WO2006117938A1

WO2006117938A1 - Optical fiber preform including a non-axisymmetric cross section

Info

Publication number: WO2006117938A1
Application number: PCT/JP2006/305483
Authority: WO
Inventors: Ryuichi Sugizaki; Akifumi Sako; Takeshi Yagi
Original assignee: The Furukawa Electric Co., Ltd
Priority date: 2005-04-26
Filing date: 2006-03-14
Publication date: 2006-11-09

Abstract

A method for manufacturing an optical fiber includes the steps of covering an outer periphery of a first glass (11) having a first softening temperature and a non-axisymmetric structure by a second glass (12, 13) having a second softening temperature which is lower than the first softening temperature, heating the first and second glasses (11, 12, 13) for fusion together to thereby obtain an optical fiber preform; and drawing the preform to the optical fiber.

Description

DESCRIPTION

OPTICAL FIBER PREFORM INCLUDING

A NON-AXISYMMETRIC CROSS SECΗON

TECHNICAL FIELD

[0001]

The present invention relates to an optical fiber preform and a method for

manufacturing the optical fiber preform, and more particularly, to an optical fiber

preform used for manufacturing an optical fiber for use in a relatively short distance

transmission.

BACKGROUND ART

[0002]

It is known that an optical fiber including OH radicals has a large absorption

peak in the vicinity wavelength of 1380nm. To prevent the occurring of such an

absorption peak, a technique is generally employed wherein the portion of the

optical fiber through which the light passes is totally synthesized so as to have less

OH radicals. Examples of the total synthesis technique include vapour-phase axial

deposition (VAD), outside vapour deposition (OVD) and modified chemical vapour

deposition (MCVD).

In general, since the light transmits in an axial symmetry, the optical fiber is

manufactured to have an axisymmetric structure by paying the full attention on the

circularity of a core of the optical fiber. In addition, the axisymmetric structure is the most desirable structure to manufacture the optical fiber. Accordingly, although

some optical fiber, such as a polarization-maintaining optical fiber, may have a non-

axisymmetric structure of a stress-applying part other than the core, even the stress-

applying part of the most of the polarization-maintaining optical fibers is generally

formed to have the axisymmetric structure.

[0003]

Techniques for manufacturing the optical fiber preform include a rot-in-tube

(RIT) technique in addition to the total synthesis technique. The RIT technique is

such that the glass rod including the core and manufactured by the total synthesis

technique is inserted into a glass tube, and the glass tube and the glass rod are

collapsed by heating to form an optical fiber preform. The techniques further

include a rot-in-cylinder (RIC) technique wherein the collapsing the glass tube and

the glass rod by heating is conducted concurrently with the drawing step.

[0004]

Since the required characteristics of the optical fiber are being complicated

in these days, a variety of optical fibers corresponding to these requirements are

proposed. The optical fibers proposed heretofore include a non-axisymmetric

optical fiber and a photonic-crystal optical fiber, the latter having a plurality of air

holes in the cladding section of the optical fiber. Patent Publication JP-2003-

34203 IA describes a preform for the photonic-crystal optical fiber and a method for

manufacturing the same. It is recited in the publication that the optical fiber

preform is manufactured by assembling and fusing a plurality of non-axisymmetric

glass rods. DISCLOSURE OF THE INVENTION

Problems to be solved by the Invention

[0005]

The optical fiber is now being used as an interconnection in an optical

circuit. This technique can utilize the high-speed characteristic of the optical signal.

In addition, the up-to-date optical circuit now uses a vertical-cavity surface-emitting

laser (VCSEL) device, which has not been used heretofore in a long-distance

optical communication system. Suppose a case wherein the light emitted from a

luminous object such as the VCSEL device is directly transmitted through an

optical fiber. If the optical fiber includes a core having a specific shape

corresponding to the light-intensity distribution of the luminous object such as a

star-shaped or polygonal distribution, for example, an extremely efficient light

transmission can be achieved.

