PATENT ABSTRACT
An optical fiber preform fabricating device is disclosed. The device includes a particle remover for removing soot from a carrier gas, the soot being particles that are not deposited on a substrate tube. The device also includes a soot collector communicating with the particle remover for containing the soot removed by the particle remover. A control valve communicates with the particle remover, and adjusts a pressure within the substrate tube.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to fabrication of an optical fiber preform. More specifically, the present invention relates to removing soot particles and controlling the diameter of the preform during a chemical vapor deposition (CVD) process.  
           [0003]    2. Description of the Related Art  
           [0004]    The present invention is useable with a process for manufacturing a preform from which optical fibers may be drawn. Such optical fibers are used for transmitting optical signals in telecommunications applications. The preform may be manufactured by a variety of methods, including the CVD process in which glassy particles (soot) are deposited onto the inside wall of a glass substrate tube. The soot generally comprises silica that has been doped to provide a desired index of refraction. During the deposition process the soot is passed longitudinally through the glass tube by a carrier gas and a heat source is passed over the outside of the glass tube. The heat from the heat source sinters the soot to provide a homogenous glass layer. Heating the tube softens the tube and the pressure must be controlled inside the tube to achieve a desired tube diameter. Without a constant target pressure, the tube diameter may detrimentally increase, decrease, or otherwise deform, thereby affecting the quality of the preform and the resulting fibers drawn from the preform.  
           [0005]    Methods for controlling the pressure inside the tube are currently unsatisfactory. For example, known methods of controlling the pressure inside the tube include using a valve to control the flow of the soot and carrier gas, and introducing a counterflow of a gas, such as oxygen, nitrogen, or other inert gas, at a downstream position relative to the flow of soot. In either example, a back-pressure is thereby created in the tube. However, such prior art methods suffer from several drawbacks including, for example, “blowback” caused by the valve sticking in a “closed” position, or imbalances that develop between the tube inlet and exit pressures. Specifically, the valve may become clogged with soot and is prevented from opening properly, some other obstruction within the apparatus may develop, or the counterflow gas may “spike” due to an unintended control loop command. Regardless of the cause, the pressure imbalance must eventually correct itself, often to the detriment of the preform. Short-term imbalances such as those described above can result in large soot agglomerations being propelled backwards into the substrate tube. These instances of blow-back cause imperfections, such as bubbles, that reduce the quality of the preform. Long-term pressure imbalances can cause catastrophic failures if the over-pressurization persists for a sufficient amount of time to cause the preform to burst.  
           [0006]    In view of the preceding discussion, a need exists for an apparatus and method for controlling the pressure in the glass substrate tube without causing imperfections or preform bursting during a CVD process.  
         ASPECTS OF THE INVENTION  
         [0007]    In a first aspect of the present invention an optical fiber preform fabricating device is provided. The preform fabricating device includes a particle remover for removing soot from a carrier gas, the soot being particles that are not deposited on a substrate tube. A soot collector communicates with the particle remover and contains the soot removed by the particle remover. Further, a control valve communicates with the particle remover. The control valve adjusts a pressure within the substrate tube.  
           [0008]    In a second aspect of the present invention an optical fiber preform fabricating device is provided. The preform fabricating device includes a particle remover, a collector and a valve. The particle remover removes soot from a carrier gas, the soot being particles that are not deposited on a substrate tube. The collector communicates with the particle remover and contains the soot removed by the particle remover. The valve adjusts a pressure within the substrate tube.  
           [0009]    In another aspect of the present invention, a method for fabricating a preform is provided. The method includes the step of removing soot from a carrier gas before the carrier gas passes through a valve, the soot being particles that are not deposited on a substrate tube. The method also includes the step of controlling a pressure and a flow rate of the carrier gas within the substrate tube.  
           [0010]    These and other 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  
       [0011]    [0011]FIG. 1 is a plan view of a preferred embodiment of the present invention.  
         [0012]    [0012]FIG. 2 is a plan view of features of the preferred embodiment of the present invention.  
