Patent Description:
This application claims the priority based on <CIT>.

In an optical transmission system using an optical fiber as a transmission line, increase in the transmission distance and the transmission capacity has been demanded. In order to satisfy such a demand, a small transmission loss of the optical fiber is desired. In the drawing process of manufacturing an optical fiber by drawing an optical fiber preform, since tension is applied to the optical fiber preform softened by heating to perform drawing, a stress parallel to the applied tension remains in a glass portion of the manufactured optical fiber. The glass portion of the optical fiber is constituted by a core and a cladding, and most of the power of the signal light propagating in the optical fiber is concentrated in the core. If a tensile stress remains in the core, the residual tensile stress increases glass structural defects in the core, which increases the transmission loss. Therefore, it is desirable that a compressive stress remains in the core.

Patent Document <NUM> discloses an optical fiber in which the stress in the core is a compressive stress. In the optical fiber disclosed in Patent Document <NUM>, since the core contains an alkali metal element, the viscosity of the core decreases, and the stress in the core is a compressive stress. In the optical fiber disclosed in Patent Document <NUM>, the cladding contains fluorine (a fluorine element) and chlorine (a chlorine element), but the distribution of the fluorine concentration in the cladding is not considered at all.

In addition, Patent Document <NUM> also discloses an optical fiber in which the stress in the core is a compressive stress. In the optical fiber disclosed in Patent Document <NUM>, since the core contains GeO<NUM> and the optical fiber obtained immediately after an optical fiber preform is drawn at an appropriate drawing speed and drawing tension passes through a heating furnace having an appropriate length, the stress in the core is a compressive stress. The glass portion of the optical fiber disclosed in Patent Document <NUM> includes a central core, an optical cladding, and a jacket. However, the outermost jacket is pure silica glass or silica glass containing chlorine and does not contain fluorine that reduces the refractive index of the glass material. Patent Document <NUM> discloses a multi-mode fiber suitable for access network or miniaturized optical apparatus having bending resistance and a method for producing the same. Patent Document <NUM> discloses optical fibers having a core comprising silica and greater than <NUM> wt% of chlorine and less than <NUM> wt% F, said core having a refractive index Δ<NUM>MAX and a inner cladding region having refractive index Δ<NUM>MIN surrounding the core, where Δ<NUM>MAX >Δ<NUM>MIN. Patent Document <NUM> relates to optical fibers with large effective area, low bending loss and low attenuation, more particularly optical fibers exhibiting high core doping concentration, high core doping uniformity and low core stress.

An optical fiber according to the present disclosure includes at least a core mainly comprised of silica glass and a cladding mainly comprised of silica glass. The core extends along the fiber axis (the central axis of the optical fiber). The cladding surrounds the outer peripheral surface of the core and has a refractive index lower than the refractive index of the core. An average value n1_ave of the refractive index of the core, a minimum value nc_min of the refractive index of the cladding, and a refractive index n0 of pure silica glass satisfy relationships of: <MAT> and <MAT> Furthermore, the cladding contains fluorine (a fluorine element), and the fluorine concentration in the cladding is adjusted to be minimum in the outermost portion of the cladding. In any small section in an outer region of the cladding, a differential coefficient XF'(r) of a fluorine concentration distribution XF(r) in the cladding at a distance r is preferably negative, and an absolute value of the differential coefficient XF'(r) is preferably <NUM> ppm/µm or more and <NUM> ppm/µm or less. Note that, the fluorine concentration distribution XF(r) in the cladding is determined by the fluorine concentration in the cladding and the distance r along a radial direction of the optical fiber from the fiber axis in a cross section of the optical fiber perpendicular to the fiber axis. The outer region of the cladding is defined as an annular region from a position where a distance along the radial direction from an inner peripheral surface of the cladding is <NUM>% of a width of the cladding to a position where a distance along the radial direction from the inner peripheral surface is <NUM>% of the width of the cladding. Furthermore, the any small section included in the outer region of the cladding has a width of <NUM> defined along the radial direction.

