U.S. patent application number 11/273216 was filed with the patent office on 2006-06-15 for method of producing a glass preform.
This patent application is currently assigned to NIPPON ELECTRIC GLASS CO., LTD.. Invention is credited to Akihiko Sakamoto, Fumio Sato.
Application Number | 20060123851 11/273216 |
Document ID | / |
Family ID | 28046080 |
Filed Date | 2006-06-15 |
United States Patent
Application |
20060123851 |
Kind Code |
A1 |
Sato; Fumio ; et
al. |
June 15, 2006 |
Method of producing a glass preform
Abstract
In a glass preform (10) to be subjected to fiber-drawing, use is
made of a cylindrical glass tube (21) whose one end in an axial
direction is sealed. A plurality of glass capillaries (22) extend
in the glass tube in the axial direction. The glass capillaries are
fused to one another into an integral structure. The glass
capillaries have air holes periodically arranged on a plane
perpendicular to the axial direction, respectively.
Inventors: |
Sato; Fumio; (Moriyama-shi,
JP) ; Sakamoto; Akihiko; (Koka-gun, JP) |
Correspondence
Address: |
Collard & Roe, P.C.
1077 Northern Boulevard
Roslyn
NY
11576
US
|
Assignee: |
NIPPON ELECTRIC GLASS CO.,
LTD.
Otsu-shi
JP
|
Family ID: |
28046080 |
Appl. No.: |
11/273216 |
Filed: |
November 14, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10389180 |
Mar 14, 2003 |
7026025 |
|
|
11273216 |
Nov 14, 2005 |
|
|
|
Current U.S.
Class: |
65/393 |
Current CPC
Class: |
C03B 37/0122 20130101;
Y10T 428/1393 20150115; C03B 2203/14 20130101; C03C 3/089 20130101;
C03C 3/091 20130101; C03B 37/01248 20130101; Y10T 428/131 20150115;
C03B 2203/42 20130101; G02B 6/02347 20130101; Y02P 40/57
20151101 |
Class at
Publication: |
065/393 |
International
Class: |
C03B 37/022 20060101
C03B037/022 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2002 |
JP |
70354/2002 |
May 22, 2002 |
JP |
148345/2002 |
Mar 7, 2003 |
JP |
61019/2003 |
Claims
1-10. (canceled)
11. A method of producing a glass preform, comprising the steps of:
preparing a cylindrical glass tube whose one end in an axial
direction is sealed; disposing a plurality of glass capillaries in
the glass tube; and heating the glass tube with its interior kept
in a reduced-pressure condition.
12. The method according to claim 11, wherein a tubular member
having an internal cavity and longitudinal opposite ends both of
which are sealed is used as each of the glass capillaries.
13. The method according to claim 11, comprising the steps of:
preparing, as each of the glass capillaries, a tubular member
having a longitudinal one end and the other end which are sealed
and opened, respectively.
14. The method according to claim 11, further comprising the step
of disposing at least one glass rod in the glass tube.
15. The method according to claim 14, further comprising the step
of using, as the at least one glass rod, a tubular member having
longitudinal opposite ends which are opened.
16. The method according to claim 14, further comprising the steps
of: using, as the at least one glass rod, a tubular member having a
longitudinal one end and the other end which are sealed and opened,
respectively, and facing the longitudinal one end of the tubular
member to the one end of the glass tube.
17. The method according to claim 14, further comprising the steps
of: using, as the at least one glass rod, a tubular member a
longitudinal one end and the other end which are sealed and opened,
respectively, and facing the other end of the tubular member to the
one end of the glass tube.
18. The method according to claim 14, further comprising the step
of using, s the at least one glass rod, a solid member without an
internal cavity.
19. The method according to claim 14, further comprising the step
of using, as the at least one glass rod, a tubular member having an
internal cavity and longitudinal opposite ends both of which are
sealed.
20. The method according to claim 14, wherein each of the glass
capillaries and the glass rod is made of a multi-component
glass.
21. The method according to claim 14, wherein each of the glass
capillaries and the glass rod is made of a borosilicate glass.
22. The method according to claim 14, wherein each of the glass
capillaries and the glass rod is made of a glass containing, by
mass %, 55-95% SiO.sub.2, 1-30% B.sub.2O.sub.3, and 0.1-10%
Na.sub.2O.
