Optical fiber

Abstract

Provided is an optical fiber comprising an inner core with a radius r1 at a center part, an outer core with a radius r2, a first cladding layer with an outermost radius r3 which is adjacent to the outer core at a radial position r2 and covers the outer periphery of the outer core, a second cladding layer with an outermost radius r4 which is adjacent to the first cladding layer at a radial position r3 and covers the outer periphery of the first cladding layer, and a third cladding layer which is adjacent to the second cladding layer at a radial position r4 and covers the outer periphery of the second cladding layer. The refractive index of the inner core, the outer core, and the first cladding layer takes on a maximum value at the center part and decreases continuously and smoothly from the center part to the outer side. The refractive index of the second cladding layer increases continuously and smoothly from the radial position r3 to the radial position r4.

Description

optical fiber

[0001] The present invention relates to optical fibers.

[0002] Patent Document 1 states that "a trench relatively far from the core may improve mode confinement, improve bending loss, permit the use of lossier "high loss" cladding material, or permit reduction in the overall fiber diameter" (Chapter 11, lines 36-40). Patent Document 2 states that "there is no abrupt composition change region at the boundary between the first cladding layer and the second cladding layer, and as a result, the refractive index profile structure has a negative gradient with respect to the outer diameter direction within the second cladding layer" (paragraph 0022). [Prior Art Literature] [Patent Documents] [Patent Document 1] U.S. Patent No. 4,852,968 [Patent Document 2] Japanese Patent No. 5,799,903 General disclosure

[0003] A first aspect of the present invention provides an optical fiber comprising an inner core of radius r1 at a center thereof, an outer core of radius r2, a first cladding layer of outermost radius r3 adjacent to the outer core at radial position r2 and covering the outer periphery thereof, a second cladding layer of outermost radius r4 adjacent to the first cladding layer at radial position r3 and covering the outer periphery thereof, and a third cladding layer adjacent to the second cladding layer at radial position r4 and covering the outer periphery thereof, wherein the refractive indices of the inner core, the outer core, and the first cladding layer are maximum at the center and decrease continuously and gradually from the center to the outside, and the refractive index of the second cladding layer increases continuously and gradually from radial position r3 to radial position r4.

[0004] In the above optical fiber, |dΔ(r) / dr|≦0.1% / μm may be satisfied in the region where the radius r is 0 to r1, |dΔ(r) / dr|≦0.15% / μm may be satisfied in the region where the radius r is r1 to r3, and |dΔ(r) / dr|≦0.1% / μm may be satisfied in the region where the radius r is r3 to r4.

[0005] In any of the above optical fibers, the inner core may have a maximum relative refractive index difference Δ1max at the center, the outer core may have a relative refractive index difference Δ2 at radial position r1, the first cladding layer may have a relative refractive index difference Δ3 at radial position r2 and a minimum relative refractive index difference Δ4min at radial position r3, and the second cladding layer may have a relative refractive index difference Δ5 at radial position r4, wherein Δ1max>Δ2>Δ3>Δ5>Δ4min and Δ2 is 0.3% or less.

[0006] In any of the above optical fibers, the gradient of the refractive index profile curve may change from negative to positive in the vicinity of the radial position r3 where the first cladding layer and the second cladding layer contact each other.

[0007] In any of the above optical fibers, the radius r1 may be 2 to 10 μm, the radius r2 may be 3 to 14 μm, and the outermost radius r3 may be 10 to 25 μm.

[0008] In any of the above optical fibers, the positive dopant added to the inner core may have a concentration distribution in the radial direction, and at the position in the concentration distribution where the concentration of the positive dopant is maximum, the relative refractive index difference of the inner core based on the average refractive index of the third cladding layer may satisfy 0.20 to 0.50%.

[0009] In any of the above optical fibers, the positive dopant may be germanium and / or chlorine.

[0010] In any of the above optical fibers, a negative dopant may be added to the first cladding layer and the second cladding layer so that the relative refractive index difference between the first cladding layer and the second cladding layer based on the average refractive index of the third cladding layer is −0.40 to −0.05%.

