Patent Application
20240418930
The present application claims priority based on Japanese Patent Application No. 2024-120905, filed on Jul. 26, 2024, the entire contents of which are incorporated herein by reference.
The present disclosure relates to an optical fiber and an optical cable.
The increase in communication traffic has made constructing an economical optical network important. In this context, from the perspective of reducing transportation and installation costs or the like there are expectations for optical cables to become smaller in diameter and higher in density. Along with this, there is also a demand for the optical fibers themselves to become smaller in diameter. Examples of small-diameter optical fibers are disclosed in U.S. Pat. Nos. 11,874,494, 11,181,687, 11,719,878, and 11,709,313. However, the reduction in diameter leads to a challenge of increased micro-bending loss occurring during cabling. Thus, it is necessary to consider the structure based on an analytical formula that can systematically estimate the impact of the optical fiber structure.
A non-patent literature (F. Cocchini, “The Lateral Rigidity of Double-Coated Optical Fibers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 13, No. 8, August 1995) discloses that the micro-bending resistance characteristics of an optical fiber are associated with the lateral rigidity D and the bending rigidity H of the optical fiber, which can be calculated using an approximation formula. WO 2018/025896 describes that the approximation formula in the literature mentioned above has been extended to more accurately reflect reality.
An optical fiber according to one embodiment of the present disclosure includes a glass fiber provided with a core and a cladding surrounding the core, and a coating layer surrounding the glass fiber. The core is formed from silica glass doped with at least one of germanium, titanium, chlorine, fluorine, and alkali metals. The cladding includes an inner cladding that is in contact with the core and surrounds the core, an outer cladding surrounding the inner cladding, and a trench placed between the inner cladding and the outer cladding in the radial direction. The coating layer includes a primary resin layer surrounding the cladding and a secondary resin layer surrounding the primary resin layer. The core has a radius of 3.6 μm or more and 5.4 μm or less, and the relative refractive index difference of the core with respect to the refractive index of the cladding is greater than the refractive index of the cladding by 0.32% or more and 0.40% or less. The volume of the trench is less than −30% μm2. The radius of the cladding is 63 μm or less. The thickness of the primary resin layer is 4 μm or more, and Young's modulus of the primary resin layer is 0.3 MPa or less. The radius of the secondary resin layer is 85 μm or less, the thickness of the secondary resin layer is 7.5 μm or more, and Young's modulus of the secondary resin layer is 1250 MPa or more. The effective cross-sectional area of the optical fiber at a wavelength of 1550 nm is 100 μm2 or less. The mode field diameter of the optical fiber at a wavelength of 1310 nm is greater than 8.2 μm. The mode field diameter of the optical fiber at a wavelength of 1550 nm is 9.40 μm or more and 10.5 μm or less. The cable cutoff wavelength of the optical fiber is less than 1420 nm. The bending loss of the optical fiber at a wavelength of 1550 nm upon being wound with a bending diameter of 10 mm is 1 dB/turn or less. The relative ratio of D/H2, which indicates the relationship between a lateral rigidity D and a bending rigidity H of the optical fiber, to D/H2 of a reference 200 μm single fiber is 540 or less. The cross-sectional area of the coating layer that excludes the primary resin layer is 4400 μm2 or more and 12000 μm2 or less. The absolute difference between the refractive index of the cladding and the refractive index of the coating layer is greater than 0.01. The optical fiber has a zero-dispersion slope of 0.092 ps/nm2/km or less.
As the diameter of optical fibers decreases, micro-bending loss may increase. Thus, it is desirable to reduce micro-bending loss even the case where the optical fiber becomes smaller in diameter.
According to the present disclosure, it is possible to reduce micro-bending loss in a small-diameter optical fiber and an optical cable including the small-diameter optical fiber.
Embodiments of the present disclosure are now listed and described.
In this optical fiber, the radius of the cladding is 63 μm or less, and the radius of the secondary resin layer is 85 μm or less, allowing for the provision of a small-diameter optical fiber. Additionally, the relationship between the lateral rigidity D and the bending rigidity H of the optical fiber, indicated as D/H2, is associated with the micro-bending loss, and can be expressed as the micro-bending loss Δαmicro=A×D/H2. In this equation, A is a constant that depends on the refractive index structure of the optical fiber. Herein, the relative ratio to D/H2 compared to a reference 200 μm single fiber is set to be 540 or less. By using an optical fiber with such a rigidity ratio, it is possible to reduce the micro-bending loss. This also reduces the cable loss when each optical fiber is incorporated into a cable. If such a rigidity ratio is greater than 540, the transmission loss increases due to slight lateral pressure, making it difficult to improve the core density when cabled or corded. Herein, the calculation conditions for D/H2 of the reference 200 μm single fiber are as follows as an example.