[0006]

In order for manufacturing a preform including a core, or stress-applying

section, having an arbitrary shape, such as a star or polygonal shape, it is generally

necessary to prevent deformation of the core shape and thereby maintain the precise

shape of the core during the heat treating. In the total synthesis technique as

described above, however, it is difficult to form a preform having such an arbitrary

shape of the core. Thus, the arbitrary shape of the core is generally obtained by

assembling and fusing together a plurality of glass members each having a specific

shape corresponding to a portion of the arbitrary shape. [0007]

For example, the polarization-maintaining optical fiber is known as such

obtained by assembling and fusing together a plurality of glass members. The

technique is such that a plurality of holes each having a specific shape are formed in

a preform, and respective glass members, or cylindrical glass rods, which are

shaped beforehand to have a small diameter, are inserted in the holes. In this

technique, it is generally difficult to obtain a desired shape with a precise

dimensional accuracy, particularly in a process for forming an axisymmetric shape

which is liable to deformation during the fusing step.

[0008]

In view of the above, it is an object of the present invention to provide an

optical fiber having a non-axisymmetric structure, which is suited for use in direct

transmission of a video image or photographic image, for example.

It is another object of the present invention to provide a method for

manufacturing an optical fiber having an arbitrary shape with a precise dimensional

accuracy, and to provide a preform used in the method.

Means for solving the Problems

[0009]

The present invention provides, in a first aspect thereof, a method for

manufacturing a preform including the steps of: covering an outer periphery of a

first glass having a single first softening temperature by a second glass having a

single second softening temperature which is lower than the first softening temperature; and heating the first and second glasses up to a heating temperature for

fusion, thereby forming an integral body of the first and second glasses.

[0010]

The present invention provides, in a second aspect thereof, a method for

manufacturing an optical fiber including the steps of: covering an outer periphery of

a first glass having a single first softening temperature by a second glass having a

single second softening temperature which is lower than the first softening

temperature; inserting an assembly of the first and second glasses in a glass tube;

and collapsing said glass tube and said assembly of said first and second glasses by

heating at the same time of drawing the optical fiber.

[0011]

The present invention provides, in a third aspect thereof, an optical fiber

preform including a plurality of glasses including first and second glasses, wherein

the first glass configures a central core section and has a non-axisymmetric structure,

the second glass configures a cladding section covering an outer periphery of the

first glass, and the first glass has a softening temperature higher than a softening

temperature of the second glass.

[0012]

The present invention provides, in a fourth aspect thereof, an optical fiber

manufactured by drawing the optical fiber preform of the present invention.

Effects of the Invention

[0013] The method of the present invention has an advantage over the prior art that

an optical fiber having a desired shape can be manufactured from the preform

wherein the outer periphery of the first glass having the first softening temperature

is covered with the second glass having the second softening temperature lower

than the first softening temperature.

[0014]

According to the optical fiber preform of the present invention, an optical

fiber including a core having a non-axisymmetric structure with a higher

dimensional accuracy can be obtained from the optical fiber preform because the

second glass configuring the cladding section has the second softening temperature

lower than the first softening temperature of the first glass configuring the core.

BRIEF DESCRIPTION OF HE DRAWINGS

Fig. 1 is a cross-sectional view of a preform according to an embodiment of

the present invention.

Fig. 2 is a cross-sectional view of a core of an optical fiber obtained by

drawing a preform in a first comparative example.

Fig. 3 is a cross-sectional view of a preform manufactured in a second

example of the present invention.

Fig. 4 is a cross-sectional view of a preform manufactured in a third

example of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION [0015]

An optical fiber preform according to a preferred embodiment of the present

invention includes a core having a polygonal shape, for example, in the cross-

section thereof, and is thus different from a conventional preform including an

axisymmetric core. An optical fiber manufactured from the optical fiber preform of

the preferred embodiment includes a core having a polygonal shape as well in the

cross-section thereof. The optical fiber thus manufactured may preferably be used

for direct transmission of a photographic image having a polygonal shape, for

example.

Moreover, the configuration wherein the softening temperature of a first

glass, such as a core section, having an arbitrary shape is higher than the softening

temperature of a second glass, such as a cladding section, covering the outer

periphery of the first glass provides an optical fiber having an arbitrary shape with a

higher accuracy.

[0016]

In an optical fiber having an arbitrary shape, the core section or stress-

applying section having the arbitrary shape is doped with a dopant, such as

germanium, boron and fluorine, and has a softening temperature lower than the

softening temperature of the glass member covering the core section or stress-

applying section.

A method for manufacturing an optical fiber according to a preferred

embodiment of the present invention includes the step of covering the outer

periphery of the first glass having a polygonal shape, for example, with the second glass having a softening temperature lower the softening temperature of the first

glass, heating first and second glasses up to a heating temperature for fusion,

thereby forming a preform of an integral body of first and second glasses, and

drawing the preform to the optical fiber.

