Abstract:
A coating die that includes an insert having a high thermal conductivity, which is greater than 1 W/cm*K. Preferably, the insert is made of diamond having a thermal conductivity in the range of 5 to 20 W/cm*K. Use of this highly conductive insert helps to efficiently dissipate the heat produced by viscous losses in the coating as most of these losses occur very close to the inner wall of the die in the land region. It also reduces to negligible level the wall temperature unbalance between opposite sides of the fiber when this fiber is off-centered thus restoring conditions prevailing at low draw rates and subsequent satisfactory centering force.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for coating optical fibers such that the coating is concentrically applied and the thickness of the coating is uniform. 
     2. Discussion of Related Art 
     After an optical fiber has been drawn from a preform, it is conventional to cover the optical fiber with a protective coating, such as an acrylate-based composition which is curable by exposure to ultraviolet (UV) light, to prevent the surface of the fiber from being damaged either during the subsequent manufacturing steps or subsequent use. This coating step is generally performed as an integral part of the drawing process using coating dies. The coating material may be applied in one or more layers. 
     In the process of applying the coating layers, it is important that the coating layers be applied concentrically to the fiber and that the diameter of the coating or coatings be consistent as well. These features are important in contributing to the ease with which the optical fibers can be spliced and connected. 
     As discussed in U.S. Pat. No. 5,366,527 to Amos et al., which is incorporated herein by reference, significant efforts have been made to ensure that the coating is applied concentrically and ever more consistently at increasing draw rates. Higher draw rates are needed to reduce the cost of manufacturing and to increase the fiber output but they may affect adversely the consistency of fiber coating if draw techniques fail to be adequately adapted. 
     A direct consequence of draw rate increase is to reduce the delay between fiber forming at temperatures close to 2000° C. and fiber coating. As a consequence, the temperature of the fiber entering the coating device may still be too high to allow good coating application unless forced cooling is applied. Various systems have been disclosed in U.S. Pat. No. 4,594,088 to Paek et al., U.S. Pat. No. 4,514,205 to Darcangelo et al. and U.S. Pat. No. 5,043,001 to Cain et al. for cooling the drawn optical fiber prior to receiving the first layer of coating. 
     High temperature of the entering fiber is not the only source of heat likely to disturb coating application: draw rate increase also results in an important increase in the heat produced by viscous losses in the coating flow. Thermal power associated with viscous losses may be computed easily knowing the draw speed and the viscous drag affecting the fiber in the coating applicator. Thermal power produced at current draw speeds of 900 to 1000 m/min for an observed drag force per applicator of 1 N is around 15 W. None of the above systems was designed to cope with or to get rid of this overly different source of heat. 
     Viscous losses occur most intensively where shear stress is highest in the coating flow, i.e. around the fiber in the applicator chamber and, especially, in the narrowest part of the sizing die, the cylindrical land region. In the latter land region, high shear rate and viscous losses are shown to be concentrated in a narrow radial range limited by the inner wall of the die. This is a consequence of optimal coating conditions in which shear rate is minimized around the fiber and is highest on the inner wall of the die as a consequence. Heat production is highest over this peripheral region. 
     This heat source may be especially detrimental to coating consistency as coating diameter and coating concentricity are mostly governed by temperature and pressure profiles in the land region as shown by numerical simulations. This fact is indirectly confirmed by U.S. Pat. No. 5,366,527 which discloses a technique by which the coating diameter is controlled by adjusting the die temperature in the land region and by PCT Publication No. WO 97/20237-A2 which discloses a fiber coating system in which concentricity of the coating is controlled by non-axisymmetrically heating the land region in the sizing die. The latter system may be capable of compensating for spurious non-axisymmetrical heating or temperature profiles occurring in the coating flow. However, these systems are very complex requiring the ability to monitor the diameter and/or the concentricity of the coating as well as the ability to control the temperature of the coating die in a localized manner (e.g., portions of the bottom surface of the die). 
     An object of the present invention is to provide a relatively simple apparatus for coating an optical fiber with coatings that are applied concentrically to the fiber to provide a consistent coated diameter. 
     An other object of the invention is to restore as much as possible coating conditions prevailing at low draw rates and to suppress most of the effect of viscous loss rather than compensate it by means of feedback. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention have been achieved by providing a coating die that includes an insert having a high thermal conductivity, which is greater than 1 W/cm*K. According to the preferred embodiment, the insert is made of diamond having a thermal conductivity in the range of 5 to 20 W/cm*K. 
