JP7097326B2 - Optical fiber - Google Patents

Description

The present invention relates to an optical fiber.

Microbend loss is known as one of the transmission losses in an optical fiber. Microbend loss is a loss caused by minute deformation or bending in a waveguide of an optical fiber. The microbend loss mainly affects the fiber characteristics in the case of a high-density optical fiber cable in which dense fibers are pressed against each other and deformed (Patent Document 1). This type of microdeformation increases the loss of individual fibers in the fiber optic cable. Such a loss is sometimes referred to as a cable loss (Non-Patent Document 1).

As the expansion of transmission capacity increases exponentially, it is required to increase the transmission capacity per cable. One solution for this is to reduce the size of the optical fiber and increase the number of optical fibers with the same cable size (Patent Document 2).

However, the smaller the diameter of the optical fiber, the greater the amount of coupling between the core mode and the clad mode. In order to reduce this effect, an optical fiber in which a trench layer, which is a layer having a refractive index lower than that of the clad, is introduced around the core has been studied (Non-Patent Document 2, Non-Patent Document 3 and Non-Patent Document 4). ). The trench fiber can confine the leak mode inside the trench and at the same time increase the effective index difference between the core mode and the clad mode. Therefore, the mode in which the basic mode can be effectively combined can be limited to the LP11 mode of the trench. In this case, if the loss of the LP11 trench mode is small and the coupling with the clad mode is very small, the loss of the core mode is also suppressed as a result (Non-Patent Document 5). Therefore, if the trench layer is made large so as to largely separate the trench mode and the clad mode, the loss in the trench mode becomes small and the loss in the core mode also becomes small (Non-Patent Document 6). Even so, since the coupling between the core mode and the clad mode is not 0, the microbend loss becomes large when the diameter of the optical fiber is reduced or the effective core cross-sectional area of the optical fiber is increased.

On the other hand, as a structure for reducing microbend loss, an optical fiber having a trench-type clad profile has been actively studied. Patent Document 3 proposes a double trench structure including a first damping clad and a second damping clad.

US Pat. No. 6,901,191 US Patent Application Publication No. 2010/0119202 Japanese Patent No. 5604028 US Patent Application Publication No. 2010/02907781 US Patent Application Publication No. 2011/018882

Nobuya Kojima, Yoshiaki Miyajima, Yasuji Murakami, Tetsuro Yabuta, Osamu Kawata, Katsuya Yamashita, and Nobuyuki Yoshizawa, “Studies on Designing of Submarine Optical Fiber Cable”, IEEE Transactions on Microwave Theory and Techniques, mtt-30, 579 (1982). S. Matsuo, M. Ikeda, and K. Himeno, “Bend-insensitive and low-splice-loss optical fiber for indoor wiring in FTTH,” OFC 2004, paper ThI 3. S. Matsuo, T. Nunome, T. Yoshita, T. Hamada, and K. Himeno, “Design optimization of trench index profile for the same dispersion characteristics with SMF,” OFC / NFOEC 2007, paper JWA2. L.-A. de Montmorillon, P. Matthijsse, F. Gooijer, F. Achten, D. Molin, N. Montaigne, and J. Maury, “Bend-optimized G.652D compatible trench-assisted single-mode fibers,” in Proc. 55th IWCS / Focus, 342-347 (2006). P. Sillard, S. Richard, L. de Montmorillon and M. Bigot-Astruc, “Micro-bend losses of trench-assisted single-mode fibers,” 36th European Conference and Exhibition on Optical Communication, Torino, 2010, pp. 1 -3. -3. Pierre Sillard, “New fibers for ultra-high capacity transport,” Optical Fiber Technology, 17, 495-502 (2011). X. Jin and F. P. Payne, “Numerical investigation of microbending loss in optical fiber,” J. Lightwave Technology 34, 1247 (2016). G. R. Hadley, “Transparent Boundary Condition for the Beam Propagation Method,” IEEE J. Quantum Electronics, 28, 363 (1992).

However, with conventional optical fibers, it has been difficult to sufficiently reduce microbend loss.