23. The method according to claim 14, wherein each of the glass
capillaries and the glass rod is made of a glass essentially
consisting of, by mass %, 55-95% SiO.sub.2, 1-30% B.sub.2O.sub.3,
and 0.1-10% Na.sub.2O, 0-10% Al.sub.2O.sub.3, 0-5% CaO, 0-10% BaO,
and 0-5% K.sub.2O.
24. The method according to claim 14, wherein each of the glass
capillaries and the glass rod is made of a glass without absorption
of light having a wavelength of 1400 nm by OH groups.
25. The method according to claim 14, wherein each of the glass
capillaries and the glass rod has a refeactive index (nd) between
1.45 and 2.00.
26. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Applicants claim priority under 35 U.S.C. 119 of Japanese
applications JP 2002-70354, filed Mar. 14, 2002; JP 2002-148345,
filed May 22, 2002; and JP 2003-61019, filed Mar. 7, 2003.
Applicants also claim priority under 35 U.S.C. 120 and 35 U.S.C.
121 as a divisional of copending parent U.S. patent application
Ser. No. 10/389,180, filed Mar. 14, 2003.
[0002] The present invention relates to a method of producing a
glass preform as a raw material for producing an optical waveguide
material, such as a photonic crystal fiber (PCF), a holey fiber, or
a photonic bandgap fiber (PBF), having a number of air holes
periodically arranged in its cross section (having a periodic
structure in its cross section).
[0003] An optical waveguide material, such as a photonic crystal
fiber, a holey fiber, or a photonic bandgap fiber, having a
periodic structure in its cross section is excellent in optical
transmission characteristic and therefore attracts attention as an
important material in a future optical communication system. In
order to produce the above-mentioned optical waveguide material,
proposal has been made of a first method of drawing a bundle of
circular cylindrical silica glass capillaries to obtain an integral
structure and a second method of preparing a bundle of polygonal
columnar silica glass capillaries as a preform and drawing the
preform to obtain an integral structure.
[0004] In the first method, however, it is difficult to precisely
arrange the capillaries in a regular periodic structure. Therefore,
it is difficult to obtain an optical waveguide material having a
periodic structure. In addition, a number of air gaps are present
between the capillaries. Therefore, the optical waveguide material
inevitably has interstitial sites between the air holes. In the
second method, it is easy to regularly arrange the capillaries.
However, in order to process the capillaries into a polygonal shape
in section, much labor is required and, therefore, production cost
is increased. If the processing accuracy is insufficient, the
optical waveguide material inevitably has interstitial sites.
SUMMARY OF THE INVENTION
[0005] It is therefore an object of the present invention to
provide a glass preform which is obtained at a low cost without
requiring capillaries or rods to be processed into a polygonal
shape in section and which is adapted to produce an optical
waveguide material having an undisturbed periodic structure in its
cross section without interstitial sites.
[0006] It is another object of the present invention to provide a
method of producing the above-mentioned glass preform.
[0007] It is still another aspect of the present invention to
provide an optical waveguide material having a periodic
structure.
[0008] Other objects of the present invention will become clear as
the description proceeds.
[0009] According to an aspect of the present invention, there is
provided a glass preform to be subjected to fiber-drawing. The
glass preform comprises a cylindrical glass tube whose one end in
an axial direction is sealed and a plurality of glass capillaries
extending in the glass tube in the axial direction and fused to one
another into an integral structure. The glass capillaries has air
holes periodically arranged on a plane perpendicular to the axial
direction, respectively.
[0010] According to another aspect of the present invention, there
is provided a method of producing a glass perform. The method
comprises the steps of preparing a cylindrical glass tube whose one
end in an axial direction is sealed, disposing a plurality of glass
capillaries in the glass tube, and heating the glass tube with its
interior kept in a reduced-pressure condition.
[0011] According to still another aspect of the present invention,
there is provided an optical waveguide material produced by
fiber-drawing of the glass preform and having a number of air holes
periodically arranged on the plane.
