阅读说明：本技术 光纤 (Optical fiber ) 是由 田村欣章 川口雄挥 佐久间洋宇 铃木雅人 于 2020-04-06 设计创作，主要内容包括：本发明的光纤在长度方向具有相同的结构。该光纤具有芯部和包层,芯部具有含碱金属元素的石英玻璃；包层具有石英玻璃,并在与长度方向垂直的剖面中包围芯部。包层的折射率比芯部的折射率低。包层在剖面中具有圆环形状的内侧包层和圆环形状的外侧包层,内侧包层包含包层的内周面,外侧包层包含包层的外周面。内侧包层含有氟。内侧包层和外侧包层具有彼此不同的折射率。外侧包层含有残余应力为拉伸应力且大小为极大的极大部。极大部与外侧包层的内周面之间的径向距离为10μm以下。(The optical fibers of the present invention have the same structure in the longitudinal direction. The optical fiber has a core and a cladding, the core having a silica glass containing an alkali metal element; the cladding has quartz glass and surrounds the core in a cross section perpendicular to the length direction. The refractive index of the cladding is lower than that of the core. The cladding has, in cross section, an inner cladding having a ring shape and an outer cladding having a ring shape, the inner cladding including an inner peripheral surface of the cladding, and the outer cladding including an outer peripheral surface of the cladding. The inner cladding contains fluorine. The inner cladding and the outer cladding have refractive indices different from each other. The outer cladding layer includes a very large portion in which the residual stress is tensile stress and the magnitude is extremely large. The radial distance between the maximum portion and the inner peripheral surface of the outer cladding is 10 [ mu ] m or less.)

1. An optical fiber having the same structure in the longitudinal direction,

the optical fiber has a core and a cladding,

the core has a silica glass containing an alkali metal element,

the cladding has quartz glass and surrounds the core in a cross section perpendicular to the length direction,

the cladding has a refractive index lower than that of the core,

the cladding has a donut-shaped inner cladding and a donut-shaped outer cladding in the cross-section,

the inner cladding comprises an inner circumferential surface of the cladding, the outer cladding comprises an outer circumferential surface of the cladding,

the inner cladding layer contains fluorine and is,

the inner cladding and the outer cladding have refractive indices different from each other,

the outer cladding contains a very large portion where the residual stress is tensile stress and the magnitude thereof is extremely large,

the radial distance between the maximum portion and the inner peripheral surface of the outer cladding is 10 [ mu ] m or less.

2. The optical fiber according to claim 1, wherein an increase in transmission loss at a wavelength of 1380nm caused by exposure to an atmosphere containing hydrogen at a partial pressure of 1kPa for 24 hours at a temperature of 80 ℃ is 0.0001dB/km or more and 0.1dB/km or less.

3. The optical fiber according to claim 1 or 2, wherein the extremely large portion has a tensile stress of 5MPa or more and 30MPa or less.

4. The optical fiber according to any of claims 1 to 3, wherein the outer cladding has a first region comprising an inner circumferential surface of the outer cladding,

the first region has a thickness of 10 μm in the radial direction,

the OH concentration of the first region is 5ppm or less.

5. The optical fiber according to any one of claims 1 to 4,

the outer cladding has a second region including an inner peripheral surface of the outer cladding,

the second region has a radial thickness of 0.1 μm or more and less than 3 μm,

the fluorine concentration in the second region is lower than the fluorine concentration in the region of the outer cladding layer other than the second region by 100ppm or more and 10000ppm or less.

6. The optical fiber of any of claims 1 to 5,

the residual stress is expressed as a function of the radial distance from the central axis of the optical fiber, and the value obtained by integrating the residual stress by the radial distance in a section between an upper limit position and a lower limit position sandwiching the maximum portion is 20MPa · μm or more and less than 120MPa · μm, the upper limit position is a position at a distance of 10 μm from the inner peripheral surface of the outer clad, and the lower limit position is a position at which the residual stress equal to the residual stress in the upper limit position is applied.

Technical Field

The present invention relates to optical fibers.

The present application claims priority from japanese application No. 2019-074782 filed on 10/4/2019, and incorporates the entire contents of the disclosure of said japanese application.

Background

An optical fiber for long-distance optical communication is made of silica glass and is manufactured through a drawing process. In the drawing step, a glass base material (preform) as an optical fiber raw material is drawn into a fiber shape while heating so that a drawing tension of 50gf (0.49N) or more is applied to the glass. As an optical fiber for long-distance optical communication in which a reduction in transmission loss is required, an optical fiber in which an alkali metal element is added to a core portion is known (see, for example, patent documents 1 and 2).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2015-157726;

patent document 2: japanese patent laid-open publication No. 2013 and 107792.

Disclosure of Invention

The optical fibers of the present invention have the same structure in the longitudinal direction. The optical fiber has a core portion including silica glass containing an alkali metal element, and a clad portion including the silica glass and surrounding the core portion in a cross section perpendicular to a longitudinal direction. The refractive index of the cladding is lower than that of the core. The cladding has, in cross section, an inner cladding having an annular shape and an outer cladding having an annular shape, the inner cladding including an inner peripheral surface of the cladding, and the outer cladding including an outer peripheral surface of the cladding. The inner cladding contains fluorine. The inner cladding and the outer cladding have refractive indices different from each other. The outer cladding layer includes a very large portion in which the residual stress is tensile stress and the magnitude is extremely large. The radial distance between the maximum portion and the inner peripheral surface of the outer cladding is 10 [ mu ] m or less. In addition, in this specification, unless otherwise specified, it is assumed that the optical fiber is axisymmetric around a central axis extending in the longitudinal direction.

