JPH10231135A - Production of optical fiber base material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the loss of a finally produced optical fiber even of a special glass by enabling the easier and surer integration of a core rod with split clad rods. SOLUTION: A concave groove is formed on one of a pair of half clad rods in the longitudinal direction over all length, a core rod is imbedded in the groove, and the half clad rod is covered with the other half clad rod to form a compound base material 3. Each of a pair of dies 4, 4 has a half circular embedding groove 41 on the facing side and a buffer layer 42 of a specified thickness having elasticity is formed on the surface of the groove. The compound base material is placed between the dies 4, 4 and they are heated under the application of stress directing the center along a diameter on the compound base material to surely integrate them. The whole part including the dies is placed in a vacuum container to dispense with troublesome controls of pressure difference and atmosphere. The integration may be performed by heating the compound base material in a state where it is placed in a tube having thermal shrinking to impart stress directing the center along a diameter or by applying drawing force on the compound base material inserted into a muffle tube to act a diameter-reducing force on it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical fiber preform having a core / cladding structure, and more particularly, to combining a glass rod having a core composition and two or more divided glass rods having a cladding composition in a predetermined arrangement. The present invention relates to a method for manufacturing an optical fiber preform by integrating the optical fiber preform.

[0002]

2. Description of the Related Art Conventionally, as a method of manufacturing an optical fiber preform, a periphery of a core rod formed of glass for a core is formed by:
Surrounded by two or more divided clad rods for clad formation formed by cladding glass, and thus a combination of a core rod and two or more divided clad rods is integrated by heating under reduced pressure, thereby A method of manufacturing an optical fiber preform having a core / clad structure is known (Japanese Patent Laid-Open No. 8-2955).
No. 24).

In addition, a fluoride optical fiber preform manufactured by integrating a core rod and a clad forming pipe formed separately from each other by heating is subjected to strain removal in a temperature range equal to or lower than the crystallization start temperature of glass. It is known (see JP-A-6-183776).

[0004]

However, the above-mentioned conventional method for manufacturing an optical fiber preform has the following problems, particularly in the integration step.

In order to effectively perform the integration (collapse), the inner space, which is the interface between the core rod and each of the divided clad rods, has a lower pressure than the outer space of each of the divided clad rods. It is necessary to reduce the pressure. However, it is difficult to control the pressure adjustment to accurately realize such a pressure difference. That is, if the pressure difference is too large, each of the divided clad rods collapses at a stretch, while if the pressure difference is too small, integration takes too much time. In addition, the above pressure difference needs to be kept constant, but between the divided clad rods and between the divided clad rods and the core rod, unless the processing accuracy of the interface between them is considerably high, the pressure difference between them is not increased. The atmosphere in the outer peripheral space enters the inner peripheral space through the gap, and it is difficult to maintain a constant pressure.

When the optical fiber preform to be manufactured is a special glass such as a fluoride glass, it is necessary to control the atmosphere without fail in order to avoid crystallization of the glass. Oxygen (O2
), The O2 and the glass component may combine to change the composition of the glass surface. In addition, if the dew point control of the inert gas used for controlling the atmosphere is insufficient, the amount of water in the atmosphere increases, and the water and the glass may react with each other to form hydrates and generate crystal nuclei. is there.

[0007] As a result, there is a possibility that the integration of each divided clad rod and the core rod may be incomplete, or in the case of special glass, crystals may be generated due to insufficient atmosphere control. As a result, bubbles or the like may be generated at the core-cladding interface of the optical fiber obtained by drawing the manufactured optical fiber preform, which may cause an increase in the transmission loss of the optical fiber. On the other hand, if the above-mentioned pressure adjustment control and atmosphere control are completely performed, the manufacturing apparatus becomes complicated.

The present invention has been made in view of such circumstances, and an object of the present invention is to make it possible to integrate a core rod and a split clad rod more easily and surely. Another object of the present invention is to reduce the loss of an optical fiber finally manufactured even with special glass.

[0009]

Means for Solving the Problems In order to achieve the above object, the invention according to claim 1 comprises forming a plurality of clad rods for forming a clad, and forming a plurality of core rods by core glass; A method of manufacturing an optical fiber preform in which after the clad rods are arranged around the outer periphery of the core rod so that the core rod is located at the center and combined, the combined preforms in the combined state are integrated with each other to form an optical fiber preform. Is assumed. In this, the combined base material is placed under a vacuum state, and the combined base material is heated while mechanically applying a radial stress from the outer peripheral side toward the core rod side. It is configured to perform an integration step.

