Optical Fiber

Abstract

To provide an optical fiber that serves as a few-mode fiber having low transmission loss while having a simple structure and being low-cost.SOLUTION: An optical fiber comprises: a core part that is composed of a center core or composed of a center core and two or less-layer core layers surrounding an outer periphery of the center core; and a cladding part that has a lower refractive index than a maximum refractive index of the core part and that of a pure quartz glass, and surrounds an outer periphery of the core part. Light is propagated in two or more propagation modes as an LP propagation mode at a wavelength of 1550 nm.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to optical fibers. [Background technology]

[0002] A few-mode fiber has been proposed that achieves an ultra-low transmission loss of 0.18 dB / km or less at a wavelength of 1550 nm by doping the cladding with a dopant that lowers the refractive index, such as fluorine (Patent Document 1). Here, a few-mode fiber is an optical fiber that propagates light in two or more propagation modes, which are LP (Linear Polarized) propagation modes.

[0003] In an optical fiber with a cladding refractive index lower than that of pure silica glass, it is possible to reduce or almost eliminate the dopant in the core, which makes it possible to reduce the Rayleigh scattering loss caused by the concentration distribution of dopants such as germanium (Ge) in the core and thereby realize low transmission loss. Patent Document 1 discloses the results of achieving a transmission loss of 0.18 dB / km or less at a wavelength of 1550 nm in all of the up to four LP propagation modes.

[0004] On the other hand, as multicore fibers that realize an ultra-low transmission loss of 0.18 dB / km or less at a wavelength of 1550 nm, many coupled-type multicore fibers as disclosed in Non-Patent Document 1 have already been reported. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2017-526960 [Non-patent literature]

[0006] [Non-Patent Document 1] Hiroyuki Sakuma et al., Institute of Electronics, Information and Communication Engineers, EXAT Study Group, EXAT2020-17 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the optical fiber disclosed in Patent Document 1 has a four-layer core structure, including a three-layer structure made of a Ge-free core and fluorine-containing silica glass, and a fluorine-doped trench layer on the outside. When the core structure has a large number of layers like this, more complicated process control is required in the optical fiber manufacturing process, resulting in a problem that the optical fiber becomes expensive.

[0008] In addition, in Patent Document 1, the difference in refractive index within the core structure is very large, specifically, the difference between Δ1 and Δ2 is very large, so a large amount of dopant is used, which also poses the problem of making the optical fiber expensive.

[0009] The present invention has been made in view of the above, and an object of the present invention is to provide an optical fiber as a few-mode fiber which has a simple structure, is inexpensive, and yet has low transmission loss. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems and achieve the object, one embodiment of the present invention is an optical fiber comprising a core portion consisting of a center core, or consisting of a center core and two or less core layers surrounding the outer periphery of the center core, and a cladding portion surrounding the outer periphery of the core portion and having a refractive index lower than the maximum refractive index of the core portion and pure silica glass, and which propagates light in two or more propagation modes, which are LP propagation modes, at a wavelength of 1550 nm.

[0011] At a wavelength of 1550 nm, the transmission loss in each of the two or more propagation modes may be 0.20 dB / km or less.

[0012] The two or more propagation modes may be a combination of two or more of an LP01 mode, an LP11 mode, an LP21 mode, and an LP02 mode.

[0013] The two or more propagation modes may be two propagation modes, an LP01 mode and an LP11 mode.

[0014] At least a portion of the cladding may be made of fluorine-doped silica glass.

[0015] The center core may be made of silica glass doped with at least one of potassium and sodium.

[0016] The radially outer side of the center core may be doped with at least one of potassium and sodium up to a radial position where the light intensity is 10% of a peak value for all of the two or more propagation modes.

[0017] At a wavelength of 1550 nm, the transmission loss in each of the two or more propagation modes may be 0.19 dB / km or less.

[0018] At a wavelength of 1550 nm, the transmission loss in each of the two or more propagation modes may be 0.18 dB / km or less.

[0019] It may have a step-type refractive index profile.

[0020] It may have a W-shaped refractive index profile.

[0021] It may have a step-type refractive index profile.