Here, c1=0.209367, c2=1.206659, and c3=0.401169, and cijk is as follows.
The approximation formula disclosed in WO 2018/025896 is calculated based on numerical analysis that limits the diameter of the glass fiber (glass diameter) to 125 μm. For this reason, it has been found that when applied to optical fibers with different glass diameters, the offset between the calculated values and the actual measured values of micro-bending loss can be significant. In other words, the optical fiber disclosed in WO 2018/025896 is sometimes unlikely to effectively suppress micro-bending loss. In contrast, in the optical fiber of the item [3] mentioned above, the lateral rigidity D and bending rigidity H satisfy Formula (3), thereby ensuring reliable suppression of micro-bending loss. In addition, due to the low coating eccentricity, the offset between the calculated value of micro-bending loss and the actual measured value is less likely to become large. Thus, it is possible to more reliably reduce micro-bending loss.
Specific examples of an optical fiber and an optical cable according to the present embodiment will be described with reference to the drawings as necessary. The present invention is not limited to these examples, and is defined by the claims, and it is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are denoted by the same reference numerals, and duplicated descriptions are omitted.
The glass fiber 10 is formed from silica glass (SiO2). The glass fiber 10 includes a core 11 and a cladding 12. The core 11 extends along the fiber axis of the optical fiber 1. The radius of the core 11 is 3.6 μm or more and 5.6 μm or less. The refractive index of the core 11 is higher than the refractive index of the cladding 12. The relative refractive index difference of the core 11 with respect to the cladding 12 is, for example, 0.32% or more and 0.40% or less. In the present embodiment, micro-bending loss is decreased, so the relative refractive index difference of the core 11 can be reduced. The core 11 is formed from silica glass to which at least one of germanium, titanium, chlorine, fluorine, and alkali metal elements is added. The alkali metal element added to the core 11 is, for example, sodium (Na), potassium (K), lithium (Li), rubidium (Rb), or cesium (Cs).
The cladding 12 surrounds the core 11 and covers the outer peripheral surface of the core 11. The cladding 12 is formed, for example, from pure silica glass. The cladding 12 has an inner cladding 13, a trench 14, and an outer cladding 15. The inner cladding 13 is in contact with the core 11 and surrounds the core 11. The outer cladding 15 surrounds the inner cladding 13. The trench 14 is placed between the inner cladding 13 and the outer cladding 15 in the radial direction. The radius of the inner cladding 13 is, for example, 7 μm or more and 20 μm or less. The radius of the trench 14 is, for example, 11 μm or more and 34 μm or less. The radius of the outer cladding 15 is, for example, 20 μm or more and 63 μm or less.
Young's modulus of the glass fiber 10 including the core 11 and the cladding 12 is, for example, 70 GPa or more and 80 GPa or less. The volume of the trench 14 may be, for example, −380%·μm2 or more and −126%·μm2 or less. The trench 14 is provided in the cladding 12 in this manner, so it is possible to reduce micro-bending loss in the optical fiber 1.
The coating resin 20 is formed from an ultraviolet-curable resin. The coating resin 20 includes a primary resin layer 21 surrounding the cladding 12 (the outer cladding 15) and a secondary resin layer 22 surrounding the primary resin layer 21. The primary resin layer 21 surrounds the glass fiber 10 and covers the outer peripheral surface of the glass fiber 10 (the outer cladding 15 of the cladding 12). The primary resin layer 21 is provided in contact with the cladding 12. The radius of the primary resin layer 21 is, for example, 40 μm or more and 70 μm or less. The thickness of the primary resin layer 21 is, for example, 4 μm or more and 30 μm or less, and may be 25 μm or less.
Young's modulus of the primary resin layer 21 is, for example, 0.05 MPa or more and 0.3 MPa or less. To reduce the micro-bending loss, it is preferable for Young's modulus of the primary resin layer 21 to be low. If Young's modulus of the primary resin layer 21 exceeds 0.3 MPa, the micro-bending loss fails to be sufficiently reduced. On the other hand, if Young's modulus of the primary resin layer 21 is too low, there is a possibility that the coating resin 20 including the primary resin layer 21 and the secondary resin layer 22 may be damaged. Setting Young's modulus of the primary resin layer 21 to 0.05 MPa or more prevents damage to the coating resin 20.
