Abstract:
Method and apparatus for producing metal-coated optical fiber involves providing a length of optical fiber having a glass fiber with or without a carbon layer surrounded by a liquid-soluble polymeric coating. The optical fiber is passed through a series of solution baths such that the fiber will contact the solution in each bath for a predetermined dwell time, the series of solution baths effecting removal of the polymer coating and subsequent electroless plating of metal on the glass fiber. The optical fiber is collected after metal plating so that a selected quantity of the metal-coated optical fiber is gathered, Preferably, the glass fiber passes through the series of solution baths without contacting anything except for the respective solution in each.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to optical fiber. More particularly, the present invention relates to metal-coated optical fiber, and techniques for manufacturing same. 
     Optical fiber is typically constructed having a polymer coating, but some applications necessitate the use of metal-coated optical fiber. One problem with metal-coated optical fiber is that it is not commercially available in long lengths. 
     Current metal-coated optical fibers are typically manufactured by a liquid freezing method or a metal plating method. The liquid freezing method is described in detail in A. Mendez &amp; T. F. Morse, Specialty Optical Fibers Handbook, Academic Press (2007), at pages 491-510 (“Metal-Coated Fibers”), which is incorporated herein by reference in its entirety for all purposes. As a brief description of the liquid freezing method, optical fiber is coated with metal by the fiber passing though a die filled with liquid metal or molten metal in line with a fiber drawing process. 
     Particularly when the thickness of metal coating is decreased less than ten micron, this freezing process has the possibility of mechanical contact of optical fiber with the coating die due to small fluctuation of the drawing tower or environmental conditions (such as temperature change, wind, vibration, etc.). A bare fiber without a thick polymer coating is fragile against handling or mechanical contact with any hard material. (A thin carbon layer is sometimes coated in line with drawing as a hermetic barrier. It will be appreciated that an optical fiber with a carbon layer less than one micron without any additional coating is still fragile against normal handling) So any mechanical contact with hard material can cause mechanical damage on the optical fiber surface and can degrade long term mechanical reliability. 
     In order to avoid mechanical contact of fiber with the coating die, the typical coating thickness of metal-coated fiber made by a liquid freezing method is larger than ten micron to obtain enough mechanical strength. However, the transmission loss of fiber with a thicker metal coating is larger due to thick metal thermal contraction. In particular, the contraction of the metal layer causes microbending loss when metal shrinks from liquid phase to solid phase due to the thermal expansion coefficient. 
     For metal-coated fiber exhibiting lower losses, thicker glass diameter needs to be selected (e.g., more than about 200 micron) to resist microbending due to metal contraction. But thick diameter of 200 micron or greater limits bending radius due to larger bending strain. 
     Difficulties in manufacturing metal plated optical fiber are due, in part, to incompatibility between the fiber drawing and polymer coating process and the metal plating process. In particular, such processes involve different process speeds and different line travel directions. Drawing, for example, is a vertical process and its line speed is typically more than 10 meter/min. On the other hand, a continuous plating process would be a horizontal process with a typical line speed of less than several meter/min. 
     So, in the production of metal-coated optical fiber, the optical fiber needs to be temporarily coated during the drawing process and taken into reel. The protective coating is applied because it is difficult to wind a drawn bare fiber into a reel as it is. As one skilled in the art will appreciate, bare fiber without a protective coating is fragile and can easily be broken by touching any hard material. After being reeled in this way, the optical fiber can be paid off from the reel for plating. 
     Referring now to  FIG. 1 , a typical plating process of the prior art is illustrated. As will be appreciated, electroplating or electroless plating is a wet process which means the fiber is immersed in liquid solution baths. (The jagged lines in each bath vessel represent the liquid surface.) The metal is coated on the glass fiber as a result of chemical reaction in a liquid. Typically, optical fiber is dipped in several baths in series for cleaning, sensitizing, activation and plating. Each wet process takes one to several minutes to occur in each of the baths. (The process time depends on coating thickness and temperature.) As illustrated by the downward arrows in  FIG. 1 , short fibers or fiber ends are dipped in a bath. (See, for example, U.S. Pat. No. 5,380,559, incorporated herein by reference in its entirety for all purposes.) Each target fiber moves vertically and is immersed in a bath. After sufficient time, it is retracted from the bath and moved horizontally to the top of the next bath. The plating will be done by soaking in a series of such baths. 
     Referring now to  FIG. 2 , conventional metal wire can be coated in a continuous process. As shown, the wire contacts one or more cathode rolls as it proceeds through the process. Additional pulleys may also be provided along the process direction to facilitate movement of the wire. One skilled in the art will appreciate that conventional wire (unlike optical fiber) is robust for handling or bending with tension because it is made of metal rather than glass. (See U.S. Pat. Nos. 5,342,503 and 3,994,786, each of which is incorporated by reference in its entirety for all purposes.) 