[0022] It may have a trench type refractive index profile.

[0023] If the relative refractive index difference of the maximum refractive index of the center core with respect to pure silica glass is Δ1, and the relative refractive index difference of the refractive index of the region adjacent to the center core with respect to pure silica glass is Δ2, (Δ1-Δ2) is 0.40% or more and 0.70% or less, and for all of the two or more propagation modes, the effective core area at a wavelength of 1550 nm is 240 μm 2 The following may also be true:

[0024] (Δ1-Δ2) may be 0.40% or more and 0.55% or less.

[0025] Compressive stress may be present in the center core and up to a radial position on the radial outside of the center core where the light intensity is 10% of a peak value for all of the two or more propagation modes.

[0026] The position where the compressive stress reaches its peak may be at least at an outer position of the center core.

[0027] At a wavelength of 1550 nm, the difference in transmission loss between the two or more propagation modes may be 0.02 dB / km or less.

[0028] The microbending loss at a wavelength of 1550 nm measured by the sandpaper method may be 1 dB / km or less. Effect of the Invention

[0029] The present invention has an effect of realizing a few-mode fiber that has a simple structure, is inexpensive, and yet has low transmission loss. [Brief description of the drawings]

[0030] [Figure 1] FIG. 1 is a schematic cross-sectional view of an optical fiber according to an embodiment, taken along a plane perpendicular to the longitudinal direction thereof. [Diagram 2] FIG. 2 is a schematic diagram of a refractive index profile of the optical fiber according to the embodiment. [Diagram 3]FIG. 3 is a diagram showing an example of the relationship between the radial position and the optical field intensity, the refractive index profile, and the K or Na doped region. [Figure 4] FIG. 4 is a diagram showing an example of the relationship between the relative light intensity to the peak at the boundary where the dopant is doped and the transmission loss. [Diagram 5] FIG. 5 is a graph showing an example of the relationship between the effective core area and the normalized microbending loss. [Figure 6] FIG. 6 is a diagram showing an example of the relationship between the core diameter and the effective core cross-sectional area. [Figure 7] FIG. 7 is a diagram showing an example of the relationship between the radial position and the residual stress and the refractive index profile. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiment described below. In addition, in each drawing, the same or corresponding components are appropriately assigned the same reference numerals. In addition, 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). In addition, other terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0032] (Embodiment) FIG. 1 is a schematic cross-sectional view of an optical fiber according to an embodiment taken along a plane perpendicular to the longitudinal direction. The optical fiber 1 is made of silica glass and includes a core 1a and a cladding 1b surrounding the outer periphery of the core 1a. The portion of the optical fiber 1 including the core 1a and the cladding 1b is a portion of the optical fiber made of glass, and may be referred to as a glass optical fiber. The optical fiber 1 also includes a coating layer 1c surrounding the outer periphery of the cladding 1b. The coating layer 1c includes a primary layer 1ca surrounding the outer periphery of the cladding 1b, and a secondary layer 1cb surrounding the outer periphery of the primary layer 1ca. The optical fiber including the coating layer 1c may be referred to as an optical fiber core.

[0033] The primary layer 1ca and the secondary layer 1cb are made of resin. This resin is, for example, an ultraviolet curing resin. The ultraviolet curing resin is, for example, a mixture of various resin materials and additives, such as oligomers, diluting monomers, photopolymerization initiators, silane coupling agents, sensitizers, and lubricants. As the oligomer, conventionally known materials such as polyether urethane acrylate, epoxy acrylate, polyester acrylate, and silicone acrylate can be used. As the diluting monomer, conventionally known materials such as monofunctional monomers and polyfunctional monomers can be used. In addition, the additives are not limited to those described above, and conventionally known additives used for ultraviolet curing resins and the like can be widely used.

[0034] The optical fiber 1 has a refractive index profile as shown in Fig. 2, for example. Figures 2(a), (b), (c), and (d) all show the refractive index profile in the radial direction from the central axis of the core 1a of the optical fiber 1. The refractive index profile is shown in terms of the relative refractive index difference with respect to pure silica glass. Here, pure silica glass refers to 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.