     Use of this highly conductive insert helps dissipate most efficiently the heat produced by viscous losses in the coating as most of these losses occur very close to the inner wall of the die in the land region. 
     It also reduces to negligible level the wall temperature unbalance between opposite sides of the fiber when this fiber is off-centered thus restoring conditions prevailing at low draw rates and subsequent satisfactory centering force. 
     It has been discovered that the high conductivity of the insert minimizes the differences or imbalances between the inner wall temperatures of the die so that the temperature of the inner wall is generally uniform. Therefore, the affects discussed above with regard to the generation of de-centering forces due to the variation in wall temperature is substantially reduced or even eliminated. In particular, whereas localized heat generated due to a non-uniform viscosity profile is relatively high in a die having a low thermal conductivity due to poor heat dissipation, the heat generated in a die having a high conductivity is quickly dissipated such that temperature uniformity is substantially improved. Therefore, even if one side of the fiber begins to be offset toward the adjacent wall of the die, there is a minimal temperature increase on that side of the fiber. As a result, the viscosity and pressure profile around the fiber remains generally uniform around the fiber in the land region so that the fiber is not drawn to one side and centering can be restored as best as possible by the centering force generated in the tapered region of the die. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematical illustration of a fiber drawing and coating apparatus according to the present invention; 
     FIG. 2 is a cross-sectional view of the coating applicator of the present invention; and 
     FIGS.  3 ( a )-( c ) are cross-sectional views showing the sizing dies associated with different embodiments of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description of preferred embodiments of the invention is offered for purposes of illustration of the principles of this invention and it is not intended to be limiting. 
     FIG. 1 is a schematic illustration showing the basic structure of a fiber forming and coating line  10 . The line  10  includes an optical fiber forming device  12 , a fiber diameter measurement device  14 , a fiber coating device  16 , a primary coating applicator  18 , a primary coating curing device,  20 , a secondary coating applicator  22 , a secondary coating curing device  24 , a coated fiber diameter measurement device  26  and a capstan  28 . Optical fiber  30  is drawn from the fiber forming device  12  and passes through the fiber diameter measurement device  14  which measures the diameter of the optical fiber  30 . The fiber  30  then passes through primary coating applicator  18  where it is coated with a primary layer. The primary coating is then cured by the primary coating curing device  20  and the fiber  30  is then passed through the secondary coating applicator  22  where a secondary coating layer is applied. The secondary coating is then cured by the secondary coating curing device  24 , after which the final diameter of the coated optical fiber is measured by the measurement device  26 . The optical fiber  30  is then wound around capstan  28  and onto a spooling device (not shown). 
     The present invention is directed to the design of the primary coating applicator  18 , illustrated in FIG.  2 . FIG. 2 is a cross-sectional view of coating applicator  18 . The coating applicator  18  includes a cylindrically-shaped main housing  32  having longitudinally-spaced, coaxial bores  34 ,  36  and  38  extending longitudinally therethrough. The main housing  32  may be formed of steel or stainless steel or any machinable metal or the like. Bores  36  and  38  meet to form an inwardly projecting shoulder  40  upon which sizing die  42  is seated. Immediately above sizing die  42  is a cylindrical, flow distribution area  44  to which the coating is supplied via inlets  46 . Bores  34  and  36  also meet to form a shoulder  48  upon which a guide die  50  is seated. The guide die  50  is conventional and includes a tapered longitudinal aperture  52  followed by a cylindrical aperture corresponding to a guide die orifice  54 . The presence of guide die orifice  54  facilitates the initial feeding of fiber  30  through the coater. It causes an inserted fiber to be centered so that it will readily pass through the coater without becoming snagged. The orifice  54  is sufficiently large that the fiber does not come into contact with it during the fiber drawing and coating operation. 
     The sizing die  42  includes a die housing  41  having a bore  56  in which a diamond insert  58  is located. With reference to FIGS.  3 ( a ) and  3 ( c ), there are three alternative designs for the sizing die. Referring to FIG.  3 ( a ), according to a first embodiment, the inlet end the sizing die  42  includes a first tapered aperture  60  having a wide opening defined by a relatively wide angle θ (of approximately 90°). In contrast, the diamond insert  58  includes a second tapered aperture  62  having a relatively narrow opening defined by an angle β (of approximately 12°) followed by a cylindrical land opening  64 . Thus, in this embodiment, the heat conductive diamond insert  58  includes both the tapered region  62  and the land region  64  of the sizing die. Thus, wall continuity and smoothness is ensured in all locations that are contacted by the high speed and high shear rate coating flow. Flow eddies are thus avoided. 
