Abstract:
The method for producing and processing a preform comprises a preliminary process phase, in which silica grain is supplied into the interior space of a silica tube having an open upper end and a closed lower end, in order to obtain an unprocessed preform, and includes a final process phase, in which the interior space of the silica tube is closed, a condition of reduced pressure is generated, the unprocessed preform is heated with a final process temperature in order to fuse the silica tube and the silica grain. According to the invention the silica grain entering the interior space is thermally treated during the preliminary process phase with an intermediate process temperature that lies under the melting point of the silica grain.

Description:
[0001]    The present invention relates to a method for producing and processing a primary, secondary or higher order preform, to such a preform and an optical fiber drawn therefrom. 
         [0002]    Fabrication of optical fibers, such as the fibers currently used in ultra high speed data communication networks, is described in [1], Mool C. Gupta, Handbook of PHOTONICS, CRC Press, 1997 Boca Raton, chapter 10.7, pages 445-449. Main process steps of optical fiber fabrication are, fabricating a preform, drawing the fiber from the preform and coating the fiber with a material that protects the fiber from handling and from environmental influences. 
         [0003]    In the drawing process, the preform is fed from above into the drawing portion of a furnace while being drawn from the bottom using tractors. The fiber is then wound onto a drum while being monitored for tensile strength. The temperature during draw is typically in the range of 2000° C. After exiting the furnace the fiber is coated with a UV-curable coating before winding on the drum. 
         [0004]    According to [1], there are basically three methods to form a preform or blank. The modified chemical vapor deposition process (MCVD), the outside vapour deposition process (OVD) and the vapour-axial deposition process (VAD). 
         [0005]    In [2], US 2007/214841 A1, and [3], WO 2005/102947 A1, a further method for producing and processing a preform is described. According to this method a primary preform is inserted into a silica tube. The free space remaining in the silica tube is then filled with silica grain. Next, a condition of reduced pressure is generated within the interior space of the silica tube that is closed, e.g. with adjoiner that holds the primary preform and the silica tube in alignment. Then the assembled unprocessed secondary preform, i.e. the silica tube with the primary preform and the silica grain, is treated with a temperature in the range of 2100° C. to 2250° C. As a result, the silica grain gets molten and fused to the primary preform, thus forming an overcladding layer on the primary preform. During this process stage, an optical fiber can simultaneously be drawn from the resulting secondary preform. Alternatively the secondary preform can completely be processed, cooled and forwarded to a further site, where the drawing process is performed. The described method advantageously allows producing preforms that are designed for drawing conventional fibers or Photonic Crystal Fibers. 
         [0006]    In [4], R. Renner-Erny, L. Di Labio et al: “A novel Technique for active fibre production” OPTICAL MATERIALS, ELSEVIER SCIENCE PUBLISHERS B.V. AMSTERDAM, NL, no. 29, pages 919-922, it is disclosed that with a modification of the methods disclosed in [2] and [3] active fiber devices can be produced. According to this method a silica glass tube forming the future core region of a fibre preform is filled with a powder mix of SiO2, Nd and Al. This tube is mounted in the centre of a larger tube forming the future cladding. The empty space between the two tubes is filled with SiO2 powder. After preheating, the evacuated preform is drawn to a fibre. According to [3], with the preparation step of evacuation and heating at a temperature of 1400° C. during one hour a drying process is performed. 
         [0007]    In [5], L. Di Labio et al: “Broadband emission from a multicore fiber fabricated with granulated oxides”, APPLIED OPTICS, OSA, OPTICAL SOCIETY OF AMERICA, WASHINGTON, DC, vol. 47, no. 10, pages 1581-1584, it is disclosed that with a further modification of the methods disclosed in [2] and [3] a fiber with seven cores can be produced, with each core being fabricated from granulated silica mixed with rare earth oxide. 
         [0008]    Also in [5] the preparation step of evacuation and heating at a temperature of 1400° C. is recommended. 
