METHOD OF MANUFACTURING FIBER PREFORM FOR SINGLE-MODE FIBERS

Abstract: A method of manufacturing a glass fiber preform in which a quartz glass rod having an outer coating containing chlorine and a core doped for higher refractive index has blown thereon a soot formed by a burner fed with quartz glass raw material and a chlorine group gas undergoing flame hydrolysis. The resulting composite is the heated and fused to collapse.

Description: The present invention relates to a method of manufacturing a glass fiber preform for single-mode optical fibers. Methods of manufacturing a glass fiber preform for single-mode fibers are well known. In these methods a glass raw material is subjected to a flame hydrolysis reaction to form a porous glass body composed of a core and a cladding. The resultant porous glass body is dehydrated and sintered to obtain a collapsed intermediate glass rod. Porous quartz glass is further formed by the above-mentioned methods at the outside of the obtained intermediate glass rod composed of the core and the cladding. The thus obtained composition is sintered or dehydrated and sintered to thereby obtain the required cladding thickness. In the above-mentioned conventional methods, the dehydration effected in producing the intermediate glass rod is very important to keep the transmission loss low when the glass fiber preform is used for making optical fibers. Therefore, a method is employed in which a gas of the chlorine group is contained in an atmosphere as a dehydrating agent. However, chlorine remains in the sintered intermediate body to result in a high refractive index of the cladding so that a difference in the refractive index is undesirably produced between the intermediate glass rod and the glass to be formed at the outside of the intermediate glass rod. As a result, the effective cut-off wavelength for a non-transmitted high order mode (hereinafter referred to simply as "cut off wavelength") is increased in the transmission characteristics of the optical fibers. Therefore, a method is employed in which the dehydration treatment is performed by using a gas of the chlorine group as a dehydrating agent when the porous glass outside the intermediate glass body is collapsed similarly as in the case of the intermediate body (U.S. Patent No. 4,486,212). In this method, as shown in Fig. 1, a porous glass body 2' is subjected to a heat treatment in an atmosphere containing a gas of the chlorine group. A heater 8 surrounds a furnace central tube 9. This method has disadvantages in that it is difficult to homogeneously incorporate the chlorine group within the porous glass body 2' because it takes a long time for the chlorine group gas to diffuse and permeate into the inside of the porous glass body 2' to react with the glass body. A further disadvantage is that if the concentration of the chlorine group gas is increased to shorten the treatment time, an excessive portion of the chlorine group gas is apt to remain as bubbles in the step of collapsing the porous glass body 2'. As described above, in the method disclosed in U.S. Patent 4,486,212, there have been such problems that when a complex of a porous glass body and an intermediate glass rod is inserted into an atmosphere containing a gas of the chlorine group in the inside of a furnace to be subjected to a treatment such as heating and dehydration (or the addition of chlorine), it takes a long time for performing the treatment homogeneously in the inside of the complex. Furthermore, if the concentration of the chlorine containing gas is made higher to shorten the treatment time, an excessive part of the chlorine gas remains so that bubbles may remain in the inside of the collapsed body in the sintering treatment. EP-A-0046281 discloses a process of manufacturing a glass having a pre-determined refractive index profile wherein one or more glass layers containing fluorine are deposited on a body and wherein the composition of the glass layers is selected such that the refractive index course in the layers in total corresponds to the pre-determined refractive index profile. It is the object of the present invention to provide a method of manufacturing a glass fiber preform for single-mode fibers in which chlorine can be homogeneously and rapidly added to a porous glass body, in which there are no residual bubbles and by which optical fibers having desirable transmission characteristics can be obtained. Said object is achieved by A method of manufacturing a glass fiber preform, comprising the steps of: preparing as starting member a glass rod comprising an outer coating made at least partially of quartz glass containing chlorine and a core made at least partially of quartz glass containing a component for increasing the refractive index of said quartz glass; introducing a quartz glass raw material and an additional gas containing chlorine into a burner to form quartz glass soot containing chlorine by a flame hydrolysis raction, the amount of the additional gas being such as to cause a chlorine concentration in said quartz glass soot comparable with the chlorine concentration in said outer coating; blowing said quartz glass soot