Abstract:
The specification describes a process and apparatus for collapsing preform tubes in Modified Chemical Vapor Deposition processes. The problem of bubble formation during tube collapse was found to be attributable to excess dopant vapor pressure emitted from the hot zone during the final stage of tube collapse. This excess pressure is controlled by cooling the tube in advance of the torch, thereby decreasing the viscosity of the dopant vapor and increasing its transport rate. This is found to reduce internal tube pressure and eliminate bubble formation in the collapsed preform.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical fiber manufacture and more specifically to improved preform fabrication techniques. 
     BACKGROUND OF THE INVENTION 
     The manufacture of optical fiber typically uses one of two fundamental approaches. Both use rotating lathes, and accumulate pure glass material on a rotating preform by chemical vapor deposition or a modification thereof. The earliest technique deposited material on the outside of a rotating preform, and the preform usually started as a hollow tube with a slowly increasing diameter as the vapor deposited glass material accumulated on the outside of the solid tube. A significant advance in this technology occurred with the introduction of the so-called Modified Chemical Vapor Deposition (MCVD) process of MacChesney et al. in which the glass forming precursers are introduced into a rotating hollow tube and the glass material is deposited on the inside wall of the hollow tube. In this way exceptionally pure material can be produced at the critical core region. It also allows better control over the reaction environment. 
     The MCVD process has evolved to a highly sophisticated manufacturing technique and is widely used in commercial practice today. However, problems in the tube collapse phase of the MCVD process persist. One of those is the spontaneous formation of a bubble in the core of the preform near the end of the collapse cycle. The bubble essentially destroys the preform. From 5-10% of preforms typically suffer this fate. 
     The dopant used to increase the core index in most commercial MCVD preforms is germanium in the form of GeO 2 . Studies of the bubble formation phenomenon have established that the source of the bubbles is GeO vapor from the GeO 2  in the doped core. GeO vapor is emitted during the entire collapse cycle but as collapse proceeds the internal tube pressure due to accumulated GeO vapor increases, sometimes to the point where it exceeds the surface tension of the softened silica tube. When that occurs the tube wall distorts in a bubble, and there is no satisfactory way of reversing the distortion. Such tubes are scrapped, resulting in a significant reduction in process yield and substantially increased cost. Collapse techniques which avoid bubble formation would represent an important advance in the commercial practice of MCVD. 
     PRIOR ART 
     Many techniques for controlling the thermodynamics of the collapse process have been proposed in the prior art. For example, U.S. Pat. No. 5,868,815, issued Feb. 9, 1999, describes a preform collapse process that employs a gas jet of air impinging on the hot zone, where the torch softens the glass, to control ovality as the preform collapses. U.S. Pat. No. 5,160,520, issued Nov. 3, 1992, describes using jets of air in the hot zone to control the temperature profile in the hot zone. In each case the gas jets contact the preform where the glass has been softened and is susceptible to deformation. 
     SUMMARY OF THE INVENTION 
     We have developed a technique for eliminating bubble formation due to excessive GeO vapor pressure in the MCVD preform tube during the final stages of collapse. Cooling means are provided just ahead of the torch used for collapse. Resulting cooling of the preform tube increases the rate of transport of the GeO vapor thereby eliminating the source of the bubble. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a diagram of an MCVD apparatus useful in practicing the invention; 
     FIGS. 2-4 are schematic representations demonstrating the development of a bubble during the late stages of preform collapse; 
     FIG. 5 is a plot showing the internal preform tube pressure for preforms with different tube diameters; and 
     FIG. 6 is a schematic diagram showing the addition of a cooling means in association with the heating torch to increase the GeO transport rate from the hot zone during preform tube collapse. 
    