     Optical fiber cannot be coated using the same coating process that would be used with metal wire. If the bare fiber is prepared and enters into wet baths by contacting with pulleys, the pulleys can damage the fiber. In particular, such pulleys are typically made of plastic or metal on the surface, which is hard and can damage the fibers through contact. Even if the pulleys are made of soft material, small dust of hard particles such as silica or metal or any solids may cause damage to the optical fiber&#39;s surface due to fiber tension when some such particles exist between fiber and pulley. The bare fiber travels along path line in contact with some pulleys and therefore mechanical damage is caused at some points along the fiber length statistically. So bare fiber is not applicable to metallic continuous plating process to achieve long metal-coated optical fiber. As a result, most application of metal plating to optical fiber is metallization of short ends of optical fiber. 
     The present invention recognizes the foregoing considerations, and others, of the prior art. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect, the present invention provides a method for producing metal-coated optical fiber. One step of the method involves providing a length of optical fiber having a glass fiber surrounded by a liquid soluble polymeric coating. The optical fiber is passed through a series of solution baths such that the glass fiber will contact the solution in each bath for a predetermined dwell time, the series of solution baths effecting removal of the polymer coating and subsequent plating of metal on the glass fiber. The optical fiber is collected after metal plating so that a selected quantity of the metal-coated optical fiber is gathered. 
     Another aspect of the present invention involves a method for producing metal-coated optical fiber. One step of the method involves providing a length of optical fiber having a glass fiber surrounded by a water soluble polymeric coating. The optical fiber is passed through a water bath to remove the polymeric coating. After removal of the polymeric coating, the optical fiber is passed through at least one solution bath such that the glass fiber will contact solution therein for a predetermined dwell time in order to achieve electroless plating of metal on the glass fiber. The optical fiber is collected after metal plating so that a selected quantity of the metal-coated optical fiber is gathered. 
     According to another aspect of the present invention, an apparatus is provided for containing a liquid through which glass fiber passes on a continuous basis during an optical fiber manufacturing process. The apparatus comprises a first vessel containing a quantity of the liquid, the first vessel defining an inlet hole and an outlet hole for allowing moving passage of the glass fiber through the first vessel. A reservoir containing reserve liquid is also provided. The apparatus further comprises a pump for drawing the reserve liquid from the reservoir to replace liquid spilled from the vessel. 
     A still further aspect of the present invention provides an apparatus for containing a liquid through which glass fiber passes on a continuous basis during an optical fiber manufacturing process. The apparatus comprises a vessel defining an inlet hole and an outlet hole for allowing moving passage of the glass fiber through the vessel. The vessel is configured to contain a quantity of the liquid and to define an ullage above the quantity of the liquid. A vacuum source is in fluid communication with the ullage to create a negative pressure therein, thus inhibiting spillage of the liquid from the inlet and outlet holes. 
     Another aspect of the present invention provides an optical fiber comprising a glass fiber including a core and a cladding with or without a carbon layer. The optical fiber further includes a water soluble polymeric coating. 
     An additional aspect of the present invention provides a metal-coated optical fiber comprising a glass fiber having a length greater than one meter, the glass fiber including a core and a cladding (with or without a carbon layer). A metal coating, which surrounds and contains the cladding, extends along substantially the entire axial length of the glass fiber. The glass fiber preferably has a diameter no greater than about 200 microns and the metal coating preferably has a thickness of no greater than about 10 microns. 
     Other objects, features and aspects of the present invention are provided by various combinations and subcombinations of the disclosed elements, as well as methods of practicing same, which are discussed in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic representation showing a batch plating process of the prior art used to create short lengths of metal-coated optical fiber. 
         FIG. 2  is a diagrammatic representation showing a continuous coating process of the prior art to coat conventional metal wire. 
         FIG. 3  is a perspective diagrammatic view of a metal-coated optical fiber with layers cut away. 
         FIG. 4  illustrates an exemplary process for drawing optical fiber and applying a temporary coating thereto. 
         FIG. 5  illustrates an exemplary process for coating optical fiber with metal in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagrammatic representation of a bath arrangement that may be used in the process of  FIG. 5  in accordance with an embodiment of the present invention. 
         FIG. 7  is a diagrammatic representation of a bath arrangement that may be used in the process of  FIG. 5  in accordance with another embodiment of the present invention. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions. 