[0011] In any of the above optical fibers, the negative dopant may be fluorine.

[0012] Any of the above optical fibers may have an attenuation of less than about 0.1845 dB / km at a wavelength of 1550 nm.

[0013] In any of the above optical fibers, bending loss at a wavelength of 1550 nm when bent at a radius of 10 mm may be 0.1 dB / turn or less.

[0014] In any of the above optical fibers, the zero dispersion wavelength may be 1300 to 1324 nm.

[0015] In any of the above optical fibers, the mode field diameter at 1310 nm may be 8.8 to 9.6 μm.

[0016] In any of the above optical fibers, the cutoff wavelength measured at a fiber length of 22 m may be 1260 nm or less.

[0017] The above summary of the invention does not list all of the features of the present invention, and subcombinations of these features may also be inventions.

[0018] 1A and 1B show a schematic cross-sectional structure of an optical fiber according to an embodiment, and an example of a refractive index profile structure of the optical fiber according to an embodiment.

[0019] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention as claimed. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0020] Fig. 1 shows an outline of the cross-sectional structure of an optical fiber 1 according to one embodiment. Fig. 2 shows an example of the refractive index profile structure of the optical fiber 1 according to one embodiment, which has low-loss transmission characteristics and bending loss equal to or greater than those of a single-mode optical fiber.

[0021] As shown in Fig. 1, the optical fiber 1 for optical communications according to this embodiment has a trench-type refractive index profile structure with a three-layer cladding structure. Specifically, the optical fiber 1 comprises an inner core 2 with a radius r1 at its center, an outer core 3 with an outermost radius r2 adjacent to the inner core 2 at the radial position r1 and surrounding the outer periphery of the inner core 2, a first cladding layer 4 with an outermost radius r3 adjacent to the outer core 3 at the radial position r2 and surrounding the outer periphery of the outer core 3, a second cladding layer 5 with an outermost radius r4 adjacent to the first cladding layer 4 at the radial position r3 and surrounding the outer periphery of the first cladding layer 4, and a third cladding layer 6 adjacent to the second cladding layer 5 at the radial position r4 and surrounding the outer periphery of the second cladding layer 5. The outer periphery of the third cladding layer 6 is located at the radial position rf, and the outer periphery of the third cladding layer 6 forms the outermost surface of the optical fiber 1. The optical fiber 1 may additionally comprise layers other than the inner core 2, the outer core 3, the first cladding layer 4, the second cladding layer 5, and the third cladding layer 6.

[0022] 2 , the refractive indexes of the inner core 2, outer core 3, and first cladding layer 4 are maximum at the center of the inner core 2 and gradually decrease from the center outward, while the refractive index of the second cladding layer 5 gradually increases from radial position r3 to radial position r4. As such, the refractive index profile of the optical fiber 1 does not have abrupt refractive index change regions in the inner core 2, outer core 3, first cladding layer 4, and second cladding layer 5. In other words, the optical fiber 1 does not have abrupt composition change regions.

[0023] In this specification, the radius of each layer is defined as follows. The refractive index at an arbitrary radial position (distance from the center of the optical fiber 1) r in the optical fiber 1 is defined as n(r). r1 is the radial position where the second derivative of the refractive index profile is minimum in the region where r<15 μm, and may be 2 to 10 μm or 3.5 to 5 μm. r2 is the radial position where the second derivative of the refractive index profile is maximum in the region where r<15 μm, and may be 3 to 14 μm or 4 to 7 μm. Furthermore, r3 is the radial position where the refractive index is minimum in the refractive index profile, and may be 10 to 25 μm or 12 to 20 μm. In the refractive index profile, the refractive index increases gradually from r3 to r4. r4 is the radial position where the refractive index first becomes n(r)=n0 from the radial position r3 outward in the refractive index profile. When the outer diameter of the fiber is defined as rf, the average refractive index from rf / 2 to rf is defined as n0.