In the case of the above configuration, in the integration step,
Because heating is performed while mechanically applying a radial stress from the outer peripheral side of the combined base material toward the core rod side, a plurality of outer clad rods are securely welded to the inner peripheral core rod. It is integrated, and there is no need to perform complicated pressure adjustment control of the inner and outer spaces for the integration. In addition, since the entire preform is placed under a vacuum, it is possible to avoid deterioration and crystallization of the glass surface due to moisture and O2 in the atmosphere even when special glass is used. There is no need to perform troublesome atmosphere control. Therefore, it is possible to easily and surely integrate the combined preforms, and to reduce the loss of the optical fiber drawn from the obtained optical fiber preform.

According to a second aspect of the present invention, there is provided a core rod forming step of forming a core rod having a uniform cross-sectional shape in the longitudinal direction from glass for a core, and separately from the core rod forming step, one core rod having a uniform cross-sectional shape in the longitudinal direction. A cladding rod forming step of forming a cladding rod with a glass for cladding, a dividing step of forming a pair of half-split clad rods by cutting the formed clad rod in a diametric direction, A groove processing step of forming a concave groove extending over substantially the entire length in the longitudinal direction at a position corresponding to the center of the cross section of the clad rod on at least one of the cut surfaces. Then, after the pair of half-split clad rods are overlapped with each other at the cut surfaces thereof in a state where the core rod is fitted into the formed groove, the combined base material in the combined state is placed in a vacuum state. The pair of half clad rods and the core rod are integrated with each other by heating while mechanically applying a radial stress from the outer peripheral side toward the core rod side to the combined base material. It is configured to perform a process.

In the case of the above construction, similarly to the first aspect of the invention, in the integration step, the combined base material is heated while mechanically applying a radial stress from the outer peripheral side to the core rod side. Therefore, the plurality of clad rods on the outer peripheral side are securely welded to and integrated with the core rod on the inner peripheral side, and it is not necessary to perform complicated pressure adjustment control of the inner and outer spaces for the integration. In addition, since the entire preform is placed under a vacuum, it is possible to avoid deterioration and crystallization of the glass surface due to moisture and O2 in the atmosphere even when special glass is used. ,
There is no need to perform troublesome atmosphere control. Therefore, it is possible to easily and surely integrate the combined preforms, and to reduce the loss of the optical fiber drawn from the obtained optical fiber preform. Only the core rod formed by the core rod forming step and one clad rod formed by the clad rod forming step are required as materials, and the combination of the core rod and the pair of half-split clad rods is also required on the half-split clad rod side. Since the positioning can be performed only by fitting the core rod into the concave groove, the steps up to the preparation of the combined base material used in the integration step are simplified.

The invention according to any one of claims 3 to 5 specifies a specific method of applying the radial stress in the invention according to claim 1 or 2, and the invention according to claim 3 According to the invention, radial stress is applied by sandwiching the outer peripheral surface of the combined base material from a side facing each other by a pair of pressing dies. In the case of this configuration, it is possible to reliably apply the radial stress to the combined base material from the outer peripheral side by the pressing die.

According to a fourth aspect of the present invention, a radial stress is applied to the outer circumference of the combined base material by heating the outer circumference of the combined base material in a state where a tube whose outer circumferential length is contracted is extrapolated. is there. In the case of this configuration, when heating is started in a state where the tube is extrapolated, the tube shrinks and the outer peripheral length gradually decreases. Is applied in the radial direction.

According to a fifth aspect of the present invention, a radial stress is applied by applying a stretching force to the combined base material in the longitudinal direction. In the case of this configuration, when the combined base material is heated while applying a stretching force in the longitudinal direction, the diameter of the combined base material is reduced as the combined base material is softened and stretched, thereby applying a radial stress toward the core rod side. Will be done.

According to a sixth aspect of the present invention, in the first or second aspect, after the integration step,
The integrated optical fiber preform is subjected to a heat treatment in a temperature range equal to or lower than a crystallization start temperature in a vacuum state, thereby performing a strain removing step of removing strain.

In the case of the above structure, by performing the strain removing step, when the surface is processed before drawing the optical fiber due to the stress applied by the heating in the integration step, the surface of the It is possible to avoid the risk that cracks and cracks are likely to occur during processing and the possibility that deformation is likely to occur during reheating during drawing, and low loss when finally drawn And a stable optical fiber can be obtained. Moreover, by specifying the temperature range during the heat treatment, crystallization due to the heat treatment for strain removal is prevented even when special glass is used.

