Method for producing porous glass preform and method for producing optical fiber

Abstract

Provided is a method for producing a porous glass preform, the method comprising: a reciprocation step in which a burner unit and a starting member are repeatedly subjected to a first travel movement, in which the burner unit and the starting member are caused to travel relatively to each other in a first direction, and then to a second travel movement, in which the burner unit and the starting member are caused to travel in a second direction over a distance different from that in the first travel movement; and a deposition step in which, simultaneously with the reciprocation step, a fine glass particle is deposited on the surface of the starting member. The travel distances in the first travel movement and the second travel movement are selected from among a plurality of travel amounts, and the plurality of travel amounts includes a basic traverse amount. When the basic traverse amount is expressed by d, the absolute value of the difference between each of the plurality of travel amounts and the basic traverse amount is 0.33d or less.

Description

Method for manufacturing porous glass preform and method for manufacturing optical fiber

[0001] This disclosure relates to a method for manufacturing a porous glass preform and a method for manufacturing an optical fiber. This application claims priority to Japanese Patent Application No. 2024-126124, filed on August 1, 2024, the contents of which are incorporated herein by reference.

[0002] Conventionally, porous glass preforms have been produced by depositing glass particles (soot) on a starting member. The porous glass preforms are used in the production of optical fibers. Patent Document 1 discloses a method in which multiple burners are installed in one burner unit and the starting member and the burner unit are reciprocated relative to each other. This method can shorten the time required for soot deposition.

[0003] Japanese Patent Publication No. 64-9821

[0004] When the burner unit and the starting member are reciprocated relative to each other, there is a moment when the relative speed becomes zero at the turning point. In other words, the burner is stopped relative to the starting member, which causes a large amount of soot to deposit locally, resulting in the problem of unevenness in the completed porous glass preform. The occurrence of such unevenness leads to a deterioration in the quality of the optical fiber.

[0005] The present disclosure has been made in consideration of these circumstances, and aims to provide a method for manufacturing a porous glass preform that can suppress the occurrence of unevenness in the soot, and a method for manufacturing an optical fiber that can suppress quality degradation caused by unevenness in the soot.

[0006] In order to solve the above problems, a method for manufacturing a porous glass preform according to a first aspect of the present disclosure includes: an arrangement step of arranging a burner unit, in which a plurality of burners are aligned, and a starting member so as to face each other; a reciprocating step of performing a first movement operation of relatively moving the burner unit and the starting member in a first direction in a longitudinal direction X of the starting member, and then performing a second movement operation of relatively moving the burner unit and the starting member in a second direction opposite to the first direction by a distance different from that of the first movement operation, repeatedly; and a deposition step of releasing a gas containing a glass raw material from each of the burners while rotating the starting member, and depositing glass particles on a surface of the starting member, simultaneously with the reciprocating step, wherein a movement distance in the first movement operation and the second movement operation is selected from a plurality of movement amounts, the plurality of movement amounts including a basic traverse amount, and when the basic traverse amount is represented by d, an absolute value of a difference between each of the plurality of movement amounts and the basic traverse amount is 0.33d or less.

[0007] A second aspect of the present disclosure is the method for manufacturing a porous glass base material according to the first aspect, wherein the reciprocating step includes a reciprocating movement in which the sum of the distances of three consecutive movements is three times the basic traverse amount d.

[0008] Aspect 3 of the present disclosure is a method for manufacturing a porous glass base material according to Aspect 1 or 2, wherein in the reciprocating step, one cycle is defined as a sequence of repeating the first movement operation and the second movement operation from a relative starting position between the burner unit and the starting member to a return to the starting position, and the number of movements included in one cycle is defined as N. When this number is defined as N, the total movement distance in one cycle is N×d.

[0009] A fourth aspect of the present disclosure is the method for manufacturing a porous glass preform according to any one of the first to third aspects, wherein an absolute value of a difference between the plurality of movement amounts excluding the basic traverse amount and the basic traverse amount is 0.17d or more.

[0010] A fifth aspect of the present disclosure is the method for manufacturing a porous glass preform according to any one of the first to fourth aspects, wherein the number of the movement amounts is five or more.

[0011] A sixth aspect of the present disclosure is the method for manufacturing a porous glass preform according to any one of the first to fifth aspects, wherein the movement amounts include five values ​​of d, d+α, d−α, d+β, and d−β, and satisfy 0.17d≦α≦0.33d and 0.17d≦β≦2 / 3α.

[0012] A seventh aspect of the present disclosure is a method for manufacturing a porous glass preform according to any one of the first to sixth aspects, wherein the time average of the position of the burner unit is approximately equal to that when the burner unit is reciprocated only by the basic traverse amount d.