to said starting member to obtain a complex of said quartz glass soot and said glass rod; and heating and fusing said complex to collapse said complex. It is preferable in the present invention to employ a gas containing one or more gases selected from the group consisting of chlorine gas, thionyl chloride gas, and carbon tetrachloride gas, or a gas obtained by adding an inert gas to one or more gases selected from the above group. Fig. 1 is a schematic view for explaining the conventional method. Fig. 2 is a schematic view for explaining an apparatus for practicing the present invention. Fig. 3 is a diagram showing the refractive index distribution of the intermediate glass rod used in one embodiment of the present invention. Fig. 4 is a diagram showing the refractive index distribution of the glass fiber preform manufactured according to the present invention. Referring to the drawings, the present invention will now be specifically described. Fig. 2 is a schematic view showing one embodiment of the present invention. A glass rod 1 is composed of an outer coating made of quartz glass containing chlorine and a core made of quartz glass containing a component for increasing the refractive index. Similarly to the conventional method, such a glass rod can be obtained through a method in which a porous glass body is dehydrated and sintered to be collapsed in an atmosphere containing chlorine. The porous glass body is produced through a method such as a VAD (vapor-phase axial deposition) method, or an OVD (outside vapor-phase deposition) method, in which a glass raw material is subjected to flame-hydrolysis. Alternatively, the porous glass body can be prepared by a method in which soot powder is formed and molded with pressure, or by any other suitable method. According to the present invention, when the porous glass 2 is formed on the outer circumference of the intermediate glass rod 1, a gas of the chlorine group 6 is fed into a burner 3 together with a flame gas 4 and a glass raw material gas 5 so that chlorine is homogeneously contained in the porous glass 2. The thus obtained complex is inserted in a furnace to be heated and fused to thereby be integrated. According to the method of the present invention, a gas containing one or more gases selected from the group consisting of chlorine gas, thionyl chloride gas, and carbon tetrachloride is preferably used as the gas containing chlorine. It is also possible to use a gas obtained by adding an inert gas such as He, or Ar, to the above-mentioned one or more gases to thereby dilute the latter. If a gas of the chlorine group is introduced together with a glass raw material to form a porous glass, chlorine can be homogeneously added into the porous glass body. Bubbles can be prevented while the porous glass body is heated to collapse in the furnace. In the conventional method, as described above, an atmosphere containing a gas of the chlorine group diffuses and penetrates into the inside of the porous glass body 2' to cause a reaction with the porous glass body. In the method according to the present invention, on the contrary, it is considered that chlorine can be homogeneously added because it forms the porous glass 2 to which chlorine is directly added, and that a generation of bubbles can be prevented because an excessive amount of the chlorine containing gas hardly remains. Adjustment of the quantity of addition of chlorine is performed by the adjustment of the mixing ratio of the glass raw material gas, the flame gas, the inert gas and the gas containing chlorine. The following examples illustrate the invention. Example 1 A sintered body produced through the VAD method and substantially composed of a core of GeO₂ - SiO₂ and a cladding of SiO₂ was subjected to drawing to produce the intermediate glass rod 1 having an outer diameter of 16 mm and a length of 500 mm. The distribution of the differential refractive index (hereinafter referred to as "Δn distribution") was such that, as shown in Fig. 3, the differential refractive index of the core 10 was 0.3%. The structure was such that the ratio of the outer diameter of the cladding 11 to that of the core 10 was 5. The residual layer 12 of chlorine owing to dehydration had a differential refractive index of 0.015%. All the above-mentioned differential refractive indices were expressed as a ratio to pure silica. Next, after being polished by an oxyhydrogen flame, the intermediate glass rod 1 was mounted on a soot forming instrument for VAD method arranged as shown in Fig. 2. The gas 4 consisting of oxygen of 44 ℓ/min, hydrogen of 40 ℓ/min, and a sealing Ar gas of 13.8 ℓ/min was fed into an oxyhydrogen burner 3 to form a flame. Additionally, silicon tetrachloride 5 of 52.3 ℓ/min was fed together with thionyl chloride gas 6 of 0.13 ℓ/min into the oxyhydrogen burner 3 so that the porous glass body 2 was formed on the intermediate glass rod 1. Then, the complex of the intermediate glass rod 1 and the porous glass body 2 was inserted into a furnace to be collapsed. The drawing furnace had an atmosphere of helium gas and a furnace temperature of 1710 °C. The drawing speed within the furnace was 8 mm/min. The