optical fiber

Abstract

The present invention provides an optical fiber, which includes a core and a cladding surrounding the core. The cladding includes an inner cladding surrounding the core and an outer cladding surrounding the inner cladding. The maximum value Δn1max of the relative refractive index difference of the core, the minimum value Δn2min of the relative refractive index difference of the inner cladding, and the maximum value Δn3max of the relative refractive index difference of the outer cladding satisfy the relationship: Δn2min<Δn3max<Δn1max. In the cladding, the residual stress σ(r) at the radial position satisfies |dσ(r) / dr|≤30MPa / μm.

Description

Technical Field

[0001] The present invention relates to an optical fiber. Background Art

[0002] It is desirable that an optical fiber for transmitting signal light has low transmission loss. In a general optical fiber, a waveguide structure is formed by a core whose refractive index is increased by adding germanium (Ge). However, in such a core, scattering loss is caused due to the presence of Ge, so it is difficult to sufficiently reduce the transmission loss. Here, in order to reduce the scattering loss of the core, a structure is proposed in which quartz glass substantially free of additives is used as the core, and the refractive index of the cladding is reduced by adding fluorine (F). By the way, even in this structure, a sufficient transmission loss reduction effect cannot be obtained. This is because the viscosity difference between the quartz glass core and the cladding containing F is large. As a result, glass defects occur due to the concentration of stress on the core during drawing.

[0003] U.S. Patent Application Publication No. 2016 / 0170137 (Patent Document 1) and U.S. Patent Application Publication No. 2017 / 0219770 (Patent Document 2) disclose methods of reducing transmission loss in an optical fiber having no Ge in the core by reducing the viscosity difference (stress difference) between the core and the cladding adjacent to the core. The optical fiber described in Patent Document 1 includes a core containing chlorine (Cl) but no Ge and a cladding containing F. The optical fiber also includes a stress relaxation layer containing F between the core and the cladding. The optical fiber described in Patent Document 2 includes a core and a cladding whose softening point difference is adjusted to be 50°C or less to match the viscosity. Summary of the invention

[0004] The optical fiber according to an embodiment of the present disclosure includes a core and a cladding surrounding the core. The cladding includes an inner cladding surrounding the core and an outer cladding surrounding the inner cladding. The maximum value Δn1max of the relative refractive index difference of the core, the minimum value Δn2min of the relative refractive index difference of the inner cladding, and the maximum value Δn3max of the relative refractive index difference of the outer cladding satisfy the relationship: Δn2min<Δn3max<Δn1max. In the cladding, the residual stress σ(r) at the radial position r satisfies |dσ(r) / dr|≤30MPa / μm.

[0005] An optical fiber according to another embodiment of the present invention includes a core and a cladding surrounding the core. The cladding includes an inner cladding surrounding the core and an outer cladding surrounding the inner cladding. The maximum value Δn1max of the relative refractive index difference of the core, the minimum value Δn2min of the relative refractive index difference of the inner cladding, and the maximum value Δn3max of the relative refractive index difference of the outer cladding respectively satisfy the relationship: -0.15%<Δn1max<0.05%, -0.5%<Δn2min<-0.2%, and 0.02%<Δn3max-Δn2min<0.2%. The core and the inner cladding contain alkali metal elements or alkaline earth metal elements. The outer cladding does not contain alkali metal elements and alkaline earth metal elements. The absolute value of the derivative of the relative refractive index difference in the inner cladding and the outer cladding with respect to the radial direction is less than 0.2% / μm. In the cladding, the residual stress σ(r) at the radial position r satisfies |dσ(r) / dr|≤30MPa / μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The foregoing and other objects, aspects and advantages will be better understood through the following detailed description of preferred embodiments of the present invention with reference to the accompanying drawings, in which:

[0007] Figure 1 is a cross-sectional view of an optical fiber according to an embodiment;

[0008] Figure 2 is a diagram showing a refractive index profile of an optical fiber according to a first example;

[0009] Figure 3A is a diagram showing residual stress distribution of an optical fiber according to a first example;

[0010] Figure 3B is a graph showing distribution of absolute values ​​of derivatives of residual stress with respect to radial directions of the optical fiber according to the first example;

