Optical fiber

Abstract

This optical fiber comprises: a core part that contains chlorine and fluorine; and a cladding part that surrounds the outer periphery of the core part, contains fluorine, and has a lower refractive index than the maximum refractive index of the core part. The core part has a first core part, and a second core part that surrounds the outer periphery of the first core part and has a higher fluorine content than the first core part. The refractive index profile of the core part has an upwardly convex curved shape in the first core part and has a downwardly convex curved shape in the second core part.

Description

Optical fiber

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

[0002] In an optical fiber conforming to ITU-T G.652 of the International Telecommunication Union (ITU), germanium (Ge) is generally contained in the core portion. However, Ge increases transmission loss caused by Rayleigh scattering of light. Therefore, from the viewpoint of low loss, a Ge-free configuration that does not contain Ge in the core portion is effective.

[0003] As a Ge-free optical fiber, a configuration in which the core portion is made of pure silica glass and the clad portion is made of fluorine-containing silica glass is known. Pure silica glass is extremely high-purity silica glass that substantially contains no dopant for changing the refractive index, and has a refractive index of about 1.444 at a wavelength of 1550 nm. However, in such a configuration, there may be a large difference in viscosity between the clad portion and the core portion. In this case, when the glass is rapidly cooled in the drawing step for producing the optical fiber, the core portion solidifies first, and then the clad portion solidifies. As a result, tensile stress may remain in the core portion. Such residual stress becomes a factor that hinders the reduction of transmission loss of the optical fiber.

[0004] Therefore, a technique is known in which by adding a dopant to the core portion within a range where the influence of Rayleigh scattering is not large, the fictive temperature of the core portion is lowered, the difference in viscosity between the core portion and the clad portion is reduced, and the transmission loss of the optical fiber is reduced. For example, in Patent Document 1, an alkali metal such as potassium is added to the core portion. Further, in Patent Document 2, chlorine and fluorine, instead of an alkali metal, are co-added to the core portion.

[0005] Japanese National Publication No.2009-517702 Japanese National Publication No.2018-516386

[0006] However, optical fibers with alkali metals added, as described in Patent Document 1, have challenges in terms of manufacturing costs and production technology, such as the difficulty in increasing the size of the optical fiber matrix and mass-producing the optical fibers. On the other hand, the technology of co-adding chlorine and fluorine, as described in Patent Document 2, can reduce manufacturing costs, but there is room for improvement in terms of reducing loss.

[0007] The present invention has been made in view of the above, and its purpose is to provide an optical fiber that is relatively low-cost while having lower transmission loss.

[0008] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention provides an optical fiber comprising a core portion containing chlorine and fluorine, and a cladding portion surrounding the outer periphery of the core portion, containing fluorine and having a refractive index lower than the maximum refractive index of the core portion, wherein the core portion comprises a first core portion and a second core portion surrounding the outer periphery of the first core portion and having a higher fluorine content than the first core portion, and the refractive index profile of the core portion has an upwardly convex curved shape in the first core portion and a downwardly convex curved shape in the second core portion.

[0009] In the optical fiber, if the relative refractive index difference with respect to pure silica glass is set as follows: the maximum relative refractive index difference of the first core portion is Δ1 (%), the relative refractive index difference of the cladding portion is Δ2 (%), the relative refractive index difference at the boundary between the first core portion and the second core portion is Δ3 (%), the relative refractive index difference at the outer circumference of the second core portion is Δ4 (%), the radius of the second core portion is r1 (μm), and the radius of the first core portion is r2 (μm), then the following conditions may hold: Δ1 > Δ3 > Δ4 > 0 > Δ2, 4 / 5 × Δ1 > Δ3 > 2 / 5 × Δ1, and 4.0 ≤ r1 / r2 ≤ 5.0.

[0010] Δ1 may satisfy 0.09 (%) ≤ Δ1 ≤ 0.12 (%), Δ2 may satisfy -0.30 (%) ≤ Δ2 ≤ -0.26 (%), Δ3 may satisfy 0.04 (%) ≤ Δ3 ≤ 0.08 (%), Δ4 may satisfy 0.033 (%) ≤ Δ4 ≤ 0.037 (%), r1 may satisfy 4.0 (μm) ≤ r1 ≤ 4.5 (μm), and r2 may satisfy 0.8 (μm) ≤ r2 ≤ 1.2 (μm).

