Single-mode optical fiber

Abstract

Provided is a single-mode optical fiber including a core layer, a first cladding layer, and a second cladding layer, the refractive index of the central portion of the core layer being n0, the radius position at which the refractive index is n0 * 0.45 being rcore, the radius position at which the first cladding layer is rcore * 2.2 being r2, the refractive index at the r2 being n2, the minimum value of the refractive index of the first cladding layer being n3, and the radius position at which the refractive index is n3 being r3. When the radius position of the boundary between the first cladding layer and the second cladding layer is r4 and the average refractive index of the second cladding layer is n4, the relative refractive index difference [delta] n0 of the core layer is 0.3% to 0.5%, the relative refractive index difference [delta] n2 is-0.12% to-0.05%, and the relative refractive index difference [delta] n3 is-0.20% to-0.12%. The relative refractive index difference of the first cladding layer continuously and gently decreases from the r core to the r3.

Description

Technical Field

[0001] The present invention relates to a single-mode optical fiber. Background Art

[0002] Patent Document 1 describes that "the low-bending-loss SMF of the present invention has the following characteristics: the cut-off wavelength is 1260 nm or less, the zero-dispersion wavelength is in the range of 1300 nm to 1324 nm, the zero-dispersion slope is 0.093 ps / nm 2 / km or less, the MFD at a wavelength of 1310 nm is in the range of 5.5 μm to 7.9 μm, and the bending loss generated when wound 10 times around a radius of 10 mm is 0.5 dB or less at a wavelength of 1550 nm" (0027). Patent Document 2 describes that "the optical fiber (1) of the present invention includes a core that includes the optical fiber axis and extends axially, a first optical cladding that surrounds the core, a second optical cladding that surrounds the first optical cladding, and a jacket that surrounds the second optical cladding, and (2) the relative refractive index difference Δ1 of the core with respect to the jacket is 0.31% to 0.37%, the relative refractive index difference Δ2 of the first optical cladding with respect to the jacket is +0.02% or more, the relative refractive index difference Δ3 of the second optical cladding with respect to the jacket is -0.2% or less, and the relationship Δ1 > Δ2 > Δ3 is satisfied" (0008). Patent Document 3 describes that "an optical fiber includes a first core, a second core, a third core, and a cladding, and satisfies 0.30% ≦ Δ1 ≦ 0.45%, -0.05% ≦ Δ2 ≦ 0.05%, -0.6% ≦ Δ3 ≦ -0.3%, 2.85 ≦ b / a, 10 μm ≦ b ≦ 15 μm, 3 μm ≦ c - b ≦ 5.5 μm, and the loss increase at a wavelength of 1550 nm when the optical fiber is wound around a mandrel with a diameter of 10 mm is 0.2 dB / turn or less" (abstract). [Prior Art Documents] [Patent Documents] [Patent Document 1] Japanese Patent No. 4833071 [Patent Document 2] Japanese Patent Laid-Open No. 2012-212115 [Patent Document 3] Japanese Patent Laid-Open No. 2013-235261 Summary of the Invention [Means for Solving the Problem]

[0003] In a first aspect of the present invention, there is provided a single-mode optical fiber including a core layer, a first cladding adjacent to the outer periphery of the core layer, and a second cladding adjacent to the outer periphery of the first cladding. In the single-mode optical fiber, when the refractive index at the center of the core layer is set to n0 and the radius position with a refractive index of n0 × 0.45 is set to r 芯 , in the first cladding, r 芯The radius position of ×2.2 is set as r2, the refractive index at r2 is set as n2, the minimum value of the refractive index of the first cladding is set as n3, the radius position with the refractive index of n3 is set as r3, the radius position at the junction of the first cladding and the second cladding is set as r4, and when the average refractive index of the second cladding is set as n4, the relative refractive index difference Δn0 of the core layer calculated from n0 and n4 is 0.3% or more and 0.5% or less, the relative refractive index difference Δn2 calculated from n2 and n4 is -0.12% or more and -0.05% or less, the relative refractive index difference Δn3 calculated from n3 and n4 is -0.20% or more and -0.12% or less, and the relative refractive index difference of the first cladding continuously and gently decreases from r 芯 to r3.