[0024] 2 , the refractive index profile from the inner core 2 to the first cladding layer 4 has a maximum value at the center of the inner core 2, decreases continuously and gently from the inside to the outside, and reaches a minimum value at radial position r3. The refractive index of the second cladding layer 5 increases continuously and gently from the inside to the outside, reaching a minimum value at radial position r3 and a maximum value at radial position r4. In other words, the slope of the refractive index profile curve changes from negative to positive near radial position r3 where the first cladding layer 4 and the second cladding layer 5 meet.

[0025] The relative refractive index difference Δ of each layer is defined as follows. The relative refractive index difference Δ(r) at radius r is defined as Δ(r)=100×(n(r)−n0) / n(r). The inner core 2 has a maximum relative refractive index difference Δ1max at the center, the outer core 3 has a relative refractive index difference Δ2 at radial position r1, the first cladding layer 4 has a relative refractive index difference Δ3 at radial position r2 and a minimum relative refractive index difference Δ4min at radial position r3, and the second cladding layer 5 has a relative refractive index difference Δ5 at radial position r4. That is, Δ1max is the maximum relative refractive index difference in the profile, Δ2 is the relative refractive index difference Δ(r1) at radial position r1, Δ3 is the relative refractive index difference Δ(r2) at radial position r2, Δ4min is the minimum relative refractive index difference Δ(r3) at radial position r3, and Δ5 is the relative refractive index difference Δ(r4) at radial position r4. It should be noted that Δ0 is the average relative refractive index difference between Δ(rf / 2) and Δ(rf).

[0026] The positive dopant added to the inner core 2 may have a concentration distribution in the radial direction of the inner core 2. The positive dopant may be germanium and / or chlorine. At the position where the concentration of the positive dopant reaches its maximum in the concentration distribution, the maximum relative refractive index difference of the inner core 2 based on the average refractive index of the third cladding layer 6 may be 0.20 to 0.50%, in order to adjust the relative refractive index difference of the core so that it falls within the MFD value specified by ITU-T G.652.D. If the maximum relative refractive index difference of the inner core 2 is less than 0.20%, the refractive index difference with the cladding becomes small, and a predetermined bending loss, for example, a bending loss of 0.1 dB / turn or less at a wavelength of 1550 nm when bending with a radius of 10 mm, cannot be obtained. Furthermore, if the maximum relative refractive index difference of the inner core 2 is greater than 0.50%, the dopant concentration in the core portion becomes high, raising concerns about increased Rayleigh scattering and worsening transmission loss.

[0027] A negative dopant may be added to the first cladding layer 4 and the second cladding layer 5 so that the relative refractive index difference between the first cladding layer 4 and the second cladding layer 5 is −0.40 to −0.05% relative to the average refractive index of the third cladding layer 6. The negative dopant may be fluorine. If the relative refractive index difference between the first cladding layer 4 and the second cladding layer 5 is greater than −0.05%, the refractive index difference with the core becomes small, and a predetermined bending loss, for example, a bending loss of 0.1 dB / turn or less at a wavelength of 1550 nm when bending with a radius of 10 mm, cannot be obtained. Furthermore, if the relative refractive index difference between the first cladding layer 4 and the second cladding layer 5 is less than −0.40%, the negative dopant concentration in the cladding becomes high, which raises concerns about increased Rayleigh scattering and thus increased transmission loss.

[0028] In an optical fiber, most light passes through the center, and the amount of light that passes decreases as the distance from the center increases. However, some of the propagating light also leaks into the cladding region. Structural imperfection loss can be reduced by minimizing the refractive index change in the core and cladding regions. In the optical fiber 1 according to this embodiment, the refractive index change is defined and normalized by |dΔ(r) / dr|, i.e., the magnitude of the radial differential value of the relative refractive index difference Δ(r), and the appropriate range of |dΔ(r) / dr| at each location has been clarified.

