Optical fibers and methods for manufacturing the same

Abstract

The purpose of the present invention is to provide: an optical fiber having low transmission loss in wide band; and a manufacturing method thereof.　This optical fiber comprises a core part (1a) including a center core doped with germanium, and a cladding part (1b) that has a lower refractive index than the maximum refractive index of the core part and surrounds the outer periphery of the core part. The cladding part has a specific refractive index difference, with respect to pure quartz glass, that is a positive value of 0.1% or lower, and is doped such that an alkaline metal element is distributed in the center core. The peak of the concentration distribution in the radial direction of the alkaline metal element is at a position separated by at least two times the radius of the center core from the center of the center core.

Description

Technical Field

[0001] The present invention relates to an optical fiber and a method for manufacturing the same.

Background Art

[0002] A technique for reducing transmission loss at a wavelength of 1550 nm by doping an alkali metal element or an alkaline earth metal element into a core portion has been disclosed (Patent Documents 1 to 5). For example, in Patent Document 2, an optical fiber in which a core is codoped with germanium (Ge) and an alkali metal element has been proposed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0004] Germanium is the most widely used and has the longest handling record as a dopant for the core portion of an optical fiber. In an optical fiber doped with germanium in the core portion, a product-level report has been made on achieving low transmission loss characteristics of 0.5 dB / km or less in a wide band including OH loss. OH loss refers to the transmission loss at the wavelength of the absorption peak of the OH group, and the wavelength is about 1383 nm.

[0005] However, no method has been proposed to further reduce transmission loss by doping with alkali metal elements, while maintaining the advantage of germanium-doped optical fibers, which is low transmission loss over a wide bandwidth, including OH loss.

[0006] The present invention has been made in view of the above, and its object is to provide an optical fiber with low transmission loss over a wide bandwidth and a method for manufacturing the same. [Means for solving the problem]

[0007] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention provides an optical fiber comprising a core portion including a germanium-doped center core, and a cladding portion having a refractive index lower than the maximum refractive index of the core portion and surrounding the outer periphery of the core portion, wherein the cladding portion has a positive specific refractive index difference of 0.1% or less with respect to pure quartz glass, is doped with alkali metal elements distributed in the center core, and the peak of the radial concentration distribution of the alkali metal elements is located at a position at a distance of at least twice the radius of the center core from the center of the center core.

[0008] The peak of the radial concentration distribution of the alkali metal element may be located at a distance of 3 to 5 times the radius of the center core from the center of the center core.

[0009] The alkali metal element may be potassium.

[0010] The average concentration of the alkali metal element in the center core may be 100 ppm or less.

[0011] The difference in relative refractive index Δ1 between the average maximum refractive index of the center core and the average refractive index of the cladding portion may be 0.2% or more and 0.6% or less.

[0012] It is also acceptable if the transmission loss at a wavelength of 1550 nm is 0.185 dB / km or less.

[0013] The transmission loss at the wavelength of the OH group absorption peak may be 0.5 dB / km or less.

[0014] In the radial direction, the peak of the lowest residual stress may be located on the outer circumference side of the center core.

[0015] The diameter 2a of the center core may be 7.9 μm or more and 13.5 μm or less, and the average maximum relative refractive index difference Δ1 of the center core with respect to the average refractive index of the cladding portion may be 0.21% or more and 0.60% or less.

[0016] The core portion may consist of the center core, the diameter 2a of the center core being 8.0 μm or more and 12.0 μm or less, and the average maximum relative refractive index difference Δ1 of the center core with respect to the average refractive index of the cladding portion being 0.30% or more and 0.60% or less.

[0017] The peak of the radial concentration distribution of the alkali metal element may be located at a distance of 2.0 to 2.8 times the radius of the center core from the center of the center core, and the average concentration of the alkali metal element in the center core may be 50 ppm to 100 ppm.

[0018] The core portion may consist of a center core and a depressed layer formed to surround the outer circumference of the center core, the depressed layer having a refractive index smaller than that of the cladding portion, the diameter 2a of the center core being 8.5 μm or more and 13.5 μm or less, the average maximum relative refractive index difference Δ1 of the center core with respect to the average refractive index of the cladding portion being 0.21% or more and 0.38% or less, the relative refractive index difference Δ2 of the average refractive index of the depressed layer with respect to the average refractive index of the cladding portion being -0.40% or more and -0.03% or less, and the ratio (b / a) of the outer diameter 2b of the depressed layer to 2a being 3.0 or more and 3.6 or less.