Drawings

Fig. 1 is a cross-sectional view of an optical fiber according to an embodiment.

Fig. 2A is a graph showing the distribution of residual stress in the case where the core portion does not contain an alkali metal element.

Fig. 2B is a graph showing the distribution of residual stress in the case where the core portion contains an alkali metal element.

Fig. 3A is a table showing the relationship of alkali concentration, first residual stress difference, and transmission loss at a wavelength of 1550 nm.

Fig. 3B is a graph showing the relationship between the first residual stress difference and the transmission loss at a wavelength of 1550 nm.

Fig. 4A is a table showing a relationship between the drawing tension and the first residual stress difference.

Fig. 4B is a graph showing a relationship between the drawing tension and the first residual stress difference.

Fig. 5A is a table showing the relationship between the alkali concentration and the maximum drawing tension.

Fig. 5B is a graph showing the relationship between the alkali concentration and the maximum drawing tension.

FIG. 6A is a table showing the relationship between drawing tension and transmission loss at alkali concentrations of 1ppm and 30 ppm.

FIG. 6B is a graph showing the relationship between the drawing tension and the transmission loss at alkali concentrations of 1ppm and 30 ppm.

FIG. 7A is a table showing the relationship between the radial position of the stress applying portion and the transmission loss at a wavelength of 1550 nm.

FIG. 7B is a graph showing the relationship between the radial position of the stress applying portion and the transmission loss at a wavelength of 1550 nm.

Fig. 8A is a table showing a relationship between a peak value of residual stress of the outer clad layer and an increase in hydrogen loss.

Fig. 8B is a graph showing a relationship between a peak value of residual stress of the outer clad layer and an increase in hydrogen loss.

FIG. 9A is a residual stress distribution of an optical fiber in which the OH concentration at the interface between the inner cladding and the outer cladding is 10ppm or more.

FIG. 9B is a residual stress distribution of an optical fiber in which the OH concentration at the interface of the inner cladding and the outer cladding is less than 1 ppm.

Fig. 10A is a table showing the relationship between the OH concentration in the first region and the second residual stress difference.

Fig. 10B is a graph showing a relationship between the OH concentration in the first region and the second residual stress difference.

FIG. 11A is a table showing the relationship between the difference in fluorine concentration, the peak value of residual stress, and the transmission loss at 1550 nm.

Fig. 11B is a graph showing a relationship between a difference in fluorine concentration and a peak value of residual stress.

FIG. 11C is a graph showing the relationship between the difference in fluorine concentration and the transmission loss at 1550 nm.

Fig. 12A is a table showing a relationship between the integrated value of the residual stress and the transmission loss.

Fig. 12B is a graph showing a relationship between the integrated value of the residual stress and the transmission loss.

Detailed Description

Problems to be solved by the invention

In the drawing step, since the viscosity of the optical fiber has unevenness due to composition and temperature distribution in the cross section, a tensile force is more likely to occur in a region having a high viscosity than in a region having a low viscosity in the cross section of the optical fiber. The tensile stress may cause deformation in the molecular structure of the glass, and scattering and glass defect loss due to the deformation may occur. When such scattering and glass defect loss occur in the core or the cladding near the core, the transmission performance of the optical fiber is degraded. One method for suppressing such a decrease in the transmission performance is to add an alkali metal element having a decreased viscosity to the core portion to suppress the occurrence of strain. However, even in an optical fiber in which an alkali metal element is added to the core, a scattering loss due to strain may occur. For example, in the case of a refractive index-increasing GeO by adding it instead to the core₂And adding fluorine (F) for lowering refractive index into the cladding to inhibit GeO addition₂In the optical fiber having the resulting transmission loss, since the viscosity is also reduced by F added to the cladding, tensile strain or strain may occur in the core or the cladding in the vicinity of the core.

Accordingly, an object of the present invention is to provide an optical fiber capable of reducing transmission loss.

Effects of the invention

According to the present invention, an optical fiber capable of reducing transmission loss can be provided.

[ description of embodiments of the invention ]

First, embodiments of the present invention will be described. The optical fibers of the embodiment have the same structure in the longitudinal direction. The optical fiber has a core portion including silica glass containing an alkali metal element, and a clad portion including the silica glass and surrounding the core portion in a cross section perpendicular to a longitudinal direction. The refractive index of the cladding is lower than that of the core. The cladding has, in cross section, an inner cladding having an annular shape and an outer cladding having an annular shape, the inner cladding including an inner peripheral surface of the cladding, and the outer cladding including an outer peripheral surface of the cladding. The inner cladding contains fluorine. The inner cladding and the outer cladding have refractive indices different from each other. The outer cladding layer includes a very large portion in which the residual stress is tensile stress and the magnitude is extremely large. The radial distance between the maximum portion and the inner peripheral surface of the outer cladding is 10 [ mu ] m or less.

In the optical fiber of the above embodiment, glass defects can be actively generated in the maximum portion, and hydrogen from the environment can be reacted with the glass defects. As a result, deterioration of transmission loss due to hydrogen generated by reaction of hydrogen from the environment with defects of the glass is suppressed in the core portion. Therefore, reduction in transmission loss can be achieved. Hereinafter, the deterioration of the transmission loss by hydrogen is simply referred to as "hydrogen deterioration".

The increase of transmission loss at a wavelength of 1380nm by exposure for 24 hours in an environment containing hydrogen at a partial pressure of 1kPa at a temperature of 80 ℃ may be 0.0001dB/km or more and 0.1dB/km or less. In this case, since hydrogen deterioration is suppressed, the transmission loss can be reliably reduced. Further, since the optical fiber can be used even in hydrogen of a higher concentration without deterioration of loss, the optical fiber material is widely selected from the viewpoint of the amount of hydrogen generated. As a result, the cost of the optical cable can be reduced.