BRIEF DESCRIPTION OF THE DRAWING
[0012] FIG. 1 is a perspective view of a glass preform according to
an embodiment of the present invention;
[0013] FIG. 2A is a perspective view of a glass tube used in the
glass preform illustrated in FIG. 1;
[0014] FIG. 2B is a perspective view of a glass capillary used in
the glass preform illustrated in FIG. 1;
[0015] FIG. 2C is a perspective view of a glass rod used in the
glass preform illustrated in FIG. 1;
[0016] FIG. 3 is a perspective view of a modification of the glass
capillary;
[0017] FIG. 4A is a perspective view of a first modification of the
glass rod;
[0018] FIG. 4B is a perspective view of a second modification of
the glass rod;
[0019] FIG. 4C is a perspective view of a third modification of the
glass rod;
[0020] FIG. 5 is a view for describing production of an optical
waveguide material by fiber-drawing of the glass preform in FIG.
1;
[0021] FIG. 6 is an optical microscope photograph of a cross
section of the glass preform of Example 1, taken along a line VI-VI
in FIG. 1; and
[0022] FIG. 7 is an SEM image photograph of a cross section of the
optical waveguide material of Example 1, taken along a line VII-VII
in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] Referring to FIG. 1, description will be made of a glass
preform according to an embodiment of the present invention.
[0024] The glass preform depicted at 10 in FIG. 1 is to be
subjected to fiber-drawing and comprises a cylindrical glass tube
21 whose one end or lower end in an axial direction is sealed, a
plurality of glass capillaries 22 extending in the glass tube 21 in
a vertical direction and fused to one another into an integral
structure, and a glass rod 23 disposed in the glass tube 21 at its
center and extending in the vertical direction.
[0025] Each of the glass capillaries 22 is sealed at longitudinal
opposite ends thereof. In the glass tube 21, the glass capillaries
22 are arranged around the glass rod 23 and kept in tight contact
with one another. Thus, air holes of the glass capillaries 22 are
periodically arranged on a predetermined plane perpendicular to the
axial direction.
[0026] In the glass preform 10, the glass capillaries 22 are fused
to one another into an integral structure in which air gaps left
between the glass capillaries 22 are reduced. By predetermined
fiber-drawing using the glass preform 10 as a raw material, an
optical waveguide material, such as a photonic bandgap fiber (PBF),
can be produced which has a periodic structure substantially
analogous to the periodic structure of the glass preform on the
predetermined plane and which has no interstitial sites. Namely,
when the glass preform 10 is heated and subjected to fiber-drawing,
the air gaps are eliminated as a result of softening of glass. By
controlling the pressure in the air holes of the glass capillaries
22 during the fiber-drawing, it is possible to adjust the size or
diameter of the air holes.
[0027] In the glass preform 10, the air gaps left between the glass
capillaries 22 are reduced. Therefore, the optical waveguide
material having no interstitial sites can easily be obtained even
if the temperature during the fiber-drawing is not so elevated.
Since the opposite ends of the glass capillaries 22 are sealed, the
size and the sectional shape of the air holes are hardly influenced
by an external air pressure and are therefore stabilized. In
particular, if one or a plurality of glass capillaries 22 are
greater in inner diameter than the glass capillaries 22
therearound, it is possible to obtain an optical waveguide
material, such as a photonic bandgap fiber (PBF), having one or a
plurality of large air holes.
[0028] In the glass tube 21, the glass rod 23 is disposed and the
glass capillaries 22 are disposed around the glass rod 23.
Therefore, by the fiber-drawing using the glass preform 10 as a raw
material, it is possible to produce an optical waveguide material,
such as a photonic crystal fiber (PCF) or a holey fiber, having a
core portion without an air hole. A plurality of glass rods 23 may
be disposed in the glass tube 21.
[0029] Each of the glass capillaries 22 and the glass rod 23 is
made of a multi-component glass, preferably, a borosilicate glass.
In this event, formation of the glass preform and fiber-drawing of
the glass preform into the optical waveguide material can be
carried out at a low temperature. Therefore, it is possible to
suppress deterioration of a forming equipment due to heat or to
perform production with a simple equipment, which is economical. In
addition, use can be made of a common production process, such as
the Danner process or the down drawing process. It is therefore
possible to easily or economically produce glass preforms in
various shapes. Furthermore, since the glass preforms in various
shapes can easily be obtained, it is possible to easily control the
periodic structure at a low temperature and to easily control
optical properties of the optical waveguide material, such as a
nonlinear optical effect or dispersion.