[0013] An eighth aspect of the present disclosure is the method for manufacturing a porous glass preform according to any one of the first to seventh aspects, wherein the basic traverse amount is equal to an average value of intervals at which the plurality of burners are arranged.

[0014] A ninth aspect of the present disclosure is the method for producing a porous glass preform according to any one of the first to eighth aspects, wherein the glass raw material is an organosilicon compound.

[0015] The method for manufacturing an optical fiber according to aspect 10 of the present disclosure includes a sintering step of sintering a porous glass preform manufactured by the method for manufacturing a porous glass preform according to any one of aspects 1 to 9 to obtain a transparent glass body, and a drawing step of drawing the transparent glass body to obtain an optical fiber.

[0016] According to the above aspects of the present disclosure, it is possible to provide a method for manufacturing a porous glass preform that can suppress the occurrence of unevenness in the soot, and a method for manufacturing an optical fiber that can suppress quality degradation caused by unevenness in the soot.

[0017] FIG. 1 is a diagram showing the configuration of a manufacturing device for a porous glass preform according to the present embodiment; FIG. 2 is a diagram explaining an example of a reciprocating process in the manufacturing method for a porous glass preform according to the present embodiment; FIG. 3 is a diagram showing the configuration of a wire drawing device according to the present embodiment; FIG. 4 is a graph showing the relationship between the shift ratio and the value of L2 / L1 for Test Examples 1 to 13; FIG. 5 is a graph showing the unevenness of soot for Test Examples 1 to 10; and FIG. 6 is a graph showing the average position of the burner unit for Comparative Example, Example 1, and Example 11.

[0018] The porous glass preform manufacturing method and optical fiber manufacturing method of this embodiment will be described below with reference to the drawings. As shown in Fig. 1, the porous glass preform manufacturing apparatus 1 includes a supply unit 2, a pair of rotary chucks 3, a burner unit 4, a starting member 5, and a control unit 6. The burner unit 4 has a plurality of burners 4a and a support base 4b. The pair of rotary chucks 3 respectively support both ends of the starting member 5. The rotary chuck 3 can rotate the starting member 5 around its axis.

[0019] [Direction Definition] In this specification, the direction in which the starting member 5 extends is referred to as the "longitudinal direction X." In the longitudinal direction X, a first direction is referred to as the "+X side." A second direction opposite to the first direction is referred to as the "-X side." The operation of the burner unit 4 shifting toward the +X side relative to the starting member 5 is referred to as the "first movement operation." The operation of the burner unit 4 shifting toward the -X side relative to the starting member 5 is referred to as the "second movement operation." In the manufacturing apparatus 1, the first movement operation and the second movement operation are alternately repeated. That is, the burner unit 4 reciprocates relative to the starting member 5 in the longitudinal direction X. The relative position of the burner unit 4 in the longitudinal direction X with respect to the starting member 5 when switching between the first movement operation and the second movement operation is referred to as the "turn-back position."

[0020] The multiple burners 4a are aligned at intervals in the longitudinal direction X. The relative positions of the burners 4a are fixed. The average value of the arrangement intervals between the burners 4a is referred to as the "average interval P" (see symbol P in Figure 1). However, the intervals between the burners 4a may be uniform or non-uniform. For example, due to individual differences between the burners 4a, the amount of glass microparticles generated by each burner 4a may vary. To prevent unevenness in the soot due to this variation, the position of each burner 4a in the longitudinal direction X may be adjusted. Through this adjustment, the interval between the burners 4a may increase or decrease within a range of 10% of the average interval P.

[0021] The support table 4b supports a plurality of burners 4a. The support table 4b is movable along the longitudinal direction X relative to the starting member 5. Therefore, the burners 4a are also movable along the longitudinal direction X relative to the starting member 5. Burners dedicated to heating that do not generate glass particles may be provided on the support table 4b. The positions of the burners dedicated to heating are not taken into account when calculating the average spacing P between the burners 4a.

[0022] In the manufacturing apparatus 1 shown here, the burner unit 4 moves in the longitudinal direction X. However, the burner unit 4 may not move, and the starting member 5 may move relative to the burner unit 4. Alternatively, both the burner unit 4 and the starting member 5 may move in the longitudinal direction X. In other words, it is sufficient that the burner unit 4 and the starting member 5 move relatively in the longitudinal direction X. As a specific example, in the "first moving operation", the burner unit 4 may move to the +X side while the starting member 5 is fixed, or the starting member 5 may move to the -X side while the burner unit 4 is fixed.