[0035] FIG. 2(a) shows a step-type refractive index profile. In FIG. 2(a), profile P11 shows the refractive index profile of the core portion 1a, and profile P12 shows the refractive index profile of the cladding portion 1b. In the step-type refractive index profile, the diameter (core diameter) of the core portion 1a is 2a, and the relative refractive index difference (maximum relative refractive index difference) of the maximum refractive index of the core portion 1a with respect to the refractive index of pure silica glass is Δ1. Also, the relative refractive index difference of the average refractive index of the cladding portion 1b with respect to the refractive index of pure silica glass is Δclad. In the case of FIG. 2(a), the center core, which is the part of the core portion 1a with the maximum average refractive index, corresponds to the entire core portion 1a. That is, the case of FIG. 2(a) is an example of a case where the core portion is made of a center core.

[0036] FIG. 2(b) shows a so-called W-shaped refractive index profile. In FIG. 2(b), 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-shaped refractive index profile, the core portion 1a is composed of a center core with a diameter of 2a and a depressed layer that is formed to surround the outer periphery of the center core and has a refractive index smaller than that of the cladding portion, an inner diameter of 2a, and an outer diameter of 2b. The center core is the part of the core portion 1a with the maximum 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 portion 1b with respect to the refractive index of pure silica glass is Δclad.

[0037] 2(b) is an example of a core portion that is composed of a center core and two or less layers (specifically, one layer) of core layers that surround the outer periphery of the center core. The depressed layer is an example of a core layer.

[0038] FIG. 2(c) shows a so-called trench-type refractive index profile. In FIG. 2(c), 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 is composed of a center core with a diameter of 2a, an intermediate layer formed to surround the outer periphery of the center core, with 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, with an inner diameter of 2b and an outer diameter of 2c, with a refractive index smaller than the refractive index of the cladding portion. The center core is the part of the core portion 1a with the maximum average refractive index difference. 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 1b 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 close to it.

[0039] 2(c) is an example of a core portion that is made up of a center core and two core layers that surround the center core. The intermediate layer and trench layer are an example of a two-layer core layer.

[0040] FIG. 2(d) shows a so-called step-type refractive index profile. In FIG. 2(d), 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 step-type refractive index profile, the core portion 1a is composed of a center core with a diameter of 2a and a step layer with an inner diameter of 2a and an outer diameter of 2b, which is formed to surround the outer periphery of the center core and has a refractive index smaller than that of the cladding portion. The center core is the part of the core portion 1a with the maximum 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 step 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 portion 1b with respect to the refractive index of pure silica glass is Δclad.

[0041] 2(d) is an example of a core portion that is composed of a center core and two or less layers (specifically, one layer) of core layers that surround the outer periphery of the center core. The step layer is an example of a core layer.

[0042] Here, the refractive index profile of the center core of the core portion 1a may not only be a step type with a geometrically ideal shape, but may also have a shape with unevenness formed due to manufacturing characteristics at the top, or may have a shape with a skirt from the top. In this case, the refractive index of the region that is approximately flat at the top of the refractive index profile within the range of the core diameter 2a of the core portion 1a in the manufacturing design is an index for determining Δ1. Even if the approximately flat region is thought to be divided into multiple places, or if a continuous change occurs and it is difficult to define the approximately flat region, it has been confirmed that it is possible to obtain characteristics close to the desired as long as at least any part of the core portion other than the part where the refractive index changes suddenly toward the adjacent layer is within the range of Δ1 below and the difference between the maximum value and the minimum value Δ is within a certain value ±30%, and there is no particular problem.

[0043] The average refractive index of the depressed layer, intermediate layer, trench layer, step layer, and cladding portion 1b is the average value of the refractive index in the radial direction of the refractive index profile. The cladding portion 1b has a refractive index lower than the maximum refractive index of the core portion 1a and pure silica glass.

[0044] This optical fiber 1 propagates light in two or more propagation modes, which are LP propagation modes, at a wavelength of 1550 nm. At a wavelength of 1550 nm, the transmission loss in each of the two or more propagation modes is preferably 0.20 dB / km or less, more preferably 0.19 dB / km or less, and even more preferably 0.18 dB / km or less.

