JP7571607B2 - Optical fiber manufacturing method - Google Patents

Description

This disclosure relates to a method for manufacturing an optical fiber base material and a method for manufacturing an optical fiber.

Patent Document 1 discloses a method for producing a glass particle deposit by depositing glass particles on a seed rod using the VAD (Vapor Phase Axial Deposition) method. The glass particle deposit thus obtained, with the seed rod attached to one end, undergoes a dehydration process and a transparent vitrification process by sintering to become a transparent glass preform. The obtained transparent glass preform is then drawn to produce an optical fiber.

JP 2017-178630 A

When a transparent glass preform is manufactured from the glass particle deposit obtained by the method disclosed in Patent Document 1, the concentration of OH groups is high in the portion of the transparent glass preform close to the seed rod, even though OH groups are removed from the glass particle deposit in the dehydration process. If such a portion with a high concentration of OH groups is used to manufacture optical fiber, the transmission loss will be large. For this reason, in the past, the portion of the transparent glass preform close to the seed rod was discarded and not used in the manufacture of optical fiber. Discarding part of the transparent glass preform ultimately contributed to increasing the manufacturing cost of optical fiber.

The objective of this disclosure is to reduce the concentration of OH groups in the portion of the transparent glass preform close to the seed rod, thereby enabling the manufacture of optical fiber with low transmission loss due to OH groups even when the portion of the transparent glass preform close to the seed rod is used.

A method for producing an optical fiber preform according to one embodiment of the present disclosure includes the steps of:
a seed rod manufacturing process for manufacturing a seed rod having an average OH group concentration of 1 ppm or less in a region at a tip end portion that is 0.05 mm or more and 0.5 mm or less away from the surface;
a glass soot deposit manufacturing step of manufacturing a glass soot deposit by supplying glass raw material to a burner to generate glass soot and depositing the glass soot on the rotating seed rod;
a dehydration step of heating the soot glass deposit body in a dehydrating gas using a heater to dehydrate the soot glass deposit body;
a transparent vitrification step of heating the soot glass deposit by the heater after the dehydration step to obtain a transparent glass base material;
Equipped with.

A method for producing an optical fiber according to one aspect of the present disclosure includes the steps of:
The present invention also includes manufacturing an optical fiber using a region of the optical fiber preform obtained by the method for manufacturing an optical fiber preform that is within 10 mm in the axial direction from the tip of the seed rod, and producing an optical fiber having a transmission loss at a wavelength of 1380 nm that is an increase of less than 40 mdB/km compared to an optical fiber manufactured using a portion of the optical fiber preform that is located 100 mm in the axial direction from the tip of the seed rod.

According to the configuration disclosed above, even if the part of the transparent glass base material close to the seed rod, which was previously discarded, is used, it is possible to manufacture an optical fiber with low transmission loss due to OH groups. As a result, the manufacturing cost of the optical fiber can be reduced.

FIG. 1 is a schematic diagram showing the diffusion of OH groups from a seed rod in the transparent vitrification process. FIG. 2 is a schematic diagram illustrating a method for manufacturing an optical fiber preform according to an embodiment of the present disclosure. FIG. 3 is a graph showing an example of the OH group concentration at the tip of a seed rod according to the present disclosure. FIG. 4 is a graph showing the diffusion of OH groups when the seed rod according to the present disclosure is heated. FIG. 5 is an example of an infrared absorption spectrum of a seed rod. FIG. 6 is a graph showing the transmission loss of optical fibers produced from the optical fiber preforms produced in each production example. FIG. 7 is a graph showing the relationship between the length of the tip of the seed rod and the incidence rate of increased OH loss.

The inventors conducted various studies to determine the cause of the problem of high OH group concentration in the part of the transparent glass base material close to the seed rod. First, the inventors focused on the dehydration process and conducted experiments by changing conditions such as the temperature and time in the dehydration process and the concentration of chlorine gas introduced during dehydration, but the problem of high OH group concentration could not be solved. The inventors also considered the possibility that outside air was entering from outside the furnace tube during the transparent vitrification process, but found that this was not related to the above problem.

