Sintering method of porous glass substrate for optical fiber

Abstract

A sintering apparatus includes: a core tube including a porous glass substrate for an optical fiber, a longitudinal direction of the porous glass substrate being along an axial direction; and a multi-stage heater in which two or more heaters surround the core tube and are arranged along the axial direction of the core tube to form a heating zone in the core tube. A sintering method includes a step of heating the substrate in the heating zone to perform a first dehydration treatment; and a step of moving the substrate so that a position determined as being least dehydrated in the longitudinal direction of the substrate is a position in the axial direction of the core tube where the temperature is highest in the heating zone, and then performing a second dehydration treatment.

Description

TECHNICAL FIELD

[0001] The present invention relates to a sintering method of a porous glass substrate for optical fiber, which is produced by dehydrating and sintering a glass substrate. BACKGROUND

[0002] A porous glass substrate formed by depositing glass particles on a target rod such as a glass rod is made transparent by dehydrating and sintering, and used as a glass substrate for optical fiber. The VAD method and the OVD method are used to produce a porous glass substrate for optical fiber. In these methods, a glass raw material is first combusted in a flame to hydrolyze and generate fine glass particles, and then the fine glass particles are adhered in the axial direction or the radial direction of a rotating target rod to produce a porous glass substrate for optical fiber.

[0003] Dehydration and sintering of a porous glass substrate for optical fiber is performed using a dehydration and sintering apparatus equipped with a furnace core tube containing a porous glass substrate for optical fiber held by a support rod and a heater arranged around the furnace core tube. A gas supply port is provided at the bottom of the furnace core tube through which a gas required for dehydration and sintering of the porous glass substrate for optical fiber, such as a halogen gas and an inert gas, is supplied. On the other hand, an exhaust pipe is provided at the top of the furnace core tube to exhaust the gas in the furnace core tube. Then, the porous glass substrate for optical fiber held by the support rod is lowered while rotating in the furnace core tube. The porous glass substrate for optical fiber is dehydrated and sintered as it passes through a heating zone of the heater.

[0004] As a method of dehydration and sintering, JP 2010-189251 A describes a two-stage vitrification method in which a porous glass substrate is dehydrated by a heating zone set to a temperature of 900-1300°C. The dehydrated porous glass substrate is then pulled up once to a predetermined position in the furnace core tube, the temperature of the heating zone is changed to 1400-1600°C, and then the glass substrate is sintered again by passing through the heating zone. However, the method in JP 2010-189251 A has the problem of long processing time and low production efficiency. To overcome this problem, a method using multiple heater stages and a wider heating zone is described in JP 2016-88821 A. SUMMARY

[0005] Problems to be Solved by the Invention

[0006] When the heaters are arranged in multiple stages, as in the method described in JP 2016-88821 A, there can be a low-temperature region between the heaters, resulting in a region where dehydration is insufficient. When a porous glass substrate for optical fiber having such a region is vitrified, the transmission loss at 1383 nm (OH absorption wavelength) increases in a part of the longitudinal direction of the manufactured glass substrate for optical fiber.

[0007] An object of the present invention is to provide a sintering method of a porous glass substrate for optical fibers that enables dehydration to be sufficiently performed as a whole even if there is a low-temperature region between heaters.

[0008] Method of solving the problem

[0009] The sintering method of a porous glass substrate for optical fibers according to the present invention is performed using a sintering apparatus including a core tube that contains a porous glass substrate for optical fibers, the longitudinal direction of the porous glass substrate for optical fibers being along an axial direction, and a multi-stage heater in which two or more heaters surround the core tube and are arranged in the axial direction of the core tube to form a heating zone in the core tube, the heaters being capable of being independently controlled in temperature, wherein the sintering method includes a first dehydration step in which the porous glass substrate for optical fibers is heated in the heating zone to perform a first dehydration process, and a second dehydration step in which, for the porous glass substrate for optical fibers after the first dehydration process, the porous glass substrate for optical fibers after the first dehydration process is moved in such a way that a position in the longitudinal direction of the porous glass substrate for optical fibers determined by a predetermined method to be least dehydrated is located in a position in the axial direction of the core tube where the temperature is highest in the heating zone, and then a second dehydration process of further heating is performed.

[0010] By adopting this method, even if dehydration is insufficient in a part of the longitudinal direction of the porous glass substrate for optical fibers due to a low-temperature region between heaters in the first dehydration process, the part where dehydration is insufficient is further dehydrated in the second dehydration process, and thus dehydration can be sufficiently performed as a whole.