         [0009]    According to [2], the applied silica grain is a synthetic silica powder that is selected according to the desired properties of the fabricated fiber. It is desired for example, that higher drawing forces can be applied, while the risk of breaking the fiber during the drawing process is reduced. 
         [0010]    With the method disclosed in [2], high-quality optical preforms can be fabricated at significantly reduced costs compared to conventional methods. However, it has been found that this method is not uncritical. Since thin-walled silica tubes are applied, there is always a risk of breakage particularly during the process step of melting the silica grain. 
         [0011]    However a breakage of a glass tube that has been filled with silica grain typically occurs before the melting point is reached, in which the processed material is softened. A breakage will typically occur during the preparation step described in [4] and [5], in which a high temperature of approx. 1400° C. is applied for drying purposes. 
         [0012]    Hence, it has been taken care that silica grain with selected properties is applied in order avoid the described problems, which however again leads to a cost increase. E.g., specific amorphous silica, but not quartz, has been used for this purpose. 
         [0013]    It would therefore be desirable to provide an improved method for fabricating preforms that involves the step of filling a silica tube with grain that is molten to become part of the fabricated preform. 
         [0014]    It would be desirable in particular to provide a method that allows fabricating primary and secondary preforms at significantly reduced cost. 
         [0015]    Still further it would be desirable to provide a method that allows the use of silica tubes with thinner walls, while avoiding the risk of breakage or rupture particularly during the step of melting the grain. 
         [0016]    Further, it would be desirable to use less costly silica grain that can be selected with reduced limitations except for the required purity grades. 
       SUMMARY OF THE INVENTION 
       [0017]    The above and other objects of the present invention are achieved by a method according to claim  1 , a preform according to claim  12  and an optical fiber according to claim  13 . 
         [0018]    The inventive method, which relates to producing and processing a preform, comprises two major process phases. In a preliminary process phase, silica grain is supplied into the interior space of a silica tube having an open upper end and a closed lower end, in order to obtain an unprocessed preform. In a final process phase the interior space of the silica tube is closed and evacuated. Then the unprocessed preform is heated with a final process temperature in order to fuse the silica tube and the silica grain. 
         [0019]    According to the invention the silica grain entering the interior space is thermally treated during the preliminary process phase with an intermediate process temperature that lies under the melting point of the silica grain. 
         [0020]    Preferably, a furnace is provided that his following the fill level of the silica grain during the filling process and that is heating the silica tube and the silica grain in the region of the fill level. 
         [0021]    With these measures at least one of the following effects is achieved. The silica grain is evenly accommodated within the interior space of the silica tube. Punctual tensions that could cause a rupture of the silica tube during labour process stages are thus avoided. 
         [0022]    Hence, the user may select silica tubes with thinner walls, thus achieving a higher average quality of the preform. The material with lower quality, resulting from the silica tube can be removed from the preform, if desired, with reduced effort. 
         [0023]    Preferably the intermediate temperature is selected in such a way that the thermal treatment causes the silica grain to change from a first state to a second state, in which the silica grain takes on a lower material density, i.e. a larger volume. For this purpose the intermediate process temperature is preferably selected in the range between approximately 576° C. and 1470° C. 
         [0024]    In the event that the silica grain consists of trigonal α-Quartz having a material density of approx. 2.65 g/cm3, then preferably the intermediate process temperature is selected between 576° C. and 870° C. so that α-Quartz is transformed into hexagonal β-Quartz having a material density of approx. 2.53 g/cm3. 
         [0025]    In the event that the silica grain consists of α-Quartz or β-Quartz, then preferably the intermediate process temperature is selected between 870° C. and 1470° C. so that α-Quartz or β-Quartz is transformed into hexagonal β-Tridymite having a material density of approx. 2.25 g/cm3. 