As a result of various studies such as those described above, the inventors have found that the above problem occurs when OH groups are released from the heated seed rod. FIG. 1 is a schematic diagram showing the diffusion of OH groups from a seed rod 10' in the transparent vitrification process. In the transparent vitrification process, a glass particle deposit M1 with a seed rod 10' attached to one end is placed in a furnace tube 31 and heated by a heater 32 to obtain a transparent glass base material. At this time, the OH groups contained in the seed rod 10' diffuse outward due to the heating, and diffuse into the core portion M11 and cladding portion M12 of the glass particle deposit M1. Finally, the OH groups remaining in the core portion M11 cause transmission loss. The present disclosure has been made based on such findings.

[Description of the embodiments of the present disclosure]
The embodiments of the present disclosure will be listed and described.
A method for producing an optical fiber preform according to one embodiment of the present disclosure includes the steps of:
a seed rod manufacturing process for manufacturing a seed rod having an average OH group concentration of 1 ppm or less in a region at a tip end portion that is 0.05 mm or more and 0.5 mm or less away from the surface;
a glass soot deposit manufacturing step of manufacturing a glass soot deposit by supplying glass raw material to a burner to generate glass soot and depositing the glass soot on the rotating seed rod;
a dehydration step of heating the soot glass deposit body in a dehydrating gas using a heater to dehydrate the soot glass deposit body;
a transparent vitrification step of heating the soot glass deposit by the heater after the dehydration step to obtain a transparent glass base material;
Equipped with.
According to this configuration, the concentration of OH groups in the transparent glass preform close to the seed rod is reduced, and even when the transparent glass preform close to the seed rod is used, it is possible to manufacture an optical fiber with low transmission loss due to OH groups, thereby reducing the manufacturing cost of the optical fiber.

In the method for producing an optical fiber preform,
The seed rod manufacturing step is preferably a step of manufacturing a seed rod having an average OH group concentration of 3 ppm or less in a region at the tip end that is 0.05 mm or more and 2.0 mm or less away from the surface.
According to this configuration, it is possible to further reduce the concentration of OH groups in the portion of the transparent glass preform close to the seed rod, and as a result, even when an optical fiber is manufactured using the portion of the transparent glass preform close to the seed rod, it is possible to stably manufacture an optical fiber with low transmission loss due to OH groups.

In the method for producing an optical fiber preform,
The length of the tip portion is preferably equal to or greater than half the length of the heater.
According to this configuration, it is possible to further reduce the concentration of OH groups in the portion of the transparent glass preform close to the seed rod, and as a result, even when an optical fiber is manufactured using the portion of the transparent glass preform close to the seed rod, it is possible to stably manufacture an optical fiber with low transmission loss due to OH groups.

A method for producing an optical fiber according to one aspect of the present disclosure includes the steps of:
The present invention also includes manufacturing an optical fiber using a region of the optical fiber preform obtained by the method for manufacturing an optical fiber preform that is within 10 mm in the axial direction from the tip of the seed rod, and producing an optical fiber having a transmission loss at a wavelength of 1380 nm that is an increase of less than 40 mdB/km compared to an optical fiber manufactured using a portion of the optical fiber preform that is located 100 mm in the axial direction from the tip of the seed rod.
According to this configuration, even if a part of the transparent glass preform close to the seed rod, which was conventionally discarded, is used, it is possible to manufacture an optical fiber with low transmission loss due to OH groups, which results in a reduction in the manufacturing cost of the optical fiber.

[Details of the embodiment of the present disclosure]
Hereinafter, examples of embodiments according to the present disclosure will be described with reference to the drawings. In the following description, the same or equivalent elements are denoted by the same reference numerals even in different drawings, and duplicated descriptions will be omitted as appropriate. In addition, the scale of each drawing used in the following description is appropriately changed so that each component can be recognized.

(Method of manufacturing optical fiber preform)
2 is a schematic diagram showing a method for manufacturing an optical fiber preform (hereinafter also referred to as a "transparent glass preform") M2 according to an embodiment of the present disclosure. As shown in FIG. 2, the method for manufacturing the optical fiber preform M2 according to the embodiment includes steps 1 to 4.