An object of the present invention is to provide an optical fiber capable of sufficiently reducing microbend loss in view of the above-mentioned problems.

According to one aspect of the present invention, it has a core portion having a first refractive index and a clad portion formed on the outer periphery of the core portion, and the clad portion is lower than the first refractive index. A base layer having a second refractive index, a trench layer having a third refractive index lower than the second refractive index, and an outer peripheral edge portion of the clad portion formed on the outside of the trench layer. It has a segment layer having a fourth refractive index lower than the first refractive index and higher than the second refractive index, and the base layer has a first base layer and a second base layer. The first base layer is formed on the outer periphery of the core portion, the trench layer is formed on the outer periphery of the first base layer, and the second base is formed. The layer is formed on the outer periphery of the trench layer, the segment layer is formed on the second base layer, the third base layer is formed on the outer periphery of the segment layer, and the fourth refractive index is formed. The difference in the specific refractive index from the second refractive index is 0.005% or more and 0.2% or less, and the difference in the specific refractive index of the third refractive index from the second refractive index is. An optical fiber characterized by having a value of −0.4% or more and −0.1% or less is provided.

According to the present invention, the microbend loss can be sufficiently reduced.

FIG. 1 is a schematic view showing an optical fiber according to an embodiment of the present invention and a refractive index profile thereof. FIG. 2 is a graph showing an example of a standard random core displacement function. FIG. 3 is a graph showing a refractive index profile obtained by simulating a trench optical fiber. FIG. 4 is a graph showing the results of calculating the microbend loss for the trench optical fiber shown in FIG. FIG. 5 is a graph showing the refractive index profile by simulation of an optical fiber in the case of introducing a segment layer and in the case of a double trench structure. FIG. 6 is a graph showing the results of calculating the microbend loss for the optical fiber when the segment layer shown in FIG. 5 is introduced and in the case of the double trench structure. FIG. 7 is a graph showing the relationship between Δn ₁ of the trench layer and the normalized microbend loss. FIG. 8 is a graph showing the relationship between Δn _c of the core portion and the normalized microbend loss.

[One Embodiment]
An optical fiber according to an embodiment of the present invention will be described with reference to FIGS. 1 to 8.

First, the configuration of the optical fiber according to the present embodiment will be described with reference to FIG. The upper part of FIG. 1 is a cross-sectional view showing an optical fiber according to the present embodiment, and shows a cross section along a plane orthogonal to the central axis of the optical fiber. The lower part of FIG. 1 is a graph showing the refractive index profile of the optical fiber according to the present embodiment. In the graph shown in the lower part of FIG. 1, the horizontal axis indicates the position r in the radial direction with respect to the central axis of the optical fiber, and the vertical axis indicates the refractive index n.

As shown in FIG. 1, the optical fiber 1 according to the present embodiment has a core portion 10, a clad portion 20 formed on the outer periphery of the core portion 10, and a covering portion 30 formed on the outer periphery of the clad portion 20. is doing. The optical fiber 1 according to the present embodiment is a single mode optical fiber. The optical fiber 1 is not limited to the single mode optical fiber, and may be a multimode optical fiber.

The cross sections of the core portion 10, the clad portion 20, and the covering portion 30 along the plane orthogonal to the central axis of the optical fiber 1 are as follows. That is, the core portion 10 has a circular cross section centered on the central axis of the optical fiber 1. The clad portion 20 is an annular cross section surrounding the core portion 10, and has a cross section concentric with the cross section of the core portion 10. The covering portion 30 has an annular cross section surrounding the clad portion 20 and has a cross section concentric with the cross section of the core portion 10. The cross sections of the core portion 10, the clad portion 20, and the covering portion 30 do not necessarily have to be concentric with each other, and the centers may be different from each other.