[0019] The peak of the concentration distribution of the alkali metal element in the radial direction is located at a position 3.0 times or more and 3.6 times or less the radius of the center core from the center of the center core, and the average concentration of the alkali metal element in the center core may be 25 ppm or more and 60 ppm or less.

[0020] The core part is formed so as to surround the outer periphery of the center core, and is composed of a center core and a stepped layer whose refractive index is smaller than the refractive index of the center core and larger than the refractive index of the clad part. The diameter 2a of the center core is 8.4 μm, the average maximum specific refractive index difference Δ1 of the center core with respect to the average refractive index of the clad part is 0.38%, the specific refractive index difference Δ2 of the average refractive index of the stepped layer with respect to the average refractive index of the clad part is 0.02%, and the ratio (b / a) of the outer diameter 2b of the stepped layer to the 2a may be 3.6.

[0021] The peak of the concentration distribution of the alkali metal element in the radial direction is located at a position 3.6 times the radius of the center core from the center of the center core, and the average concentration of the alkali metal element in the center core may be 20 ppm.

[0022] The core part is formed so as to surround the outer periphery of the center core, and includes an intermediate layer having a refractive index smaller than the maximum refractive index of the center core, and a trench layer formed so as to surround the outer periphery of the intermediate layer and having a refractive index smaller than the refractive index of the cladding part. The diameter 2a of the center core is 7.9 μm or more and 11.8 μm or less. The average maximum specific refractive index difference Δ1 of the center core with respect to the average refractive index of the cladding part is 0.27% or more and 0.40% or less. The specific refractive index difference Δ2 of the intermediate layer with respect to the average refractive index of the cladding part is -0.05% or more and 0.05% or less. The specific refractive index difference Δ3 of the trench layer with respect to the average refractive index of the cladding part is -0.60% or more and -0.12% or less. The ratio (b / a) of the outer diameter 2b of the intermediate layer to 2a is 2.0 or more and 3.0 or less. The ratio (c / a) of the outer diameter 2c of the trench layer to 2a is 3.0 or more and 5.0 or less.

[0023] The peak of the concentration distribution of the alkali metal element in the radial direction is located at a position 3.0 times or more and 5.0 times or less the radius of the center core from the center of the center core, and the average concentration of the alkali metal element in the center core is 5 ppm or more and 55 ppm or less.

[0024] One aspect of the present invention is a method for manufacturing the optical fiber, comprising: synthesizing, in a batch synthesis process, a portion corresponding to the center core and a portion corresponding to a position 2 times or more the radius of the portion corresponding to the center core from the center of the portion corresponding to the center core to produce a center core rod; disposing, on the outer periphery of the center core rod, a glass pipe having an inner surface doped with an alkali metal element; diffusing the alkali metal element to a site corresponding to the center core; and drawing an optical fiber from an optical fiber preform including the center core rod and the glass pipe.

[0025] The batch synthesis process may be a VAD (Vapor-phase Axial Deposition) method.

Advantages of the Invention

[0026] The present invention offers the advantage of realizing optical fibers with low transmission loss over a wide bandwidth. [Brief explanation of the drawing]

[0027] [Figure 1] Figure 1 is a schematic cross-sectional view of an optical fiber according to an embodiment, in a plane perpendicular to the longitudinal direction. [Figure 2A] Figure 2A is a schematic diagram of the refractive index profile of an optical fiber according to the embodiment. [Figure 2B] Figure 2B is a schematic diagram of the refractive index profile of an optical fiber according to the embodiment. [Figure 2C] Figure 2C is a schematic diagram of the refractive index profile of an optical fiber according to the embodiment. [Figure 2D] Figure 2D is a schematic diagram of the refractive index profile of the optical fiber according to the embodiment. [Figure 3] Figure 3 shows an example of the relationship between radial position, refractive index profile, and K concentration. [Figure 4] Figure 4 shows an example of the relationship between the center core radius ratio at the alkali-doped peak position and the OH loss and 1550 nm loss. [Figure 5] Figure 5 shows an example of the relationship between radial position, K concentration, and residual stress. [Modes for carrying out the invention]

[0028] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to the embodiments described below. In each drawing, the same or corresponding components are appropriately denoted by the same reference numerals. In this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength (λcc) as defined in ITU-T G.650.1 of the International Telecommunication Union (ITU). Furthermore, terms not specifically defined in this specification shall be defined and measured according to the definitions and measurement methods in G.650.1 and G.650.2.