[0030] Specifically, each of the glass capillaries 22 and the glass
rod 23 is made of a glass containing, by mass %, 55-95% SiO.sub.2,
1-30% B.sub.2O.sub.3, and 0.1-10% Na.sub.2O. Preferably, each of
the glass capillaries 22 and the glass rod 23 is made of a glass
essentially consisting of, by mass %, 55-95% SiO.sub.2, 1-30%
B.sub.2O.sub.3, and 0.1-10% Na.sub.2O, 0-10% Al.sub.2O.sub.3, 0-5%
CaO, 0-10% BaO, and 0-5% K.sub.2O.
[0031] The glass tube 21 may be made of a multi-component glass
same as that used for the glass capillaries 22 and the glass rod
23. In this event, the glass preform or the optical waveguide
material can easily be formed.
[0032] Next, description will be made of respective components of
the multi-component glass.
[0033] SiO.sub.2 is a component forming a backbone structure (i.e.,
a network former) of the glass. The content of SiO.sub.2 is 55-95%,
preferably 60-90%, more preferably 65-80%. If the content of
SiO.sub.2 is greater than 95%, the viscosity of the glass is
increased and the forming temperature during production of the
glass capillaries, the glass rod, and the glass tube tends to
become very high. The content smaller than 55% is not unfavorable
because weather resistance, such as acid resistance and water
resistance, is considerably degraded.
[0034] B.sub.2O.sub.3 has an effect of lowering the viscosity of
the glass. The content of B.sub.2O.sub.3 is 1-30%, preferably
1-25%, more preferably 2-20%. If the content of B.sub.2O.sub.3 is
greater than 30%, the weather resistance is considerably degraded.
If the content of B.sub.2O.sub.3 is smaller than 1%, the viscosity
of the glass is increased so that the forming temperature during
production of the glass capillaries, the glass rod, and the glass
tube tends to become very high.
[0035] Na.sub.2O has an effect of lowering the viscosity of the
glass. The content of Na.sub.2O is 0.1-10%, preferably 0.5-8%. If
the content of Na.sub.2O is greater than 10%, the weather
resistance is considerably degraded so that the surface of the
optical waveguide material is remarkably deteriorated in a
high-temperature high-humidity condition. If the content is smaller
than 0.1%, the viscosity of the glass is increased so that the
productivity is decreased.
[0036] Al.sub.2O.sub.3 is a component forming the backbone
structure of the glass, together with SiO.sub.2, and has an effect
of improving the weather resistance. The content of Al.sub.2O.sub.3
is 0-10%, preferably 0.5-8%. If the content of Al.sub.2O.sub.3 is
greater than 10%, phase separation tends to occur. This results in
occurrence of devitrification during production of the glass.
[0037] Each of CaO and BaO has an effect of lowering the viscosity.
However, if the content is excessive, the weather resistance of the
product is considerably degraded. In view of the above, the content
of CaO is 0-5%, preferably 0-3%. Similarly, the content of BaO is
0-10%, preferably 0-5%.
[0038] K.sub.2O has an effect of lowering the softening point of
the glass. However, if the content is excessive, devitrification
occurs during production of the glass and the productivity is
decreased. In view of the above, the content of K.sub.2O is 0-5%,
preferably 0-3%.
[0039] In the above-mentioned preform 10, each of the glass
capillaries 22 and the glass rod 23 is preferably made of a glass
without absorption of light having a wavelength of 1400 nm by OH
groups. In this event, the optical waveguide material produced by
the fiber-drawing has a reduced optical loss at an E-band frequency
(1360 to 1460 nm).
[0040] Each of the glass capillaries 22 and the glass rod 23 is
preferably made of a glass having a refractive index (nd) between
1.45 and 2.00, preferably between 1.47 and 2.00. In the optical
waveguide material produced by the use of the above-mentioned
glass, an effective difference in refractive index between a core
and a cladding is great. This makes it possible to obtain a
dispersion property which has not been achieved by an existing
silica-based optical fiber.
[0041] Referring to FIGS. 2A to 2D, description will be made of a
method of producing the glass preform 10 in FIG. 1.
