PREFORM FOR OPTICAL FIBER AND MANUFACTURING METHOD OF OPTICAL FIBER

Abstract

A preform for optical fiber made of a quartz glass sintered body which is made from quartz glass powder as a main raw material, in which at least a portion of the quartz glass sintered body is an opaque body, the opaque body has a visible light transmittance of 90% or less at a length of 5 mm or less in one direction, and the opaque body has a bulk density of 2.1 g/cm.sup.3 or more, or the opaque body has a visible light transmittance of 90% or less at a length of 5 mm or less in one direction, and the opaque body has an open porosity of 3.5% or less.

Claims

1-11. (canceled)

12. A preform for optical fiber made of a quartz glass sintered body which is made from quartz glass powder as a main raw material, wherein: (a) at least a portion of the quartz glass sintered body is an opaque body, the opaque body has a visible light transmittance of 90% or less at a length of 5 mm or less in one direction, and the opaque body has a bulk density of 2.1 g/cm.sup.3 or more; (b) at least a portion of the quartz glass sintered body is an opaque body, the opaque body has a visible light transmittance of 90% or less at a length of 5 mm or less in one direction, and the opaque body has an open porosity of 3.5% or less; or (c) at least a portion of the quartz glass sintered body is an opaque body, the opaque body has a visible light transmittance of 2.5% or more and 90% or less at a length of 5 mm or less in one direction.

13. A preform for optical fiber made of a columnar quartz glass sintered body which is made from quartz glass powder as a main raw material, wherein: (a) the preform includes a portion having a ratio of outer roundness to outer diameter of 0.5% or less and a bulk density of 2.1 g/cm.sup.3 or more; (b) the preform includes a portion having a ratio of outer roundness to outer diameter of 0.5% or less and an open porosity of 3.5% or less; or (c) the preform includes a portion having a ratio of outer roundness to outer diameter of 0.5% or less.

14. The preform for optical fiber according to claim 12, wherein the quartz glass sintered body is a clad preform having a through-hole for inserting a core material.

15. The preform for optical fiber according to claim 12, wherein the quartz glass sintered body is a core preform.

16. An optical fiber preform comprising: a clad preform having a through-hole; and a core preform inserted into the through-hole, wherein at least one of the clad preform and the core preform is the preform for optical fiber according to claim 12.

17. A manufacturing method of an optical fiber preform, the method comprising a step of manufacturing an optical fiber preform by inserting a core preform into a through-hole of a clad preform having the through-hole, and a step of sintering the clad preform and the core preform, wherein at least one of the clad preform and the core preform is the preform for optical fiber according to claim 12.

18. A method of manufacturing an optical fiber, the method comprising a step of drawing the optical fiber preform according to claim 16.

19. The preform for optical fiber according to claim 13, wherein the quartz glass sintered body is a clad preform having a through-hole for inserting a core material.

20. The preform for optical fiber according to claim 13, wherein the quartz glass sintered body is a core preform.

21. An optical fiber preform comprising: a clad preform having a through-hole; and a core preform inserted into the through-hole, wherein at least one of the clad preform and the core preform is the preform for optical fiber according to claim 13.

22. A manufacturing method of an optical fiber preform, the method comprising a step of manufacturing an optical fiber preform by inserting a core preform into a through-hole of a clad preform having the through-hole, and a step of sintering the clad preform and the core preform, wherein at least one of the clad preform and the core preform is the preform for optical fiber according to claim 13.

23. A method of manufacturing an optical fiber, the method comprising a step of drawing the optical fiber preform according to claim 21.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a schematic temperature profile in a step of manufacturing an optical fiber from a clad preform according to an embodiment of the present invention.

[0028] FIG. 2 is a cross-sectional view of a clad preform.

[0029] FIG. 3 is a graph showing the relationships between the sintering temperature and the light transmittance, the sintering temperature and the outer roundness, and the sintering temperature and the hole roundness of clad preforms obtained from samples 1 to 5 used in Manufacturing Example 1.

[0030] FIG. 4 is a graph showing the relationships between the sintering temperature and the light transmittance, the sintering temperature and the outer roundness, and the sintering temperature and the hole roundness of clad preforms obtained from samples 6 to 9 used in Manufacturing Example 2.