The optical fiber 1 according to the present embodiment is, for example, a quartz glass optical fiber in which the core portion 10 and the clad portion 20 are made of quartz glass. The material of the optical fiber 1 is not particularly limited. The optical fiber 1 is, for example, a multi-component glass optical fiber in which the core portion 10 and the clad portion 20 are made of multi-component glass, and a plastic optical fiber in which the core portion 10 and the clad portion 20 are made of plastic. May be good. The core portion 10 and the clad portion 20 in the optical fiber 1 form an optical waveguide, and in order to propagate light to the core portion 10, a difference in refractive index is provided with each other.

The core portion 10 has a refractive index n ₁ higher than the refractive index of the clad portion 20. That is, the refractive index n ₁ of the core portion 10 is higher than any of the refractive index n ₂ of the base layer 202, the refractive index n ₃ of the trench layer 204, and the refractive index n ₄ of the segment layer 206 in the clad portion 20, which will be described later. ing. The refractive index distribution of the core portion 10 in the radial direction of the optical fiber ₁ is, for example, a constant step shape with a refractive index n1. A dopant that increases the refractive index, such as germanium or phosphorus, is added to the core portion 10 at a predetermined concentration so as to have such _a refractive index n1. The refractive index distribution of the core portion 10 in the radial direction does not necessarily have to be stepped. The refractive index distribution of the core portion 10 in the radial direction gradually becomes the maximum value of the refractive index n ₁ from the refractive index n ₂ of the base layer 202 toward the center of the core portion 10 at the end portion on the outer peripheral side, for example. It may be increased to. In this case, as the refractive index of the core portion 10, representative values such as the maximum value and the average value of the refractive index in the refractive index distribution of the core portion 10 can be used.

Further, the core portion 10 has a radius r ₁ of, for example, 2 μm or more and 10 μm or less.

The clad portion 20 has a base layer 202, a trench layer 204, and a segment layer 206 as layers having different refractive indexes from each other. The base layer 202 includes a first base layer 202a, a second base layer 202b, and a third base layer 202c as layers having the same refractive index.

The first base layer 202a is formed on the outer periphery of the core portion 10. A trench layer 204 is formed on the outer periphery of the first base layer 202a. A second base layer 202b is formed on the outer periphery of the trench layer 204. A segment layer 206 is formed on the outer periphery of the second base layer 202b. A third base layer 202c is formed on the outer periphery of the segment layer 206. The segment layer 206 is formed on the outside of the trench layer 204 inside the outer peripheral edge portion of the clad portion 20.

The cross sections of the first base layer 202a, the trench layer 204, the second base layer 202b, the segment layer 206, and the third base layer 202c along the plane orthogonal to the central axis of the optical fiber 1 are as follows. be. That is, the first base layer 202a has an annular cross section surrounding the core portion 10 and has a cross section concentric with the cross section of the core portion 10. The trench layer 204 is an annular cross section surrounding the first base layer 202a, and has a cross section concentric with the cross section of the core portion 10. The second base layer 202b is an annular cross section surrounding the trench layer 204, and has a cross section concentric with the cross section of the core portion 10. The segment layer 206 is an annular cross section surrounding the second base layer 202b, and has a cross section concentric with the cross section of the core portion 10. The third base layer 202c is an annular cross section surrounding the segment layer 206, and has a cross section concentric with the cross section of the core portion 10. The first base layer 202a, the trench layer 204, the second base layer 202b, the segment layer 206, and the third base layer 202c do not necessarily have to be concentric with each other, and may have different centers from each other.

The base layer 202 has a refractive index n ₂ lower than the refractive index n ₁ of the core portion 10. That is, the first base layer 202a, the second base layer 202b, and the third base layer 202c _each have a refractive index n2. The radial refractive index distribution of the optical fiber 1 in each of the first base layer 202a, the _second base layer 202b, and the third base layer 202c is, for example, constant at the refractive index n2. The first base layer 202a, the _second base layer 202b, and the third base layer 202c are added with a dopant that lowers the refractive index, such as fluorine and boron, at a predetermined concentration so as to have such a refractive index n2. Has been done.

