Abstract:
A coated optical fiber adjusted to operate at a predetermined temperature range. The coated fiber includes an optical fiber, a first polymer coating generally concentrically surrounding the optical fiber and a second polymer coating generally concentrically surrounding the first polymer coating, wherein the first polymer coating includes substantially no coefficient of thermal expansion stresses when the optical fiber is placed at a lower end of the temperature range.

Description:
This is a continuation of application Ser. No. 09/780,983 filed Feb. 9, 2001. 
    
    
     STATEMENT AS TO FEDERALLY SPONSORED RESEARCH 
     This invention was made pursuant to DARPA Contract No. DAAH 01-95-C-R128. Accordingly, the federal government may have rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to manufacturing coated optical fibers. 
     Optical fibers typically are silica-based. To improve the moisture resistance and mechanical properties of the fiber, the fiber is often provided with multiple polymeric coatings disposed concentrically about the fiber, with the coating nearest the fiber being more flexible than the outermost coating(s). 
     To form the coatings, a photopolymerizable composition typically is applied to the fiber and polymerized by exposure to actinic radiation, e.g., ultraviolet radiation, to form a first polymer coating. Next, a second photopolymerizable composition is applied to the first polymer coating and likewise exposed to actinic radiation to form a second polymer coating. 
     One problem encountered with such coated fibers is that both polymerization processes generate heat. The heat generated during the second polymerization process can lead to the development of tensile stresses in the first polymer coating. These tensile stresses, in turn, can cause the first polymer coating to fracture, or delaminate, or both, thereby compromising the strength and moisture resistance of the fiber. In addition, in the case of telecommunications fibers and stress-sensitive fibers such as polarization maintaining (PM) and polarizing (PZ) fibers, these tensile stresses can manifest themselves as microbending losses or other effects on the optical signal, thereby degrading the overall performance of the fiber. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention features a method for coating an optical fiber that includes: (a) applying a photopolymerizable composition to an optical fiber having a surface coated with a first polymer coating; and (b) exposing the photopolymerizable composition to a source of actinic radiation to form a second polymer coating under conditions which inhibit the production of thermally induced tensile stresses in the first polymer coating. 
     In preferred embodiments, the fiber is cooled prior to application of the photopolymerizable composition. Preferably, this is accomplished by exposing the fiber to a chilled stream of gas (e.g., an inert gas such as helium). 
     Inhibiting the production of thermally induced tensile stresses in the first polymer coating during exposure may be accomplished in several ways. For example, the fiber may be cooled with a chilled stream of gas such as helium during exposure. Another protocol involves providing the source of actinic radiation with a dichroic reflector that transmits infrared radiation generated by the radiation source away from the fiber. Yet another useful protocol includes placing a water-cooled jacket concentrically about the fiber. The surface of the jacket may be further provided with an infrared radiation-absorbing coating. In another embodiment, a tube (e.g., a quartz tube) having a surface coated with an infrared radiation-absorbing coating is disposed concentrically about the fiber. 
     Each of these protocols may be used alone, or in combination with any, or all, of the others. 
     The actinic radiation preferably is ultraviolet radiation. The first polymer coating preferably includes an acrylate-functional silicone polymer, while the photopolymerizable composition preferably includes a photopolymerizable acrylate-functional epoxy or acrylate-functional urethane composition. 
     In a second aspect, the invention features a method for coating an optical fiber featuring a surface coated with a first polymer coating where the fiber is essentially free of a hermetic carbon coating underlying the first polymer coating. The method includes (a) cooling the fiber (e.g., by exposing the fiber to a chilled stream of gas such as helium gas); (b) applying a photopolymerizable composition to the first polymer coating; and (c) exposing the photopolymerizable composition to a source of actinic radiation to form a second polymer coating. Preferably, the method further includes inhibiting the production of thermally induced tensile stresses during exposure according to the procedures described above. 
     The invention provides optical fibers having multiple polymer coatings in which the production of tensile stresses within an individual polymer coating is minimized. The fibers exhibit good moisture resistance and mechanical properties, and resist delamination. The ability to minimize tensile stresses, and thus the defects associated with such stresses, makes the fibers particularly useful in defect-sensitive applications such as interferometric fiber optic gyroscopes. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of an apparatus for manufacturing coated optical fibers according to the invention. 
     FIG. 2 a  is an expanded schematic drawing of the apparatus shown in FIG. 1 illustrating the equipment used to polymerize the second photopolymerizable composition. 
     FIG. 2 b  is a top view of the equipment depicted in FIG. 2 a.   
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1 and 2, there is shown an apparatus  10  for manufacturing a coated optical fiber having a plurality of polymer coatings disposed concentrically about the fiber core. As shown in FIG. 1, an optical fiber  12  provided with a first photopolymerizable coating disposed concentrically about the fiber core is exposed to actinic radiation (e.g., ultraviolet or visible radiation) from a lamp  14  to polymerize the coating. Examples of suitable materials for the first coating include relatively flexible polymers such as acrylate-functional silicone polymers. The particular type of actinic radiation and the exposure conditions are selected based upon the particular photopolymerizable coating employed. 
     Once polymerization is complete, the coated optical fiber may be cooled at a cooling station  16  by exposing the coated fiber to a chilled stream of gas. Preferably, the gas is inert with respect to the coated fiber. A number of gases can be used, including helium, nitrogen, argon, carbon dioxide, and combinations thereof. Because helium has a high thermal conductivity, it is particularly effective for cooling the coated fiber. The gas may be cooled, e.g., by running it through a coil of copper tubing submerged in a dry ice/propanol bath. Cooling the coated fiber prior to application of the second photopolymerizable coating is advantageous because it shrinks the dimensions of the coated fiber, thereby minimizing the production of tensile stresses following coating and polymerization of the second photopolymerizable coating. 