[0029] (Embodiment) Figure 1 is a schematic cross-sectional view of an optical fiber according to an embodiment, in a plane perpendicular to the longitudinal direction. The optical fiber 1 comprises a core portion 1a and a cladding portion 1b surrounding the outer circumference of the core portion 1a. The portion of the optical fiber 1 comprising the core portion 1a and the cladding portion 1b is made of glass and may be referred to as a glass optical fiber. The optical fiber 1 also comprises a coating layer 1c surrounding the outer circumference of the cladding portion 1b. The coating layer 1c has a primary layer 1ca surrounding the outer circumference of the cladding portion 1b and a secondary layer 1cb surrounding the outer circumference of the primary layer 1ca. The optical fiber 1 comprising the coating layer 1c may be referred to as an optical fiber core.

[0030] The primary layer 1ca and the secondary layer 1cb are made of resin. This resin is, for example, an ultraviolet-curable resin. The ultraviolet-curable resin is a mixture of various resin materials and additives, such as oligomers, diluent monomers, photopolymerization initiators, silane coupling agents, sensitizers, and lubricants. As the oligomer, conventionally known materials such as polyether-based urethane acrylate, epoxy acrylate, polyester acrylate, and silicone acrylate can be used. As the diluent monomer, conventionally known materials such as monofunctional monomers and polyfunctional monomers can be used. Furthermore, the additives are not limited to those mentioned above, and a wide range of conventionally known additives used for ultraviolet-curable resins can be used.

[0031] The optical fiber 1 has a refractive index profile as shown in Figures 2A, 2B, 2C, and 2D. Figures 2A, 2B, 2C, and 2D all show the refractive index profile in the radial direction from the central axis of the core portion 1a of the optical fiber 1. The refractive index profile is shown as the difference in relative refractive index with respect to pure quartz glass. Here, pure quartz glass is an extremely high-purity quartz glass that substantially does not contain dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm.

[0032] Figure 2A shows a step-type refractive index profile. In Figure 2A, profile P11 shows the refractive index profile of the core 1a, and profile P12 shows the refractive index profile of the cladding 1b. In a step-type refractive index profile, the diameter of the core 1a is 2a, and the relative refractive index difference (maximum relative refractive index difference) of the average maximum refractive index of the core 1a to the average refractive index of the cladding 1b is Δ1. Also, the relative refractive index difference of the average refractive index of the cladding 1b to the refractive index of the pure quartz glass is Δclad. In the case of Figure 2(a), the center core, which is the part of the core 1a with the highest average refractive index, corresponds to the entire core 1a. That is, Figure 2A is an example where the core consists of a center core.

[0033] Figure 2B shows a so-called W-type refractive index profile. In Figure 2B, profile P21 shows the refractive index profile of the core portion 1a, and profile P22 shows the refractive index profile of the cladding portion 1b. In the W-type refractive index profile, the core portion 1a consists of a center core with a diameter of 2a and a depressed layer formed to surround the outer circumference of the center core. The depressed layer has a refractive index smaller than that of the cladding portion, with an inner diameter of 2a and an outer diameter of 2b. The center core is the part of the core portion 1a with the highest average refractive index. The difference in the average maximum relative refractive index of the center core compared to the average refractive index of the cladding portion 1b is Δ1. The difference in relative refractive index of the average refractive index of the depressed layer compared to the average refractive index of the cladding portion 1b is Δ2. Furthermore, the difference in relative refractive index of the average refractive index of the cladding portion 1b compared to the refractive index of the pure quartz glass is Δclad.

[0034] Figure 2B shows an example where the core section includes a center core and a depressed layer.

[0035] Figure 2C shows a so-called trench-type refractive index profile. In Figure 2C, profile P31 shows the refractive index profile of the core portion 1a, and profile P32 shows the refractive index profile of the cladding portion 1b. In the trench-type refractive index profile, the core portion 1a consists of a center core with a diameter of 2a, an intermediate layer formed to surround the outer circumference of the center core with a refractive index smaller than the maximum refractive index of the center core, an inner diameter of 2a and an outer diameter of 2b, and a trench layer formed to surround the outer circumference of the intermediate layer with a refractive index smaller than the refractive index of the cladding portion, an inner diameter of 2b and an outer diameter of 2c. The center core is the part of the core portion 1a with the maximum average refractive index. The difference in the average maximum specific refractive index of the center core with respect to the average refractive index of the cladding portion 1b is Δ1. The difference in the specific refractive index of the intermediate layer with respect to the average refractive index of the cladding portion 1b is Δ2. The difference in the specific refractive index of the trench layer with respect to the average refractive index of the cladding portion 1b is Δ3. Furthermore, the relative refractive index difference between the average refractive index of the cladding 1b and the refractive index of the pure quartz glass is Δclad. Note that Δ2 is usually set to be equal to or near 0%. Equivalent to or near 0% means, for example, in the range of -0.05% to 0.05%. It has been found that if Δ2 is in the range of -0.05% to 0.05%, it does not significantly affect the optical properties of the optical fiber.

