Dehydration-sintering furnace, a manufacturing method of an optical fiber preform utilizing the furnace and an optical fiber preform manufactured by the method

A dehydration-sintering furnace for dehydrating and/or sintering an optical fiber preform for use in production of an optical fiber includes a muffle for accommodating the optical fiber preform, a heater for heating the muffle, and a pressure fluctuation absorbing apparatus connected to the muffle. Since the pressure fluctuation absorbing apparatus is thermally insulated from a room temperature atmosphere or heated, vapor produced in a dehydration-sintering process is prevented from condensing (liquefying) in a pressure fluctuation absorbing apparatus, thereby preventing reduced dehydration effectiveness in the muffle and reduced quality of the optical fiber preform.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority under 35 U.S.C. §119 to Japanese Patent Application No. 2003-409068, the entire contents of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dehydration-sintering furnace, more particularly, a dehydration-sintering furnace for use in a dehydration and/or a sinter process of a porous optical fiber (glass fiber) preform.

2. Description of the Related Art

A porous optical fiber preform produced by Vapor Phase Axial Deposition (VAD) method, Outside Vapor Deposition (OVD) method and the like is dehydrated and/or sintered by accommodating the preform in a muffle of a dehydration-sintering furnace, supplying to the muffle dehydration agents such as chlorine, thionyl chloride or the like and an inert gas such as helium or the like, and then heating the muffle with a heater.

An example of a dehydration-sintering furnace for dehydrating and/or sintering the porous optical fiber preform is a furnace that has a muffle connected to a pressure fluctuation absorbing apparatus such as a balloon type pressure buffer, a gas-flow buffering chamber, or a pressure fluctuation absorption chamber utilizing an electro magneto valve and the like in order to suppress a rather large pressure fluctuation and swiftly keep the muffle pressure constant (Refer to, for example, Publication of Examined Utility Model Application H06-50513 and Japanese Patent Application Laid-open Publications H05-4828, H06-127964, and H10-120428).

While the pressure fluctuation absorbing apparatus connected to the muffle is advantageous in absorbing a short-term pressure fluctuation in the muffle, the absorbing apparatus is somewhat disadvantageous as described below.

When the pressure in the muffle is raised, the gas therein is pressed out into the pressure fluctuation absorbing apparatus from the muffle. On the other hand, when the pressure is lowered, the gas in the pressure fluctuation absorbing apparatus flows back to the muffle.

Since the pressure fluctuation absorbing apparatus is positioned outside of the furnace and exposed to a room temperature atmosphere, vapor that includes substances produced during a dehydration and/or sinter process is condensed (liquefied) in the pressure fluctuation absorbing apparatus. The larger amount of moisture exists in the porous optic fiber preform, the larger amount of condensation takes place.

The liquid produced by condensation (liquefaction) in the pressure fluctuation absorbing apparatus will flow back to the muffle when the pressure in the furnace decreases, thereby deteriorating the dehydration effectiveness in the muffle.

In addition, the liquid condensed in the pressure fluctuation absorbing apparatus may drop on the porous optical fiber preform and then taint the surface of the preform. Also, the dropped liquid may make brittle the portion of soot on which the liquid drops and then the portion may fall apart therefrom. Those will deteriorate the quality of the optical fiber preform and reduce the production yield of the same.

Also, when the liquid condensed in the pressure fluctuation absorbing apparatus does not flow back to the muffle but stays at a duct and the like connecting the muffle and the pressure fluctuation absorbing apparatus, since substances produced in the dehydration process contain strong acids such as hydrogen chloride and sulfuric acid, the duct and the like will be eroded, thereby shortening an operating life of the furnace.

The objective of the present invention is to provide a dehydration-sintering furnace having a pressure fluctuation absorbing apparatus, wherein vapor that contains substances produced in the dehydration and/or sinter process is prevented from condensing in the pressure fluctuation absorbing apparatus and thereby the flowing of condensed liquid back to the muffle of the dehydration-sintering furnace is essentially prevented.

Another objective of the present invention is to provide a manufacturing method of optical fiber preform, utilizing the above dehydration-sintering furnace, the method enabling a stable production of a high quality optical fiber preform.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a dehydration-sintering furnace having a muffle for dehydrating and/or sintering an optical fiber preform for use in production of an optical fiber, wherein a pressure fluctuation absorbing apparatus is connected to said muffle and said pressure fluctuation absorbing apparatus is covered with a thermally insulating material.

