Apparatus for producing glass preform for optical fiber

Abstract

This apparatus for producing a glass preform for an optical fiber comprises a furnace core tube and an upper box part. The furnace core tube is capable of accommodating a support rod and a glass fine particle deposit supported by the support rod, and has a first through-hole formed in the upper part thereof through which the support rod can be inserted. The upper box part is provided on the upper part of the furnace core tube and, together with the upper part of the furnace core tube, defines an upper box space communicating with the internal space of the furnace core tube via the first through-hole. The upper box part is provided with a first discharge port, an opening, and a second through-hole. The first discharge port is formed in a side surface of the upper box part, and air in the upper box space is discharged therethrough. The opening is formed in a side surface of the upper box part and communicates with the upper box space and the outside air surrounding the upper box part and the furnace core tube. The second through-hole is formed in the upper part of the upper box part, and the support rod is inserted therethrough. The furnace core tube can heat the glass fine particle deposit while introducing a corrosive gas into the furnace core tube, and air is discharged from the first discharge port while the glass fine particle deposit is heated in the furnace core tube.

Description

Apparatus for manufacturing glass preform for optical fiber

[0001] The present disclosure relates to an apparatus for manufacturing a glass preform for an optical fiber. The present application claims priority based on Japanese Patent Application No. 2025-053312 filed on March 27, 2025, and incorporates all the content described in the said application by reference.

[0002] Patent Document 1 discloses a configuration for maintaining a constant pressure inside the core tube of a sintering furnace.

[0003] Japanese Unexamined Patent Publication No. 05-022532

[0004] An apparatus for manufacturing a glass preform for an optical fiber according to one aspect of the present disclosure includes: a core tube capable of accommodating a support rod and a glass particulate deposit supported by the support rod, the core tube having a first through hole formed in an upper portion thereof into which the support rod can be inserted; and an upper box portion provided on an upper portion of the core tube and defining an upper box space that communicates with an internal space of the core tube via the first through hole together with the upper portion of the core tube, wherein the upper box portion has: a first discharge port formed on a side surface of the upper box portion for discharging air in the upper box space; an opening formed on the side surface of the upper box portion and communicating the upper box space with outside air of the upper box portion and the core tube; and a second through hole formed in an upper portion of the upper box portion into which the support rod is inserted, the core tube is capable of heating the glass particulate deposit while introducing a corrosive gas into the interior thereof, and air is discharged from the first discharge port while heating the glass particulate deposit in the core tube.

[0005] Figure 1 illustrates an apparatus for manufacturing a glass preform for an optical fiber. Figure 2 is an enlarged view of section II in Figure 1. Figure 3 is a top view of a cross section orthogonal to the support rod of the upper box portion. Figure 4 illustrates a manufacturing apparatus according to a modified example. Figure 5 is a graph showing transmission loss at each wavelength of an optical fiber manufactured from a glass preform under a first condition. Figure 6 is a graph showing transmission loss at each wavelength of an optical fiber manufactured from a glass preform under a second condition.

[0006] Support rods are inserted into the reactor core tubes. Therefore, a gap is inevitably formed between the reactor core tubes and the support rods. Maintaining positive pressure relative to atmospheric pressure inside the reactor core tubes makes it difficult for impurities to enter through this gap.

[0007] However, the reactor core tubes contain chlorine (Cl ２ ) and silicon tetrafluoride (SiF ４ Corrosive gases such as ) are sometimes supplied into the reactor core tube. In this case, the glass base material was manufactured while maintaining negative pressure inside the reactor core tube to prevent corrosive gases from leaking out through the gaps. As a result, impurities and other contaminants were easily introduced through these gaps, which tended to degrade the quality of the glass.

[0008] This disclosure aims to provide a manufacturing apparatus for optical fiber glass substrates that is less prone to the leakage of corrosive gases.