Step 1 is a seed rod manufacturing step for manufacturing the seed rod 10. The seed rod 10 satisfies the below-described condition 1 regarding the average OH group concentration at least in the tip 11 (the portion between the tip P2 and the position P1). The axial length a of the tip 11 is preferably, for example, half or more of the axial length b of the heater 32. This is because it is preferable that the tip P2 of the seed rod 10 descends to near the center of the heater 32 in the below-described step 4 (transparent vitrification step). In this case, the portion up to a position axially a distance b/2 from the tip P2 will enter the heat zone of the heater 32 in the transparent vitrification step. The portion that enters the heat zone is heated more strongly than other portions of the seed rod 10, so that the OH groups are more likely to diffuse into the transparent glass base material M2, but such diffusion can be prevented by making the length a of the tip 11 half or more of the length b of the heater 32.

Figure 3 is a graph showing an example of the OH group concentration at the tip 11 of the seed rod 10. The seed rod 10 satisfies condition 1 that the average OH group concentration in the region R1, which is 0.05 mm or more and 0.5 mm or less from the surface, is 1 ppm or less. Conventionally, the OH groups that were the cause of transmission loss were mainly due to the OH groups present in the region R1. Therefore, by making the average OH group concentration in the region R1 1 ppm or less, it is possible to prevent the OH groups originating from the seed rod 10 from diffusing and remaining in the transparent glass base material M2. In addition to condition 1, it is preferable that the seed rod 10 satisfies condition 2 that the average OH group concentration in the region R2, which is 0.05 mm or more and 2.0 mm or less from the surface, is 3 ppm or less.

In addition, in region R3, which is at a distance from the surface of 0 mm or more and less than 0.05 mm, there is no condition regarding the average OH group concentration, and the average OH group concentration may reach several thousand ppm, for example. This is because the OH groups contained in region R3 diffuse from seed rod 10 to glass particle deposit M1 in step 2 (dehydration step), and are further removed from glass particle deposit M1.

Figure 4 is a graph showing the diffusion of OH groups when the seed rod 10 is heated, and is a theoretical graph calculated using a diffusion equation. As shown in Figure 4, OH groups present in an area more than 0.5 mm away from the surface remain inside the seed rod 10 even after heating for 4 hours, and are unlikely to diffuse outside the seed rod 10. This shows that the average OH group concentration in the area R1, which is 0.05 mm or more and 0.5 mm or less away from the surface, is the most important.

The average OH group concentration of the seed rod 10 can be measured using a conventionally known method. For example, the average OH group concentration of the seed rod 10 can be calculated by observing the O-H stretching vibration of Si-OH in an infrared absorption spectrum.

Fig. 5 is an example of an infrared absorption spectrum of a seed rod. The average OH group concentration of the seed rod can be calculated by the Lambert-Beer formula (1) below using the absorption peak I at a wave number of 3670 cm ^-1 corresponding to the O-H stretching vibration shown in Fig. 5 and the base I ₀ on the baseline.

C _OH =α/εt...(1)

In the above formula (1), the absorbance α is calculated by "α = Log10 (I0/I)". The molar absorption coefficient ε can be calculated by the value "ε = 77.5 L/mol cm" given in the paper by G. Stephenson et al. (G. Stephenson, et al., Trans. Br. Ceram. Soc., 59, 397 (1960)). The sample thickness t can be measured with a caliper or the like.

The distribution of the OH group concentration in the radial direction of the seed rod 10 can be calculated, for example, by the following method. First, the transmittance in the radial direction of the seed rod 10 is measured by the above formula (1), and the average OH group concentration in the radial direction of the seed rod 10 is obtained. Next, the surface of the seed rod 10 is etched by immersing the seed rod 10 in a hydrofluoric acid solution, for example. After that, the average OH group concentration of the seed rod 10 is obtained again by the above formula (1). From the outer diameter and the average OH group concentration of the seed rod 10 before and after etching, the OH group concentration of the etched part is calculated by the following formula (2). By sequentially repeating etching and measurement in the same procedure, the distribution of the OH group concentration in the radial direction of the seed rod 10 can be obtained.

(DC-D'C')/(DD')...(2)

In the above formula (2), "D" is the outer diameter of the seed rod 10 before etching. "D'" is the outer diameter of the seed rod 10 after etching. "C" is the average OH group concentration of the seed rod 10 before etching. "C'" is the average OH group concentration of the seed rod 10 after etching.