     The present invention provides various improvements in metal-coated optical fiber and methods of making the same. In particular, metal plating may be applied along continuous lengths of optical fiber (such as lengths up to ten kilometers) with sufficient mechanical strength along the whole length. According to an important aspect of the present invention, metal-coated optical fibers may be coated by a continuous plating process in which a bare fiber enters into several liquid baths without physical contact with any solid materials. 
     Referring now to  FIG. 3 , an exemplary metal-coated fiber  10  is illustrated. Fiber  10  includes a glass fiber having a core  12  and a cladding  14 . A metal coating  16  surrounds and contains the cladding/core combination. Metal coating  16  may be formed of any suitable metal which may be applied by electroless plating, such as nickel, copper, gold, silver, or suitable alloys. In accordance with a preferred embodiment, the diameter of the cladding/core combination may often be less than about 200 micron with the metal coating  16  often having a diameter less than about ten micron. 
     Certain aspects of the present invention involve novel methodology and materials for the temporary protective coating applied during the drawing process. In this regard,  FIG. 4  illustrates an exemplary fiber drawing process during which a temporary protective coating is applied. Toward this end, various types of possible temporary coatings are contemplated. One skilled in the art will appreciate that the temporary coating should provide enough protection against handling while being easily removable. Normal accrylate coating may be removed by solvent such as dichlormethane, MEK and acetone. While dichloromethane is good for removing acclylate coating, its use presents certain concerns. MEK and acetone are generally effective but each takes long time to remove coating by dissolving and it is also difficult to remove the coating with only MEK or acetone without mechanical touch. Moreover, MEK and acetone are flammable which also presents concerns. 
     Another candidate for the temporary coating material is wax. For example, wax having a melting temperature of less than 100 deg. C. might be used because it can be removed with hot water. However, the resulting fiber, even after most of the wax is removed, may not have enough elasticity for handling. 
     To eliminate these issues, some preferred embodiments of the present invention utilize a water soluble polymer for the temporary coating. The water soluble polymer used for the temporary coating has enough elasticity and gives enough protective coating for handling and mechanical strength. In accordance with exemplary methodology, optical fiber coated with water soluble polymer is paid off and enters into a removing bath. Bare fiber without any polymer thus comes out just before proceeding to the plating baths. The resulting bare fiber preferably goes through several liquid baths without physical contact with any solid materials (only liquid) and its fiber surface is processed in liquids such as cleaning, rinsing, dry, sinsetizing and plating. In particular, the fiber preferably travels through the bath solutions without any physical contact with any hard material until the fiber becomes enough robust with metalic deposition growth. 
     Water soluble polymer becomes solid when it is dried and it becomes liquid when immersed in water. As for water soluble polymer, sodium polyacrylater, plyacrylamide, polyvinyl alcohol, polyethyleneimine, polyethylene glycol, polyvinylpyrrolidone etc. are applicable. Viscosity of the water solution is preferably controlled to be 1-5000 mPa-s is good for application of fiber coating. 
     One example of a water soluble polymer believed to be suitable for this purpose is vinyl alcohol copolymer (product name nichigo G-polymer OKS-8049, sold by nippon gohsei). In this regard,  FIG. 4  illustrates a vertical fiber drawing process where the fiber  18  is continuously coated with water soluble polymer as a temporary protective coating. First, a water soluble polymer solution is prepared with appropriate viscosity by mixing water with G-polymer. Then, water soluble polymer is applied to normal drawing process (as shown in  FIG. 4 ). In one case, for example, optical fiber glass diameter is 125 micron, drawing speed is around 25 m/min, oven temperature is 200 deg. C. The water soluble polymer may be applied in a double layer coating as shown, yielding a coating thickness of about 9 micron. (The optical fiber may or may not have a carbon layer.) Cured fiber is them taken up into reel  20  in desired lengths of several kilometers or more. Typically, there will be no mechanical damage will occur to the fiber during drawing. 
       FIG. 5  is a diagrammatic representation of an exemplary process in accordance with the present invention for producing metal-coated optical fiber on a continuous basis. According to this embodiment, the fiber  18  is paid off of reel  20  and first passed through a coating removing bath  22  in a manner that does not contact any hard materials. The coating bath contains a suitable liquid to remove the polymer. For example, if the polymer requires removal by chemical solvent (e.g., accrylate removal by MEK or acetone), bath  22  will contain the appropriate solvent. In the case of water soluble polymer coating, bath  22  contains water to remove the polymer. Fiber  18  remains in contact with the removal liquid for a sufficient dwell time for the particular temperature. For example, with the exemplary water soluble polymer referenced above, the water may be at room temperature (i.e., unheated) with a dwell time of no more than about one minute in order to completely remove the polymer. In the case of chemical solvents for removal of other temporary polymer coatings, the process time for removing may be up to approximately 5 min. 