First, the second glass fuses and adheres onto the first glass by heating.

The heating temperature at which the second glass is fused is set below the

softening temperature of the first glass, whereby the first glass maintains the

original polygonal shape in the cross-section thereof. This achieves a higher

accuracy of the polygonal shape of the first glass, thereby providing an optical fiber

having a desired polygonal shape with a higher accuracy.

[0017]

A glass tube may be used as an outer member for the second glass member.

In this case, the first and second glasses are arranged or assembled within the glass

tube, and are heated. This prevents deformation from the polygonal shape to obtain

a preform having a desired shape. In case the glass tube attached outside the second

glass configuring a cladding, the glass tube is configured as a part of the cladding

after drawing the preform to an optical fiber. Use of the glass tube allows the

cladding shape to be maintained with ease. The softening temperature of the glass

tube is preferably higher than the softening temperature of the second glass, and

may be preferably equal to the softening temperature of the first glass.

EXAMPLES

[First Example] [0018]

Fig. 1 shows a cross-sectional view of a preform according to a first

example of the present invention. Glass members 11, 12 and 13 having the shape as

shown in Fig. 1 were obtained by grinding glass materials, and were assembled and

fused together to form an optical fiber preform. The glass member 11 had a shape

of a pole having a square cross-section, and a pair of glass members 13 each had a

shape of a pole having a rectangular cross-section, wherein one of the shorter sides

of the rectangle configured a circular arc, which formed a part of the cylindrical

surface of the cladding. A pair of glass members 12 each had a substantially half-

cylindrical surface, which was assembled with the circular arc of the glass members

13 so that a substantially complete cylindrical surface was formed from the glass

members 12 and 13. These glass members 11, 12 and 13 were assembled together

such that the glass members 13 were disposed first so as to oppose each other, with

the glass member 11 being sandwiched therebetween, and the glass members 12

were then disposed, with the flat surface of the glass members 12 abutting to the flat

surface of the glass members 11 and 13. The resultant preform had a shape of a

cylinder.

[0019]

The glass member 11 was made of a material having a softening

temperature higher than the softening temperature of the glass members 12 and 13,

and thus was not liable to deformation at a heating temperature lower the softening

temperature thereof. As a concrete example, the glass member 11 was made of

pure quartz, whereas the glass members 12 and 13 were made of quartz doped with fluorine. In general, the glass material is easily deformed at a temperature

exceeding the softening temperature thereof. The glass member 11 had a softening

temperature of 1800 degrees C, whereas the glass members 12 and 13 had a

softening temperature of 1600 degrees C due to doping the quartz with 2wt.-percent

fluorine. Those glass members 11, 12 and 13 were assembled together, as shown in

Fig. 1, and fused together by heating in an electric furnace. A specific case of

fusing together these members at a temperature of 1650 degrees C provided an

optimum cross-sectional shape having a least deformation.

[0020]

In the process of the present embodiment, an electric furnace was used for

fusing, but not limited thereto. A burner providing a flame may be used for this

purpose so long as the burner provides a uniform temperature distribution during

the fusion coupling of the glass members. The softening temperature, 1650 degrees

C, is only an example, and a softening temperature in a range between the softening

temperatures of both the glass materials may be used instead. An excessively lower

temperature dose not provide a suitable fusion and a peeling-off may occur. On the

other hand, an excessively higher temperature involves a larger deformation. Thus,

it is preferable that a temperature about 50 degrees C higher than the lower

softening temperature be used for the fusing.

[0021]

The preform as described above is drawn to an optical fiber in a drawing

furnace at a temperature of 1820 degrees C. The resultant optical fiber had no

substantial deformation in the core. In general, a core having a square cross-section, such as shown in Fig. 1, as well as another core having a polygonal cross-section

may be evaluated for the degree of deformation thereof by expressing the vertex

angle at each corner or apex of the cross-section. In the resultant optical fiber, the

corner of the cross section of the core was maintained at a substantially right angle

without incurring a problem in the deformation. In addition, the non-circularity of

the cladding in the resultant optical fiber was as low as about 1.0%, without

incurring a substantial problem in the circularity of the optical fiber.