[0036] Figure 2C shows an example where the core section includes a center core, an intermediate layer, and a trench layer.

[0037] Figure 2D shows a so-called stepped refractive index profile. In Figure 2D, profile P41 shows the refractive index profile of the core portion 1a, and profile P42 shows the refractive index profile of the cladding portion 1b. In the stepped refractive index profile, the core portion 1a consists of a center core with a diameter of 2a and a stepped layer surrounding the outer circumference of the center core. This stepped layer has an inner diameter of 2a and an outer diameter of 2b, and its refractive index is smaller than that of the center core but larger than that of the cladding portion. The center core is the part of the core portion 1a with the highest average refractive index. The difference in the average maximum relative refractive index of the center core compared to the average refractive index of the cladding portion 1b is Δ1. The difference in the relative refractive index of the average refractive index of the stepped layer compared to the average refractive index of the cladding portion 1b is Δ2. Furthermore, the difference in the relative refractive index of the average refractive index of the cladding portion 1b compared to the refractive index of pure quartz glass is Δclad.

[0038] Figure 2D shows an example where the core section includes a center core and a stepped layer.

[0039] Here, the refractive index profile of the center core of the core part 1a may not only be a geometrically ideal step shape, but the shape of the top may not be flat, but may have irregularities due to manufacturing characteristics, or it may have a shape that extends from the top to the bottom. In this case, the refractive index of the region where the top of the refractive index profile is approximately flat within the range of the core diameter 2a of the core part 1a in the manufacturing design serves as an indicator for determining Δ1. Even if there appear to be multiple approximately flat regions, or if a continuous change occurs and it is difficult to define an approximately flat region, we have confirmed that it is possible to obtain characteristics close to the desired ones as long as at least one part of the core other than the part where the refractive index changes abruptly toward the adjacent layer falls within the range of Δ1 described below, and the difference between the maximum and minimum values ​​of Δ is within a certain value ±30%, so there is no particular problem.

[0040] Furthermore, the average refractive index of the depressed layer, intermediate layer, trench layer, stepped layer, and cladding 1b is the average value of the refractive index in the radial direction of the refractive index profile. The refractive index of cladding 1b is lower than the maximum refractive index of core 1a.

[0041] Next, the constituent materials of the core portion 1a and cladding portion 1b of the optical fiber 1 will be described. First, the cladding portion 1b is made of a silica-based glass in which the relative refractive index difference with respect to pure silica glass is a positive value of 0.1% or less, for example, due to chlorine (Cl). The cladding portion 1b does not necessarily have to contain any dopants other than Cl that change the refractive index.

[0042] Next, the center core of the core section 1a is made of quartz glass doped with Ge or alkali metal elements. Examples of alkali metal elements include potassium (K) and sodium (Na). Alkali metal elements act as dopants that increase the refractive index of the quartz glass and decrease its viscosity. Note that the alkali metal elements may also be doped as compounds, such as potassium compounds or sodium compounds. The center core may also be doped with Cl.

[0043] The stepped layer of the core 1a is made of quartz glass doped with Ge or alkali metal elements. The stepped layer may also be doped with Cl.

[0044] The depressed layer and trench layer of the core portion 1a are made of quartz glass doped with fluorine or boron, which are refractive index-reducing dopants that lower the refractive index. The intermediate layer is made of quartz glass with a refractive index that is the same as or close to that of the cladding portion 1b. Here, fluorine is more preferable as the refractive index-reducing dopant from the viewpoint of manufacturability. Note that fluorine may be doped as a fluorine compound. Furthermore, the depressed layer, trench layer, and intermediate layer may be doped with Cl.

[0045] Furthermore, if the desired refractive index profile is achieved, the layers other than the center core and the stepped layer in the core portion 1a, or the cladding portion 1b, may be doped with alkali metal elements.

[0046] Next, we will specifically explain the concentration distribution of alkali metal elements in optical fiber 1. Optical fiber 1 is doped with alkali metal elements so that they are distributed in the center core of the core portion 1a, and the peak of the radial concentration distribution of alkali metal elements is located at a position more than twice the radius of the center core from the center of the center core. In the following explanation, we will assume that the alkali metal element is potassium (K).