[0004] In the above single-mode optical fiber, when the radius position of ×0.8 in the core layer is set as r1, for the differential value dΔ(r) / dr of the relative refractive index difference Δ(r) (%) with respect to the distance r (μm) from the center of the core layer, dΔ(r) / dr ≥ -0.08% / μm can be satisfied within the range where the radius position r is from 0 to r1. 芯 ×0.8 is set as r1, for the differential value dΔ(r) / dr of the relative refractive index difference Δ(r) (%) with respect to the distance r (μm) from the center of the core layer, dΔ(r) / dr ≥ -0.08% / μm can be satisfied within the range where the radius position r is from 0 to r1.

[0005] In any of the above single-mode optical fibers, r 芯 can be 3 to 6 μm, and r4 can be 14 to 20 μm.

[0006] The bending loss at a wavelength of 1550 nm when any of the above single-mode optical fibers is bent with a bending radius of 10 mm can be 0.10 dB / turn or less.

[0007] In any of the above single-mode optical fibers, the mode field diameter at a wavelength of 1310 nm can be 8.2 to 9.4 μm.

[0008] In any of the above single-mode optical fibers, the zero-dispersion wavelength can be in the range of 1300 to 1324 nm, the dispersion slope at the zero-dispersion wavelength can be 0.091 ps / nm 2 / km or less, and the wavelength dispersion value at a wavelength of 1550 nm can be 17.5 ps / nm / km or less.

[0009] In any of the above single-mode optical fibers, the cut-off wavelength measured with a fiber length of 22 m can be 1260 nm or less.

[0010] In any of the above single-mode optical fibers, germanium, fluorine, and chlorine can be added to the core layer and the first cladding, and chlorine can be added to the second cladding.

[0011] In addition, the above-described summary of the invention does not list all the features of the present invention. Additionally, sub-combinations of these feature groups can also form an invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 Shows an example of the refractive index distribution structure of the optical fiber of the present embodiment. Figure 2 Shows the refractive index distribution structure of the optical fiber in Example 1. Figure 3 Shows dΔ(r) / dr in Example 1. Figure 4 Shows the refractive index distribution structure of the optical fiber in Comparative Example 1-1. Figure 5 Shows dΔ(r) / dr in Comparative Example 1-1. Figure 6 Shows the refractive index distribution structure of the optical fiber in Comparative Example 1-2. Figure 7 Shows dΔ(r) / dr in Comparative Example 1-2. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, the present invention will be described by way of embodiments of the invention. However, the following embodiments do not limit the invention of the claims. Additionally, not all combinations of the features described in the embodiments are necessary for the solution means of the invention.

[0014] Figure 1 Shows an example of the refractive index distribution structure of the single-mode optical fiber of the present embodiment. The single-mode optical fiber of the present embodiment realizes characteristics of low bending loss and low transmission loss by adjusting the refractive index distribution shapes of the core region and the inner cladding region, and reduces the dispersion slope at the zero-dispersion wavelength, that is, the optical fiber with zero-dispersion slope and wavelength dispersion value.

[0015] In the single-mode optical fiber of the present embodiment, the radius r of the core layer 芯 is the radius position where the refractive index is n0×0.45 with respect to the center of the core layer (r = 0) (n0 is the refractive index at the center of the core layer). In the core layer, r 芯 ×0.8 of the radius position is set as r1, the refractive index at r1 is set as n1, in the first cladding, r 芯 ×2.2 of the radius position is set as r2, the refractive index at r2 is set as n2, the minimum value of the refractive index of the first cladding is set as n3, the radius position of the refractive index n3 is set as r3, the boundary radius position between the first cladding and the second cladding is set as r4, and the average refractive index of the second cladding is set as n4.

