Method for producing a base material for an optical fiber

Abstract

A method for producing a base material for an optical fiber comprising the step: Reciprocating a starting element and burners (3) relative to each other and depositing glass microparticles on a surface of the starting element while rotating the starting element, which is formed by fusing both ends of a core rod (1) with dummy rods (2), about an axis of the starting element, the method further comprising the steps of: Specifying two or more axes as the reciprocating axes that enable reciprocating movement relative to the initial element; Providing a burner (3) facing the output element on each of the axes; Causing each burner (3) to move over the output element to one end of the output element; Changing a position where at least two burners (3) pass each other during the continuous movement, in a longitudinal direction of the output element and Changing the movement speeds of the at least two burners (3) during the overtravel movement, so that the position where the burners (3) pass each other is different each time an overtravel movement occurs.

Description

BACKGROUND 1. Technical field

[0001] The present invention relates to a method for producing an optical fiber base material, which realizes high-speed production of the optical fiber base material and produces an optical fiber base material having a uniform thickness in the longitudinal direction of a starting member. 2. Related technology

[0002] Conventionally, various methods for producing an optical fiber base material have been proposed. One of these methods is an OVD (Outside Vapor Deposition) method for producing an optical fiber base material, which comprises rotating a starting member, which is a core rod fused to dummy rods at both ends, around its axis, reciprocating the starting member and a plurality of burners relative to each other, depositing glass microparticles on the surface of the starting member, removing moisture from the resulting material in an electric furnace, and performing a sintering process to obtain the optical fiber base material. This OVD method is widely used because it is suitable for mass production of an optical fiber base material having a desired refractive index distribution and a large aperture diameter.

[0003] With this OVD technology, ideas have been considered to improve plant productivity, such as increasing the number of burners and increasing the deposition rate.

[0004] Japanese patent JP 2 809 905 B2 proposes a method comprising arranging burners along the entire region of the starting element that is to become the final product and improving the deposition rate by partially passing the burners over this portion of the starting element.

[0005] With this method, the number of burners can be significantly increased and the deposition rate can be improved, but because each burner only covers a predetermined section of the product area, there is a significant effect due to individual differences between the burners and the variation of the flow rates, and there is a fluctuation in the deposition thickness of the glass microparticles at different positions in the longitudinal direction of the starting element, so that it is necessary to carry out post-processing, which includes an additional step of grinding the surface of the base material.

[0006] On the other hand, a method for improving the deposition rate has been proposed by causing each burner to travel over the entire product area, the method comprising arranging a plurality of burners with axes of movement along the exit element and around the circumference of the exit element.

[0007] With this method, because the burners pass over the entire product area, variations in the deposition thickness of the glass microparticles in the longitudinal direction are less likely to occur due to changes in gas flow rates or individual differences between the burners. However, the burners must pass each other during deposition, and in the sections where the burners pass each other, a decrease in deposition efficiency occurs due to the interaction between the flames of the burners and a variation in the deposition in the longitudinal direction.

[0008] To solve this problem, Japanese patent JP 3 581 764 B2 proposes a method that involves increasing the speed at which the burners pass each other to reduce the effect of interaction when the burners pass each other.

[0009] However, with this method, the burners pass each other at the same position each time they pass over the output element, so slight fluctuations can occur between the positions where the burners pass each other and the positions where they do not. In particular, new-generation base materials have become larger, and even if only a small fluctuation occurs, this small fluctuation is amplified, resulting in a large fluctuation.

[0010] As another example, Japanese patent JP 4 690 979 B2 proposes a method that involves changing the range of movement traversed by the torches for each axis of movement, but this method results in an increase in the amount of non-product regions, which are the conical regions at the ends.

[0011] JP H10 - 81 537 A describes that a porous preform for optical fibers is produced by slowly moving several burners from one side of a target to the other, maintaining fixed distances between the burners. During this movement, glassoot is generated in the flame at the burner tip and applied to the outer circumference of the target. After reaching the opposite side, the burners return to the starting point at a higher speed, approximately 1.5 times the slow movement speed. The speed of subsequent burners is reduced to enable immediate restart without waiting times. JP H04 - 292 434 A also describes that a burner is controlled with regard to its movement speed. And JP 2013 - 249 233 A describes that, in one method in question, burners are provided at different locations in different cycles.