The trench layer 204 has a refractive index n ₃ that is lower than the refractive index n ₂ of the base layer 202. The refractive index distribution of the trench layer 204 in the radial direction of the optical fiber ₁ has a refractive index n3 and is in a constant step shape. A dopant that lowers the refractive index _, such as fluorine and boron, is added to the trench layer 204 at a predetermined concentration higher than that of the base layer 202 so as to have such a refractive index n3. The refractive index distribution of the trench layer 204 in the radial direction does not necessarily have to be stepped. The refractive index distribution of the trench layer 204 in the radial direction becomes, for example, the minimum value of the refractive index n ₃ from the refractive index n ₂ of the base layer 202 toward the center of the trench layer 204 at both ends on the center side and the outer peripheral side. It may be a gradual decrease. In this case, as the refractive index of the trench layer 204, representative values such as the minimum value and the average value of the refractive index in the refractive index distribution of the trench layer 204 can be used.

The trench layer 204 located between the first base layer 202a and the second base layer 202b has, for example, an inner radius r ₂ of 3 μm or more and 20 μm or less, and an outer radius r ₃ of, for example, 4 μm or more and 25 μm or less. ing. Further, the width Δr _tr of the radial trench layer 204 calculated by r ₃ − r ₂ is, for example, 2 μm or more and 15 μm or less.

The segment layer 206 has a refractive index n ₄ which is lower than the refractive index n ₁ of the core portion 10 and higher than the refractive index n ₂ of the base layer 202. The refractive index distribution of the segment layer 206 in the radial direction of the optical fiber ₁ is, for example, a constant step shape with a refractive index n4. A dopant that lowers the refractive index, such as fluorine and boron, is added to the segment layer 206 at a predetermined concentration lower than that of the base layer 202 so as to have such a refractive index _n4 . The refractive index distribution of the segment layer 206 in the radial direction does not necessarily have to be stepped. The refractive index distribution of the segment layer 206 in the radial direction becomes, for example, the maximum value of the refractive index n ₄ from the refractive index n ₂ of the base layer 202 toward the center of the segment layer 206 at both ends on the center side and the outer peripheral side. It may be such that it gradually increases. In this case, as the refractive index of the segment layer 206, representative values such as the maximum value and the average value of the refractive index in the refractive index distribution of the segment layer 206 can be used.

The segment layer 206 located between the second base layer 202b and the third base layer 202c has, for example, an inner radius r ₄ of 5 μm or more and 30 μm or less, and an outer radius r ₅ of, for example, 6 μm or more and 35 μm or less. ing. The width Δr _sg of the radial segment layer 206 calculated by r5 _{- r4} is, for example, 2 μm or more and 15 μm or less.

The third base layer 202c has, for example, an outer radius r6 of 20 μm or more and ₆₀ μm or less. In the optical fiber 1 according to the present embodiment, since the microbend loss can be reduced as described later, the outer diameter of the optical waveguide composed of the core portion 10 and the clad portion 20, that is, 2r ₆ can be reduced. .. For example, 2r ₆ can be 80 μm or less.

The difference Δn _c in the refractive index of the core portion 10 with respect to the base layer 202 is not particularly limited, but is 0.34% or more and 0.39% or less from the viewpoint of confining light in the core portion 10. Is preferable. In addition, Δn _c is a specific refractive index difference defined by the following equation (1).
Δn _c = {(n ₁ − n ₂ ) / n ₁ } × 100 [%] …… (1)

The difference in refractive index Δn ₁ of the trench layer 204 with respect to the base layer 202 is not particularly limited, but is −0.6% or more and −0.02% from the viewpoint of manufacturing and reducing microbend loss. It is preferably less than or equal to, and more preferably −0.4% or more and −0.1% or less. In addition, Δn ₁ is a specific refractive index difference defined by the following equation (2).
Δn ₁ = {(n ₃ -n ₂ ) / n ₃ } × 100 [%] …… (2)