The primary resin layer 21 may employ, for example, a polyether-based or polyester-based urethane acrylate. The primary resin layer 21 may also contain a reactive diluent monomer and a photoinitiator as needed. Young's modulus of the primary resin layer 21 is adjustable, for example, based on the molecular weight of the polyether portion of the ultraviolet-curable resin and the type of diluent monomer used.
The primary resin layer 21 includes, for example, a photoinitiator-containing phosphorus in an amount of 0.3% by mass or more and 2.0% by mass or less. The primary resin layer 21 includes, for example, polypropylene glycol with a mass average molecular weight of 1000 or more and 5000 or less. As a result, in the optical fiber 1 having the primary resin layer 21 with a relatively low Young's modulus, even if the optical fiber 1 is subjected to an external force during processes such as rewinding or unitization, delamination at the interface between the glass fiber 10 and the primary resin layer 21 or destruction of the resin coating is less likely to occur.
The secondary resin layer 22 surrounds the primary resin layer 21 and covers the outer peripheral surface of the primary resin layer 21. The radius of the secondary resin layer 22 is, for example, 50 μm or more and 85 μm or less. The radius of the secondary resin layer 22 may be 81 μm or less, or may be 51 μm or less. The diameter of the secondary resin layer 22 corresponds to the diameter (coating diameter) of the coating resin 20 when no additional colored layer (ink layer) is provided on the outer periphery of the secondary resin layer 22. The thickness of the secondary resin layer 22 is, for example, 7.5 μm or more and 25 μm or less, and may be 10 μm or more. Young's modulus of the secondary resin layer 22 is, for example, 1250 MPa or more and 3000 MPa or less.
The secondary resin layer 22 employs, for example, polyether-based or polyester-based urethane acrylate. The secondary resin layer 22 may also contain a reactive diluent monomer and a photoinitiator as necessary. Young's modulus of the secondary resin layer 22 can be adjusted, for example, based on the molecular weight of the polyether portion of the ultraviolet-curable resin and the type of diluent monomer.
As illustrated in
In the optical fiber 1, the total cross-sectional area of the secondary resin layer 22 and the colored layer 23 (if formed) of the coating resin 20 is, for example, 4400 μm2 or more and 12000 μm2 or less. The term “total cross-sectional area of the secondary resin layer 22 and the colored layer 23 (if formed)” herein corresponds to the cross-sectional area of the coating resin 20 that excludes the primary resin layer 21. If the colored layer 23 is not formed, the total cross-sectional area of the secondary resin layer 22 of the coating resin 20 is, for example, 4400 μm2 or more and 12000 μm2 or less. In addition, the ratio of the thickness of the primary resin layer 21 to the total thickness of the secondary resin layer 22 and the colored layer 23 (if formed) is, for example, 0.25 or more and 1.50 or less.
The effective cross-sectional area of the optical fiber 1 at a wavelength of 1550 nm is, for example, 100 μm2 or less. The mode field diameter (MFD) of the optical fiber 1 at a wavelength of 1310 nm is, for example, greater than 8.2 μm and equal to or less than 9.50 μm. The mode field diameter (MFD) of the optical fiber 1 at a wavelength of 1550 nm is, for example, 9.40 μm or more and 10.5 μm or less, and may be 10 μm or more. The mode field diameter (MFD) of the optical fiber 1 at a wavelength of 1625 nm is, for example, 9.70 μm or more and 10.90 μm or less. The cable cutoff wavelength of the optical fiber 1 is, for example, less than 1420 nm, and may also be less than 1310 nm.
In the optical fiber 1, the bending loss of the optical fiber 1 at a wavelength of 1550 nm when wound with a bending diameter of 10 mm is, for example, 1 dB/turn or less. In the optical fiber 1, the bending loss of the optical fiber 1 at a wavelength of 1550 nm when wound with a bending diameter of 15 mm is, for example, 0.3 dB/turn or less. In the optical fiber 1, the bending loss of the optical fiber 1 at a wavelength of 1550 nm when wound with a bending diameter of 20 mm is, for example, 0.05 dB/turn or less. In the optical fiber 1, the bending loss of the optical fiber 1 at a wavelength of 1550 nm when wound with a bending diameter of 30 mm is, for example, 0.01 dB/turn or less. In the optical fiber 1, the bending loss of the optical fiber 1 at a wavelength of 1625 nm when wound with a bending diameter of 10 mm is, for example, 5.0 dB/turn or less, and may also be 3 dB/turn or less.
In the optical fiber 1, the absolute difference between the refractive index of the cladding 12 and the refractive index of the coating resin 20 including the primary resin layer 21 and the secondary resin layer 22 is greater than 0.01. The optical fiber 1 has a zero-dispersion slope of 0.092 ps/nm2/km or less.