     After the temporary protective coating is removed, the fiber passes through a series of baths to achieve electroless metal plating. In this example, the successive baths are cleaning bath  24 , activator bath  26 , accelerator bath  28 , and plating bath  30 . As will be explained below, each bath preferably has a configuration similar to the coating removing bath which prevents contact of bare fiber with any hard materials. The solution of each bath depends on metal and plating objectives. According to one example, nickel may be plated as thick as  3  micron using nickel phosphorus solution. One skilled in the art will appreciate that the fiber&#39;s duration of transit through the process is set so that the fiber will have sufficient dwell time in each bath. Of course, the geometry of the respective baths also contributes to the dwell time in each. In one example, the following parameters may be used: line speed of 0.1 m/min, cleaning 5 min, activator time of 4 min, accelerator 3 min, and plating time of 13 min at 80 deg. C. After plating in bath  30 , the continuous process ends at take-up reel (pulley)  32 . 
     As noted, the fiber  18  passes through each of the baths  22 ,  24 ,  26 ,  28 , and  30  without contacting anything other than water or the process liquid. At feed reel (pulley)  20 , the fiber will have the temporary polymer coating which prevents contact between the optical fiber glass and the pulley. At take-up pulley  32 , the fiber has been sufficiently strengthened by the metal coating in order to contacted again. 
       FIG. 6  illustrates one configuration of an arrangement that can be used in the process of  FIG. 5  to ensure that the optical fiber does not contact anything except the water or process solution (depending on which bath). In this case, fiber  18  passes through exits (i.e., fiber inlet and outlet) of a bath where liquid flows out and below the level of liquid. The bath arrangement includes dual cells, an inner cell (vessel)  40  and an outer cell (vessel)  42 . Inner cell  40  contains sufficient liquid such that it flows over from exits at each end (as shown). Outer cell  42  receives the liquid which flows out from inner cell  40  for recirculation. The liquid received by outer cell  42  flows to solution reservoir  44 . A slight pulling tension applying to fiber will give straight passing through holes or slits of walls without touching. 
     As shown, solution in reservoir  44  is pumped up into inner cell  40  to keep the fiber immersed in a liquid of the cell. It will be appreciated that the fiber will have a tendency to sag between pulleys  20  and  32  due to gravity. Because the inlet into inner cell  40  from the pump is located at bottom of the cell, this tends to push the fiber up by the flow of the liquid. The upward force counteracts the sagging due to gravity and prevents the fiber from contacting hard components, such as the bottom or walls of inner cell  40 . The fiber&#39;s vertical position will preferably be controlled to keep constant against sag by monitoring position and adjusting the flow rate of the incoming solution, if necessary. 
     In an alternative embodiment, fibers may be passed through wet baths without mechanical contact utilizing pressure difference. For example, the inner cell may be enclosed and the ullage space in the cell evacuated. In such an arrangement, a pressure difference is caused between ambient pressure of the outer cell and the ullage space of the inner cell. As a result, the liquid does not flow out from the exit or inlet holes which are located under the level of liquid. A configuration of this concept is shown in  FIG. 7 . 
     One skilled in the art will appreciate that various advantages are achieved by a system configured in accordance with the present invention. Notably: 
     (1) Bare fiber is coated with metal without any mechanical contact with hard materials such as die or pulleys. The mechanical reliability of produced fiber is not consequently degraded. A long fiber without mechanical defects can thus be produced. 
     (2) The inlet or outlet size of the bath vessels is large enough for a fiber to pass through compared with the size of a die. The coating thickness is not determined by the size of the hole and is instead determined by the process time. Therefore, a thin metal coating can be achieved without mechanical damage. 
     (3) A thin metal coating achievable according to the process described herein will not cause significant fiber shrinkage due to thermal contraction because of thin layer and low thermal gap. (Because the process temperature is less than 100 deg. C. in preferred embodiments, low loss fiber is achieved.) 
     Additional advantages are realized by the use of a water soluble polymer for the temporary coating: 
     (1) Water soluble polymer is coated well in drawing process as well as thermal cure polymer because liquid polymer becomes solid by drying. Good compatibility with conventional thermal cure polymer coating. 
     (2) The removing solution is just water. It is safe and easy handling. 
     (3) Dissolution speed is very high. Typical dissolution speed of 10 micron polymer is within 1 min in room temperature water. Hotter water may be used to achieve faster dissolution times. 
     While preferred embodiments of the invention have been shown and described, modifications and variations may be made thereto by those of ordinary skill in the art without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to be limitative of the invention as further described in the appended claims.