         [0026]    Still further, an intermediate process temperature above 1470° C. can be applied to transform silica grain with an initial configuration of α-Quartz, β-Quartz or β-Tridymite into β-Cristobalite having a material density of approx. 2.20 g/cm3. 
         [0027]    Due to the thermal treatment the silica grain is evenly accommodated within the interior space of the silica tube and assumes a lower material density that is maintained sufficiently long, even if the temperature is lowered again. 
         [0028]    Hence, during the final process phase, in which the final process temperature is applied and the silica grain is molten, an expansion of the mass of the silica grain that could break the silica tube is avoided. 
         [0029]    The inventive method therefore yields several advantages and options. First of all, the process reliability is improved, avoiding process failures caused by the breakage of silica tubes. Further, since the forces occurring during the heating and melting process are strongly reduced, the applicant may select silica tubes with thinner walls. 
         [0030]    Still further, the user may select the silica grain from a larger variety of products offered by the industry. Considerations concerning the dynamic property of the material can be neglected. Hence, the user may select material such as α-Quartz at lower cost. 
         [0031]    The results can further be improved by rotating the silica tube during the filling process with a speed between approximately 50 and 120 turns per minute. Optimal results are achieved in the range of 80-100 turns per minute. With the rotation of the silica tube in a defined range quick and uniform distribution of the silica grain is achieved while avoiding a radial segregation of particles with different sizes, which could occur with higher turning speeds. 
         [0032]    The result of the first process phase is an unprocessed preform that consists of the silica tube, which has been filled with thermally processed and evenly distributed silica grain. 
         [0033]    The unprocessed preform can further be processed immediately without applying a cooling phase. Hence, after completion of the preliminary process phase, the final process phase can immediately be started by evacuating the silica tube and by fusing the silica tube and the silica grain. 
         [0034]    Alternatively the unprocessed preform can be cooled, removed and reinstalled later at the same or another site in order to perform the final process phase. 
         [0035]    The inventive method can be used to produce primary, secondary or higher order preforms. Further, preforms can be produced, from which photonic fibers can be drawn. 
         [0036]    In the event that a secondary preform shall be produced, then a primary preform or silica blank is inserted into the silica tube and aligned along its longitudinal axis. Then, in the preliminary process phase, silica grain is supplied into the interior space of the silica tube that has been reduced by the volume of the primary preform. 
         [0037]    In the event that a preform for photonic fibers shall be produced, then auxiliary silica tubes and/or auxiliary removable rods are inserted into the silica tube and aligned in parallel to its longitudinal axis. Then, in the preliminary process phase, silica grain is supplied into the interior space of the silica tube that has been reduced by the volume of the auxiliary silica tubes and/or auxiliary removable rods, preferably carbon rods. The auxiliary silica tubes and/or auxiliary removable rods are arranged in an at least substantially two-dimensionally periodic structure as required for the photonic fibers. After the final process phase has been completed, the carbon rods are removed leaving longitudinal cylindrical openings in the preform. 
         [0038]    In the event that auxiliary silica tubes had been entered into the silica tube to define cylindrical openings in the preform, then it must be taken care that no deformations occur, which would alter the properties of the photonic fiber. Also in this application the use of silica tubes with thinner walls is desirable and can be achieved by applying the inventive method. Using the inventive filling procedure prevents the mass of grain from deforming the outer silica tube and the inner auxiliary silica tubes. Hence, the inventive method is particularly advantages in processes that serve for the production of photonic fibers. 
         [0039]    Secondary preforms and preforms designed for photonic fibers can be further processed in the different ways. 
         [0040]    The final process phase can be executed and the processed preform can be removed for later handling. 
         [0041]    However, the drawing phase can also be applied immediately after termination of the final process phase. In the final process phase, the furnace can be moved along the preform, e.g. from the lower to the upper end of the preform in order to fuse the silica tube and the silica grain. Subsequently the furnace is moved again to the lower end of the preform, which then is heated to a softened state, in which the optical fiber can be drawn from the preform. 