[0011] The aforementioned Δ3 may satisfy the condition 0.05 (%) ≤ Δ3 ≤ 0.07 (%).

[0012] The above r2 may satisfy 0.9 (μm) ≤ r2 ≤ 1.0 (μm).

[0013] In the core portion, the chlorine concentration is 9,000 ppm or more and 12,000 ppm or less; in the first core portion, the fluorine concentration is less than 1,000 ppm; and in the second core portion, the fluorine concentration may be 1,000 ppm or more and 3,000 ppm or less.

[0014] The present invention offers the advantage of realizing optical fibers with lower transmission loss while maintaining relatively low costs.

[0015] Figure 1 is a schematic cross-sectional view of an optical fiber according to the embodiment. Figure 2 is a schematic diagram of the refractive index profile of the optical fiber according to the embodiment.

[0016] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to the embodiments described below. In each drawing, the same or corresponding components are appropriately denoted by the same reference numerals. Furthermore, in this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength as defined in ITU-T G. 650.1 of the ITU. In addition, for other terms not specifically defined in this specification, the definitions and measurement methods in G. 650.1 and G. 650.2 shall apply.

[0017] (Embodiment) Figure 1 is a schematic cross-sectional view of an optical fiber according to an embodiment. The optical fiber 10 comprises a core portion 1 made of silica glass and a cladding portion 2 made of silica glass having a refractive index lower than the maximum refractive index of the core portion 1, and surrounding the outer circumference of the core portion 1. The core portion 1 has a first core portion 1a and a second core portion 1b surrounding the outer circumference of the first core portion 1a.

[0018] Figure 2 is a schematic diagram of the refractive index profile of an optical fiber according to an embodiment, showing the refractive index profile in the radial direction from the central axis of the optical fiber 10. The refractive index profile is shown as the difference in relative refractive index with respect to pure silica glass.

[0019] In Figure 2, Δ1 is the maximum relative refractive index difference of the first core portion 1a. Δ2 is the relative refractive index difference of the cladding portion 2. Point P is the position of the boundary between the first core portion 1a and the second core portion 1b, and Δ3 is the relative refractive index difference at the boundary between the first core portion 1a and the second core portion 1b. Δ4 is ​​the relative refractive index difference of the outer circumference of the second core portion 1b. r1 is the radius of the second core portion 1b. r2 is the radius of the first core portion 1a.

[0020] As shown in Figure 2, the refractive index profile of the core portion 1 has an upwardly convex curve shape in the first core portion 1a and a downwardly convex curve shape in the second core portion 1b. Such a refractive index profile is achieved, for example, by the first core portion 1a containing chlorine and fluorine, and the second core portion 1b containing chlorine and fluorine, but with a higher fluorine content than the first core portion 1a. The chlorine concentration in the core portion 1 is, for example, almost uniform in the radial direction. Also, for example, at the boundary between the first core portion 1a and the second core portion 1b, the fluorine concentration decreases continuously from the second core portion 1b toward the first core portion 1a. Here, chlorine functions as a dopant that increases the refractive index of the silica glass. Fluorine also functions as a dopant that decreases the refractive index of the silica glass.

[0021] The specific refractive index difference of Δ2 in the cladding portion 2 is achieved by the inclusion of fluorine in the cladding portion 2.

[0022] As described above, the optical fiber 10 is constructed without alkali metals and is therefore relatively low-cost. Furthermore, in the optical fiber 10, the fluorine content is relatively low in the first core portion 1a and higher in the second core portion 1b. As a result, Rayleigh scattering due to fluorine is suppressed in the first core portion 1a, where the power of the transmitted light is higher, while the viscosity difference between the second core portion 1b and the cladding portion 2 is reduced, and the virtual temperature of the second core portion 1b is also reduced. As a result, the optical fiber 10 achieves lower transmission loss. Moreover, the refractive index profile of the core portion 1 has an upward-convex curve shape in the first core portion 1a and a downward-convex curve shape in the second core portion 1b, so the viscosity and specific refractive index difference change smoothly in the radial direction. As a result, for example, tensile stress does not concentrate in the first core portion 1a but is smoothly distributed and relieved towards the second core portion 1b. As a result, the optical fiber 10 achieves lower transmission loss. The transmission loss achieved is, for example, 0.164 dB / km or less at a wavelength of 1550 nm.