[0011] Figure 4 is a diagram showing a refractive index profile of an optical fiber according to a second example;

[0012] Figure 5A is a diagram showing residual stress distribution of an optical fiber according to a second example;

[0013] Figure 5B is a graph showing distribution of absolute values ​​of derivatives of residual stress with respect to radial directions of optical fibers according to a second example;

[0014] Figure 6 is a graph showing the relationship between the transmission loss and the absolute value of the maximum derivative of the residual stress;

[0015] Figure 7 is a diagram showing a refractive index profile and a potassium profile of an optical fiber according to a third example and a fourth example;

[0016] Fig. 8A is a diagram showing residual stress distribution of an optical fiber according to a third example;

[0017] Figure 8B is a diagram showing distribution of the absolute value phase of the derivative of the residual stress with respect to the radial direction of the optical fiber according to the third example;

[0018] Fig. 9A is a diagram showing residual stress distribution of an optical fiber according to a fourth example; and

[0019] Fig. 9B : is a graph showing the distribution of the absolute value of the derivative of the residual stress with respect to the radial direction of the optical fiber according to the fourth example. DETAILED DESCRIPTION

[0020] [Problems to be solved by the present invention]

[0021] In order to increase the communication capacity, optical fibers that reduce nonlinearity by increasing the effective area and optical fibers that propagate multiple modes to the core have been proposed. These transmission optical fibers also need to have low bending loss characteristics. In order to improve the bending loss characteristics, optical fibers with a refractive index profile called W-type or trench type are generally used, in which the cladding consists of two or more layers.

[0022] The conventional method of reducing transmission loss by viscosity matching focuses on the difference in viscosity (stress) between the core and the cladding where the optical power is concentrated, but the optical power actually has a wide distribution. In particular, in an optical fiber having a multi-layer cladding, the stress difference in the cladding may be large. In this case, the transmission loss increases due to the irregularity of the structure. Therefore, in such an optical fiber, if the viscosity (stress) difference of the cladding is not taken into account, a sufficient transmission loss reduction effect cannot be obtained.

[0023] An object of the present invention is to provide an optical fiber having low transmission loss while improving bending loss characteristics.

[0024] [Advantageous Effects of the Present Disclosure]

[0025] According to the present disclosure, it is possible to provide an optical fiber having low transmission loss while improving bending loss characteristics.

[0026] [Description of each embodiment of the present disclosure]

[0027] First, various embodiments of the present disclosure will be listed and described. The optical fiber according to the embodiment includes a core and a cladding surrounding the core. The cladding includes an inner cladding surrounding the core and an outer cladding surrounding the inner cladding. The maximum value Δn1max of the relative refractive index difference of the core, the minimum value Δn2min of the relative refractive index difference of the inner cladding, and the maximum value Δn3max of the relative refractive index difference of the outer cladding satisfy the relationship: Δn2min<Δn3max<Δn1max. In the cladding, the residual stress σ(r) at the radial position r satisfies |dσ(r) / dr|≤30MPa / μm.

[0028] In the optical fiber according to the above embodiment, the bending loss characteristics can be improved. In addition, since the residual stress in the cladding changes gently in a manner of |dσ(r) / dr|≤30 MPa / μm, an increase in transmission loss due to glass defects or structural irregularities can be suppressed.

[0029] In the cladding, the residual stress σ(r) may satisfy |dσ(r) / dr|≤20 MPa / μm. In this case, an increase in transmission loss can be further suppressed.

[0030] The effective area of ​​the fundamental mode at 1550nm wavelength can be 100μm 2 In this case, the communication capacity can be increased.

[0031] The optical cable cut-off wavelength can be below 1530nm. In this case, single-mode operation can be performed in the entire C-band.

[0032] Compressive stress may be maintained in the entire region of the core. When tensile stress is maintained in the core, the peripheral glass including the core is stretched. As a result, glass defects increase, thereby increasing transmission loss. In this case, since compressive stress is maintained in the entire region of the core, the increase in transmission loss is suppressed.