         [0042]    As a further alternative, the fiber can be drawn from the preform simultaneously during execution of the final process phase. In this application, the fiber is drawn from the preform, while the silica tube and the silica grain are molten. 
         [0043]    In all described variations the inventive method facilitates the handling of the process and provides better process reliability at reduced costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    Some of the objects and advantages of the present invention have been stated, others will appear when the following description is considered together with the accompanying drawings, in which: 
           [0045]      FIG. 1   a  shows a thin-walled silica tube  11 , having a primary axis x, an interior space  110  and a conical closure  111  at its lower end; 
           [0046]      FIG. 1   b  shows the silica tube  11  of  FIG. 1   a  with the interior space  110  being completely filled in a conventional way with silica grain  5  in order to obtain an unprocessed primary preform  1   p′;    
           [0047]      FIG. 1   c  shows the unprocessed primary preform  1   p ′ of  FIG. 1   b  being closed by means of an adjoiner  3 , through which the interior space  110  of the silica tube  11  has been evacuated, and a furnace  23  that is guided along the primary preform  1   p ′ in order to fuse the silica tube  11  and the silica grain  5  at a temperature between 2100° C. and 2350° C.; 
           [0048]      FIG. 2   a  shows the silica tube  11  of  FIG. 1   a  with the interior space  110  being filled with grain  5   a  that is exposed to a temperature below the melting point during the process of filling in order to obtain an unprocessed primary preform  1   p;    
           [0049]      FIG. 2   b  shows the primary preform  1   p  of  FIG. 2   a  being closed by means of an adjoiner  3 , through which the interior space  110  of the silica tube  11  has been evacuated, and a furnace  23  that is guided along the primary preform  1   p  in order to fuse the silica tube  11  and the thermally pre-treated grain  5   b  at a temperature between 2100° C. and 2350° C.; 
           [0050]      FIG. 3   a  shows the silica tube  11  of  FIG. 1   a  with a primary preform  1   p ,  1   p ′ in the interior space  110  that is filled with grain  5   a  that is exposed to a temperature below the melting point during the process of filling in order to obtain an unprocessed secondary preform  1   s;    
           [0051]      FIG. 3   b  shows the unprocessed secondary preform is of  FIG. 3   a  after completion of the filling and heating procedures; 
           [0052]      FIG. 3   c  shows the unprocessed secondary preform is of  FIG. 3   b  being closed by means of an adjoiner  3 , through which the interior space  110  of the silica tube  11  has been evacuated, and a furnace  23  that is guided along the secondary preform is in order to fuse the silica tube  11  and the thermally pre-treated grain  5   b  at a temperature between 2100° C. and 2350° C.; 
           [0053]      FIG. 4   a - 4   c  show the treatment of the processed primary or secondary preform  1   p ,  1   s , during which a peripheral layer of the preform  1   p ,  1   s , is removed, which consists of material originating from the silica tube  11 ; 
           [0054]      FIG. 5  shows an apparatus  2  used for drawing an optical fiber  8  from an inventive secondary preform is as shown in  FIG. 3   b  or  FIG. 4   c ; and 
           [0055]      FIG. 6  shows the apparatus  2  of  FIG. 5  with an inventive secondary preform  1   s , from which a photonic fiber  8  is drawn. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0056]      FIG. 1   a  shows a thin-walled silica tube  11  made of SiO2 and having a primary axis x, an interior space  110  and a conical closure  111  at its lower end. The diameter d 10  of the walls of the silica tube  11  is very small compared to the diameter of the silica tube  11 , sold that a relatively large part of the preform will consist of high-quality silica grain. 
         [0057]      FIG. 1   b  shows the silica tube  11  of  FIG. 1   a  with the interior space  110  being completely filled with silica grain  5  in order to obtain an unprocessed primary preform  1   p ′. As shown in  FIG. 1   b  the filling process is not accompanied by a heating process. 