The seed rod 10 can be manufactured by heat treating a glass rod of fused quartz glass manufactured by the electric melting method or synthetic quartz glass manufactured by the soot method in a dry atmosphere to reduce the average OH group concentration in region R1 to 1 ppm or less. Quartz glass manufactured by the flame fusion method or direct method can also be used as the seed rod 10, but since the OH group concentration contained in the glass is high, it takes time to reduce the average OH group concentration. In addition, synthetic quartz glass sintered in a vacuum in the soot method is preferred because it is easy to obtain a low average OH group concentration in region R1. Furthermore, synthetic quartz glass sintered by dehydration treatment in the soot method is preferred because it is possible to obtain a low average OH group concentration not only in region R1 but also near the center (e.g., region R2).

The seed rod manufacturing process may include a step of flame polishing the surface of the seed rod 10. By performing flame polishing, the surface of the seed rod 10 is cleaned and smoothed, and it is possible to prevent crystallization or residual air bubbles at the interface with the deposited glass particles. Crystallization and residual air bubbles can cause cracks, so it is preferable to prevent these occurrences.

In addition, because flame polishing involves applying a burner flame to the surface of the seed rod 10, the OH group concentration on the surface of the seed rod 10 increases as a result of flame polishing. However, this increase in OH group concentration is within a range of 0.05 mm from the surface of the seed rod 10. As mentioned above, within this range, the OH groups are released from the surface of the seed rod 10 by heating during the dehydration process, and the OH group concentration is reduced during the transparent vitrification process, so there is no effect on the transmission loss of the optical fiber.

Returning to FIG. 2, steps 2 to 4 will now be described. Step 2 is a glass particle deposit manufacturing process for manufacturing a glass particle deposit M1. In the glass particle deposit manufacturing process, for example, the VAD method is used to manufacture the glass particle deposit M1. Specifically, glass raw material gas G1 is supplied to the core burner 21, and the core glass particles generated by the flame hydrolysis reaction are deposited on the rotating seed rod 10. In addition, glass raw material gas G2 is supplied to the clad burner 22, and the glass particles for the clad core generated by the flame hydrolysis reaction are deposited on the core glass particles. In this manner, the glass particle deposit M1 is manufactured.

The glass particle deposit M1 may also be manufactured using the OVD (outside vapor deposition) method. In the OVD method, a mandrel is inserted into a pipe-shaped seed rod, glass particles are deposited on the outside of the seed rod and mandrel, and after the deposition of the glass particles is completed, the mandrel is pulled out and, with the seed rod attached to one end, a dehydration process and a transparent vitrification process by sintering are carried out to produce a transparent glass base material. Therefore, when the OVD method is used, the average OH group concentration in the area 0.05 mm to 0.5 mm away from the surface on both the inner and outer surfaces of the pipe-shaped seed rod is set to 1 ppm or less.

The glass raw material gas G1 is a gas for core, which contains, for example, silicon tetrachloride (SiCl ₄ ) or siloxane, as well as a dopant such as germanium tetrachloride (GeCl ₄ ). The glass raw material gas G2 is a gas for cladding, which contains, for example, silicon tetrachloride or siloxane, but does not contain a dopant. In the glass soot deposit manufacturing process, a conventionally known configuration can be appropriately adopted.

Step 3 is a dehydration step for dehydrating the glass soot deposit M1. In the dehydration step, the glass soot deposit M1 is placed in the furnace core tube 31. Then, a dehydration gas G3 (for example, a mixed gas of an inert gas such as nitrogen and chlorine gas) is supplied to the furnace core tube 31, and the glass soot deposit M1 is lowered in the furnace core tube 31. The glass soot deposit M1 is heated by a heater 32 arranged around the furnace core tube 31 in an atmosphere containing the dehydration gas G3, and a dehydration process is performed. The dehydration process removes OH groups contained in the glass soot deposit M1, etc. The conditions in the dehydration step, such as temperature and time, can be set appropriately based on conventionally known conditions.

Step 4 is a transparent vitrification step in which the glass particle deposit M1 is heated by the heater 32 to obtain a transparent glass base material M2. In the transparent vitrification step, the glass particle deposit M1, which moved downward in the core tube 31 in the dehydration step, is raised to the upper part of the core tube 31. Then, an inert gas G4 (e.g., nitrogen or helium) is supplied to the core tube 31, and the glass particle deposit M1 is lowered in the core tube 31. The glass particle deposit M1 is heated and sintered by the heater 32 in an atmosphere containing the inert gas G4, and becomes a transparent glass base material M2. In the transparent vitrification step, it is preferable to lower the tip P2 of the seed rod 10 to near the center of the heater 32. The conditions such as temperature and time in the transparent vitrification step can be appropriately set based on conventionally known conditions.