[0011] For example, the second dehydration process can be performed for two hours or more.

[0012] This allows more reliable dehydration.

[0013] In the first dehydration process, the porous glass substrate for optical fibers can be heated while being repeatedly reciprocated in the axial direction of the core tube.

[0014] By heating, when reciprocated in this way repeatedly in the first dehydration process, the number and range of regions where dehydration is insufficient can be reduced at least compared to the case where heating is performed in a stationary state.

[0015] In the second dehydration process, the porous glass substrate for optical fibers that has been moved after the first dehydration process can be further heated while being repeatedly reciprocated in the axial direction of the core tube around the position where it has been moved.

[0016] By heating, when the reciprocating motion is repeated in this manner in the second dehydration treatment, the portions in the low temperature region can be uniformly heated at the highest temperature of the heating zone by the multi-stage heater. As a result, sufficient dehydration can be achieved more reliably overall.

[0017] The predetermined method of identifying the position of the most insufficient dehydration of the porous glass substrate for optical fiber after the first dehydration treatment can include, for example, subjecting another porous glass substrate for optical fiber, which is manufactured in the same manner as the porous glass substrate for optical fiber, to the first dehydration treatment, then measuring a distribution of a predetermined optical property of the produced glass substrate for optical fiber, which is heated and sintered at a temperature higher than the heating temperature in the first dehydration treatment, in the longitudinal direction, and identifying the position based on the measurement result.

[0018] The sintered glass substrate for optical fiber shrinks with respect to the porous glass substrate for optical fiber before sintering, but the relative positional relationship between the two is maintained in the longitudinal direction. Therefore, by ascertaining the optical property of each position in the longitudinal direction of the porous glass substrate for optical fiber after sintering, the optical property of each position in the longitudinal direction of the porous glass substrate for optical fiber before sintering can be ascertained. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 A schematic cross-sectional view of a sintering apparatus of a porous glass substrate for optical fiber and a temperature distribution in the axial direction of the furnace core tube are shown.

[0020] Figure 2 How the porous glass substrate for optical fiber is moved in the second dehydration treatment is shown.

[0021] Figure 3 A relationship between the execution time of the second dehydration treatment and the lowest refractive index in the longitudinal direction of the porous glass substrate for optical fiber is shown.

[0022] Figure 4 A refractive index distribution in the longitudinal direction of the glass substrate for optical fiber in each of the comparative example, Example 1, and Example 2 is shown. DETAILED DESCRIPTION

[0023] Hereinafter, an embodiment of the present application is described. Common components in each drawing are denoted by the same reference numerals.

[0024] Figure 1 (a) of FIG. 1 shows a schematic cross-sectional view of a sintering apparatus 100 of a porous glass substrate for optical fiber for performing a sintering method of the porous glass substrate for optical fiber according to the present application.

[0025] The sintering apparatus 100 of the porous glass substrate for optical fiber is an apparatus for dehydrating and sintering the porous glass substrate for optical fiber, and is equipped with a support rod 110, a furnace core tube 120, a multi-stage heater 130, and a lifting device 140.

[0026] The porous glass substrate B for the optical fiber to be sintered is prefabricated by any method such as the VAD method and has a core formed at its center and a cladding formed around it.

[0027] The support rod 110 supports the porous glass substrate B for optical fibers at one end.

[0028] The core tube 120 includes a porous glass substrate B for optical fibers supported by a support rod 110, such that the longitudinal direction L of the porous glass substrate for optical fibers coincides with the axial direction D of the core tube 120. The core tube 120 is a cylindrical core tube made of, for example, quartz glass, and has an openable and closable top cover. The core tube 120 is equipped with a gas supply port 121 at the bottom for supplying gases required for dehydration and sintering, and a gas exhaust port 122 at the top for venting gases from the core tube 120 to the outside.

[0029] The multi-stage heater 130 has heaters 131 and 132 that can control the temperature of each other independently. Heaters 131 and 132 are respectively arranged around the core tube 120 and along the axial direction D of the core tube 120 to form a heating zone in the core tube 120. The example here shows a two-stage heater, but three or more stages are also acceptable.

[0030] The lifting device 140 grips the other end of the support rod 110 and, if necessary, raises and lowers the porous glass substrate B for optical fibers along the axial direction D of the furnace core tube 120 via the support rod 110.

