Patent Description:
Patent Literature <NUM> and Patent Literature <NUM> disclose a method for manufacturing an optical fiber preform by adding an alkali metal to an optical fiber preform whereby attenuation of an optical fiber manufactured from the preform can be reduced. In the method for manufacturing an optical fiber preform described in Patent Literature <NUM>, an alkali metal such as potassium is added to an inner surface of a silica-based glass pipe, and thereafter etching of the inner surface and a collapsing process are performed to manufacture an optical fiber preform. Patent Literature <NUM> is concerned with an optical fiber having a silica-based core comprising an alkali metal oxide a silica-based core, said core comprising an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof in an average concentration in said core between about <NUM> and <NUM> ppm by weight, and a silica-based cladding surrounding and directly adjacent the core, said fiber comprising a cable cutoff less than <NUM> chromatic dispersion at <NUM> between about <NUM> and <NUM> ps/nm/km and a zero dispersion wavelength less than about <NUM>. Patent Literature <NUM> relates to a method for producing an optical fiber preform which includes a collapse step of collapsing a silica-based glass tube by heating with a heat source continuously traversed in the longitudinal direction of the glass tube to form a first glass rod to be formed into a core part or part of a core part of an optical fiber, the glass tube having an inner surface doped with an alkali metal, in which the glass tube has a maximum alkali metal concentration of <NUM> to <NUM>,<NUM> atomic ppm, a maximum chlorine concentration of <NUM> to <NUM> atomic ppm, and a maximum fluorine concentration of <NUM> to <NUM>,<NUM> atomic ppm, and in which in the collapse step, the maximum temperature of the outer surface of the glass tube is <NUM> to <NUM>, and the traverse speed of the heat source is <NUM>/min to <NUM>/min.

The present disclosure provides a method for manufacturing an optical fiber preform according to claim <NUM> including a core part and a cladding part. The method includes: adding an alkali metal to an inner surface of a silica-based glass pipe; etching the inner surface of the silica-based glass pipe to which the alkali metal is added; making a glass rod by collapsing the silica-based glass pipe after the etching; and making an optical fiber preform using the glass rod. The silica-based glass pipe is heated in the adding of the alkali metal such that a surface temperature of the silica-based glass pipe falls within a temperature range of <NUM> or higher to <NUM> or lower.

The present disclosure provides a method for manufacturing an optical fiber according to claim <NUM>. This method is a method for manufacturing an optical fiber comprising the above method for manufacturing an optical fiber preform. The method for manufacturing an optical fiber further includes drawing the optical fiber preform to manufacture an optical fiber.

When an alkali metal is added to a silica-based glass pipe for an optical fiber preform, the added alkali metal may cause transition of glass to a crystalline structure. In the method for manufacturing an optical fiber preform described in Patent Literature <NUM>, an outer surface of a silica-based glass pipe is heated by an oxyhydrogen burner to <NUM> or higher when an alkali metal is added to the silica-based glass pipe. This prevents devitrification of glass due to the crystalline structure. When the heating temperature for the silica-based glass pipe is high, the crystallization of glass can be suppressed but the silica-based glass pipe may be softened and become deformed or non-circular. It is therefore desired that while devitrification of the glass pipe for an optical fiber preform is suppressed, its deformation is also suppressed.

According to the present disclosure, while devitrification of the glass pipe used for an optical fiber preform is suppressed, the deformation of the glass pipe also can be suppressed.

Embodiments of the present disclosure will be described. A method for manufacturing an optical fiber preform according to an embodiment of the present invention is a method for manufacturing an optical fiber preform including a core part and a cladding part. The method includes: adding an alkali metal to an inner surface of a silica-based glass pipe; etching the inner surface of the silica-based glass pipe to which the alkali metal is added; making a glass rod by collapsing the silica-based glass pipe after the etching; and making an optical fiber preform using the glass rod. The silica-based glass pipe is heated in the adding such that the surface temperature of the silica-based glass pipe falls within a temperature range of <NUM> or higher to <NUM> or lower.