The difference Δn ₂ in the refractive index of the segment layer 206 with respect to the base layer 202 is not particularly limited, but is preferably 0.005% or more and 0.2% or less from the viewpoint of reducing microbend loss. It is more preferably 0.01% or more and 0.12% or less. In addition, Δn ₂ is a specific refractive index difference defined by the following equation (3).
Δn ₂ = {(n ₄ -n ₂ ) / n ₄ } × 100 [%] …… (3)

The covering portion 30 is a layer for protecting the core portion 10 and the clad portion 20, and is made of a resin such as an ultraviolet curable resin. The covering portion 30 may include a plurality of covering layers having different Young's modulus. For example, the covering portion 30 is formed on the outer periphery of the primary coating layer 302 formed on the outer periphery of the clad portion 20 and the primary covering layer 302, and has a Young's modulus higher than the Young's modulus of the primary covering layer 302. It has a next coating layer 304.

In this way, the optical fiber 1 according to the present embodiment is configured.

For example, in a high-density optical fiber cable, minute deformation frequently occurs in the waveguide of the optical fiber. This results in the coupling of the core mode to other modes propagating outside the core in the fiber optic in the fiber optic cable, resulting in a waveguide region from the core to the clad and finally to the glass region and the like. Power leak loss to the outside will occur. As described above, in the optical fiber, microbend loss, which is a loss due to minute deformation of the waveguide, can occur.

In the optical fiber 1 according to the present embodiment, in order to avoid microbend loss, the core has a refractive index distribution that can effectively suppress the power coupling between the core mode and the mode outside the core (for example, the clad mode). It is introduced to the outside of the portion 10. That is, in the optical fiber 1 according to the present embodiment, inside the outer peripheral edge portion of the clad portion 20, it is lower than the refractive index n ₁ of the core portion 10 and below the refractive index n ₂ of the base layer 202 on the outside of the trench layer 204. Also introduced is a segment layer 206 with a high index of refraction n ₄ . The optical fiber 1 according to the present embodiment can reduce the microbend loss due to the refractive index distribution including the segment layer 206 in addition to the trench layer 204. This point will be described in detail below.

The inventors of the present application used the Finite Difference Beam Propagation Method (FDBPM) (Non-Patent Document 7) to evaluate the microbend loss of optical fibers having different refractive index profiles. According to this method, the lateral propagation field flow through the boundary is determined and the loss is evaluated by solving the wave equation by applying the Transparent Boundary Condition (TBC) (Non-Patent Document 8). Can be done.

In FDBPM, the input field of the calculation is the basic core mode held by the linear waveguide. In order to imitate the minute deformation of the optical fiber, a so-called random core displacement function (RCDF) represented by f (z) was applied along the optical fiber (Non-Patent Document 7). The index of refraction profile is laterally rearranged according to the actual f (z) function value at each Δz step while the propagation is calculated by FDBPM. This RCDF can be defined by two parameters, the correlation length L _c and the standard deviation σ of the core displacement. σ indicates the amplitude of fiber displacement, and L _c is related to the number of bends at a particular fiber length. FIG. 2 is a graph showing a standard RCDF. The RCDF shown in FIG. 2 is an RCDF of an optical fiber having a fiber length of 50 mm, σ is 0.01 μm, and L _c is 0.8 mm.

Using the RCDF model above, for example, assuming the same mechanical deformation, investigate the microbend loss of optical fibers with different refractive index profiles, which optical fiber has the smallest microbend under the same RCDF. It is possible to calculate whether it indicates a loss. It is known that the microbend loss also depends largely on the coating (Patent Document 2, Patent Document 4, Patent Document 5). On the other hand, in the present embodiment, an approach is adopted in which the optical cross section of the optical fiber, specifically, the refractive index distribution of the optical fiber is changed to reduce the microbend loss.

The refractive index distribution by simulation was examined by reproducing the structure obtained in the actual optical fabber drawing experiment as much as possible. Specifically, in order to describe the refractive index profile of the optical fiber, Gaussian functions of different orders were applied to construct all layers of the entire optical fiber. The core was well approximated by a 7th-order super Gaussian function. The trench in the following calculation is approximated by a first-order Gaussian function.

