METHOD OF MANUFACTURING OPTICAL FIBER PREFORM, AND OPTICAL FIBER

The present invention relates to a preform manufacturing method and others for effectively reducing variation in refractive index due to chlorine used in manufacture of an optical fiber preform. The manufacturing method includes a dechlorination step carried out between a point of an end time of a dehydration step and a point of a start time of a sintering step, the dechlorination step being a step of heating a porous preform after dehydrated, in an atmosphere containing no chlorine-based dehydrating agent, for a given length of time while maintaining a temperature lower than a sintering temperature, thereby removing chlorine from the porous preform after dehydrated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. The same elements will be denoted by the same reference signs in the description of the drawings, without redundant description.

FIGS. 1A to 1Dare drawings showing a sectional structure of an optical fiber according to an embodiment of the present invention, and various refractive index profiles applicable to the optical fiber. Specifically,FIG. 1Ais a drawing showing the typical sectional structure of the optical fiber according to the present embodiment, and this optical fiber100has at least a core region110extending along a predetermined axis (coincident with the optical axis AX), and a cladding region120provided on the outer periphery of the core region110. The core region110and the cladding region120may be comprised of respective glass regions with different refractive indices.

FIGS. 1B to 1Dshow examples of various refractive index profiles of optical fibers applicable to the optical fiber100shown inFIG. 1A. Namely, the optical fiber100to be applied herein can be a multimode optical fiber having a GI type refractive index profile150shown inFIG. 1B, a multimode optical fiber having a step index type refractive index profile160shown inFIG. 1C, or a single-mode optical fiber suitable for long-haul optical communication, having a step type refractive index profile170shown inFIG. 1D. The refractive index profiles150to170shown inFIGS. 1B to 1Dindicate refractive indices of respective portions on a line L (coinciding with a radial direction of the optical fiber100) perpendicular to the optical axis AX coincident with a center of the core region110, inFIG. 1A.

A manufacturing method of an optical fiber preform, for obtaining the optical fiber100having the sectional shape and refractive index profile as described above, will be described below in detail.

For obtaining the optical fiber100, an optical fiber preform600(cf.FIG. 9) is first manufactured. This optical fiber preform600is obtained by initially manufacturing a core preform corresponding to the core region110and thereafter further providing a transparent glass body corresponding to the cladding region120on the outer periphery of the core preform. In the present embodiment, the core preform is obtained by manufacturing a porous preform, for example, doped with GeO2(germanium dioxide) by the OVD process or by the VAD process and subjecting the porous preform to steps including dehydration, dechlorination, sintering, and extension. Furthermore, a porous glass body is deposited on the outer periphery of the obtained core preform by the VAD process or the like, and the resulting body is processed through the same steps of dehydration, sintering, and others as those described above, to obtain an optical fiber preform in which a transparent glass body corresponding to the cladding region120is provided on the outer peripheral surface of the core preform.

FIG. 2Ashows a device configuration for carrying out the OVD process applied to the manufacturing step of the core preform corresponding to the core region110of the optical fiber100(the first half step in the manufacturing process of the optical fiber preform).FIG. 2Bis a drawing for explaining a device configuration for carrying out the VAD process. As an example, where the optical fiber100having the GI type refractive index profile150is manufactured, the core preform manufactured by the OVD process or by the VAD process is a portion to make the core region110with the refractive index profile having the alpha value in the range of 1.9 to 2.2 after drawn.

First, when the OVD process is applied to the deposition step of manufacturing the porous preform wherein at least its peripheral region is covered by a porous glass body, a porous preform510is manufactured by a soot-depositing device shown inFIG. 2A. This soot-depositing device has a structure for holding a central shaft310(which functions as a support mechanism and which may be a hollow glass tube) in a rotatable state in the direction indicated by arrows S1. The soot-depositing device is provided with a burner320for growing the porous preform510along the central shaft310(support mechanism), and a gas supply system330for supplying a source gas. The burner320can be moved in directions indicated by arrows S2aand S2binFIG. 2A, by a predetermined moving mechanism.

