Patent Application
20090252468
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.
It is known that an optical fiber including OH radicals has a large absorption peak in the vicinity wavelength of 1380 nm. 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.
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.
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-2003-342031A 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.
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.
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.
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.
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.
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.
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.
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.
The present invention provides, in a fourth aspect thereof, an optical fiber manufactured by drawing the optical fiber preform of the present invention.
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.
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.
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.
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.
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.
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 2 wt.-percent fluorine. Those glass members 11, 12 and 13 were assembled together, as shown in
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.
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
In a first comparative example of the preform having a structure similar to the structure shown in
After assembling the glass members 31 to 34 in the structure as shown in
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.
After assembling the glass members 41 to 44 in the structure as shown in
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.
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.
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.
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
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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-127905 | Apr 2005 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP06/05483 | Mar 2006 | US |
| Child | 11500958 | US |