A purpose of the present disclosure is to provide a low loss optical fiber capable of being manufactured inexpensively and easily. First, the details of the embodiment of the present disclosure are individually listed and described.

As one aspect of the present disclosure, a relative refractive index difference Δ+ of the average value of refractive index of the core with respect to the depressed portion is preferably <NUM>% or more and <NUM>% or less. Furthermore, as one aspect of the present disclosure, a relative refractive index difference Δ2 of an average value of refractive index of the depressed portion with respect to the minimum refractive index of the cladding is preferably -<NUM>% or more and -<NUM>% or less. In addition, a ratio r2/r1 of a radius r2 of the depressed portion to a radius r1 of the core is preferably <NUM> or more and <NUM> or less.

Each aspect listed in [Description of Embodiment of Present Disclosure] is applicable to each of the other aspects or all combinations of the other aspects. The optical fiber in the present disclosure can be manufactured inexpensively and easily and reduce the transmission loss.

Hereinafter, a specific configuration of an optical fiber according to the present disclosure is described in detail with reference to the attached drawings. Note that, the present invention is not limited to examples to be described, is represented by claims, and includes all modifications within the meaning and scope equivalent to claims. In the description of the drawings, identical elements are denoted by the same reference signs, and overlapped descriptions are omitted.

In the optical fiber disclosed in Patent Document <NUM>, since an alkali metal element is doped to the core the manufacturing cost increases. In the optical fiber disclosed in Patent Document <NUM>, it is necessary to dope GeO<NUM> to the core to increase the refractive index of the core in order to confine the signal light in the core. However, if the concentration of GeO<NUM> is high, the Rayleigh scattering due to fluctuation in the concentration of GeO<NUM> increases, and the transmission loss increases. In addition, in order to manufacture the optical fiber disclosed in Patent Document <NUM>, the cost of installing a heating furnace is required, and the length and drawing speed of the heating furnace must be appropriately set, but there is a limit to the increase in the drawing speed.

<FIG> is a diagram showing a configuration of an optical fiber <NUM>. <FIG> shows a structure of the optical fiber <NUM> in a cross section perpendicular to a fiber axis AX (central axis), a refractive index profile along a radial direction from the fiber axis AX as the center, and a fluorine concentration distribution along the radial direction. The optical fiber <NUM> includes a core <NUM> having a radius r1 extending along the fiber axis AX, and a cladding <NUM> having a radius rc and surrounding the outer peripheral surface of the core <NUM>. The core <NUM> and the cladding <NUM> each mainly comprised of silica glass. The refractive index of the core <NUM> is higher than the refractive index of the cladding <NUM>. An average value n1_ave of the refractive index of the core <NUM>, a minimum value nc_min of the refractive index of the cladding <NUM>, and a refractive index n0 of pure silica glass satisfy, in the optical fiber <NUM> (the core <NUM> and the cladding <NUM>), relationships represented by the following expressions (<NUM>) and (<NUM>): <MAT> and <MAT>.

<FIG> is a diagram showing a configuration of an optical fiber <NUM>. <FIG> shows a structure of the optical fiber <NUM> in a cross section perpendicular to the fiber axis AX (central axis), a refractive index profile along the radial direction from the fiber axis AX as the center, and a fluorine concentration distribution along the radial direction. The optical fiber <NUM> includes a core <NUM> having a radius r1 extending along the fiber axis AX, a depressed portion <NUM> having a radius r2 and surrounding the outer peripheral surface of the core <NUM>, and a cladding <NUM> having a radius rc and surrounding the outer peripheral surface of the depressed portion <NUM>. The optical fiber <NUM> includes the depressed portion <NUM> provided between the core <NUM> and the cladding <NUM>. The core <NUM>, the depressed portion <NUM>, and the cladding <NUM> each mainly comprised of silica glass. The refractive index of the core <NUM> is higher than the refractive index of the cladding <NUM>. The refractive index of the depressed portion <NUM> is lower than the refractive index of the cladding <NUM>. An average value n2_ave of the refractive index of the depressed portion <NUM> in the optical fiber <NUM> (the core <NUM>, the depressed portion <NUM>, and the cladding <NUM>) satisfies, in addition to the relationships represented by the above expressions (<NUM>) and (<NUM>), the following expression (<NUM>): <MAT>.