[0012] It is an object of the present invention to provide a method for producing a base material for an optical fiber which enables the production of a base material for an optical fiber at a high speed and the formation of a uniform deposition thickness in the longitudinal direction even when the burners are arranged on multiple axes of movement. SHORT DESCRIPTION

[0013] The invention is defined by claim 1. Embodiments can be found in the subclaims.

[0014] According to one aspect of the present invention, there is provided a method for manufacturing an optical fiber base material, which comprises reciprocating a starting member and burners relative to each other and depositing glass microparticles on a surface of the starting member while rotating the starting member, which is formed by fusing both ends of a core rod with dummy rods, about an axis of the starting member, the method comprising setting two or more axes as the reciprocating axes that enable reciprocating movement relative to the starting member, providing a burner facing the starting member on each of the axes, causing each burner to cross the starting member to one end of the starting member, and changing a position at which at least two burners cross each other during the cross-movement in the longitudinal direction of the starting member. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic plan view of an example of an apparatus for producing a base material for an optical fiber according to the present invention; Fig. 2 shows a schematic side view of the Fig. 1 illustrated device; Fig. 3 is a graph showing positions where burners pass each other and an overtravel state of the burners in an embodiment in which two axes are set as the moving axes of the burners; Fig. 4 shows a graph showing the change in the moving speed of the burner (3-A) moving on the A-axis; Fig. 5 shows a graph showing the change in the moving speed of the burner (3-B) moving on the B-axis; Fig. 6 is a graph showing the change in deposition thickness in the longitudinal direction according to the base material obtained by the embodiment; Fig. 7 is a graph showing the change in the moving speed of the burner (3-A) moving on the A-axis in a comparative example in which two axes are set as the moving axes of the burners; Fig. 8 shows a graph showing the change in the moving speed of the burner (3-B) moving on the B-axis; Fig. Fig. 9 is a graph showing the position at which burners pass each other and the overrun state of the burners in a comparative example; and Fig. 10 is a graph showing the change in deposition thickness in the longitudinal direction of a base material obtained according to a comparative example. DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0015] During the deposition of glass microparticles on the surface of a starting element by burners moving along axes of motion in two or more axial directions, interaction occurs between the flames as these burners pass each other, and deposition efficiency decreases in the areas where the burners pass each other. If deposition continues while the burners pass each other at the same position each time, the areas where a decrease in deposition efficiency occurs overlap, resulting in a large variation in deposition thickness in the longitudinal direction.

[0016] Therefore, it was found that in order to achieve a uniform deposit thickness, it is very important to gradually change the positions where the burners pass each other so that they do not overlap in the longitudinal direction.

[0017] Therefore, in the manufacture of an optical fiber base material by rotating a starting element obtained by fusing a core rod with dummy rods at both ends around its axis, reciprocating the starting element and the burners relative to each other, and depositing glass microparticles on the surface of the starting element, if two or more axes are set as the axes for the relative reciprocating movement, a burner is provided on each axis and facing the starting element, and the positions at which the burners pass each other in the longitudinal direction of the starting material are changed by changing the moving speed of the burner on each axis every time the burners pass the starting element, the positions at which the burners pass each other are different from each other and do not overlap even if the number of burners is increased.so that a base material for an optical fiber can be obtained which has a uniform deposition thickness without variations in the deposition thickness in the longitudinal direction.,

[0018] Methods contemplated for changing the positions at which the burners pass each other include, in addition to a method for adjusting the moving speed of the burners, a method that includes adjusting the time during which the burners stop at the ends of the base material and a method that includes adjusting the positions over which the burners pass, the former method having a problem in that productivity is reduced by increasing the time during which no deposition takes place while the burners stop, and the latter method having a problem in that the length of the ineffective cone portions at the ends of the base material increases, thereby decreasing the product yield.

[0019] With the method according to the invention, which includes adjusting the movement speed of the burners, the above problems do not occur and the adjustment of the movement speed has little influence on the deposition speed and the deposition density, so that problems such as cracking during deposition or layer detachment during vitrification hardly occur when the movement speed changes significantly for each deposited layer.