A second aspect of the present invention provides a dehydration-sintering furnace having a muffle for dehydrating and/or sintering an optical fiber preform for use in production of an optical fiber, wherein a pressure fluctuation absorbing apparatus is connected to said muffle and said pressure fluctuation absorbing apparatus is accommodated in a thermally insulated container.

A third aspect of the present invention provides a dehydration-sintering furnace having a muffle for dehydrating and/or sintering an optical fiber preform for use in production of an optical fiber, wherein a pressure fluctuation absorbing apparatus is connected to said muffle and said furnace is provided with heating means configured to heat said pressure fluctuation absorbing apparatus.

A fourth aspect of the present invention provides a manufacturing method of an optical fiber preform, utilizing any of said dehydration-sintering furnaces, wherein a gas temperature is at 100 degrees Celsius or higher in said pressure fluctuation absorbing apparatus.

The dehydration-sintering furnace according to the present invention is capable of preventing vapor produced in a dehydration-sintering process from condensing (liquefying) in a pressure fluctuation absorbing apparatus since the pressure fluctuation absorbing apparatus is thermally insulated from a room temperature atmosphere or heated.

Since the above structure is able to essentially prevent the liquid condensed in the pressure fluctuation absorbing apparatus from flowing back to the muffle and/or dropping on the optical fiber preform accommodated in the muffle, a dehydration effectiveness in the muffle and a quality of the optical fiber preform are not deteriorated, thereby enabling a stable production of an optical fiber preform.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A First Embodiment

Referring toFIG. 1, a hydration-sintering furnace according to a first embodiment of the present invention will be described in the following. The hydration-sintering furnace inFIG. 1has a quartz-made, hermetically constructed muffle11as the furnace main body. The muffle11accommodates a porous optical fiber preform W so as to suspend the preform W with a seed rod100in the interior13of the muffle11.

Outside of the muffle11is provided a furnace body17in which a heater15for heating the interior13, that is, the optical fiber preform W in the interior13is provided. An inert gas such as helium, argon and the like as a purge gas is supplied from a purge gas inlet19into the furnace body17. The purge gas supplied into the furnace body17is discharged out from an outlet21.

The muffle11has at the bottom thereof a gas inlet tube23that allows dehydration agents such as chlorine gas, thionyl chloride or the like and inert gas such as helium or the like to be supplied from a gas supplier (not shown) into the interior13.

In addition, the muffle11has at the top thereof a pressure adjustment port25. To the pressure adjustment port25, a pressure fluctuation absorbing apparatus29is connected via a duct27. The pressure fluctuation absorbing apparatus29can be a well-known one such as a balloon type pressure buffer, a gas-flow buffer chamber, or a pressure fluctuation absorption chamber utilizing an electro magneto valve and the like, about which a detailed explanation is eliminated.

The pressure fluctuation absorbing apparatus29is covered and thermally insulated with a thermally insulating material31. Also, the duct27connecting the muffle11and the pressure fluctuation absorbing apparatus29is covered with a thermally insulating material33to thermally insulate the duct27.

The thermally insulating materials31,33are formed of fibrous insulation material, such as an alumina fiber, having high heat resisting properties and totally cover the pressure fluctuation absorbing apparatus29and the duct27for thermal insulation.

In the dehydration-sintering furnace having the above structure, the optical fiber preform W is dehydrated and/or sintered in the interior13by heating the muffle11with the heater15.

When the pressure in the interior13is raised during a dehydration and sinter process, the gas in the interior13is pressed out into the pressure fluctuation absorbing apparatus29via the duct27, while the gas in the pressure fluctuation absorbing apparatus29flows back to the muffle11via the duct27when the pressure in the interior13is lowered, thereby suppressing the pressure fluctuation in the interior13.

The gas flow between the interior13and the pressure fluctuation absorbing apparatus29contains vapor that contains moisture emerging from the optical fiber preform W in the interior13and substances produced in the dehydration and sinter process. Since the pressure fluctuation absorbing apparatus29and the duct27are insulated from a room temperature atmosphere with the thermally insulating materials31,33, such vapor is prevented from condensing in the pressure fluctuation absorbing apparatus29and the duct27.

