Abstract:
A preform overcladding device is provided. The preform overcladding device includes an injector for providing particles having a predetermined material, and a preheater for preheating the particles provided by the injector. The preform overcladding device also includes a heater for heating and depositing onto a preform the particles preheated by the preheater.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to plasma deposition of particles onto a preform. More specifically, the present invention relates to spheroidizing the particles online prior to depositing the particles on the preform.  
           [0003]    The present invention is useable with a process for manufacturing a preform from which optical fibers may be drawn. Such optical fibers are used in telecommunications and are generally drawn from a preform on which one or more layers of an overcladding material have been deposited. The present invention deposits such an overcladding material using, in part, a preheater for spheroidizing particles of the overcladding material, reducing a size distribution of the particles, and preventing undue contamination of the particles. The particles are preferably heated by the preheater prior to being deposited and fused onto the preform.  
           [0004]    2. Description of the Related Art  
           [0005]    The preform is often constructed by depositing particles onto a primary preform through a process known as ‘overcladding’. The primary preform may be overclad by injecting synthetic or natural quartz particles into a plasma plume using a carrier gas, then directing the particles onto the primary preform. The plasma plume further heats the particles deposited onto the primary preform and fuses the deposited particles into a generally homogenous overcladding layer around the primary preform.  
           [0006]    The quality of the overcladding layer and the rate at which the overcladding layer is deposited are functions, in part, of the particles&#39; size and shape distributions. For example, when the particles have a relatively large size distribution, relatively big particles may be incompletely molten when the particles are deposited onto the surface of the primary preform. Consequently, the particles are not fused by the plasma torch and do not form a useful overcladding layer. By contrast, relatively small particles may be evaporated by the heat of the plasma plume, thereby decreasing the energy available for heating and fusing other particles, and decreasing the flow rate of particles deposited onto the primary preform. For at least the foregoing reasons, a relatively large particle size distribution reduces the effectiveness and efficiency of the overcladding process.  
           [0007]    The particles&#39; shape distribution also has an influence on the effectiveness and efficiency of the overcladding process. For example, randomly shaped particles can not be packed into a relatively dense layer onto the primary preform. In contrast, spherical particles can be efficiently packed into a relatively dense layer on the primary preform, and may then be readily fused into a generally homogeneous overcladding layer. A layer of densely packed spherical particles can thus be fused into an overcladding layer more effectively and efficiently than a layer of randomly sized particles.  
           [0008]    The particles&#39; size and shape distributions are generally a result of how the particles were created. Mechanical processing, such as crushing and grinding, is a conventional method of creating the particles to be deposited onto the primary preform. Particles produced by mechanical processing generally do not have a spherical shape or a uniform size, and must undergo additional processing to reduce the particles&#39; size and shape distributions. Such additional processing may include mechanical sorting and chemical processing operations that may contaminate the particles, thereby reducing the optical quality and performance of the overcladding layer. Additional processing also increases the particles&#39; manufacturing costs. Thus, controlling the particles&#39; size and shape distributions is difficult and costly during the particles&#39; manufacture, and further processing for reducing the particles&#39; size and shape distributions often results in contamination of the particles.  
           [0009]    In view of the proceeding discussion, a need exists for an apparatus and method for decreasing the size and shape distributions of particles to be overclad onto a primary preform. Further, a need exists for a method and apparatus for inexpensively reducing the particles&#39; size and shape distributions without contaminating the particles.  
         OBJECTS OF THE INVENTION  
         [0010]    In a first aspect of the present invention, a preform overcladding device is provided and includes an injector for providing particles having a predetermined material. The preform overcladding device also includes a preheater for preheating the particles provided by the injector, and a heater for heating and depositing onto a preform the particles preheated by the preheater.  
           [0011]    In another aspect of the present invention, a method for overcladding a preform is provided. The method includes the steps of injecting particles having a predetermined material and preheating the particles provided during the injecting step. The method further includes the step of heating and depositing onto a preform the particles preheated during the preheating step.  