[0022]

[First Comparative Example]

In a first comparative example of the preform having a structure similar to

the structure shown in Fig. 1, the glass member 11 was made of quartz doped with

germanium and the glass members 12 and 13 were made of pure quartz. In this

structure, the first glass member 11 had a softening temperature of 1600 degrees C

whereas the glass members 12 and 13 had a softening temperature of 1800 degrees

C. Fusing of these glass members 11, 12 and 13 was conducted at a temperature of

1650 degrees, which is 50 degrees C higher than the softening temperature of the

glass member 11. This provided a preform having a least deformation among other

fusing processes for the same structure at different temperatures. The resultant

preform was drawn to an optical fiber at a temperature of 1900 degrees C, which

was about 80 degrees C higher than the drawing temperature used in the first

example.

[0023]

Fig. 2 shows the cross-section of the core of the optical fiber obtained in the first comparative example. The core HA obtained from the glass member 11 of the

first comparative example had a significant deformation. The deformation was

such that each of the corners of the core 11 in the preform was melted, flowed out

into the gap between the glass member 12 and the glass member 13 configuring a

part of the cladding, and solidified in the as-melted state within the gap . If the

optical fiber obtained from this comparative example is used for direct transmission

of a photographic image, the transmitted image will have a significant deformation

whereby the optical fiber does not achieve the object of the present invention. The

optical fiber obtained in the first comparative example had a non-circularity of 0.9%

which was comparable to the non-circularity obtained in the first example of the

present invention.

[0024]

[Second Example]

Fig. 3 shows a preform according to a second example of the present

invention. Glass members 31 to 33 were similar to the glass members 11 to 13

shown in Fig. 1. Another glass member 34 is a glass tube made of a material

similar to the material of the glass member 31 (11). The glass members 31 to 33

were assembled together and inserted into the glass member 34. More precisely,

the glass members of the present example had a dimensional ratio different from the

dimensional ratio of the glass members 11 to 13 in the first example, for achieving

the final diameter of the core that is comparable to the final diameter of the core in

the first example. This was achieved by the configuration wherein the dimensions

of the glass member 31 were equal to the dimensions of the glass member 11, and the outer diameter of the glass member 34 was equal to the outer diameter of the

preform shown in Fig. 1.

[0025]

After assembling the glass members 31 to 34 in the structure as shown in

Fig. 3, the assembly of glass members 31 to 34 was drawn to an optical fiber at a

temperature similar to the temperature used in the first example. The resultant

optical fiber had a deformation in the core comparable to the deformation in the first

example; however, the non-circularity of the cladding was 0.2% which was superior

to that of the first example. Comparison of the structure obtained by inserting the

glass members 31 to 33 assembled and fused beforehand into the glass tube 34

against the structure obtained by inserting the as-assembled glass members 31 to 33

into the glass tube 34 did not provide a significant difference after drawing to the

optical fiber.

[0026]

The core used in the above examples had a square cross-section; however,

the core may have any contour such as polygon or star shape for achieving a similar

result, which was confirmed in other experiments.

[0027]

[Third Example]

Fig. 4 shows a preform according to a third example. The glass members

42 to 44 in Fig. 4 are similar to glass members 32 to 34 shown in Fig. 3. The glass

member 41a is of a cylinder inserted in the glass member 41b having a square

cross-section in the outline. The glass member 41a has a softening temperature lower than the softening temperature of the glass member 41b, which is higher than

the softening temperature of the other glass members 42 to 44. This configuration

achieves a drawing process incurring substantially no deformation.

[0028]

After assembling the glass members 41 to 44 in the structure as shown in

Fig. 4, the assembly of glass members 41 to 44 was drawn to an optical fiber at a

temperature similar to the temperature used in the first example. The relationship in

the softening temperature between the glass member 41a and the glass member 41b

is similar to the relationship in the normal optical fiber. That is, the glass member

41a has a softening temperature lower than the softening temperature of the glass

member 41b. In this structure, the axial symmetry of the glass member 41a

provided substantially no deformation in the glass members 41a and 41b after

drawing to an optical fiber.

[0029]

Samples having a structure of the first example and a softening temperature

of 1700 degrees C in the glass members 12 and 13 were prepared, and subjected to

a fusing process at temperatures between 1700 degrees C and 1800 degrees C. A

softening temperature of the glass members 11 is 1800 degrees C. Results of the

structure with respect to the core deformation and degree of fusion of the glass

members after the fusing process are shown in the following table 1. Table 1

[0030]

In the table 1, "G" indicates Good, "NG" indicates No Good, and "--"

indicates not measurable. In the above results, the fusion itself was achieved

without a problem so long as the heating temperature was higher than the softening

temperature of the glass members 12 and 13 by 50 degrees C or higher. However, a

heating temperature of 1760 degrees C or higher resulted in deformation of the core.

