cladding portion is doped with alkali metal or alkaline earth metal in a radial distribution having a third peak at a position radially spaced from the second peak.

4. The optical fiber according to claim 3, wherein said cladding portion is doped with an alkali metal or an alkaline earth metal in a radial distribution such that a fourth peak is located at a position radially spaced from said third peak.

5. The optical fiber according to claim 3 or 4, wherein the distance between adjacent peaks in the radial direction is 40 μm or less.

6. The optical fiber according to claim 5, wherein the interval is 20 μm or less.

7. The optical fiber according to claim 1, wherein said alkali metal is potassium.

8. The optical fiber according to claim 1, wherein the core portion is doped with at least one of fluorine and chlorine.

9. The optical fiber according to any one of claims 1 to 4, wherein the peak value of the first peak is greater than the peak value of the second peak.

10. The optical fiber according to claim 3 or 4, wherein the peak value of the first peak is greater than the peak value of the second peak, and the peak value of the second peak is greater than the peak value of the third peak.

11. The optical fiber according to claim 10, wherein the peak value of the second peak is 95% or less of the peak value of the first peak, and the peak value of the third peak is 90% or less of the peak value of the first peak.

12. The optical fiber according to claim 4, wherein the peak value of the first peak is greater than the peak value of the second peak, the peak value of the second peak is greater than the peak value of the third peak, and the peak value of the third peak is greater than the peak value of the fourth peak.

13. The effective core area at the wavelength of the input light is 70 μm ２ 180 μm or more ２ The optical fiber of claim 1 , wherein:

14. The effective core area at the wavelength of the input light is 100 μm ２ 180 μm or more ２ The optical fiber of claim 1 , wherein:

15. The effective core area at the wavelength of the input light is 140 μm ２ 180 μm or more ２ The optical fiber of claim 1 , wherein:

16. The optical fiber according to claim 1, having a transmission loss of 0.174 dB / km or less at a wavelength of 1550 nm.

17. The optical fiber according to claim 1, having a transmission loss of 0.164 dB / km or less at a wavelength of 1550 nm.

18. The optical fiber according to claim 1, having a transmission loss of 0.154 dB / km or less at a wavelength of 1550 nm.

19. The optical fiber according to claim 1, having a cutoff wavelength of 1530 nm or less.

20. The optical fiber according to claim 1, wherein the cladding is doped with at least one of fluorine and chlorine.

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