WO2015079987A1

WO2015079987A1 - Optical fiber and optical fiber preform

Info

Publication number: WO2015079987A1
Application number: PCT/JP2014/080590
Authority: WO
Inventors: 春名　徹也; 平野　正晃; 欣章田村
Original assignee: 住友電気工業株式会社
Priority date: 2013-11-29
Filing date: 2014-11-19
Publication date: 2015-06-04
Also published as: JP2015105199A

Abstract

Provided is an optical fiber with low transmission loss, and an optical fiber preform which is used for obtaining such an optical fiber and in which crystals are not prone to form in the core. This optical fiber is provided with a core that includes the central axis, and a cladding that surrounds the core and has a lower refractive index than that of the core. The core contains an alkali metal element and elemental chlorine. The average concentration of the alkali metal element in the core is 0.1-20 atomic ppm, the average concentration of the elemental chlorine in the core is less than or equal to 800 atomic ppm, and the transmission loss at a wavelength of 1550nm is 0.185dB/km or less.

Description

Optical fiber and optical fiber preform

The present invention relates to an optical fiber containing an alkali metal element and an optical fiber preform.

A silica glass optical fiber containing an alkali metal element in the core is known (Japanese Patent Publication No. 2005-537210 (Patent Document 1), US Patent Application Publication No. 2006/0130530 (Patent Document 2), Table 2007-504080 (Patent Document 3), JP-T 2008-536190 (Patent Document 4), JP-T 2010-501894 (Patent Document 5), JP-T 2009-541796 (Patent Document 6) JP-T 2010-526749 (Patent Document 7), International Publication No. 98/002389 (Patent Document 8), US Pat. No. 5,146,534 (Patent Document 9), JP 2012-229150 (Patent Document) 10), Japanese Patent Laid-Open No. 2013-61620 (Patent Document 11)). If the core of the optical fiber preform contains an alkali metal element, the viscosity of the core decreases when the optical fiber preform is drawn, and the relaxation of the silica glass network structure proceeds. It is said that loss can be reduced.

Patent Documents

1 and 2 describe a diffusion method which is a method of adding an alkali metal element into silica glass. The diffusion method is to heat the glass pipe with an external heat source or to generate plasma in the glass pipe while introducing the raw material vapor such as alkali metal element or alkali metal salt as the raw material into the glass pipe. An alkali metal element is diffusely added to the inner surface of the glass pipe.

後 After adding the alkali metal element in the vicinity of the inner surface of the glass pipe in this way, the glass pipe is heated to reduce the diameter. After the diameter reduction, a certain thickness of the inner surface of the glass pipe is etched for the purpose of removing transition metal elements such as Ni and Fe which are added simultaneously with the addition of the alkali metal element. Since the alkali metal element diffuses faster than the transition metal element, the alkali metal element can remain even if the transition metal element is removed by etching the glass surface with a certain thickness. After the etching, the glass rod is heated and solidified to produce a core rod containing an alkali metal element. An optical fiber preform is manufactured by synthesizing a cladding portion having a refractive index lower than that of the core portion including the core rod outside the core rod including the alkali metal element. And an optical fiber can be manufactured by drawing this optical fiber preform by a known method.

The present invention aims to provide an optical fiber with low transmission loss and an optical fiber preform that is used to obtain such an optical fiber and hardly generates crystals in the core.

An optical fiber comprising a core including a central axis and a clad surrounding the core and having a refractive index smaller than the refractive index of the core, wherein the core includes an alkali metal element and a chlorine element, and the alkali metal element in the core An optical fiber having an average concentration of 0.1 atomic ppm or more and 20 atomic ppm or less, an average concentration of chlorine element in the core of 800 atomic ppm or less, and a transmission loss at a wavelength of 1550 nm of 0.185 dB / km or less is provided. Is done.

In the optical fiber of the present invention, the average chlorine concentration in the core may be 300 atomic ppm or more. In the optical fiber of the present invention, the core may further contain a fluorine element, and the average concentration of dopants other than alkali metal elements, chlorine elements, and fluorine elements in the core may be 10 atomic ppm or less. In the optical fiber of the present invention, the core may contain a potassium element as an alkali metal element.

As another aspect of the invention, a silica part used for obtaining the above optical fiber, comprising a core part including a central axis, and a clad part surrounding the core part and having a refractive index smaller than the refractive index of the core A glass-based optical fiber preform, wherein the core portion includes a first core portion and a second core portion surrounding the first core portion, and the alkali metal element in the core portion includes the first core portion and the second core portion. Of the first core portion, the concentration of the alkali metal element in the second core portion is 10 atomic ppm or less, and the average concentration of the chlorine element in the first core portion is the second core portion. An optical fiber preform smaller than the average concentration of elemental chlorine in the part is provided.