[0029] FIG. 2 shows an example of a refractive index distribution structure, in which the horizontal axis is the radius r of the optical fiber 1, the vertical axis of the top diagram is the relative refractive index difference Δ, the vertical axis of the middle diagram is the first differential value |dΔ(r) / dr| calculated from the refractive index distribution structure, and the vertical axis of the bottom diagram is the second differential value |dΔ ２ (r) / dr ２ |. The refractive index distribution structure is obtained by preparing an optical fiber with a diameter of 125 μm, calculating the relative refractive index difference Δ(r) at 0.15 μm pitches, and plotting this. Then, dΔ(r) / dr can be calculated by differentiating the diameter distribution Δ(r) of the relative refractive index difference. Furthermore, to ignore the direction of change (increase or decrease) and focus only on the steepness of the refractive index change, |dΔ(r) / dr|, the absolute value of dΔ(r) / dr, is obtained.

[0030] Below, we will explain the appropriate range of |dΔ(r) / dr| for each region and the manufacturing method considerations for achieving this range. First, |dΔ(r) / dr| in the (0 to r1) radius region may be 0.1% / μm or less. Because a large amount of light passes through the inner core region, preventing such a sudden change in refractive index, i.e., suppressing large changes in the glass composition, can reduce structural imperfection loss in the optical fiber 1. To reduce |dΔ(r) / dr| in this region, for example, the pull-down speed of the preform during sintering may be adjusted. Specifically, slowing the pull-down speed may promote diffusion of central Ge into the cladding, thereby reducing |dΔ(r) / dr| in that radius region. Furthermore, for example, increasing the chlorine concentration in the atmosphere during dehydration may reduce |dΔ(r) / dr| in that radius region.

[0031] Next, |dΔ(r) / dr| in the (r1 to r3) radius region may be 0.15% / μm or less. This suppresses abrupt changes in refractive index in the optical fiber 1, thereby reducing structural imperfection loss. If |dΔ(r) / dr| in the (r1 to r3) radius region is greater than 0.15% / μm, structural imperfection loss increases due to the abrupt change in refractive index. To reduce |dΔ(r) / dr| in the (r1 to r3) radius region, for example, fluorine may be doped during the dehydration process. By doping with fluorine when the density of the porous preform is low, abrupt changes in doping from the center to the outside may be suppressed, thereby reducing |dΔ(r) / dr| in that radius region.

[0032] In the (r3 to r4) radius region, |dΔ(r) / dr| may be 0.1% / μm or less. Because light also seeps through and passes through this radius region near the third cladding, preventing abrupt changes in refractive index can further reduce structural imperfection loss in the optical fiber 1. In order to reduce |dΔ(r) / dr| in this radius region, for example, a step of removing fluorine from the surface of the porous glass preform may be inserted between the dehydration step and the vitrification step.

[0033] In the optical fiber 1 according to this embodiment, the relative refractive index differences Δ of the layers have the relationship Δ1max > Δ2 > Δ3 > Δ5 > Δ4min. It is preferable that the relative refractive index difference Δ2 at the radial position r1 is 0.3% or less. This is because the vicinity of Δ2 is the position where the power volume of light is maximum, and a high Ge concentration in this portion leads to an increase in Rayleigh scattering.

[0034] When the inner core is manufactured using the OVD method or the like, a drop in refractive index (center dip) exists in the center, but the central portion refers to the region excluding the center dip.

[0035] Example 1 Next, a method for manufacturing a single-mode optical fiber according to the present invention will be described. First, a porous glass preform consisting of a core and an intermediate layer is integrally synthesized using the VAD method. At this time, the core is doped with germanium to increase the refractive index. At this time, the bulk density of the glass particles (soot) can be adjusted by controlling the soot deposition temperature. The higher the bulk density, the more easily the amount of fluorine doping can be reduced in the subsequent sintering process in a fluorine atmosphere.