         [0058]      FIG. 1   c  shows the unprocessed primary preform  1   p ′ of  FIG. 1   b  being closed by means of an adjoiner  3 . The adjoiner  3  comprises a first and the second channel  31 ;  32 . The first channel  31 , which is designed to optionally receive a primary preform  1   p ′,  1   p  or a glass blank is closed by a cap  4 . The second channel  32  is connected to a vacuum pump  22  that evacuates the silica tube  11  before the final process phase is performed. In the final process phase a furnace  23  is guided along the unprocessed primary preform  1   p ′ in order to fuse the silica tube  11  and the silica grain  5  at a temperature between 2100° C. and 2350° C. 
         [0059]    With the method illustrated in  FIGS. 1   b  and  1   c , which has previously been applied, the problem occurs that a breakage of the thin-walled silica tube  11  can occur due to the expansion of the silica grain  5 . With the use of amorphous silica this problem can be reduced. However, in the event that α-Quartz would be used, then the filled mass of silica grain  5  in the silica tube  11  would heavily expand under the impact of heat possibly causing a rupture of the silica tube  11 . 
         [0060]      FIG. 2   a  shows the silica tube  11  of  FIG. 1   a  with the interior space  110  being filled with silica grain  5   a , that for example is α-Quartz, which can be purchased at a relatively low price, but with high purity. Simultaneously with the filling process a heating process is performed by means of a furnace  23 , which along the silica tube  11  is preferably following the fill level of the silica grain  5   a . As illustrated in  FIG. 2   a , the silica grain  5   a  that has entered the silica tube  11  changes its structure under the impact of the heat applied by the furnace  23 . E.g., an intermediate process temperature of approximately 600° C. is applied, under which the α-Quartz  5   a  is transformed to β-Quartz  5   b . Higher temperatures may be applied, which transform the silica grain  5   a  into β-Tridymite or β-Cristobalite. The intermediate process temperature is selected according to the process parameters, particularly depending on the diameter of the walls of the silica tube  11 , the placement of auxiliary silica tubes and the silica grain  5  applied. In the event that thin silica tubes, particularly auxiliary silica tubes, are applied it is recommended that transform the α-Quartz or β-Quartz into β-Tridymite or β-Cristobalite. 
         [0061]    The material density of the silica grain  5   b  is therefore reduced and changed to a lower level. The resulting unprocessed primary preform  1   p  can therefore be processed in the final process phase shown in  FIG. 2   b  with a significantly reduced risk of process failure. 
         [0062]      FIG. 3   a  shows the silica tube  11  of  FIG. 1   a  with a primary preform  1   p ,  1   p ′ in the interior space  110  of the silica tube  11  that is being filled with grain  5   a , e.g. α-Quartz. Preferably, the processed primary preform  1   p  resulting from the final process phase shown in  FIG. 2   b  is entered into the silica tube  11 . However any other primary preform  1   p  such as a high-quality glass blank, produced e.g. with the modified chemical vapor deposition process (MCVD), the outside vapour deposition process (OVD) or the vapour-axial deposition process (VAD), can be used. 
         [0063]    As described in conjunction with  FIG. 2   a , simultaneously with the filling process a heating process is performed by means of a furnace  23 , which along the silica tube  11  is following the fill level  50  of the silica grain  5   a  in order to achieve the desired change of the structure of the silica grain  5   a.    
         [0064]      FIG. 3   b  shows the unprocessed secondary preform is of  FIG. 3   a  after completion of the preliminary process phase that has been performed according to the inventive method. In this state the preform can be cooled down and delivered to another site, there the final process phase and the drawing processes are performed. Alternatively, the unprocessed secondary preform is can immediately be further processed, e.g. before it is cooled down. 
         [0065]    As shown in  FIG. 3   b , the unprocessed secondary preform is may optionally comprise auxiliary silica tubes  10  or removable rods preferably made of carbon that define longitudinal cylindrical spaces or voids within the secondary preform is. From secondary preforms  1   s  of this kind, photonic fibers  8  can be drawn as shown in  FIG. 6 . 