Further, the invention according to claim 7 is the invention according to claim 2.
In the invention described, the cutting position of the clad rod in the dividing step is shifted from the center point of the clad rod by a dimension corresponding to the radius of the core rod, and the formation of the concave groove in the groove processing step is performed by a pair of half-clad clad rods. Only one of them.

In the case of the above configuration, the work of combining the core rod and the pair of half clad rods is completed only by fitting the other half clad rod after fitting the core rod into the concave groove, and moving the core rod to the center position. The combined base material in the positioned state can be obtained reliably and easily. Moreover, compared with the case where the clad rod is cut at a diameter passing through the center point in the dividing step, and the groove is formed in each of the two half-clad rods in the groove processing step, and the core rod is sandwiched between the two concave grooves, In this case, it is possible to reduce the risk of breakage caused by the core rod and each half clad rod rubbing each other.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a method for manufacturing an optical fiber preform according to a first embodiment of the present invention. The method for manufacturing the optical fiber preform includes a clad rod forming step P1, a dividing step P2, a groove processing step P3, and polishing / polishing for the clad rod.
While performing the etching step P4, each of these steps P1
Apart from P4, a core rod forming step P5, a drawing / stretching step P6, and an etching step P7 are performed on the core rod, and a pair of half-split rods and a reduced diameter core rod are combined to form a combined base material described later. Thereafter, the dewatering step P8, the integrating step P9, the strain removing step P10, and the resurfacing step P11 are performed on the combined preform, thereby completing the production of the optical fiber preform for drawing the optical fiber. Then, a drawing step P12 is performed using this optical fiber preform to obtain an optical fiber. Hereinafter, each of the steps P1 to P11 will be described in detail.

(Clad rod forming step P1-polishing / etching step P4) First, the clad rod forming step P1
Then, a clad rod 1 having a predetermined diameter (for example, a diameter of 10 mm) having a uniform circular cross section in the longitudinal direction is formed using glass having a clad composition. The clad rod 1 may be formed by, for example, a casting method in which a cylindrical mold having a circular cross section is used, and a glass melt having a clad composition is cast in the mold and gradually cooled.

In the dividing step P2, as shown in FIG. 2, the clad rod 1 is cut at the position of the cutting line CL in the diameter direction to form a pair of half clad rods 11 and 12.
The cutting line CL is parallel to one of the orthogonal lines X and Y intersecting the center line C of the cross section, and is made to have a diameter-reduced core rod (for example, having a diameter d = 300 μm) after processing by an etching process P7 described later. ) Is set at a position shifted by a dimension corresponding to the radius of () (150 μm in the above example). Thus, the first half clad rod 11 whose dimension R1 along the other orthogonal line Y is longer than the radius R of the clad rod 1 by d / 2, and the dimension R2 along the orthogonal line Y is d /
It is divided into a second half clad rod 12 which is shorter by two. Here, the relationship between R1 and R2 is R1−d = R
It becomes 2.

In the groove processing step P3, a concave groove 112 having a predetermined dimension is formed on the cut surface 111 of the first half clad rod 11 so as to pass through the center line C over the entire length in the longitudinal direction. As shown in FIG. 4, the dimensions of the groove 112 are such that the core rod 21 having the diameter d described above is just inserted into the groove width × depth (d × d), and a square groove is formed. . The concave groove 112 may be formed by using a high-speed rotating disk grindstone such as a precision processing machine for forming an optical fiber waveguide. In this case, the thickness of the disk grindstone may be set to a value corresponding to the groove width. The shape of the concave groove 112 is not limited to the above square shape, and may be any shape as long as the diameter-reduced core rod 21 having the diameter d just fits therein. For example, a U-shape as shown in FIG. V like the bottom
It may be in the shape of a letter.

Then, in the polishing / etching step P4,
The cut surface 111 and the inner surface of the groove 112 of the first half clad rod 11 and the second half clad rod 12
First, polishing is performed as a mechanical surface treatment, and then etching is performed as a chemical surface treatment. Etching is, for example, 1 m
It may be carried out by dipping in an ol / l hydrochloric acid (HCl) solution.

(Core rod forming step P5 to etching step P7) On the other hand, in the core rod forming step P5, a core rod 2 having a predetermined diameter (for example, a diameter of 10 mm) having a uniform circular cross section in the longitudinal direction is formed using glass having a core composition. . The core rod 2 may be formed by a casting method, for example, as in the case of the clad rod forming step P1 described above.