First, as an example for optimizing the microbend loss, the microbend loss was calculated for a plurality of trench optical fibers having different trench layer widths. Microbend loss was calculated using FDBPM by adding the losses resulting from the microdeformation of the core position at a given microlength in the length direction.

FIG. 3 is a graph showing the refractive index profile of the trench optical fiber in which the microbend loss is calculated by simulation. In the graph shown in FIG. 3, the horizontal axis indicates the radial position with respect to the central axis of the trench optical fiber, and the vertical axis represents the specific refractive index as the difference in the relative refractive index with respect to the base layer of the clad portion. It shows the difference.

As shown in FIG. 3, the width of the trench layer Δr _tr = r ₃ −r while maintaining the position (r ₃ + r ₂ ) / 2 of the center of the trench layer having the inner radius r ₂ and the outer radius r ₃ at the same position. ₂ was changed and the microbend loss was calculated for each Δr _tr .

FIG. 4 is a graph showing the results of calculating the microbend loss for the trench optical fiber shown in FIG. In the graph shown in FIG. 4, the horizontal axis represents the width Δr _tr of the trench layer, and the vertical axis represents the microbend loss.

As shown in FIG. 4, the wider the width Δr _tr of the trench layer, the lower the microbend loss. The microbend loss decreases exponentially as the width Δr _tr of the trench layer increases. This result is due to the fact that the clad mode or radiation mode is more separated from the core mode and the mode confined by the trench layer. Therefore, the wider the trench layer, the smaller the overlap between the core mode and the clad mode.

In order to further reduce the microbend loss reduced by the trench layer as described above, one or more optical structures can be additionally introduced into the clad portion of the fiber. Intuitively, it is conceivable to add another trench layer to the optical fiber as such an optical structure to increase the suppression of coupling between the basic core mode and the radiated cladding mode. However, the separation of these two modes can be more effective by introducing a resonance structure, that is, a segment layer, which has a photonic bandgap for waves propagating laterally from the core portion. .. In this way, it is conceivable to prohibit the appearance of these lateral waves on the outside of the trench layer, which is an optical structure that separates the core mode and the clad mode.

In order to substantiate the above idea, when a segment layer having a higher refractive index than the base layer is introduced outside the first trench layer, and when a second trench layer is further introduced outside the first trench layer. The microbend loss was calculated for the trench structure. The segment layer or the second trench layer was introduced to the outside of the first trench layer while keeping the width and position in the radial direction constant.

FIG. 5 is a graph showing the refractive index profile by simulation of an optical fiber in the case of introducing a segment layer for which the microbend loss is calculated and in the case of a double trench structure. In the graph shown in FIG. 5, the horizontal axis indicates the radial position with respect to the central axis of the optical fiber, and the vertical axis represents the relative refractive index difference as the relative refractive index difference with respect to the base layer of the clad portion. Is shown. Δn ₂ is the difference in the specific refractive index from the refractive index n ₂ of the base layer having the refractive index n ₄ of the introduced segment layer or the second trench layer {(n ₄ -n ₂ ) / n ₄ } × 100 [%]. Is. When Δn ₂ is a positive value, it means that a segment layer has been introduced. When Δn ₂ has a negative value, it means that a second trench layer has been introduced. When Δn ₂ is 0, neither the segment layer nor the second trench layer is introduced.

FIG. 6 is a graph showing the results of calculating the microbend loss for the case shown in FIG. In the graph shown in FIG. 6, the horizontal axis represents Δn ₂ and the vertical axis represents the microbend loss. Further, the range in which Δn ₂ has a positive value indicates the case where the segment layer is introduced, and the range in which Δn ₂ has a negative value indicates the case of a double trench structure.