[0045] The two or more propagation modes are, for example, a combination of two or more of the LP01 mode, the LP11 mode, the LP21 mode, and the LP02 mode. For example, when there are two propagation modes, there are two, the LP01 mode and the LP11 mode.

[0046] Furthermore, at a wavelength of 1550 nm, it is preferable that the difference in transmission loss between two or more propagation modes is 0.02 dB / km or less, thereby reducing the difference in characteristics between the propagation modes.

[0047] Next, the constituent materials of the core 1a and cladding 1b of the optical fiber 1 will be described. The center core and step layers of the core 1a are made of silica glass doped with chlorine (Cl) and at least one of potassium (K) and sodium (Na). Cl and alkali metal elements such as K and Na are dopants that increase the refractive index of silica glass and reduce its viscosity. K and Na may be doped as compounds, such as potassium compounds and sodium compounds.

[0048] On the other hand, at least a part of the cladding portion 1b is made of quartz glass containing a dopant that reduces the refractive index, such as fluorine (F) or boron (B). On the other hand, the depressed layer and the trench layer are made of quartz glass containing more fluorine or boron, which is a refractive index reducing dopant, than the cladding portion 1b. The intermediate layer is made of quartz glass having the same composition as the cladding portion 1b or a composition close to that. Here, fluorine is more preferable as the dopant that reduces the refractive index from the viewpoint of manufacturability. Fluorine may be doped as a fluorine compound.

[0049] As will be described below, as long as a desired refractive index profile is realized, the core layers other than the center core and the step layers in the core region 1a, or the cladding region 1b may be doped with K, Na or Cl.

[0050] Next, a preferred state in the case where the optical fiber 1 is doped with at least one of potassium and sodium will be described.

[0051] FIG. 3 is a diagram showing an example of the relationship between the radial position of the optical fiber 1, the optical field intensity (hereinafter sometimes referred to as the optical intensity), the refractive index profile, and the K or Na doped region. The radial position of 0 μm means the position of the central axis of the center core. FIG. 3 also shows a case where the refractive index profile is step-type, (Δ1-Δclad) is 0.45%, and the core radius (1 / 2 the core diameter) is 8 μm. The optical field intensity is normalized by the peak value.

[0052] As shown in Fig. 3, the field intensity of the LP01 mode light is less than 10% of the peak at the core radius in the radial direction. Therefore, as shown in "known doped region", if at least one of potassium and sodium is doped from the central axis to the core radius (the interface between the core portion 1a, which is the center core, and the cladding portion 1b), the effect of reducing loss by potassium or sodium can be sufficiently obtained.

[0053] In contrast, in the case of a few-mode fiber such as optical fiber 1, for the higher modes LP11, LP21, and LP02, the optical field intensity is 10% or more of the peak even at the core radius in the radial direction, as shown in Figure 3.

[0054] In this case, as shown in "Doped region of embodiment", it is preferable that at least one of potassium and sodium is doped on the radial outside of the center core up to the radial position where the light intensity is 10% of the peak value for all propagation modes in the optical fiber 1. The radial position where the light intensity is 10% of the peak value is the radial position where the normalized intensity is 0.1. This allows the effect of reducing loss by potassium or sodium to be sufficiently obtained even in a few-mode fiber. Note that if at least one of potassium and sodium is doped at 5 ppm or more up to the radial position where the light intensity is 10% of the peak value, the effect of reducing loss can be more suitably obtained.

[0055] Note that, although FIG. 3 shows a case where the refractive index profile is step-type with a predetermined core radius and a predetermined (Δ1-Δclad), the refractive index profile may be other refractive index profiles such as W-type, trench-type, step-type, etc., and the core radius and Δ1 are not particularly limited.