[0023] Each burner 4a is supplied with multiple types of gas (e.g., combustible gas, carrier gas, glass raw material, oxygen) from the supply unit 2. Specific examples of combustible gas include oxyhydrogen gas and methane gas. When the gas supplied to the burner 4a burns, a flame is generated at the outlet of the burner 4a. The glass raw material reacts in the flame, producing glass particles. The reaction may be, for example, an oxidation reaction or a hydrolysis reaction. The glass particles are deposited on the surface of the starting member 5, forming a deposition layer of glass particles (soot 10). This results in a porous glass preform. As shown in FIG. 1 , the soot 10 has an effective portion 11 and a tapered portion 12. The tapered portions 12 are located at both ends of the effective portion 11 in the longitudinal direction X. The thickness of the tapered portions 12 decreases toward the outside in the longitudinal direction X.

[0024] The effective portion 11 is a portion where the thickness of the soot 10 is stable and the optical fiber can be used as a product when the porous glass preform is drawn. The tapered portion 12 is a portion where the thickness is 5% to 93% of the average soot thickness in the effective portion 11. The optical fiber obtained from the tapered portion 12 cannot be used as a product. If the thickness is 93% or more of the average value of the effective portion 11, there is a high probability that the optical fiber obtained from that portion can be used as a product. Portions with a thickness less than 5% of the average value of the effective portion 11 can be removed by wiping after deposition (the deposition process described below), and therefore are not included in the tapered portion 12.

[0025] The extent of the spread of the flame generated by the burners 4a varies depending on the structure of the burners 4a. The greater the spread of the flame in the longitudinal direction X, the more extensively the glass particles generated from one burner 4a are deposited on the starting member 5. If the distance between the burners 4a is too close, the flames may interfere with each other, reducing the deposition efficiency of the glass particles. Therefore, the distance between the burners 4a is determined taking the deposition efficiency into consideration.

[0026] Silicon tetrachloride (SiCl4), an organic silicon compound, or the like can be used as the glass raw material supplied to the burner 4a. The use of an organic silicon compound has the following advantages: Because organic silicon compounds are flammable, they tend to increase the temperature around the starting member 5. Therefore, the bulk density of the soot tends to be higher than when silicon tetrachloride is used as the raw material. The higher the bulk density, the smaller the soot volume for the same weight. Therefore, when an organic silicon compound is used, the difference in soot volume between the turn-back position and other positions is reduced. The smaller the difference in soot volume depending on the position in the longitudinal direction X, the less likely unevenness will occur in the soot. As a result, using an organic silicon compound as the raw material makes it easier to obtain a porous glass preform with fewer unevenness than using silicon tetrachloride.

[0027] Specific examples of organic silicon compounds include alkylcyclosiloxanes. Octamethylcyclotetrasiloxane (OMCTS) is particularly suitable. OMCTS is also called "D4," where "D" is the (CH3)2-Si-O- unit, and D4 refers to a structure in which four D units are connected in a ring. D4 (C8H 24 O4Si4) is widely used industrially and is easily available, but D3 (C6H 18 O3Si3), D5(C 10 H 30 O5Si5) may also be used. D3, D4, and D5 may be used alone or in combination. Organosilicon compounds do not generate hydrochloric acid even when subjected to an oxidation reaction, which contributes to reducing the environmental impact and reducing production costs by eliminating the need for hydrochloric acid treatment facilities.

[0028] The starting member 5 is, for example, a silica glass rod including a portion that will become the core of the optical fiber. The control unit 6 controls at least the reciprocating process described below. Specifically, the control unit 6 operates the starting member 5 or the burner unit 4 in a predetermined direction and by a predetermined amount of movement. The manufacturing apparatus 1 is equipped with an actuator (motor, etc.) (not shown) that is controlled by the control unit 6. The control unit 6 outputs a control signal to the actuator, thereby executing the reciprocating process, etc.

[0029] The control unit 6 includes a processor such as a CPU (Central Processing Unit) and a memory. The processor performs arithmetic processing to execute the functions of the control unit 6. The memory stores a rewritable program that describes the functions executed by the CPU. The control unit 6 may realize these functions using hardware (including circuitry) such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a GPU (Graphics Processing Unit). Alternatively, the functions of the control unit 6 may be realized by a combination of software and hardware.

[0030] [Method for manufacturing a porous glass preform] Next, a method for manufacturing a porous glass preform and a method for manufacturing an optical fiber will be described. The method for manufacturing a porous glass preform in this embodiment includes (1) a placement step, (2) a reciprocating step, and (3) a deposition step. The method for manufacturing an optical fiber in this embodiment includes (4) a sintering step, and (5) a drawing step.