The optical fiber <NUM> (<FIG>) corresponds to the structure in which r2 equals to r1 in the optical fiber <NUM> (<FIG>) (the structure in which the depressed portion <NUM> is removed). Each of the radii r1 and r2 is determined as a value when the differential coefficient of the refractive index at the distance r along the radial direction from the fiber axis AX is minimum or maximum.

In each of the optical fiber <NUM> (<FIG>) and optical fiber <NUM> (<FIG>), the cladding <NUM> contains fluorine. The fluorine concentration in the cladding <NUM> is adjusted to be minimum in the outermost portion. The "outermost portion" is an annular region including the outer peripheral surface of the cladding <NUM> and positioned outside the position where the distance along the radial direction from the inner peripheral surface of the cladding <NUM> (the position of r2 in each of <FIG> and <FIG>) is <NUM>% of the width of the cladding (rc - r2). Silica glass has higher viscosity as the fluorine concentration is smaller. Therefore, the tension applied to the glass in the drawing process is mainly borne by the vicinity of the outermost portion of the cladding <NUM>, and the tension applied to the core is relatively small. As a result, a tensile stress acting perpendicular to the cross section perpendicular to the fiber axis remains in the vicinity of the outermost portion of the cladding <NUM>. However, since the stress in the core in which most of the power of the signal light propagating in the optical fiber is present can be a compressive stress, the transmission loss of the optical fiber as a whole can be controlled to be low.

When the end face of the optical fiber <NUM> or the optical fiber <NUM> is fusion-spliced to the end face of another optical fiber, the end portion of the optical fiber is cleaved. At this time, a small fluorine concentration in the outermost portion of the cladding <NUM> of the optical fiber <NUM> or the optical fiber <NUM> makes the cleaving of the optical fiber easy. That is, it is possible to obtain a cleaved surface that is more perpendicular to the fiber axis AX and is flatter.

A fluorine concentration XFc_outer[ppm] in the outermost portion of the cladding <NUM> and a maximum value XFc_max[ppm] of the fluorine concentration in the cladding <NUM> satisfy a relationship represented by the following expression (<NUM>): <MAT>.

In addition, it is preferable that they satisfy a relationship represented by the following (<NUM>): <MAT>.

It is more preferable that they satisfy a relationship represented by the following expression (<NUM>): <MAT>.

Furthermore, it is most preferably that they satisfy a relationship represented by the following expression (<NUM>): <MAT>.

By satisfying the relationships represented by the above expressions (<NUM>) to (<NUM>), the viscosity of glass in the outermost portion of the cladding <NUM> can be further increased.

When the fluorine concentration distribution of the cladding <NUM> (the distribution defined by the distance r along the radial direction and the fluorine concentration) is XF(r), in any small section (a section having a width of <NUM> along the radial direction) of the outer region of the cladding <NUM>, a differential coefficient XF'(r) of the fluorine concentration distribution XF(r) at the distance r is negative and that the absolute value of the differential coefficient XF'(r) is <NUM> ppm/µm or more and <NUM> ppm/µm or less. Note that, the outer region of the cladding <NUM> satisfies a relationship represented by the following expression (<NUM>): <MAT> and is defined as an annular region from a position where a distance along the radial direction from the inner peripheral surface of the cladding <NUM> is <NUM>% of the width of the cladding (rc - r2) to a position where a distance along the radial direction from the inner peripheral surface is <NUM>% of the width of the cladding.

As the absolute value of the differential coefficient XF'(r) is larger, the fluorine concentration in the outermost portion of the cladding <NUM> is smaller. This is preferable to increase the viscosity of the glass in the outermost portion of the cladding <NUM>. On the other hand, if the absolute value of the differential coefficient XF'(r) is excessively large, the change in the residual stress becomes large along the radial direction. Such an increase in the change in the residual stress leads to non-uniformity of the glass structure, and the transmission loss is increased. Therefore, it is preferable that, in any small section having a width of <NUM> in the outer region of the cladding <NUM>, the differential coefficient XF'(r) is negative and that the absolute value of the differential coefficient XF'(r) is <NUM> ppm/µm or more and <NUM> ppm/µm.