Therefore, no liquid is produced by condensation in the pressure29and the duct27, thereby essentially preventing the flowing of any liquid by condensation back to the muffle11. This is how a reduction in dehydration effectiveness and a phenomenon such as the liquid by condensation dropping on the optical fiber preform W are prevented, thereby enabling a stable production of a high quality optical fiber preform.

Optical fibers manufactured by drawing the optical fiber preform that has been dehydrated and sintered in the dehydration-sintering furnace having a pressure fluctuation absorbing apparatus29covered with a thermally insulating material show an average transmission loss of as low as about 0.286 dB/km at a wavelength of 1385 nm. For comparison, other optical fibers are manufactured under the same conditions but a use of a dehydration-sintering furnace having a pressure fluctuation absorbing apparatus with no thermally insulating material covered. As a result, the optical fibers for comparison show an average transmission loss of about 0.292 dB/km at a wavelength of 1385 nm. From this comparison, it is understood that the dehydration-sintering furnace according to this embodiment and a manufacturing method of an optical fiber preform utilizing the same exert a superior operational effect.

In addition, since any liquid by condensation in the pressure fluctuation absorbing apparatus29does not flow back to the muffle11or stay in the duct27, the duct27is prevented from erosion in a short term by strong acids such as hydrogen chloride and sulfuric acid, thereby lengthening the life of the furnace.

A Second Embodiment

Referring toFIG. 2, a dehydration-sintering furnace according to a second embodiment of the present invention will be described. InFIG. 2, a member or component corresponding to that inFIG. 1will be represented by a same reference mark used inFIG. 1to eliminate an undue repetition of explanation.

In the second embodiment, a pressure fluctuation absorbing apparatus29is totally housed in a hermetically constructed, thermally insulated container35. The container35is composed of a hermetic box37and a thermally insulating material39, such as an alumina fiber and the like, which covers the hermetic box37to thermally insulate substantially the entire box37.

In this embodiment, the pressure fluctuation absorbing apparatus29and a duct27are thermally insulated from a room temperature atmosphere by the hermetic box37and the thermally insulating material33, thereby preventing vapor that contains substances produced in a dehydration-sintering process from condensing (liquefying) in the pressure fluctuation absorbing apparatus29and the duct27.

Therefore, since any liquid by condensation is not produced in the pressure fluctuation absorbing apparatus29and the duct27, any liquid by condensation does not flow back to the muffle11, thereby essentially preventing the flowing of condensed liquid back to the muffle11and enabling a stable production of a high quality optical fiber preform.

A Third Embodiment

Referring toFIG. 3, a dehydration-sintering furnace according to a third embodiment of the present invention will be described. Also inFIG. 3, a member or component corresponding to that inFIG. 1will be represented by a same reference mark used inFIG. 1to eliminate an undue repetition of explanation.

In the third embodiment, a pressure fluctuation absorbing apparatus29is housed in a container41. Outside of the container41, a heater43is provided as a heating means surrounding the container41. Electric power supplied to the heater43is regulated by a heater controlling means45, which controls the heating value from the heater43to heat the container41. In this way, the temperature inside the container41is raised and the pressure fluctuation absorbing apparatus29is heated indirectly with the heater43.

The container41has a temperature sensor47therein that detects a temperature inside the container41. The heater controlling means45inputs a signal from the temperature sensor47and accordingly regulates electric power to be supplied to the heater43in order to keep the temperature inside the container41at a predetermined temperature (a constant temperature) needed to avoid condensation.

With the above construction, the pressure fluctuation absorbing apparatus29is kept at a temperature equal to or higher than the temperature needed to avoid condensation, thereby preventing a generation of liquid by condensation in the pressure fluctuation absorbing apparatus29. Therefore, the flowing of condensed liquid back to the muffle11is essentially prevented, thereby enabling a stable production of a high quality optical fiber preform.

Incidentally, the pressure fluctuation absorbing apparatus29may be constructed such that the apparatus29is directly heated by the heater43. This construction also demonstrates the same operational advantages as the foregoing construction in this embodiment. Also, an additional heater can be provided for the duct27.