The residual stress of the extremely large portion may be a tensile stress of 5MPa or more and 30MPa or less. In this case, the transmission loss can be further reduced.

The outer cladding may have a first region including an inner circumferential surface of the outer cladding. The first region may have a thickness of 10 μm in the radial direction. The OH concentration of the first region may be 5ppm or less. In this case, since the extremely large portion of the tensile force is reliably formed in the first region, the transmission loss can be reliably reduced.

The outer cladding may have a second region including an inner circumferential surface of the outer cladding. The thickness of the second region in the radial direction may be 0.1 μm or more and less than 3 μm. The fluorine concentration in the second region may be lower by 100ppm or more and 10000ppm or less than the fluorine concentration in the region of the outer cladding other than the second region. In this case, an extremely large part of the tensile tension is reliably formed even in the second region or the region including the second region, and therefore the transmission loss can be reliably reduced. In addition, a region of the outer clad layer including the inner peripheral surface of the outer clad layer may be the first region and the second region.

The residual stress is expressed as a function of the radial distance from the central axis of the optical fiber, and the value obtained by integrating the residual stress with the radial distance in the section between the upper limit position and the lower limit position sandwiching the maximum portion may be 20MPa · μm or more and less than 120MPa · μm, the upper limit position being a position spaced apart by 10 μm from the inner peripheral surface of the outer clad, and the lower limit position being a position at which the residual stress equal to the residual stress at the upper limit position is applied. In this case, the transmission loss can be further reduced.

[ details of embodiments of the present invention ]

Specific examples of the optical fiber of the present invention will be described below with reference to the drawings. The present invention is not limited to these examples, but is defined by the scope of the claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of the claims. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

Fig. 1 is a cross-sectional view of an optical fiber according to an embodiment. As shown in fig. 1, an optical fiber 1 of the present embodiment includes a core 10 and a cladding 20. The cross-sectional view of fig. 1 shows a cross-section perpendicular to the central axis (optical axis) of the optical fiber 1. The optical fibers 1 have the same structure in the longitudinal direction. Here, the same structure also includes a structure that differs within a manufacturing error range. That is, the optical fibers 1 have substantially the same structure in the longitudinal direction.

The core 10 contains a quartz glass containing an alkali metal element as a main component (base material). Examples of the alkali metal element include sodium (Na), potassium (K), cesium (Cs), and rubidium (Rb). The core 10 is provided, for example, in a region including the central axis of the optical fiber 1. The core 10 has an outer peripheral surface 10 a. The central axis of the core 10 coincides with the central axis of the optical fiber 1, for example. The outer diameter (core diameter) of the core 10 is, for example, 8 μm or more and 15 μm or less.

The clad layer 20 contains quartz glass as a main component (base material). The clad 20 surrounds the core 10 in a cross section perpendicular to the central axis direction (longitudinal direction) of the optical fiber 1, and covers the outer peripheral surface 10a of the core 10. The cladding 20 has an outer peripheral surface 20a and an inner peripheral surface 20 b. The outer peripheral surface 20a constitutes the outer peripheral surface of the optical fiber 1. The inner peripheral surface 20b is in contact with the outer peripheral surface 10a of the core 10. The outer diameter (cladding diameter) of the cladding 20 is equal to the outer diameter (fiber diameter) of the optical fiber 1, and is, for example, 124 μm or more and 126 μm or less. The length (thickness) of the cladding 20 in the radial direction is, for example, 55 μm or more and 59 μm or less. The outer peripheral surface of the optical fiber 1 may be coated with an ultraviolet curable resin, and in the present specification and the drawings, the coating is omitted as long as it is not particularly described. As is well known to those skilled in the art, the coating can prevent damage to the outer peripheral surface of the optical fiber 1 and optimize the rigidity of the optical fiber 1.

The cladding 20 has an inner cladding 21 and an outer cladding 22. In a cross section perpendicular to the central axis direction (longitudinal direction) of the optical fiber 1, both the inner cladding 21 and the outer cladding 22 have a circular ring shape. The inner cladding 21 and the outer cladding 22 have refractive indices different from each other. As described below, although the inner cladding 21 and the outer cladding 22 are in contact with each other in the present embodiment, the cladding 20 may have a cladding other than the inner cladding 21 and the outer cladding 22 between the inner cladding 21 and the outer cladding 22.

The inner cladding 21 surrounds the core 10 in a cross section perpendicular to the central axis direction (longitudinal direction) of the optical fiber 1, and covers the outer peripheral surface 10 a. The inner cladding 21 has an outer peripheral surface 21a and an inner peripheral surface 21 b. The outer peripheral surface 21a is in contact with the outer cladding 22. The inner peripheral surface 21b of the inner cladding 21 constitutes an inner peripheral surface 20b of the cladding 20. That is, the inner cladding 21 includes the inner peripheral surface 20 b. The inner cladding 21 is an innermost cladding. The outer diameter of the inner cladding 21 is, for example, 20 μm or more and 70 μm or less. The thickness of the inner cladding 21 in the radial direction is, for example, 5 μm or more and 30 μm or less. In one example, the inner cladding 21 has an outer diameter of 35 μm and a radial thickness of 12.5 μm.