[0031] FIG. 5 is a graph showing the relationships between the sintering temperature and the outer diameters, and the sintering temperature and the hole diameters of clad preforms obtained from samples 10 to 20 used in Manufacturing Example 3.

[0032] FIG. 6 is a graph showing the relationships between the sintering temperature and the outer diameters, the sintering temperature and the bulk density, and the sintering temperature and the apparent density of the clad preforms obtained from the samples 10 to 20 used in Manufacturing Example 3.

[0033] FIG. 7 is a graph showing the relationships between the sintering temperature and the bulk density, and the sintering temperature and the light transmittance of the clad preforms obtained from the samples 10 to 20 used in Manufacturing Example 3.

[0034] FIG. 8 is a graph showing the relationship between the bulk density and the open porosity of the clad preforms obtained from the samples 10 to 20 used in Manufacturing Example 3.

[0035] FIG. 9 is a graph showing the relationships between the sintering temperature and the bulk density, and the sintering temperature and the open porosity of the clad preforms obtained from the samples 10 to 20 used in Manufacturing Example 3.

[0036] FIG. 10 is a graph showing the relationships between the sintering temperature and the open porosity, and the sintering temperature and the light transmittance of the clad preforms obtained from the samples 10 to 20 used in Manufacturing Example 3.

[0037] FIG. 11 is a graph showing the relationships between the sintering temperature and the contraction ratios, and the sintering temperature and the light transmittance of the clad preforms obtained from the samples 10 to 20 used in Manufacturing Example 3. FIG. 12 is a graph showing the relationships between the sintering temperature and the bulk density, and the sintering temperature and the Vickers hardness of the clad preforms obtained from the samples 10 to 20 used in Manufacturing Example 3.

DESCRIPTION OF EMBODIMENTS

[0038] The present invention relates to a preform for optical fiber made of a quartz glass sintered body which is made from quartz glass powder as a main raw material. The preform for optical fiber according to the present invention includes an optical fiber preform (optical fiber base material), a core preform (core base material), and a clad preform (clad base material).

[0039] The preform for optical fiber according to the present invention is characterized in that at least a portion of the quartz glass sintered body is an opaque body, the opaque body has a visible light transmittance of 90% or less at a length of 5 mm or less in one direction, and the opaque body has a bulk density of 2.1 g/cm.sup.3 or more, or that at least a portion of the quartz glass sintered body is an opaque body, the opaque body has a visible light transmittance of 90% or less at a length of 5 mm or less in one direction, and the opaque body has an open porosity of 3.5% or less, or that at least a portion of the quartz glass sintered body is an opaque body, the opaque body has a visible light transmittance of 2.5% or more and 90% or less at a length of 5 mm or less in one direction.

[0040] If the preform for optical fiber according to the present invention has a columnar shape, the preform for optical fiber is characterized in that the preform includes a portion having a ratio of the outer roundness to the outer diameter of 0.5% or less and a bulk density of 2.1 g/cm.sup.3 or more, or includes a portion having a ratio of the outer roundness to the outer diameter of 0.5% or less and an open porosity of 3.5% or less, or includes a portion having a ratio of the outer roundness to the outer diameter of 0.5% or less. Here, the portion having the ratio of the outer roundness is 0.5% or less includes a portion having the ratio of the outer roundness of approximately 0%, that is, a columnar preform for optical fiber the cross-sectional shape of which is approximately a complete round.

[0041] If the preform for optical fiber according to the present invention is an optical fiber preform, the optical fiber preform includes a core preform and a clad preform. Both the core preform and the clad preform may satisfy the requirements for the preform for optical fiber according to the present invention, or only one of the core preform and the clad preform may satisfy the requirements for the preform for optical fiber according to the present invention.

[0042] Hereinafter, an embodiment in which the preform for optical fiber according to the present invention is applied to a clad preform will be described.

[0043] FIG. 1 illustrates a schematic temperature profile in steps of manufacturing an optical fiber from a clad preform of the present embodiment, where a horizontal axis represents time and a vertical axis represents temperature. As shown in FIG. 1, a clad preform 100 of the present embodiment includes a columnar glass sintered body, and has, inside the glass sintered body, a hole 10 penetrating both end surfaces. Although FIG. 1 shows the clad preform 100 having one hole 10, the clad preform 100 may have a plurality of holes. The outer shape of the clad preform is not limited to a columnar shape, and may be a prismatic shape. A clad preform having no hole penetrating both end surfaces may be used. In the case of this clad preform, a hole penetrating both end surfaces can be formed by cutting or the like.