As shown in FIG. 6, when the segment layer is introduced, the microbend loss is reduced by one order of magnitude or more, and by two orders of magnitude depending on the value of Δn ₂ , as compared with the case of the single trench structure and the case of the double trench structure. Is done. Specifically, in the range where Δn ₂ is 0.005% or more and 0.2% or less, the microbend loss is at least an order of magnitude smaller than that in the case of the single trench structure having only the first trench layer. In the range where Δn ₂ is 0.01% or more and 0.12% or less, the microbend loss is reduced as compared with the case of the double trench structure. Further, the microbend loss is minimized when Δn ₂ is about 0.035%. When Δn ₂ is about 0.035%, a microbend loss well below 10 ^-4 dB / km is achieved. As described above, it can be seen that the optical fiber in which the segment layer is formed on the outer side of the first trench layer can dramatically reduce the conventional microbend loss by an order of magnitude or more.

Note that FIG. 7 is a graph showing the relationship between Δn ₁ of the trench layer and the normalized microbend loss. As shown in FIG. 7, when the normalized microbend loss is 20 or more, practical inconvenience may occur. Therefore, Δn ₁ is more preferably −0.1% or less. Further, if the absolute value of Δn ₁ becomes excessively large, manufacturing becomes difficult and transmission loss increases. Therefore, it is more preferable that Δn ₁ is −0.4% or more.

Further, FIG. 8 is a graph showing the relationship between Δn _c of the core portion and the normalized microbend loss. As shown in FIG. 8, when the normalized microbend loss is 20 or more, practical inconvenience may occur. Therefore, Δn _c is preferably 0.34% or more. Further, if Δn _c becomes excessively large, manufacturing becomes difficult and transmission loss increases, so that it is preferably 0.39% or less.

In the optical fiber 1 according to the present embodiment, since the segment layer 206 is formed on the outer side of the trench layer 204 as described above, the microbend loss can be sufficiently reduced. As described above, in the present embodiment, since the microbend loss can be sufficiently reduced, the outer diameter of the optical waveguide composed of the core portion 10 and the clad portion 20 can be reduced to, for example, 80 μm or less. On the other hand, if the outer diameter of the optical waveguide is maintained at the same level as that of the single trench optical fiber, the microbend loss can be sufficiently reduced. Therefore, a low-cost coating having low resistance to the microbend loss is applied to the covering portion 30. Can be used as.

The optical fiber 1 according to the present embodiment can be manufactured by various methods. For example, it can be manufactured by the following method. First, a preform used as a base material of an optical fiber 1 is manufactured by a MCVD (Modified Chemical Vapor Deposition) method, an OVD (Outside Vapor Deposition) method, a VAD (Vapor Phase Axial Deposition) method, or the like. In the production of the preform, the flow rate ratio of the raw material gas containing the dopant gas for adjusting the refractive index is appropriately changed. As a result, the refractive index distribution corresponding to the core portion 10 of the optical fiber 1 and the clad portion 20 including the trench layer 204 and the segment layer 206 is formed in the preform. Next, the preform is stretched into a thread shape by a drawing device, coated with a resin such as an ultraviolet curable resin, and wound around a bobbin. In this way, the optical fiber 1 according to the present embodiment can be manufactured.

Further, in the above, the case where the segment layer 206 is formed on the outer side of the single trench layer 204 has been described, but the present invention is not limited to this. The segment layer 206 can be introduced into any trench profile structure. For example, the segment layer 206 can be introduced outside the double trench structure in which the trench layer is further formed outside the trench layer 204 in the clad portion 20.

As described above, according to the present embodiment, in the clad portion 20 formed on the outer periphery of the core portion 10, the segment layer 206 is formed on the outer side of the trench layer 204, so that the microbend loss can be sufficiently reduced. Can be done.

The optical fiber 1 according to the present embodiment can be used to form, for example, an optical fiber cable as an optical transmission medium. The optical fiber cable using the optical fiber 1 according to the present embodiment has a reduced transmission loss including a microbend loss, and can be suitably used for long-distance transmission.

[Example]
In order to clarify the difference between the different refractive index profiles of the optical fiber with respect to the microbend loss, the microbend loss was calculated for the optical fiber having different refractive index profiles from the standard step profile to the refractive index profile according to the above embodiment. The calculation was performed on the optical fibers having a clad diameter of 80 μm in Comparative Examples 1-4 and Example 1 described below.