The outer clad 22 surrounds the inner clad 21 in a cross section perpendicular to the central axis direction (longitudinal direction) of the optical fiber 1, and covers the outer peripheral surface 21a of the inner clad 21. The outer cladding 22 has an outer peripheral surface 22a and an inner peripheral surface 22 b. The outer peripheral surface 22a constitutes the outer peripheral surface 20a of the cladding 20. That is, the outer cladding 22 includes an outer peripheral surface 20 a. The outer cladding 22 is an outermost cladding. The inner peripheral surface 22b of the outer cladding 22 is in contact with the outer peripheral surface 21a of the inner cladding 21. The outer diameter of the outer cladding 22 is equal to the outer diameter of the cladding 20. The thickness of the outer cladding 22 in the radial direction is, for example, 27 μm or more and 53 μm or less. In one example, the outer diameter of inner cladding 21 is 35 μm and the thickness of outer cladding 22 is 45 μm.

The outer cladding 22 has a first region including an inner peripheral surface 22 b. The first region surrounds the inner cladding 21. The first region has a thickness of 10 μm in the radial direction. The residual stress of the first region is tensile stress. In the present specification, unless otherwise specified, the residual stress refers to a component in the central axis direction of stress acting on a cross section perpendicular to the central axis direction (fiber drawing direction) of the residual stress after the fiber after drawing is cooled to room temperature. The residual stress is represented by a positive sign in the case of tension and by a negative sign in the case of compression. The residual stress is a function of radial position, and unless otherwise specified, the value of the residual stress is defined as the value averaged over a region of 1 μm in diameter, 1 μm being a typical measurement resolution of the residual stress. Further, unless otherwise specified, when the direction of the residual stress is tensile, the absolute value thereof is referred to as tensile stress. The residual stress of the outer cladding 22 is maximum in the first region. That is, the outer clad layer 22 includes the extremely large portion 30 in which the residual stress is tensile stress and the magnitude thereof is extremely large in the first region. The radial distance between the extremely large portion 30 and the inner peripheral surface 22b is 10 μm or less.

The residual stress of the extremely large portion 30 is, for example, a tensile stress of 5MPa to 30 MPa. As described later, the residual stress of the maximum portion 30 is set to 30MPa or less, whereby deterioration of the transmission loss of the optical fiber 1 can be suppressed. Further, as described later, by setting the residual stress of the extremely large portion 30 to 5MPa or more, hydrogen deterioration of the core portion 10 can be suppressed. As a result of the inhibition of hydrogen deterioration, the increase of transmission loss at a wavelength of 1380nm caused by the exposure of the optical fiber 1 to an atmosphere containing hydrogen at a temperature of 80 ℃ and a partial pressure of 1kPa for 24 hours was 0.1dB/km or less. Although an increase in transmission loss of 0.0001dB/km or more may occur, it does not pose a hindrance in most applications.

It is known that hydrogen degradation is a factor that deteriorates the transmission loss of an optical fiber. Hydrogen degradation is produced by the reaction of hydrogen molecules from the environment with glass defects. The hydrogen molecules from the environment are hydrogen from the environment outside the glass such as resin. Glass defects are caused by tensile stress on the glass, with the result that bonds of the glass molecules are broken. It is well known that glass defects, particularly the transmission loss of optical fibers, are the main cause of deterioration with time. The peak of the transmission loss at the wavelength 1380nm due to the absorption of OH groups generated by the defects of the glass increases with time.

In the core 10, the transmission loss due to the glass defect may increase due to the tensile stress. Therefore, the optical fiber 1 is provided with the maximum portion 30 as a stress applying portion for applying tensile stress, and glass defects are actively generated in the maximum portion 30. Thus, the reaction of hydrogen from the environment with glass defects increases in the maxima 30. The outer clad layer 22 provided with the extremely large portion 30 is considered to function as a barrier layer that blocks hydrogen deterioration of the core 10, and hydrogen deterioration of the core 10 can be suppressed. The maximum portion 30 is provided at a position sufficiently far from the core 10 to the extent that the signal light is not affected even when the signal light is diffused, for example, at a position having a radial distance of 30 μm or more from the center of the core 10. Further, in order to avoid an excessive thickening of the outer diameter of the optical fiber 1, the maximum-diameter portion 30 is provided at a position having a radial distance of 60 μm or less from the outer peripheral surface 10a of the core portion 10.

In general, in order to confine light to propagate in the core, the refractive index of the cladding needs to be lower than that of the core. For this reason, a structure in which a dopant for increasing the refractive index, such as germanium (Ge), is contained in the core, and a structure in which a dopant for decreasing the refractive index, such as fluorine (F), is contained in the cladding, are considered. The latter structure can also be applied to, for example, a pure silica core optical fiber having no dopant in the core. When the refractive index of the pure quartz core is set to n_oThe refractive index of the cladding is n_iThe relative refractive index difference is represented by formula (1).

[ formula 1]

Δ％＝(n_i ²-n₀ ²)/2n₀ ²×100 (1)

The optical fiber 1 of the present embodiment has the latter structure, and contains F in an amount of 1000ppm to 100000ppm in the entire cladding 20. Thus, the refractive index of the cladding 20 is lower than that of the core 10. Of the cladding layers 20, at least the inner cladding layer 21 contains F. The F concentration of cladding 20 is highest in inner cladding 21. In the present invention, the F concentration is expressed as the mass fraction of F, that is, the ratio of the mass of F to the mass of the whole. The relative refractive index difference between the core 10 and the inner cladding 21 is 0.2% or more. The refractive index of the core 10 is equal to that of the pure quartz core.

The OH concentration in the first region is 5ppm or less. In the present invention, the OH concentration is expressed as the mass fraction of OH, that is, the ratio of the mass of OH to the mass of the whole. This enables the extremely large portion 30 to be formed in the first region as described later. The OH concentration of the first region is, for example, the average OH concentration of the first region.