[0023] Next, we will illustrate the preferred ranges for the structural parameters Δ1 to Δ4, r1, and r2. First, for Δ1 to Δ4, it is preferable that the following equation holds: Δ1 > Δ3 > Δ4 > 0 > Δ2

[0024] It is preferable that the following equations hold true for Δ1 and Δ3: 4 / 5 × Δ1 > Δ3 > 2 / 5 × Δ1. This suppresses abrupt changes in viscosity and specific refractive index difference from the first core portion 1a to the second core portion 1b.

[0025] Preferably, Δ1 satisfies 0.09 (%) ≤ Δ1 ≤ 0.12 (%). This suppresses residual stress in the first core portion 1a and Rayleigh scattering, thereby achieving low transmission loss. Furthermore, within the above range of Δ1, the chlorine concentration is not excessively high, thus maintaining manufacturing stability.

[0026] It is preferable that Δ2 satisfies the condition -0.30 (%) ≤ Δ2 ≤ -0.26 (%). This makes it easier to satisfy the optical properties defined in ITU-T G. 652B.

[0027] Preferably, Δ3 satisfies the condition 0.04 (%) ≤ Δ3 ≤ 0.08 (%), and more preferably 0.05 (%) ≤ Δ3 ≤ 0.07 (%). This suppresses abrupt changes in viscosity and specific refractive index difference from the first core portion 1a to the second core portion 1b.

[0028] Preferably, Δ4 satisfies the condition 0.033 (%) ≤ Δ4 ≤ 0.037 (%). This improves the viscosity consistency between the second core portion 1b and the cladding portion 2. Furthermore, if the fluorine concentration satisfies the above range of Δ4, there is not an excessive amount of fluorine, so the increase in transmission loss due to Rayleigh scattering in the second core portion 1b can be suppressed.

[0029] It is preferable that r1 satisfies the condition 4.0 (μm) ≤ r1 ≤ 4.5 (μm). This makes it possible to achieve a mode field diameter comparable to that of optical fibers conforming to ITU-T G. 652B.

[0030] It is preferable that r2 satisfies 0.8 (μm) ≤ r2 ≤ 1.2 (μm), and more preferably 0.9 (μm) ≤ r2 ≤ 1.0 (μm). This makes it possible to have a relatively wide region of the first core portion 1a where the effect of Rayleigh scattering is small, while ensuring a sufficient thickness of the second core portion 1b to match the viscosity with the cladding portion 2.

[0031] Furthermore, it is preferable that 4.0 ≤ r1 / r2 ≤ 5.0 holds true. This makes it easier to manufacture using the manufacturing method described later, which is preferable in terms of manufacturability and cost.

[0032] Furthermore, in the optical fiber 10, the chlorine concentration in the core portion 1 is, for example, 9,000 ppm or more and 12,000 ppm or less, the fluorine concentration in the first core portion 1a is less than 1,000 ppm, and the fluorine concentration in the second core portion 1b is 1,000 ppm or more and 3,000 ppm or less.

[0033] Furthermore, the optical properties realized in the optical fiber 10 are, for example, the optical properties defined in ITU-T G. 652B. Therefore, the realized optical properties are, for example, a zero-dispersion wavelength of 1300 nm or more and 1324 nm or less, and a dispersion slope of 0.092 ps / nm at the zero-dispersion wavelength. ２ The frequency is less than or equal to / km, the mode field diameter at a wavelength of 1310 nm is between 8.6 μm and 9.5 μm, or the cutoff wavelength is 1260 nm or less.

[0034] The optical fiber 10 can be manufactured, for example, as follows. First, a core matrix is ​​manufactured using a known VAD (Vapor Axial Deposition) method. Specifically, a core suit is first manufactured using a known VAD apparatus. At this time, the first suit, which will be the basis of the first core portion 1a, is synthesized in the first burner of the VAD apparatus, and the second suit, which will be the basis of the second core portion 1b, is synthesized in the second burner, which is located outside the first burner. For example, the first burner contains SiCl, which is a raw material gas that will be used as a raw material for the first suit. ４ Oxygen gas and hydrogen gas are supplied to the second burner. Additionally, the second burner is supplied with, for example, SiCl, which is the raw material gas for the second suit. ４ and SIF ４ Then, oxygen and hydrogen gases are supplied. As a result, fluorine is added to the second suit.