[0056] The inventors assumed various refractive index profiles, such as step type, W type, trench type, and staircase type, and structural parameters that define the refractive index profiles, and assumed potassium or sodium as a dopant, and performed comprehensive simulation calculations and experiments to obtain the minimum transmission loss in each propagation mode. Note that, since the core part of the step type, W type, trench type, and staircase type consists of a center core, or the core part consists of a center core and two or less core layers surrounding the center core, the core structure is called a one-layer structure, two-layer structure, or three-layer structure. These core structures have a relatively small number of layers, so that the process control is easier in the optical fiber manufacturing process, and as a result, the optical fiber is inexpensive.

[0057] The inventors obtained the results shown in Table 1. As shown in Table 1, in a so-called three-layer or less structure in which the core part is composed of a center core, such as a step type, W type, trench type, or staircase type, or the core part is composed of a center core and two or less core layers surrounding the center core, it was confirmed that there is no solution that achieves 0.20 dB / km or less in a propagation mode (for example, LP31) other than the modes LP01, LP11, LP21, and LP02, no matter how much optimization is performed. Therefore, the two or more propagation modes in the optical fiber 1 are, for example, a combination of two or more of the LP01 mode, LP11 mode, LP21 mode, and LP02 mode. Also, for example, when there are two propagation modes, the LP01 mode and the LP11 mode are preferable in terms of obtaining a low transmission loss of 0.18 dB / km or less.

[0058] [Table 1]

[0059] Next, Fig. 4 shows an example of the relationship between the peak-to-peak optical intensity and the transmission loss at the boundary where the dopant is doped, which was obtained as a result of many experiments. Potassium was selected as the dopant.

[0060] As shown in Figure 4, it was experimentally confirmed that for each propagation mode, potassium doping can reduce transmission loss up to the radial position where the light intensity is 10% of the peak value. The same tendency was confirmed in the results of an experiment in which the dopant was changed from potassium to sodium.

[0061] In addition, when the refractive index profile is step-type, (Δ1-Δclad) is 0.45%, and the core radius (1 / 2 the core diameter) is 8 μm as shown in Figure 3, the radial position where the light intensity is 10% of the peak value is 9.2 μm in the LP11 mode, 10.4 μm in the LP21 mode, and 8.8 μm in the LP02 mode. Therefore, the region to be doped with potassium or sodium can be selected according to the mode in which the light is propagated.

[0062] Here, in an optical fiber, if Aeff is too large, there is a risk of microbending loss increasing. The microbending loss can be a value measured by a sandpaper method similar to the fixed diameter drum method defined in JIS C6823:2010. The sandpaper method defines the difference between the transmission loss in state A, where an optical fiber of 500 m length is wound in a single layer without overlapping with a tension of 100 gf around a fixed drum wrapped with sandpaper of #1000, and the transmission loss of the optical fiber in state B of the wound bundle, as the value of the microbending loss. Here, the transmission loss of the optical fiber in state B does not include the microbending loss, and is considered to be a transmission loss inherent to the optical fiber itself. In addition, in this measurement method, the transmission loss is measured at a wavelength of 1550 nm, for example, so the microbending loss is also a value at the wavelength of 1550 nm. Hereinafter, unless otherwise specified, the microbending loss is a value at the wavelength of 1550 nm. The microbending loss at a wavelength of 1550 nm measured by the sandpaper method is preferably 1 dB / km or less.

[0063] Therefore, the present inventors have investigated the relationship between Aeff in the LP01 mode at a wavelength of 1550 nm and the normalized microbending loss for an optical fiber with a step-type refractive index profile. Here, the normalized microbending loss is a value obtained by normalizing the microbending loss with the microbending loss in a standard SMF. A standard SMF is a single-mode optical fiber having characteristics conforming to the standard defined by ITU-T G.652, and here, the Aeff at a wavelength of 1550 nm is further normalized to 80 μm. 2 I selected the following.