[0044] In this embodiment, the clad preform 100 is manufactured by a slurry cast method. Manufacturing steps of the slurry cast method includes the steps of slurry blending, molding, demolding, drying, degreasing, and sintering. FIG. 1 shows the sintering step, which is the final step of the slurry cast method.

[0045] In the slurry cast method, first, a glass raw material solution containing a quartz glass powder, a solvent, a dispersant, and a hardening resin is placed in a ball mill and mixed over a predetermined time to blend a slurry (blending step). As the solvent, distilled water is usually used. The glass raw material solution may contain an additive (titanium dioxide (TiO.sub.2), aluminum oxide (Al.sub.2O.sub.3), fluorine (F), or the like) for adjusting the refractive index of the clad. A slurry for manufacturing the core preform may contain germanium dioxide (GeO.sub.2) in addition to the above-described additive as an additive for adjusting the refractive index of the core. It is not excluded that an unintended impurity is contained in the glass raw material solution. The slurry taken out from the ball mill is filled in a molding die in which a hole forming rod is disposed after a hardener is added.

[0046] When the slurry filled in the molding die is left at room temperature, the hardening resin is hardened (molding step). When the molded body is formed in the molding die by hardening the hardening resin, the molding die and the hole forming rod are removed from the molded body (demolding step). Subsequently, the molded body having the hole is dried, degreased, and sintered. In the drying step, the solvent (distilled water) in the molded body is mainly removed, and in the degreasing step, the hardening resin in the molded body is mainly removed. Then, a clad preform which is a glass sintered body having a hole is obtained by the sintering step.

[0047] The clad preform produced as described above is processed to be an optical fiber through a drawing step by a rod-in tube method, that is, after a core material is inserted into the hole of the clad preform. For the purpose of removing contaminants on a surface of the preform, flame polishing may be carried out before drawing. The temperature profile of drying, degreasing, sintering, flame polishing, and drawing is set in advance according to the composition of the slurry, the size and shape of the clad preform, and the like.

[0048] In the rod-in tube method, because the core preform is inserted into the hole of the clad preform, if the position of the hole of the clad preform is not as designed, if the interval between such holes is not as designed when the clad preform has a plurality of holes, or if the roundness of the hole is poor and a gap is generated around the core material inserted into the hole, the transmission characteristics of the obtained optical fiber may be deteriorated. If the roundness of the hole of the clad preform is poor, it may be necessary to etch the inner periphery of the hole of the clad preform and the outer periphery of the core preform. Therefore, high dimensional accuracy and shape accuracy are required for the clad preform.

[0049] As a result of studying conditions for obtaining a preform for optical fiber with high dimensional accuracy and shape accuracy, the present inventor has found that the sintering step may be completed in a state where densification of quartz glass powder, which is a main raw material of the glass sintered body, is almost completed, the glass does not need to be completely transparent, and the shape accuracy and the dimensional accuracy are rather deteriorated when sintering is performed until the glass is completely transparent. The preform for optical fiber according to the present invention has been made based on such findings.

[0050] Therefore, a major difference between the clad preform of the present embodiment and a conventional clad preform is that the former includes a glass sintered body that is incompletely transparent, whereas the latter includes a glass sintered body that is completely transparent. The glass sintered body that is incompletely transparent refers to a glass sintered body at least a portion of which includes an opaque portion. In the present embodiment, a portion having a visible light transmittance of 90% or less at a thickness of 5 mm or less is defined as a portion that is incompletely transparent, and a portion having a visible light transmittance of more than 90% is defined as a portion that is completely transparent.

[0051] The present inventor has found that bulk density and open porosity are effective as indices indicating that the densification of the quartz glass powder is substantially completed, so that the clad preform of the present embodiment has been defined with any of these indices and light transmittance.