    
     DETAILED DESCRIPTION 
     The basic MCVD process is well known, as is the equipment used in the process. See for example, J. B. MacChesney et al., “Preparation of Low Loss Optical Fibers Using Simultaneous Vapor Phase Deposition and Fusion”, Xth Int. Congress on Glass, Kyoto, Japan (1973) 6-40. As seen in FIG. 1, the silica tube  11  is mounted for rotation in an MCVD glass lathe (not shown). Glass precursor gases, e.g. SiCl 4 , GeCl 4 , O 2 , are passed down the rotating tube while the tube is heated with an oxy-hydrogen torch  12 . When deposition and consolidation are complete the tube is collapsed by known techniques, i.e. heating the tube to well above the glass softening temperature, i.e. &gt;2000-2400° C. to allow the surface tension of the glass tube to slowly shrink the tube diameter, finally resulting, after multiple passes of the torch, in the desired solid preform. The temperature of the torch is controlled by the ratio of hydrogen to oxygen, and their absolute flow rates in the fuel mixture supplied to the torch. The gas flow control, shown at  13  in FIG. 1, controls the flow rate of H 2  and 02 independently, and thus the ratio of hydrogen to oxygen, and the resulting metered gas streams are supplied to the torch  12 . The gases are mixed at the flame according to well known techniques. 
     In the MCVD process, the last deposited soot layer is typically silica doped with GeO 2 , the latter for increasing the refractive index of the silica in the core of the preform. We have observed experimentally that at the last torch pass during the collapse, a section of the substrate tube evolves into a large glass bubble. We have demonstrated experimentally that the bubble formation is due to the vaporization of GeO 2  dopant in the core region. The silica tube collapse is conducted at very high temperatures, sufficient to soften the silica glass and allow the tube to collapse in a controlled fashion under the influence of surface tension on the glass surface. However, these temperatures are also high enough to vaporize significant amounts of GeO 2  and form GeO vapor. When the transport rate of the GeO vapor from the hot zone of the collapsing tube is less than the vaporization rate, an internal pressure develops in the tube. Bubbles form when this internal pressure exceeds the capillary force due to the silica surface tension. 
     These effects are illustrated by FIGS. 2-5. FIG. 2 shows the silica glass preform  21  and the traversing torch  22 , wherein the preform is in the final stage of collapse, i.e. during the last torch pass. These figures are schematic diagrams and the torch is a schematic representation of an apparatus more fully described by FIG.  1 . In FIG. 2, part of the preform is shown properly collapsed. As the final pass proceeds, as shown in FIG. 3, vaporization of GeO begins to exceed the transport rate of GeO vapor in region  23  of the tube and a bulge develops. FIG. 4 shows the tube after completion of the final pass, wherein the GeO vapor is entrapped in the tube and a large bubble  15  has formed. 
     It is known from Chemical Engineering textbooks, e.g. page 46 in “Transport Phenomena” by R. B. Bird, W. E. Stewart and E. N. Lightfoot, John Wiley and Sons, Inc. NY 1966, that at a given vaporization rate, Q, of GeO from GeO 2 , the internal pressure or the pressure difference P L −P 0 , across a transport distance, L, is given as 
     Equation (1):            P   L     -     P   0       =       8   π          L     R   i   4          Q                 μ                            
     where R i  is the radius of the substrate opening and μ is the viscosity of the GeO vapor phase inside the substrate tube. 
     The above internal pressure is constrained by the capillary force, F CAP , due to the silica surface tension, and the capillary force is given as 
     Equation (2):          F   cap     =     γ        [       1     R   i       +     1     R   o         ]                              
     A bubble forms when the internal pressure exceeds the capillary force, i.e., 
     Equation (3):            8   π          L     R   i   4          Q                 μ     &gt;     γ        [       1     R   i       +     1     R   o         ]                              
     It is further known that the vapor phase viscosity has a temperature dependence given as 
     Equation (4): 
     
       
         μ=μ 0 T 0.64874    
       
     
     where μ 0  is the material-dependent constant. 
     The rate of GeO 2  evaporation, Q, is related to the core burn-off radius, r, as well as the GeO 2  dopant concentration, the core delta, Δ, and the torch traverse velocity V. The evaporation rate is given as 
     Equation (5): 
     
