Patent Application
20240004126
The present disclosure relates to an optical fiber.
An optical fiber that achieves ultra-low transmission loss by doping a dopant to reduce the refractive index of glass, such as fluorine, in a cladding portion has been disclosed (Japanese Patent No. 6690296). In this kind of optical fiber in which the refractive index of the cladding portion is reduced, it is possible to reduce, or almost eliminate a dopant to be doped in a core portion. As a result, the Rayleigh scattering loss originated from concentration distribution of dopant in the core portion is reduced, and an optical fiber with ultra-low transmission loss may thereby be achieved. With this kind of optical fiber, it is possible to achieve a transmission loss of 0.18 dB/km or less at a wavelength of 1550 nm, which has been difficult to be achieved, for example, with an optical fiber having a cladding portion made from silica glass not doped with a dopant to reduce the refractive index.
If fluorine or a dopant that is relatively expensive and difficult to handle, such as boron, which is another dopant that reduces the refractive index of glass, is doped in the cladding portion, there is a problem that the material cost and manufacturing cost of the cladding portion increase, resulting in increased cost for an optical fiber.
There is a need for an optical fiber that has low transmission loss and is relatively inexpensive.
According to one aspect of the present disclosure, there is provided an optical fiber including: a core portion; and a cladding portion configured to surround an outer periphery of the core portion, the cladding portion having a refractive index lower than a maximum refractive index of the core portion, and containing a dopant to reduce the refractive index, wherein the core portion includes a center core in which an average refractive index is maximized in the optical fiber, a relative refractive-index difference of the average refractive index of the center core with respect to an average refractive index of the cladding portion is equal to or higher than 0.20% and equal to or lower than 0.50%, an outer diameter of the cladding portion is equal to or larger than 70 μm and equal to or smaller than 120 μm, a cable cutoff wavelength is equal to or smaller than 1530 nm, and a transmission loss at a wavelength of 1550 nm is equal to or less than 0.18 dB/km.
Hereinafter, an embodiment will be explained with reference to the drawings. It is noted that the embodiment explained below is not intended to limit the present disclosure. Moreover, in the respective drawings, identical reference symbols are assigned to identical or corresponding components as appropriate. Furthermore, in the present specification, cutoff wavelength or effective cutoff wavelength refers to cable cutoff wavelength defined by ITU-T G.650.1 of the International Telecommunication Union (ITU). Moreover, terms not specifically defined in the present specification are to follow the definition and measurement method in G.650.1 and G.650.2.
The optical fiber 1 has a refractive index profile, for example, as shown in
Moreover, a relative refractive-index difference of an average refractive index of the cladding portion 1b with respect to the refractive index of pure silica glass is ΔClad. In the case of
The refractive index profile of the core portion 1a is not only a geometrically ideal shape of step index, but may also be a shape in which a shape of a top portion is not flat but has unevenness due to manufacturing characteristics, or in a sloping shape tapering downward from the top like a hem. In this case, the refractive index of a region that is substantially flat at the top portion of the refractive index profile within a range of the core diameter 2a of the core portion 1a based on the manufacturing design is to be an index to determine Δ1. Also in a case in which a substantially flat region seems to be separated into plural parts, or in a case in which the definition of substantially flat region is difficult because continuous changes occur, it has been confirmed that desired characteristics may be achieved as long as at least either part of the core portion excluding portions in which the refractive index changes abruptly toward an adjacent layer is within the range of Δ1 described below, and a difference of A between the maximum value and the minimum value is within a value ±30%, and there are no particular problems.
Furthermore, the average refractive index of the depressed layer, the intermediate layer, the trench layer, or the cladding portion 1b is an average value of the refractive index in the diameter direction of the refractive index profile.
Constituent materials of the optical fiber 1 will be explained. The center core of the core portion 1a is constituted of, for example, pure silica glass, or silica glass that includes at least one of Chlorine (Cl), fluorine (F), germanium (Ge), potassium (K), and sodium (Na). F reduces the refractive index of silica glass, and Ge, Cl, K, and Na are dopants that increase the refractive index of silica glass.
On the other hand, as for the cladding portion 1b, at least a part thereof is made from silica glass that includes, for example, fluorine or boron (B), which is a dopant to reduce the refractive index. On the other hand, the depressed layer and the trench layer are made from silica glass that includes more fluorine or boron, which is a dopant to reduce the refractive index, than the cladding portion. The intermediate layer is made from silica glass of the same components as the cladding portion 1b or components similar thereto. As the dopant to reduce the refractive index, it is more preferable to use fluorine in terms of manufacturability.