     The extension of the insert to include part of the tapered region  62  of the die is beneficial but not essential for heat transfer. For example, referring to FIG.  3 ( b ), according to a second embodiment, the sizing die  42  includes a single tapered opening  66  that is initially defined by a relative wide angle θ (approximately 90°) but tapers down to a smaller angle β (approximately 12°). The insert  58  is located on the downstream side of the sizing die  42  and includes just a land region  68  having a sizing orifice  70 . The size of orifice  70  is determined by various parameters including the diameter of the optical fiber to be coated, the thickness of the coating and the particular coating material employed. 
     Thus, in this particular embodiment the heat-conductive insert includes only the cylindrical land region of the sizing die. The sizing die  42  is made of a cast metal, such as cast iron, or a similar machinable material. This design is sufficient to allow dissipation of most of the heat produced by viscous losses as these losses occur mainly over the inner wall of the land region  68  of the die insert  58 . Special care must be given to avoid discontinuities of the inner wall at the up stream side of the insert  58 . Turbulence in the coating flow may be otherwise initiated, as this limit is located in a region of extreme speed and shear rate in the coating flow. Provided successful manufacturing, this embodiment may be cheaper than state of the art Tungsten Carbide dies. 
     Referring to FIG.  3 ( c ), a third, altogether different embodiment of the invention could be realized by replacing the heat conductive insert  58  with a sufficiently thick layer  70  of heat conductive material like pyrolithic diamond over part or the entire inner wall of the sizing die. 
     According to an important aspect of the invention, the die insert  58  in each embodiment is made of a material which has a high thermal conductivity, greater than 1 W/cm*K. Examples of such a material include all varieties of diamond, natural or synthetic (having a thermal conductivity in the range of 5 to 20 W/cm*K) or isotopically pure C12 diamond (with a conductivity of 50 W/cm*K). Another suitable but less performant material could be Silicon Carbide (with a conductivity in the range of 1 to 1.3 W/cm*K at room temperature). 
     This is in contrast to conventional coating die inserts having a thermal conductivity of only 0.15 W/cm*K. It has been discovered that the high conductivity of the insert minimizes the differences or imbalances between the inner wall temperatures of the die so that the temperature of the inner wall is generally uniform around the circumference of the die insert. Therefore, the effects discussed above with regard to the generation of de-centering forces due to the variation in wall temperature is substantially reduced or even eliminated. In particular, whereas localized heat generated due to a non-uniform viscosity profile is relatively high in a die having a low thermal conductivity due to poor heat dissipation, the heat generated in a die having a high conductivity is quickly dissipated such that localized heating is substantially reduced. Therefore, even if one side of the fiber begins to be offset toward the adjacent wall of the die, there is a minimal amount of heat remaining in the liquid on that side of the fiber. As a result, the pressure profile around the fiber remains generally uniform in the land region so that the fiber is not drawn to one side. 
     Thus, the present invention provides a much simpler solution that the prior art systems discussed above in which the ability to monitor the concentricity of the coating on-line is required. 
     Specifically, it has been confirmed by numerical simulation that a die insert having a high thermal conductivity is advantageous in rapidly dissipating heat generated by viscous heating (the result of the conversion of mechanical to thermal energy via fluid friction). Viscous heating tends to occur most where shear rate is highest in the coating flow. In the land region of the sizing die, high shear rate and subsequent heating are highest over the inner wall of the sizing die. This heat can be extracted easily if the heat conductivity of the die is sufficient. 
     In addition, simulations show that shear rate and viscous dissipation on opposite sides of the fiber become unbalanced if the fiber is off-centered with respect to the sizing-die, a situation which often occurs in actual fiber manufacturing. This initial unbalance induces subsequent unbalances between temperatures, coating viscosities, and pressures on opposite sides of the fiber unless die heat conductivity is sufficiently high to both evacuate heat radially and to conduct it around the inner wall of the die to equalize temperatures as best as possible. Results of simulation show that in case of use of a high conductivity insert substantial centering force is generated in the tapered region of the die pulling back the off-centered fiber. To the contrary, if temperatures on either sides of the fiber are left unbalanced and uncontrolled, flow rates are likely to become in turn unbalanced, especially in the land region, and the global centering force is mitigated producing a loss of coating concentricity.