The coating eccentricity of the optical fiber 1 is, for example, 8 μm or less. Herein, the coating eccentricity is defined as the distance from the central axis of the coating resin 20 (secondary resin layer 22) based on the outer periphery to the central axis of the glass fiber 10. In a cross-section perpendicular to the fiber axis, the offset between the center of the outer perimeter of the coating resin 20 and the center of the glass fiber 10 is the coating eccentricity. The coating eccentricity may be 6 μm or less. The coating eccentricity is likely to vary in the longitudinal direction of the optical fiber 1, and thus it is desirable to measure it at multiple points in the longitudinal direction of the optical fiber 1. Preferably, the average of the values measured at 500 or more points at intervals of 1 mm to 100 mm may be used as the coating eccentricity.
As described above, when the approximation formula disclosed in WO 2018/025896 is applied to an optical fiber having a glass diameter other than 125 μm, there may be a large discrepancy between the calculated value of the micro-bending loss and the actual measured value. It has been found that the micro-bending loss calculated by the approximation formula disclosed in WO 2018/025896 is lower than the actual value, especially in a small-diameter optical fiber. Thus, the present inventors have derived an approximation formula that extends to a range in which the glass diameter is smaller than 125 μm or the coating diameter is smaller than 160 μm, and specified an optical fiber 1 that can reliably suppress micro-bending loss even in the case of a small diameter, as described below.
In other words, given that the radius of the glass fiber 10 is R0 [m], Young's modulus of the glass fiber 10 is E0 [N/m2], the radius of the primary resin layer 21 is R1 [m], Young's modulus of the primary resin layer 21 is E1 [N/m2], the radius of the secondary resin layer 22 is R2 [m], and Young's modulus of the secondary resin layer 22 is E2 [N/m2], then, the relationship between the lateral rigidity (lateral elastic modulus) D [N/m2] of the optical fiber, indicated in Formula (1) and the bending rigidity (bending elastic modulus) H [Nm2] of the optical fiber, indicated in Formula (2), satisfies Formula (3).
Here, c1=0.209367, c2=1.206659, and c3=0.401169, and cijk is as follows.
Formula (2) is the equation used for the bending rigidity H indicated in the non-patent literature (F. Cocchini, “The Lateral Rigidity of Double-Coated Optical Fibers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 13, No. 8, August 1995). The derivation methods of Formulas (1) and (3) are described below.
In general, the micro-bending loss α of an optical fiber is expressed using the lateral rigidity D, the bending rigidity H, and a constant A due to the optical properties of the optical fiber in the approximation formula of Formula (4).
The micro-bending loss α of a small-diameter optical fiber is desirably 5.0 dB/km or less, preferably 3.0 dB/km or less, and more preferably 1.0 dB/km or less. Thus, the constant A in Formula (4) is calculated from the actual measured value of the micro-bending loss α, and the results of D/H2 that satisfies the above value are indicated in Table 1. From the results indicated in Table 1, Formula (3) was obtained as the conditional formula for D/H2 that suppresses the micro-bending loss α to 5.0 dB/km or less.
Based on the technique described in the non-patent literature (F. Cocchini, “The Lateral Rigidity of Double-Coated Optical Fibers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 13, No. 8, August 1995), two-dimensional finite element method (FEM) calculations were performed using the analysis software MSC. Nastran 2020 sp1 for 378 combinations where the cladding diameter (2R0) is 75 μm or more and 130 μm or less, the primary diameter (2R1) is 0 μm or more and 210 μm or less, the secondary diameter (2R2) is 110 μm or more and 210 μm less, Young's modulus of the primary resin layer E1 is 0.05 MPa or more and 0.7 MPa or less, and the Young's modulus of the secondary resin layer E2 is 1000 MPa or more and 3000 MPa or less.
Subsequently, the lateral rigidity D of each structure was calculated from the analysis results using the following formula.
In this formula, F is the lateral pressure (1 MPa), θ is the stress application angle (0 degrees or more and 9 degrees or less), and uy* is the displacement of the pressurized part in each structure.
Furthermore, from the results of the lateral rigidity D obtained for each structure, the following analytical formula was obtained with R0, R1, R2, E0, E1, and E2 as explanatory variables. Moreover, c1, c2, c3, and cijk are as described above.
By rearranging these Formulas (5) and (6), Formula (1) was obtained. By using Formula (1), the lateral rigidity D can be calculated, and D/H2 in Formula (3) can be obtained.