[0052] Specifically, the clad preform according to the present embodiment is made of a columnar quartz glass sintered body which is made from quartz glass powder as a main raw material, the clad preform including a portion having a visible light transmittance of 90% or less at a thickness of 5 mm or less and a bulk density of 2.1 g/cm.sup.3 or more, or a portion having a visible light transmittance of 90% or less at a thickness of 5 mm or less and an open porosity of 3.5% or less. Of course, the entire clad preform according to the present embodiment may satisfy the above requirements (light transmittance and bulk density, light transmittance and open porosity).

[0053] If the columnar glass sintered body is sufficiently densified, the outer roundness of the glass sintered body is good. Therefore, the clad preform of the present embodiment can be defined by either the outer roundness or the indices described above.

[0054] Specifically, the clad preform according to the present embodiment is made of a columnar quartz glass sintered body which is made from quartz glass powder as a main raw material, the clad preform including a portion having a ratio of the outer roundness to the outer diameter of 0.5% or less and a bulk density of 2.1 g/cm.sup.3 or more, or a portion having a ratio of the outer roundness to the outer diameter of 0.5% or less and an open porosity of 3.5% or less. Also in this case, the entire clad preform according to the present embodiment may satisfy the above requirements (ratio of outer roundness to outer diameter and bulk density, ratio of outer roundness to outer diameter and open porosity).

[0055] The bulk density and the open porosity can be measured using the Archimedes method (boiling method). When the dry mass of a sample (test piece) obtained by cutting out a portion of the clad preform is W.sub.1 [g], the mass in water is W.sub.2 [g], the mass of saturated water is W.sub.3 [g], the bulk density is Pb [g/cm.sup.3], and the open porosity is P.sub.0 [%], the bulk density Pb and the open porosity P.sub.0 are expressed by the following Equations (1) and (2), respectively:

Pb={W.sub.1/(W.sub.3−W.sub.2)}×ρ  (1)

P.sub.0={(W.sub.3−W.sub.1)/(W.sub.3−W.sub.2)}×100  (2)

[0056] In Equation (1), p represents the density [g/cm.sup.3] of distilled water used for measurement.

[0057] The bulk density corresponds to a value obtained by dividing the mass of the sample by the total volume (outer volume) including closed pores and open pores when the sample has the open pores and the closed pores. The open porosity is a percentage ratio of the volume occupied by the volume of the open pore portion when the outer volume of the sample is 1.

[0058] Next, a manufacturing example performed to examine the manufacturing conditions of the clad preform according to the present embodiment will be described. The manufacturing conditions include the shape of the clad preform, the composition of the slurry, and the temperature conditions of each step from the slurry blending to the sintering.

Manufacturing Example 1

[0059] In Manufacturing Example 1, five samples 1 to 5 were prepared in which only the temperature condition of the sintering step was varied among manufacturing conditions. The manufacturing conditions of these samples 1 to 5 are as shown in Table 1. As shown in Table 1, in Manufacturing Example 1, the manufacturing conditions were set so as to obtain, from each of the samples 1 to 5, a clad preform including a columnar glass sintered body having seven holes penetrating both end surfaces, the clad preform having an outer diameter of 30 mm, a length of 400 mm, and an inner diameter of each hole of 5.85 mm. Although not shown in Table 1, the steps from molding to drying were performed under general temperature conditions.

TABLE-US-00001 TABLE 1 Item Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Average particle size of quartz powder 2 μm Outer diameter × length [mm] φ30 × 400 φ30 × 400 φ30 × 400 φ30 × 400 φ30 × 400 Hole diameter [mm] φ5.85 φ5.85 φ5.85 φ5.85 φ5.85 Number of holes 7 7 7 7 7 Blending ratio of slurry Quartz powder [wt %] 71.5% 71.5% 71.5% 71.5% 71.5% Water [wt %] 18.5% 18.5% 18.5% 18.5% 18.5% Resin [wt %]  8.4%  8.4%  8.4%  8.4%  8.4% Dispersant [wt %]  1.6%  1.6%  1.6%  1.6%  1.6% Degreasing condition  850° C.  850° C.  850° C.  850° C.  850° C. Sintering condition 1690° C. 1600° C. 1550° C. 1500° C. 1450° C. Hold 1 Hold 30 Hold 30 Hold 30 Hold 30 minute minutes minutes minutes minutes