[0031] In this invention, the porous glass substrate B for optical fiber is dehydrated and sintered by sequentially performing the first dehydration process, the second dehydration process, and the sintering process described below using the sintering equipment 100 for the porous glass substrate for optical fiber described above, in order to produce the glass substrate for optical fiber.

[0032] <First Dehydration Process>

[0033] In the first dehydration process, one end of the support rod 110 is connected to the porous glass substrate B for optical fibers, and the other end of the support rod 110 is connected to the lifting device 140. Then, the porous glass substrate B for optical fibers in this state is inserted into the furnace core tube 120, and the top cover is closed.

[0034] Next, the porous glass substrate B for optical fiber is positioned at a predetermined location in the furnace core tube 120 using the lifting device 140, and the heaters 131 and 132 are heated so that the heating zone of the porous glass substrate B for optical fiber is within a predetermined temperature range. For example, a temperature range of 1000-1300°C is suitable.

[0035] At this time, halogen gas or a mixture of halogen gas and inert gas is supplied to the furnace core tube 120 from the gas supply port 121. An appropriate amount of the supplied gas is discharged from the gas discharge port 122, maintaining the pressure in the furnace core tube 120 at a constant level. The volume concentration of the supplied halogen gas should preferably be 20-100%. For dehydration treatment, Cl2 or SiCl4 is a preferred halogen gas, and He, Ar, or N2 is a preferred inert gas.

[0036] When heating the porous glass substrate B for optical fiber in the first dehydration process, the porous glass substrate B for optical fiber can be heated while reciprocating in the axial direction D of the furnace core tube 120 via the lifting device 140.

[0037] For those formed in Figure 1 The heating zone in the core tube 120 shown in (a) Figure 1 (b) shows an example of the temperature distribution corresponding to a position in the axial direction D of the heating zone. Figure 1 As can be seen from (b), there is a temperature peak near the center of each of the heaters constituting the multi-stage heater 130, while there are low-temperature regions between the heaters.

[0038] If the porous glass substrate B for optical fiber is heated by keeping it stationary in a heating zone with this temperature distribution, this will not only lead to insufficient dehydration in the low-temperature regions, but in some cases, it will also lead to insufficient dehydration in multiple locations or over a large area. However, by heating the porous glass substrate B while repeatedly moving it back and forth in the axial direction D of the furnace core tube 120, the porous glass substrate B for optical fiber is heated uniformly in the longitudinal direction L. Therefore, at least compared with the case of heating in a stationary state, the number of locations and the size of the area of ​​insufficient dehydration can be reduced.

[0039] The width of the reciprocating motion and the number or duration of the reciprocating motion can be appropriately set so that the straight body of the porous glass substrate B for optical fiber is uniformly heated in the heating zone.

[0040] <Second Dehydration Process>

[0041] In the second dehydration process, the porous glass substrate B for optical fibers, after the first dehydration process, is moved such that the position of the least dehydrated porous glass substrate B, determined by a predetermined method, in the longitudinal direction is located at the position in the axial direction of the furnace core tube with the highest temperature in the heating zone. Then, a second dehydration process is performed, in which the porous glass substrate for optical fibers after the first dehydration process is further heated.

[0042] For example, suppose that the position X of the least dehydrated porous glass substrate B for optical fiber in the longitudinal direction L after the first dehydration treatment is determined by... Figure 2 The bold line in (a) indicates the location. If according to Figure 2 The temperature distribution at the location of the heating zone formed in the furnace core tube 120 in the axial direction D, as shown in (a), is... Figure 2 If the state shown in (c) is such that the porous glass substrate B for the optical fiber after the first dehydration treatment is moved, the position X with the least dehydration reaches the position shown by the dashed line, which is the position with the highest temperature in the heating zone of the furnace core tube 120.

[0043] A predetermined method for determining the location of the least dehydrated porous glass substrate B for optical fibers after the first dehydration treatment may include, for example, performing a first dehydration treatment on another porous glass substrate for optical fibers manufactured in the same manner as the porous glass substrate B for optical fibers, then measuring the distribution of predetermined optical properties of the optical fiber glass substrate produced by heating and sintering at a temperature higher than the heating temperature in the first dehydration treatment in the longitudinal direction, and determining the location based on the measurement results.

[0044] The optical properties of the fiber optic glass substrate used to determine the location X in the porous glass substrate B for optical fiber that is least dehydrated after the first dehydration treatment include, for example, the refractive index distribution and infrared absorption in the longitudinal direction of the cladding portion.