[0016] In addition, the relative refractive index difference of each layer is defined as follows. The relative refractive index difference Δ(r) at a distance r from the center of the core layer is based on the refractive index n4 of the second cladding layer, and is set as Δ(r) = 100×(n(r) - n4) / n(r). Therefore, the relative refractive index difference Δn0 of the core layer is calculated from the central refractive index n0 of the core layer and the refractive index n4 of the second cladding layer, the relative refractive index difference Δn2 of the first cladding layer is calculated from the refractive index n2 of the first cladding layer and the refractive index n4 of the second cladding layer, and the relative refractive index difference Δn3 of the first cladding layer is calculated from the refractive index n3 of the first cladding layer and the refractive index n4 of the second cladding layer.

[0017] In addition, the relative refractive index difference Δ(r) is calculated by preparing a single-mode optical fiber with a diameter of 125 μm and measuring the refractive index at intervals of 0.15 μm.

[0018] Here, the wavelength dispersion will be briefly described.

[0019] Wavelength dispersion is a phenomenon in which the propagation times of lights with different wavelengths are different because the propagation speeds of lights with different wavelengths in a single-mode optical fiber are different. Wavelength dispersion will distort the optical signal waveform and become a reason for limiting the transmission capacity. Wavelength dispersion has wavelength dependence and is represented by the sum of material dispersion and structural dispersion. Regarding material dispersion, since it is caused by the wavelength dependence of the refractive index of the glass material used for the single-mode optical fiber, it is difficult to control. On the other hand, structural dispersion is caused by the refractive index distribution shape, so it is important to appropriately control the refractive index distribution shape. The wavelength (λ0) at which the wavelength dispersion is zero is the zero-dispersion wavelength, and the zero-dispersion slope specified in the ITU-T standard represents the slope of the wavelength dispersion at the zero-dispersion wavelength.

[0020] The single-mode optical fiber of this embodiment reduces the zero-dispersion slope and the wavelength dispersion at a wavelength of 1550 nm (hereinafter referred to as 1550-nm wavelength dispersion) by adjusting the refractive index distribution shapes of the core region and the inner cladding region. The zero-dispersion slope and the 1550-nm wavelength dispersion are usually negatively correlated with the zero-dispersion wavelength. In order to reduce the zero-dispersion slope and the 1550-nm wavelength dispersion simultaneously, it is effective to increase the zero-dispersion wavelength.

[0021] Furthermore, in order to determine which part of the refractive index distribution shape is effective for the zero-dispersion slope and the 1550-nm wavelength dispersion, while changing the shape of a specific part of the refractive index distribution shape, the estimated values of the optical characteristics are calculated. When calculating the estimated values, the scalar wave equation is solved using the finite element method.

[0022] As a result of the estimation calculation, it is known that with respect to the core diameter r 芯 , r 芯The relative refractive index difference Δn2 at the radius position r2 of ×2.2 is one of the important factors. By increasing Δn2, the zero-dispersion wavelength can be increased, and the zero-dispersion slope and the dispersion at the 1550 nm wavelength can be reduced simultaneously. On the other hand, if Δn2 is increased too much, the refractive index difference from the core layer becomes small, and the bending loss specified in ITU-T G.657.A2 cannot be obtained. Regarding the relative refractive index difference Δn2 in the first cladding layer, it is preferably set to -0.12 to -0.05%.

[0023] The relative refractive index difference Δn0 of the core layer is preferably set to 0.30% to 0.50%. If the relative refractive index difference is less than 0.3%, the refractive index difference from the cladding layer becomes small, and the bending loss specified in ITU-T G.657.A2 cannot be obtained. In addition, if the relative refractive index difference is 0.5% or more, the dopant concentration in the core portion becomes high, and there is a concern that the transmission loss deteriorates due to an increase in Rayleigh scattering.