As still another aspect, the silica glass includes a core portion including a central axis, and a clad portion surrounding the core portion and having a refractive index smaller than the refractive index of the core, and used for obtaining the above optical fiber. An optical fiber preform of the system, wherein the core portion contains an alkali metal element and a chlorine element, and the average concentration of the alkali metal element in the core portion is 5 atomic ppm or more and 100 atomic ppm or less. An optical fiber preform is provided having an average concentration of less than 1000 atomic ppm.

According to the present invention, it is possible to provide an optical fiber having a low transmission loss and an optical fiber preform that is used to obtain such an optical fiber and hardly generates crystals in the core portion.

It is sectional drawing of the optical fiber preform | base_material of embodiment of this invention.

It is a graph which shows the potassium element concentration and chlorine element concentration dependence of crystallization generation | occurrence | production in the core part of an optical fiber preform | base_material.

It is a graph which shows concentration distribution of the alkali metal element and chlorine element in a core part in case a chlorine element density | concentration is substantially uniform from a 1st core part to a 2nd core part.

It is a graph which shows concentration distribution of the alkali metal element and chlorine element in a core part in case chlorine element concentration differs in a 1st core part and a 2nd core part.

It is a figure which shows typically the example of the refractive index profile of an optical fiber preform | base_material.

It is a graph which shows the relationship between the average density | concentration of the potassium element in the core part of an optical fiber preform | base_material, and the average concentration of potassium element in the core of the optical fiber which drawn this optical fiber preform | base_material.

It is a graph which shows the relationship between the average concentration of the chlorine element in the core part of an optical fiber preform, and the average concentration of the chlorine element in the core of the optical fiber which drawn this optical fiber preform.

However, in the techniques described in Patent Documents 1 to 11, crystals are easily generated in the core portion of the optical fiber preform in the manufacturing process. An optical fiber obtained by drawing an optical fiber preform in which a crystal is generated in the core portion as described above may have a large transmission loss.

DETAILED DESCRIPTION Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

The present inventor has obtained the following knowledge in the course of conducting research on an optical fiber made of silica glass containing an alkali metal element in the core. It has been found that silica-based glass containing an alkali metal element is easily crystallized because the glass transition point is lowered to 1000 to 1400 ° C. and the crystallization speed is increased. In addition, if the core rod containing an alkali metal element contains at least one of an alkali metal element and a halogen element in a high concentration, a lot of halogen compounds of the alkali metal element are contained in the glass in the optical fiber preform manufacturing process. As a result, it was found that crystals are easily generated in the glass.

FIG. 1 is a cross-sectional view of an optical fiber preform 1 according to an embodiment of the present invention. The optical fiber preform 1 is made of silica glass, and includes a core portion 10 and a clad portion 20 that surrounds the core portion 10. The core part 10 includes the central axis of the optical fiber preform 1, and the refractive index of the core part 10 is larger than the refractive index of the cladding part 20. The core part 10 includes a first core part 11 and a second core part 12 surrounding the first core part 11. The clad part 20 includes a first clad part (optical clad part) 21 surrounding the second core part 12 and a second clad part (physical clad part) 22 surrounding the first clad part 21.

Next, the concentration range of the alkali metal element and the chlorine element in the core portion 10 will be described. FIG. 2 is a graph showing the dependency of crystallization generation in the core 10 on the potassium (K) element concentration and the chlorine (Cl) element concentration. According to this, in order to prevent the occurrence of crystallization, it is desirable that the average concentration of potassium element in the core portion 10 is 100 atomic ppm or less and the average concentration of chlorine element is less than 1000 atomic ppm. As shown in FIGS. 6 and 7, when the optical fiber preform 1 is drawn, both the alkali metal element and the chlorine element contained in the core portion 10 diffuse from the core portion 10 and the average concentration decreases. Therefore, it is desirable that the preferable value of the average concentration of the alkali metal element in the core in the fiber state is 20 atomic ppm or less, and the preferable value of the average concentration of the chlorine element is 800 atomic ppm or less. Further, if the concentration of the elemental chlorine is too thin, defects increase and cause an increase in transmission loss. Therefore, it is desirable that the concentration is at least 300 atomic ppm or more.