During the manufacture of the porous preform510, fine glass particles are made by hydrolysis reaction of the source gas supplied from the gas supply system330, in flame of the burner320, and these fine glass particles are deposited on the side face of the central shaft310. During this step, the central shaft310is rotated in the direction indicated by arrows S1, while the burner320is moved in the directions indicated by arrows S2a, S2b.This operation causes a porous glass body to grow along the central shaft310, obtaining the porous preform510(soot preform) to make the core region110. The soot-depositing device shown inFIG. 2Acan also be applied to manufacture of a porous glass body to make the cladding region120, which is to be formed on the outer peripheral surface of the core preform obtained in the end.

On the other hand, when the VAD process is applied to the deposition step of manufacturing the porous preform wherein at least the peripheral region is covered by the porous glass body, the porous preform510(porous glass body) is formed by the soot-depositing device shown inFIG. 2B. This soot-depositing device is provided with a vessel315having at least an exhaust port315a, and a support mechanism310for supporting the porous preform510. Namely, the support mechanism310is provided with a support shaft rotatable in the direction indicated by arrow S1, and a starting rod for growth of a porous glass body (soot body) to make the porous preform510is attached to the tip of the support shaft.

The soot-depositing device inFIG. 2Bis provided with a burner320for depositing the porous glass body (soot body), and the gas supply system330supplies a desired source gas (e.g., GeCl4, SiCl4, etc.), a combustion gas (H2and O2), and a carrier gas such as Ar or He.

During the manufacture of the porous preform510, fine glass particles are made by hydrolysis reaction of the source gas supplied from the gas supply system330, in flame of the burner320, and these fine glass particles are deposited on the bottom surface of the starting rod. During this step, the support mechanism310performs an operation of once moving the starting rod disposed at the tip of the support mechanism, in the direction indicated by arrow S2a, and thereafter pulling up the starting rod along the direction indicated by arrow S2b(the longitudinal direction of the porous preform510) while rotating it in the direction indicated by arrow S1. This operation causes the porous glass body to grow downward from the starting rod on the bottom surface of the starting rod, obtaining the porous preform (soot preform)510to make the core region110eventually. The soot-depositing device shown inFIG. 2Bcan also be applied to manufacture of the porous glass body to make the cladding region120, which is to be formed on the outer peripheral surface of the core preform obtained in the end.

Next, the dehydration step, dechlorination step, and sintering step are successively carried out for the porous preform510obtained as described above.FIGS. 3A to 3Care drawings for explaining device configurations for carrying out the dehydration step, the dechlorination step, and the sintering step, respectively, in the present embodiment.

First, the porous preform510is manufactured by the OVD process or by the VAD process as described above (deposition step), and then the porous preform510is subjected to the dehydration step by the device shown inFIG. 3A. Namely, the porous preform510thus manufactured is set in a heating vessel350(traverse furnace) with a heater360, which is shown inFIG. 3A, and is heated in a chlorine-containing atmosphere in a state in which the in-furnace temperature is maintained at a predetermined temperature, for a given length of time. In the case where the porous preform510is manufactured by the OVD process, the central shaft310is removed from the porous preform510before execution of the dehydration step; when the central shaft310is a hollow glass tube, it may be removed by flowing an etchant gas into the hollow glass tube after the sintering step.

The heating vessel350is provided with an inlet port350afor supplying a gas containing chlorine and an exhaust port350b. During this dehydration step, while rotating the porous preform510in the direction indicated by arrow S4around the central axis AX of the porous preform510(coincident with the optical axis of the optical fiber to be obtained), the support mechanism340further moves the whole porous preform510in the directions indicated by arrows S3a, S3b, thereby to change the relative position of the porous preform510to the heater360. Through this step, hydroxyl groups (—OH) are removed to make the porous preform520in which a predetermined amount of chlorine is added.