thus obtained collapsed preform was subjected to drawing and the Δn distribution was measured. As shown in Fig. 4, no stepped portion were found in the refractive index of the cladding but the Δn distribution was equal longitudinally as well as radially. The glass fiber preform was flame-polished and was subjected to drawing to be made into fibers. As the result of measurement, the effective cut-off wavelength of the fibers was 1.26 µm which was a proper value matching other properties of the transmission characteristics. Comparative Example 1 A glass fiber preform was produced under the same conditions except that the thionyl chloride 6 was not supplied for forming the porous glass body 2'. The resultant glass fiber preform was made into fibers. The effective cut-off wavelength of the fibers was measured. The effective cut-off wavelength was 1.30 µm which was larger by 0.04 µm than that in Example 1 of the invention. This effective cut-off wavelength of the comparative example is an undesirable value in view of matching properties with respect to other characteristic values. Comparative Example 2 The porous glass body 2' was produced under the same conditions as in comparative Example 1 and was inserted into a furnace as shown in Fig. 1 to be collapsed in an atmosphere of a mixture of a chlorine gas of 0.3 ℓ/min and a helium gas of 15 ℓ/min, at a furnace temperature of 1710 °C and at a drawing speed of 8 mm/min. A number of minute bubbles remained in the boundary surface between the intermediate glass rod 1 and the porous glass body 2'. Further, the content of chlorine was investigated through EDAX. The result proved that the distribution of the chlorine was not homogeneous both radially and longitudinally. It was presumed that the glass fiber preform had undesirable characteristics for optical fibers. Example 2 A collapsed preform was produced under the same conditions as in example 1 except that a mixed gas of a helium gas of 0.1 ℓ/min and a chlorine gas of 0.16 ℓ/min were used instead of thionyl chloride gas. The obtained solid mother material was subjected to drawing and the Δn distribution was measured. As shown in Fig. 4, no step was found in the refractive index of the cladding but the Δn distribution was equal longitudinally as well as radially. The glass fiber preform was flame-polished and was subjected to drawing to be made into fibers. As the result of measurement, the effective cut-off wavelength of the fibers was 1.26 µm which was the same value as that in the Example 1 and which was a proper value with respect to other transmission characteristic values. As described above, according to the method of the present invention chlorine can be homogeneously added in the forming portion of the porous glass body with no residual bubbles, so that good characteristics as optical fibers can be obtained. In the present invention when the outer cladding is formed on the intermediate glass rod having a cladding of substantially pure silica to which chlorine has been added through dehydration by a gas containing chlorine, and when the substantially pure silica has a higher refractive index than that of the original pure silica glass owing to the content of chlorine or any other reason, chlorine is added in order to improve the characteristics of the glass fiber preform as optical fibers. The use of the intermediate glass rod itself is not limited to the VAD method but applicable to any other manufacturing method. Further, in forming the outer cladding portion, the invention is not limited to the VAD method shown in Fig. 2 but other well-known methods such as OVD, may be used by adjusting the ratio of the flame gas, the glass raw material gas, and the chlorine gas to be fed into the burner.

Claims: A method of manufacturing a glass fiber preform, comprising the steps of: preparing as starting member a glass rod comprising an outer coating made at least partially of quartz glass containing chlorine and a core made at least partially of quartz glass containing a component for increasing the refractive index of said quartz glass; introducing a quartz glass raw material and an additional gas containing chlorine into a burner to form quartz glass soot containing chlorine by a flame hydrolysis reaction, the amount of the additional gas being such as to cause a chlorine concentration in said quartz glass soot comparable with the chlorine concentration in said outer coating; blowing said quartz glass soot to said starting member to obtain a complex of said quartz glass soot and said glass rod; and heating and fusing said complex to collapse said complex. The method of claim 1, wherein said additional gas containing chlorine comprises one or more gases selected from the group consisting of chlorine gas, thionyl chloride gas, and carbon tetrachloride gas. The method of claim 2, wherein said additional gas containing chlorine further comprises an inert gas.

Inventors: ENDOH, SHIGEKI YOKOHAMA WORKS, KYOGOKU, TAKESHI YOKOHAMA WORKS, MIZUTANI, FUTOSHI YOKOHAMA WORKS, SAITOH, TATSUO YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1987
Main classification: C03B37
IPC codes: C03B20, C03B37, C03B8, G02B6