The fluorine concentration distribution along the radial direction in the optical fiber can be measured by an electron probe micro analyzer (EPMA). The EPMA irradiates the optical fiber, which is the object to be measured, with an electron beam to measure the intensity of the X-ray generated at that time. By measuring a sample whose fluorine concentration is known in advance (to obtain a calibration curve between the X-ray intensity and the fluorine concentration), the X-ray intensity measured from the optical fiber can be converted into the fluorine concentration.

The stress remaining in the outer region of the cladding <NUM> (the range in which the distance along the radial direction from the inner peripheral surface of the cladding is <NUM>% or more and <NUM>% or less of the width of the cladding) is preferably a tensile stress. As a result, the tension at the time of drawing can be borne in a wide region of the cladding <NUM>, and the stress in the core <NUM> can be a compressive stress. Alternatively, the stress in the entire region of the cladding <NUM> may be a tensile stress. Here, the distribution of the residual stress along the radial direction in the optical fiber can be measured based on the birefringence in the optical fiber using two beams of polarized light orthogonal to each other as disclosed in, for example, Patent Document <NUM>.

The transmission loss of each of the optical fibers <NUM> and <NUM> in the present disclosure at a wavelength of <NUM> is <NUM> dB/km or less, preferably <NUM> dB/km or less, and more preferably <NUM> dB/km or less.

A conventionally known single mode optical fiber (SMF) has an effective area of about <NUM><NUM> at a wavelength of <NUM> and a cable cutoff wavelength of <NUM> or less. In addition, a conventionally known non-zero dispersion shifted fiber (NZDSF) has an effective area of about <NUM><NUM> or more and <NUM><NUM> or less at a wavelength of <NUM> and a cable cutoff wavelength of <NUM> or less. Unlike these conventional SMF and NZDSF, each of the optical fibers <NUM> and <NUM> in the present disclosure preferably has an effective area of <NUM><NUM> or more and <NUM><NUM> or less at a wavelength of <NUM> and a cable cutoff wavelength of <NUM> or less.

In each of the optical fibers <NUM> and <NUM> in the present disclosure, the refractive index is locally high in the outermost portion of the cladding <NUM>, and the difference between the refractive index of the outermost portion of the cladding <NUM> and the refractive index of the core is small. When a part of the power of the signal light extends to the vicinity of the outermost portion of the cladding <NUM>, the effective relative refractive index difference between the core <NUM> and the cladding <NUM> becomes small. Therefore, it is preferable that most of the power of the signal light is present only in the core <NUM> and the inner region of the cladding <NUM> adjacent to the core <NUM>. Furthermore, in the present embodiment, it is preferable that the effective area is larger because the outer diameter of the core <NUM> is larger, and the larger part of the power of the signal light is present only in the core <NUM> and the inner region of the cladding <NUM> adjacent to the core <NUM>. Meanwhile, if the effective area is excessively large, the bending loss easily increases. Therefore, the effective area at a wavelength of <NUM> is preferably <NUM><NUM> or more and <NUM><NUM> or less.

It is preferable that the cable cutoff wavelength is longer because the power of the signal light in the fundamental mode is more strongly confined in the core <NUM>. On the other hand, in order for the signal light to propagate in the single mode in the C-band (wavelength <NUM> or more and <NUM> or less), the cable cutoff wavelength is preferably <NUM> or less. The bending loss at wavelengths of <NUM> and <NUM> when the optical fiber is wound <NUM> turns with a bending diameter of <NUM> is preferably <NUM> dB or less. This bending loss is more preferably <NUM> dB or less and most preferably <NUM> dB or less.

Other characteristics of the optical fibers <NUM> and <NUM> in the present disclosure may comply with the international recommendations ITU-T G.