[0009] Embodiments of the present disclosure will be listed and described first. (1) A manufacturing apparatus for a glass matrix for optical fibers according to one aspect of the present disclosure is a furnace tube capable of housing a support rod and a glass microparticle deposition supported by the support rod, the furnace tube having a first through-hole formed at its upper part into which the support rod can be inserted, and an upper box portion provided at the upper part of the furnace tube and defining an upper box space together with the upper part of the furnace tube, communicating with the internal space of the furnace tube through the first through-hole, wherein the upper box portion has a first exhaust port formed on the side surface of the upper box portion for discharging air from the upper box space, an opening formed on the side surface of the upper box portion communicating with the upper box space, the upper box portion and the outside air of the furnace tube, and a second through-hole formed at the upper part of the upper box portion into which the support rod can be inserted, the furnace tube is capable of heating the glass microparticle deposition while introducing a corrosive gas into it, and while heating the glass microparticle deposition inside the furnace tube, air is discharged from the first exhaust port.

[0010] According to the above-described manufacturing apparatus for optical fiber glass substrates, the upper box section discharges air from the first outlet, and air flows into the upper box space through the second through-hole and opening. As a result, even if corrosive gas leaks out from the first through-hole, it is difficult for the corrosive gas to leak out of the reactor core tube and the upper box section through the second through-hole. Furthermore, even if the flow rate of air discharged from the first outlet fluctuates, only the flow rate of outside air flowing in through the second through-hole and opening fluctuates, and the pressure in the upper box space does not easily fluctuate. As a result, the pressure inside the reactor core tube does not easily fluctuate even through the first through-hole.

[0011] (2) In the apparatus for manufacturing glass preforms for optical fibers according to (1) above, a lid portion defining a small chamber is provided at the top of the furnace tube, the small chamber is formed at the top of the lid portion and communicates with the upper box space through a third through hole into which the support rod can be inserted, a second outlet is formed in the small chamber for discharging the air inside the small chamber, and a flow regulator capable of adjusting the flow rate of the discharged air may be provided in the discharge pipe extending from the second outlet to the outside of the small chamber.

[0012] According to the above-described manufacturing apparatus for optical fiber glass substrates, a second outlet is provided in the small chamber, and a flow regulator capable of adjusting the flow rate of the discharged air is installed in the exhaust pipe extending from the second outlet to the outside of the small chamber. By adjusting the flow rate of the discharged air, the internal pressure in the small chamber and the reactor core tube is adjusted. However, when the flow regulator adjusts the air flow rate, the air in the exhaust pipe may flow backward, and impurities may also flow into the small chamber along with the airflow. However, since the backflowing air does not directly flow into the reactor core tube, it becomes difficult for impurities to flow into the reactor core tube.

[0013] (3) In the apparatus for manufacturing optical fiber glass substrates according to (1) or (2) above, the opening may be located in the opposite direction to the first outlet with respect to the support rod when viewed from above.

[0014] According to the above-described manufacturing apparatus for optical fiber glass substrates, the opening is located opposite the first outlet to the support rod, so the air flowing in from the opening flows towards the support rod. Since corrosive gases flow along the support rod, the air flowing in from the opening comes into contact with the support rod, making it easier for the corrosive gases to separate from the support rod and flow towards the first outlet. As a result, it becomes difficult for corrosive gases to leak out of the reactor core tube and upper box through the second through-hole.

[0015] (4) In a manufacturing apparatus for optical fiber glass substrates according to any of (1) to (3) above, the air in the upper box space may be discharged such that the airflow velocity at the center of the opening is 3 m / s or more.

[0016] According to the above-described manufacturing apparatus for optical fiber glass substrates, a flow velocity of 3 m / s or more at the center of the opening makes it difficult for corrosive gases to leak out from the second through-hole and the opening.

[0017] (5) In a manufacturing apparatus for optical fiber glass preforms according to any of (1) to (4) above, a groove is formed on the lower surface of the upper box, and the groove may be in communication with the outside air of the upper box and the furnace tube.

[0018] According to the above-described manufacturing apparatus for optical fiber glass substrates, a groove is formed on the contact surface between the upper box and the core tube, and the groove communicates with the outside air of both the upper box and the core tube. This allows the contact surface of the upper box, which is prone to overheating, to be cooled.

[0019] (Details of Embodiments of the Disclosure) Specific examples of optical fiber manufacturing apparatus according to embodiments of the Disclosure are described below with reference to the drawings. However, the Disclosure is not limited to these examples and is intended to include all modifications within the meaning and scope of the claims, as indicated by the claims.