In this method for manufacturing an optical fiber preform, when an alkali metal is added to the glass pipe, the silica-based glass pipe is heated by adjusting the surface temperature of the silica-based glass pipe within a temperature range of <NUM> or higher to <NUM> or lower. In this case, according to the study by the inventors of the present invention, it was confirmed that the glass pipe with the addition of an alkali metal is not devitrified and deformation of the glass pipe is suppressed. Thus, the surface temperature of the glass pipe is adjusted in a temperature range of <NUM> or higher to <NUM> or lower when an alkali metal is added, whereby devitrification of the glass pipe used for an optical fiber preform is suppressed and its deformation also can be suppressed, resulting in an optical fiber preform for manufacturing an optical fiber with low attenuation.

In the adding, the heating time per traverse with which the surface temperature of a predetermined area of the silica-based glass pipe falls within a temperature range of <NUM> or higher to <NUM> or lower is <NUM> minute or longer and shorter than <NUM> minutes. In this case, while devitrification of the glass pipe used for an optical fiber preform is suppressed, deformation or becoming non-circular of the glass pipe can be further suppressed. As used herein, "per traverse" refers to one traverse movement in one direction (single path).

In the adding, the silica-based glass pipe is heated by a heating burner such that the surface temperature of the silica-based glass pipe falls within a temperature range of <NUM> or higher to <NUM> or lower, and the width that achieves a temperature zone of <NUM> or higher in the heating temperature profile of the heating burner is kept to be not more than six times an outer diameter of the silica-based glass pipe. In this case, the silica-based glass pipe is heated more locally, so that the addition of an alkali metal to the glass pipe can be performed while deformation of the glass pipe due to the spread of the heating area is further suppressed.

In one aspect of the present embodiment, in the adding, a space in which the silica-based glass pipe is held may be kept at a positive pressure, and the internal pressure in the space may be greater than <NUM> Pa and <NUM> Pa or less. In this case, deformation of the glass pipe by heating can be further suppressed.

In one aspect of the present embodiment, the alkali metal added in the adding may be potassium, and the silica-based glass pipe may be repeatedly heated in the adding such that the ratio d2/d1 in the optical fiber preform is <NUM> or more and less than <NUM>, where d1 is the diameter of an area in which the potassium concentration in the optical fiber preform is <NUM> [atomic ppm] or more, and d2 is the diameter of an area in which the potassium concentration in the optical fiber preform is <NUM> [atomic ppm] or less and the chlorine concentration is <NUM> [atomic ppm] or less. In this case, crystallization of glass can be suppressed while the potassium-added area that can reduce attenuation of the optical fiber is increased.

In another aspect, the present embodiment relates to a method for manufacturing an optical fiber. The method is a method for manufacturing an optical fiber comprising the method for manufacturing an optical fiber preform according to any one or a combination of the aspects described above. The method for manufacturing an optical fiber further includes drawing the optical fiber preform to manufacture an optical fiber. In this case, since an optical fiber is manufactured using the glass rod made from the glass pipe in which devitrification and deformation are suppressed, an optical fiber with lower loss can be obtained.

Specific examples of the method for manufacturing an optical fiber preform according to an embodiment of the present disclosure and the method for manufacturing an optical fiber using the optical fiber preform manufactured by this method will be described below with reference to the drawings. The present invention is not limited by such examples but is defined by the appended claims. In the following description, the same elements in a description of the drawings are denoted by the same reference signs and an overlapping description will be omitted.

<FIG> is a cross-sectional view of an optical fiber preform manufactured by the method for manufacturing an optical fiber preform according to the present embodiment. An optical fiber preform <NUM> is formed of a silica-based glass and includes a core part <NUM> and a cladding part <NUM> surrounding the core part <NUM>. The refractive index of the core part <NUM> is higher than the refractive index of the cladding part <NUM>. The core part <NUM> has a first core part <NUM> and a second core part <NUM> surrounding the first core part <NUM>. The cladding part <NUM> has a first cladding part <NUM> surrounding the core part <NUM> and a second cladding part <NUM> surrounding the first cladding part <NUM>. An alkali metal (for example, potassium) is added to the first core part <NUM> by the manufacturing method described later.

<FIG> is a flowchart for explaining the method for manufacturing an optical fiber preform according to the present embodiment. In the method for manufacturing an optical fiber preform, as illustrated in <FIG>, a preparation step S1, an addition step S2, a diameter-reducing step S3, an etching step S4, a collapsing step S5, a first stretching and grinding step S6, a first rod-in collapse step S7, a second stretching and grinding step S8, a second rod-in collapse step S9, and an OVD step S10 are performed in order, to manufacture the optical fiber preform <NUM> illustrated in <FIG>. <FIG> is a schematic diagram illustrating the process in the addition step S2 in the method for manufacturing an optical fiber preform.