In the dehydration step in the present embodiment, the temperature in the heating furnace350is maintained at 1000° C. and a gas mixture containing chlorine gas (Cl2) at a mixture ratio of 8.5% and He gas at a mixture ratio of 91.5% is supplied through the inlet port350ainto the heating vessel350. As a result, we obtain the porous preform520inside which the predetermined amount of chlorine remains. Each value of the above-described gasses is merely an example of a mixture ratio. Also, a combination of Cl2and Ar, a combination of Cl2and N2, and so on is applicable to a gas mixture to be supplied.

FIGS. 4A to 4Cshow an intrusion process of chlorine into the porous preform510. At the beginning of dehydration, no chlorine intrudes into the porous preform510, as shown inFIG. 4A, at each of parts of the preform indicated by arrows A1to A3inFIG. 3A. However, with progress of dehydration, chlorine gradually intrudes in directions toward the center (central axis AX) of the porous preform510as a target of dehydration. In the porous preform520at a point of an end time of the dehydration step, i.e., in the porous preform520after the dehydration, a considerable amount of chlorine remains. InFIGS. 4A to 4C, the axis of abscissas represents the radial distance r from the central axis AX of the porous preform510(520), and the axis of ordinates the chlorine concentration.

Subsequently, in the present embodiment, the porous preform520after dehydrated is subjected to the dechlorination step by the device shown inFIG. 3B. Namely, the porous preform520after dehydrated is set in the heating vessel350(traverse furnace) with the heater360shown inFIG. 3B, and is heated in an atmosphere not containing chlorine (e.g., in an inert gas), in a state in which the interior of the furnace is maintained at a predetermined temperature of not more than 1300° C., for a given length of time.

The heating vessel350shown inFIG. 3Bis also provided with the inlet port350afor supplying a gas not containing chlorine (e.g., He gas, N2gas, Ar gas, a gas mixture of He and Ar, a gas mixture of He and N2, and so on), and the exhaust port350b. During this dechlorination step, while rotating the porous preform520after dehydrated, in the direction indicated by arrow S4around the central axis of the porous preform520, the support mechanism340further moves the whole porous preform520in the directions indicated by arrows S3a, S3b, thereby to change the relative position of the porous preform520to the heater360. Through this step, the chlorine having remained in the porous preform520after dehydrated is removed.

In the dechlorination step in the present embodiment, the temperature in the heating furnace350is maintained at 1000° C. (in-furnace temperature) which is the same as in the dehydration step, and a gas containing only He is supplied through the inlet port350ainto the heating vessel350, thereby to remove the chlorine having remained in the porous preform520after dehydrated. The preferred temperature in the dechlorination step is not more than 1300° C. The temperature range over 1300° C. is a range in which the increase in density and sintering of porous glass begins to proceed, and the progress of the density increase and sintering will impede the removal of chlorine based on the diffusion thereof out of the porous glass.

The porous preform520after dechlorinated, which was obtained through the foregoing dechlorination step, is then sintered in the heating vessel350shown inFIG. 3C(to be transparentized). Namely, as shown inFIG. 3C, the porous preform520after dechlorinated is set in the heating vessel350(traverse furnace) in a state in which it is supported by the support mechanism340. At this time, the temperature in the heating vessel350(in-furnace temperature) is maintained at 1500° C. higher than the temperature at which the dechlorination step is executed, and He gas is supplied through the inlet port350ainto the heating vessel350. The gas to be supplied into the heating vessel350is not limited to He gas. Instead of He gas, N2gas, Ar gas, a gas mixture of He and at least either one of these gasses, or the like may be used as a gas to be supplied into the heating vessel.