[0047] Figure 3 shows an example of the relationship between radial position, refractive index profile, and K concentration. The position where the radial position is zero is the central axis of core 1a, which is the central axis of the center core. Also, the region with a large specific refractive index in the refractive index profile is the center core. As shown in Figure 3, K is doped to be distributed in the center core, and the peak of the K concentration is located at a position more than twice the radius of the center core from the center of the center core.

[0048] When manufacturing such an optical fiber 1, for example, a center core rod made of silica-based glass is first synthesized using a single synthesis process, which involves synthesizing the portion corresponding to the center core and the portion corresponding to the area at a distance of more than twice the radius of the center core from the center of the center core portion. Here, the single synthesis process refers to methods such as VAD (Vapor-phase Axial Deposition) or MCVD (Modified Chemical Vapor Deposition).

[0049] Next, a glass pipe doped with alkali metal element K on its inner surface is placed around the outer circumference of the center core rod, and the center core rod and the glass pipe are integrated by heat treatment. Through this heat treatment for integration, or through heat treatment for integration and additional treatment, K diffuses to the area corresponding to the center core. After that, an optical fiber 1 is drawn from the optical fiber base material including the center core rod and the glass pipe.

[0050] According to the above manufacturing process, OH groups may be introduced to the surface of the center core rod, but the surface of the center core rod is located at a distance of more than twice the radius of the part corresponding to the center core from the center of the part corresponding to the center core. Therefore, in the manufactured optical fiber 1, the location where OH groups exist is also at a distance of more than twice the radius of the center core from the center of the center core. As a result, since the OH groups are located away from the region of high light intensity in optical fiber 1, OH loss is suppressed. Furthermore, since the center core is doped with K, the transmission loss at a wavelength of 1550 nm in optical fiber 1 is also suppressed.

[0051] As described above, the optical fiber 1 according to Embodiment 1 has suppressed transmission loss and OH loss at a wavelength of 1550 nm, and is an optical fiber with low transmission loss over a wide bandwidth.

[0052] Furthermore, in optical fiber 1, the transmission loss at a wavelength of 1550 nm is, for example, 0.185 dB / km or less. Also, the OH loss is, for example, 0.5 dB / km or less.

[0053] Next, we will explain the relationship between the peak position of the K concentration and the transmission loss and OH loss at a wavelength of 1550 nm. Figure 4 shows an example of the relationship between the center core radius ratio of the alkali-doped peak position and the OH loss and 1550 nm loss. The center core radius ratio of the alkali-doped peak position (hereinafter sometimes abbreviated as the center core radius ratio) is the value obtained by normalizing the distance from the center of the center core to the position of the K concentration peak in the radial direction by the radius of the center core. Also, the 1550 nm loss refers to the transmission loss at a wavelength of 1550 nm.

[0054] As shown in Figure 4, increasing the center-core radius ratio from 1 reduces the OH loss (OH peak loss). However, when the center-core radius ratio is increased from 1, the 1550nm loss initially decreases but then increases. This is thought to be because increasing the center-core radius ratio too much from 1 reduces the structural relaxation-promoting effect of alkali metal elements during demarcation in the region of high light intensity in optical fiber 1. Therefore, a center-core radius ratio of 3 or more and 5 or less, that is, when the peak of the radial concentration distribution of alkali metal elements is located at a distance of 3 to 5 times the radius of the center core from the center core, is preferable in terms of balancing the effect of reducing OH groups and the effect of reducing 1550nm loss.

[0055] Next, the inventors investigated the change in average transmission loss at a wavelength of 1550 nm when the Δ1 of the center core and the average concentration of K in the center core (average K concentration) were varied. Here, the average K concentration refers to the average concentration of K in the radial direction. The refractive index profiles of the optical fiber samples used in the investigation were step type, W type, stepped type, and trench type. The core diameter was adjusted to (1) a cable cutoff wavelength of 1200 nm, or (2) a cable cutoff wavelength of 1500 nm. The average transmission loss mentioned above is the average transmission loss of the samples fabricated under these various conditions.

[0056] The investigation revealed a strong correlation between Δ1 and average K concentration and average transmission loss, but no strong correlation was found between refractive index profile or cable cutoff and average transmission loss.