           [0012]    In yet another aspect of the present invention, a preform overcladding device is provided. The preform overcladding device includes an injecting mechanism for providing particles having a predetermined material, and a preheating mechanism for preheating the particles provided by the injecting means. The preform overcladding device also includes a heating mechanism for heating and depositing onto a preform the particles preheated by the preheating mechanism.  
           [0013]    These and other objects, aspects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a plan view of a preferred embodiment of the present invention.  
         [0015]    [0015]FIG. 2 is a sectional view of a preform with overcladding layers deposited in accordance with the preferred embodiment of the present invention.  
         [0016]    [0016]FIG. 3 is a sectional view of features of the preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    As explained below in detail, preferred embodiments of the present invention provide an apparatus and method for depositing particles onto a preform using an overcladding unit with, among other features, a preheater for on-line spheroidization of the particles. Of course, the present invention should not be limited solely to such features. These and other features of the preferred embodiments of the present invention are described below with reference to the drawings.  
         [0018]    A preform overcladding unit  10  is illustrated in FIG. 1. The preform overcladding unit  10  includes a preform support  12  having a centerpiece  14  having two opposing supports  16 . During an initial overcladding operation, a preform  18  is mounted to the supports  16  such that the supports  16  rotate the preform  18  about the preform  18 &#39;s longitudinal axis. By way of example, the supports  16  rotate the preform  18  with a rotational velocity of from approximately 1 to 50 revolutions per minute, and more preferably from 3 to 30 revolutions per minute. The preform support  12  may be, by way of example, a conventional overcladding glass working lathe.  
         [0019]    With reference to FIG. 2, the preform  18  secured by the supports  16  includes a primary preform  20 . An initial overcladding layer  22  and one or more subsequent overcladding layers  24  may be deposited onto the primary preform  20  in the manner discussed below. The primary preform  20  is preferably a solid, cylindrical rod of, for example, natural or synthetic silica glass. The primary preform  20  preferably has a length of from approximately 0.5 to 1.5 meters. The diameter of the primary preform  20  is preferably from approximately 15 to 40 millimeters. The primary preform  20  may have other dimensions while remaining within the scope of the present invention.  
         [0020]    In FIG. 1, the centerpiece  14  is carried and supported by the preform support  12  such that the centerpiece  14  moves from an initial position to a return position while the supports  16  rotate the preform  18  about the preform  18 &#39;s longitudinal axis. Movement of the centerpiece  14  is controlled by a controller  25  such as a central processing unit having a memory with executable code. The controller  25  controls the centerpiece  14  by adjusting the speed and direction of revolution of a threaded rod (not shown) along which the centerpiece  14  travels. The speed and travel direction of the centerpiece  14  may be controlled by the controller  25  in accordance with, for example, the rotation speed of the preform  18 , and the desired thickness of the preform  18 . Of course, the speed and travel direction of the centerpiece  14  may be controlled by other structures and may be controlled in accordance with other parameters.  
         [0021]    Centerpiece  14 , and the preform  18 , are moved past a depositing unit for depositing material onto the preform  18 . The depositing unit includes an injector  28  for delivering particles entrained in a gas. The particles entrained in the gas are the particles to be fused onto the outer surface of the primary preform  20  to form the initial overcladding layer  22  and the subsequent overcladding layer  24 . (See FIG. 2). The particles are supplied to the injector  28  from a particle reservoir  30  such as a conventional hopper which, for example, feeds the particles onto a moving belt. The particles are then entrained in a gas stream supplied by a gas supply  32 , such as a conventional compressor or other known gas supply, and the gas and particles mixture then exits the injector  28  toward a preheater  34  discussed below in detail. The particles may be, for example, grains of natural or synthetic silica or quartz having predetermined optical properties. Of course, the particles may contain a dopant of any other material, such as TiO 2  or Al 2 O 3 , and may have any desired physical properties. The gas is preferably air or a mixture of air, nitrogen and oxygen. The gas may also be air and a fluorinated or chlorinated gas, or even the fluorinated or chlorinated gas in a pure state, as disclosed in U.S. Pat. No. 6,269,663 to Drouart et al. incorporated herein, in its entirety, by reference. The fluorinated gas may be sulfur hexafluoride SF 6 , or a Freon generally selected from those gasses authorized under European regulations, such as C 2 F 6 . The chlorinated gas may be chlorine gas Cl 2 , for example. The gas is preferably delivered at a mass flow rate of from approximately 100 to 2000 cubic centimeters per minute (cm 3 /min.), and more preferably from to 200 to 1000 cm 3 /min. Of course, other suitable gasses and mass flow rates are considered to be within the scope of the present invention.  