On the other hand, when the core portion 10 contains potassium element, the viscosity of the core portion 10 is reduced when the optical fiber preform 1 is drawn, and the relaxation of the network structure of the silica glass proceeds. Transmission loss can be reduced. From this, it is desirable that the average concentration of potassium element in at least the core portion 10 is 5 atomic ppm or more. As shown in FIG. 6, the preferred value of the average concentration of the alkali metal element in the core in the fiber state is 0.1 atomic ppm or more.

Patent Documents 5 and 6 do not substantially contain a metal oxide (GeO ₂ , Al ₂ O _3, etc.) for increasing the refractive index, and in an optical fiber having a core made of a glass containing an alkali metal element. Lowering the chlorine element concentration can reduce transmission loss, not containing any chlorine element, or desirably having a chlorine element (Cl) concentration of 500 ppm by weight (850 atomic ppm) or less, and oxidation. It describes that the potassium (K ₂ O) concentration is preferably 50 to 500 ppm by weight (the potassium element (K) concentration is 65 to 650 atomic ppm). Here, a chlorine element weight concentration of 1 ppm by weight was converted to a chlorine element molar concentration of 1.7 atomic ppm, and a potassium oxide weight concentration of 1 ppm by weight was converted to a potassium element molar concentration of 1.3 atomic ppm. Therefore, according to the techniques described in Patent Documents 5 and 6, crystals are generated in the glass in the manufacturing process of the optical fiber preform, and the transmission loss of the optical fiber tends to increase.

As shown in FIG. 3, for example, the concentration distribution of the alkali metal element and the chlorine element in the core portion 10 may be such that the concentration of the chlorine element in the entire core portion 10 is substantially uniform. In FIG. 3, potassium element and chlorine element are included so as to decrease outward with the central portion of the core portion 10 as a peak. The average concentration of potassium element in the entire core portion 10 is 100 atomic ppm or less. The average concentration of chlorine element in the entire core portion 10 is 1000 atomic ppm or less. Specifically, the average concentration of potassium element in the entire core portion 10 is 19.3 atomic ppm, and the average concentration of chlorine element is 924 atomic ppm. The peak concentration of potassium element in the entire core portion 10 is 750 atomic ppm, and the peak concentration of chlorine element is 1270 atomic ppm. The average concentration of potassium element in the core of the optical fiber obtained by drawing the optical fiber preform 1 was 2 atomic ppm, and the average concentration of chlorine element was 600 ppm.

Moreover, the concentration distribution of the alkali metal element and the chlorine element in the core part 10 may be different in the chlorine element concentration between the first core part 11 and the second core part 12, for example, as shown in FIG. . In FIG. 4, the potassium element has a maximum concentration in the first core portion 11 of the first core portion 11 and the second core portion 12. The peak concentration of potassium element in the first core portion 11 is higher than 1000 atomic ppm, and the average concentration is 250 atomic ppm or more. On the contrary, the average concentration of chlorine element in the first core portion 11 is lower than 80 atomic ppm. The average concentration of alkali metal element (potassium element) in the second core portion 12 is preferably 10 atomic ppm or less (in this example, 1 atomic ppm or less), and the average concentration of chlorine element is 1000 atomic ppm or more. It has become. Further, the average concentration of potassium element in the entire core portion 10 including the first core portion 11 and the second core portion 12 is 100 atomic ppm or less, and the average concentration of chlorine element is 1000 atomic ppm or less.

Specifically, the average concentration of potassium element in the first core portion 11 is 260 atomic ppm, and the average concentration of chlorine element is 77 atomic ppm. The peak concentration of potassium element in the first core portion 11 is 1141 atomic ppm. The average concentration of potassium element in the entire core portion 10 is 7.7 atomic ppm, and the average concentration of chlorine element is 906 atomic ppm. The peak concentration of chlorine element in the entire core portion 10 is 1180 atomic ppm. The average concentration of potassium element in the core of the optical fiber obtained by drawing the optical fiber preform was 1 atomic ppm, and the average concentration of chlorine element was 750 atomic ppm.

It is known that the optical fiber has a refractive index that increases with compressive stress and decreases with tensile stress due to the photoelastic effect. Therefore, it is desirable that the average refractive index of the second clad provided on the outer periphery of the first clad is 0.01% or more higher in relative refractive index difference than the average refractive index of the first clad.