The material of the core <NUM> may be pure silica glass. The material of the core <NUM> may be silica glass containing any one of GeO<NUM>, chlorine (a chlorine element), and fluorine (a fluorine element), and may be silica glass containing two or more of these dopants. It is more preferable that the core <NUM> contains these elements because the viscosity of the core <NUM> is reduced, and the stress in the core <NUM> can be a compressive stress Furthermore, the core <NUM> containing GeO<NUM> or chlorine makes the refractive index of the core <NUM> high. This is preferable because the signal light can be more strongly confined in the core <NUM>. However, if the core <NUM> excessively contains these dopants, the Rayleigh scattering loss due to fluctuation in the concentration of the doped element increases. Therefore, a relative refractive index difference Δcore of the average value of refractive index of the core <NUM> with respect to pure silica glass preferably fall within the range of -<NUM>% or more and + <NUM>% or less. The Δcore is more preferably -<NUM>% or more and + <NUM>% or less, and most preferably <NUM>% or more and + <NUM>% or less.

A chlorine concentration XCl_outer[ppm] in the outermost portion of the core <NUM> and a maximum value XCl_max[ppm] of the chlorine concentration in the core <NUM> preferably satisfy a relationship represented by the following expression (<NUM>): <MAT>.

As a result, the change in the viscosity in the boundary surface between the core and the cladding can be controlled to be small. Furthermore, the chlorine concentration may satisfy a relationship represented by the following expression (<NUM>): <MAT>.

It is more preferable if the core <NUM> contains both GeO<NUM> and chlorine. The Rayleigh scattering loss due to fluctuation in the concentration of GeO<NUM> more easily increases than fluctuation in the concentration of chlorine. However, the core <NUM> containing both GeO<NUM> and chlorine can increase the refractive index of the core <NUM>, and the signal light can be more strongly confined in the core without excessively doping GeO<NUM>. Furthermore, the viscosity of the core <NUM> can be reduced to increase the stress in the core <NUM>. Preferably, the average value of the concentration of GeO<NUM> contained in the core <NUM> is <NUM> ppm or more and <NUM> ppm or less (a concentration that increases the relative value of the refractive index variation with respect to the refractive index of pure silica glass by <NUM> or more and <NUM>% or less), and the average value of the concentration of chlorine is <NUM> ppm or more and <NUM> ppm or less (a concentration that increases the relative value of the refractive index variation with respect to the refractive index of pure silica glass by <NUM>% or more and <NUM>% or less). More preferably, the average value of the concentration of GeO<NUM> contained in the core is <NUM> ppm or more and <NUM> ppm or less (a concentration that increases the relative value of the refractive index variation with respect to the refractive index of pure silica glass by <NUM>% or more and <NUM>% or less), and the average value of the concentration of chlorine is <NUM> ppm or more and <NUM> ppm or less (a concentration that increases the relative value of the refractive index variation with respect to the refractive index of pure silica glass by <NUM>% or more and <NUM>% or less).

It is preferable that the radius r1 of the core <NUM> is larger because the power of the signal light can be more strongly confined in the core <NUM>. Furthermore, it is preferable that a relative refractive index difference Δ<NUM> of the average value of refractive index of the core <NUM> with respect to the minimum refractive index of the cladding <NUM> is larger because the power of the signal light can be more strongly confined in the core <NUM>. However, if both the radius r1 and the relative refractive index difference Δ1 are excessively large, the cable cutoff wavelength becomes long, and the signal light does not propagate in the single mode in a signal light wavelength band such as the C-band. Therefore, the radius r1 is preferably <NUM> or more and <NUM> or less, and the relative refractive index difference Δ1 is preferably <NUM>% or more and <NUM>% or less. As a result, the above effective area, cable cutoff wavelength, and fiber characteristics can be implemented.

Compared with the configuration of the optical fiber <NUM> (<FIG>) including the core <NUM> and the cladding <NUM>, the configuration of the optical fiber <NUM> (<FIG>) further including the depressed portion <NUM> is more preferable. A relative refractive index difference Δ+ of the average value of refractive index of the core <NUM> with respect to the depressed portion <NUM> is preferably <NUM>% or more and <NUM>% or less. As a result, most of the power of the signal light is confined in the core <NUM> and the depressed portion <NUM>. That is, the relative refractive index difference Δ1 may be small, and the fluorine concentration in the cladding <NUM> may be small. At this time, the relative refractive index difference Δ1 may be <NUM>% or more and <NUM>% or less.