[0020] More preferably, the positions at which the burners pass each other are set in advance such that the positions at which the burners pass each other do not overlap in the longitudinal direction, and a pattern for the moving speed of the burners is set in advance for each individual pass over the starting material such that for each moving axis, the burners pass each other at the predetermined positions.

[0021] Some embodiments of the present invention are described below. These embodiments are not intended to limit the invention as defined in the claims, and various aspects are possible.

[0022] The apparatus and method for producing a base material for an optical fiber according to the present invention will be described in detail with reference to the drawings.

[0023] Fig. Fig. 1 is a schematic plan view of an example of a manufacturing apparatus used for manufacturing an optical fiber base material according to the present invention, and Fig. 2 shows a schematic side view of the same device. In Fig. 1, the starting element is obtained by fusing a core rod 1 with dummy rods 2 at both ends and held by a base holding element (not shown) in such a way that it can rotate freely about its axis.

[0024] Two axes (A and B) are defined as the movement axes of the burners along the output element, and the burners 3 (3-A and 3-B) are configured to be freely moved along the output element by burner guide mechanisms 4 (4-A and 4-B). Movement motors 5 (5-A and 5-B) are provided separately on the respective movement axes, and each movement motor can be operated independently.

[0025] A porous optical fiber base material is manufactured by supplying an optical fiber raw material, such as SiCl4 vapor, and a combustion gas (hydrogen gas and oxygen gas) to burners 3 using normal oxygen-hydrogen, synthesizing glass microparticles (soot) by hydrolysis in the hydrogen-oxygen flames, and discharging the soot toward the starting element to deposit it on the starting element. The exhaust gas and the remaining silica microparticles are exhausted to the outside of the system through an exhaust hood 6 located above.

[0026] The method for producing the optical fiber base material of the present invention using the OVD (Outside Vapor Deposition) method will be described below.

[0027] First, while the starting element, which is held by a base holding section (not shown), is rotated on its axis using a rotary motor, flames are ejected from the burners 3 toward the starting element, and soot is deposited on the starting element. The porous optical fiber base material is then manufactured by forming the deposition layer using the burner guide mechanisms 4 to reciprocate the burners 3 along the longitudinal direction of the starting element.

[0028] The porous optical fiber base material thus obtained is passed through a heating furnace made of a heater and a heat-insulating material and subjected to dehydration and transparent vitrification to obtain the optical fiber base material. Examples of implementationFirst embodiment

[0029] A porous base material for an optical fiber was prepared according to an OVD method using, for example, a Fig. 1 and Fig. 2. The starting material was obtained by fusing a core rod with an outer diameter of 50 mm and a length of 3000 mm at both ends with dummy rods, each with an outer diameter of 50 mm and a length of 500 mm, and soot deposition was carried out until an outer diameter of 300 mm was reached.

[0030] The burners used were multi-nozzle deposition burners in which small diameter combustion-supporting gas outlet nozzles are housed in a discharge opening for a combustible gas, with three of these burners arranged at intervals of 150 mm on the A-axis and the B-axis.

[0031] As in Fig. 3, the deposition was carried out in such a way that the movement speed on the A-axis was as in Fig. 4 and the movement speed on the B-axis as shown in Fig. 5, so that the positions where the burners pass each other on the A-axis and on the B-axis do not overlap in the longitudinal direction of the output element. Fig. 3, the solid line represents the movement path of torch 3-A on the A-axis, the dashed line represents the movement path of torch 3-B on the B-axis, and the black circles indicate the positions where the torches pass each other, which are each set to different positions when the torches pass the output element. The settings for the movement speeds on the A-axis and on the B-axis in the Fig. 4 and Fig. 5 are not on the Fig. 3, but can be changed in many ways as long as the positions where the burners pass each other do not overlap.

[0032] If the soot is deposited on the starting element in this way, as in Fig. 6, the thickness at the ends is somewhat insufficient, but the deposit thickness over the entire product area except the end sections is constant, so that a highly advantageous base material is obtained. Fig. 6 shows the change in the deposition thickness at positions in the longitudinal direction of the product area using a ratio in which the deposition thickness at a position of 500 mm in the longitudinal direction is set to 1. In this way, by changing the moving speed on the A-axis and the moving speed on the B-axis to change the positions where the burners pass each other each time the burners pass the exit member, the positions where the burners pass each other in the longitudinal direction of the base material do not overlap. In this way, advantageous effects are realized, for example, a porous optical fiber base material with a uniform deposition thickness in the longitudinal direction is obtained and the porous optical fiber base material can be produced at a high speed. [First comparison example]

[0033] A porous optical fiber base material was manufactured according to an OVD method using the same starting element and apparatus as in the first embodiment.