The outer cladding 22 has a second region including an inner peripheral surface 22 b. The second region has a radial thickness of 0.1 μm or more and less than 3 μm. The F concentration in the second region is lower than the F concentration in the region of the outer cladding 22 other than the second region by 100ppm to 10000 ppm. This makes it possible to form the maximum portion 30 that can suppress the transmission loss of the optical fiber 1, as will be described later. The F concentration in the second region is, for example, the minimum value (lowest value) of the F concentration in the second region. The F concentration in the region of the outer clad layer 22 other than the second region is, for example, the minimum value (lowest value) of the F concentration in the region of the outer clad layer 22 other than the second region.

In a section between the upper limit position and the lower limit position sandwiching the extremely large section 30, a value obtained by integrating the residual stress expressed by a function of the radial distance from the central axis of the optical fiber 1 by the radial distance is 20MPa · μm or more and less than 120MPa · μm. When the radial distance from the central axis of the optical fiber 1 is defined as r, the residual stress is defined as p (r), and the integration section is defined as radial distances r1 to r2, the integrated value is expressed by the following formula (2). r1 is the lower limit of the radial distance, and r2 is the upper limit of the radial distance.

[ formula 2]

Here, the upper limit position of the integration section coincides with the outer edge of the first region, and the radial distance between the upper limit position of the integration section and the inner peripheral surface 22b is 10 μm. Further, the residual stresses in the lower limit position and the upper limit position of the integration section are equal to each other. That is, P (r1) ═ P (r 2). As will be described later, the maximum amplitude portion 30 capable of suppressing the transmission loss of the optical fiber 1 can be formed by setting the integrated value in the integration section in such a range to be 20MPa · μm or more and less than 120MPa · μm.

Fig. 2A is a graph showing the distribution of residual stress in the case where the core portion does not contain an alkali metal element. Fig. 2B is a graph showing the distribution of residual stress in the case where the core portion contains an alkali metal element. In fig. 2A and 2B, for the residual stress, the tensile stress is represented by + and the compressive stress is represented by-. In fig. 2A and 2B, a portion corresponding to the core is denoted by "a", a portion corresponding to the inner cladding is denoted by "B", a portion corresponding to the outer cladding is denoted by "C", portions corresponding to the first region are denoted by "C1" and "C2", and the first residual stress difference is denoted by "Δ σ". The first residual stress difference refers to a difference between the residual stress of the core and the residual stress of the inner cladding (a value obtained by subtracting the residual stress of the inner cladding from the residual stress of the core).

From the graphs shown in fig. 2A and 2B, it is expected that by making the core portion contain an alkali metal element, the viscosity of the core portion decreases, the stress of the core portion becomes compressive stress, and as a result, deformation caused by tensile stress is less likely to occur in the core portion. The first residual stress difference is reduced by making the core portion contain an alkali metal element. However, as shown in fig. 2B, even in the case where the viscosity of the core is reduced, the lowest residual stress is not the core but the inner cladding outside the core. Therefore, it is found that the residual stress of the core is in a range not becoming tensile stress, but sometimes protrudes toward the tensile stress side when compared with the residual stress of the inner clad.

The reason why the residual stress of the core portion protrudes to the tensile stress side can be considered as follows. That is, the alkali metal element diffuses into the cladding portion depending on the production conditions such as the addition concentration of the alkali metal element and the drawing speed. Thus, the inner cladding layer contains both the alkali metal element and F at the maximum concentration. The lowest viscosity is in such an inner cladding. It is considered that since the core is relatively hard, the fiber may be subjected to tensile stress during cooling in the drawing step.

Next, the first residual stress difference was changed by changing the alkali concentration, and the relationship between the first residual stress difference and the transmission loss was examined. The alkali concentration means the average alkali metal element concentration in the core portion. In the present invention, the alkali metal element concentration is expressed as a mass fraction of the alkali metal element, that is, a ratio of the mass of the alkali metal element to the mass of the whole. Fig. 3A is a table showing the relationship of alkali concentration, first residual stress difference, and transmission loss at a wavelength of 1550 nm. Fig. 3B is a graph showing the relationship between the first residual stress difference and the transmission loss at a wavelength of 1550 nm. Here, the diameter (added diameter) of the region in which the alkali metal element is present in the core portion at the base material stage before drawing is set to 20% of the core portion diameter.

The synthesis of the core portion to which the alkali metal element is added can be performed by a diffusion method with reference to the method of patent document 1, for example. The diffusion method is a method of diffusing and adding an alkali metal element into glass by heating a glass tube prepared in advance from the outside while supplying an alkali vapor. The adjustment of the addition diameter can be performed by, for example, applying glass containing no alkali metal element to be the second core portion on the outside of the core portion by using a bushing hot-melt technique (コラプス method). The adjustment of the addition diameter can also be performed by cutting the core portion to which the alkali metal element is added.

Here, K is added as an alkali metal element. In the case of adding an alkali metal element other than K, the first residual stress difference can be made to be the same degree as in the case of adding K by changing the addition diameter in accordance with the diffusion rate of the alkali metal element. For example, when Rb is added as an alkali metal element with an addition diameter of 80% to produce an optical fiber, the first residual stress difference is 15MPa, and the transmission loss is also the same as in the case of K. It can be considered that there is a correlation between the first residual stress difference and the transmission loss regardless of the additive element.