[0031] (1) Arranging Step The arranging step is a step of arranging the burner unit 4 and the starting member 5 so as to face each other. More specifically, the starting member 5 is attached to the pair of rotary chucks 3.

[0032] (2) Reciprocating Step The reciprocating step is a step in which, after a first movement operation, a second movement operation is repeated, in which the burner unit 4 and the starting member 5 are moved relative to each other by a distance different from that of the first movement operation. In other words, the movement distances on the outward and return paths are different in the reciprocating motion. Therefore, the relative positions of the burner unit 4 and the starting member 5 do not return to their original positions in a single reciprocating motion, but are shifted in the longitudinal direction X. In this way, when a reciprocating motion that shifts the positions in the longitudinal direction X is repeated a predetermined number of times, the relative positions of the burner unit 4 and the starting member 5 return to the positions at the time of departure (starting positions).

[0033] As described above, the reciprocating motion from the starting position to the return to the original starting position is referred to as "one cycle" in this specification. The number of movements included in one cycle is represented by N, which is a natural number.

[0034] In the reciprocating process, the movement distances in the first movement operation and the second movement operation are selected from among a plurality of movement amounts. The movement amounts include a "basic traverse amount d." The basic traverse amount d is an arbitrary value. As an example, the basic traverse amount d may be the same as the average interval P of the burners 4a. As an example, the "plural movement amounts" may be three values: d, d+α, and d-α. Here, α is also referred to as the "shift amount."

[0035] (3) Deposition Step The deposition step is carried out simultaneously with the reciprocating step. In the deposition step, a gas containing glass raw material is released from each burner 4a and reacted in a flame, thereby depositing glass particles on the surface of the starting member 5. At this time, the starting member 5 is rotated around its axis by the rotating chuck 3. By depositing the glass particles in this manner, soot 10 is formed. This results in a porous glass base material.

[0036] Here, an example of a reciprocating process will be described using FIG. 2. In FIG. 2, there are three "multiple movement amounts": d, d+α, and d-α, where α = 0.33d. Also, d = P. In other words, in FIG. 2, the basic traverse amount d is equal to the average interval P of the burners 4a. The (a) box in FIG. 2 shows the passage of time. The (b) box in FIG. 2 shows the movement amount corresponding to the passage of time. The (c) box in FIG. 2 shows the movement path corresponding to the passage of time. Time t0 is the start time of the movement. Point A0 in the (c) box is the relative position of the burner 4a and the starting member 5 at the start time t0.

[0037] For example, the amount of movement from time t0 to t1 is d. Focusing on the burner 4a located at the leftmost position (-X side) in FIG. 2, its relative position with respect to the starting member 5 changes from point A0 to point A1 between time t0 and t1. The distance from point A0 to point A1 in the longitudinal direction X is the basic traverse amount d, and the direction of movement is the +X side. The burner 4a moves from point A1 to point A2 between time t1 and t2. The distance from point A1 to point A2 is d+α, and the direction of movement is the -X side. In other words, the amount of movement and the direction of movement are different between the first movement operation (time: t0 → t1, movement amount: d, direction: +X side) and the second movement operation (time: t1 → t2, movement amount: d+α, direction: -X side). Thereafter, it moves by the amount d-α toward the +X side from time t2 to t3, and by the amount d toward the -X side from time t3 to t4. It then moves by the amount d+α toward the +X side from time t4 to t5, and by the amount d-α toward the -X side from time t5 to t6.

[0038] As shown in Figure 2(c), the leftmost burner 4a starts from point A0 and moves to points A1, A2, A3, A4, and A5 in this order. At time t6, the relative positions of the burner unit 4 and the starting member 5 return to point A0, which is the starting position. Therefore, in the example of Figure 2, the period from time t0 to t6 is "one cycle." One cycle includes six movements.

[0039] Here, if the shift amount α is not used, i.e., if the burner unit 4 simply travels back and forth at the basic traverse amount d, the turn-back positions are fixed at points A0 and A1. At the turn-back positions, there is a moment when the relative speed between the burner unit 4 and the starting member 5 becomes zero. In other words, there is a time period when the burner unit 4 is stopped relative to the starting member 5. During this time period, the generated glass particles continue to accumulate at the same location in the longitudinal direction X, resulting in a large amount of accumulation at that location. As a result, the amount of soot 10 deposited at the turn-back position is larger than the amount deposited at other positions, causing unevenness in the soot 10. Furthermore, when the burner unit 4 is stopped relative to the starting member 5, the soot 10 is locally heated. In other words, the temperature at the turn-back position is higher than that at other locations, causing defects due to temperature differences.