[0064] FIG. 5 is a diagram showing an example of the relationship between Aeff and normalized microbending loss. As shown in FIG. 5, the normalized microbending loss is expressed as a logarithmic function of Aeff. Note that the relationship between Aeff and normalized microbending loss showed a similar tendency for modes higher than the LP01 mode. It was also confirmed that if the normalized microbending loss is 100 or more, it is impossible to achieve a transmission loss of 0.20 dB / km or less even if other structural parameters and manufacturing conditions are optimized. From this point of view, for each propagation mode, Aeff at a wavelength of 1550 nm is set to 240 μm 2 The following is preferred:

[0065] Therefore, the inventor set Aeff to 240 μm 2 A systematic study was conducted on the optimal structural parameters (also called profile parameters) for achieving the following. As a result, it was found that Aeff is closely related to Δ1 and Δ2. Note that Δ2 can be considered as the relative refractive index difference of the refractive index of the region adjacent to the center core with respect to pure silica glass, so in the case of a step type, Δ2 can be replaced with Δclad.

[0066] Fig. 6 is a diagram showing an example of the relationship between the core diameter and Aeff at a wavelength of 1550 nm. Fig. 6 shows the relationship between the core diameter and Aeff in each propagation mode in the various different refractive index profiles shown in Fig. 2, with (Δ1-Δ2) fixed at 0.4%, and the values ​​of the different refractive index profiles were calculated, and then averaged. When the calculation as in Fig. 6 was performed for various values ​​of (Δ1-Δ2), it was found that when (Δ1-Δ2) was 0.40% or more, Aeff was stable at 240 μm. 2 It was confirmed that when (Δ1-Δ2) becomes larger than 0.40%, Aeff becomes smaller when compared at the same core diameter, and 2 It tends to be in the following range: Therefore, (Δ1-Δ2) is preferably 0.40% or more.

[0067] Moreover, Table 2 shows the transmission losses in the LP01 and LP11 modes of optical fibers obtained by optimizing the structural parameters and manufacturing process for various values ​​of (Δ1-Δ2). As shown in Table 2, by optimizing the structural parameters and manufacturing process, if (Δ1-Δ2) is 0.7% or less, the transmission loss in each mode can be preferably kept to 0.20 dB / km or less. Furthermore, if (Δ1-Δ2) is 0.55% or less, the transmission loss in each mode can be preferably kept to 0.18 dB / km or less, or even 0.17 dB / km or less.

[0068] [Table 2]

[0069] Next, in the optical fiber 1 of the embodiment, it is preferable that compressive stress exists in the center core and in a radial position on the radial outside of the center core where the light intensity is 10% of the peak value for all of the two or more propagation modes. In the optical fiber 1, the residual stress in a certain region being compressive stress indicates that the structural relaxation during drawing of the optical fiber 1 is more advanced in that region than in other regions due to the region being doped with potassium or sodium. Such structural relaxation is preferable in terms of suppressing Rayleigh scattering loss and therefore transmission loss.

[0070] Fig. 7 is a diagram showing an example of the relationship between the radial position and the residual stress and refractive index profile of an optical fiber manufactured as an example of optical fiber 1. The radial position of 0 μm means the position of the central axis of the center core. The optical fiber in Fig. 7 has a step-type refractive index profile, (Δ1-Δclad) is 0.4%, and the core radius (1 / 2 the core diameter) is 7 μm.

[0071] In the optical fiber shown in Figure 7, light propagates in two propagation modes, LP01 mode and LP11 mode. In this case, the radial position where the light intensity in LP11 mode is 10% of the peak value is 8.8 μm, but beyond that radial position to the outside of the center core, a range of about 11 μm is subject to compressive stress (negative residual stress). In addition, the light intensity in LP11 mode at the radial position of 11 μm is very small, at about 2% of the peak value. As a result, the optical fiber shown in Figure 7 has an extremely low transmission loss in LP11 mode of 0.165 dB / km or less.

[0072] 7, the compressive stress peaks are located at 0 μm and approximately 8 μm in the radial direction, but the peak at approximately 8 μm is at least a peak located outside the center core. Such a peak is believed to be formed at a position where the concentration of potassium or sodium is high, for example, at a position where potassium or sodium is doped.

[0073] As described above, according to the optical fibers of the embodiments and examples, it is possible to realize a few-mode fiber that has a low transmission loss of 0.20 dB / km or less, 0.19 dB / km or less, or even 0.18 dB / km or less at a wavelength of 155 nm, while still being inexpensive and having a simple structure of three layers or less.