[0036] Next, the soot base material is sintered in the following procedure. First, as a de-OH treatment of the soot base material, the entire length of the soot base material is heat-treated in a gas atmosphere of an in-furnace gas of Ar = 20 L / min and Cl2 = 0.5 L / min at a sintering temperature of 1200 °C and a pulling-down speed of 10 mm / min. Next, as a transparent vitrification treatment and a fluorine doping treatment, the in-furnace gas is heated in a gas atmosphere of He = 20 L / min and SiF ４ The entire length of the soot base material is heat-treated in a mixed gas atmosphere of SiO 2 = 0.3 L / min at a sintering temperature of 1500°C and a feed rate of 5 mm / min. Note that tetrafluoromethane, hexafluoroethane, or the like may be used instead of tetrafluorosilane gas.

[0037] The transparent glass core preform obtained in this manner is drawn to a predetermined diameter on a glass lathe to make the outer diameter in the longitudinal direction uniform. During this process, OH groups are incorporated into the surface of the preform due to the influence of the oxyhydrogen flame of the glass lathe. These can be removed by immersing the transparent glass core preform in an aqueous hydrofluoric acid solution to melt the surface. When drawing on the glass lathe, a plasma flame may be used as the heating source. In this case, OH groups are not incorporated into the surface of the core preform, so the treatment with hydrofluoric acid can be omitted.

[0038] The transparent core preform thus produced is used as a target, and a cladding layer is further added by the OVD method to obtain a porous preform. The obtained porous preform is then sintered and vitrified to produce an optical fiber preform. The obtained optical fiber preform is heated to approximately 2100°C and spun to obtain an optical fiber with a diameter of 125 μm. 100 60 km optical fibers were obtained. Note that the diameter may be 125 μm or less. By reducing the diameter, the fiber density within the cable can be increased, thereby increasing the energy density.

[0039] Example 2: First, a porous glass preform consisting of a core and an intermediate layer was synthesized using the VAD method. The core was doped with germanium to increase its refractive index. This porous glass preform was heated to approximately 1200°C in a mixed gas flow atmosphere consisting of 1.5 liters per minute of chlorine gas, 0.3 liters per minute of tetrafluorosilane gas, and 20 liters per minute of Ar gas. The porous glass preform was then pulled down at 10 mm / min to dehydrate and dope with fluorine. He was then flowed at 20 liters per minute. The preform was then heated to approximately 1500°C to form a solid, transparent glass core preform. Note that tetrafluorosilane gas may be replaced with tetrafluoromethane or hexafluoroethane. A third cladding layer was then added using the OVD method. The resulting porous preform was then sintered and vitrified to form a transparent optical fiber preform. The preform was spun to obtain 100 optical fibers each having a diameter of 125 μm and a length of 60 km.

[0040] Example 3: First, a porous glass preform consisting of a core and an intermediate layer was synthesized using the VAD method. The core was doped with germanium to increase its refractive index. This porous glass preform was heated to approximately 1200°C in a mixed gas flow atmosphere consisting of chlorine gas at 3 liters per minute, tetrafluorosilane gas at 0.3 liters per minute, and Ar gas at 20 liters per minute. The porous glass preform was then lowered at 10 mm / min to dehydrate and dope with fluorine. Subsequently, a process of heating the porous glass preform at 1300°C for 1 hour was added to remove surface fluorine. During this process, He was flowed at 20 liters per minute. The preform was then heated to approximately 1500°C to form a solid, transparent glass core preform. Note that tetrafluorosilane gas may be replaced with tetrafluoromethane or hexafluoroethane. The third cladding layer was then added by the OVD method, and the resulting porous preform was sintered and vitrified to produce an optical fiber preform. The preform was then spun to obtain 100 60 km long optical fibers with a diameter of 125 μm.