If the ratio r2/r1 of the radius r2 of the depressed portion <NUM> to the radius r1 of the core <NUM> is excessively small, the signal light in the fundamental mode leaks to the cladding <NUM>, and the confinement in the core becomes weak. Alternatively, if the ratio r2/r1 is excessively large, light of unnecessary high-order modes is also confined in the core <NUM> and the depressed portion <NUM>, and the signal light does not propagate in the single mode in the signal light wavelength band. Therefore, the ratio r2/r1 is preferably <NUM> or more and <NUM> or less, and more preferably <NUM> or more and <NUM> or less.

A relative refractive index difference Δ2 of the average value of refractive index of the depressed portion with respect to the minimum refractive index of the cladding <NUM> is negative. When Δ+ is constant, the high-order mode is easily to be confined if the absolute value of Δ2 is excessively small. That is, the signal light does not propagate in the single mode in the signal light wavelength band. Meanwhile, if the absolute value of Δ2 is excessively large, the signal light in the fundamental mode is cut off, or a loss due to leakage occurs. Therefore, Δ2 is preferably -<NUM>% or more and -<NUM> or less.

A relative refractive index difference Δc_outer of the outermost portion of the cladding <NUM> with respect to pure silica glass and a relative refractive index difference Δc_min of the minimum refractive index of the cladding <NUM> with respect to pure silica glass satisfy a relationship represented by the following expression (<NUM>): <MAT>.

It is further preferable that they satisfy a relationship represented by the following expression (<NUM>): <MAT>.

It is most preferable that they satisfy a relationship represented by the following expression (<NUM>): <MAT>.

Here, a relative refractive index difference Δcore of the average value of refractive index of the core <NUM> with respect to pure silica glass is represented by the following expression (<NUM>): <MAT>.

A relative refractive index difference Δ<NUM> of the average value of refractive index of the core <NUM> with respect to the minimum refractive index of the cladding <NUM> is represented by the following expression (<NUM>): <MAT>.

A relative refractive index difference Δ<NUM> of the average value of refractive index of the depressed portion <NUM> with respect to the minimum refractive index of the cladding <NUM> is represented by the following expression (<NUM>): <MAT>.

A relative refractive index difference Δ+ of the average value of refractive index of the core <NUM> with respect to the depressed portion <NUM> is represented by the following expression (<NUM>): <MAT>.

When the refractive index of the outermost portion of the cladding <NUM> is nc_outer, the relative refractive index difference Δc_outer of the outermost portion of the cladding <NUM> with respect to pure silica glass is represented by the following expression (<NUM>): <MAT>.

When the minimum refractive index of the cladding <NUM> is nc_min, the relative refractive index difference Δc_min of the minimum refractive index of the cladding <NUM> with respect to pure silica glass is represented by the following expression (<NUM>): <MAT>.

The core <NUM> of each of the optical fibers <NUM> and <NUM> may have any index of refraction profile, and a type A to type H refractive index profiles as shown in <FIG> can be applied to the core <NUM>, for example. Regardless of the refractive index profile, the average value of the refractive index of the core <NUM> is n1_ave.

Here, the refractive index profile of the core <NUM> may be an α-th power profile (the type B, the type C, the type G, the type H, or the like in <FIG>) in which the refractive index n(r) at the radius r[um] is approximated by the expression of: <MAT> <MAT>.