As can be seen from the tables and graphs shown in fig. 3A and 3B, in the case where the first residual stress difference is greater than 15MPa, the transmission loss is deteriorated. It is difficult to measure the first residual stress difference of less than 5Mpa because of insufficient measurement accuracy. However, the smaller the first residual stress difference is, the smaller the difference between the viscosity of the core and the viscosity of the inner cladding is, and therefore it is considered that the transmission loss can be reduced.

As a method of preventing the tensile stress from being applied to the core portion as described above, for example, a method of suppressing the drawing tension to be low may be considered. Fig. 4A is a table showing a relationship between the drawing tension and the first residual stress difference. The drawing tension is a tension applied to the glass portion at the time of drawing. Fig. 4B is a graph showing the relationship thereof. Here, a base material having an alkali concentration of 1ppm was used.

As is clear from the tables and graphs shown in fig. 4A and 4B, the first residual stress difference can be made 15MPa or less by setting the drawing tension to 100g or less. It is considered that when the alkali concentration is high, the first residual stress difference can be realized to be 15MPa or less in a wider range of drawing tension. For example, if the alkali concentration is 30ppm, the first residual stress difference can be made 15MPa or less even if the drawing tension is 150g, and if the alkali concentration is 15ppm, the first residual stress difference can be made 15MPa or less even if the drawing tension is 130 g. In this manner, the alkali concentration and the maximum value of the drawing tension (maximum drawing tension) at which the first residual stress difference is 15MPa or less are positively correlated with each other.

Fig. 5A is a table showing the relationship between the alkali concentration and the maximum drawing tension (i.e., the alkali concentration dependence of the maximum drawing tension). Fig. 5B is a graph showing the relationship thereof. From the tables and graphs shown in fig. 5A and 5B, it is presumed that if the drawing tension is 100g or less, the first residual stress difference of 15MPa or less can be achieved regardless of the alkali concentration.

FIG. 6A is a table showing the relationship between drawing tension and transmission loss at alkali concentrations of 1ppm and 30 ppm. Fig. 6B is a graph showing the relationship thereof. As shown in fig. 6A and 6B, when the drawing tension is less than 20g or less, the transmission loss rapidly increases regardless of the alkali concentration. When the drawing tension is too low, the fiber diameter is unstable due to fiber vibration in the drawing step. It can be presumed that the transmission loss deteriorates as a result. Therefore, the drawing tension of the optical fiber having the alkali concentration of 1ppm or more can be set to 20g or more and 100g or less regardless of the alkali concentration.

However, when the drawing step is performed at a low drawing tension, the heating temperature of the base material needs to be increased or the drawing speed needs to be decreased. Therefore, production efficiency may be reduced. In the optical fiber 1 of the present embodiment, the cladding 20 is formed of a plurality of claddings, and the outer cladding 22, which is the outermost cladding, functions as a stress applying layer to which tensile stress is applied. For example, by reducing the F concentration of the outer clad layer 22, the viscosity of the outer clad layer 22 can be increased to form a stress applying layer. Further, the outer clad layer 22 has an interface with different viscosity, and thereby diffusion of the alkali metal element can be suppressed (refer to patent document 2).

When the residual stress of the outer clad layer is uniform (flat) over the entire outer clad layer (for example, when the difference between the maximum value and the minimum value of the residual stress in the outer clad layer is 5MPa or less), the occurrence of strain in the core can be suppressed. However, it is impossible to form a barrier layer for blocking a glass defect loss described later. Therefore, transmission loss from glass defects increases.

For example, when the outer cladding is formed of pure quartz containing no F, the residual stress becomes tensile stress in the entire outer cladding. Therefore, the tensile force is dispersed throughout the outer clad, and the tensile stress per unit cross-sectional area is reduced. Therefore, in the outer clad layer, the residual stress becomes low tensile stress, and a peak (maximum) of the residual stress is not formed, and the effect of the barrier layer is not obtained. Therefore, it is considered necessary to provide a stress peak by adding some additives to the outer clad layer to impart a difference in viscosity to the inside of the layer.

Examples of the additive include chlorine (Cl) and F. Since Cl increases the refractive index of the cladding, the refractive index of the cladding is higher than that of the core, and light may leak into the cladding. When the amount of Cl added (i.e., the Cl concentration of the clad) exceeds 5000ppm, the outer clad becomes less viscous, and the core is subjected to tensile stress. Therefore, the amount of Cl added can be set to 5000ppm or less. In the present invention, the Cl concentration is expressed as a mass fraction of Cl, that is, a ratio of the mass of Cl to the mass of the whole. The amount of Cl added may be 3000ppm or less. In this case, the stress applying portion to which a higher tensile stress is applied can be formed in the outer cladding layer. However, as described later, by providing a concentration difference of 100ppm or more in the outer clad layer, a stress applying portion capable of reducing transmission loss can be formed.

In order to reduce the transmission loss, a heating step may be performed, in which the fiber drawn out from the drawing furnace in the drawing step is reheated in a heating furnace (see japanese patent application laid-open No. 2014-114195). According to such a heating step, particularly in the case where the stress applying portion is formed only on the outer side of the radial middle of the outer clad layer, the stress applying portion may be remelted and apply tensile stress to the core portion. Therefore, the stress applying portion can be formed at a position further inward than the radial middle of the outer clad layer.

FIG. 7A is a table showing the relationship between the radial position of the stress applying portion and the transmission loss at a wavelength of 1550 nm. Fig. 7B is a graph showing the relationship thereof. The radial position is determined according to the radial distance between the stress applying portion and the inner peripheral surface of the outer cladding. The position of the stress applying portion is, for example, a position where the residual stress is tensile stress and the magnitude thereof is extremely large. Here, the thickness of the outer clad layer in the radial direction (the thickness of the outer clad layer) was 50 μm.