A Fourth Embodiment

Referring toFIG. 4, a dehydration-sintering furnace according to a fourth embodiment of the present invention will be described. InFIG. 4, a member or component corresponding to that inFIG. 3will be represented by a same reference mark used inFIG. 3to eliminate an undue repetition of explanation.

In the fourth embodiment, a heat exchanger49as a heating means is provided so as to surround the outside of the container41. The heat exchanger49has a heat transfer medium such as hot water, steam, and the like flowing therein.

With this construction, the container41is heated and then the temperature inside the container41is raised, thereby heating indirectly the pressure fluctuation absorbing apparatus29by the heat exchanger49.

Accordingly, the pressure fluctuation absorbing apparatus29is kept at a temperature equal to or higher than the temperature needed to avoid condensation, thereby preventing liquid by condensation from being produced in the pressure fluctuation absorbing apparatus29. Therefore, the flowing of liquid by condensation back to the muffle11is essentially prevented, thereby enabling a stable production of a high quality optical fiber preform.

Incidentally, the pressure fluctuation absorbing apparatus29may be constructed such that the apparatus29is directly heated by the heat exchanger49, which is capable of demonstrating the same operational advantages as the above-mentioned construction in this embodiment. In addition, the heat transfer medium supplied to the heat exchanger49can be a hot purge gas exhausted from the furnace body17. Furthermore, an additional heat exchanger can be provided for the duct27.

A Fifth Embodiment

Referring toFIG. 5, a dehydration-sintering furnace according to a fifth embodiment of the present invention will be described. In the fifth embodiment, a container41has an inlet51and an outlet53, through which a heat transfer medium such as steam is supplied to the container41. With this construction, the interior of the container41is heated so that the temperature thereof is raised, thereby heating the pressure fluctuation absorbing apparatus29.

Accordingly, the pressure fluctuation absorbing apparatus29is kept at a temperature equal to or higher than the temperature needed to avoid condensation, thereby preventing liquid by condensation from being produced in the pressure fluctuation absorbing apparatus29. Therefore, the flowing of liquid by condensation back to the muffle11is essentially prevented, thereby enabling a stable production of a high quality optical fiber preform.

In addition, the duct27connecting the muffle11and the pressure fluctuation absorbing apparatus29may be coaxially surrounded by an outer pipe55to form a double tube. The heat transfer medium such as steam is also supplied to pass through a path57between the duct27and the outer pipe55, thereby heating the duct27. The heat transfer medium flowing through the duct keeps a temperature of the duct equal to or higher than the temperature needed to prevent condensation, thereby preventing liquid by condensation from being produced in the duct. By the way, the heat transfer medium supplied to the container41and the path57can just as readily be hot water instead of steam.

A Sixth Embodiment

Referring toFIG. 6, a dehydration-sintering furnace according to a sixth embodiment of the present invention will be described. In the sixth embodiment, a container41and a path57are connected to a gas purge discharge port21of a furnace body17via ducts59,61,63. With this construction, the purge gas discharged from the furnace body17is supplied to the container41and the path57as a heat transfer medium, thereby raising the temperature inside the container41and the path57, and then also raising the temperature of the pressure fluctuation absorbing apparatus29and the duct27.

Therefore, the pressure fluctuation absorbing apparatus29and the duct27are kept at a temperature equal to or higher than the temperature needed to avoid condensation, thereby preventing liquid by condensation from being produced in the pressure fluctuation absorbing apparatus29and the duct27. Therefore, the flowing of liquid by condensation back to the muffle11is essentially prevented, thereby enabling a stable production of a high quality optical fiber preform.

The gas temperature inside the pressure fluctuation absorbing apparatus29may be kept at 100 degrees Celsius or higher, or may be kept at 300 degrees Celsius or higher. When the temperature is 100 degrees Celsius or higher, moisture in the gas does not condense. When the temperature is 300 degrees Celsius or higher, even if the dehydration agents contain sulfur atoms and oxygen atoms and then sulfuric acid or sulfuric acid gas is produced by dehydration reaction, sulfuric acid does not condense in the pressure fluctuation absorbing apparatus29, thereby enabling a stable production of a high quality optical fiber preform.

Although the above exemplary embodiments of the present invention have been described, it will be understood by those skilled in the art that the present invention should not be limited to the described exemplary embodiments, but that various changes and modifications can be made within the spirit and scope of the present invention.