[0057] Table 1 shows the average transmission loss at a wavelength of 1550 nm when Δ1 and the average K concentration are varied. As can be seen from Table 1, when Δ1 is between 0.2% and 0.6% or when the average K concentration is 100 ppm or less, it is preferable to keep the transmission loss at a wavelength of 1550 nm below 0.185 dB / km. This is because when Δ1 is 0.2% or more, the increase in transmission loss due to bending loss is less likely to occur, and when Δ1 is 0.6% or less, the effect of Rayleigh scattering loss due to the center core dopant is small.

[0058] [Table 1]

[0059] Next, the residual stress in the optical fiber 1 according to the embodiment will be described. During the manufacturing process of the optical fiber 1, K thermally diffuses from the doped area, generating residual compressive stress in a wide area including the center core. Figure 5 shows an example of the relationship between the radial position, K concentration, and residual stress in an optical fiber fabricated as an embodiment of optical fiber 1. The center core radius of the fabricated optical fiber is approximately 4 μm. Note that tensile stress is shown as a positive value and compressive stress as a negative value for residual stress.

[0060] In Figure 5, the peak of the K concentration corresponds to the location where K is doped (the location on the surface of the center core rod). In other words, Figure 5 shows that residual compressive stress exists in the optical fiber over a wide region, with the peak at the location where K is doped. This means that structural relaxation is progressing in this region during fiber drawing, which is reflected in the reduction of transmission loss at a wavelength of 1550 nm.

[0061] As shown in Figure 5, a desirable condition is when the peak of the lowest residual stress is located on the outer circumference of the center core in the radial direction.

[0062] (Examples) As an embodiment of the present invention, an optical fiber similar to the optical fiber according to the embodiment was manufactured by either method (1) or method (2) below.

[0063] Method (1): First, a core rod (an example of a center core rod) having a portion corresponding to the core of an optical fiber and a portion corresponding to a part of the cladding was manufactured by batch synthesis using a known VAD apparatus. Next, a tube corresponding to the remaining cladding portion was prepared by a tube manufacturing method. Then, potassium chloride (KCl) raw material was heated above its melting point in an electric furnace to melt and evaporate it, and aerosol particles were generated by a cooling gas. These were then transported into the inside of the tube with an Ar carrier gas, and potassium was deposited on the inner surface. After that, the core rod was inserted into the tube, the inside was evacuated, and a collapse treatment was performed by applying an oxyhydrogen flame to the outside of the tube to obtain an optical fiber preform. Next, an optical fiber was drawn from this optical fiber preform. Note that the potassium diffuses both in the central direction (center core direction) and radially outward (cladding side) by each heat treatment process performed after deposition (especially the heat treatment during the collapse treatment), and is doped into desired areas such as the center core. Furthermore, during the line drawing process, line drawing conditions such as line drawing speed and line drawing tension were optimized to minimize transmission loss.

[0064] Method (2): Similar to Method (1), a core rod having a portion corresponding to the core of the optical fiber and a portion corresponding to part of the cladding was manufactured by batch synthesis using a known VAD apparatus. Subsequently, aerosol particles generated in the same manner as in Method (1) were mixed with oxyhydrogen gas and passed through a VAD burner to deposit potassium as uniformly as possible on the entire surface of the core rod. Then, the portion corresponding to the remaining cladding was formed using the VAD method or the jacket method to obtain an optical fiber preform. Subsequently, an optical fiber was drawn from this optical fiber preform.

[0065] Table 2 shows the design parameters and optical properties of the optical fibers of the manufactured samples No. 1 to 16. In Table 2, "Alkali concentration peak position" is the distance from the center of the center core to the position of the K concentration peak in the radial direction, normalized by the radius of the center core. "Center core alkali concentration average value" is the average concentration of alkali metal elements in the center core. "Aeff" is the effective core cross-sectional area. Regarding the refractive index profile, Nos. 1 to 5 are step type, Nos. 6 to 10 are W type, No. 11 is step type, and Nos. 12 to 16 are trench type.

[0066] [Table 2]

[0067] All of the optical fibers from No. 1 to 16 exhibited low transmission loss and OH loss at a wavelength of 1550 nm. Furthermore, various values ​​for λcc and Aeff were achieved.

[0068] In particular, optical fiber No. 7 has a Δ1 of 0.38%, a Δ2 of -0.05%, an alkali concentration peak position of 3.2, a b / a ratio of 3.2, a 2a ratio of 8.6 μm, and an average center core alkali concentration of 40 ppm. As a result, the transmission loss is 0.173 dB / km, the OH loss is 0.34 dB / km, the λcc is 1192 nm, and the Aeff is 72 μm. 2 This characteristic is obtained, and it is desirable that good characteristics are achieved with a small absolute value of Δ2.