[0024] The relative refractive index difference Δn3 in the first cladding layer is preferably set to -0.20 to -0.12%. If the relative refractive index difference is less than -0.20%, the negative dopant concentration in the cladding layer becomes high, and there is a concern that the transmission loss deteriorates due to an increase in Rayleigh scattering. In addition, if it becomes greater than -0.12%, the refractive index difference from the core layer becomes small, and the bending loss specified in ITU-T G.657.A2 cannot be obtained.

[0025] The core shape is also one of the important factors for the zero-dispersion slope and the dispersion at the 1550 nm wavelength. A core shape in which the refractive index rises sharply toward the core center can increase the zero-dispersion wavelength and reduce the dispersion at the 1550 nm wavelength, but it causes an increase in the zero-dispersion slope. When the radius position of r 芯 ×0.8 is set to r1, regarding dΔ(r) / dr, it is preferably that dΔ(r) / dr ≧ -0.08% / μm holds in the range where the distance from the center of the core layer, that is, the radius position r is 0 to r1.

[0026] The relative refractive index difference of the first cladding layer is preferably continuously and gently reduced from the radius position r 芯 to r3 to suppress a sharp change in the refractive index, that is, a sharp change in the composition.

[0027] Here, a comparative example for comparison with the single-mode optical fiber of the present embodiment will be described. The single-mode optical fiber of the comparative example has the following characteristics: The signal light propagates in the core of the optical fiber, and the signal can be propagated even when the optical fiber is slightly bent. In a single-mode optical fiber, as the bending radius becomes smaller, the proportion of light that does not fully propagate and leaks from the core increases exponentially, resulting in an increase in transmission loss, which is called bending loss. In recent years, single-mode optical fibers may be used in a bent state with a curvature radius of 15 mm or less to about 10 mm. On the other hand, a single-mode optical fiber with lower loss is sought. As a standard related to low bending loss of single-mode optical fibers, the standard defined in ITU-T G.657 is known. For example, in the G.657.A2 standard, it is specified that the bending loss at a wavelength of 1550 nm when bent with a radius of 10 mm is 0.10 dB / turn or less.

[0028] In order to reduce the bending loss, it is effective to increase the refractive index of the core and confine the light in the core. This can be improved by reducing the mode field diameter (MFD). Therefore, single-mode optical fibers with an MFD of about 8.2 to 8.8 μm are mostly used.

[0029] In addition, by adding a negative dopant such as fluorine to the cladding on the outer periphery of the core layer and adopting a trench-type refractive index distribution with a reduced refractive index, the light can also be confined in the core, and the bending loss of the single-mode optical fiber can be reduced.

[0030] In order to form the trench layer, a method of manufacturing by coating a fluorine-doped glass tube after forming the core and the inner cladding is known, but the manufacturing process of the base material increases and becomes complicated. Further, near the interface of the trench layer, impurities such as OH groups are likely to be mixed in. In addition, there is also a compositional difference (undulation of the interface) accompanied by a sharp refractive index change at the interface, so there is a possibility of an increase in transmission loss.

[0031] In order to deepen the trench layer, it is necessary to increase the fluorine addition amount. On the other hand, if the fluorine addition amount is increased, the zero-dispersion wavelength decreases, and the zero-dispersion slope and the wavelength dispersion value decrease. In order for the single-mode optical fiber to have the characteristics of low bending loss and low transmission loss and to make the various optical characteristics of the single-mode optical fiber fall within the required range, it is important to appropriately control the refractive index distribution shapes of the core region and the inner cladding region.

[0032] In contrast, the single-mode optical fiber according to the present embodiment, as described above, has the characteristics of low bending loss and low transmission loss by adjusting the refractive index distribution shapes of the core region and the inner cladding region, and reduces the zero-dispersion slope and the wavelength dispersion value.