       
         Q=Q 0 r 2 VTΔ 
       
     
     Based on the above equations, the internal pressure at three different core burn-off radii, as well as the capillary force, versus the diameter of the preform tube opening is related as shown in FIG.  5 . FIG. 5 shows the conditions where bubble formation becomes most likely, i.e. when the internal tube pressure becomes excessive. The data is for a preform tube having a core delta of 0.3%. The torch traverse speed was 10 mm/min. Curve 51 represents the silica tube surface tension. Curve  52  gives data for a 2 mm burn-off radius. Curve  53  gives data for a 4 mm burn-off radius, and curve  54  gives data for a 6 mm burn-off radius. It is shown that that bubbles are likely to occur when the substrate tube opening is less than 1.5-2.5 mm diameter. 
     Given this understanding of the bubble forming mechanism, several options for eliminating or reducing bubble formation can be considered: (i) the MCVD preform tube opening can be kept large; (ii) the torch velocity can be reduced; (iii) the burn-off radius can be kept small; and (iv) the collapse temperature can be reduced. These expedients reduce the internal pressure in the preform tube and reduce or prevent bubble formation. 
     However, each of the expedients enumerated above carry significant disadvantages. For example, the risk of core ovality increases when the tube is collapsed with a large opening. Core ovality can produce unwanted polarization-mode dispersion (PMD) and excessive splice loss. A slower torch velocity increases process time and decreases productivity. Limiting the burn-off radius also increases process time and reduces throughput, as does a lower collapse temperature. 
     The preferred solution, according to the invention, is to increase the rate of GeO vapor transport from the hot zone in the preform tube. This can be achieved by reducing the temperature of the preform tube in advance of the heating torch, which reduces the viscosity μ of the GeO vapor and, as evident from Equation (1), proportionally reduces the internal tube pressure. The temperature of the preform ahead of the torch can be reduced by positioning a cooling jet in advance of the heating torch as shown schematically in FIG.  6 . In FIG. 6, the preform tube is indicated at  61 , the heating torch at  62 , and the cooling jet at  63 . The cooling jet can be an air jet, or a jet of an inert gas, e.g. N 2 . The flow rate of gas from the cooling jet will depend on the temperature of the gas and the apparatus. These can easily be determined by those skilled in the art, and will typically be in the range 20 to 50 liters/min. 
     Studies have shown that when the tube temperature is reduced from 1600° C., to 1400° C., the internal pressure in the preform tube is reduced by 17%. More importantly, the GeO vapor condenses to a solid state at a lower temperature. The effect of the cooling jet also reduces loss of germania from the preform core and helps control the well known index depression effect due to excessive germania loss. 
     While FIG. 6 schematically shows a single gas nozzle or jet  63 , multiple nozzles can be used and placed at other locations around the tube diameter. One or more gas jets may also be incorporated into the torch assembly so that the movement of the cooling jet ahead of the heating torch is easily coordinated with the movement of the torch. The distance between the gas jets for the traversing heat source and the gas jets for the cooling means will typically be in the range 10 to 15 cm. 
     Although the process as described above uses a flame torch and a fuel of mixed oxygen and hydrogen, plasma torches using, for example, a microwave plasma ring, can also be used in MCVD processes. Also gas torches other than oxy-hydrogen torches can be used. The process of the invention involves providing a cooling means ahead of the heat source regardless of the heat source used. 
     In the foregoing description, the dopant used in the core was GeO 2 . Other dopants, e.g. P 2 O 5 , B 2 O 3 , and Al 2 O 3 , cause similar bubble forming problems. Accordingly, the invention is not to be construed as limited to any species of dopant. 
     The heat source and the cooling means of the invention are described herein as traversing the tube. As recognized in the art, this relative motion can be achieved by moving either the heat source/cooling means or the tube. 
     The temperature of the air or other gas from source  63  is at room temperature, or at most is substantially less than the softening temperature of the glass tube. A cooled gas supply can also be used, e.g. nitrogen from a liquid nitrogen supply. The application of cooled gas to the tube ahead of the hot zone is found not to interfere with the collapse rate of the preform tube. 
     While the use of externally applied cooling gas applied to the exterior of the tube ahead of the torch is effective, and is a preferred embodiment of the invention, the designer may choose to use internally applied cooling gas to achieve the objectives of the invention. To implement this embodiment the nozzle  63  is simply placed inside the rotating tube. This alternative can also be implemented by cooling the oxygen and chlorine gases introduced into the interior of the tube to control water contamination of the preform tube during collapse. The flow rate of these gases may also be increased for more effective cooling. 
     Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.