[0069] Furthermore, for No. 14, Δ1 is 0.27%, Δ2 is 0%, Δ3 is -0.17%, the alkali concentration peak position is 4.0, b / a is 2.5, c / a is 4.0, 2a is 11.8 μm, and the average center core alkali concentration is 5 ppm. As a result, the transmission loss is 0.178 dB / km, the OH loss is 0.32 dB / km, λcc is 1503 nm, and Aeff is 123 μm. 2 This characteristic is achieved, and it is desirable that good characteristics are realized while maintaining a high Aeff.

[0070] Furthermore, these optical fibers did not pose any particular problems when used in experiments involving related technologies such as cabling and connection.

[0071] Furthermore, a preferred example is when 2a is between 7.9 μm and 13.5 μm, and Δ1 is between 0.21% and 0.60%.

[0072] In particular, a preferred example for the step-type refractive index profile is when 2a is between 8.0 μm and 12.0 μm, and Δ1 is between 0.30% and 0.60%. In the step-type case, for example, the alkali concentration peak position is between 2.0 and 2.8, and the average center core alkali concentration is between 50 ppm and 100 ppm.

[0073] Furthermore, a suitable example for a W-type refractive index profile is when 2a is between 8.5 μm and 13.5 μm, Δ1 is between 0.21% and 0.38%, Δ2 is between -0.40% and -0.03%, and b / a is between 3.0 and 3.6. For a W-type profile, for example, the alkali concentration peak position is between 3.0 and 3.6, and the average center core alkali concentration is between 25 ppm and 60 ppm.

[0074] Furthermore, a suitable example for a stepped refractive index profile is one where 2a is 8.4 μm, Δ1 is 0.38%, Δ2 is 0.02%, and b / a is 3.6. In the case of a stepped profile, for example, the alkali concentration peak position is 3.6 and the average center core alkali concentration is 20 ppm.

[0075] Furthermore, a suitable example for a trench-type refractive index profile is when 2a is 7.9 μm or more and 11.8 μm or less, Δ1 is 0.27% or more and 0.40% or less, Δ2 is -0.05% or more and 0.05% or less, Δ3 is -0.60% or more and -0.12% or less, b / a is 2.0 or more and 3.0 or less, and c / a is 3.0 or more and 5.0 or less. In the case of a trench type, for example, the alkali concentration peak position is 3.0 or more and 5.0 or less, and the average center core alkali concentration is 5 ppm or more and 55 ppm or less.

[0076] The method of potassium doping is not limited to the method described in the above examples. For example, when manufacturing a core rod, silica soot may be prepared first, then pre-sintered in a temperature range where densification does not occur, and potassium may be doped into the pre-sintered body by liquid immersion. Potassium nitrate, iodide, bromide, etc., may also be used instead of potassium chloride. Furthermore, when doping with sodium instead of potassium, various sodium compounds can be used.

[0077] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible. [Explanation of Symbols]

[0078] 1: Optical fiber 1a: Core section 1b: Clad section 1c: Covering layer 1ca: Primary layer 1cb: Secondary layer

Claims

1. The core section includes a germanium-doped center core, The cladding portion has a refractive index lower than the maximum refractive index of the core portion and surrounds the outer circumference of the core portion, Equipped with, The cladding portion has a positive value of 0.1% or less in relative refractive index difference with respect to pure quartz glass. Alkali metal elements are doped so that they are distributed in the region including the center core. The peak of the radial concentration distribution of the alkali metal element is located at a position that is more than 2 times but less than 5 times the radius of the center core from the center of the center core. Optical fiber.

2. The peak of the radial concentration distribution of the alkali metal element is located at a position at a distance of three times or more the radius of the center core from the center of the center core. The optical fiber according to claim 1.

3. The alkali metal element is potassium. The optical fiber according to claim 1.

4. The average concentration of the alkali metal element in the center core is 100 ppm or less. The optical fiber according to claim 1.

5. The relative refractive index difference Δ1 between the average maximum refractive index of the center core and the average refractive index of the cladding portion is 0.2% or more and 0.6% or less. The optical fiber according to claim 1.

6. The transmission loss at a wavelength of 1550 nm is 0.185 dB / km or less. The optical fiber according to claim 1.

7. The transmission loss at the wavelength of the absorption peak of the OH group is 0.5 dB / km or less. The optical fiber according to claim 1.

8. In the radial direction, the peak of the lowest residual stress is located on the outer circumference side of the center core. The optical fiber according to claim 1.