[0020] In the attached drawing, arrow F indicates the front direction of the illustrated structure. Arrow B indicates the rear direction of the illustrated structure. Arrow U indicates the upward direction of the illustrated structure. Arrow D indicates the downward direction of the illustrated structure. Arrow R indicates the right direction of the illustrated structure. Arrow L indicates the left direction of the illustrated structure. These directional expressions are used for explanatory purposes only and do not limit the actual orientation or direction of the illustrated structure in use.

[0021] Figure 1 illustrates a manufacturing apparatus 1 for optical fiber glass preforms. The manufacturing apparatus 1 is a device that manufactures glass preforms by heating and dehydrating a deposit of glass microparticles G1 supported by a support rod 2. The manufacturing apparatus 1 comprises a furnace tube 10 and an upper box section 30.

[0022] The reactor core tube 10 is formed to accommodate a support rod 2 and a deposit of glass microparticles G1 supported by the support rod 2. A heater 3 is provided around the reactor core tube 10. A reactor core tube inlet 10a is formed at the bottom of the reactor core tube 10. A reactor core tube outlet 10b is formed at the top of the reactor core tube 10. The reactor core tube 10 can heat the deposit of glass microparticles G1 while introducing corrosive gas into the interior of the reactor core tube 10 from the reactor core tube inlet 10a.

[0023] Figure 2 is an enlarged view of part II in Figure 1. As illustrated in Figure 2, the reactor core tube 10 includes a lid portion 20. The lid portion 20 is provided on the upper part of the reactor core tube 10. The lid portion 20 forms one or more small chambers in its internal space. In this embodiment, the lid portion 20 forms three small chambers: a first small chamber S1, a second small chamber S2, and a third small chamber S3. The first small chamber S1 is located below the second small chamber S2 and the third small chamber S3. The second small chamber S2 is located above the first small chamber S1 and below the third small chamber S3.

[0024] A first through-hole H1 is formed in the partition wall 21 separating the internal space of the reactor core tube 10 from the first small chamber S1 for the support rod 2 to pass through. A through-hole is formed in the partition wall 22 separating the first small chamber S1 from the second small chamber S2 for the support rod 2 to pass through. A through-hole is formed in the partition wall 23 separating the second small chamber S2 from the third small chamber S3 for the support rod 2 to pass through. Ideally, airtightness in the small chambers should be maintained when the support rod 2 passes through any of the through-holes, but in reality, gaps are created in the through-holes. Therefore, the internal space of the reactor core tube 10 is in communication with the first small chamber S1, the first small chamber S1 is in communication with the second small chamber S2, and the second small chamber S2 is in communication with the third small chamber S3 via the through-holes.

[0025] A third through-hole H3 is formed in the partition wall 24 at the top of the lid portion 20 and the top of the third small chamber S3, into which the support rod 2 can be inserted.

[0026] The first chamber S1 and the third chamber S3 are connected to intake pipes that can introduce gases such as purge gas. The purge gas is, for example, nitrogen gas.

[0027] The second chamber S2 has a second outlet D2 for discharging the air inside the second chamber S2. The discharge pipe extending from the second outlet D2 to the outside of the second chamber S2 is equipped with a flow regulator A that can adjust the flow rate of the discharged air.

[0028] The upper box portion 30 is provided on the upper part of the core tube 10. In this embodiment, the upper box portion 30 is provided on the upper part of the lid portion 20. The upper box portion 30 is made of polycarbonate, nickel, or the like. The upper box portion 30 has a cylindrical housing shape for vertically inserting the support rod 2. The upper box portion 30, together with the partition wall 24 which is the upper part of the lid portion 20, defines a cylindrical upper box space S4.

[0029] On the side of the upper box section 30, there is a first exhaust port D1 for discharging air from the upper box space S4, and an opening O that communicates the upper box space S4 with the outside air of the upper box section 30 and the core tube 10. An exhaust pipe for discharging air is connected to the first exhaust port D1. The first exhaust port D1 discharges air from the upper box space S4 during the manufacturing of the glass base material. In other words, air is discharged from the first exhaust port D1 while the glass particle deposition G1 is heated inside the core tube 10.