As shown in fig. 7A and 7B, when the radial distance between the stress imparting portion and the inner peripheral surface of the outer clad exceeds 10 μm, the transmission loss deteriorates as the radial distance increases. In particular, when the stress applying portion is provided within a range of 10 μm or less from the outer peripheral surface of the outer clad in the radial direction (a range in which the radial distance between the stress applying portion and the inner peripheral surface of the outer clad is 40 μm or more and 50 μm or less), the transmission loss is rapidly deteriorated. This is presumably because the stress applying portion is melted again in the heating step, and the residual stress of the stress applying portion is released, thereby causing tensile deformation in the core portion.

The thickness of the outer cladding is a value optimized according to the thickness and refractive index of the inner cladding, and is not limited to 50 μm. However, the thicker the thickness of the outer cladding, the wider the region where the F concentration as an additive is low, and therefore, fibers can be produced at low cost. On the other hand, when the thickness of the outer clad exceeds 60 μm, the stress applying portion approaches the core when the outer diameter of the clad is fixed to 125 μm, and thus a part of the spread signal light propagating through the core reaches the stress applying portion. Therefore, the transmission loss may be deteriorated by the influence of the glass defect loss generated in the stress applying portion. When the thickness of the outer clad layer is less than 10 μm, the stress applying portion is melted again by the heating step. Therefore, the thickness of the outer cladding layer needs to be 10 μm or more.

In the case of conducting the same study with the thickness of the outer clad layer set to 60 μm, the transmission loss did not deteriorate until the radial distance between the stress applying portion and the inner peripheral surface of the outer clad layer was 15 μm. From this, it is presumed that, when the radial distance is less than 10 μm, deterioration of the transmission loss can be suppressed regardless of the thickness of the outer clad.

Fig. 8A is a table showing a relationship between a peak value of residual stress of the outer clad layer and an increase in hydrogen loss. Fig. 8B is a graph showing the relationship thereof. The peak value (maximum value) of the residual stress of the outer clad layer is the residual stress of the stress applying portion. The increase in hydrogen loss is a change in transmission loss (absorption loss) at a wavelength of 1380nm before and after the hydrogen deterioration test. In the hydrogen deterioration test, the optical fiber was exposed to an atmosphere containing hydrogen gas at a partial pressure of 1kPa at a temperature of 80 ℃ for 24 hours. Here, the outer cladding starts at a radial distance of 30 μm from the core. The stress applying portion is provided at a position having a radial distance of 10 [ mu ] m or less from the inner peripheral surface of the outer clad.

As shown in fig. 8A and 8B, when the peak value (maximum value) is 0MPa or more, hydrogen degradation is greatly suppressed. Further, when the peak value is 5MPa or more, hydrogen deterioration is reduced to a level that cannot be measured. On the other hand, when the peak exceeds 30MPa, the transmission loss deteriorates. This is presumably because the tensile stress is too strong, and the amount of glass defects generated becomes too large. That is, it is presumed that even when the radial distance from the core of the outer cladding is 30 μm, the signal light which has diffused only a little is affected by the defect loss to deteriorate the transmission loss.

As a method of forming a clad layer composed of a plurality of different refractive indices, there is a method of spraying glass fine particles onto a glass rod including an inner clad layer of a core to form a loose body and then sintering the loose body. However, in this method, after the base material is synthesized, moisture is added by a burner flame used when forming a loose body on the surface of the glass rod. This moisture also diffuses in the direction of the outer clad layer in the drawing step, and therefore the OH concentration in the vicinity of the interface between the inner clad layer and the outer clad layer may increase. When such a glass base material is drawn, the viscosity near the interface is locally reduced. Therefore, in the outer clad layer, the residual stress on the inner peripheral surface side is reduced.

FIG. 9A is a residual stress distribution of an optical fiber in which the OH concentration at the interface between the inner cladding and the outer cladding is 10ppm or more. FIG. 9B is a residual stress distribution of an optical fiber in which the OH concentration at the interface between the inner cladding and the outer cladding is 1ppm or less. In fig. 9A and 9B, for the residual stress, tensile stress is represented by + and compressive stress is represented by-and the position of the interface is represented by a dotted line.

In the optical fiber of fig. 9B, the cladding is not formed by the deposition method, but the optical fiber is obtained by synthesizing an optical fiber base material by applying an outer cladding to which F is previously added by the jacket hot-melt technique (コラプス method), and drawing the optical fiber base material. In the process of synthesizing an optical fiber base material by the jacket tube hot-melt technique (コラプス method), first, a glass tube to which F is added in advance is prepared, and a glass rod including an inner cladding of a core is inserted into the glass tube. Then, the glass tube and the glass rod are integrated by hot-melting (コラプス) the jacket tube while vacuum-evacuating, and an optical fiber base material containing K in the core portion is synthesized. Instead of synthesizing an optical fiber base material by the jacket tube hot-melt technique (コラプス method), a glass rod including an inner cladding of a core may be inserted into a glass tube and integrated and drawn.

In the optical fiber of fig. 9A, the residual stress decreases near the interface, and the residual stress of the outer cladding decreases monotonically toward the inner cladding. On the other hand, in the optical fiber of fig. 9B, the stress applying portion is formed in the outer cladding at a position within a radial distance of 10 μm from the inner cladding.