It is preferable that the average concentration of the alkali metal element in the core of the optical fiber is 0.1 atomic ppm or more so that the transmission loss is reduced to 0.185 dB / km or less. On the other hand, if it is contained in a high concentration, the radiation resistance and hydrogen resistance are significantly deteriorated, so that the average concentration of the fiber for submarine cables is preferably 20 atomic ppm or less. The average concentration of dopants other than alkali metal elements, chlorine elements and fluorine elements in the core of the optical fiber is desirably 10 atomic ppm or less.

The average concentration of the alkali metal element in the cocoon core part 10 is preferably 100 atomic ppm or less as described above from the viewpoint of suppressing crystallization. By doing in this way, it is possible to improve the manufacturability of the core rod containing an alkali metal element. In order to sufficiently reduce the transmission loss, the average concentration is preferably 5 atomic ppm or more.

The maximum value of the refractive index of the core portion 10 based on the refractive index of the cladding portion 20 (in the case where the cladding portion 20 has a multilayer structure, the refractive index at a radial position that is about three times the outer periphery of the core portion 10). The relative refractive index difference (Δc2) may be 0.25 to 0.55%. The radius of the core part 10 may be not less than 3.0 μm and not more than 7.0 μm. The transmission loss is preferably as low as possible. The transmission loss at a wavelength of 1550 nm is preferably lower than 0.180 dB / km, more preferably 0.175 dB / km or less, and most preferably 0.170 dB / km or less.

The cocoon core portion 10 is preferably silica-based glass containing an alkali metal element such as a halogen element such as chlorine element or fluorine (F) element, potassium element, sodium (Na) element, or rubidium (Rb) element. Concentrations of dopants other than alkali metal elements such as germanium (Ge) elements, typical metal elements such as aluminum (Al) elements, transition metal elements such as nickel (Ni) and copper (Cu), chlorine elements and fluorine elements Is preferably 10 atom ppm or less, more preferably 1 atom ppm or less, and most preferably 0.1 atom ppm or less.

The transmission loss at a wavelength of 1380 nm is preferably lower than 0.8 dB / km, more preferably 0.4 dB / km or less, and most preferably 0.3 dB / km or less. PMD may be 0.2 ps / √km or less. The cable cutoff wavelength is preferably 1520 nm or less, and more preferably 1450 nm or less, which is the pump wavelength used for Raman amplification. The refractive index of the core part 10 and the clad part 20 does not need to be uniform in each region, and may have various refractive index profiles as shown in FIG. There is no limit.
Example 1

In Example 1, an optical fiber preform and an optical fiber were manufactured as follows, and the transmission characteristics of the optical fiber were examined.

1. As a pipe for diffusing an alkali metal element, 970 atomic ppm of chlorine element and 6000 atomic ppm of fluorine element are included as dopants, and the concentration of other elements is 10 atomic ppm or less. Then, a pipe (pure silica glass pipe) that is a glass that does not substantially contain a metal oxide (GeO ₂ , Al ₂ O _3, etc.) for increasing the refractive index is referred to as “pure quartz glass” is prepared. did. This glass pipe had an outer diameter of 35 mm and an inner diameter of about 20 mm.

2. Potassium bromide (KBr) was used as an alkali metal raw material, and this was heated to 840 ° C. by an external heat source to generate potassium bromide vapor. Then, potassium bromide vapor is added together with oxygen at a flow rate of 1 SLM introduced as a carrier gas (1 liter / min in terms of standard state). While being introduced into the pure silica glass pipe, the outer surface of the pure silica glass pipe was heated to 2050 ° C. by a thermal plasma flame as an external heat source. The thermal plasma flame was traversed at a speed of 30 mm / min, heated for a total of 12 turns, and potassium element was diffused and added to the inner surface of the pure silica glass pipe.

3.2. While flowing oxygen (2SLM) into a pure silica glass pipe containing potassium element, the outer surface of the pure silica glass pipe was heated to 2100 ° C. by a thermal plasma flame as an external heat source. The thermal plasma flame was traversed at a speed of 40 mm / min and heated for a total of 6 turns to reduce the diameter of a pure silica glass pipe containing potassium element until the inner diameter became 3 mm.

4.3. While introducing a mixed gas of SF ₆ (0.05 SLM), chlorine (0.5 SLM) and helium (0.5 SLM) into a pure silica glass pipe containing potassium elements and oxygen molecules, the gas phase is heated by an external heat source The inner diameter of the pure silica glass pipe was set to 3.4 mm by etching.