[0045] The sintered optical fiber glass substrate shrinks relative to the unsintered porous optical fiber glass substrate, but their relative positional relationship in the longitudinal direction is maintained. Therefore, by examining the optical properties of each position in the longitudinal direction of the sintered porous optical fiber glass substrate, the optical properties of each position in the longitudinal direction of the unsintered porous optical fiber glass substrate can be determined. For example, due to residual chlorine from the dehydration process gas, the refractive index of the cladding portion becomes higher at locations with insufficient dehydration. Therefore, by examining the longitudinal refractive index distribution of the cladding portion of the sintered optical fiber glass substrate, the location with the least dehydration in the unsintered porous optical fiber glass substrate can be determined. Furthermore, since infrared absorption increases at locations with insufficient dehydration and high OH concentration, the location with the least dehydration can also be determined by examining the infrared absorption in the longitudinal direction.

[0046] Figure 3 The relationship between the execution time of the second dehydration treatment and the lowest refractive index in the longitudinal direction is shown. From Figure 3 It can be seen that when the time is less than 2 hours, the minimum refractive index increases over time, which means that insufficient dehydration is gradually being resolved. After 2 hours, the refractive index is almost constant, which means that sufficient dehydration has been achieved. Therefore, it is preferable to perform the second dehydration treatment for at least two hours.

[0047] By performing a second dehydration process in addition to the first dehydration process, even if some portions of the porous glass substrate for optical fibers are not sufficiently dehydrated in the longitudinal direction due to the presence of the low-temperature region between the heaters in the first dehydration process, the insufficiently dehydrated portions are further dehydrated in the second dehydration process, so that overall sufficient dehydration is achieved.

[0048] In the second dehydration process, the porous glass substrate B for optical fibers, which has already moved after the first dehydration process, can be further heated while repeatedly reciprocating around its moved position in the axial direction of the furnace core tube.

[0049] As mentioned above, if the optical fiber is heated in the first dehydration process by keeping it stationary in the heating zone, the heating zone has Figure 1 The temperature distribution shown in (b) not only leads to insufficient dehydration in the low-temperature regions, but in some cases, it also results in insufficient dehydration in multiple locations or over large areas. However, after the first dehydration treatment, the substrate is moved to the highest temperature position in the heating zone along the axial direction D of the core tube 120. Simultaneously, the porous glass substrate B for optical fibers is heated by repeatedly reciprocating along the axial direction D of the core tube 120 around the highest temperature position in the heating zone. The highest temperature in the heating zone of the multi-stage heater 130 can be used to uniformly heat the portions in the low-temperature regions, thereby enabling more reliable and complete dehydration overall.

[0050] The range of motion for the reciprocating motion can, for example, be determined based on measurements of the optical property distribution along the longitudinal direction L of the glass substrate for optical fibers, performed to determine the position X where dehydration is least adequate. This allows for the determination of the range of positions along the longitudinal direction L where dehydration is insufficient above a certain level, and the determined range of positions is used as the range of motion. For example, if the determined range of positions is... Figure 2 The shaded area shown in (a) is as follows Figure 2 The optical fiber is moved using the porous glass substrate B as shown in (b), and then reciprocated so that the shaded area repeatedly passes through the position shown by the dashed line, which is the position of the highest temperature in the heating zone of the core tube 120.

[0051] <Sintering Treatment>

[0052] Following the second dehydration process, heater 132 is further heated to form a high-temperature heating zone that is hotter than the heating zones in the first and second dehydration processes. For example, a heating temperature of 1000-1300°C is suitable. The porous glass substrate for optical fiber, after the second dehydration process, is then lowered and passed through the high-temperature heating zone to sequentially sinter the porous glass substrate from the bottom up, thereby producing a glass substrate for optical fiber.

[0053] According to the above-mentioned sintering method for porous glass substrates for optical fibers, dehydration can be fully achieved as a whole, even in the presence of low-temperature regions between heaters.

[0054] Example

[0055] Examples and comparative examples that support the effects of this invention are described below.

[0056] (Comparative Example)

[0057] After a first dehydration treatment of the porous glass substrate for optical fibers, a sintering process is performed to produce the glass substrate for optical fibers. Figure 4 In the image, the refractive index distribution of the glass substrate for optical fibers produced in this manner is shown in the longitudinal direction using long dashed lines. This refractive index distribution indicates the existence of a region in the longitudinal direction where the refractive index is significantly reduced due to insufficient water removal.