[0074] (Manufacturing method) The optical fiber of the embodiment shown in Fig. 7 was manufactured as follows. First, silica soot was produced using silicon tetrachloride (SiCl4) gas, hydrogen gas, oxygen gas, and inert gas using a known VAD (Vapor-phase Axial Deposition) device. After that, helium gas and chlorine gas were flowed during sintering and vitrifying the silica soot to dope it with a high concentration of chlorine. The chlorine-based gas flowed during sintering may be a chlorine compound such as silicon tetrachloride. This resulted in the formation of a core preform with a Δ1 of 0.1%.

[0075] Next, a tube doped with fluorine was prepared by a tube manufacturing method so that the relative refractive index difference with respect to pure quartz glass was -0.3%. Then, potassium chloride (KCl) raw material was heated above its melting point in an electric furnace to melt and evaporate, and then aerosol particles were generated by cooling gas and transported to the inside of the tube by Ar carrier gas, and potassium was deposited on the surface. Then, a core preform was inserted into the tube, the inside was evacuated, and an oxyhydrogen flame was applied to the outside of the tube to perform a collapse process, thereby obtaining a core / clad integrated preform. Then, a tube was further placed over the preform so that the core diameter of the optical fiber after drawing would be 14 μm, and an optical fiber preform was produced. Next, an optical fiber was drawn from this optical fiber preform. Note that potassium diffuses both toward the center (toward the center core) and toward the outside in the radial direction (toward the clad portion) by each heat treatment process performed after deposition, and is doped in the desired region.

[0076] The method of doping potassium is not limited to the above. For example, after producing silica soot, it may be pre-sintered in a temperature range where densification does not occur, and the pre-sintered body may be doped with potassium by a liquid immersion method. Potassium nitrate, iodide, bromide, etc. may be used instead of potassium chloride. Furthermore, when doping with sodium instead of potassium, various sodium compounds may be used.

[0077] Table 3 shows the optical characteristics of the optical fiber in each propagation mode of the above example. The optical characteristics were measured using the shift input method, and considering the effect of mode coupling, the average value of multiple measurements was taken for long measurements. Microbending loss was measured using the sandpaper method. In Table 3, "MFD" stands for mode field diameter.

[0078] As shown in Table 3, good transmission loss characteristics of about 0.16 dB / km were obtained at a wavelength of 1550 nm for both the LP01 mode and the LP11 mode. In addition, the Aeff at the wavelength of 1550 nm was 240 μm. 2The microbending loss was also low, at 0.5 dB / km or less. In other words, it was confirmed that the increase in transmission loss of this optical fiber in the normal bobbin-wound state is sufficiently small compared to the wound bundle state.

[0079] [Table 3]

[0080] Next, optical fibers of samples No. 1 to No. 10 were fabricated in the same manner as in the above embodiment, and the optical characteristics in each propagation mode were measured. The results are shown in Tables 4 and 5. In Tables 4 and 5, "ΔC" means the relative refractive index difference of the cladding part with respect to the pure silica glass, and is the same as Δclad in FIG. 2. Also, "dopant (1 layer / 2 layers)" indicates the type of alkali dopant doped in the first layer (center core) and at least a part of the outer layer. Therefore, for example, in the optical fiber No. 5, the center core and the depressed layer are doped with both K and Na. Also, "transmission loss", "microbending loss", "Aeff", and "MFD" are all values ​​at a wavelength of 1550 nm. Also, regarding the refractive index profile, No. 1 to No. 4 are step type, No. 5 to No. 7 are W type, No. 8 is step type, and No. 9 and No. 10 are trench type.