         [0022]    In a preferred embodiment illustrated in FIGS. 1 and 3, the preheater  34  is located near the injector  28  and provides a plume  26  of heat. The injector  28  injects the stream of gas and particles into the plume  26  to be heated, thereby reducing the size and shape distributions of the particles. Specifically, relatively small particles are evaporated when passing through the plume  26 , and the remaining particles are heated to a spheroidization temperature. At the spheroidization temperature the surface of the particles are sufficiently melted so that surface tension of the individual particles draws the corresponding particle into a sphere. Thus, the relatively small particles are removed from the gas and particle stream, and the remaining particles undergo sphereoidization when passing through the plume  26  from the preheater  34 . The particles&#39; size and shape distributions are thereby decreased. Further, contamination of the particles and the initial overcladding layer  22  is prevented because the preheater  34  sphereoidizes the particles during the actual overcladding process, rather than using mechanical or chemical processes prior to the overcladding process.  
         [0023]    In a preferred embodiment, the preheater  34  is a plasma torch and supplies a plume  26  having a temperature of from, for example, 2000 to 5500° Celsius. By way of example, the plasma torch of the preheater  34  may be a water cooled, double enveloped plasma torch with swirled gas operation. Of course, another plasma torch or heater may also be used without deviating from the scope of the present invention. The surface of the particles is preferably heated by the preheater  34  to a spheroidization temperature of, for example 2000 to 3500° Celsius by passing through the plume  26  of the preheater  34 . The particles exit the plasma plume  26  with a diameter of from approximately 50 to 500 microns, and more preferably from about 50 to 300 microns. Of course the diameter of the particles may be larger or smaller than the stated ranges while still remaining within the scope of the invention.  
         [0024]    An alternative embodiment of the injector  28  and the preheater  34  is illustrated in FIG. 4. As shown, the injector  28  is incorporated into the preheater  34  such that the stream of gas and particles is carried along a longitudinal axis of the preheater  34 . The stream of gas and particles is then injected directly into the plume  26 .  
         [0025]    As shown in FIGS. 1 and 3, the gas and particles pass through a plasma plume  35  from a plasma torch  36  after passing through the plume  26  from the preheater  34 . The plasma torch  36  is preferably located near the preheater  34  and provides the plasma plume  35  which heats the particles exiting the preheater  34  to a temperature at which the particles may be fused. By way of example, the plasma torch  36  may be a water cooled, double enveloped plasma torch employing swirled gas operation. Of course, other plasma torches or heat supplies may also be used without deviating from the scope of the present invention. The plume  35  from the plasma torch  36  directs the particles toward, and deposits the particles onto, the primary preform  20 . Because the particles deposited onto the primary preform  20  are generally spherical in shape and have a generally uniform size, the particles are efficiently and densely packed onto the primary preform  20 . After depositing the particles onto the primary preform  20 , the plasma torch  36  further heats the particles thereby fusing the particles into the uniform initial overcladding layer  22  on the surface of the primary preform  20 . (See FIG. 2). During the overcladding operation, the primary preform  20  is rotated by the supports  16  of the centerpiece  14  while the centerpiece  14  is controlled by the controller  25  to travel adjacent to the preheater  34  and the plasma torch  36  for a distance of approximately the length of the preform  18 . By this arrangement, the entire length and circumference of the primary preform  20  is overclad with the initial overcladding layer  22 . After the centerpiece  14  has moved the length of the preform  18 , the controller  25  controls the preform support  12  to move the centerpiece  14 , and thus the preform  18 , a predetermined distance away from the plasma torch  36 . For example, the centerpiece  14 , and the preform  18 , may be moved about 3 millimeters along the longitudinal axis of the plasma torch  36  in a direction away from the plasma torch  36 . Of course, the centerpiece  14 , and preform  18 , may be moved any other predetermined distance. In this manner the outer surface of the preform  18  is maintained at the predetermined distance from the plasma torch  36 .  