In the preparation step S1, a silica-based glass pipe <NUM> (see <FIG>) into which an alkali metal element is to be diffused is first prepared. The silica-based glass pipe <NUM> is a pipe that contains, for example, a predetermined amount of chlorine (Cl) and fluorine (F) and in which the concentrations of other dopants and impurities are kept to a predetermined amount or lower. The silica-based glass pipe <NUM>, for example, has an outer diameter of <NUM> and an inner diameter of <NUM>.

Subsequently, in the addition step S2, an alkali metal is added to the inner peripheral surface of the silica-based glass pipe <NUM>. For example, potassium (K), sodium (Na), rubidium (Rb), or cesium (Cs) can be used as the alkali metal added in the addition step. For example, when potassium bromide (KBr) is used as alkali metal material <NUM>, as illustrated in <FIG>, potassium bromide is heated by an external heat source <NUM> to generate KBr vapor. Then, while KBr vapor is introduced together with the supplied carrier gas to the inner peripheral of the silica-based glass pipe <NUM>, the outer surface of the silica-based glass pipe <NUM> is heated by a heating burner <NUM>. In this heating process, the heating burner <NUM> is reciprocatively traversed multiple times (for example <NUM> turns) in the direction of the arrow in the drawing at a predetermined speed (for example <NUM>/min) to diffusively add the potassium metal into an inner surface 31a of the silica-based glass pipe <NUM>. In this case, for example, when KBr vapor is introduced to the inside of the silica-based glass pipe <NUM> together with a carrier gas in which oxygen is introduced at a flow rate of <NUM> SLM (<NUM> liter/min in terms of a normal condition), the maximum value of potassium concentration of the silica-based glass pipe <NUM> to which the alkali metal is added can be set to <NUM> [atomic ppm].

In the heating process by the heating burner <NUM> in the addition step in the present embodiment, heating is performed by adjusting the heating burner <NUM> such that the surface temperature of the glass pipe <NUM> is <NUM> or higher and <NUM> or lower. In other words, the heating process is performed such that the surface temperature of the glass pipe <NUM> does not exceed <NUM>. The heating burner <NUM> has a preset prescribed temperature profile (see <FIG>) and is adjusted such that the width D in which the heating temperature is <NUM> or higher is not increased. In the heating burner <NUM>, for example, the width D in the temperature profile is kept to be not more than six times the diameter of the glass pipe <NUM> heated. In the addition step, in addition to limiting the range of heating temperature, heating is performed by adjusting the traverse speed of the heating burner <NUM> such that the heating time (burner heating time) in each area of the glass pipe <NUM> by the heating burner <NUM> is within a predetermined range of <NUM> minute or longer and shorter than <NUM> minutes. That is, the traverse speed is adjusted such that the heating time per traverse with which the surface temperature of a certain area of the silica-based glass pipe <NUM> falls within a temperature range of <NUM> or higher to <NUM> or lower is <NUM> minute or longer and shorter than <NUM> minutes. As used herein, "per traverse" refers to one traverse movement in one direction (single path).

In the addition step in the present embodiment, the heating temperature and the heating time by the heating burner <NUM> are adjusted to a predetermined range, and then an alkali metal is diffusively added to the inner surface 31a of the silica-based glass pipe <NUM>. The adjustment of heating in this manner suppresses devitrification due to crystallization and thermal deformation of the glass pipe <NUM>. For example, an oxyhydrogen burner can be used as the heating burner <NUM>. The addition step may be performed in an environment in which a space in which the silica-based glass pipe <NUM> is held is kept at a positive pressure and the internal pressure of the space is greater than <NUM> Pa and <NUM> Pa or less.

Subsequently, in the diameter-reducing step S3, after the supply of an alkali metal such as KBr vapor used in the addition step S2 is stopped, the diameter of the silica-based glass pipe <NUM> to which the alkali metal is added is reduced. In doing so, while oxygen (for example, a flow rate of <NUM> SLM) is introduced to the inside of the silica-based glass pipe <NUM>, the silica-based glass pipe <NUM> is heated by an external heat source such that the outer surface of the silica-based glass pipe <NUM> attains <NUM> to <NUM>. Heating is performed by traversing the heating burner <NUM>, for example, about <NUM> turns, and the diameter of the silica-based glass pipe <NUM> to which the alkali metal is added is reduced until the inner diameter attains <NUM>.