[0041] [Comparative Example] First, a porous glass preform consisting of a core and an intermediate layer was synthesized using the VAD method. The core was doped with germanium to increase the refractive index. This porous glass preform was heated to approximately 1200°C in a mixed gas flow atmosphere consisting of chlorine gas at 0.5 liters per minute, tetrafluorosilane gas at 0.3 liters per minute, and Ar gas at 20 liters per minute. The porous glass preform was then pulled down at 10 mm / min to dehydrate and dope with fluorine. Next, 20 liters of helium gas was introduced and heated to approximately 1500°C to form a solid transparent glass core preform. A third cladding layer was then added using the OVD method, and the resulting porous preform was sintered and vitrified to produce an optical fiber preform. The preform was then spun to obtain 100 60 km optical fibers with a diameter of 125 μm. Table 1 shows various parameters of the optical fibers obtained in Examples 1, 2, 3, and the Comparative Example. The various parameters shown in Table 1 were measured for 100 of the resulting 60 km optical fibers. Average values ​​were calculated for λcc, λ0, and MFD. The transmission loss and bending loss were calculated as the percentages of those less than 0.1845 dB / km and 0.1 dB / Turn or less, respectively.

[0042] In Example 1, the rate at which the transmission loss at a wavelength of 1550 nm was less than 0.1845 dB / km was 54%. This is because the refractive index reaches its maximum at the center and gradually decreases from the center outward, and the refractive index of the second cladding layer gradually increases from the radial position r3 to the radial position r4, thereby sufficiently reducing structural misalignment loss. Meanwhile, the rate at which the bending loss at a radius of 10 mm (R10 × 1 turn) was 0.1 dB / turn or less was 99%. This is in accordance with the ITU-T G.657.A2 recommendation.

[0043] In the above-described Example 1, the average value of the zero-dispersion wavelength λ0 was 1300 nm, which was within the range of 1300 to 1324 nm. The average value of the mode field diameter at 1310 nm was 8.92 μm, which was within the range of 8.8 to 9.6 μm. The average value of the cutoff wavelength λcc measured over a fiber length of 22 m was 1244 nm, which was below 1260 nm. These characteristics comply with ITU-T Recommendation G.652.D.

[0044] In Example 2, the rate of transmission loss at a wavelength of 1550 nm being less than 0.1845 dB / km was 78%. This is because increasing the chlorine concentration during dehydration promoted the diffusion of Ge into the first cladding, reducing |dΔ(r) / dr| in the radius range (0 to r1) and |dΔ(r) / dr| in the radius range (r1 to r3). ​​This further reduced structural imperfection loss, resulting in lower transmission loss at a wavelength of 1550 nm. Meanwhile, the rate of bending loss at a radius of 10 mm (R10 × 1 turn) being 0.1 dB / turn or less was 99%. This is in accordance with the ITU-T G.657.A2 recommendation.

[0045] In Example 2, the average zero-dispersion wavelength λ0 was 1302 nm, which was within the range of 1300 to 1324 nm. The average mode field diameter at 1310 nm was 8.85 μm, which was within the range of 8.8 to 9.6 μm. The average cutoff wavelength λcc measured over a 22 m fiber length was 1239 nm, which was below 1260 nm. These characteristics comply with ITU-T Recommendation G.652.D.

[0046] In Example 3, the rate of transmission loss at a wavelength of 1550 nm of less than 0.1845 dB / km was 99%. This was because the chlorine concentration during dehydration was further increased from that in Example 2, which promoted the diffusion of Ge into the first cladding, thereby reducing |dΔ(r) / dr| in the (0 to r1) radius range and |dΔ(r) / dr| in the (r1 to r3) radius range. Furthermore, by providing a fluorine removal process after fluorine doping, the diffusion of F into the second cladding was promoted, thereby reducing |dΔ(r) / dr| in the (r3 to r4) radius range. This further reduced structural imperfection loss, resulting in lower transmission loss at a wavelength of 1550 nm. Furthermore, by setting Δ2 to 0.3% or less, Rayleigh scattering could be suppressed, resulting in lower transmission loss at a wavelength of 1550 nm. On the other hand, the bending loss at a radius of 10 mm (R10 x 1 turn) was 0.1 dB / turn or less in 99% of cases. This is in accordance with the ITU-T G.657.A2 recommendation.