[0040] In contrast, in the movement path using the shift amount α, the turn-back position changes as shown in Fig. 2(c). By changing the turn-back position, the locations where the deposition amount is locally increased are dispersed, making it possible to reduce the magnitude of unevenness in the entire soot 10. Furthermore, by changing the turn-back position, the locations that are locally heated are also dispersed, making it possible to suppress defects caused by temperature differences.

[0041] FIG. 2 shows an example of three movement amounts: d, d+α, and d-α, but more than three movement amounts may be used. For example, a second shift amount β may be used in addition to the shift amount α. The second shift amount β is a value different from the shift amount α. As an example, the "plural movement amounts" may be five: d, d+α, d-α, d+β, and d-β. In this case, a cycle using the shift amount α as shown in FIG. 2 may be performed once, followed by a cycle using the second shift amount β, and the process may be repeated. The order of operations in the "cycle using the second shift amount β" may be the order in which α shown in FIG. 2 is replaced with β.

[0042] The turning position in the cycle using the second shift amount β is different from the turning position in the cycle using the shift amount α. Therefore, by using the second shift amount β in addition to the shift amount α, the turning positions can be further dispersed, thereby suppressing the occurrence of unevenness and temperature differences in the soot 10.

[0043] (4) Sintering Step The porous glass base material obtained as described above is subjected to a sintering process or the like to obtain a transparent glass body 102 (see FIG. 3). The porous glass base material may be subjected to a dehydration process, a doping process, or the like, as necessary.

[0044] (5) Drawing Process: An optical fiber is obtained by drawing the transparent glass body 102. The drawing process is performed by, for example, a drawing apparatus 200 as shown in FIG. 3 . The drawing apparatus 200 includes a spinning unit 110, an outer diameter measurement unit 120, a cooling unit 130, a coating unit 140, a curing unit 150, a take-up unit 160, a winding unit 170, and the like. The spinning unit 110 includes a heating furnace 112. The heating furnace 112 heats the transparent glass body 102 and melt-spins it to form a bare optical fiber 103. The outer diameter of the bare optical fiber 103 may be measured in the outer diameter measurement unit 120. The bare optical fiber 103 may be cooled in the cooling unit 130. In the coating unit 140, a coating material is applied (coated) to the outer periphery of the bare optical fiber 103 to form a coating layer, thereby obtaining a coated optical fiber 104. In the curing unit 150, the coating layer is cured to obtain an optical fiber strand 105. The optical fiber strand 105 is taken up by the take-up unit 160 and wound up by the winding unit 170 .

[0045] The above embodiment will be described below using specific examples, but the present disclosure is not limited to the following examples.

[0046] The burner unit 4 had four burners 4a. The average spacing P between the burners 4a was 180 mm. The basic traverse amount d was 180 mm. That is, in this example, the basic traverse amount d and the average spacing P were the same. OMCTS was used as the raw material, and argon was used as the carrier gas. OMCTS and argon were introduced into the vaporization units to generate a mixed gas. This mixed gas was mixed with oxygen to form a premixed gas, and the premixed gas was supplied to each burner 4a via the supply unit 2. At the same time, oxyhydrogen gas was supplied to the burners 4a and burned to generate an oxyhydrogen flame. By releasing the premixed gas into the oxyhydrogen flame, glass microparticles were generated and deposited on the starting member 5.

[0047]

[0048] The reciprocating step and the deposition step were performed under different operating conditions as shown in Table 1. The shift amount α and the second shift amount β were not set for Test Example 1. That is, for Test Example 1, the movement amount was fixed to 180 mm, which is the basic traverse amount d, and the burner unit 4 was reciprocated relative to the starting member 5 by this movement amount.

[0049] For test examples 2 to 10, three movement amounts were used: d, d+α, and d-α. The operation order was repeated as shown in Figure 2: d, d+α, d-α, d, d+α, d-α. The shift amount α was also varied within the range of 25 to 90 mm. The "shift ratio" in Table 1 is the ratio of the shift amount α to the basic traverse amount d. For example, for test example 2, the shift amount α was 25 mm and the basic traverse amount d was 180 mm, so the shift ratio was 25 ÷ 180 × 100 ≒ 14%.

[0050] For Test Examples 11 to 13, the movement amounts were set to five values: d, d+α, d-α, d+β, and d-β. The operation order was repeated as follows: d, d+α, d-α, d, d+α, d-α, d, d+β, d-β, d, d+β, and d-β. The shift amount α was varied within a range of 25 to 35 mm, and the second shift amount β was varied within a range of 50 to 70 mm. The "Shift Ratio" column lists the values ​​corresponding to the shift amount α and the second shift amount β. For example, for Test Example 11, the shift ratio corresponding to the shift amount α (25 mm) was 14%, and the shift ratio corresponding to the second shift amount β (50 mm) was 28%.