[0060] The outer diameter, the outer roundness, the ratio (%) of the outer roundness to the outer diameter, the hole diameter, the hole roundness, and the light transmittance of the clad preform obtained from each of the samples 1 to 5 are shown in Table 2. A cross section of the clad preform obtained from each of the samples 1 to 5 is shown in FIG. 2. The outer diameter and the outer roundness of the clad preform are average values of values measured at positions of 10 mm, 100 mm, 200 mm, 300 mm, and 390 mm from one of both ends in the longitudinal direction of the clad preform. The hole diameter is an average value of the inner diameters of the seven holes measured at the positions of 10 mm and 390 mm, and the hole roundness is an average value of the roundness of the seven holes. The light transmittance was defined as a light transmittance when an end portion of the obtained clad preform was cut, and both end surfaces of the cut clad preforms were mirror-polished to obtain a thin piece having a thickness of 2 mm and a thin piece having a thickness of 5 mm, and the surfaces of the respective thin pieces were irradiated with visible light (beam diameter: about 3 mm) having a wavelength of 530 nm from a direction perpendicular to the surfaces. A column in which “-” is displayed in Table 2 indicates that the item is not measured. The portion indicated by a black circle in FIG. 2 is the portion irradiated with the visible light beam.

TABLE-US-00002 TABLE 2 Item Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Outer diameter [mm] 29.94 29.93 30.09 30.07 30.03 Outer roundness [mm] 0.430 0.362 0.196 0.109 0.070 Outer roundness/outer diameter [%] 1.44 1.21 0.65 0.36 0.23 Hole diameter [mm] 5.79 5.83 5.89 5.89 5.91 Hole roundness [mm] 0.155 0.094 0.076 0.033 0.021 Light transmittance [%] λ = 530 nm 91.6% 91.4% 91.6% 87.8% — (Thickness 2 mm) Light transmittance [%] λ = 530 nm 92.4% 92.7% 92.5% — 83.2% (Thickness 5 mm)

[0061] FIG. 3 shows the relationships between the outer roundness and the sintering temperature, the hole roundness and the sintering temperature, and the light transmittance (thickness 5 mm, thickness 2 mm) and the sintering temperature (see Tables 1 and 2) of the clad preforms obtained from the samples 1 to 5. As can be seen from FIG. 3, as the sintering temperature was higher, the outer roundness and the hole roundness deteriorated (the value of the roundness increased). When the sintering temperature was lower than 1550° C., the light transmittance increased as the sintering temperature was higher, but when the sintering temperature was 1550° C. or higher, the light transmittance was substantially constant. From this, it can be said that the clad preforms obtained by setting the sintering temperature to 1550° C. or higher are sufficiently densified.

Manufacturing Example 2

[0062] In Manufacturing Example 2, four samples 6 to 9 were prepared in which only the temperature condition of the sintering step was varied among manufacturing conditions. The manufacturing conditions of these samples 6 to 9 are as shown in Table 3. As shown in Table 3, in Manufacturing Example 2, the manufacturing conditions were set so as to obtain, from each of the samples 6 to 9, a clad preform including a columnar glass sintered body having seven holes penetrating both end surfaces, the clad preform having an outer diameter of 28.7 mm, a length of 400 mm, and an inner diameter of each hole of 5.6 mm. Although not shown in Table 3, the steps from molding to drying were performed under general temperature conditions.

TABLE-US-00003 TABLE 3 Item Sample 6 Sample 7 Sample 8 Sample 9 Average particle size of quartz powder 0.1 μm Outer diameter × length [mm] φ28.7 × 400 φ28.7 × 400 φ28.7 × 400 φ28.7 × 400 Hole diameter [mm] 5.6 5.6 5.6 5.6 Number of holes 7 7 7 7 Blending ratio of slurry Quartz powder [wt %] 66.2% 66.2% 66.2% 66.2% Water [wt %] 23.9% 23.9% 23.9% 23.9% Resin [wt %]  7.6%  7.6%  7.6%  7.6% Dispersant [wt %]  2.3%  2.3%  2.3%  2.3% Degreasing condition  850° C.  850° C.  850° C.  850° C. Sintering condition 1690° C. 1550° C. 1500° C. 1450° C. Hold 1 Hold 30 Hold 30 Hold 30 minute minutes minutes minutes

[0063] The outer diameter, the outer roundness, the ratio (%) of the outer roundness to the outer diameter, the hole diameter, the hole roundness, and the light transmittance of the clad preform obtained from each of the samples 6 to 9 are shown in Table 4. The method of obtaining each value, the method of producing a thin piece in which the light transmittance was measured, and the measurement point of the light transmittance were the same as in Manufacturing Example 1.