[0033] In the single-mode optical fiber of the present embodiment, r 芯It may be 3 to 6 μm, and r4 may be 14 to 20 μm. The bending loss at a wavelength of 1550 nm when bent with a bending radius of 10 mm may also be 0.10 dB / turn or less. The mode field diameter at a wavelength of 1310 nm may also be 8.2 to 9.4 μm.

[0034] In the single-mode optical fiber of this embodiment, the zero-dispersion wavelength (λ0) may be in the range of 1300 to 1324 nm, and the dispersion slope at the zero-dispersion wavelength may be 0.091 ps / nm 2 / km or less, and the wavelength dispersion value at a wavelength of 1550 nm may also be 17.5 ps / nm / km or less. The cut-off wavelength (λcc) measured with a fiber length of 22 m may also be 1260 nm or less.

[0035] In the single-mode optical fiber of this embodiment, germanium, fluorine, and chlorine may also be added to the core layer and the first cladding layer, and chlorine may also be added to the second cladding layer.

[0036] Next, a method for manufacturing the single-mode optical fiber of the present invention will be described. First, a porous glass preform including a core layer and a first cladding layer is synthesized by the VAD method. At this time, a burner for forming the core layer and a burner for forming the first cladding layer are used. In addition to supplying hydrogen, oxygen, and SiCl4 as a source gas to the burner for forming the core layer, GeCl4 is also supplied, and germanium for increasing the refractive index is doped. Hydrogen, oxygen, and SiCl4 as a source gas are supplied to the burner for forming the first cladding layer, and the first cladding layer is deposited to produce a soot preform.

[0037] In addition, by adjusting the gas flow rate supplied to the burner for forming the core layer, the setting of the burner, etc., the core shape can be controlled.

[0038] Subsequently, the produced soot preform is sintered through the following steps. First, as dehydration treatment and fluorine doping treatment of the soot preform, in a mixed gas atmosphere of Ar = 25 L / min, Cl2 = 1.0 L / min, and SiF4 = 0.25 L / min in the furnace, the entire length of the soot preform is heat-treated at a sintering temperature of 1200 °C and a descending speed of 10 mm / min. Subsequently, as transparent vitrification treatment, in a gas atmosphere of He = 20 L / min in the furnace, the entire length of the soot preform is heat-treated at a sintering temperature of 1500 °C and a descending speed of 5 mm / min.

[0039] The transparent core preform thus produced is used as a target, and the second cladding layer is attached by the OVD method. The obtained porous preform is sintered and transparently vitrified to obtain an optical fiber preform. Then, the optical fiber preform is heated to about 2100 °C and spun to obtain an optical fiber with a diameter of 125 μm. [Examples]

[0040] [Example 1] First, a porous glass preform including a core and a first cladding is synthesized by the VAD method. Germanium for increasing the refractive index is doped in the core. For this porous glass preform, chlorine is supplied at 1 liter per minute, tetrafluorosilane gas is supplied at 0.25 liter per minute, and further argon is supplied at 25 liters per minute. It is heated to about 1200 °C, and the porous glass preform is drawn down at 10 mm / min to perform dehydration and fluorine doping. Then, helium is supplied at 20 liters per minute, heated to about 1500 °C, and the porous glass preform is drawn down at 5 mm / min to produce a transparent glass core preform. In addition, tetrafluoromethane or hexafluoroethane etc. can also be used in place of tetrafluorosilane gas.

[0041] The transparent core preform including the core and the first cladding thus produced is used as a target, and a second cladding is externally attached by the OVD method. The obtained porous preform is sintered to be vitrified transparently, thereby producing an optical fiber preform. The obtained preform is heated to about 2100 °C and spun to obtain an optical fiber with a diameter of 125 μm. The refractive index distribution of the obtained optical fiber is shown in Figure 2 , Figure 3 where dΔ(r) / dr is shown.