[0058] (Example 1)

[0059] For the porous glass substrate for optical fiber after the first dehydration treatment under the same conditions as the comparative example, the location with the least sufficient dehydration, determined based on the refractive index distribution measured in the longitudinal direction of the cladding portion of the optical fiber glass substrate produced in the comparative example, was moved to the location with the highest temperature in the heating zone of the multi-stage heater. Then, a second dehydration treatment was performed for two hours, during which the porous glass substrate for optical fiber was heated while remaining in that location. The porous glass substrate for optical fiber after the second dehydration treatment was then further subjected to a sintering process to produce a glass substrate for optical fiber. Figure 4 In the diagram, the longitudinal refractive index distribution of the glass substrate used for optical fibers produced in this manner is represented by a solid line. This refractive index distribution indicates that the substrate is sufficiently dehydrated throughout the entire region, without any significant decrease in refractive index due to inadequate dehydration.

[0060] (Example 2)

[0061] For the porous glass substrate for optical fiber after the first dehydration treatment under the same conditions as the comparative example, the position with the least dehydration, determined based on the refractive index distribution measured in the longitudinal direction of the cladding portion of the optical fiber glass substrate produced in the comparative example, was moved to the position with the highest temperature in the heating zone of the multi-stage heater. Then, the upper position of the reciprocating motion of the porous glass substrate for optical fiber was determined to be the position where the lower position of the region with insufficient dehydration above a certain level coincided with the position of the highest temperature in the heating zone. The lower position of the reciprocating motion was determined to be the position where the upper position of the region with insufficient dehydration above a certain level coincided with the position of the highest temperature in the heating zone. The second dehydration treatment was performed for two hours, during which the porous glass substrate was heated while repeating the reciprocating motion in this manner. Then, the porous glass substrate for optical fiber after the second dehydration treatment was further sintered to produce an optical fiber glass substrate. Figure 4 In the diagram, the longitudinal refractive index distribution of the glass substrate for optical fibers produced in this manner is shown by the short dashed line. This refractive index distribution indicates that the substrate is sufficiently dehydrated throughout the region, without any significant decrease in refractive index due to insufficient dehydration.

[0062] This invention is not limited to the above embodiments and examples, but any changes that have a configuration that is substantially the same as the technical concept described in the claims of this invention and produce similar effects are included within the technical scope of this invention.

Claims

1. A method for sintering a porous glass substrate for optical fibers, characterized in that, The sintering method is performed using a sintering apparatus including: a core tube containing a porous glass substrate for optical fiber, a longitudinal direction of the porous glass substrate being along an axial direction; and a multi-stage heater in which two or more heaters surround the core tube and are arranged in the axial direction of the core tube to form a heating zone in the core tube, the heaters being capable of being independently controlled in temperature from each other, wherein the sintering method includes: a first dehydration step in which the porous glass substrate for optical fiber is heated in the heating zone to perform a first dehydration treatment; a second dehydration step in which, for the porous glass substrate for optical fiber after the first dehydration treatment, the porous glass substrate for optical fiber after the first dehydration treatment is moved in such a manner that a position in the longitudinal direction of the porous glass substrate for optical fiber determined by a predetermined method as being least dehydrated is located at a position in the axial direction of the core tube where the temperature is highest in the heating zone, and then a second dehydration treatment of further heating is performed; and the porous glass substrate for optical fiber after the second dehydration treatment is further subjected to a sintering treatment to produce a glass substrate for optical fiber, wherein, in the second dehydration treatment, when the porous glass substrate for optical fiber that has been moved after the first dehydration treatment is further heated, reciprocating movement is repeatedly performed in the axial direction of the core tube with the position after the movement as a center.

2. The sintering method of porous glass substrates for optical fibers according to claim 1, wherein, The second dehydration treatment is performed for 2 hours or more.

3. The sintering method of porous glass substrates for optical fibers according to claim 1, wherein, In the first dehydration treatment, the porous glass substrate for optical fiber is heated while reciprocating movement is repeatedly performed in the axial direction of the core tube.

4. The sintering method of porous glass substrates for optical fibers according to claim 1, wherein, The predetermined method of determining the position of the porous glass substrate for optical fiber where dehydration is least sufficient after the first dehydration treatment is a method in which another porous glass substrate for optical fiber manufactured in the same manner as the porous glass substrate for optical fiber is subjected to the first dehydration treatment, then a distribution of a predetermined optical property of a glass substrate for optical fiber produced by heating and sintering at a temperature higher than the heating temperature in the first dehydration treatment is measured in the longitudinal direction, and the position is determined based on the measurement result.

Images