         [0013]    [0013]FIG. 3 is a cross-sectional view of a glass tube during a CVD operation. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    As explained below in detail, preferred embodiments of the present invention provide an apparatus and method for removing soot particles from a carrier gas and for controlling the diameter of a glass substrate tube during a CVD process. Of course the invention should not be limited solely to such features. These and other features of the preferred embodiments of the present invention are discussed below in detail.  
         [0015]    A deposition apparatus  10  for performing a CVD operation in accordance with the present invention is illustrated in FIG. 1. The deposition apparatus  10  includes a glass working lathe  12  having a headstock  14  and a tailstock  16 . The headstock  14  and tailstock  16  support a substrate tube  18  in such a manner that the substrate tube  18  may be rotated about its longitudinal axis. The substrate tube  18  is mounted in the headstock  14  and tailstock  16  such that a stream of reactants, collectively referred to as soot, entrained in a carrier gas passes longitudinally through the substrate tube  18 . Specifically, the reactants and the carrier gas are fed through the headstock  14 , they react and form soot particles in the substrate tube  18 , and the effluent, which includes carrier gasses and undeposited soot, flows through the tail stock  16 . The soot includes dopants, such as germanium, for affecting optical properties of the finished preform. In a preferred embodiment the soot may also include phosphorous, fluorine, or any other desired materials.  
         [0016]    The glass working lathe  12  also includes a heat source  20  such as a hydrogen and oxygen burner that directs heat against the substrate tube  18  in a defined heated area. The heated area creates a reaction zone within the substrate tube  18 . The heat source  20  is moved co-axially along the rotating substrate tube  18  during the CVD process, thereby causing the reaction zone to move along the substrate tube  18 . Soot is deposited onto the inner surface of the substrate tube  18  within and downstream of the reaction zone and is fused into a homogenous layer by the heat from the heat source  20 . Moving the heat source  20  along the substrate tube  18  is repeated one or more times to deposit additional layers of soot onto the inner wall of the substrate tube  18 . The wall of the substrate tube  18  is thereby increased to a desired thickness. See FIG. 3.  
         [0017]    Not all of the soot is deposited onto the wall of the substrate tube  18  when the soot and carrier gas mixture passes through the reaction zone. The remaining soot and carrier gas mixture exits the substrate tube  18  through the tailstock  16 . As shown in FIGS. 1 and 2, the soot and gas mixture passes through the tailstock  16  and enters a particle removing device  22  for removing the soot particles from the gas stream. Soot particles are thereby prevented from passing back into the substrate tube  18  if there is any inadvertent counterflow of the carrier gas. The particle removing device  22  may be any suitable mechanism for removing the soot particles. Examples of such a device include separators such as a cyclone, impaction box, impingement separator, filter, scrubber, thermal separator or a settling chamber. In a preferred embodiment, a soot collector  24  communicates with the particle removing device  22  and collects the soot particles for later disposal or recycling. The soot collector  24  may be configured as a removable drawer, a bag or box, or any other easily replaceable or easily cleanable structure. Of course the invention is not limited to the above-described structures, and other devices for separating and holding the soot particles are also considered to be within the scope of the present invention.  
         [0018]    In the preferred embodiment, the gas stream exits the particle removing device  22  then passes across a pressure transducer  26 . The pressure transducer  26  may be any known pressure transducer. The pressure transducer  26  detects the pressure of the carrier gas as it exits the particle removing device  22  and, from this detected pressure, the pressure inside the substrate tube  18  may be closely approximated. The position shown in FIGS. 1 and 2 for pressure transducer  26  is only one example. Other ports for pressure measurement can be placed at various points within the system. Also, for increased accuracy, multiple pressure transducers  26  may be used to detect the pressure at multiple points within the deposition apparatus  10 . For example, a pressure transducer  26  may be incorporated into one or more of the particle removing device  22 , the tailstock  16  and the headstock  14 . The pressure within the substrate tube  18  may thus be accurately approximated in accordance with measurements from one or more of the pressure transducers  26 .  