[0051] The "Results" column in Table 1 shows the dimensions of the soot 10 obtained under each operating condition. "Length of effective portion L1" is the length of the effective portion 11 in the longitudinal direction X. "Length of tapered portion L2" is the average value of the lengths of the two tapered portions 12 in the longitudinal direction X. "L2 / L1" is the ratio of the length L2 of the tapered portion to the length L1 of the effective portion. Figure 4 plots the results of Table 1. The horizontal axis of Figure 4 represents the shift ratio, and the vertical axis represents L2 / L1 in Table 1. For Test Example 1, the shift amount α = 0 was assumed and plotted in Figure 3. For Test Examples 11 to 13, the larger of the shift amount α and the second shift amount β was plotted as the value on the horizontal axis.

[0052] Here, a large value of L2 / L1 means that a small portion of the porous glass preform can be used as a product (optical fiber). In other words, the smaller the value of L2 / L1, the better. As shown in Figure 4, when the shift ratio of the shift amount α or the second shift amount β exceeds 33%, the value of L2 / L1 increases significantly. From the above results, it is preferable to set the shift amount α or the second shift amount β so that its ratio to the basic traverse amount d is 33% or less.

[0053] Next, the relationship between the shift ratio and the magnitude of the unevenness of the soot 10 will be explained. FIG. 5 shows the results of measuring the shape of the soot 10 corresponding to Test Examples 1 to 10 in Table 1. FIGS. 5(a) to 5(j) correspond to Test Examples 1 to 10, respectively. The horizontal axes of FIGS. 5(a) to 5(j) indicate the position of the soot 10 in the longitudinal direction X. The vertical axes of FIGS. 5(a) to 5(j) indicate the magnitude of fluctuation in the outer diameter when the average outer diameter of the effective portion 11 of the soot 10 is taken as the reference (value 1.0). Therefore, the flatter the graphs of FIGS. 5(a) to 5(j), the smaller the unevenness of the soot 10 (i.e., the outer diameter fluctuation).

[0054] As shown in Figure 5(h), Test Example 8, which had a shift ratio of 33%, had the smallest unevenness. As the shift ratio decreased or increased from 33%, the unevenness tended to increase.

[0055] Next, the results of further investigation into the amount of movement will be explained using Table 2. Table 2 shows how the unevenness of the soot 10 changes when the amount of movement and the order of operations are changed. The "standard deviation" in Table 2 is an index showing the magnitude of the unevenness of the soot 10. Specifically, the outer diameter of the effective portion 11 of the soot 10 was measured at predetermined intervals in the longitudinal direction X, and the standard deviation was calculated. The smaller the standard deviation, the smaller the outer diameter fluctuation, i.e., the smaller the unevenness.

[0056]

[0057] From Table 2, the following can be seen. The "multiple movement amounts" in Examples 1 to 11 include the basic traverse amount d, and are selected so that the absolute value of the difference between each movement amount and the basic traverse amount d is 0.33d or less. Furthermore, as shown in the "order of operations," three consecutive reciprocating movements are included in which the sum of each movement amount is three times the basic traverse amount d. In Examples 1 to 11, the standard deviation of the unevenness is 0.12 to 0.27. On the other hand, when the same reciprocating operation is performed using only the basic traverse amount d, as in the comparative example, the standard deviation of the unevenness is 0.39. In the comparative example, the amount of soot deposition at the turn-back position increased locally, resulting in large unevenness. In contrast, in Examples 1 to 11, the turn-back positions were dispersed, resulting in small unevenness.

[0058] Furthermore, when Examples 1 to 3 are compared with Example 4, Examples 1 to 3 have smaller unevenness. This is due to the difference in the shift amount α. That is, the shift amount α in Examples 1 to 3 is 0.17d to 0.33d, which is larger than the shift amount α of Example 4, which is 0.08d. It can be seen that by setting the shift amount α in this manner, unevenness in the soot 10 can be further suppressed. When the shift amount α is smaller than 0.17d, under the conditions of this example, the amount of change in the turning position is insufficient relative to the flame spread width of the burner 4a, and it is thought that the effect of suppressing unevenness is relatively small. When the shift amount α is larger than 0.33d, it is thought that the area where the soot deposition positions of adjacent burners 4a overlap increases, and the unevenness actually becomes larger.