[0042] At first, preparation is made of the cylindrical glass tube
21, a plurality of the glass capillaries 22, and the glass rod 23.
As illustrated in FIG. 2A, the cylindrical glass tube 21 has one
end 21a and the other end 21b in the axial direction which are
sealed and opened, respectively. As illustrated in FIG. 2B, each of
the glass capillaries 22 has a longitudinal one end 22a and the
other end 22b both of which are sealed. As illustrated in FIG. 2C,
the glass rod 23 has no internal cavity. Next, the glass
capillaries 22 and the glass rod 23 are inserted and disposed in
the glass tube 21. The glass capillaries 22 are disposed in the
glass tube 21 around the glass rod 23 and are kept in tight contact
with one another without leaving any substantial gap.
[0043] In the above-mentioned state, the glass tube 21 is heated
with its interior kept in a reduced-pressure or low-pressure
condition. At this time, the interior of the glass tube 21 is
preferably kept at pressure lower than -100 mmHg, preferably lower
than -500 mmHg. In this event, the gaps between the glass
capillaries 22 are completely eliminated and, simultaneously, the
glass capillaries 22 are automatically packed into a close-packed
state which is most stable. Thus, the glass preform having a highly
regular periodic structure in its cross section is easily
obtained.
[0044] It is preferable to use the glass capillary 22 with its
opposite ends 22a and 22b preliminarily sealed as described above.
Alternatively, each of the glass capillaries 22 may have a
structure in which the one end 22a alone is sealed while the other
end 22b is opened, as illustrated in FIG. 3. In this event, the
glass capillary 22 is disposed so that the other end 22b is faced
to a bottom portion, i.e., the one end 21a of the glass tube 21. By
heating and softening the bottom portion of the glass tube 21, the
other end 22b of the glass capillary 22 is sealed by softening of
the glass. Thereafter, the glass tube 21 is heated from the bottom
portion towards an upper portion, i.e., the other end 21b. In this
manner, a hole 22c of the glass capillary 22 is not collapsed even
if heated in the reduced-pressure condition. By heating the glass
tube 21 successively and gradually from the bottom portion towards
the upper portion, air gaps are hardly left between the glass
capillaries 22 and between the glass capillaries 22 and the glass
rod 23.
[0045] Referring to FIG. 4A, the glass rod 23 may comprise a long
tubular member having a longitudinal one end 23a and the other end
23b which are sealed and not sealed, respectively, i.e., having a
cavity which is opened at its one end. Referring to FIG. 4B, the
glass rod 23 may comprise a long tubular member having opposite
ends 23a and 23b both of which are opened. By the fiber-drawing
using the glass preform as a raw material, the cavity within the
tubular member is substantially completely lost. Thus, in this case
also, it is possible to produce an optical waveguide material, such
as a photonic crystal fiber (PCF) and a holey fiber, having a core
portion without an air hole, as in the case where the glass rod 23
comprises a solid rod without any internal cavity.
[0046] Referring to FIG. 4C, the glass rod 23 may comprise a
tubular member having longitudinal opposite ends 23a and 23b both
of which are sealed, and a closed hole 23c formed inside. By the
fiber-drawing using the glass preform as a raw material, it is
possible to produce an optical waveguide material having a large
hole in a core portion, i.e., a so-called photonic bandgap fiber
(PBF).
[0047] It is also possible to produce the PBF with the glass rod 23
of FIG. 4A being disposed so that the other end 23b is faced to a
bottom portion, i.e., the one end 21a of the glass tube 21. By
heating and softening the bottom portion of the glass tube 21, the
other end 23b of the glass rod 23 is sealed by softening of the
glass. As a result of heating, the glass rod 23 of FIG. 4A is
modified into that is similar to the glass rod of FIG. 4C.
Therefore, the PBF can be manufactured by the use of the glass rod
23 of FIG. 4A.
[0048] During the fiber-drawing, the glass preform is heated at a
heating temperature T.sub.H preferably within a range given by
(T.sub.S -200.degree. C.)<T.sub.H<(T.sub.S +200.degree. C.)
where T.sub.S represents the softening point of the glass. If the
heating temperature T.sub.H is equal to or lower than (T.sub.S
-200.degree. C.), the air gaps between the glass capillaries are
not filled. On the other hand, if the heating temperature is equal
to or higher than (T.sub.S +200.degree. C.), the glass is
excessively softened so that the periodic structure is
disturbed.