[0030] Figure 3 is a view from above of a cross-section of the upper box portion 30 perpendicular to the support rod 2. As illustrated in Figure 3, the opening O is located in the opposite direction to the first discharge port D1 with respect to the support rod 2. In this embodiment, the first discharge port D1 is located to the left of the support rod 2, and the opening O is located to the right of the support rod 2.

[0031] Returning to Figure 2, a second through-hole H2 into which the support rod 2 is inserted is formed in the upper part of the upper box section 30. Ideally, the upper box space S4 should be airtight to the outside air when the support rod 2 is passing through the second through-hole H2, but in reality, a gap is created in the second through-hole H2.

[0032] The third small chamber S3 is in communication with the upper box space S4 via the third through-hole H3. Since the internal space of the core tube 10 and the third small chamber S3 are indirectly in communication, the upper box space S4 is in communication with the internal space of the core tube 10 via the first through-hole H1.

[0033] A groove P is formed on the lower surface 31 of the upper box portion 30. The groove P communicates with the outside air between the upper box portion 30 and the reactor core tube 10. The groove P also communicates with the upper box space S4. Therefore, outside air between the upper box portion 30 and the reactor core tube 10 flows into the upper box space S4 through the groove P.

[0034] Next, we will explain the airflow inside the manufacturing apparatus 1 during the manufacturing of the glass base material. During the manufacturing of the glass base material, the air in the upper box space S4 is drawn in and discharged from the first outlet D1. As a result, the upper box space S4 is under negative pressure.

[0035] As illustrated in Figure 2, when the internal space of the reactor core tube 10 is under positive pressure, the air inside the reactor core tube 10 containing corrosive gas (hereinafter simply referred to as corrosive gas) flows into the first small chamber S1 through the first through-hole H1, as shown in flow F1. The corrosive gas flows into the third small chamber S3, which is indirectly connected to the first small chamber S1. Furthermore, the corrosive gas flows into the upper box space S4 through the third through-hole H3. The corrosive gas flows upward along the support rod 2 inside the upper box space S4, as shown in flow F2.

[0036] When the upper box space S4 is under negative pressure due to the discharge of air from the first outlet D1, outside air from the upper box 30 flows into the upper box space S4 through the second through-hole H2, as shown in flow F3. Also, outside air from the upper box 30 flows into the upper box space S4 through the opening O, as shown in flow F4. At this time, the air in the upper box space S4 may be discharged such that the air velocity vc at the center position C of the opening O is 3 m / s or more.

[0037] Thus, within the upper box space S4, there are flows F2 in which corrosive gas rises along the support rod 2, flows F3 in which outside air flows in through the second through-hole H2, and flows F4 in which outside air flows in through the opening O. For this reason, corrosive gas does not easily flow in the direction of the second through-hole H2 and the opening O, and the stagnant corrosive gas easily flows towards the first outlet D1.

[0038] Here, since the opening O is located in the opposite direction to the first outlet D1 relative to the support rod 2, the flow F4 flowing from the opening O flows so as to hit the support rod 2. Flow F4 has a larger flow rate than flow F2, and the flow F2 rising along the support rod 2 is blocked by flow F4. As a result, the corrosive gas is less likely to flow above flow F4, and the stagnant corrosive gas is more likely to flow towards the first outlet D1, as in flow F5.

[0039] According to the glass preform manufacturing apparatus 1 of the present embodiment, the upper box portion 30 discharges air from the first discharge port D1, so air flows into the upper box space S4 through the second through hole H2 and the opening O. Accordingly, even if corrosive gas flows out from the first through hole H1, the corrosive gas is less likely to flow out to the outside of the furnace core tube 10 and the upper box portion 30 through the second through hole H2.

[0040] Even if corrosive gas leaks from the furnace core tube 10, since the corrosive gas is less likely to leak from the manufacturing apparatus 1, it becomes possible to maintain a positive pressure inside the furnace core tube 10 during manufacturing of the glass preform. Generally, when corrosive gas is introduced during manufacturing of a glass preform, the pressure inside the furnace core tube 10 is often set to a negative pressure to make it difficult for corrosive gas to leak from the furnace core tube 10. However, setting the pressure inside the furnace core tube 10 to a negative pressure has been one of the causes for outside air to flow in through the first through hole H1 and the like. When outside air flows into the furnace core tube 10, impurities adhere to the glass particulate deposit G1 and the glass preform, which causes deterioration in the quality of optical fibers manufactured from the glass preform.