The primary layer 1ca and the secondary layer 1cb are made from resin. This resin is, for example, ultraviolet-curable resin. The ultraviolet-curable resin is a mixture of various kinds of resin materials, such as oligomer, diluent monomer, photopolymerization initiator, silane coupling agent, sensitizer, and lubricant, and an additive. As oligomer, conventionally known materials, such as polyether-based urethane acrylate, epoxy acrylate, polyester acrylate, and silicone acrylate may be used. As diluent monomer, conventionally known materials, such as monofunctional monomer and multifunctional monomer, may be used. Moreover, the additive is not limited to the ones described above, but a wide variety of conventionally known additives used for ultraviolet curable resin or the like may be used.
Because the cladding portion 1b includes a dopant to reduce the refractive index in this optical fiber 1, low transmission loss may be achieved, and for example, the transmission loss at a wavelength of 1550 nm is 0.18 dB/km or less. While the dopants described above are provided as examples of a dopant to be included in the cladding portion 1b or the core portion 1a, the dopant is not limited to these examples as long as the transmission loss at the wavelength of 1550 nm is 0.18 dB/km or less. Moreover, in this optical fiber 1, a cable cutoff wavelength is 1530 nm or smaller, and it enables light of a wavelength in a 1550 nm band (for example, 1530 nm to 1565 nm) or longer to be transmitted in a single mode and, therefore, it is practical.
Furthermore, in this optical fiber 1, the outer diameter (cladding diameter) of the cladding portion 1b is equal to or larger than 70 μm and equal to or smaller than 120 μm, and is smaller than 125 μm, which is a typical cladding diameter. This enables to exponentially reduce the volume of the cladding portion 1b that includes an expensive dopant, such as fluorine and boron, to reduce the refractive index, and leads to significant reduction of cost.
The inventors investigated the reduction rate of volume of a cladding portion relative to an optical fiber having a cladding diameter of 125 μm when the cladding portion includes a dopant to reduce the refractive index, and the cladding diameter of three kinds of optical fibers having effective core areas different from one another was changed. The effective core areas (Aeff) were set to 80 μm2, 110 μm2, and 125 μm2, which are all typical values. Moreover, the cladding diameter was changed in a 10 μm interval from 70 μm to 120 μm. The results are presented in Table 1. As shown in Table 1, it was confirmed that when the cladding diameter was 120 μm, the volume of the cladding portion 1b could be reduced by approximately 8%, and when 80 μm, approximately 60% could be reduced. Accordingly, it was confirmed that reduction of diameter of the cladding portion 1b is a highly effective means for cost reduction.
A concern when the cladding diameter of the cladding portion 1b is reduced in the optical fiber 1 is increase of the microbending loss. Therefore, the inventors have conducted earnest investigation on the microbending loss.
Generally, in an optical fiber, when a glass diameter, that is, the cladding diameter, is reduced, the microbending loss (also called side pressure loss) increases. Normally, the transmission loss of an optical fiber increases in a form of optical fiber cable. An amount of increase of the transmission loss at this time is closely related to microbending loss, and when the microbending loss is large, the amount of increase is also large.
Herein, referring to single-mode optical fiber having characteristics that comply with the standard defined by ITU-T G.652 as standard optical fiber, it is denoted as standard SMF. Such a standard SMF normally has a coating layer constituted of a resin having a thickness of approximately 62.5 μm on an outer periphery of the cladding portion. The coating layer is constituted of, for example, in the case of two layer structure, a primary layer that has a thickness of approximately 37.5 μm and a secondary layer that surrounds the outer periphery of the primary layer, and that has a thickness of approximately 25 μm. Therefore, the outer diameter of the coating layer is approximately 250 μm.
In the optical fiber 1, if the microbending loss at a wavelength of 1550 nm is 20 times lower than the microbending loss of standard SMF, it may be considered as the microbending loss of a practical level. If it is assumed that a value standardized by the microbending loss in the standard SMF is defined as normalized microbending loss, the normalized microbending loss of the optical fiber 1 according to the present embodiment is preferable to be 20 or less, and is more preferable to be 10 or less. A value 20 of a case in which the normalized microbending loss is to be 20 or less is a value that may suppress the microbending loss to a practical level even after it is manufactured into a cable.