[0035] Next, the core soot is vitrified in a known vitrification furnace under general conditions and temperature range to form the core base material. ４ By vitrifying the core soot under this atmosphere, a high concentration of chlorine is added to the entire core matrix. Furthermore, the heat generated during vitrification causes the fluorine added to the second soot to diffuse towards the first soot, resulting in a smooth shape in the refractive index profile that gradually decreases radially outward from the center, consisting of upwardly convex and downwardly convex curves.

[0036] Next, a cladding soot, which will form the cladding portion 2, is formed around the outer circumference of the core base material using a known OVD (Outside Vapor Deposition) apparatus, and then the cladding soot is vitrified to manufacture the optical fiber base material. The raw material gas for the cladding soot is, for example, SiCl ４ For example, vitrification is performed using SiF ４ Fluorine is added by performing the process under a specific atmosphere. This results in the formation of a step-type refractive index profile.

[0037] Next, the optical fiber preform is drawn using a known drawing device. This allows the optical fiber 10 to be manufactured.

[0038] Furthermore, there are known VAD devices that synthesize suits in which a portion of the cladding suit, which will form the cladding portion, is formed around a core suit, which will form the core portion. When manufacturing the optical fiber 10, a burner for forming the core suit may be used as the first burner and a burner for forming a portion of the cladding suit may be used as the second burner in such a VAD device. In this case, the core suit of the optical fiber 10 can be easily manufactured using a known VAD device. In particular, if 4.0 ≤ r1 / r2 ≤ 5.0 is satisfied, it is easier to manufacture using a known VAD device, which is preferable in terms of manufacturability and cost.

[0039] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible.

[0040] This invention can be used in optical fibers.

[0041] 1: Core section 1a: First core section 1b: Second core section 2: Cladding section 10: Optical fiber P: Point

Claims

1. An optical fiber comprising: a core portion containing chlorine and fluorine; and a cladding portion surrounding the outer periphery of the core portion, containing fluorine and having a refractive index lower than the maximum refractive index of the core portion, wherein the core portion comprises a first core portion and a second core portion surrounding the outer periphery of the first core portion and having a higher fluorine content than the first core portion, and the refractive index profile of the core portion having an upwardly convex curved shape in the first core portion and a downwardly convex curved shape in the second core portion.

2. The optical fiber according to claim 1, wherein, as the relative refractive index difference with respect to pure silica glass, the maximum relative refractive index difference of the first core portion is Δ1 (%), the relative refractive index difference of the cladding portion is Δ2 (%), the relative refractive index difference at the boundary between the first core portion and the second core portion is Δ3 (%), the relative refractive index difference at the outer circumference of the second core portion is Δ4 (%), the radius of the second core portion is r1 (μm), and the radius of the first core portion is r2 (μm), the following conditions hold: Δ1 > Δ3 > Δ4 > 0 > Δ2, 4 / 5 × Δ1 > Δ3 > 2 / 5 × Δ1, and 4.0 ≤ r1 / r2 ≤ 5.

0.

3. The optical fiber according to claim 2, wherein Δ1 satisfies 0.09 (%) ≤ Δ1 ≤ 0.12 (%), Δ2 satisfies -0.30 (%) ≤ Δ2 ≤ -0.26 (%), Δ3 satisfies 0.04 (%) ≤ Δ3 ≤ 0.08 (%), Δ4 satisfies 0.033 (%) ≤ Δ4 ≤ 0.037 (%), r1 satisfies 4.0 (μm) ≤ r1 ≤ 4.5 (μm), and r2 satisfies 0.8 (μm) ≤ r2 ≤ 1.2 (μm).

4. The optical fiber according to claim 3, wherein Δ3 satisfies 0.05 (%) ≤ Δ3 ≤ 0.07 (%).

5. The optical fiber according to claim 3, wherein r2 satisfies 0.9 (μm) ≤ r2 ≤ 1.0 (μm).

6. The optical fiber according to claim 3, wherein the concentration of chlorine in the core portion is 9,000 ppm or more and 12,000 ppm or less, the concentration of fluorine in the first core portion is less than 1,000 ppm, and the concentration of fluorine in the second core portion is 1,000 ppm or more and 3,000 ppm or less.

Images