[0059] Furthermore, Examples 5 and 6 have the same offset amount α of 0.33d, but different second offset amounts β. The second offset amount β of Example 5 is 0.25d, while the second offset amount β of Example 6 is 0.17d. The unevenness of the soot 10 is smaller in Example 6 than in Example 5. From these results, it is preferable to select the second offset amount β so that β≦2 / 3α. This is because if the "multiple movement amounts" include combinations with similar values, the change in the turnaround position becomes small, and the setting of the second offset amount β does not work sufficiently. It has also been found that when the relative movement between the burner 4a and the starting member 5 stops, the width of the localized soot deposition approximately corresponds to the diameter of the burner 4a. If the offset amount α is sufficiently large compared to the diameter of the burner 4a, it is preferable that the offset amount β be larger than the diameter of the burner 4a. For example, when the diameter of the burner 4a is 20 mm and the offset amount α is 60 mm, the second offset amount β may be 22 mm.

[0060] Next, the deposition position of the soot 10 in the longitudinal direction X will be described. In FIGS. 6( a) and 6(b), the horizontal axis represents time, and the vertical axis represents the position of the burner unit 4 in the longitudinal direction X. FIG. 6(a) compares the comparative example with Example 1. FIG. 6(b) compares the comparative example with Example 11. The "average position" in FIGS. 6(a) and 6(b) indicates the average position of the burner unit 4 with respect to time. As shown in FIG. 6(a), the average position of the burner unit 4 in Example 1 matches that of the comparative example. In contrast, as shown in FIG. 6(b), the average position of the burner unit 4 in Example 11 does not match that of the comparative example. In other words, in Example 11, the soot 10 is deposited on the starting member 5 at a position shifted in the longitudinal direction X compared to that of the comparative example. Therefore, if the manufacturing apparatus 1 is designed so that the relative positions of the starting member 5 and the burner unit 4 in the longitudinal direction X are suitable when the burner 4a is operated with only the basic traverse amount d, soot cannot be deposited in the center of the starting member 5.

[0061] Taking this into consideration, it is preferable that the average position of the burner unit 4 approximately coincides with that when the burner unit 4 is operated with only the basic traverse amount d. Here, "approximately coincides" means that the difference is within 50 mm. If the deviation is within this range, even if the soot 10 deposition position is deviated from the ideal position, the deviated portion can be consumed while stabilizing operation in the subsequent fiber drawing process, etc. Therefore, this does not pose a substantial problem.

[0062] As described above, the method for manufacturing a porous glass preform according to this embodiment includes: an arrangement step of arranging the burner unit 4, in which a plurality of burners 4a are aligned, and the starting member 5 so as to face each other; a reciprocating step of performing a first movement operation of relatively moving the burner unit 4 and the starting member 5 in a first direction (+X side) in the longitudinal direction X of the starting member 5, followed by a second movement operation of relatively moving the burner unit 4 and the starting member 5 in a second direction (-X side) opposite to the first direction by a distance different from that of the first movement operation, repeatedly; and a deposition step of releasing a glass frit-containing gas from each burner 4a while rotating the starting member 5 simultaneously with the reciprocating step, thereby depositing glass particles on the surface of the starting member 5. The movement distances in the first movement operation and the second movement operation are selected from a plurality of movement amounts (e.g., d, d+α, d-α), the plurality of movement amounts including a basic traverse amount d, and the absolute value of the difference between each of the plurality of movement amounts and the basic traverse amount d is 0.33d or less.

[0063] According to the above manufacturing method, the turning position changes during the reciprocating process. By changing the turning position, the locations where the deposition amount of soot 10 is locally increased are dispersed, making it possible to reduce the magnitude of unevenness across the entire soot 10. Furthermore, by changing the turning position, the locations that are locally heated are also dispersed, making it possible to suppress defects caused by temperature differences.

[0064] The method for manufacturing an optical fiber according to this embodiment includes a sintering step of sintering a porous glass preform to obtain a transparent glass body 102, and a drawing step of drawing the transparent glass body 102 to obtain an optical fiber (a bare optical fiber 103 or an optical fiber strand 105). This manufacturing method uses a porous glass preform with minimal irregularities in the soot 10, and therefore can manufacture a high-quality optical fiber.

[0065] The reciprocating process may also include a reciprocating movement in which the sum of the distances of three consecutive movements is three times the basic traverse amount d. By employing such a reciprocating process, it is possible to more reliably suppress unevenness of the soot 10.

[0066] Furthermore, in the reciprocating process, one cycle is defined as the process of repeating the first and second movement operations from the relative starting position of the burner unit 4 and the starting member 5 until the return to the starting position, and the number of movements included in one cycle is defined as N. The total movement distance in one cycle may be N x d. For example, Examples 1 to 11 in Table 2 satisfy this condition. In this case, it is possible to prevent the relative positions of the burner unit 4 and the starting member 5 from shifting as the reciprocating process is repeated.