[0041] According to the glass preform manufacturing apparatus 1 of the present embodiment, corrosive gas is less likely to flow out of the manufacturing apparatus 1, so even when corrosive gas is introduced into the furnace core tube 10, the inside of the furnace core tube 10 can be maintained at a positive pressure. Accordingly, manufacturing of the glass preform is performed in an environment where impurities are less likely to flow into the furnace core tube 10, so the quality of optical fibers manufactured from the glass preform is improved.

[0042] The flow rate of air discharged from the first discharge port D1 is not constant and may fluctuate. According to the glass preform manufacturing apparatus 1 of the present embodiment, even if the flow rate of air discharged from the first discharge port D1 fluctuates, only the flow rate of outside air flowing in through the second through hole H2 and the opening O fluctuates, and the pressure in the upper box space S4 is less likely to fluctuate. Accordingly, the pressure inside the furnace core tube 10 is less likely to fluctuate even via the first through hole H1.

[0043] According to the glass preform manufacturing apparatus 1 of the present embodiment, the second sub-chamber S2 is provided with a second discharge port D2, and a discharge pipe extending from the second discharge port D2 to the outside of the second sub-chamber S2 is provided with a flow rate regulator A capable of adjusting the flow rate of discharged air. By adjusting the flow rate of air discharged by the flow rate regulator A, the internal pressure in the second sub-chamber S2 and the furnace core tube 10 is adjusted. Here, when the flow rate regulator A adjusts the air flow rate, the air in the discharge pipe may flow backward, and impurities may also flow into the second sub-chamber S2 along with the air flow. However, since the backflowing air does not directly flow into the furnace core tube 10, impurities are less likely to flow into the furnace core tube 10.

[0044] According to the glass preform manufacturing apparatus 1 of the present embodiment, the opening O is located in the opposite direction to the first discharge port D1 with respect to the support rod 2, so the air flowing in from the opening O flows so as to hit the support rod 2. Since the corrosive gas flows along the support rod 2, the air flowing in from the opening O hits the support rod 2, which makes the corrosive gas easily peel off from the support rod 2 and easily flow toward the first discharge port D1. This makes it difficult for corrosive gas to flow out of the furnace core tube 10 and the upper box portion 30 through the second through hole H2.

[0045] According to the glass preform manufacturing apparatus 1 of the present embodiment, when the flow velocity vc of air at the center position C in the opening O is 3 m / s or more, it becomes difficult for corrosive gas to flow out from the second through hole H2 and the opening O.

[0046] The lower surface 31 of the upper box portion 30 is a surface to be installed on the furnace core tube 10, and therefore is a portion that tends to reach a high temperature. For example, when the upper box portion 30 is formed of polycarbonate, it is preferable to take measures so that the portion of the lower surface 31 is less likely to deform or melt. According to the glass preform manufacturing apparatus 1 of the present embodiment, a groove P is formed in the lower surface 31, which is the installation surface of the upper box portion 30 with the furnace core tube 10. The groove P communicates with the outside air of the upper box portion 30 and the furnace core tube 10. This makes it possible to cool the easily heated contact surface in the upper box portion 30.

[0047] In particular, when the groove P is also in communication with the upper box space S4, the discharge of air from the first outlet D1 from the upper box space S4 causes outside air from the upper box section 30 to flow into the upper box space S4 through the groove P, as shown in flow F6. As a result, the lower surface 31 of the upper box section 30 is more easily cooled.

[0048] Figure 4 illustrates a modified manufacturing apparatus 101. The modified manufacturing apparatus 101 differs from the manufacturing apparatus 1 illustrated in Figures 1 and 2 in that a small chamber is not formed in the lid of the core tube 110. For this reason, in the modified manufacturing apparatus 101, the upper box portion 130 is installed directly on top of the core tube 110, and the internal space of the core tube 110 and the upper box space S4 of the upper box portion 30 are in communication via the first through-hole H1 and the third through-hole H3.