     Next, the cooled, coated fiber enters a coating station  18  where it is coated with a second photopolymerizable composition using conventional techniques such as die coating. The second photopolymerizable composition is designed to produce a second polymer coating concentrically disposed about the first polymer coating. The second polymer coating preferably is more rigid than the first polymer coating to provide mechanical reinforcement. Typical photopolymerizable compositions for preparing the second polymer coating include photopolymerizable acrylate or methacrylate-based compositions such as photopolymerizable acrylate-functional epoxy or urethane resins. Upon exposure to actinic radiation such as ultraviolet or visible radiation, the acrylate groups polymerize to form an acrylate polymer. 
     Following application of the second photopolymerizable composition, the fiber enters an enclosure  24  housing an actinic radiation source  20  and a water-jacketed quartz tube  22  designed to cool the fiber during actinic radiation exposure. These features are shown in greater detail in FIGS.  2 ( a ) and  2 ( b ). 
     As shown in FIGS.  2 ( a ) and  2 ( b ), enclosure  24  includes, as the actinic radiation source, an electrodeless ultraviolet lamp  20 . Within enclosure  24 , the fiber is exposed to ultraviolet radiation from lamp  20  as it moves through water-jacketed quartz tube  22 . The particular exposure conditions are selected based upon the photopolymerizable composition. Upon exposure, the second photopolymerizable composition coated on the fiber polymerizes to form a second polymer coating. 
     Water circulating through the jacket absorbs heat and infrared radiation generated by the lamp during exposure, thereby preventing it from reaching the fiber. To enhance the heat absorption function, the outer surface of tube  22  may be further provided with an infrared-absorbing, ultraviolet-transmitting coating. 
     The fiber is further cooled during exposure by means of chilled helium gas supplied via a port  26 . The helium may be cooled prior to contact with the fiber, e.g., by running it through a coil of copper tubing submerged in a dry ice/propanol bath. 
     A dichroic reflector  28  located within enclosure  24  and positioned around lamp  20  and tube  22  further assists inhibiting the formation of thermally induced tensile stresses in the first polymer coating during polymerization to form the second polymer coating. Reflector  28  reflects ultraviolet radiation generated in lamp  20  toward tube  22  but transmits infrared radiation away from tube  22 , thereby reducing the amount of infrared radiation reaching the fiber. 
     The invention will now be described further by way of the following examples. 
     EXAMPLES 
     Comparative Example A 
     A freshly drawn silica fiber lacking a hermetic carbon coating was initially die-coated with a photopolymerizable, acrylate-functional, silicone composition (commercially available from Shin-Etsu under the designation “OF206”) using a primary die size of 179 micrometers and a line speed of 1 m/sec. The composition was polymerized by exposing the coated fiber at a line speed of 1 m/sec to ultraviolet radiation supplied from a Fusion Systems I 256 irradiator with an F10-T housing equipped with an R350 reflector, a “D” bulb, and a VPS-6 variable power supply. The maximum output of the lamp (i.e., when the power level was set at 100%) was 375 watts/inch. 
     After formation of the first polymer coating, the fiber was die-coated with a second photopolymerizable composition using a primary die size of 199 micrometers. The composition was an acrylate-functional epoxy resin commercially available from DSM Desotech under the designation “3471-2-137.” Following coating, the fiber was exposed to ultraviolet radiation using the above-described Fusion Systems equipment. The power supply was set at 80% power during exposure. 
     Following ultraviolet radiation exposure, approximately 2 meters of the resulting fiber were wrapped under low tension on a 2.5 inch diameter, 0.25 inch thick aluminum cylinder. The cylinder was then mounted horizontally in a temperature-controlled chamber and the free ends of the fiber were affixed to free hanging 25 gram weights. The resulting structure was then cycled between −55° C. and 70° C. for a total of 30 cycles, after which the fiber was examined microscopically for defects such as delaminations and fractures. Examination revealed a total of nine defects in the form of delaminations and fractures. 
     Example 1 
     The procedure of Comparative Example A was followed except that prior to application of the second photopolymerizable composition, the fiber was cooled by exposing it to a stream of chilled helium gas in a cooling unit measuring 10 in. long. The final product displayed no evidence of delamination or fracture. 
     Example 2 
     The procedure of Comparative Example A was followed except that a Fusion Systems dichroic reflector was positioned around the fiber and the ultraviolet lamp. The dichroic reflector reduced the amount of infrared radiation reaching the fiber during exposure. The final product displayed no evidence of delamination or fracture. 
     Example 3 
     The procedure of Comparative Example A was followed except that during exposure the fiber was cooled by exposing it to a stream of chilled helium gas. The final product displayed no evidence of delamination or fracture. 
     Example 4 
     The procedure of Comparative Example A was followed except that during exposure the fiber was cooled by encasing it in a water-cooled jacket. The final product displayed no evidence of delamination or fracture. 
     Example 5 
     The procedure of Comparative Example A was followed except that the exposure conditions were adjusted by reducing the power level setting to 60%. The final product displayed no evidence of delamination or fracture. 
     Example 6 
     The procedure of Comparative Example A was followed except that prior to application of the second photopolymerizable composition, the fiber was cooled by exposing it to a stream of chilled helium gas, as described in Example 1. In addition, a dichroic reflector was positioned around the fiber and the ultraviolet lamp to reduce the amount of infrared radiation reaching the fiber, as described in Example 2. Moreover, during exposure the fiber was cooled by encasing it in a water jacket, as described in Example 3, and exposing it to a stream of chilled helium gas, as described in Example 4. The exposure conditions were the same as described in Example 5. The final product displayed no evidence of delamination or fracture.