[0067] Furthermore, the absolute value of the difference between the basic traverse amount d and a plurality of movement amounts (for example, d+α, d−α) excluding the basic traverse amount d may be 0.17d or more. In this case, it is possible to avoid an insufficient change in the turning position and more reliably suppress unevenness of the suit 10.

[0068] The number of shift amounts may be five or more. That is, a second shift amount β may be adopted in addition to the shift amount α. In this case, the turning positions can be further dispersed, and unevenness of the soot 10 can be more reliably suppressed.

[0069] Furthermore, the movement amount may include five values, d, d+α, d-α, d+β, and d-β, and may satisfy 0.17d≦α≦0.33d and 0.17d≦β≦2 / 3α. In this case, it is possible to prevent the second shift amount β from being ineffective due to the difference between the shift amount α and the second shift amount β being too small.

[0070] Furthermore, the time average of the position of the burner unit 4 may be approximately the same as when the burner unit 4 is reciprocated only by the basic traverse amount d. In this case, it is possible to prevent the soot 10 from being deposited at a position deviated from the target in the longitudinal direction X with respect to the starting member 5.

[0071] Furthermore, the basic traverse amount d may be the same as the average value (average interval P) of the intervals at which the plurality of burners 4a are arranged.

[0072] The glass raw material may also be an organic silicon compound, which makes it easier to obtain a porous glass base material with less irregularities than when silicon tetrachloride is used, for example.

[0073] The technical scope of the present disclosure is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present disclosure.

[0074] 1, the number of burners 4a is four. However, the number of burners 4a may be changed. Furthermore, the basic traverse amount d may be different from the average interval P between the burners 4a.

[0075] In addition, within the scope of the present disclosure, it is possible to replace the components in the above-described embodiments with well-known components as appropriate, and the above-described embodiments and variations may be combined as appropriate.

[0076] 4: burner unit 4a: burner 5: starting member 102: transparent glass body d: basic traverse amount X: longitudinal direction

Claims

1. A method for manufacturing a porous glass preform, comprising: an arrangement step of arranging a burner unit having a plurality of aligned burners and a starting member so that they face each other; a reciprocating step of performing a first movement operation to relatively move the burner unit and the starting member in a first direction in the longitudinal direction X of the starting member, and then repeating a second movement operation to relatively move them in a second direction opposite to the first direction by a distance different from that of the first movement operation; and a deposition step of releasing a gas containing glass raw material from each of the burners while rotating the starting member simultaneously with the reciprocating step, and depositing glass fine particles on the surface of the starting member, wherein the movement distances in the first movement operation and the second movement operation are selected from a plurality of movement amounts, and the plurality of movement amounts include a basic traverse amount, and when the basic traverse amount is represented by d, the absolute value of the difference between each of the plurality of movement amounts and the basic traverse amount is 0.33d or less.

2. The method for manufacturing a porous glass base material according to claim 1, wherein the reciprocating step includes a reciprocating movement in which the sum of the distances in three successive movements is three times the basic traverse amount d.

3. The method for manufacturing a porous glass base material according to claim 1 or 2, wherein in the reciprocating process, one cycle is defined as a sequence of repeating the first movement operation and the second movement operation from a relative starting position between the burner unit and the starting member until returning to the starting position, and the number of movements included in one cycle is defined as N, and the total movement distance in one cycle is N x d.

4. A method for manufacturing a porous glass base material according to any one of claims 1 to 3, wherein the absolute value of the difference between the plurality of movement amounts excluding the basic traverse amount and the basic traverse amount is 0.17d or more.

5. The method for manufacturing a porous glass base material according to any one of claims 1 to 4, wherein the number of the movement amounts is five or more.

6. A method for manufacturing a porous glass base material according to any one of claims 1 to 5, wherein the movement amounts include five values: d, d+α, d-α, d+β, and d-β, and satisfy the relationships 0.17d≦α≦0.33d and 0.17d≦β≦2 / 3α.

7. A method for manufacturing a porous glass base material according to any one of claims 1 to 6, wherein the time average of the position of the burner unit is approximately the same as that when the burner unit is reciprocated only by the basic traverse amount d.

8. A method for manufacturing a porous glass preform according to any one of claims 1 to 7, wherein the basic traverse amount is the same as the average value of the intervals at which the plurality of burners are arranged.

9. The method for producing a porous glass base material according to any one of claims 1 to 8, wherein the glass raw material is an organic silicon compound.

10. A method for manufacturing an optical fiber, comprising: a sintering step of sintering a porous glass preform manufactured by the manufacturing method described in any one of claims 1 to 9 to obtain a transparent glass body; and a drawing step of drawing the transparent glass body to obtain an optical fiber.

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