[0033] The core may contain an alkali metal element or an alkaline earth metal element. In this case, when an optical fiber is manufactured by drawing an optical fiber preform, the viscosity of the core can be reduced. Therefore, the relaxation of the glass structure is promoted, and the transmission loss can be reduced.

[0034] The transmission loss at a wavelength of 1550 nm may be 0.174 dB / km or less, 0.164 dB / km or less, or 0.154 dB / km or less.

[0035] An optical fiber according to another embodiment includes a core and a cladding surrounding the core. The cladding includes an inner cladding surrounding the core and an outer cladding surrounding the inner cladding. The maximum value Δn1max of the relative refractive index difference of the core, the minimum value Δn2min of the relative refractive index difference of the inner cladding, and the maximum value Δn3max of the relative refractive index difference of the outer cladding respectively satisfy the relationship: -0.15%<Δn1max<0.05%, -0.5%<Δn2min<-0.2%, and 0.02%<Δn3max-Δn2min<0.2%. The core and the inner cladding contain alkali metal elements or alkaline earth metal elements. The outer cladding does not contain alkali metal elements and alkaline earth metal elements. The absolute value of the derivative of the relative refractive index difference in the inner cladding and the outer cladding with respect to the radial direction is less than 0.2% / μm. In the cladding, the residual stress σ(r) at the radial position r satisfies |dσ(r) / dr|≤30MPa / μm.

[0036] In the optical fiber according to another embodiment, the bending loss characteristics can be improved. In addition, since the residual stress in the cladding changes gently in a manner of |dσ(r) / dr|≤30 MPa / μm, an increase in transmission loss due to glass defects or structural irregularities can be suppressed.

[0037] [Detailed description of each embodiment of the present disclosure]

[0038] Detailed examples of the optical fiber of the present disclosure will be described below with reference to the accompanying drawings. In addition, the present invention is not limited to these examples, but is expressed by the claims. It is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same components will be represented by the same reference numerals, and repeated descriptions will be omitted.

[0039] Figure 1 is a cross-sectional view of an optical fiber according to an embodiment. Figure 1 As shown in FIG. 1 , the optical fiber 1 of this embodiment includes a core 10, a cladding 20 surrounding the core 10, and a resin coating surrounding the cladding 20. Figure 1 The resin coating is not shown in the figure. In the optical fiber 1, the cladding 20 has a multilayer structure of two or more layers. In this embodiment, the cladding 20 includes an inner cladding 21 surrounding the core 10 and an outer cladding 22 surrounding the inner cladding 21. Figure 1 The cross-sectional view shows a cross section perpendicular to the axial direction of the optical fiber 1 .

[0040] The core diameter (outer diameter of the core 10) is, for example, 15 μm. The inner cladding diameter (outer diameter of the inner cladding 21) is, for example, 50 μm. The outer cladding diameter (outer diameter of the outer cladding 22) is, for example, 125 μm. In this embodiment, the outer cladding diameter is also the cladding diameter (outer diameter of the cladding 20).

[0041] Compressive stress is maintained in the entire region of the core 10. In the optical fiber 1, when the maximum value of the relative refractive index difference of the core 10 is Δn1max, the minimum value of the relative refractive index difference of the inner cladding 21 is Δn2min, and the maximum value of the relative refractive index difference of the outer cladding 22 is Δn3max, the relationship of Δn2min<Δn3max<Δn1max is satisfied and a W-type refractive index profile is obtained. The relative refractive index difference is based on the refractive index of pure silica glass. In addition, the relationship of -0.15%<Δn1max<0.05%, -0.5%<Δn2min<-0.2%, and 0.02%<Δn3max-Δn2min<0.2% can be satisfied. Therefore, the bending loss characteristics can be improved. The optical fiber 1 can have a "groove type" refractive index profile, and the "groove type" refractive index profile includes a pedestal between the core 10 and the inner cladding 21. The pedestal has a relative refractive index difference substantially equal to the relative refractive index difference of the outer cladding 22. The optical fiber 1 may have a "vacancy-providing" refractive index profile in which the inner cladding 21 includes voids.