Since the optical fiber in the present disclosure has a cable cutoff wavelength of <NUM> at a maximum, light of two or more modes can propagate in the O-band (wavelength <NUM> or more and <NUM> or less). Therefore, the optical fiber in the present disclosure can be applied to mode division multiplexing transmission in which light of a plurality of modes propagates in one fiber. It is preferable that the profile shape of the core <NUM> has the α-th power because the differential mode delays (DMD) between modes can be reduced and the load of multiple-input and multiple-output (MIMO) processing in the receiver is reduced. <FIG> is a graph showing DMDs in an LP01 mode and an LP11 mode at a wavelength of <NUM> in various refractive index profiles by varying an exponent α as r2/r1 = <NUM>, re = <NUM>, and Δ2 = -<NUM>%. Here, Δmax and the core radius r1 are adjusted so that an effective area Aeff is <NUM><NUM> and the cable cutoff wavelength is <NUM>. This shows that the exponent α is preferably <NUM> or more and <NUM> or less in order to keep the absolute value of DMD small. More preferably, the exponent α is <NUM> or more and <NUM> or less. Most preferably, the exponent α is <NUM> or more and <NUM> or less.

Furthermore, the depressed portion <NUM> of the optical fiber <NUM> may have any refractive index profile and may have, for example, the type A or type B refractive index profile shown in <FIG>. Regardless of the refractive index profile, the average value of the refractive index of the depressed portion <NUM> is n2_ave.

The radius rc of the cladding <NUM> falls within the range of <NUM> or more and <NUM> or less. Each of the optical fibers <NUM> and <NUM> includes a resin coating layer provided so as to be in contact with the outer peripheral surface of the cladding <NUM>. <FIG> is a diagram showing a cross-sectional structure of the optical fiber <NUM> including the resin coating layer. The resin coating layer has a two-layer structure and includes a primary coating layer <NUM> provided on the inner side and functioning so that external force is not directly transmitted to the cladding <NUM>, and a secondary coating layer <NUM> provided on the outside of the primary coating layer <NUM> functioning to prevent damage. The outer radius of the primary coating layer <NUM> is <NUM> or more and <NUM> or less, and the outer radius of the secondary coating layer <NUM> may be <NUM> or more and <NUM> or less. Alternatively, the outer radius of the primary coating layer <NUM> is <NUM> or more and <NUM> or less, and the outer radius of the secondary coating layer <NUM> may be <NUM> or more and <NUM> or less.

The optical fibers <NUM> and <NUM> in the present disclosure can reduce the transmission loss without the core <NUM> containing a special element such as an alkali metal element. Furthermore, since the cladding <NUM> contains fluorine, the refractive index of the cladding <NUM> is reduced. Therefore, it is not necessary to dope Ge to the core in order to increase the refractive index of the core <NUM>, or the amount of Ge doped to the core <NUM> can be small. When the optical fiber in the present disclosure is manufactured by drawing an optical fiber preform, it is not necessary to provide a heating furnace directly under the drawing furnace. The optical fiber in the present disclosure can be manufactured inexpensively and easily and reduce the transmission loss.

Next, a specific example of the optical fiber according to the present embodiment will be described. An optical fiber according to a specific example <NUM> has the same structure as the optical fiber <NUM> (<FIG>). <FIG> is a diagram showing a refractive index profile of the optical fiber according to the specific example <NUM>. <FIG> is a diagram showing the fluorine concentration distribution XF(r) of the optical fiber according to the specific example <NUM>. <FIG> is a graph plotting the differential coefficient XF'(r) of the fluorine concentration distribution XF(r) of the optical fiber according to the specific example <NUM>. <FIG> is a diagram showing a residual stress distribution of the optical fiber according to the specific example <NUM>. In <FIG>, a positive residual stress means a tensile stress, and a negative residual stress means a compressive stress.

Structural parameters of the optical fiber according to the specific example <NUM> are set as follows: r1 = <NUM>, r2/r1 = <NUM>, rc = <NUM>, Δcore = <NUM>%, Δ1 = <NUM>%, Δ2 = -<NUM>%, and Δ+ = <NUM>%. The core is comprised of silica glass containing chlorine. The cladding is comprised of silica glass containing fluorine. XFc_max is 2560ppm, and XFc_outer is 570ppm. The absolute value of the differential coefficient XF'(r) of the fluorine concentration distribution XF(r) in the outer region of the cladding (the range in which the distance along the radial direction from the inner peripheral surface of the cladding is <NUM>% or more and <NUM>% or less of the width of the cladding) is <NUM> ppm/µm or more and <NUM> ppm/µm or less. The stress in the entire region of the cladding is the tensile stress and the stress in the entire region of the core is the compressive stress. The transmission loss at a wavelength of <NUM> is <NUM> dB/km. The effective area at a wavelength of <NUM> is <NUM><NUM>. The cable cutoff wavelength is <NUM>. The bending loss at wavelengths of <NUM> and <NUM> when the optical fiber is wound <NUM> turns with a diameter of <NUM> is <NUM> dB or less. The other fiber characteristics comply with the international recommendations ITU-T G.