Subsequently, in the etching step S4, the inner peripheral surface of the silica-based glass pipe having its diameter reduced is etched. In the etching step, while a gas mixture of SF<NUM> (for example, a flow rate of <NUM> SLM) and chlorine (for example, a flow rate of <NUM> SLM) is introduced to the inside of the silica-based glass pipe having its diameter reduced, the silica-based glass pipe is heated by an external heat source to perform a vapor phase etching of the inner peripheral surface. In this process, the inner peripheral surface of the glass pipe is etched at a thickness of about <NUM> to <NUM> to remove the pipe inner surface containing a high concentration of impurities added together with the alkali metal in the addition step. The impurities are thus removed from the glass pipe.

Subsequently, in the collapsing step S5, the silica-based glass pipe having its diameter reduced and having the etched inner peripheral surface is collapsed. In the collapsing step, the silica-based glass pipe is collapsed by reducing the absolute pressure inside the silica-based glass pipe to <NUM> kPa or lower while introducing a gas mixture of oxygen (for example, a flow rate of <NUM> SLM) and helium (for example, a flow rate of <NUM> SLM) to the inside of the silica-based glass pipe, and bringing the surface temperature of the silica-based glass pipe to <NUM> to <NUM> using an external heat source. This collapsing step results in a first glass rod (for example, an outer diameter of <NUM>) of a transparent silica-based glass containing an alkali metal. The alkali metal is diffusively added in the first glass rod.

Subsequently, in the first stretching and grinding step S6, the first glass rod obtained by collapsing is stretched, for example, to a diameter of <NUM>, and the outer peripheral portion is further ground to a diameter of <NUM>, resulting in the first core part <NUM>. In doing so, as illustrated in <FIG>, when the diameter in which the K concentration is <NUM> [atomic ppm] or more is d1, and the outer diameter in which the K concentration is <NUM> [atomic ppm] or less and the Cl concentration is <NUM> [atomic ppm] or less is d2, the ratio of d2/d1 in the first core part <NUM> can be set between <NUM> or more and less than <NUM>.

Subsequently, in the first rod-in collapse step S7, the second core part <NUM> is provided on the outside of the first core part <NUM>, resulting in a second glass rod. In step S7, the second glass rod is manufactured by a rod-in collapse method in which the first core part <NUM> is inserted to the inside of a silica-based glass pipe (second core part <NUM>) having an outer diameter of <NUM> and in which a predetermined amount of chlorine atoms is added, and they are heated to be integrated by an external heat source.

Subsequently, in the second stretching and grinding step S8, the second glass rod is stretched to a diameter of <NUM>, and the outer peripheral portion is further ground to a diameter of <NUM>. A combination of the first core part <NUM> and the second core part <NUM> is the core part <NUM>. When the diameter of the core part <NUM> is d3, the ratio of d3/d1 in the core part <NUM> can be set to <NUM> to <NUM>.

Subsequently, in the second rod-in collapse step S9, the first cladding part <NUM> is provided on the core part <NUM>. In this step, a rod-in collapse method is used in which the core part <NUM> is inserted to the inside of a silica-based glass pipe (corresponding to the first cladding part <NUM>) in which a predetermined amount of fluoride is added, and they are heated to be integrated by an external heat source. The relative ratio refractive index difference between the second core part <NUM> and the first cladding part <NUM> is, for example, at most about <NUM>%. As a result of the synthesis by this rod-in collapse method, the moisture in the core part <NUM> and the neighboring first cladding part <NUM> can be suppressed to be sufficiently low.

Subsequently, in the OVD step S10, the glass rod formed by integrating the core part <NUM> and the first cladding part <NUM> is stretched to a predetermined diameter, and thereafter the second cladding part <NUM> containing fluoride is synthesized on the outside of the glass rod by the OVD method to manufacture the optical fiber preform <NUM>. In the resultant optical fiber preform <NUM>, for example, the outer diameter of the first cladding part <NUM> is <NUM>, and the outer diameter of the second cladding part <NUM> is <NUM>. The relative ratio refractive index difference between the second core part <NUM> and the second cladding part <NUM> is at most about <NUM>%. The concentration of OH group on the outside of the first cladding part <NUM> can be measured using infrared absorption spectroscopy and is about <NUM> [mol ppm].