[0042] As described by PL Chu and T. Whitbread in "Measurement of stresses in optical fiber and preform" Appl. Opt. 21, 4241, 1982, the phase change of light transmitted through the optical fiber can be used to measure the residual stress of the optical fiber. In particular, this document is incorporated herein by reference. Unless otherwise specified, here, residual stress refers to the stress component acting on the cross section perpendicular to the axis of the optical fiber.

[0043] In the optical fiber 1, the effective area (Aeff) of the fundamental mode at a wavelength of 1550 nm (hereinafter referred to as "effective area") is, for example, 100 μm 2 Above. Therefore, the communication capacity can be increased. The optical cable cutoff wavelength is, for example, below 1530nm. For this reason, single-mode operation can be performed in the entire C-band. The transmission loss at a wavelength of 1550nm can be below 0.174dB / km, or below 0.164dB / km, or below 0.154dB / km.

[0044] The core 10 contains an alkali metal element or an alkaline earth metal element. It can be said that the core 10 contains at least one or more elements selected from the element group including the alkali metal element and the alkaline earth metal element. The core 10 contains, for example, potassium (K). The inner cladding 21 contains an alkali metal element or an alkaline earth metal element. It can be said that the inner cladding 21 contains at least one or more elements selected from the element group including the alkali metal element and the alkaline earth metal element. The inner cladding 21 contains, for example, potassium (K). The outer cladding 22 does not contain an alkali metal element and an alkaline earth metal element.

[0045] The absolute value of the derivative of the relative refractive index difference in the inner cladding 21 and the outer cladding 22 with respect to the radial direction is 0.2% / μm or less. That is, when the relative refractive index difference of the optical fiber 1 is Δn and the radial position is r, the inner cladding 21 and the outer cladding 22 have a relationship of |dΔn / dr|≤0.2% / μm. Therefore, since the viscosity difference between the inner cladding 21 and the outer cladding 22 hardly occurs, a sharp change in residual stress is suppressed. As a result, since glass defects or structural irregularities are suppressed, an increase in transmission loss can be suppressed.

[0046] When the residual stress is σ and the radial position is r, the absolute value of the derivative of the residual stress with respect to the radial direction in the cladding 20, |dσ / dr|, is 30 MPa / μm or less. That is, the relationship of |dσ / dr|≤30 MPa / μm is satisfied in the cladding 20. In this way, since the change (gradient) of the residual stress in the cladding 20 becomes gentle, the increase in transmission loss due to glass defects or structural irregularities can be suppressed. In the cladding 20, |dσ / dr| can be 20 MPa / μm or less. That is, the relationship of |dσ / dr|≤20 MPa / μm can be satisfied in the cladding 20. In this case, since the change (gradient) of the residual stress in the cladding 20 is gentle, the increase in transmission loss can be further suppressed.

[0047] The optical fiber 1 is, for example, a single-mode optical fiber (SMF). The optical fiber 1 is not limited to a single-mode optical fiber, but may be an optical fiber that propagates a plurality of modes as signal light. In a high-order mode, the effective cross-sectional area is large. Therefore, in the transmission loss of the high-order mode, the influence of the transmission loss caused by the residual stress distribution of the cladding 20 tends to increase. As a result, the mode-dependent loss tends to increase. However, according to the optical fiber 1, the mode-dependent loss can be reduced.

[0048] The core 10 may have an annular core shape having a refractive index reduction portion disposed at the center and a ring surrounding the refractive index reduction portion. By adopting the annular core shape, the effective area can be increased while maintaining the mode field diameter (MFD). That is, with a general single-mode optical fiber, nonlinearity can be reduced while suppressing an increase in connection loss. The cladding 20 may have three or more layers, as in a trench-type refractive index profile.

[0049] Hereinafter, various examples will be described. Figure 2 is a diagram showing the refractive index distribution of the optical fiber according to the first example. Figure 2 In FIG. 1 , the horizontal axis represents the radial position r, and the vertical axis represents the relative refractive index difference based on the refractive index of pure silica glass. For reference, Figure 2In the figure, reference numerals "10", "20", "21" and "22" are assigned to positions corresponding to the core, cladding, inner cladding and outer cladding. The optical fiber of the first example has a W-type refractive index distribution. In the optical fiber of the first example, the effective area is 150 μm 2 , and the optical cable cutoff wavelength is below 1420nm. The core contains 100ppm of potassium by mass. The core diameter is 15μm, and the inner cladding diameter is 50μm.