In the drawing and drawing step P6, the core rod 2 is drawn to a predetermined diameter (for example, 400 μm to 500 μm in diameter) by drawing using, for example, a spinning furnace to form a core rod 21 having a reduced diameter.

Then, in the etching step P7, an etching process is performed on the above-mentioned core rod 21 having a reduced diameter, just in case. This etching treatment may be performed, for example, by immersing in a 1 mol / l hydrochloric acid (HCl) solution as described above.
The core rod 2 is made of a fluoride glass containing ZrF4 and BaF2 as main components, in particular, ZrF4-BaF2-LaF3-Al doped with Pr (praseodymium).
When it is made of F3-NaF (ZBLAN) -based fluoride glass or fluorophosphoric acid-based glass, it is, for example, 0.1 mm.
It may be immersed in a mixed solution of 4 mol / l ZrOCl2 and 1 mol / l HCl. By this etching process, the diameter-reduced core rod 21 is reduced to a diameter d (300 μm) that matches the size of the concave groove 112.
When this step P7 is omitted, the drawing may be performed so that the diameter becomes d in the drawing and drawing step P6.

(Dehydration Step P8) Next, the diameter-reduced core rod 21 after the etching step P7 and the pair of half clad rods 11, 1 after the polishing and etching step P4.
2 is dewatered by vacuuming or using chlorine gas to remove moisture adhering to the surface. Thereafter, as shown in FIG. 3, the core rod 21 having a reduced diameter is fitted into the concave groove 112 of the first half clad rod 11, and the cut surface 121 of the second half clad rod 12 is superposed thereon. combine. The combined preform 3 serving as the original form of the optical fiber preform held in this combined state is integrated in the next integrating step P9.

(Integration Step P9) In the integration step P9,
The whole of the combined base material 3 is placed under a predetermined vacuum state, and heated while applying a radial stress from the outer peripheral side toward the diameter-reduced core rod 21 to the combined base material, thereby forming a pair of halves. The clad rods 11, 12 and the core rod 21 are integrated with each other. As the above-mentioned vacuum state, it is preferable that the ultimate degree of vacuum is 10 ⁻² Torr or less, and it is more preferable that the degree of vacuum is about 10 ⁻³ to 10 ⁻⁴ Torr.

There are the following first to third methods for applying the above-mentioned radial stress, and any one of the methods may be adopted.

The first method is to integrate the combination base material 3 by using a pair of pressing dies 4 and 4 as shown in FIG. 7, which is the most preferable method. This press mold 4,4
Are semi-circular concave fitting grooves 4 formed of a good heat conductor such as metal or ceramics on the mutually facing surfaces.
1 and 41 are formed, and have fixing means for holding the two pressing dies 4 and 4 pressed against the opposing sides. A buffer layer 42 made of a material having elasticity (cushioning property) such as silicone rubber or Teflon (trade name: polytetrafluoroethylene) and having heat resistance is disposed on the surface of each fitting groove 41. The combined base material 3 is sandwiched between a pair of press forms 4 and 4 via the buffer layers 42 and 42. As shown in FIG. 8, the inner diameters D2 formed by the inner peripheral surfaces of the two buffer layers 42, 42 in a state where the two pressing dies 4, 4 are pressed by the fixing means as shown in FIG. Each buffer layer 42 has a predetermined dimension smaller than the outer diameter D1 formed by the outer peripheral surface of the base material 3.
Is set.

In the case of the first method, the whole combined base material 3 is heated in a vacuum vessel (not shown) in a state where the combined base material 3 is sandwiched and fixed between the buffer layers 42, 42. Thus, the combined base material 3 is heated while receiving the restoring force from the contracted buffer layers 42, 42 inward in the radial direction, and the outer peripheral surface of the core rod 21 having a reduced diameter and the clad rods 11, 12 are divided. The cut surfaces 111 and 121 of the two half clad rods 11 and 12 are adhered and integrated.

The second method uses a heat-shrinkable tube 5 as shown in FIG. The tube 5 is formed of, for example, the above-described material such as Teflon, and is formed so as to contract mainly in the circumferential direction when heated to a predetermined temperature or higher. The shrinkage start temperature is determined by the temperature characteristics of the glass material of the combination base material 3. As shown below, the glass transition temperature Tg of the glass material is higher than the shrinkage start temperature of the tube 5 and the shrinkage force of the tube 5 May be determined to be lower than the use limit temperature at which the temperature is lost.