In the subsequent drawing step, the optical fiber preform <NUM> manufactured by the manufacturing method as described above is drawn to obtain an optical fiber. The drawing speed is, for example, <NUM>/min, and the drawing tension can be <NUM> N. As described above, an optical fiber with low attenuation by addition of an alkali metal, and an optical fiber preform for the optical fiber can be manufactured.

The degree of crystallization (devitrification) and deformation (becoming non-circular) of the optical fiber preform manufactured by the manufacturing method described above will now be described. As described above, in the method for manufacturing an optical fiber preform according to the present embodiment, in the addition step S2 of adding an alkali metal, the heating temperature in adding an alkali metal such as potassium to the silica-based glass pipe <NUM> is adjusted to fall within a range of <NUM> or higher to <NUM> or lower. That is, the heating process is adjusted such that the surface temperature of the silica-based glass pipe <NUM> quickly reaches <NUM> and does not exceed <NUM>. In this way, the glass temperature of the surface of the silica-based glass pipe is adjusted not to low temperatures (<NUM> or higher to lower than <NUM>) but to high temperatures (<NUM> or higher), so that formation or growth of crystalline nuclei of silica (SiO<NUM>) forming the glass pipe is not promoted, thereby preventing the crystalline structure from being kept. With such adjustment, even when glass is easily crystallized due to the alkali metal added to reduce the attenuation of the optical fiber, devitrification of the silica-based glass pipe used for manufacturing an optical fiber preform can be suppressed.

Diffusion and penetration of an alkali metal in the glass proceed by keeping a high surface temperature of the glass pipe to which the alkali metal is added. However, it has been found that when the glass heating temperature is further higher (<NUM> or higher), the fiber preform itself is deformed or becomes non-circular due to heat. Then, in the present embodiment, the upper limit of the heating temperature in the addition step is defined to <NUM> or lower. This can suppress heating of the glass pipe to a certain degree when an alkali metal is added to the silica-based glass pipe and can also suppress deformation or becoming non-circular of the optical fiber preform. The glass heating temperature is adjusted to <NUM> or lower as in the method for manufacturing an optical fiber preform according to the present embodiment, whereby deformation or becoming non-circular of the optical fiber preform can be suppressed, and an optical fiber manufactured using such an optical fiber preform achieves low attenuation.

In the method for manufacturing an optical fiber preform according to the present embodiment, in the addition step, the heating time per traverse with which the surface temperature of a predetermined area of the silica-based glass pipe <NUM> falls within a temperature range of <NUM> or higher to <NUM> or lower is set to <NUM> minute or longer to shorter than <NUM> minutes. In this case, while devitrification of the glass pipe for an optical fiber preform is suppressed, deformation or becoming non-circular of the glass pipe can be further suppressed.

In the method for manufacturing an optical fiber preform according to the present embodiment, in the addition step, the silica-based glass pipe <NUM> is heated by the heating burner <NUM> such that the surface temperature of the silica-based glass pipe <NUM> falls within a temperature range of <NUM> or higher to <NUM> or lower, and the width D that achieves a temperature zone of <NUM> or higher in the heating temperature profile (see <FIG>) of the heating burner <NUM> is kept to be not more than six times the diameter of the silica-based glass pipe <NUM>. In this case, the silica-based glass pipe <NUM> is heated more locally, so that the addition of an alkali metal to the glass pipe can be performed while deformation of the glass pipe due to the spread of heating is further suppressed.

In the method for manufacturing an optical fiber preform according to the present embodiment, in the addition step, an space in which the silica-based glass pipe <NUM> is held may be kept at a positive pressure, and the internal pressure of the space may be greater than <NUM> Pa and <NUM> Pa or less. In such a pressure state, the deformation of the glass pipe by heating can be further suppressed.