5.4. While introducing oxygen (1SLM) into the pure silica glass pipe, the absolute pressure in the pure silica glass pipe was reduced to 1 kPa, and the surface temperature was solidified to 1400 ° C. by an external heat source, and the outer diameter was 28 mm. A core glass rod containing a metal element was used. The core glass rod had a maximum potassium element concentration of 1800 atomic ppm, and the outer diameter of a region containing 10 atomic ppm or more of potassium element was 12 mm.

6.5. The core glass rod containing the alkali metal element was ground at an outer peripheral portion so as to have an outer diameter of 20 mm to obtain a core glass.

7.6. A silica-based glass (first clad portion) containing elemental fluorine was synthesized outside the core glass to obtain a core glass with an optical clad portion. Relative refractive index difference ((n _a −n _b1 ) / n _SiO2 × 100 between the core part (refractive index: n _a ) and the first cladding part (refractive index: n _b1 ), n _SiO2 : refractive index of pure SiO ₂ ) Was about 0.37% at the maximum. 5. When synthesizing the first cladding part, prepare a silica-based glass pipe containing fluorine element; A known rod-in collapse method was used in which the core glass was inserted and heated and integrated by an external heat source. As a result of the synthesis by the rod-in collapse method, it was possible to suppress the moisture content of the core glass and the first clad portion in the vicinity thereof sufficiently low.

8.7. After processing the core glass with an optical cladding part into a predetermined diameter by stretching or the like, an optical fiber preform having a physical cladding part is obtained by synthesizing a silica-based glass (second cladding part) containing fluorine element on the outside. . The optical fiber preform had an outer diameter of the first cladding part of 36 mm and an outer diameter of the second cladding part of 140 mm. Relative refractive index difference ((n _a −n _b2 ) / n _{SiO 2} × 100 between the core part (refractive index: n _a ) and the second cladding part (refractive index: n _b2 ), n _SiO2 : refractive index of pure SiO ₂ ) Was about 0.38% at the maximum. A known OVD method was used for the synthesis of the second cladding part. Further, in the optical fiber preform at this time, no crystal was observed in the glass.

9.8. An optical fiber was manufactured by drawing the optical fiber preform of the above by a known method. At this time, the processing speed for forming an optical fiber during drawing was 2300 m / min, and the tension applied to the optical fiber was 0.5 N.

The average concentration of potassium element in the core of the optical fiber manufactured as described above was about 2 atomic ppm. Various characteristics are as shown in Table 1, and an optical fiber with low transmission loss was obtained.

(Example 2)

In Example 2, an optical fiber preform and an optical fiber were manufactured as follows, and the transmission characteristics of the optical fiber were examined.

1. As a pipe for diffusing an alkali metal element, a pipe containing 930 atomic ppm of chlorine element and 6000 atomic ppm of fluorine element as a dopant, and the concentration of other elements is 10 atomic ppm or less, and is substantially pure silica glass. Was prepared (pure silica glass pipe). This glass pipe had an outer diameter of 35 mm and an inner diameter of about 20 mm.

2. In the same manner as in Example 1, 1. While being introduced into the pure silica glass pipe, the outer surface of the pure silica glass pipe was heated to 2050 ° C. by a thermal plasma flame as an external heat source. The thermal plasma flame was traversed at a speed of 30 mm / min, heated for a total of 20 turns, and potassium element was diffused and added to the inner surface of the pure silica glass pipe.

3. In the same manner as in Example 1, a pure silica glass pipe containing potassium element was reduced in diameter until the inner diameter became 3 mm.

4. In the same manner as in Example 1, the inner diameter of the pure silica glass pipe was 3.4 mm.

5.4. The pure silica glass pipe was solidified by the same method as in Example 1 to obtain a core glass rod containing an alkali metal element having an outer diameter of 28 mm. The core glass rod had a maximum potassium element concentration of 1140 atomic ppm, and the diameter of the region containing 10 atomic ppm or more of potassium element was 12 mm.

6.5. The core glass rod containing the alkali metal element was stretched using a known method so as to have an outer diameter of 20 mm, and then the outer peripheral portion was ground so that the outer diameter was 12 mm (first core portion).