An optical fiber according to a specific example <NUM> also has the same structure as the optical fiber <NUM> (<FIG>). Structural parameters of the optical fiber according to the specific example <NUM> are set as follows: r1 = <NUM>, r2/r1 = <NUM>, rc = <NUM>, Δcore = <NUM>%, Δ1 = <NUM>%, Δ2 = -<NUM>%, and Δ+ = <NUM>%. The core is comprised of silica glass doped with GeO<NUM> and chlorine. The average value of the concentration of GeO<NUM> doped to the core is <NUM> ppm. The average value of the concentration of chlorine doped to the core is <NUM> ppm. The cladding contains fluorine. XFc_max is 2400ppm, and XFc_outer is 1020ppm. The value of |XF'(r)| in the outer region of the cladding (the range in which the distance along the radial direction from the inner peripheral surface of the cladding is <NUM>% or more and <NUM>% or less of the width of the cladding) is <NUM> ppm/µm or more and <NUM> ppm/µm or less. The stress in the entire region of the cladding is the tensile stress and the stress in the entire region of the core is the compressive stress. The transmission loss at a wavelength <NUM> is <NUM> dB/km, the effective area Aeff at a wavelength <NUM> is <NUM><NUM>, the cable cutoff wavelength is <NUM>, the bending loss at wavelengths <NUM> and <NUM> when the optical fiber is wound <NUM> turns with a diameter of <NUM> is <NUM> dB or less. The other fiber characteristics comply with the international recommendations ITU-T G.

Claim 1:
An optical fiber (<NUM>; <NUM>) comprising:
a core (<NUM>) mainly comprised of silica glass and extending along a fiber axis (AX); and
a cladding (<NUM>) mainly comprised of silica glass, the cladding (<NUM>) surrounding an outer peripheral surface of the core (<NUM>) and having a refractive index lower than a refractive index of the core (<NUM>), wherein
an average value n1_ave of the refractive index of the core (<NUM>), a minimum value nc_min of the refractive index of the cladding (<NUM>), and a refractive index n0 of pure silica glass satisfy relationships of: <MAT> and <MAT>
the cladding (<NUM>) contains fluorine, and
a fluorine concentration in the cladding (<NUM>) is adjusted to be minimum in an outermost portion of the cladding (<NUM>);
wherein the outermost portion is an annular region including the outer peripheral surface of the cladding (<NUM>) and is positioned outside the position where the distance along the radial direction from the inner peripheral surface of the cladding (<NUM>) is <NUM>% of the width of the cladding (<NUM>); and
characterized in that
a fluorine concentration distribution XF(r) is determined by the fluorine concentration in the cladding (<NUM>) and a distance r along a radial direction of the optical fiber (<NUM>; <NUM>) from the fiber axis (AX), in a cross section of the optical fiber (<NUM>; <NUM>) perpendicular to the fiber axis (AX), and
a differential coefficient XF'(r) of the fluorine concentration distribution XF(r) at the distance r is negative, and an absolute value of the differential coefficient XF'(r) is <NUM> ppm/µm or more and <NUM> ppm/µm or less in any small section, the small section having a width of <NUM> defined along the radial direction, in an outer region of the cladding (<NUM>), the outer region being defined as an annular region from a position where a distance along the radial direction from an inner peripheral surface of the cladding (<NUM>) is <NUM>% of a width of the cladding (<NUM>) to a position where a distance along the radial direction from the inner peripheral surface is <NUM>% of the width of the cladding (<NUM>).