An optical fiber preform was manufactured based on the manufacturing method described above, and the presence/absence of devitrification of the glass pipe and the presence/absence of deformation of the glass pipe were evaluated. The prepared silica-based glass pipe had an outer diameter of <NUM> and an inner diameter of <NUM>. Potassium bromide (KBr) as an alkali metal for adding to the glass pipe was heated by an external heat source to <NUM> to generate KBr vapor. Then, while KBr vapor was introduced into the silica-based glass pipe <NUM> together with a carrier gas in which oxygen was introduced at a flow rate of <NUM> SLM, heating was performed from the outside by an oxyhydrogen burner (heating burner <NUM>) traversing such that the surface of the silica-based glass pipe <NUM> reached a range of <NUM> or higher to <NUM> or lower. The heating time by the oxyhydrogen burner (burner heating time) is a value obtained by dividing the burner heating range by a traverse speed and means a heating time during which a certain point (area) in the glass pipe is heated to <NUM> or higher. The temperature profile (see <FIG>) of the burner was set such that the heating range (the width D illustrated in <FIG>) by the oxyhydrogen burner was <NUM> (the range of temperatures of <NUM> or higher). The number of turns (the number of reciprocations) of traverse by the oxyhydrogen burner is set to <NUM>.

Then, as illustrated below, the presence/absence of devitrification and the presence/absence of deformation of the glass pipe were evaluated by adjusting the traverse speed of the oxyhydrogen burner to change the burner heating time stepwise from <NUM> minute to <NUM> minutes. The evaluation result is illustrated in Table <NUM> below.

In the presence/absence of devitrification in Table <NUM>, "A" indicates a state in which no devitrification occurs and "B" indicates a state in which devitrification partially occurs. "A" in the presence/absence of deformation indicates a state in which the deformation of the glass pipe (an effective length of <NUM>) is <NUM> or less, and "B" in the presence/absence of deformation indicates a state in which the deformation of the glass pipe is <NUM> or more and <NUM> or less.

As is clear from Example above, it was confirmed that, when the surface temperature of the glass pipe is set to <NUM> or higher to <NUM> or lower and an alkali metal is added to the glass pipe, devitrification due to crystallization of the glass pipe can be suppressed and deformation due to heating of the glass pipe can be suppressed. In addition, the time during which the surface temperature of the glass pipe is set to <NUM> or higher to <NUM> or lower is set to <NUM> minute or longer and shorter than <NUM> minutes when an alkali metal is added to the glass pipe, whereby thermal deformation of the optical fiber preform can be further suppressed.

A method for manufacturing an optical fiber preform according to the present embodiment and a method for manufacturing an optical fiber using an optical fiber preform manufactured by this method have been described above. However, the present invention is not limited thereto and a variety of modifications can be applied. For example, in the addition step S2 in the above method for manufacturing an optical fiber preform, the glass pipe <NUM> is heated by the heating burner <NUM> with only both ends thereof supported. However, in order to further suppress deformation of the glass pipe <NUM>, for example, as illustrated in <FIG>, two rotatable support rollers <NUM> having an axis parallel to the axis of the glass pipe <NUM> may be provided under the glass pipe <NUM> to support the glass pipe <NUM>. In this case, deformation of the glass pipe <NUM> by heat can be further suppressed. In this case, the rotatable support rollers <NUM> may be disposed immediately outside of the range in which the heating burner is traversed, or the rotatable support rollers <NUM> may be disposed at a certain distance from the heating burner, and the rotatable support rollers <NUM> and the heating burner may be traversed simultaneously.

Claim 1:
A method for manufacturing an optical fiber preform (<NUM>) including a core part (<NUM>, <NUM>, <NUM>) and a cladding part (<NUM>, <NUM>, <NUM>), comprising:
adding an alkali metal (<NUM>) to an inner surface of a silica-based glass pipe (<NUM>);
etching the inner surface of the silica-based glass pipe (<NUM>) to which the alkali metal (<NUM>) is added;
making a glass rod by collapsing the silica-based glass pipe (<NUM>) after the etching; and
making an optical fiber preform (<NUM>) using the glass rod,
wherein the silica-based glass pipe (<NUM>) is heated in the adding such that a surface temperature of the silica-based glass pipe (<NUM>) falls within a temperature range of <NUM> or higher to <NUM> or lower, wherein the heating time per traverse with which a surface temperature of a predetermined area of the silica-based glass pipe (<NUM>) falls within a temperature range of <NUM> or higher to <NUM> or lower is <NUM> minute or longer and shorter than <NUM> minutes;
wherein in the adding,
the silica-based glass pipe (<NUM>) is heated by a heating burner (<NUM>) such that the surface temperature of the silica-based glass pipe (<NUM>) falls within a temperature range of <NUM> or higher to lower than <NUM>, and
the width (D) that achieves a temperature zone of <NUM> or higher in the heating temperature profile of the heating burner (<NUM>) is kept to be not more than six times a diameter of the silica-based glass pipe (<NUM>).