[0049] In such a manufacturing apparatus 101, as air is discharged from the first outlet D1 while the glass particle deposit G1 is heated inside the furnace tube 110, corrosive gases are less likely to leak out of the manufacturing apparatus 101.

[0050] Next, the flow rate of air discharged from the first outlet D1 was varied, and the flow velocity vc at the center position C of the opening O and the flow velocity vH at the second through-hole H2 were measured to verify whether or not there was leakage of corrosive gas from the production apparatus 1 under each condition.

[0051]

[0052] Table 1 shows the flow velocity vc and vH, and the presence or absence of corrosive gas leakage under each condition. In the case of gas leakage, Y indicates that corrosive gas leakage was detected. In the case of gas leakage, N indicates that no corrosive gas leakage was detected. － Although no gas leaks of corrosive gases were detected, this indicates that the amount of air discharged from the first outlet D1 was too high, preventing a positive pressure from being created inside the reactor core tube 10.

[0053] Under all conditions, the flow velocity vc is slower than the flow velocity vH. This is thought to be because the opening O is wider than the void formed in the second through-hole H2 into which the support rod 2 is inserted. Therefore, it is thought that corrosive gases are more likely to leak out from the opening O.

[0054] Under condition H, the flow velocity vc at the center position C of the opening O was 2.5 m / s, and in this case, gas leakage of corrosive gas was detected. On the other hand, under condition G, the flow velocity vc at the center position C of the opening O was 3.2 m / s, and in this case, gas leakage of corrosive gas was not detected. Therefore, it is considered that corrosive gas is unlikely to leak out if the flow velocity vc is at least equal to or greater than that of condition G. Based on these verification results and further detailed verification, it was confirmed that when air is discharged from the first outlet D1 so that the flow velocity vc is 3.0 m / s or higher, corrosive gas is unlikely to leak out.

[0055] Next, we investigated the effect of creating a positive pressure inside the furnace tube 10 on the quality of optical fibers manufactured from the glass matrix. In this investigation, the transmission loss at each wavelength of the optical fiber was examined as a measure of the quality of the manufactured optical fiber. The manufacturing of the glass matrix includes a dehydration process, a fluorine addition process, and a sintering process. We examined the transmission loss at each wavelength of the optical fiber under two conditions with different pressures inside the furnace tube 10 during the dehydration process, the fluorine addition process, and the sintering process.

[0056] The first condition was that the glass base material was manufactured under conditions where the pressure inside the furnace tube 10 was relatively positive. The average internal pressure inside the furnace tube during the dewatering process was -84 ± 100 Pa. The average internal pressure inside the furnace tube during the fluorine addition process was -10 ± 100 Pa. The average internal pressure inside the furnace tube during the sintering process was -11 ± 100 Pa.

[0057] The second condition was that the glass base material was manufactured in the conventional manner, such that the pressure inside the furnace tube 10 became negative. The average internal pressure inside the furnace tube during the dewatering process was -175 ± 100 Pa. The average internal pressure inside the furnace tube during the fluorine addition process was -208 ± 100 Pa. The average internal pressure inside the furnace tube during the sintering process was -241 ± 100 Pa.

[0058] In both the first and second conditions, the temperature, the flow rate of the inert gas introduced, the flow rate of the corrosive gas introduced, and the processing time in each step remained unchanged. In the dewatering step, the temperature was 1240°C, the flow rate of the inert gas introduced was 15.5 slm of helium (He), and the corrosive gas introduced was chlorine (Cl). ２ The volume is 3.0 slm, and the processing time is 6 hours. In the fluorine addition process, the temperature is 1270°C, the flow rate of the inert gas introduced is helium (He) at 20.0 slm, and the corrosive gas introduced is silicon tetrafluoride (SiF). ４ The volume is 0.9 slm, and the processing time is 12 hours. In the dewatering process, the temperature is 1460°C, the flow rate of the inert gas introduced is helium (He) at 20.0 slm, and the corrosive gas introduced is silicon tetrafluoride (SiF). ４ The volume is 0.9 slm, and the processing time is 3 hours.