[0050] Figure 3A is a diagram showing the residual stress distribution of the optical fiber according to the first example. Figure 3A , the horizontal axis represents radial position (r), and the vertical axis represents residual stress (σ). Figure 3B : is a graph showing the distribution of the absolute value of the derivative of the residual stress with respect to the radial direction of the optical fiber according to the first example. Figure 3B In , the horizontal axis represents the radial position (r), and the vertical axis represents the absolute value of the derivative of the residual stress with respect to the radial direction (|dσ(r) / dr|). Figure 3A In the equation, the negative sign corresponds to compressive stress and the positive sign corresponds to tensile stress.

[0051] like Figure 3A As shown in FIG. 1 , compressive stress is maintained in the entire region of the core. Compressive stress is maintained in the inner cladding, while tensile stress is maintained in the outer cladding. Figure 3B As shown, the maximum value of |dσ(r) / dr| is 42 MPa / μm.

[0052] In the optical fiber of the first example, a refractive index difference is provided between the inner cladding and the outer cladding by adjusting the fluorine concentration. At the same time, a viscosity difference also occurs in the inner cladding and the outer cladding. Therefore, the residual stress between the inner cladding and the outer cladding (in the interface) changes sharply. The sharp change in residual stress increases the transmission loss caused by glass defects or structural irregularities. The transmission loss of the optical fiber of the first example is 0.156 dB / km.

[0053] Figure 4 is a diagram showing the refractive index distribution of the optical fiber according to the second example. Figure 4 In FIG. 1 , the horizontal axis represents the radial position r, and the vertical axis represents the relative refractive index difference based on the refractive index of pure silica glass. For reference, Figure 4 In the figure, reference numerals "10", "20", "21" and "22" are assigned to positions corresponding to the core, cladding, inner cladding and outer cladding. Figure 4 As shown in FIG. 1 , the optical fiber of the second embodiment is manufactured from a preform whose fluorine concentration profile is adjusted so that the residual stress between the inner cladding and the outer cladding gradually changes. The optical fiber of the second embodiment has a W-type refractive index profile. In the optical fiber of the second embodiment, the effective area is 150 μm2 , and the optical cable cutoff wavelength is below 1420nm. The core contains 100ppm of potassium by mass. The core diameter is 15μm, and the inner cladding diameter is 50μm.

[0054] Figure 5A is a diagram showing the residual stress distribution of the optical fiber according to the second example. Figure 5A In , the horizontal axis represents the radial position (r), and the vertical axis represents the residual stress (σ). Figure 5A In the equation, the negative sign corresponds to compressive stress and the positive sign corresponds to tensile stress. Figure 5B : is a graph showing the distribution of the absolute value of the derivative of the residual stress with respect to the radial direction of the optical fiber according to the second example. Figure 5B , the horizontal axis represents the radial position (r), and the vertical axis represents the absolute value of the derivative of the residual stress with respect to the radial direction (|dσ / dr|).

[0055] like Figure 5A As shown in FIG. 1 , compressive stress is maintained in the entire region of the core. Compressive stress is maintained in the portion of the inner cladding near the core. In the portion of the inner cladding near the outer cladding, the residual stress gradually changes from compressive stress to tensile stress. Tensile stress is maintained in the outer cladding. Figure 5B As shown, the maximum value of |dσ / dr| is 24 MPa / μm. The transmission loss of the optical fiber of the second example is 0.152 dB / km.

[0056] Figure 6 is a graph showing the relationship between the transmission loss and the absolute value of the maximum derivative of the residual stress. Figure 6 In the figure, the horizontal axis represents the maximum value of |dσ / dr| of the cladding, and the vertical axis represents the transmission loss. Figure 6 As shown in FIG. 1 , by setting |dσ / dr| to 30 MPa / μm or less, the transmission loss can be reduced. By setting |dσ / dr| to 20 MPa / μm or less, the transmission loss can be further reduced.