7.6. Silica-based glass (second core portion) containing a chlorine element having a concentration of 1000 atomic ppm was provided outside the core glass rod containing the alkali metal element so as to have an outer diameter of 65 mm. Then, it extended | stretched using the well-known method so that it might become 24 mm of outer diameters. Furthermore, the outer peripheral portion was ground so that the outer diameter was 20 mm to obtain a core glass. The first core portion and the second core portion are combined to form the core portion of the optical fiber preform. The average concentration of the alkali metal element in the core portion was 8 atomic ppm. For the synthesis of the second core portion, here, a silica-based glass pipe containing a chlorine element having a concentration of 6000 atomic ppm is prepared. A known rod-in collapse method in which a core glass rod containing an alkali metal element is inserted and heated and integrated by an external heat source was used. As a result of the synthesis by the rod in collapse method, the ratio of the diameter (D1) of the first core part to the diameter (D2) of the second core part containing a high concentration of chlorine element: D2 / D1 was 5.8.

8.7. A silica-based glass (first clad portion) containing elemental fluorine was synthesized outside the core glass to obtain a core glass with an optical clad portion. Relative refractive index difference ((n _a2 −n _b1 ) / n _SiO2 between the second core part (n _a2 : refractive index of the second core part) and the first cladding part (n _b1 : refractive index of the first cladding part) × _{100, n SiO2:} refractive index of pure SiO ₂₎ was the maximum of about 0.34%. A known rod-in collapse method was used for the synthesis of the first cladding portion. As a result of the synthesis by the rod-in collapse method, it was possible to suppress the moisture content of the core glass and the first clad portion in the vicinity thereof sufficiently low.

9.8. After the core glass with an optical cladding portion was processed into a predetermined diameter by stretching or the like, silica-based glass (second cladding portion) containing fluorine was synthesized outside thereof to obtain an optical fiber preform further having a physical cladding portion. The optical fiber preform had an outer diameter of the first cladding part of 36 mm and an outer diameter of the second cladding part of 140 mm. Relative refractive index difference ((n _a2 −n _b2 ) / n _SiO ₂ between the second core part (n _a2 : refractive index of the second core part) and the second cladding part (n _b2 : refractive index of the second cladding part) × _{100, n SiO2:} refractive index of pure SiO ²⁾ was the maximum of about 0.30%. A known VAD method was used for the synthesis of the second cladding part. Further, in the optical fiber preform at this time, no crystal was observed in the glass.

10.9. An optical fiber was manufactured by drawing the optical fiber preform of the above by a known method. At this time, the processing speed for forming an optical fiber during drawing was 2300 m / min, and the tension applied to the optical fiber was 0.5 N.

The average concentration of potassium element in the core of the optical fiber manufactured as described above was about 1 atomic ppm. Various characteristics are as shown in Table 1, and an optical fiber with low transmission loss was obtained.

Used as an optical fiber for submarine cables and an optical fiber for land cables.

Claims

An optical fiber comprising a core including a central axis, and a clad surrounding the core and having a refractive index smaller than the refractive index of the core,
The core includes an alkali metal element and a chlorine element,
The average concentration of the alkali metal element in the core is 0.1 atomic ppm or more and 20 atomic ppm or less,
The average concentration of elemental chlorine in the core is 800 atomic ppm or less,
An optical fiber having a transmission loss of 0.185 dB / km or less at a wavelength of 1550 nm.
The optical fiber according to claim 1, wherein an average concentration of chlorine element in the core is 300 atomic ppm or more.
The core further contains elemental fluorine,
The optical fiber according to claim 1 or 2, wherein an average concentration of dopants other than alkali metal elements, chlorine elements and fluorine elements in the core is 10 atomic ppm or less.
The optical fiber according to any one of claims 1 to 3, wherein the core includes a potassium element as an alkali metal element.
To obtain the optical fiber according to any one of claims 1 to 4, further comprising: a core portion including a central axis; and a clad portion surrounding the core portion and having a refractive index smaller than a refractive index of the core. A silica glass optical fiber preform used,
The core part has a first core part and a second core part surrounding the first core part,
The alkali metal element in the core part has a maximum concentration in the first core part among the first core part and the second core part,
The average concentration of the alkali metal element in the second core part is 10 atomic ppm or less,
An optical fiber preform in which an average concentration of chlorine element in the first core portion is smaller than an average concentration of chlorine element in the second core portion.
To obtain the optical fiber according to any one of claims 1 to 4, further comprising: a core portion including a central axis; and a clad portion surrounding the core portion and having a refractive index smaller than a refractive index of the core. A silica glass optical fiber preform used,
The core portion includes an alkali metal element and a chlorine element,
The average concentration of the alkali metal element in the core portion is 5 atomic ppm or more and 100 atomic ppm or less,
An optical fiber preform in which an average concentration of chlorine element in the core portion is less than 1000 atomic ppm.