In addition, in the optical fiber 1 according to the present embodiment, the relative ratio of D/H2, which indicates the relationship between the lateral rigidity D and the bending rigidity H of the optical fiber 1, to D/H2 of the 200 μm single fiber is adjusted to be 540 or less. The lateral rigidity D and the bending rigidity H referred to herein can be obtained from the above-mentioned Formulas (1) and (2). The calculation conditions for D/H2 of the reference 200 μm single fiber are as follows.
In this case, the bending rigidity H of the 200 μm single fiber is calculated to be 3.63808×10−12, the lateral rigidity D [MPa] of the 200 μm single fiber is calculated to be 3.821657426, and D/H2 is calculated to be 2.88739×1023.
In addition, a plurality of optical fibers 1 with such a structure may be prepared to form an optical cable 30 as illustrated in
An experimental example of the optical fiber according to the present embodiment is now described. In the following experimental examples, the optical fibers 1 with the structures indicated in Tables 2 to 6 were prototyped, and various indices were obtained from each of the optical fibers that were actually prototyped. These indices are indicated in Tables 7 to 11.
The relative lateral pressure sensitivity coefficient (the ratio of D/H2 to that of a 200 μm single fiber) in the above-mentioned Experimental Examples 1 to 25 was as indicated in Table 12 below. In other words, for Experimental Examples 1 to 23 and 25, the relative lateral pressure sensitivity coefficient was 540 or less. On the other hand, in Experimental Example 24, the relative lateral pressure sensitivity coefficient was a value exceeding 3000, resulting in a significantly large value. This means that the micro-bending loss and cable loss became large, making it difficult to meet, for example, the standard cable loss (less than 0.3 dB/km). In Table 12, evaluation A in the cable loss of Table 12 indicates that the standard cable loss (less than 0.3 dB/km) is satisfied, while evaluation B indicates that the standard cable loss (less than 0.3 dB/km) is not satisfied.
Table 12 also indicates the results of the tensile test for the optical fiber 1. In this test, a tensile tester having a first mandrel and a second mandrel was prepared, and sandpaper with an average particle size of 15 μm or more and 25 μm or less was wrapped around the first mandrel. The first mandrel and the second mandrel were arranged to be spaced apart from each other by a predetermined distance. Then, both ends of the optical fiber 1 were wound around the first mandrel and the second mandrel to perform the tensile test. The first mandrel has one end of the optical fiber 1 wrapped on the sandpaper described above. In this test, the median breaking stress of the optical fibers of the above-mentioned Experimental Example 1 to Experimental Example 25 was measured. As indicated in Table 12, it was confirmed that the median breaking stress was 1.5 GPa or more, which is the standard value, in Experimental Example 1 to Experimental Example 23. On the other hand, in Experimental Examples 24 and 25, the median breaking stress was smaller than the standard value of 1.5 GPa. This is considered to be due to the fact that in Experimental Examples 24 and 25, in addition to the fact that the fibers are small in diameter, the thickness of the primary resin layer is 4 μm or less, and the thickness of the secondary resin layer is 6 μm or less. Thus, with the optical fibers according to Experimental Examples 1 to 23, it is possible to reduce the diameter of the optical fiber, reduce micro-bending loss, and still maintain the required tensile strength.
As described above, the optical fiber 1 satisfies at least the following requirements.
As described above, in the optical fiber 1, it is possible to reduce micro-bending loss in a small-diameter optical fiber. This allows for a reduction in cable loss when the optical fiber is incorporated into a cable.
Furthermore, in the optical fiber 1, the lateral rigidity D and the bending rigidity H satisfy Formula (3), so micro-bending loss can be reliably suppressed. As the coating eccentricity increases, the discrepancy between the calculated and actual measured values of micro-bending loss becomes larger. In the optical fiber 1, the coating eccentricity is 8 μm or less, so the discrepancy between the calculated and actual measured values of micro-bending loss is suppressed. Thus, the micro-bending loss can be more reliably suppressed. In the case where the coating eccentricity was 5 μm, the relative error between the calculated value and the actual measured value was 5.3%. In the case where the coating eccentricity was 8 μm, the relative error between the calculated value and the actual measured value was 9.8%. In the case where the coating eccentricity was 10 μm, the relative error between the calculated value and the actual measured value was 21.8%. A relative error of 10% or less is desirable, so a coating eccentricity of 8 μm or less is preferable. In order to further reduce the relative error, a lower coating eccentricity is better.
Although the embodiments of the present disclosure have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2024-120905 | Jul 2024 | JP | national |