[0057] In the second example, as a method of adjusting the change of residual stress to be gentle, the fluorine concentration in the cladding is adjusted, but the method of adjusting the residual stress is not limited thereto. The concentration distribution of other halogen elements such as chlorine or bromine, germanium, phosphorus (P), alkali metal elements, and alkaline earth metal elements may be adjusted. In addition, for example, by adjusting conditions such as drawing tension or drawing speed to control diffusion, elements such as sodium (Na) or potassium having a high diffusion rate in glass can have a desired residual stress distribution.

[0058] Consider the case where the core portion of an optical fiber preform (glass preform) contains potassium. The diffusion rate of potassium is relatively fast. Therefore, even if potassium is distributed in the central portion of the core portion of the optical fiber preform, potassium diffuses by heating during the drawing process, especially in the drawdown portion (the portion where the outer diameter is reduced). As a result, potassium is distributed in the entire region in a fibrous state.

[0059] For example, by increasing the drawing speed, the residence time of potassium in the drawing portion where diffusion is likely to occur is shortened. Therefore, the diffusion of potassium can be suppressed. In addition, by increasing the tension during the drawing process, the diffusion of potassium can be suppressed. This is because when the temperature required to draw the optical fiber from the optical fiber preform decreases as the tension increases, the influence of thermal diffusion becomes weaker.

[0060] Figure 7 is a diagram showing the refractive index profile and potassium profile of the optical fiber according to the third example and the fourth example. Figure 7 In FIG. 1 , the horizontal axis represents the radial position r, and the vertical axis represents the relative refractive index difference based on the refractive index of pure quartz glass. The vertical axis represents the concentration of potassium (au). For reference, Figure 7 , reference numerals "10", "20", "21", and "22" are assigned to positions corresponding to the core, cladding, inner cladding, and outer cladding. The refractive index profiles of the optical fibers of the third and fourth examples are indicated by solid lines, the potassium profile of the optical fiber of the third example is indicated by dotted lines, and the potassium profile of the optical fiber of the fourth example is indicated by single-point dashed lines. The optical fibers of the third and fourth examples have a W-type refractive index profile. In the optical fibers of the third and fourth examples, the effective area is 150 μm 2 , the optical cable cut-off wavelength is below 1420nm. The core contains 100ppm of potassium by mass. The core diameter is 15μm, and the inner cladding diameter is 50μm.

[0061] The optical fiber according to the third example is manufactured without adjusting the above-mentioned drawing conditions (drawing conditions are not adjusted). The optical fiber according to the fourth embodiment is manufactured by adjusting the above-mentioned drawing conditions (drawing conditions are adjusted). The optical fibers according to the third example and the fourth example have the same refractive index distribution and different potassium distributions. Potassium has the effect of reducing the viscosity of glass. Therefore, by controlling the diffusion of potassium, the residual stress distribution of the cladding can be adjusted so that the potassium distribution range becomes appropriate. Specifically, at the boundary between the potassium-containing region and the potassium-free region, the residual stress is discontinuous. Therefore, the change (gradient) of the residual stress in the entire cladding can be made gentle.

[0062] Fig. 8A is a diagram showing the residual stress distribution of the optical fiber according to the third example. Fig. 8A , the horizontal axis represents radial position (r), and the vertical axis represents residual stress (σ). Figure 8B : is a graph showing the distribution of the absolute value of the derivative of the residual stress with respect to the radial direction of the optical fiber according to the third example. Figure 8B In , the horizontal axis represents the radial position (r), and the vertical axis represents the absolute value of the derivative of the residual stress with respect to the radial direction (|dσ / dr|). Fig. 8A In the equation, the negative sign corresponds to compressive stress and the positive sign corresponds to tensile stress.

[0063] Fig. 9A is a diagram showing the residual stress distribution of the optical fiber according to the fourth example. Fig. 9A , the horizontal axis represents radial position (r), and the vertical axis represents residual stress (σ). Fig. 9B : is a graph showing the distribution of the absolute value of the derivative of the residual stress with respect to the radial direction of the optical fiber according to the fourth example. Fig. 9B In , the horizontal axis represents the radial position (r), and the vertical axis represents the absolute value of the derivative of the residual stress with respect to the radial direction (|dσ / dr|). Fig. 9A In the equation, the negative sign corresponds to compressive stress and the positive sign corresponds to tensile stress.