TABLE-US-00004 TABLE 4 Item Sample 6 Sample 7 Sample 8 Sample 9 Outer diameter [mm] 28.74 28.74 28.79 28.80 Outer roundness [mm] 0.259 0.166 0.087 0.058 Outer roundness/outer diameter [%] 0.90 0.58 0.30 0.20 Hole diameter [mm] 5.61 5.63 5.64 5.65 Hole roundness [mm] 0.099 0.043 0.023 0.012 Light transmittance [%] λ = 530 nm 94.2% 93.9% 93.6% 92.2% (Thickness 2 mm)

[0064] FIG. 4 shows the relationships between the outer roundness and the sintering temperature, the hole roundness and the sintering temperature, and the light transmittance and the sintering temperature (see Tables 3 and 4) of the clad preforms obtained from the samples 6 to 9. As can be seen from FIG. 4, also in Manufacturing Example 2, as the sintering temperature was higher, the outer roundness and the hole roundness deteriorated (the value of the roundness increased). In Manufacturing Example 2, the light transmittance was substantially constant at a sintering temperature of 1500° C. or higher. From this, it can be said that the clad preform obtained by setting the sintering temperature to 1500° C. or higher is sufficiently densified.

Manufacturing Example 3

[0065] In Manufacturing Example 3, 11 samples 10 to 20 were prepared in which only the temperature condition of the sintering step was varied among manufacturing conditions. The manufacturing conditions of these samples 10 to 20 are as shown in Table 5. As shown in Table 5, in Manufacturing Example 3, the manufacturing conditions were set so as to obtain, from each of the samples 10 to 19, a clad preform including a columnar glass sintered body having one hole penetrating both end surfaces, the clad preform having an outer diameter of 20 mm, a length of 400 mm, and an inner diameter of each hole of 5.85 mm. The manufacturing conditions were set so as to obtain, from the sample 20, a clad preform including a (solid) columnar glass sintered body having no hole, the clad preform having an outer diameter of 20 mm and a length of 400 mm. Although not shown in Table 5, the steps from molding to drying were performed under general temperature conditions.

TABLE-US-00005 TABLE 5 Item Sample 10 Sample 11 Sample 12 Sample 13 Sample 14 Sample 15 Sample 16 Sample 17 Sample 18 Sample 19 Sample 20 Average particle 2 μm size of quartz powder Outer diameter × φ20 × φ20 × φ20 × φ20 × φ20 × φ20 × φ20 × φ20 × φ20 × φ20 × φ20 × length [mm] 400 400 400 400 400 400 400 400 400 400 400 Hole diameter φ5.85 φ5.85 φ5.85 φ5.85 φ5.85 φ5.85 φ5.85 φ5.85 φ5.85 φ5.85 — [mm] Number of holes 1 1 1 1 1 1 1 1 1 1 — Blending ratio of slurry Quartz powder 71.5% 71.5% 71.5% 71.5% 71.5% 71.5% 71.5% 71.5% 71.5% 71.5% 71.5% [wt %] Water [wt %] 18.5% 18.5% 18.5% 18.5% 18.5% 18.5% 18.5% 18.5% 18.5% 18.5% 18.5% Resin [wt %] 8.4% 8.4% 8.4% 8.4% 8.4% 8.4% 8.4% 8.4% 8.4% 8.4% 8.4% Dispersant 1.6% 1.6% 1.6% 1.6% 1.6% 1.6% 1.6% 1.6% 1.6% 1.6% 1.6% [wt %] Degreasing 850° C. 850° C. 850° C. 850° C. 850° C. 850° C. 850° C. 850° C. 850° C. 850° C. 850° C. condition Sintering 1200 1300 1350 1400 1400 1400 1450 1450 1500 1550 1690 temperature [° C.] Sintering time 30 30 30 30 30 30 30 30 30 30 1 [min]

[0066] The outer diameter and the hole diameter (the inner diameter of the hole), the apparent density, the bulk density, the open porosity, the Vickers hardness, and the light transmittance of the clad preform obtained from each of the samples 10 to 20 were measured.