The optical fiber of Comparative Example 1 is a single-mode optical fiber in which neither a trench layer nor a segment layer is formed.

The optical fiber of Comparative Example 2 is a single-mode optical fiber having a single trench structure in which a trench layer having a width of 0.8 μm in the radial direction is formed. No segment layer is formed in the optical fiber of Comparative Example 2.

The optical fiber of Comparative Example 3 is a single-mode optical fiber having a single trench structure in which a trench layer having a width of 1.2 μm in the radial direction is formed, which is wider than that of Comparative Example 2. No segment layer is formed in the optical fiber of Comparative Example 3.

The optical fiber of Comparative Example 4 is a single-mode optical fiber having a double trench structure in which two trench layers having a width of 0.8 μm in the radial direction are formed. No segment layer is formed in the optical fiber of Comparative Example 4.

In the optical fiber of Example 1, a segment layer having Δn ₁ of −0.2% and a radial width of 2 μm was formed on the outside of a trench layer having a radial width of 1.5 μm and Δn ₂ of 0.04%. It is a single mode optical fiber. The optical fiber of Example 1 is the case where Δn ₂ = 0.04% shown in FIG.

Table 1 shows the results of calculating the microbend loss for the optical fibers of Comparative Example 1-4 and Example 1. Table 1 shows the calculation results and various parameters shown in the lower part of FIG. 1 for each optical fiber. The microbend loss value shown in Table 1 is a relative value when the microbend loss value calculated for the optical fiber of Comparative Example 1 is used as a reference. For the optical fiber of Comparative Example 1, a microbend loss of about 30 dB / km was measured.

As shown in Table 1, the optical fiber of Example 1 in which the segment layer is formed on the outside of the trench layer is higher than the optical fiber of Comparative Example 1 of the standard step refractive index profile which does not include the trench layer in the clad portion. , Almost 6 orders of magnitude lower microbend loss was achieved.

The present invention is not limited to the above embodiment, and can be appropriately modified without departing from the spirit of the present invention.

1 ... Optical fiber 10 ... Core portion 20 ... Clad portion 30 ... Covering portion 202 ... Base layer 202a ... First base layer 202b ... Second base layer 202c ... Third base layer 204 ... Trench layer 206 ... Segment layer 302 … Primary coating layer 304… Secondary coating layer

Claims (5)

The core part having the first refractive index and
It has a clad portion formed on the outer periphery of the core portion, and has a clad portion.
The clad portion is
A base layer having a second refractive index lower than the first refractive index,
A trench layer having a third refractive index lower than the second refractive index,
It has a segment layer formed on the outside of the trench layer inside the outer peripheral edge portion of the clad portion and having a fourth refractive index lower than the first refractive index and higher than the second refractive index. death,
The base layer is
The first base layer and
With the second base layer,
Including the third base layer
The first base layer is formed on the outer periphery of the core portion, and is formed on the outer periphery thereof.
The trench layer is formed on the outer periphery of the first base layer, and is formed on the outer periphery thereof.
The second base layer is formed on the outer periphery of the trench layer, and is formed on the outer periphery of the trench layer.
The segment layer is formed on the second base layer, and the segment layer is formed on the second base layer.
The third base layer is formed on the outer periphery of the segment layer.
The specific refractive index difference of the fourth refractive index from the second refractive index is 0.005% or more and 0.2% or less .
An optical fiber having a specific refractive index difference of the third refractive index from the second refractive index of −0.4% or more and −0.1% or less . The optical fiber according to claim 1, wherein the difference in the specific refractive index of the third refractive index from the second refractive index is −0.25% or more and −0.1% or less. The optical fiber according to claim 1 or 2 , wherein the microbend loss is smaller than that in the case where the segment layer is not formed. The optical fiber according to any one of claims 1 to 3 , wherein the microbend loss [dB / km] is at least 1/10 or less as compared with the case where the segment layer is not formed. The optical fiber according to any one of claims 1 to 4 , wherein the optical fiber is a single mode optical fiber.

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