[0047] In Example 3, the average zero-dispersion wavelength λ0 was 1310 nm, which was within the range of 1300 to 1324 nm. The average mode field diameter at 1310 nm was 9.17 μm, which was within the range of 8.8 to 9.6 μm. The average cutoff wavelength λcc measured over a 22 m fiber length was 1241 nm, which was below 1260 nm. These characteristics comply with ITU-T Recommendation G.652.D.

[0048] In the comparative example, the percentage of samples with a transmission loss of less than 0.1845 dB / km at a wavelength of 1550 nm was 1%, while the percentage of samples with a bending loss of 0.1 dB / turn or less at a radius of 10 mm (R10×1 turn) was 5%.

[0049] As a comparative example of the optical fiber 1 according to the embodiment described above, a single-mode optical fiber has the characteristic that signal light propagates through the core of the optical fiber and can transmit signals even when the optical fiber is slightly bent. Generally, in a single-mode optical fiber, as the bending radius becomes smaller, the proportion of light that cannot propagate and leaks from the core increases exponentially, resulting in increased transmission loss. This is the bending loss mentioned above. In recent years, optical fibers may be used in a bent state with a curvature radius of 10 mm or less to 15 mm, but optical fibers with even lower loss are required.

[0050] To reduce bending loss, it is effective to increase the refractive index of the core to better focus the light into the core. This can be improved by reducing the mode field diameter (MFD). For this reason, optical fibers with an MFD of approximately 8.2 to 8.8 μm have been used. By doing this, for example, when an optical fiber is wound around a mandrel (cylinder) with a radius r of 10 mm, bending loss of 0.1 dB / turn or less at a wavelength of 1550 nm has been achieved.

[0051] However, since the MFD of optical fibers conforming to the ITU-T G.652.D standard, which are generally used in long-distance optical communications, is approximately 8.8 to 9.6 μm, when an optical fiber with reduced bending loss as described above is spliced ​​with an optical fiber conforming to the above standard, there is a problem that the difference in MFD results in a large splice loss.

[0052] In other comparative examples, in order to solve this problem, a trench-type optical fiber is used to reduce bending loss while designing it to have a large MFD. This is a well-known technique that has been known for a long time, but in recent years, its excellent bending loss characteristics have attracted attention.

[0053] However, in the case of an optical fiber with a trench-type refractive index profile, there is an interface where the glass composition changes significantly, and residual stress occurs in the region where the refractive index changes significantly, which causes an increase in transmission loss. Since this transmission loss has little wavelength dependency, it is not absorption loss caused by specific impurities but the structural imperfection loss mentioned above.

[0054] In still another comparative example, the inclination of a part of the cladding portion of the trench was specified in order to reduce the structural imperfection loss, but this did not sufficiently reduce the structural imperfection loss.

[0055] In contrast, the optical fiber 1 according to the present embodiment provides an optical fiber having a refractive index profile with few structural imperfections. The refractive index of at least the inner core 2, the outer core 3, and the first cladding layer 4 is maximized at the center of the inner core 2 and gradually decreases from the center outward, while the refractive index of the second cladding layer 5 gradually increases from the radial position r3 to the radial position r4. The optical fiber 1 having such a configuration can reduce bending-induced transmission loss and transmission loss at each wavelength while maintaining transmission characteristics such as cutoff wavelength, mode field diameter, and zero-dispersion wavelength equivalent to those of a single-mode optical fiber. For example, as described above, the optical fiber 1 reduces |dΔ(r) / dr| in the (0 to r1) radius region and the (r1 to r3) radius region, thereby suppressing abrupt refractive index changes. The structural imperfection loss is reduced, resulting in low transmission loss. The bending loss can be reduced while maintaining an MFD of 8.8 to 9.6 μm.

[0056] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the claims that such modifications and improvements can also be included within the technical scope of the present invention.