[0064] like Fig. 8A and Fig. 9A As shown, in any of the optical fibers of the third example and the fourth example, compressive stress is maintained in the entire region of the core. Compressive stress is also maintained in the portion of the inner cladding close to the core. In the portion of the inner cladding close to the outer cladding, the residual stress changes from compressive stress to tensile stress. Tensile stress is maintained in the outer cladding. In the optical fiber of the fourth example, the change in the residual stress in the portion of the inner cladding close to the outer cladding is more gradual than that in the optical fiber of the third example.

[0065] like Figure 8B and Fig. 9B As shown, in the optical fiber of the third example, the maximum value of |dσ / dr| is 45 MPa / μm, but in the optical fiber of the fourth example, the maximum value of |dσ / dr| is 28 MPa / μm. The transmission loss of the optical fiber of the third example is 0.161 dB / km, but the transmission loss of the optical fiber of the fourth example is 0.154 dB / km.

[0066] This application is based upon and claims the benefit of priority from Japanese patent application No. P2019-112800 filed on Jun. 18, 2019, the entire contents of which are incorporated herein by reference.

Claims

1. An optical fiber, comprising: core; as well as a cladding surrounding the core, wherein the cladding comprises an inner cladding surrounding the core and an outer cladding surrounding the inner cladding, The maximum value Δn1max of the relative refractive index difference of the core, the minimum value Δn2min of the relative refractive index difference of the inner cladding, and the maximum value Δn3max of the relative refractive index difference of the outer cladding satisfy the relationship: Δn2min<Δn3max<Δn1max, and In the cladding, a residual stress σ(r) at a radial position r satisfies |dσ(r) / dr|≤30 MPa / μm.

2. The optical fiber according to claim 1, in, In the cladding, the residual stress σ(r) satisfies the formula |dσ(r) / dr|≤20MPa / μm.

3. The optical fiber according to claim 1, in, The effective area of ​​the fundamental mode at 1550nm is 100μm 2 above.

4. The optical fiber according to claim 2, in, The effective area of ​​the fundamental mode at 1550nm is 100μm 2 above.

5. The optical fiber according to claim 3, in, The cut-off wavelength of the optical cable is below 1530nm.

6. The optical fiber according to claim 4, in, The cut-off wavelength of the optical cable is below 1530nm.

7. The optical fiber according to any one of claims 1 to 6, in, The compressive stress is maintained over the entire area of ​​the core.

8. The optical fiber according to any one of claims 1 to 6, in, The core contains an alkali metal element or an alkaline earth metal element.

9. The optical fiber according to any one of claims 1 to 6, in, The transmission loss at a wavelength of 1550nm is less than 0.174dB / km.

10. The optical fiber according to any one of claims 1 to 6, in, The transmission loss at a wavelength of 1550nm is less than 0.164dB / km.

11. The optical fiber according to any one of claims 1 to 6, in, The transmission loss at a wavelength of 1550nm is less than 0.154dB / km.

12. An optical fiber, comprising: core; as well as a cladding surrounding the core, wherein the cladding comprises an inner cladding surrounding the core and an outer cladding surrounding the inner cladding, The maximum value Δn1max of the relative refractive index difference of the core, the minimum value Δn2min of the relative refractive index difference of the inner cladding, and the maximum value Δn3max of the relative refractive index difference of the outer cladding respectively satisfy the relationship: -0.15%<Δn1max<0.05%, -0.5%<Δn2min<-0.2%, and 0.02%<Δn3max-Δn2min<0.2%, The core and the inner cladding contain an alkali metal element or an alkaline earth metal element, The outer cladding does not contain alkali metal elements and alkaline earth metal elements, The absolute value of the derivative of the relative refractive index difference between the inner cladding and the outer cladding with respect to the radial direction is 0.2% / μm or less, and In the cladding, a residual stress σ(r) at a radial position r satisfies |dσ(r) / dr|≤30 MPa / μm.

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