[0042] [Comparative Example 1-1] First, a porous glass preform including a core and a first cladding is synthesized by the VAD method. Germanium for increasing the refractive index is doped in the core. At this time, the gas flow rate supplied to the burner for forming the core layer and the setting of the burner are changed so as to be Figure 4 adjusted to have the core shape shown in Figure 4 . Figure 5 For this porous glass preform, chlorine is supplied at 1 liter per minute, tetrafluorosilane gas is supplied at 0.35 liter per minute, and further argon is supplied at 25 liters per minute. It is heated to about 1230 °C, and the porous glass preform is drawn down at 10 mm / min and dehydration and fluorine doping are performed. Then, helium is supplied at 20 liters per minute, heated to about 1500 °C, and the porous glass preform is drawn down at 5 mm / min to produce a transparent glass core preform. Then, a second cladding is externally attached in the same manner as in Example 1, vitrified transparently to produce an optical fiber preform, and spun to obtain an optical fiber. The refractive index distribution of the obtained optical fiber is shown in

[0043] [Comparative Example 1-2] First, a porous glass preform including a core and a first cladding is synthesized by the VAD method. Germanium for increasing the refractive index is doped in the core. At this time, the gas flow rate supplied to the burner for forming the core layer and the setting of the burner are changed so as to be Figure 6It is adjusted in the manner of the core shape shown. For this porous glass base material, chlorine is supplied at a rate of 1 liter per minute, silane tetrafluoride gas is supplied at a rate of 0.15 liters per minute, and further argon is supplied at a rate of 25 liters per minute. It is heated to about 1180 °C, and the porous glass base material is pulled down at 10 mm / min to perform dehydration and fluorination. Subsequently, helium is supplied at a rate of 20 liters per minute, heated to about 1500 °C, and the porous glass base material is pulled down at 5 mm / min to produce a transparent glass core base material. Then, a second cladding is attached in the same manner as in Example 1, and it is made transparent to produce an optical fiber base material, and spun to obtain an optical fiber. The refractive index distribution of the obtained optical fiber is shown in Figure 6 , Figure 7 where dΔ(r) / dr is shown.

[0044] Table 1 shows various parameters of the optical fibers obtained in the examples and comparative examples.

[0045] [Table 1]

[0046] In Example 1, the transmission loss at a wavelength of 1310 nm was 0.327 dB / km, and the bending loss at a wavelength of 1550 nm when bent with a bending radius of 10 mm was 0.04 dB / turn, satisfying the bending loss value specified in ITU-T's G.657.A2. In addition, the zero dispersion slope was 0.089 ps / nm 2 / km, and the wavelength dispersion at a wavelength of 1550 nm was 17.3 ps / nm / km, both obtaining good values.

[0047] Compared with Example 1 in Comparative Example 1-1, fluorination was carried out to the inside of the first cladding, and Δn2 or Δn3 became smaller, so the bending loss became smaller. The transmission loss at a wavelength of 1310 nm was 0.334 dB / km, and the bending loss at a wavelength of 1550 nm when bent with a bending radius of 10 mm was 0.02 dB / turn, satisfying the bending loss value specified in ITU-T's G1.657.A2. On the other hand, since Δn2 became smaller, the zero dispersion slope was 0.093 ps / nm 2 / km, exceeding the ITU-T standard. In addition, the wavelength dispersion at a wavelength of 1550 nm was large, at 17.9 ps / nm / km.

[0048] In addition, the core shape became steeper from r1 to the center refractive index, and in the range where r was from 0 to r1, the minimum value of dΔ(r) / dr was -0.09% / μm. Further, due to the influence of the increased fluorine addition amount, the transmission loss at a wavelength of 1310 nm was also greater than that in Example 1.