For the purpose of fusing, the viscosity of the core glass should be lowered to some

extent. That is, a temperature 50 degrees C lower than the softening temperature of

the glass member 11, at which the glass member is on the verge of melting, is most

suitable in the view point of prevention of deformation.

[0031]

Table 2 shows heating temperatures dependency of the deformation of core

for the glass members 12 and 13 having softening temperatures between 1600 degrees C and 1720 degrees C. A softening temperature of the glass members 11 is

1800 degrees C.

Table 2

[0032]

In the table 2, "G" indicates Good, "NG" indicates No Good, and "--" indicates not measurable. A heating temperature less than 50 degrees C higher than

the softening temperature of the glass members 12 and 13 achieved insufficient

fusion whereby a suitable preform was not formed. On the other hand, a

temperature 50 degrees or above higher than the softening temperature of the glass

members 12 and 13 achieved a suitable fusion without a problem. In these

experiments, since the heating temperature was set at 1750 degrees C or lower,

which is 50 degrees C lower than the softening temperature of the glass members

11, in consideration of the results shown in Fig. 1, all the samples that achieved a

suitable fusion exhibited no significant deformation.

In addition, although not shown in Table 2, a lower heating temperature

provides a lower degree of deformation of the core, and thus should be employed so

long as fusion itself is achieved. This means a heating temperature 50 degrees C

higher than the softening temperature of the glass members 12 and 13 is optimum.

Drawing of all the preforms manufactured in these experiments provided optical

fibers having a degree of deformation of the core and non-circularity of the cladding

comparable to those achieved in the first example.

[0033]

From the above results, the heating temperature for fusion coupling should

be preferably 50 degrees C or above higher than the softening temperature of the

glass members 12 and 13, and also 50 degrees C or above lower than the softening

temperature of the glass member 11. Thus, the softening temperature of the glass

members 12 and 13 should preferably be 100 degrees C or above lower than the

softening temperature of the glass member 11. [0034]

Since the above embodiment and examples are described only for

exemplification purposes, the present invention is not limited to the above

embodiment or examples and various modifications or alterations can be easily

made therefrom by those skilled in the art without departing from the scope of the

present invention.

Claims

1. A method for manufacturing a preform comprising the steps of:

covering an outer periphery of a first glass (11) having a single first

softening temperature by a second glass (12, 13) having a single second softening

temperature which is lower than said first softening temperature; and

heating said first and second glasses (11, 12, 13) up to a heating temperature

for fusion, thereby forming an integral body of said first and second glasses.

2. The method according to claim 1, wherein said second glass includes at

least two glass members (12, 13) assembled.

3. The method according to claim 1, wherein said second softening

temperature is 100 degrees C or above lower than said first softening temperature.

4. The method according to claim 1, wherein said heating temperature is 50

degrees C or above higher than said second softening temperature, and 50 degrees

C or above lower than said first softening temperature.

5. The method according to any one of claims 1 to 4, wherein said first glass

has a non-axisymmetric structure.

6. The method according to any one of claims 1 to 5, wherein said first glass

(11) configures a core section.

7. A method for manufacturing an optical fiber comprising the steps of:

covering an outer periphery of a first glass (31) having a single first

softening temperature by a second glass (32, 33) having a single second softening

temperature which is lower than said first softening temperature;

inserting an assembly of said first and second glasses (31, 32, 33) in a glass

tube (34); and

collapsing said glass tube (34) and said assembly of said first and second

glasses (31, 32, 33) by heating at the same time of drawing the optical fiber.

8. The method according to claim 7, wherein said first glass (31) has a non-

axisymmetric structure.

9. The method according to claim 7 or 8, wherein said first glass (31)

configures a core section.

10. An optical fiber preform comprising a plurality of glasses including first and

second glasses (11, 12, 13), wherein said first glass (11) configures a central core

section and has a non-axisymmetric structure, said second glass (12, 13) configures

a cladding section covering an outer periphery of said first glass (11), and said first

glass (11) has a softening temperature higher than a softening temperature of said

second glass (12, 13).

11. The optical fiber preform according to claim 10, wherein said first glass

(11) has a polygonal cross-section.

12. An optical fiber manufactured by drawing the optical fiber preform

according to claim 10 or 11.