As for the microbending loss, a value that is measured by using a sandpaper method similar to the fixed diameter drum method defined in JIS C6823:2010 may be adopted. The sandpaper method is to define a difference between a transmission loss in a state A in which an optical fiber having a length of 500 m is wound in a single layer without overlap on a stationary drum wrapped with, for example, a #1000 grit sandpaper with a tension of 100 gf and a transmission loss of an optical fiber in a state B in which it is wound on the same bobbin as that in the state A but without the sandpaper wrapped with the same tension and the same length as the state A, as a value of the microbending loss. The transmission loss of the optical fiber in the state B does not include a microbending loss, and may be regarded as an inherent transmission of the optical fiber itself. Moreover, in this measurement method, because a transmission loss is measured at, for example, a wavelength of 1550 nm, a microbending loss is also a value at a wavelength of 1550 nm. Hereinafter, unless otherwise specified, a microbending loss is a value at a wavelength of 1550 nm.
For the microbending loss, a value that is measured by using other methods, such as a wire mesh method, may also be adopted.
Moreover, generally, in an optical fiber, a leakage loss in which light propagating through the core portion leaks out from the cladding portion may occur when the cladding diameter is reduced. As a value of the leakage loss, it is preferable to be a substantially ignorable level, such as 0.001 dB/km at a wavelength of 1625 nm.
Furthermore, in an optical fiber that includes a dopant to reduce the refractive index in the cladding portion 1b as the optical fiber 1, there is a possibility that stress distributions differ between the core portion and the cladding portion, compared with an optical fiber in which the cladding is made from mainly pure silica glass. Therefore, to achieve low transmission loss, it is necessary to suppress increase of the transmission loss based on such a difference in stress distributions. The inventors conducted a systematic and comprehensive investigation based on simulation calculations and experiments and confirmed that such an increase of the transmission loss may be achieved by adjustment of structural parameters in the step-index type, the W-shaped type, and the trench type, which are the refractive index profiles including the core portion 1a.
In the following, it will be specifically explained.
As shown in
Next,
As shown in
Subsequently, the microbending loss is considered to be affected not only by the glass diameter, but also by the primary thickness and the secondary thickness of the coating layer, and is considered to be significantly affected particularly by the primary thickness. For this, the inventors have conducted investigations by simulation calculations and experiments.
As shown in
Next, systematic investigations was conducted while changing Aeff, and changing a refractive index profile and the structural parameters (Δ1, Δ2, Δ3, 2a, b/a, c/a, and the like) of the refractive index profile, the glass diameter, and the primary thickness, with Aeff thus changed.
From
Furthermore, from
Moreover, from
The inventors have found, from the systematic investigations as described above, that the low-leakage-loss threshold value, the microbending loss of 20 or less, and the cable cutoff wavelength of 1530 nm or smaller may be satisfied at the same time while making the fiber diameter 210 μm or smaller, within an optimal parameter range of the glass diameter being equal to or larger than 70 μm and equal to or smaller than 120 μm, the primary thickness being equal to or larger than 10 μm and equal to or smaller than 50 μm, Δ1 being equal to or higher than 0.20% and equal to or lower than 0.50%, and Aeff being equal to or smaller than 150 μm2.
As described above, it is preferable that the glass diameter is small because reduced cost of the optical fiber is achieved, and because reduced cost by reduced resin materials and high density integration of optical fibers in an optical fiber cable are achieved with a reduced fiber diameter.
As for Aeff, it is not particularly problematic to be small from the viewpoint of the leakage loss or the microbending loss. However, when considering optical non-linearity, or connectivity with other optical fibers, such as a typical optical fiber having Aeff of about 80 μm2, Aeff is preferable to be equal to or larger than 50 μm2, and, for example, be equal to or larger than 70 μm2.
Next, the transmission loss will be further explained. As for the transmission loss, in addition to combinations of various parameters as described above, various parameters in a manufacturing process (process parameters), such as a residual stress distribution occurring at the time of drawing of an optical fiber, affect. Therefore, the inventors made many prototypes of optical fiber under conditions that meet the optimal parameter range described above, and investigated the transmission loss characteristics thereof. In manufacturing of the prototypes, process parameters were optimized under respective conditions.
As for the coating layer, an ultraviolet-curable resin material having an elastic modulus of equal to or higher than 0.2 MPa to equal to or lower than 3.0 MPa was used for the primary layer, and an ultraviolet-curable resin material having an elastic modulus of equal to or higher than 5.0 MPa to equal to or lower than 2000 MPa was used for the secondary layer. These ultraviolet-curable resin materials are typical materials used for a coating layer of a standard SMF.
As shown in
Moreover, although it fluctuates depending on a material to be doped in the core portion or the cladding portion or on a refractive index profile even in optical fibers having the same value of microbending loss, it was experimentally confirmed that a limit in achieving the desired characteristics by the optimization is a range in which the normalized microbending loss is 20 or less.