During this sintering step, while rotating the porous preform520after dechlorinated, in the direction indicated by arrow S4around the central axis of the porous preform520, the support mechanism340further moves the whole porous preform520in the directions indicated by arrows S3a, S3b, thereby to change the relative position of the porous preform520to the heater360. Through this step, we obtain a transparent glass body530with the diameter D1.

The foregoing dehydration step, dechlorination step, and sintering step were executed in the traverse furnace (heating vessel350), but each of these steps may be executed in a heating vessel350A (soaking furnace) shown inFIG. 5.FIG. 5is a drawing for explaining another device configuration (soaking furnace) for carrying out the dehydration step, the dechlorination step, and the sintering step in the present embodiment.

InFIG. 5, the heating vessel350A as a soaking furnace is provided with an inlet port350Aa for supply of gas and an exhaust port350Ab, as in the aforementioned traverse furnace, and the heating vessel350A is further provided with a heater360A to simultaneously heat the whole porous preform set in the heating vessel350A. In each of the steps of dehydration, dechlorination, and sintering with application of this heating vessel350A as a soaking furnace, the conditions including the supply gas, the heating temperature, and so on are the same as those with application of the heating vessel350as a traverse furnace shown inFIGS. 3A to 3C.

Next,FIG. 6shows the structure of the transparent glass body530(core preform before extension or unextended core preform) obtained through the aforementioned dehydration step, dechlorination step, and sintering step.FIG. 8shows the results of measurement of residual chlorine at respective parts of the unextended core preform530.FIG. 7is a drawing showing chlorine concentrations at respective parts of the core preform after sintered, which was obtained through the conventional preform manufacturing process, as a comparative example.

In the conventional preform manufacturing method, the dehydration step and the sintering step are successively carried out for the porous preform manufactured in the deposition step. These steps are carried out by the same devices as those shown inFIGS. 3A and 3C, respectively. Namely, in the dehydration step in the conventional technology, the in-furnace temperature is maintained at 1000° C. and the gas mixture containing chlorine gas (Cl2) at the mixture ratio of 8.5% and He gas at the mixture ratio of 91.5% is supplied through the inlet port into the heating vessel. Thereafter, the in-furnace temperature is immediately raised from 1000° C. to 1500° C. and then the sintering step is carried out in a 100% He gas atmosphere.FIG. 7is the drawing showing changes of residual chlorine concentrations along the radial direction r at the respective parts of the core preform obtained by the above-described conventional preform manufacturing method (which correspond to parts B1to B3of the unextended core preform shown inFIG. 6). Namely, inFIG. 7, graph G710represents the residual chlorine concentrations at the sintering beginning end side B3of the core preform, graph G720the residual chlorine concentrations at an intermediate part B2of the core preform, and graph G730the residual chlorine concentrations at the sintering finish end side B1of the core preform.

On the other hand, in the preform manufacturing method according to the present embodiment, the dehydration step, the dechlorination step, and the sintering step are successively carried out for the porous preform manufactured in the deposition step. Namely, in the dehydration step in the present embodiment, the in-furnace temperature is maintained at 1000° C. and the gas mixture containing chlorine gas (Cl2) at the mixture ratio of 8.5% and He gas at the mixture ratio of 91.5% is supplied through the inlet port350ainto the heating vessel350. In the dechlorination step, He gas (100% He gas atmosphere not containing chlorine) is supplied in the state in which the in-furnace temperature is maintained at 1000° C., into the heating vessel350in which the porous preform after dehydrated is set. Thereafter, the in-furnace temperature is raised from the in-furnace temperature in the dechlorination step to 1500° C. and then the sintering step is carried out in the 100% He gas atmosphere.FIG. 8is a drawing showing changes of residual chlorine concentrations along the radial direction r at the respective parts of the core preform obtained by the preform manufacturing method of the present embodiment as described above (which correspond to the parts B1to B3of the unextended core preform530shown inFIG. 6). Namely, inFIG. 8, graph G810represents the residual chlorine concentrations at the sintering beginning end side B3of the unextended core preform530, graph G820the residual chlorine concentrations at the intermediate part B2of the unextended core preform530, and graph G830represents the residual chlorine concentrations at the sintering finish end side B1of the unextended core preform530.