Tube shrinkage start temperature <Tg <Tube use limit temperature For example, when Teflon (FEP) is used, the shrinkage start temperature is 180 ° C and the use limit temperature is 320 to 330 ° C.
Becomes

Then, in the case of the second method, the tubes 5 are extrapolated to the combined base material 3 and they are heated in a vacuum vessel. The heating may be any of zone heating in which the heater is moved relative to the outside of the vacuum vessel for each part, and whole heating in which the entire body is heated simultaneously. Due to this heating, the tube 5 shrinks and is softened while receiving a radial stress by the shrinking force, so that the combined preform 3 is integrated.

In the third method, as shown in FIG. 10, the combined base material 3 is passed through the muffle tube 6, and the upper end side of the combined base material 3 is held by, for example, a cap 7 made of silicone. Then, the lower end of the combined base material 3 is zone-heated by the annular heater 8 while being stretched by a stretching jig (not shown). In this case, the combined base material 3 is stretched at the stretching speed V2 while the entire muffle tube 6 is relatively moved downward at the feeding speed V1 with respect to the annular heater 8, and the stretching speed V2 is higher than the feeding speed V1. Also set to be faster. When performing the whole heating instead of the zone heating, it is only necessary to set the stretching speed to a predetermined stretching speed corresponding to the radial stress. In FIG. 10, reference numeral 9 denotes a tube for evacuation.

In the third method, the entire interior of the muffle tube 6 is evacuated by evacuating the tube 9, and the combined base material 3 is heated while being stretched under this vacuum, so that the tube is stretched. A diameter-reducing force for reducing the diameter of the combined base material 3 with respect to the combined base material 3 based on the combined base material 3, that is, a stress directed inward in the radial direction is generated,
Under such a state where the radial stress is applied, the combination base material 3 is softened and integrated.

By using the same apparatus as that of the third method, the combined base material 3 is hung downward, and the combined base material 3 in the muffle tube 6 is owed to the weight of the combined base material 3 in the hung part. It is also possible to apply a diameter reducing force to 3, and it is also possible to positively add a weight to the lower end of the above-mentioned hanging portion to adjust the diameter reducing force.

(Distortion Removal Step 10) In the distortion removal step 10, the integrated optical fiber preform after the integration step P9 is subjected to heat treatment in a vacuum vessel at a predetermined temperature Ts for a predetermined time (for example, 30 minutes). This removes distortion. In this case, for ordinary glass, the temperature Ts may be set to a temperature range equal to or higher than the glass transition temperature Tg. In particular, in the case of fluoride glass, the temperature is further lower than the crystallization start temperature Tx to prevent crystallization. May be added to the condition (Tg <Ts <Tx
).

In the distortion removing step P10, since the heat treatment has already been performed in the integration step P9, and the distortion has been removed by the heating treatment in the integration step P9, the distortion removing step P10 may be omitted. It is possible.

(Resurfacing Step P12) In the resurfacing step P12, the same processing as in the above-described polishing / etching step P4 is performed, and the production of the optical fiber preform is completed.

<Other Embodiments> The present invention is not limited to the above embodiment, but includes various other embodiments. That is, in the above-described embodiment, a pair of half clad rods 11 and 12 in which a circular clad is halved are used as the plurality of clad rods arranged around the core rod 21. However, the present invention is not limited to this. May be arranged so as to surround the outer periphery of the core rod adjusted to a predetermined diameter to form a combined base material.

[0044]

As described above, according to the method for manufacturing an optical fiber preform according to the first aspect of the present invention, in the integration step, the whole is placed under a vacuum state and the outer circumference of the combined preform is Since heating is performed while mechanically applying a radial stress toward the core rod side, there is no need to perform conventional complicated pressure adjustment control and atmosphere control, and integration into the optical fiber preform is easy and easy. As a result, the loss of the optical fiber drawn from the obtained optical fiber preform can be reduced.

According to the second aspect of the present invention, similarly to the first aspect of the present invention, there is no need to perform the conventional troublesome pressure adjustment control and atmosphere control, and the integration into the optical fiber preform is facilitated. As a result, the loss of the optical fiber drawn from the obtained optical fiber preform can be reduced. In addition, the steps up to the preparation of the combined base material used in the integration step can be simplified.

According to the third or fifth aspect of the present invention, the radial stress can be specifically and reliably applied, and the effect of the first or second aspect of the present invention can be obtained. Can be obtained reliably.