[0059] Figure 5 is a graph showing the transmission loss at each wavelength of an optical fiber manufactured from a glass matrix under the first condition. Figure 6 is a graph showing the transmission loss at each wavelength of an optical fiber manufactured from a glass matrix under the second condition. As shown in Figures 5 and 6, the transmission loss of the optical fiber manufactured from the glass matrix under the first condition is reduced compared to the transmission loss of the optical fiber manufactured from the glass matrix under the second condition.

[0060] Here, as an example of transmission loss in optical fibers, we will compare in detail the transmission loss under each condition for wavelengths of 1380 nm and 1550 nm. The transmission loss at a wavelength of 1380 nm under the first condition was 0.303 dB / km. The transmission loss at a wavelength of 1380 nm under the second condition was 0.403 dB / km. Therefore, it was confirmed that by making the pressure inside the reactor core relatively positive, from the second condition to the first condition, the transmission loss of the optical fiber at a wavelength of 1380 nm was improved by 0.100 dB / km.

[0061] The transmission loss at a wavelength of 1550 nm under the first condition was 0.152 dB / km. The transmission loss at a wavelength of 1550 nm under the second condition was 0.154 dB / km. Therefore, it was confirmed that by making the pressure inside the reactor tube relatively positive, from the second condition to the first condition, the transmission loss of the optical fiber at a wavelength of 1550 nm was improved by 0.002 dB / km.

[0062] Thus, it was confirmed that increasing the pressure inside the core tube 10 improves the quality of the glass matrix, and consequently, the quality of the optical fibers manufactured from the glass matrix also improves.

[0063] Although this disclosure has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of this disclosure. Furthermore, the number, position, shape, etc. of the components described above are not limited to the embodiments described above and can be changed to a number, position, shape, etc. that is suitable for carrying out this disclosure.

[0064] 1,101 Manufacturing equipment 2 Support rod 3 Heater 10,110 Core tube 10a Core tube introduction section 10b Core tube discharge section 20 Lid section 21, 22, 23, 24 Partition wall 30, 130 Upper box section 31 Lower surface S1 First small chamber S2 Second small chamber S3 Third small chamber S4 Upper box space H1 First through hole H2 Second through hole H3 Third through hole D1 First discharge port D2 Second discharge port O Opening P Groove G1 Glass particle deposit body

Claims

1. A furnace tube capable of housing a support rod and a glass microparticle deposition supported by the support rod, the furnace tube having a first through-hole formed at its upper part into which the support rod can be inserted; an upper box portion provided at the upper part of the furnace tube and together with the upper part of the furnace tube, defining an upper box space that communicates with the internal space of the furnace tube through the first through-hole, wherein the upper box portion has a first exhaust port formed on the side surface of the upper box portion for discharging air from the upper box space, an opening formed on the side surface of the upper box portion that communicates with the upper box space, the upper box portion, and the outside air of the furnace tube, and a second through-hole formed at the upper part of the upper box portion into which the support rod can be inserted; the furnace tube is capable of heating the glass microparticle deposition while introducing a corrosive gas into it; and while heating the glass microparticle deposition inside the furnace tube, air is discharged from the first exhaust port, wherein the furnace tube is capable of heating the glass microparticle deposition while air is discharged from the first exhaust port.

2. The upper part of the core tube is provided with a lid that defines a small chamber, the small chamber is formed on the upper part of the lid and communicates with the upper box space through a third through-hole into which the support rod can be inserted, the small chamber is provided with a second outlet for discharging air from inside the small chamber, and the discharge pipe extending from the second outlet to the outside of the small chamber is provided with a flow regulator capable of adjusting the flow rate of the discharged air, the apparatus for manufacturing a glass preform for optical fibers according to claim 1.

3. In a top view, the opening is located in the opposite direction to the first outlet relative to the support rod, the apparatus for manufacturing a glass preform for optical fibers according to claim 1 or 2.

4. The apparatus for manufacturing a glass preform for optical fibers according to any one of claims 1 to 3, wherein the air in the upper box space is discharged such that the airflow velocity at the center of the opening is 3 m / s or more.

5. A groove is formed on the lower surface of the upper box portion, and the groove is in communication with the outside air of the upper box portion and the furnace core tube, the apparatus for manufacturing a glass preform for optical fibers according to any one of claims 1 to 4.

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