         [0066]      FIG. 3   c  shows the unprocessed secondary preform is of  FIG. 3   b  with the silica tube  11  being closed and evacuated as described in conjunction with  FIG. 2   a . A furnace  23  is guided along the secondary preform is in order to fuse the silica tube and the thermally pre-treated grain  5   b  at a temperature between 2100° C. and 2350° C. subsequently obtaining the processed secondary preform is. 
         [0067]      FIGS. 4   a - 4   c  show the mechanical treatment of the heat processed primary preform  1   p  of  FIG. 2   b  or the secondary preform is of  FIG. 3   c . During this mechanical treatment a peripheral layer is removed, which consists of material originating from the silica tube  11  that may not have the desired quality.  FIG. 4   a  shows the processed primary or secondary preform  1   p  or is before the treatment.  FIG. 4   b  shows the processed primary or secondary preform  1   p ; is during the grinding process, preferably executed by an automated grinding tool.  FIG. 4   c  shows the processed primary preform  1  after the completion of the grinding process, which is recommended to be performed in the event, that the material of the primary silica tube  11  does not favourably contribute to the properties of primary preform  1  or the optical fibers derived therefrom. 
         [0068]      FIG. 5  shows an apparatus  2  used for drawing an optical fiber  8  from an inventive secondary preform is as shown in  FIG. 3   b  or  FIG. 4   c . As stated above, the drawing process can be performed simultaneously with or after the final process phase as shown in  FIG. 3   c.    
         [0069]    Once the lower end of the secondary preform is has been heated to its melting point and a fiber  8  has been pulled, an angular area called “neck-down” is formed. A single optical fiber  8  emerges from the secondary preform is in a semi-molten state and passes through a diameter monitor  24 . The optical fiber  8  continues to be pulled downward and passes through a coating applicator  25  that applies a coating to protect the optical fiber  8 . The optical fiber  8  also passes through other units  26 ,  27  that cure the optical coating and monitor the overall diameter after the coating has been applied. The optical fiber  8  then encounters a spinning apparatus  28  which may comprise a roller that imparts a spin into the optical fiber  8 . The optical fiber  8  then eventually encounters a series of rollers (not shown) pulling the optical fiber  8  before it is then wrapped around a drum or spool  29 . The secondary preform is mounted in a holding device  21 , which allows controlled vertical movement along and preferably rotation around its axis. 
         [0070]    Furthermore the holding device  21  of the apparatus  2 , which can be used in the preliminary process phase and in the final process phase, may be designed to apply a vibration onto the installed preform  1   p , is in order to condense the silica grain  5   a ,  5   b.    
         [0071]      FIG. 6  shows the apparatus  2  used for drawing an inventive optical fiber  8 , such as a photonic crystal fiber from a secondary preform is that comprises longitudinal cylindrical voids  500  that originate from auxiliary silica tubes or rods, e.g. carbon rods that have been removed after the preliminary or final process phase. 
       REFERENCES 
       [0000]    
       
         [1] Mool C. Gupta, Handbook of PHOTONICS, CRC Press, 1997 Boca Raton, chapter 10.7, pages 445-449 
         [2] US 2007/214841 A1 
         [3] WO 2005/102947 A 
         [4] R. Renner-Erny, L. Di Labio et al: “A novel Technique for active fibre production” OPTICAL MATERIALS, ELSEVIER SCIENCE PUBLISHERS B.V. AMSTERDAM, NL, no. 29, pages 919-922 
         [5] L. Di Labio et al: “Broadband emission from a multicore fiber fabricated with granulated oxides”, APPLIED OPTICS, OSA, OPTICAL SOCIETY OF AMERICA, WASHINGTON, DC, vol. 47, no. 10, pages 1581-1584