The transparent glass base material M2 manufactured through steps 1 to 4 above has a lower OH group concentration in region M21 close to the seed rod 10 than when manufactured using conventional methods. Therefore, region M21, which was previously discarded, can also be used as a base material for optical fiber. Region M21 is the region between positions P5 and P6. Position P5 corresponds to the tip P2 of the seed rod 10. Position P6 is, for example, a position 100 mm axially downward from position P5.

(Method of manufacturing optical fiber)
The method for manufacturing an optical fiber according to this embodiment will be described below. The method for manufacturing an optical fiber according to this embodiment is a method for manufacturing an optical fiber using a region within 10 mm in the axial direction from the tip P2 of the seed rod 10 of the optical fiber preform M2 obtained by the above-mentioned method for manufacturing an optical fiber preform, and for manufacturing an optical fiber in which the increase in transmission loss at a wavelength of 1380 nm is less than 40 mdB/km is based on an optical fiber manufactured using a portion of the optical fiber preform M2 located 100 mm in the axial direction from the tip P2 of the seed rod 10. Note that an increase of less than 40 mdB/km from the reference also includes a case in which the transmission loss is lower than the reference.

The optical fiber preform M2 becomes an optical fiber through a conventionally known process for manufacturing an optical fiber from a transparent glass preform M2, such as a drawing process. The region M21 of the optical fiber preform M2 obtained by the above-mentioned manufacturing method for an optical fiber preform has a low average OH group concentration, so that the transmission loss at a wavelength of 1380 nm of an optical fiber obtained by using a region within 10 mm in the axial direction from the tip P2 of the seed rod 10 (hereinafter also referred to as transmission loss T ₁₀ ) increases by less than 40 mdB/km compared with the transmission loss (hereinafter also referred to as transmission loss T ₁₀₀ ) of an optical fiber manufactured by using a portion located 100 mm in the axial direction from the tip P2 of the seed rod 10. That is, "T ₁₀ -T ₁₀₀ < 40 mdB/km". The manufacturing method for an optical fiber according to this embodiment also includes manufacturing an optical fiber by using a region within 10 to 100 mm in the axial direction from the tip P2 of the seed rod 10 or a region below the region M21 shown in FIG. 2.

[Example]
The present disclosure will be described in more detail below by showing examples according to the present disclosure. Note that the present disclosure is not limited to the following examples.

(Production Examples 1 to 5)
The synthetic quartz glass produced by the soot method was heat-treated in a dry atmosphere to produce three low-OH seed rods, one of which was confirmed to satisfy the above-mentioned condition 1 but not to satisfy condition 2. The synthetic quartz glass produced by the soot method was heat-treated in a dry atmosphere to dehydrate it to produce three low-OH seed rods, one of which was confirmed to satisfy the above-mentioned conditions 1 and 2. Two normal seed rods, which were commercially available fused quartz glass produced by the electric melting method, were also prepared. From one of the normal seed rods, it was confirmed that the average OH group concentration in the region at the tip from 0.05 mm to 0.5 mm away from the surface was 130 ppm.

The above-mentioned glass particle deposit manufacturing process, dehydration process, and transparent vitrification process were carried out under the same conditions for each of the four low OH seed rods and one normal seed rod, to produce five transparent glass preforms M2. Next, each of the obtained transparent glass preforms M2 was drawn under the same conditions to produce optical fibers. The transmission loss at 1380 nm was measured for each of the obtained optical fibers. The results are shown in Figure 6. Note that Production Examples 1 and 2 are production examples using low OH seed rods that satisfy condition 1 but do not satisfy condition 2. Production Examples 3 and 4 are production examples using low OH seed rods that satisfy conditions 1 and 2. Production Example 5 is a production example using a normal seed rod.