[0057] It should be noted that the order of execution of each process, such as operations, procedures, steps, and stages, in the devices, systems, programs, and methods shown in the claims, specifications, and drawings is not specifically stated as "before," "prior to," etc., and that the processes can be performed in any order unless the output of a previous process is used in a subsequent process. Even if the operational flow in the claims, specifications, and drawings is described using "first," "next," etc. for convenience, this does not mean that the processes must be performed in this order.

[0058] REFERENCE SIGNS LIST 1 Optical fiber 2 Inner core 3 Outer core 4 First cladding layer 5 Second cladding layer 6 Third cladding layer

Claims

1. An optical fiber comprising an inner core with radius r1 in the center, an outer core with radius r2, a first cladding layer with outermost radius r3 adjacent to the outer core at radial position r2 and covering its outer circumference, a second cladding layer with outermost radius r4 adjacent to the first cladding layer at radial position r3 and covering its outer circumference, and a third cladding layer adjacent to the second cladding layer at radial position r4 and covering its outer circumference, wherein the refractive index of the inner core, the outer core, and the first cladding layer takes its maximum value at the center and decreases continuously and smoothly outward from the center, and the refractive index of the second cladding layer increases continuously and smoothly from radial position r3 to radial position r4.

2. The optical fiber according to claim 1, wherein |dΔ(r) / dr| ≤ 0.1% / μm is satisfied in the region where radius r is 0 to r1, |dΔ(r) / dr| ≤ 0.15% / μm is satisfied in the region where radius r is r1 to r3, and |dΔ(r) / dr| ≤ 0.1% / μm is satisfied in the region where radius r is r3 to r4.

3. The optical fiber according to claim 1 or 2, wherein the inner core has a maximum relative refractive index difference Δ1max at the center, the outer core has a relative refractive index difference Δ2 at radial position r1, the first cladding layer has a relative refractive index difference Δ3 at radial position r2 and a minimum relative refractive index difference Δ4min at radial position r3, the second cladding layer has a relative refractive index difference Δ5 at radial position r4, and Δ1max > Δ2 > Δ3 > Δ5 > Δ4min, and Δ2 is 0.3% or less.

4. The optical fiber according to claim 1 or 2, wherein the slope of the refractive index distribution shape curve changes from negative to positive near the radial position r3 where the first cladding layer and the second cladding layer are in contact.

5. The optical fiber according to claim 1 or 2, wherein the radius r1 is 2 to 10 μm, the radius r2 is 3 to 14 μm, and the outermost radius r3 is 10 to 25 μm.

6. The optical fiber according to claim 1 or 2, wherein the positive dopant added to the inner core has a concentration distribution in the radial direction, and at the position where the concentration of the positive dopant is maximum in the concentration distribution, the difference in the specific refractive index of the inner core with respect to the average refractive index of the third cladding layer is 0.20 to 0.50%.

7. The optical fiber according to claim 6, wherein the positive dopant is germanium and / or chlorine.

8. The optical fiber according to claim 1 or 2, wherein a negative dopant is added to the first cladding layer and the second cladding layer such that the difference in relative refractive index between the first cladding layer and the second cladding layer, based on the average refractive index of the third cladding layer, is -0.40 to -0.05%.

9. The optical fiber according to claim 8, wherein the negative dopant is fluorine.

10. The optical fiber according to claim 1 or 2, having an attenuation of less than approximately 0.1845 dB / km at a wavelength of 1550 nm.

11. The optical fiber according to claim 1 or 2, wherein the bending loss at a wavelength of 1550 nm when bent with a radius of 10 mm is 0.1 dB / turn or less.

12. The optical fiber according to claim 1 or 2, wherein the zero-dispersion wavelength is 1300 to 1324 nm.

13. The optical fiber according to claim 1 or 2, wherein the mode field diameter at 1310 nm is 8.8 to 9.6 μm.

14. The optical fiber according to claim 1 or 2, wherein the cutoff wavelength measured with a fiber length of 22 m is 1260 nm or less.