[0081] All of the optical fibers No. 1 to No. 10 had good transmission loss of 0.20 dB / km or less in each propagation mode. 2 For example, in the case of optical fiber No. 1, Δ1-ΔC (=Δ1-Δ2) is 0.45%, 2a is 14.0 μm, and the dopant is K, the transmission loss in the LP01 mode is 0.156 dB / km, the microbending loss is 0.020 dB / km, and the Aeff is 123 μm. 2 For the LP11 mode, the transmission loss was 0.167 dB / km, the microbending loss was 0.293 dB / km, and the Aeff was 193 μm. 2, and the MFD was 10.0 μm. Thus, optical fiber No. 1 is the best example from the viewpoint of transmission loss. In addition, the difference in transmission loss between each propagation mode at a wavelength of 1550 nm for all of optical fibers No. 1 to 10 was 0.02 dB / km or less, which is believed to be the result of potassium being doped also in the cladding. In other words, by doping the cladding with potassium, which has a transmission loss reducing effect, the transmission loss reducing effect of potassium is believed to be fully exerted even for light in high-order modes in which the light field seeps into the cladding to a large extent.

[0082] [Table 4] [Table 5]

[0083] The present invention is not limited to the above-mentioned embodiment. The present invention also includes a configuration in which the above-mentioned components are appropriately combined. 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-mentioned embodiment, and various modifications are possible. [Explanation of symbols]

[0084] 1: Optical fiber 1a: Core part 1b: Clad section 1c: Covering layer 1ca: Primary layer 1cb: Secondary layer

Claims

1. a core portion consisting of a center core or consisting of a center core and two or less core layers surrounding the outer periphery of the center core; a cladding portion having a refractive index lower than the maximum refractive index of the core portion and pure silica glass, the cladding portion surrounding the outer periphery of the core portion; Equipped with If the relative refractive index difference of the maximum refractive index of the center core with respect to pure silica glass is Δ1, and the relative refractive index difference of the refractive index of the region adjacent to the center core with respect to pure silica glass is Δ2, (Δ1-Δ2) is 0.40% or more and 0.70% or less, The radius of the center core is 5.5 μm or more and 7.5 μm or less, having a step, W, or trench type refractive index profile; At a wavelength of 1550 nm, light is propagated in two or more propagation modes, which are LP propagation modes. Optical fiber.

2. At a wavelength of 1550 nm, the transmission loss in each of the two or more propagation modes is 0.20 dB / km or less.

2. The optical fiber of claim 1.

3. The two or more propagation modes are a combination of two or more of an LP01 mode, an LP11 mode, an LP21 mode, and an LP02 mode.

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

4. The two or more propagation modes are two propagation modes, LP01 mode and LP11 mode. The optical fiber according to any one of claims 1 to 3.

5. At least a part of the cladding portion is made of fluorine-doped silica glass. The optical fiber according to any one of claims 1 to 4.

6. The center core is made of quartz glass doped with at least one of potassium and sodium. The optical fiber according to any one of claims 1 to 5.

7. The center core is doped with at least one of potassium and sodium by 5 ppm or more up to a radial position where the light intensity is 10% of a peak value for all of the two or more propagation modes on the radial outside of the center core. The optical fiber according to any one of claims 1 to 6.

8. At a wavelength of 1550 nm, the transmission loss in each of the two or more propagation modes is 0.19 dB / km or less. The optical fiber according to any one of claims 1 to 7.

9. At a wavelength of 1550 nm, the transmission loss in each of the two or more propagation modes is 0.18 dB / km or less. The optical fiber according to any one of claims 1 to 8.

10. For all of the two or more propagation modes, the effective core area at a wavelength of 1550 nm is 240 μm ２ is less than or equal to The optical fiber according to any one of claims 1 to 9.

11. (Δ1-Δ2) is 0.40% or more and 0.55% or less The optical fiber according to any one of claims 1 to 10.

12. The center core and a radial position on the radial outside of the center core where the light intensity is 10% of the peak value for all of the two or more propagation modes are doped with at least one of potassium and sodium at 5 ppm or more, and compressive stress exists. The optical fiber according to any one of claims 1 to 11.

13. The position where the compressive stress reaches its peak is at least on the outer side of the center core. The optical fiber according to any one of claims 1 to 12.

14. At a wavelength of 1550 nm, the difference in transmission loss between the two or more propagation modes is 0.02 dB / km or less. The optical fiber according to any one of claims 1 to 13.

15. The microbending loss at a wavelength of 1550 nm measured by the sandpaper method is 1 dB / km or less. The optical fiber according to any one of claims 1 to 14.

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