A broken line L1 is a curve obtained by approximating data points by a quintic function using the least squares method. The broken line L1 is represented by y=2.8341×10−8x5−1.8284×10−6x4+4.0977×10−5×3−3.4584×10−4x2+1.777 9×10−3x+0.1607. y is transmission loss, and x is normalized microbending loss.
A value when x=0 in the quintic function above, that is, 0.1607 dB/km is thought to be a transmission loss when there is no influence of microbending loss, that is, in a state in which no side pressure is applied at all.
Therefore, considering 0.18−0.1607=0.0193, it is preferable that an optical fiber be configured by selecting a refractive index profile or a cross-sectional structure with which an increase of transmission loss caused by the microbending loss is 0.0193 dB/km or less on average when it is in an actual operating condition with side pressure applied, such as in a state in which an optical fiber is wound around a bobbin or is turned into a cable form, to achieve the transmission loss of 0.18 dB/km or less.
A broken line L2 shows an example of a relationship between the normalized microbending loss and the transmission loss of an optical fiber in which the cladding portion does not include a dopant to reduce the refractive index, and the cladding portion is made thin. The broken lines L1 and L2 are both quintic function, and the tendency of those are similar but the manner of change is different from each other, and it is found that an optimal range or an obtained value of transmission loss are different.
As shown in
Moreover, the inventors have found, based on numerous experiments, that a relative refractive-index difference of an average refractive index of a center core in which the average refractive index is maximized in the core portion with respect to the refractive index of pure silica glass significantly affects the transmission loss in an optical fiber with the cladding portion including a dopant to reduce the refractive index.
Furthermore, as may be seen from
As described, Δ1′ (absolute value of a relative refractive-index difference of a center core) with respect to the silica level and the transmission loss of a thin optical fiber have a close relationship, and there is an optical range specific to having a thin diameter. Therefore, manufacturing an optical fiber in the optical range of the present disclosure is significantly important to reduce the transmission loss of a thin optical fiber that includes a dopant to reduce the refractive index in the cladding portion.
The optical fiber 1 according to the present embodiment may be manufactured easily by fabricating an optical fiber preform by a publicly known method using the vapor axial deposition (VAD) method, the outside vapor deposition (OVD) method, the modified chemical vapor deposition (MCVD) method, the plasma CVD method, and the like, and by drawing fibers from this optical fiber preform into the optical fiber 1.
For example, dopants, such as germanium, fluorine, potassium, and sodium, may be added to an optical fiber preform by using a gas including the dopant while synthesizing soot. Moreover, potassium and sodium may be doped in glass by the gas-phase method, the immersion method, or the like, utilizing their speedy diffusion, not while synthesizing soot. Furthermore, chlorine may be added to an optical fiber preform by causing a chlorine gas that is used in a dehydration process to remain. Moreover, fluorine may be added to an optical fiber preform by flowing a fluorine gas in a vitrification structure.
As an example, optical fibers of sample Nos. 1 to 22 were drawn from an optical fiber preform fabricated by using the VAD method, and their optical characteristics were measured. In the optical fibers, a coating layer of an ultraviolet-curable resin constituted of the primary layer and the secondary layer was formed. The Young's modulus of the primary layer was 0.4 MPa, and the Young's modulus of the secondary layer was 1000 MPa.
Structures and optical characteristics of optical fibers of sample Nos. 1 to 22 are shown in Tables 2A (2A-1 and 2A-2) and 2B (2B-1 and 2B-2). In Tables 2A and 2B, “Δ1′”, “Δ2′”, and “Δ3′” are a relative refractive-index difference of the center core with respect to the silica level, a relative refractive-index difference of the depressed layer or the intermediate layer with respect to the silica level, and a relative refractive-index difference of the trench layer with respect to the silica level, respectively. Moreover, “primary diameter” is an outer diameter of the primary layer, and “fiber diameter” is an outer diameter of the secondary layer. Furthermore, as for the dopants, for example, “Cl2+K+Na/F” signifies that chlorine, potassium, and sodium are doped in the center core, and fluorine is doped in the cladding portion.
In either one of sample Nos. 1 to 22, the cable cutoff wavelength was 1530 nm or smaller. Therefore, either one of sample Nos. 1 to 22 was to function as a single mode optical fiber at a wavelength of 1550 nm.
Moreover, in either one of sample Nos. 1 to 22, the cladding diameter was equal to or larger than 70 μm and equal to or smaller than 120 μm, and the transmission loss at the wavelength of 1550 nm was 0.18 dB/km or less.
Furthermore, in either one of sample Nos. 1 to 22, Aeff at the wavelength of 1550 nm was equal to or smaller than 150 μm2, and equal to or larger than 50 μm2.