[0049] In Comparative Examples 1-2, the transmission loss at a wavelength of 1310 nm was 0.329 dB / km, and the bending loss at a wavelength of 1550 nm when bent with a bending radius of 10 mm was 0.12 dB / turn, exceeding the standard of ITU-T G.657.A2. The zero dispersion slope was 0.087 ps / nm 2 / km, and the wavelength dispersion at a wavelength of 1550 nm was 17.1 ps / nm / km.

[0050] Compared with Example 1, in Comparative Examples 1-2, the fluorine doping did not reach the inside of the first cladding, and Δn2 or Δn3 became larger, so the bending loss became larger.

[0051] The present invention has been described above using embodiments, but the scope of protection of the present invention is not limited to the scope described in the above embodiments. Those skilled in the art will know that various changes or improvements can be made to the above embodiments. From the description of the claims, the embodiments incorporating these changes or improvements can also be included within the scope of protection of the present invention.

[0052] It should be noted that the execution order of each process such as actions, steps, procedures, and stages in the devices, systems, programs, and methods shown in the claims, the specification, and the drawings is not specifically specified as "before...", "prior to...", etc. Additionally, as long as the output of the previous process is not used in the subsequent process, it can be implemented in any order. Regarding the action flow in the claims, the specification, and the drawings, even if it is described using "first," "subsequently," etc. for convenience, it does not mean that it must be implemented in that order.

Claims

1. A single-mode optical fiber, characterized in that: comprising a core layer, a first cladding layer adjacent to an outer periphery of the core layer, and a second cladding layer adjacent to an outer periphery of the first cladding layer, and When the refractive index of the center of the core layer is n0, the radial position where the refractive index is n0×0.45 is r 芯 , the first cladding layer r 芯 ×2.2 is set to r2, the refractive index at r2 is set to n2, the minimum value of the refractive index of the first cladding is set to n3, the radial position of the refractive index n3 is set to r3, the radial position of the boundary between the first cladding and the second cladding is set to r4, and the average refractive index of the second cladding is set to n4, the relative refractive index difference Δn0 of the core layer calculated according to n0 and n4 is 0.3% to 0.5%, the relative refractive index difference Δn2 calculated according to n2 and n4 is -0.12% to -0.05%, the relative refractive index difference Δn3 calculated according to n3 and n4 is -0.20% to -0.12%, and the relative refractive index difference of the first cladding is from r 芯 Then, r3 decreases continuously and gently.

2. The single-mode optical fiber according to claim 1, characterized in that In the core layer, when r 芯 When the radial position of ×0.8 is set to r1, the differential value dΔ(r) / dr of the relative refractive index difference Δ(r) (%) relative to the distance r (μm) from the center of the core layer satisfies dΔ(r) / dr≧-0.08% / μm within the range of the radial position r of 0 to r1.

3. The single-mode optical fiber according to claim 1 or 2, characterized in that: The r 芯 It is 3 to 6 μm, and r4 is 14 to 20 μm.

4. The single-mode optical fiber according to any one of claims 1 to 3, characterized in that: The bending loss at a wavelength of 1550 nm when bent with a bending radius of 10 mm is 0.10 dB / turn or less.

5. The single-mode optical fiber according to any one of claims 1 to 4, characterized in that: The mode field diameter at a wavelength of 1310 nm is 8.2 to 9.4 μm.

6. The single-mode optical fiber according to any one of claims 1 to 5, characterized in that: The zero dispersion wavelength is in the range of 1300-1324nm, and the dispersion slope at the zero dispersion wavelength is 0.091ps / nm 2 / km or less, and the wavelength dispersion value at a wavelength of 1550nm is less than 17.5ps / nm / km.

7. The single-mode optical fiber according to any one of claims 1 to 6, characterized in that: The cut-off wavelength measured with an optical fiber length of 22 m is below 1260 nm.

8. The single-mode optical fiber according to any one of claims 1 to 7, characterized in that: Germanium, fluorine, and chlorine are added to the core layer and the first cladding layer, and chlorine is added to the second cladding layer.

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