Figure 6 is a graph showing the transmission loss of optical fibers manufactured from the optical fiber preform M2 manufactured in each manufacturing example. Each plot point on the graph in Figure 6 corresponds to the transparent glass preform M2 shown at the top of the graph. For example, the transmission loss of an optical fiber manufactured using position P6 as the preform is the value on the line indicating position P6.

As shown in FIG. 6, when a normal seed rod not satisfying condition 1 is used, the optical fiber manufactured using region M21 shows a significant increase in transmission loss. On the other hand, when a low-OH seed rod satisfying condition 1 is used, even an optical fiber manufactured using region M21 does not show a significant increase in transmission loss. In particular, Manufacturing Examples 3 and 4 using low-OH seed rods satisfying conditions 1 and 2 show low transmission loss values. Specifically, the _T10 - _T100 values are 3 mdB/km for Manufacturing Example 1, 3 mdB/km for Manufacturing Example 2, 3 mdB/km for Manufacturing Example 3, 3 mdB/km for Manufacturing Example 4, and 46 mdB/km for Manufacturing Example 5. In addition, in Manufacturing Examples 1 to 4, the transmission loss of the optical fiber manufactured using region M21 is 0.31 dB/km or less.

(Production Examples 6 to 10)
Transparent glass preforms M2 were manufactured under the same conditions except that the length a of the tip 11 of the seed rod 10 was changed to 0.1 times, 0.2 times, 0.5 times, 0.8 times, and 1 times the length b of the heater 32. In the transparent vitrification process, the tip P2 of the seed rod 10 was lowered to near the center of the heater 32.

Next, each of the obtained transparent glass preforms M2 was drawn under the same conditions to produce optical fibers. For each transparent glass preform M2, the optical fibers produced using the portion located 10 mm axially downward from position P5 were judged for the presence or absence of an increase in OH loss (transmission loss due to OH groups). An increase in OH loss was judged to have occurred when the transmission loss at 1380 nm increased by 40 mdB/km or more, based on the optical fiber produced using the portion located 100 mm axially downward from position P5.

The transparent glass preform M2 was manufactured multiple times under each of the above conditions for the length a of the tip portion 11, and the occurrence rate of increased OH loss under each condition (number of transparent glass preforms M2 manufactured under the same conditions that experienced increased OH loss/number of transparent glass preforms M2 manufactured under the same conditions) was calculated. The results are shown in Figure 7.

From Figure 7, it can be seen that the occurrence rate of increased OH loss can be reduced by making the length a of the tip portion 11 0.5 times or more the length b of the heater 32.

Although the present invention 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 the present invention. Furthermore, the number, position, shape, etc. of the components described above are not limited to the above embodiment, and can be changed to any number, position, shape, etc. suitable for implementing the present invention.

10', 10: seed rod 11: tip 21: core burner 22: clad burner 31: furnace core tube 32: heater

G1, G2: Glass raw material gas G3: Dehydration gas G4: Inert gas M1: Glass particle deposit M11: Core part M12: Cladding part M2: Transparent glass base material (base material for optical fiber)
M21: Area R1, R2, R3: Area

Claims (3)

a seed rod manufacturing process for manufacturing a seed rod having an average OH group concentration of 1 ppm or less in a region at a tip end portion that is 0.05 mm or more and 0.5 mm or less away from the surface;
a glass soot deposit manufacturing step of manufacturing a glass soot deposit by supplying glass raw material to a burner to generate glass soot and depositing the glass soot on the rotating seed rod;
a dehydration step of heating the soot glass deposit body in a dehydrating gas using a heater to dehydrate the soot glass deposit body;
a transparent vitrification step of heating the soot glass deposit by the heater after the dehydration step to obtain a transparent glass base material;
using a region of the optical fiber preform obtained by the method for producing an optical fiber preform comprising the steps of:
and manufacturing an optical fiber having a transmission loss at a wavelength of 1380 nm that increases by less than 40 mdB/km based on an optical fiber manufactured using a portion of the optical fiber preform that is located 100 mm axially from the tip of the seed rod . 2. The method for producing an optical fiber according to claim 1, wherein the seed rod production step is a step of producing a seed rod having an average OH group concentration of 3 ppm or less in a region at a distance of 0.05 mm or more and 2.0 mm or less from the surface at the tip portion. 3. The method for producing an optical fiber according to claim 1, wherein the length of the tip portion is equal to or greater than half the length of the heater.

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