According to the sixth aspect of the present invention, in addition to the effect of the first or second aspect of the present invention, since a distortion removing step is performed, a low level is obtained when a final line is drawn. Loss and stable optical fiber can be reliably obtained, and by specifying the temperature range at the time of heat treatment, crystallization due to heat treatment for strain removal can be prevented even when special glass is used. .

Further, according to the invention of claim 7, in addition to the effect of the invention of claim 2, it is possible to reduce the risk of occurrence of breakage when a pair of half clad rods and a core rod are combined. The combination base material can be obtained reliably and easily.

[Brief description of the drawings]

FIG. 1 is a process explanatory view showing an embodiment of the present invention.

FIG. 2 is a view showing a cutting position of a clad rod in a dividing step.

FIG. 3 is a cross-sectional view illustrating a method for combining a pair of half clad rods and a core rod.

4 is a partially enlarged view of FIG. 3 showing a shape of a concave groove;

5 is a diagram corresponding to FIG. 4, showing a groove having a shape different from that of FIG. 4;

FIG. 6 is a diagram corresponding to FIG. 4, showing a concave groove having a shape different from those of FIGS. 4 and 5;

FIG. 7 is a perspective view illustrating a first method of the integration step.

FIG. 8 is a partially enlarged cross-sectional view when the double press die of FIG. 7 is fixed.

FIG. 9 is a partially cutaway perspective view illustrating a second method of the integration step.

FIG. 10 is a schematic sectional view illustrating a third method of the integration step.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Clad rod 3 Combination base material 4 Press type 5 Tube 11 and 21 Half clad rod 21 Thin core rod (core rod) 111 Cutting surface 112 Groove P1 Clad rod forming step P2 Division step P3 Groove processing step P5 Core rod forming step P6 Drawing process (Core rod forming process) P9 Integration process P10 Destraining process

Claims (7)

[Claims] 1. A plurality of clad rods for forming a clad are formed, while a core rod is formed of glass for a core, and the plurality of clad rods are arranged and combined around the outer periphery of the core rod such that the core rod is located at the center. Thereafter, in a method for manufacturing an optical fiber preform in which the combined preforms in the combined state are integrated with each other to form an optical fiber preform, the above-described combined preform is placed under vacuum, and A method of manufacturing an optical fiber preform, wherein the step of integrating the preform is performed by heating while mechanically applying a radial stress from the outer peripheral side toward the core rod. 2. A core rod forming step of forming a core rod having a uniform cross-sectional shape in the longitudinal direction from glass for a core, and apart from the core rod forming step, one clad rod having a uniform cross-sectional shape in the longitudinal direction is formed by a glass for cladding. Forming a pair of half clad rods by cutting the formed clad rods in the diametric direction, and forming at least one cut surface of the pair of half clad rods On the other hand, a groove processing step of forming a concave groove extending substantially the entire length in the longitudinal direction at a position corresponding to the center of the cross section of the clad rod, and a pair of half-split clad rods in a state where the core rod is fitted into the formed concave groove. After polymerizing and combining the cut surfaces of each other, the combination in this combined state The base material is placed under vacuum, and the combined base material is heated by mechanically applying a radial stress from the outer peripheral side toward the core rod side, thereby heating the pair of half clad rods and the core rod. And an integrating step of integrating the optical fibers with each other. 3. The light according to claim 1, wherein a radial stress is applied by sandwiching an outer peripheral surface of the combined base material from a side opposite to each other by a pair of pressing dies. Manufacturing method of fiber preform. 4. The method according to claim 1 or 2, wherein a radial stress is applied by heating the outer periphery of the combined base material in a state where a tube whose outer peripheral length shrinks by heating is extrapolated. A method of manufacturing an optical fiber preform. 5. The method of manufacturing an optical fiber preform according to claim 1, wherein a radial stress is applied by applying a stretching force to the combined preform in the longitudinal direction. 6. The strain removing method according to claim 1, wherein the integrated optical fiber preform is subjected to a heat treatment under a vacuum state at a temperature equal to or lower than a crystallization start temperature after the integrating step. A method of manufacturing an optical fiber preform, comprising performing a strain removing step. 7. The method according to claim 2, wherein a cutting position of the clad rod in the dividing step is shifted from a center point of the clad rod by a dimension corresponding to a radius of the core rod, and the formation of the groove in the groove processing step is performed by a pair of grooves. A method for manufacturing an optical fiber preform, wherein the method is performed on only one of the half clad rods.