         [0019]    As shown in FIG. 2, the carrier gas passes through a control valve  28  after passing across the pressure transducer  26 . The control valve  28  controls the flow rate of the gas stream as the gas stream passes through the deposition apparatus  10  and thereby controls the gas pressure inside the substrate tube  18 . Specifically, the control valve  28  is adjusted by a controller  30  to regulate the flow rate of the gas in accordance with the pressure detected by the pressure transducer  26 . By way of example, the controller  30  may include a central processing unit and memory with executable code for manipulating data received from the pressure transducer  26  and for outputting a corresponding control signal to the control valve  28 . The control valve  28  may be any conventional pressure proportioning or flow control valve assembly, or any other variable aperture device useable for regulating the flow of gas and/or pressure in response to a control signal or other input. The controller  30  controls the control valve  28  such that the pressure of the carrier gas inside substrate tube  18  reaches and maintains a desired value. The pressure of the carrier gas in the substrate tube  18  is decreased by controlling the control valve  28  to adjust to a more open position. The flow rate of the carrier gas through the control valve  28  and out of the substrate tube  18  is thereby increased, and the pressure within the substrate tube  18  is decreased. Alternatively, the pressure of the carrier gas in the substrate tube  18  is increased by controlling the control valve  28  to adjust to a more closed position. The flow rate of the carrier gas through the control valve  28  and out of the substrate tube  18  is thereby decreased, and the pressure within the substrate tube  18  increases. In accordance with a preferred embodiment of the present invention, the control valve  28  does not suffer performance degradation caused by soot accretion. Instead, the particle removing device  22  removes the soot from the carrier gas before the carrier gas enters the control valve  28 , and the control valve  28  is thus protected from becoming clogged or fouled. Blow-back of gas into the substrate tube  18  resulting from the control valve  28  sticking in the closed position, leading to an imbalance in gas pressures, is thereby prevented. If more than one pressure transducer  26  is used to monitor pressures within the deposition apparatus  10 , the controller  30  adjusts the control valve  28  in accordance with the pressures detected by each of the pressure transducers  26 , or by pre-determined combinations of the pressure transducers  26 .  
         [0020]    The gas stream exits the control valve  28  and passes to downstream components such as a scrubber  32  for further removing components from the carrier gas. For example, the scrubber  32  removes any remaining particles, chlorine gases, germanium, silica, byproducts of reactions in the reaction zone, or any other predetermined components of the carrier gas.  
         [0021]    As described above in detail, a preferred embodiment of the present invention prevents fouling of the control valve  28 , and prevents blow-back of the carrier gas and soot into the substrate tube  18 . In addition to preventing fouling and blow-back, a preferred embodiment of the present invention also controls the diameter of the substrate tube  18 . Specifically, the wall of the substrate tube  18  in the reaction zone is softened when the wall is heated by the heat source  20 . The pressure of the carrier gas is detected by one or more of the pressure transducers  26  and the pressure within the reaction zone of the substrate tube  18  is approximated. The controller  30  then controls the control valve  28  to increase or decrease the flow rate of the carrier gas through the substrate tube  18  in accordance with the pressure detected by the pressure transducer. The difference in pressure between the carrier gas in the substrate tube  18  and ambient pressure outside the substrate tube  18  causes the softened walls of the substrate tube  18  to expand or collapse to reach a desired diameter. Further, unwanted and potentially dangerous expansion of the substrate tube  18  resulting from obstructions caused by soot build-up in the exhaust apparatus is prevented in the manner previously discussed. Specifically, the control valve  28  is prevented from becoming fouled, and an unwanted pressure differential does not occur, because the particle removing device  22  removes unwanted soot particles from the carrier gas stream before the carrier gas stream passes through the control valve  28 . In the foregoing manner the diameter of the substrate tube  18  in the reaction zone is reliably controlled. As the heat source  20  moves along the substrate tube  18  the reaction zone also moves along the substrate tube  18 . The diameter of the entire substrate tube  18  is then controlled in the foregoing manner.  
         [0022]    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.