Moreover, in either one of sample Nos. 1 to 22, the fiber diameter was 210 μm or smaller.
Furthermore, in either one of sample Nos. 1 to 22, the thickness of the primary layer was 10 μm or larger. For example, in even sample No. 22 having the thinnest primary layer, it was 17.5 μm.
Moreover, in either one of sample Nos. 1 to 22, the normalized microbending loss at the wavelength of 1550 nm was 20 or less.
Furthermore, in either one of sample Nos. 1 to 22, Δ1 was equal to or higher than 0.20% and equal to or lower than 0.50%, and Δ1′ was equal to or higher than −0.15% and equal to or lower than 0.17%.
Moreover, in either one of sample Nos. 1 to 22, ΔClad was equal to or higher than −0.50% and was 0.13%.
Specifically, for sample No. 1, when the refractive index was the step-index type, Δ1 was 0.42%, ΔClad was −0.33%, 2a was 9 μm, the glass diameter was 118 μm, the primary diameter was 169 μm, the fiber diameter was 207 μm, the dopant of the center core was Cl2, and the dopant of the cladding portion was F, preferable values were obtained as the transmission loss of 0.167 dB/km, the normalized microbending loss of 1.6, and Aeff of 75 μm2, and it was a particularly preferable combination of core dopants from the viewpoint of manufacturability and low loss.
Moreover, for sample No. 3, when the refractive index was the step-index type, Δ1 was 0.39%, ΔClad was −0.39%, 2a was 10 μm, the glass diameter was 112 μm, the primary diameter was 163 μm, the fiber diameter was 197 μm, the dopant of the center core was K, and the dopant of the cladding portion was F, preferable values were obtained as the transmission loss of 0.156 dB/km, the normalized microbending loss of 1.9, and Aeff of 86 μm2, and it was a particularly preferable combination of core dopants from the viewpoint of manufacturability and low loss, and was a particularly preferable combination of profile structures from the viewpoint of manufacturability and preferable optical characteristics.
Furthermore, for sample No. 4, when the refractive index was the W-shape type, Δ1 was 0.36%, Δ2 was −0.4%, ΔClad was −0.23%, b/a was 2, 2a was 9 μm, the glass diameter was 110 μm, the primary diameter was 155 μm, the fiber diameter was 193 μm, the dopant of the center core was C12+K, and the dopant of the cladding portion was F, preferable values were obtained as the transmission loss of 0.153 dB/km, the normalized microbending loss of 1.1, and Aeff of 58 μm2, and it was a particularly preferable combination of core dopants from the viewpoint of manufacturability and low loss.
Moreover, for sample No. 7, when the refractive index was the W-shape type, Δ1 was 0.27%, Δ2 was −0.11%, ΔClad was −0.14%, b/a was 2.9, 2a was 12.6 μm, the glass diameter was 102 μm, the primary diameter was 146 μm, the fiber diameter was 182 μm, the dopant of the center core was Cl2+K+Na, and the dopant of the cladding portion was F, preferable values were obtained as the transmission loss of 0.158 dB/km, the normalized microbending loss of 3.5, and Aeff of 111 μm2, and it was a particularly preferable combination of profile structures from the viewpoint of manufacturability and optical characteristics.
Furthermore, for sample No. 9, when the refractive index was the W-shape type, Δ1 was 0.25%, Δ2 was −0.17%, ΔClad was −0.3%, b/a was 2.8, 2a was 14.1 μm, the glass diameter was 97 μm, the primary diameter was 140 μm, the fiber diameter was 174 μm, the dopant of the center core was F+Na, and the dopant of the cladding portion was F, preferable values were obtained as the transmission loss of 0.18 dB/km, the normalized microbending loss of 8.9, and Aeff of 127 μm2, and it was a particularly preferable combination of profile structures from the viewpoint of manufacturability and optical characteristics.
The present disclosure may be used for optical fibers.
According to the present disclosure, an effect that an optical fiber that has low transmission loss and is relatively inexpensive may be achieved is produced.
Note that the present disclosure is not limited to the above embodiment. The present disclosure also includes structures that are formed by appropriately combining the respective components described above. Moreover, more effects and modifications may be easily derived by those skilled in the art. Therefore, broader embodiments are not limited to the embodiment described above, and allow various modifications.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-056156 | Mar 2021 | JP | national |
This application is a continuation of International Application No. PCT/JP2022/014387, filed on Mar. 25, 2022 which claims the benefit of priority of the prior Japanese Patent Application No. 2021-056156, filed on Mar. 29, 2021, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2022/014387 | Mar 2022 | US |
| Child | 18469607 | US |