[0049] By heating the glass tube 21 while the interior of the glass
tube 21 is reduced in pressure or by heating the glass tube 21
after the interior of the glass tube 21 is reduced in pressure and
then the upper portion of the glass tube 21 is sealed, the glass
preform 10 can be formed.
[0050] Referring to FIG. 5, the fiber-drawing will be
described.
[0051] The glass preform 10 is inserted into an electric furnace
31. The glass preform 10 is heated and pulled by a roller 32 in a
direction depicted by an arrow 33. By the fiber-drawing in the
above-mentioned manner, an optical waveguide material 10a having a
desired diameter and extending long is produced.
[0052] In the optical waveguide material 10a thus obtained, a
number of small air holes deriving from the glass capillaries 22
are periodically arranged on a plane perpendicular to a
longitudinal direction of the optical waveguide material 10a. In
other words, the optical waveguide material 10a has a periodic
structure substantially analogous to the above-mentioned periodic
structure in the glass preform and having high regularity. Thus, it
is possible to obtain an optical waveguide material, such as a
photonic crystal fiber (PCF), a holey fiber, and a photonic bandgap
fiber (PBF), which has no interstitial site and which is uniform in
shape and size of the air holes.
[0053] Hereinafter, description will be made in conjunction with
Examples 1 to 3 and Comparative Examples 1 and 2.
EXAMPLE 1
[0054] Preparation was made of the glass tube 21, the glass
capillaries 22, 449 in number, and the glass rod 23. As illustrated
in FIG. 2A, the glass tube 21 had a cylindrical shape with its
bottom portion 21a sealed. The glass tube 21 had an outer diameter
of 30 mm.phi. and an inner diameter of 24 mm.phi.. As illustrated
in FIG. 2B, each of the glass capillaries 22 had the opposite ends
22a and 22b both of which are sealed. Each of the glass capillaries
22 had an outer diameter of 1 mm.phi. and an inner diameter of 125
.mu.m.phi.. The glass rod 3 as illustrated in FIG. 2C had an outer
diameter of 1 mm.phi.. Each of the glass tube 21, the glass
capillaries 22, and the glass tube 23 was made of a glass
essentially consisting of, by mass %, 72.5% SiO.sub.2, 6.8%
Al.sub.2O.sub.3, 10.9% B.sub.2O.sub.3, 0.7% CaO, 1.2% BaO, 5.9%
Na.sub.2O, 1.8% K.sub.2O, and 0.2% Sb.sub.2O.sub.3. The glass had a
refractive index (nd) of 1.495.
[0055] Next, the glass rod 23 was disposed in the glass tube 21 at
a substantial center thereof. Around the glass rod 21, the glass
capillaries 22, 449 in number, were disposed so as to leave no
substantial gap.
[0056] Then, the interior of the glass tube 21 is reduced in
pressure to -750 mmHg by the use of a vacuum pump. With the
above-mentioned reduced pressure maintained, the glass tube 21 was
heated to 780.degree. C. successively or gradually from the bottom
portion towards an open end or the upper portion and contracted.
After the glass tube 21 was heated to the open end and contracted,
the glass tube 21 was gradually cooled to the room temperature.
Thereafter, a normal pressure was recovered. Thus, the glass
preform 10 illustrated in FIG. 1 was produced. Next, as illustrated
in FIG. 5, the glass preform 10 was inserted into the electric
furnace 31 and pulled by the roller 32 to be subjected to the
fiber-drawing. In the above-mentioned manner, the optical waveguide
material 10a was produced.
EXAMPLE 2
[0057] Use was made of the glass capillaries, 110 in number, each
of which was made of a glass essentially consisting of, by mass %,
70.5% SiO.sub.2, 6.0% Al.sub.2O.sub.3, 12.6% B.sub.2O.sub.3, 0.7%
CaO, 2.1% BaO, 6.6% Na.sub.2O, 1.3% K.sub.2O, and 0.2%
Sb.sub.2O.sub.3. The glass had a refractive index (nd) of 1.493.