[0067] The measurement results for the samples 10 to 20 are shown in Table 6. The method of obtaining the outer diameter of the clad preform is the same as that in Manufacturing Example 1. The apparent density, the bulk density, and the open porosity were all measured by the boiling method (JIS 1634) (for the bulk density and the open porosity, see Equations (1) and (2) described above). A column in which “-” is displayed in Table 6 indicates that the item is not measured. For the samples 10 and 11, since the entire glass sintered bodies obtained were white, and the light transmittance of the glass sintered body obtained from the sample 12 having a higher sintering temperature than the samples 10 and 11 was 0.9%, the light transmittance was expected to be approximately 0%, and thus the light transmittance was not measured. Since the densification of the glass sintered bodies obtained from the samples 10 and 11 having low sintering temperatures did not proceed and it was expected that measurement of the Vickers hardness would be difficult, the measurement was omitted.

[0068] In Manufacturing Example 3, an optical fiber was obtained by inserting a core preform into the hole of the clad preform obtained from each of the samples 14, 15, and 18 and performing drawing, and a transmission loss of the optical fiber was measured. The samples 14 and 18 were flame polished before drawing, and the sample 15 was not flame polished. The results are also shown in Table 6.

TABLE-US-00006 TABLE 6 Item Sample 10 Sample 11 Sample 12 Sample 13 Sample 14 Sample 15 Sample 16 Sample 17 Sample 18 Sample 19 Sample 20 Outer diameter 23.84 22.77 20.97 20.04 20.04 20.17 20.02 20.01 20.02 20.00 20.03 [mm] Inner diameter 7.07 6.75 6.20 5.97 5.95 6.00 5.91 5.91 5.91 5.90 — [mm] Apparent density 2.15 2.17 2.13 2.19 2.18 2.15 2.19 2.19 2.19 2.20 2.20 [g/cm.sup.3] Bulk density 1.31 1.49 1.88 2.17 2.17 2.13 2.17 2.16 2.17 2.17 2.19 [g/cm.sup.3] Open porosity 39.3 31.5 11.6 0.8 0.8 1.1 1.0 1.3 1.0 1.7 0.5 [%] Vickers hardness 414 633 636 659 763 801 791 — — [kgf/mm.sup.2] Light — — 0.9 10.0 9.8 2.9 83.7 85.1 92.1 92.8 — transmittance [%] λ = 530 nm Carrying out — — — — ◯ ◯ — — ◯ — — drawing (With (Without (With flame flame flame polishing) polishing) polishing) Optical fiber — — — — 0.21 0.25 — — 0.19 — — transmission loss [dB/km]

[0069] FIGS. 5 to 12 are graphs showing the relationships between the outer diameters, the hole diameters, the bulk density, the light transmittance, the open porosity, the contraction ratios, and the sintering temperature (see Tables 5 and 6) of the clad preforms obtained in Manufacturing Example 3.

[0070] From the results in FIGS. 5 to 12, it was presumed that densification of glass starts when the sintering temperature exceeds 1400° C., and transparentizing starts when the sintering temperature exceeds 1450° C. Therefore, it can be said that if the characteristics of the clad preform obtained by setting the sintering temperature to 1400° C. to 1450° C. are satisfied, a clad preform of a dense and opaque glass sintered body is obtained.

[0071] From FIG. 8, since there is a high correlation between the bulk density and the open porosity, it can be said that the clad preform of the present embodiment can be defined regardless of whether the bulk density or the open porosity is used.

[0072] Furthermore, when optical fibers were produced by drawing the clad preforms (samples 14, 15) obtained by setting the sintering temperature to 1400° C. and the clad preform (sample 18) obtained by setting the sintering temperature to 1500° C., it was found that the samples 14 and 15 had slightly poorer transmission losses than the sample 18, but there was no problem. From this, it can be said that the clad preform of the present embodiment is useful as a preform of optical fiber.

REFERENCE SIGNS LIST

[0073] 10 . . . Hole [0074] 100 . . . Clad Preform