It is seen fromFIGS. 7 and 8that concentration differences between residual chlorine at the sintering beginning end side B3and residual chlorine at the sintering finish end side B1, i.e., concentration differences of residual chlorine due to the time difference during the sintering are smaller in the unextended core preform530(FIG. 8) obtained by the preform manufacturing method according to the present embodiment. In other words, the dispersion of the chlorine concentration distribution along the longitudinal direction (direction along the central axis AX) is more suppressed in the unextended core preform530(FIG. 8) obtained by the preform manufacturing method of the present embodiment than in the unextended core preform (FIG. 7) obtained by the conventional preform manufacturing method. Specifically, in the residual chlorine concentration distribution shown inFIG. 8, a maximum concentration of residual chlorine in the unextended core preform530is not more than 0.15 wt %. Furthermore, a variation along the longitudinal direction of the unextended core preform530, of the maximum concentration of residual chlorine in the unextended core preform530(a difference between graph G810and graph G830at the central axis AX coincident with the core center) falls within not more than +0.05 wt % with respect to the residual chlorine concentration at any part on the central axis AX. Moreover, a difference between a maximum and a minimum of residual chlorine concentrations in the unextended core preform530, along the radial direction of the unextended core preform530(or the direction perpendicular to the central axis AX) is not more than 0.12 wt % at any part on the central axis AX (in all of graphs G810to G830, the difference between the maximum and the minimum thereof is not more than 0.12 wt %). Therefore, the shape of the residual chlorine concentration distribution shown inFIG. 8is also almost maintained in an optical fiber obtained by drawing the optical fiber preform including the pertinent unextended core preform530, and it is thus contemplated that it is feasible to suppress the unintended refractive index variation (refractive index variation due to residual chlorine) along the longitudinal direction at least in the core region110.

Subsequently, in order to finally obtain the optical fiber preform600as shown inFIG. 9, a porous glass body (preform region to make the cladding region120after drawing) is further deposited on the outer peripheral surface of the core preform obtained by extending the transparent glass body530, thereby manufacturing a new starting preform (second deposition step). This second deposition step can be carried out by either of the OVD process and the VAD process as described above.FIG. 9is a drawing for explaining a device configuration for carrying out a drawing step of the optical fiber preform obtained.

The dehydration step (FIG. 3A) and the sintering step (FIG. 3C) are again carried out for the porous preform obtained through the second deposition step. Then, the optical fiber preform600obtained through the above steps has an inside region610to make the core region110after drawing and a peripheral region620to make the cladding region120, as shown inFIG. 9. In the fiber drawing step shown inFIG. 9, one end of the optical fiber preform600is drawn in the direction indicated by arrow S7, while heated by a heater630, to obtain the optical fiber100having the sectional structure shown inFIG. 1A.

Since the unintended refractive index variation along the longitudinal direction, i.e., the refractive index variation due to the chlorine having remained in the preform manufacture, is removed or reduced in the optical fiber100manufactured as described above, the optical fiber100has stable fiber characteristics and the manufacture yield thereof can also be improved.

According to the present invention, the manufacture of the optical fiber preform includes the step of heating the porous preform at the temperature lower than the sintering temperature to remove the chlorine having remained in the porous preform, prior to the step of sintering the porous preform dehydrated in the chlorine-containing atmosphere, and therefore, the unintended refractive index variation (refractive index variation due to residual chlorine) along the longitudinal direction of the optical fiber obtained finally is effectively reduced and we can expect the stabilization of fiber characteristics and the improvement in manufacture yield.

From the above description of the present invention, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all improvements as would be obvious to those skilled in the art are intended for inclusion within the scope of the following claims.