Each of the glass capillaries had an outer diameter of 2 mm.phi.
and an inner diameter of 250 .mu.m.phi. and was sealed only at one
end thereof. The glass capillaries were packed in the glass tube so
that the one ends as sealed ends are faced to the open end of the
glass tube. Before the interior of the glass tube was reduced in
pressure, the bottom portion of the glass tube was heated and
softened so that the other end or unsealed ends of the glass
capillaries were sealed as a result of softening of the glass tube.
The glass preform and the optical waveguide material were produced
in the manner similar to Example 1 except the above.
EXAMPLE 3
[0058] In Example 3, use was made of the glass capillaries having
opposite ends both of which were not sealed. The heating
temperature was 700.degree. C. The glass preform and the optical
waveguide material were produced in the manner similar to Example 1
except the above.
COMPARATIVE EXAMPLE 1
[0059] Use was made of the glass capillaries having opposite ends
both of which were not sealed. The glass tube was heated without
reducing the pressure in the glass tube. The glass preform and the
optical waveguide material were produced in the manner similar to
Example 1 except the above.
COMPARATIVE EXAMPLE 2
[0060] Use was made of the glass capillaries and the glass rod each
of which was polished into a regular hexagonal cylinder having a
longest diagonal of 2 mm in section. The glass tube was heated
without reducing the pressure in the glass tube. The glass preform
and the optical waveguide material were produced in the manner
similar to Example 2 except the above.
[0061] Each of the glass preforms obtained in Examples 1 to 3 and
Comparative Examples 1 and 2 was cut in a transversal direction.
The transversal or cross section was observed by an optical
microscope to evaluate an air hole interval, an air gap between the
glass capillaries, variation in size of the air holes, and the
shape of the air holes. For the optical waveguide material, its
cross section in a SEM (Scanning Electron Microscope) image was
observed to evaluate the air hole interval, the interstitial site,
variation in size of the air holes, and the shape of the air holes.
The result of evaluation is shown in Table 1. TABLE-US-00001 TABLE
1 Comparative Example Example 1 2 3 1 2 glass air hole uniform
uniform uniform non- non- preform interval uniform uniform air gap
not not present present present between present present glass
capillaries variation no no no varied varied in size of air holes
shape of true true true ellipse ellipse air holes circle circle
circle optical air hole uniform uniform uniform non- non- waveguide
interval uniform uniform material interstitial not not not present
present site present present present variation no no no varied
varied in size of air holes shape of true true true ellipse ellipse
air holes circle circle circle
[0062] Referring to FIGS. 6 and 7, the glass preform in each of
Examples 1 to 3 had a close-packed structure in which the glass
capillaries were closely packed. In the glass preform and the
optical waveguide material, air hole intervals D and d were uniform
and air holes 41 and 51 were uniform in size and kept in a shape of
a true circle. No disturbance in the periodic structure was
observed. In the cross section of the glass preform in FIG. 6, no
air gap was present between the glass capillaries (Examples 1 and
2) and little air gaps were left between the glass capillaries
(Example 3). Referring to FIG. 7, the optical waveguide material
produced by the use of each of these glass preforms, no
interstitial site was present (although Examples 2 and 3 are not
shown).
[0063] Thus, in each of the glass preforms in Examples 1 to 3, each
of the glass capillaries and the glass rod need not be processed
into a polygonal cylindrical shape. Therefore, the glass preform is
obtained at a low cost. By the fiber-drawing of the glass preform,
it is possible to produce the optical waveguide material having an
undisturbed periodic structure in its cross section without any
interstitial site. Thus, the glass preform is suitable as a raw
material of the optical waveguide material, such as a photonic
crystal fiber (PCF), a holey fiber, and a photonic bandgap fiber
(PBF), having high accuracy.
[0064] On the other hand, in Comparative Example 1, the
close-packed structure was not obtained because of presence of a
large amount of air gaps between the glass capillaries, as shown in
Table 1. The air hole interval was considerably irregular or
nonuniform. The variation in size of the air holes was large.
Furthermore, the air holes were deformed into an elliptical shape
or some of the air holes were collapsed (not shown). In Comparative
Example 2, the close-packed structure was partly obtained. However,
the air gaps were locally present between the glass capillaries.
The air hole interval was nonuniform and the variation in size of
the air holes was observed. The shape of the air hole was
elliptical (not shown).
* * * * *