Fig. 10A is a table showing the relationship between the OH concentration in the first region and the second residual stress difference. Fig. 10B is a graph showing the relationship. The second residual stress difference is a difference between the residual stress in the first region and the residual stress in the central portion of the outer clad layer in the thickness direction (a value obtained by subtracting the residual stress in the central portion of the outer clad layer in the thickness direction from the residual stress in the first region). Here, the thickness of the outer cladding layer was 60 μm. The OH concentration was determined as an average value of points having a diameter of about 10 μm by a microscopic infrared spectroscopy method.

As shown in fig. 10A and 10B, in the case where the OH concentration of the first region is higher than 10ppm, the second residual stress difference is a negative value. That is, the residual stress in the first region is smaller than the residual stress in the central portion of the outer clad layer in the thickness direction, and the stress applying portion cannot be formed in the first region. On the other hand, when the OH concentration in the first region is 1ppm or less, the second residual stress difference is fixed to 5MPa without being affected by the OH concentration. That is, the residual stress in the first region is 5MPa greater than the residual stress in the central portion in the thickness direction of the outer clad layer. When the OH concentration in the first region is 5ppm or less, the second residual stress difference is 2MPa or more. That is, the residual stress in the first region is greater than the residual stress in the central portion in the thickness direction of the outer clad layer by 2MPa or more.

FIG. 11A is a table showing the relationship between the difference in F concentration, the peak value of residual stress, and the transmission loss at 1550 nm. Fig. 11B is a graph showing a relationship between the difference in F concentration and the peak value of residual stress. FIG. 11C is a graph showing the relationship between the difference in F concentration and the transmission loss at 1550 nm. The difference in F concentration is a difference between the minimum value of F concentration in a region smaller than 3 μm from the inner peripheral surface of the outer clad in the radial direction and the F concentration at a position 3 μm from the inner peripheral surface of the outer clad in the radial direction (a value obtained by subtracting the minimum value of F concentration in a region smaller than 3 μm from the inner peripheral surface of the outer clad in the radial direction from the F concentration at a position 3 μm from the inner peripheral surface of the outer clad in the radial direction). The peak value of the residual stress refers to a peak value of the residual stress of the first region. The OH concentration at the interface between the inner cladding and the outer cladding is 1ppm or less.

As shown in fig. 11A to 11C, when the difference in F concentration is 100ppm or more, the peak value of the residual stress is 5MPa or more, and the transmission loss is also low. On the other hand, when the difference in F concentration is 3000ppm or more, the peak value of the residual stress is 30MPa or more, and the transmission loss rapidly deteriorates.

F is a dopant which reduces the viscosity of the glass. Therefore, when the F concentration of the cladding has a distribution that monotonically increases toward the central axis of the optical fiber, the stress applying portion cannot be formed in the first region of the outer cladding. Forming the stress applying portion in the first region requires that the F concentration be lower in the first region than in other regions of the outer cladding layer. When the diameter of the region with a low F concentration exceeds 3 μm, not only the residual stress but also the peak of the relative refractive index difference itself exhibits the function of blocking a signal like a core, and it is considered that the transmission characteristics of the signal are affected by the increase in the wavelength of the cutoff wavelength, the deterioration of the bending loss, and the like. Therefore, the diameter of the region with a low F concentration needs to be 3 μm or less. The minimum value of the F concentration in the region with a low F concentration needs to be lower than the minimum value of the F concentration in the outer region by 100ppm or more.

Fig. 12A is a table showing a relationship between the integrated value of the residual stress and the transmission loss. Fig. 12B is a graph showing the relationship thereof. As shown in fig. 12A and 12B, when the integrated value of the residual stress is 20MPa · μm or more, a sufficient stress applying portion is formed, and the transmission loss can be kept low. On the other hand, it is found that when the integrated value of the residual stress is 120MPa · μm or more, the transmission loss deteriorates again. This is presumably because the strain of the stress applying portion is excessively large, causing a defect loss and deteriorating the transmission loss.

The graph of the residual stress P (r) in the integration interval (r 1. ltoreq. r.ltoreq.r 2) is formed into a triangular shape in which the peak values P and P (r2) of P (r1) and P (r) (r2) are connected by straight lines, the peak values are 30MPa, and the calculated integrated value is 150 MPa.mu.m, that is, 120 MPa.mu.m or more. R2 represents a position of 10 μm from the inner surface of the outer cladding, and r1 represents a position of P (r1) ═ P (r 2).

The reason why the integrated value is 120MPa · μm or more in this manner is presumed to be that the following formula (3) is used to apply the maximum value P of the residual stress_maxRadius r of_maxWhen the peripheral residual stress distribution is approximated, if the exponent α is 1 or more and the integral value is larger than the distribution of the area of the triangle, there is no abrupt change in stress, and a sufficient stress applying portion is not formed. Therefore, α can be made smaller than 1 as the distribution shape of the stress applying portion. In this way, the stress applying unit needs to specify not only the peak value but also the distribution of the area value (i.e., the integral value) and the exponent α. In the formula (3), a is a radial distance 10(μm) between the upper limit position of the integration interval and the inner peripheral surface 22b, and b is expressed by the following formula (4).

[ formula 3]

P(r)＝P_max·(1-b·[(r-r_max)/a]^α) (3)

[ formula 4]

b＝(1-[P(r1)/P_max])·[a/(r1-r_max)]^α (4)

Description of the reference numerals

1: an optical fiber;

10: a core;

10 a: an outer peripheral surface;

20: a cladding layer;

20 a: an outer peripheral surface;

20 b: an inner peripheral surface;

21: an inner cladding;

21 a: an outer peripheral surface;

21 b: an inner peripheral surface;

22: an outer cladding;

22 a: an outer peripheral surface;

22 b: an inner peripheral surface;

30: the most big part.