9. The diameter 2a of the center core is 7.9 μm or more and 13.5 μm or less, and the average maximum relative refractive index difference Δ1 of the center core with respect to the average refractive index of the cladding portion is 0.21% or more and 0.60% or less. The optical fiber according to claim 1.

10. The core portion consists of the center core, The diameter 2a of the center core is 8.0 μm or more and 12.0 μm or less. The average maximum relative refractive index difference Δ1 between the center core and the average refractive index of the cladding portion is 0.30% or more and 0.60% or less. The optical fiber according to claim 9.

11. The peak of the radial concentration distribution of the alkali metal element is located at a distance of 2.0 to 2.8 times the radius of the center core from the center of the center core. The average concentration of the alkali metal element in the center core is 50 ppm or more and 100 ppm or less. The optical fiber according to claim 10.

12. The core portion is composed of the center core and a depressed layer formed to surround the outer circumference of the center core, the depressed layer having a refractive index smaller than that of the cladding portion. The diameter 2a of the center core is 8.5 μm or more and 13.5 μm or less, and the average maximum relative refractive index difference Δ1 of the center core with respect to the average refractive index of the cladding portion is 0.21% or more and 0.38% or less. The relative refractive index difference Δ2 between the average refractive index of the cladding portion and the average refractive index of the depressed layer is -0.40% or more and -0.03% or less. The ratio (b / a) of the outer diameter 2b of the depressed layer to 2a is 3.0 or more and 3.6 or less. The optical fiber according to claim 9.

13. The peak of the radial concentration distribution of the alkali metal element is located at a distance of 3.0 to 3.6 times the radius of the center core from the center of the center core. The average concentration of the alkali metal element in the center core is 25 ppm or more and 60 ppm or less. The optical fiber according to claim 12.

14. The core portion is composed of the center core and a stepped layer formed to surround the outer circumference of the center core, the stepped layer having a refractive index lower than that of the center core and higher than that of the cladding portion. The diameter 2a of the center core is 8.4 μm, and the average maximum relative refractive index difference Δ1 of the center core with respect to the average refractive index of the cladding portion is 0.38%. The relative refractive index difference Δ2 between the average refractive index of the cladding portion and the average refractive index of the stepped layer is 0.02%. The ratio (b / a) of the outer diameter 2b of the step layer to 2a is 3.

6. The optical fiber according to claim 9.

15. The peak of the radial concentration distribution of the alkali metal element is located at a position 3.6 times the radius of the center core from the center of the center core. The average concentration of the alkali metal element in the center core is 20 ppm. The optical fiber according to claim 14.

16. The core portion is composed of the center core, an intermediate layer formed to surround the outer periphery of the center core and having a refractive index smaller than the maximum refractive index of the center core, and a trench layer formed to surround the outer periphery of the intermediate layer and having a refractive index smaller than the refractive index of the cladding portion. The diameter 2a of the center core is 7.9 μm or more and 11.8 μm or less, and the average maximum relative refractive index difference Δ1 of the center core with respect to the average refractive index of the cladding portion is 0.27% or more and 0.40% or less. The difference in relative refractive index Δ2 between the average refractive index of the cladding portion and the intermediate layer is -0.05% or more and 0.05% or less. The difference in specific refractive index Δ3 between the average refractive index of the cladding portion and the trench layer is -0.60% or more and -0.12% or less. The ratio (b / a) of the outer diameter 2b of the intermediate layer to 2a is 2.0 or more and 3.0 or less. The ratio (c / a) of the outer diameter 2c of the trench layer to 2a is 3.0 or more and 5.0 or less. The optical fiber according to claim 9.

17. The peak of the radial concentration distribution of the alkali metal element is located at a distance of 3.0 to 5.0 times the radius of the center core from the center of the center core. The average concentration of the alkali metal element in the center core is 5 ppm or more and 55 ppm or less. The optical fiber according to claim 16.

18. A method for manufacturing an optical fiber according to claim 1, A center core rod is manufactured by synthesizing a portion corresponding to the center core and a portion corresponding to a position at a distance of more than twice the radius of the portion corresponding to the center core from the center of the portion corresponding to the center core in a single synthesis process. A glass pipe doped with an alkali metal element on its inner surface is placed on the outer circumference of the aforementioned center core rod. The alkali metal element is diffused to the area corresponding to the center core. An optical fiber is drawn from the optical fiber base material, which includes the center core rod and the glass pipe. A method for manufacturing optical fibers.

19. The aforementioned batch synthesis process is the VAD (Vapor-phase Axial Deposition) method. The method for manufacturing an optical fiber according to claim 18.

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