         [0026]    The preceding discussion explains in detail the deposition of the initial overcladding layer  22  onto the primary preform  20 . Subsequent overcladding layers  24  may be added on top of the initial overcladding layer  22  in a similar manner. Specifically, the centerpiece  14 , carrying the preform  18 , is controlled by the controller  25  to reverse direction and move in the direction from which the centerpiece  14  previously traveled while depositing another layer of particles onto the preform  18 . The preform  18  is rotated about its longitudinal axis as the centerpiece  14 , and the preform  18 , are moved parallel to the longitudinal axis of the preform  18 . As the centerpiece  14  and the preform  18  travel in the reverse direction the particle reservoir  30  continues to provide particles to the injector  28  and the particles are entrained in the gas stream provided by the gas supply  32 . The entrained particles exit the injector  28  and enter the plume  26  from the preheater  34 . When passing through the plume  26  from the preheater  34  the relatively small particles are evaporated and the remaining particles are spheroidized, thereby reducing the particles&#39; size and shape distributions. After exiting the plume  26  from preheater  34 , the spheroidized particles enter the plasma plume  35  from plasma torch  36 . The plasma plume  35  from plasma torch  36  further heats the particles, then deposits and fuses the particles onto the initial overcladding layer  22  while the preform  18  is rotated about its longitudinal axis by supports  16 . After the centerpiece  14 , and the preform  18 , have reached the initial position, the controller  25  controls the preform support  12  to move the centerpiece  14 , and the preform  18 , the predetermined distance away from the plasma torch  36 . Such overcladding operations may be repeated any desired number of times to create a preform  18  having the desired dimensions and characteristics. For example, the perform  18  may have from 2 to 50 overcladding layers, or more, and more preferably may have 8 to 12 layers. Of course, the number of layers is not restricted to these numbers and more or fewer layers may also be deposited while still remaining within the scope of the invention.  
         [0027]    Although the depositing operation has been described above as depositing particles to form a subsequent overcladding layer  24  when moving in a reverse direction, the present invention also encompasses returning the centerpiece  14 , and the preform  18 , to the initial start position before beginning the subsequent overcladding operation. Specifically, the centerpiece  14  may reverse direction after forming the initial overcladding layer  22  and may be controlled by the controller  25  to travel parallel to the longitudinal axis of the preform  18  to the initial position without depositing the subsequent overcladding layer  24 . After the centerpiece  14  reaches the initial position the centerpiece  14  may be controlled to again reverse direction and the subsequent overcladding layer  24  may be deposited starting from the initial position in the manner previously discussed.  
         [0028]    The overcladding layers  22  and  24  may be formed into any predetermined thickness by controlling operating parameters such as the mass flow rate of the particles and the velocity of the centerpiece  14  with respect to the preheater  34  and the plasma torch  36 . In this manner, the preform  18  may be built up to any predetermined diameter. Also, the preheater  34 , the plasma torch  36  and the centerpiece  14  may be controlled by the controller  25  to deposit an overcladding layer along only a predetermined length of the preform  18 . Thus, localized fluctuations in the diameter of the preform  18  may be normalized. Further, the rotation of the preform  18  and the movement of the centerpiece  14  may be controlled to deposit particles on limited areas along the circumference of the preform  18  in order to, for example, selectively build-up portions of the preform  18 .  
         [0029]    Although specific embodiments of the present invention have been described above in detail, it will be understood that this description is merely for illustration purposes. Various modifications of and equivalent structures corresponding to the disclosed aspects of the preferred embodiments in addition to those described above may be made by those skilled in the art without departing from the spirit of the present invention which is defined in the following claims, the scope of which is to be accorded the broadest interpretation so as to encompass such modifications and equivalent structures.