[0034] As in the Fig. 7 and Fig. As shown in Fig. 8, deposition was carried out in the same manner as in the first embodiment, except that the A-axis moving speed and the B-axis moving speed were each set to 100 mm / min.

[0035] As a result, as shown in Fig. 9, the positions where the burners pass each other on the A-axis and on the B-axis, at the longitudinal position of 2000 mm of the base material each time the burners passed the output element, so that, as shown in Fig. 10, the thickness of the deposit layer was reduced near the central region where the positions where the burners pass each other overlap.

[0036] In the foregoing, an example of a method for manufacturing the base material for an optical fiber according to the present invention has been described, in which two axes are used as the moving axes of the burners and the moving speed is different for each axis. Instead, when the phase difference between the burners is greater than 0° and less than 180°, the moving speed of each of the burners may be the same. In a case where the moving speed of each of the burners is the same, a phase difference of 0° between the burners indicates a state in which the burner 3-A arranged on the A axis and the burner 3-B arranged on the B axis move at the same positions in the longitudinal directions. Furthermore, in a case where the moving speed of each of the burners is the same, a phase difference of 180° between the burners indicates the state in Fig. 9 shows the first comparative example.

[0037] That is, in a case where the phase difference between the burner 3-A arranged on the A-axis and the burner 3-B arranged on the B-axis is greater than 0° and less than 180°, the positions where the burner 3-A and the burner 3-B pass each other are two points shifted to the right and left of the center, respectively, instead of both being located at the center of the output element. In this way, even if the moving speeds of the burners are different, the burners are caused to pass each other at two different positions, rather than at one position when the phases of the burners are different. In this way, it is possible to limit the decrease in deposit thickness at the position where the burners pass each other.The operation of setting the phase difference between the two burners to a value greater than 0° and less than 180° can be combined with the operation of setting the movement speeds of the burners to different values.

[0038] The above embodiment describes an example in which the movement speeds of the burners are changed each time the burners pass over the output element, but the movement speeds of the burners are fixed during a single pass. However, the present invention is not limited to this; the movement speeds of the burners can be changed during the pass.

[0039] While an example of a method for manufacturing an optical fiber base material according to the present embodiment has been described, where two axes are used as the moving axes of the torches, three or four axes may be used as the moving axes instead. In such a case, it is only required that the positions where torches arranged on at least two of the axes pass each other do not overlap. As the number of axes increases, it becomes more complicated to adjust the moving speed on each axis so that none of the positions where torches arranged on any of the axes pass each other overlap, but this adjustment can still be achieved in the same way.In this way, it is possible to obtain an optical fiber base material having a uniform deposition thickness in the longitudinal direction and to produce the optical fiber base material at a high speed. [List of reference symbols]

[0040] 1: Core rod, 2: Dummy rod, 3: Burner, 4: Burner guide mechanism, 5: Movement motor, 6: Extraction hood

Claims

[1] A method for producing a base material for an optical fiber comprising the step of: Reciprocating a starting element and burners (3) relative to each other and depositing glass microparticles on a surface of the starting element while rotating the starting element, which is formed by fusing both ends of a core rod (1) with dummy rods (2), about an axis of the starting element, the method further comprising the steps of: Specifying two or more axes as the reciprocating axes that enable reciprocating movement relative to the initial element; Providing a burner (3) facing the output element on each of the axes; Causing each burner (3) to move over the output element to one end of the output element; Changing a position where at least two burners (3) pass each other during the continuous movement, in a longitudinal direction of the output element and Changing the movement speeds of the at least two burners (3) during the overtravel movement, so that the position where the burners (3) pass each other is different each time an overtravel movement occurs. [2] Method according to claim 1, wherein a phase difference between the at least two burners (3) during the overtravel movement is greater than 0° and less than 180°. [3] Method according to claim 1 or 2, comprising the step: Ensure that the movement speeds of the at least two burners (3) are different from one another during the crossing movement.

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