Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
1999-10-28
2003-12-09
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06661502
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and apparatus for measuring the diameter and/or eccentricity of a coating layer of a coated optical fiber and, more particularly, to a method and apparatus that are capable of being utilized to measure the diameter and/or eccentricity of a primary coating layer of a coated optical fiber during an optical fiber cable manufacturing process.
BACKGROUND OF THE INVENTION
The successful implementation of a light wave communication system requires high quality light guide fibers having mechanical properties sufficient to withstand the stresses to which they are subjected. Each fiber must be capable of withstanding over its entire length a maximum stress level to which the fiber will be exposed during installation and service. The importance of fiber strength becomes apparent when one considers that a single fiber failure will result in the loss of several hundreds of circuits.
The failure of light guide fibers in tension is commonly associated with surface flaws which cause stress concentrations and lower the tensile strength below that of pristine unflawed glass. The size of the flaw determines the level of stress concentration and, hence, the failure stress: Even micron-sized surface flaws cause stress concentrations which significantly reduce the tensile strength of the fibers.
Optical fibers are normally made in a continuous process which involves drawing a thin glass strand of fiber from a partially molten glass preform and thereafter applying the coating layers. A furnace is used to partially melt the preform to permit the fiber to be drawn. The heat of the furnace and the rate of draw of the fiber must be in proper balance so that the optical fiber can be drawn continuously under uniform conditions. Long lengths of light guide fibers have considerable potential strength, but the strength is diminished by airlines or holes occurring in the optical fibers. Furthermore, airlines in optical fibers also interfere with the light-propagation properties of the optical fibers.
Soon after an optical fiber is drawn, the optical fiber is coated with a primary layer of coating material and a secondary layer of coating material. The primary layer of coating material surrounds the optical fiber and serves as a soft cushion for the optical fiber to prevent micro-bending losses. The primary coating layer also seals the outer surface of the optical fiber from environmental conditions, such as atmospheric moisture. This secondary coating layer surrounds the primary coating layer. The secondary coating layer is harder than the primary coating layer and serves to shield the fiber from surface abrasion, which could occur as a result of subsequent manufacturing processes and handling during installation. The secondary coating layer also provides protection against corrosive environments and atmospheric moisture. U.S. Pat. Nos. 5,880,825, 5,828,448 and 5,786,891, which are incorporated herein by reference, are directed to detecting, and/or distinguishing between, defects in an optical fiber coating.
The primary and secondary coating layers are often applied by a dual coating applicator during the fiber drawing process. Therefore, the dual coating applicator applies both the secondary and primary coating layers. The coating layers are subsequently cured in an ultraviolet (UV) lamp system as the coated optical fiber is drawn through the UV lamp system. It is desirable to use a dual applicator for applying both of the coating layers because less coating material is lost than if separate coating applicators are used for applying each of the coating layers. When separate applicators are used, the optical fiber having the primary coating layer thereon must be drawn into the secondary coating applicator and uncured primary coating material may fall away from the optical fiber as it is strung between the applicators. Using a dual applicator eliminates or reduces this problem.
Since the primary and secondary coating layers are applied in a dual applicator, it is not possible to physically obtain access the primary coating layer without removing a portion of the secondary coating layer. Currently, the diameters of the optical fiber and of the secondary coating layer are measured with diameter gauges at the ends of the coated fiber. Measuring these diameters, however, does not provide information about the diameter of the primary coating layer.
It would be desirable to provide a technique for measuring the diameter of the primary coating layer after the primary and secondary coating layers have been applied to the optical fiber without having to physically remove a portion of the secondary coating layer. The primary coating layer must meet certain requirements in order to be deemed satisfactory. One of these requirements is that the primary coating layer must have a proper thickness, or diameter. Another requirement relating to the primary coating layer is that it should have an eccentricity that is within desired or prescribed limits. Therefore, a need exists for a technique for measuring the diameter of the primary coating layer to ensure that it is within desirable or prescribed limits in terms of its diameter and/or in terms of its eccentricity. Furthermore, it would be desirable to perform the technique quickly during the optical fiber cable manufacturing process so that information relating to the diameter of the primary coating layer can be utilized by the draw tower to alter, if necessary, the manufacturing conditions in real time to ensure that the amount of cable having an improper primary coating layer diameter, if any, is minimized or eliminated.
U.S. Pat. No. 5,208,645 to Inoue, et al discloses a method and an apparatus for optically measuring the thickness of an inner coating layer of an optical fiber. The technique disclosed in Inoue, et al. utilizes an observation that, when light is projected through an optical fiber having a coating layer thereon, an intensity distribution having two peaks occurs. It was further observed that these peaks have levels that could be correlated to the thickness of a carbon coating layer on the optical fiber. In order to apply the technique, the thickness of the carbon coating layer is measured by another technique, such as measurement of the electrical resistance of the coating layer. A calibration curve is generated that correlates the electrical resistance to the peaks associated with the intensity distribution. By correlating the peaks of the intensity distribution to the calibration curve, the thickness of the coating layer can be ascertained.
One of the disadvantages of this technique is that it provides no way of determining the thickness of an inner coating layer when more than one coating layer surrounds the optical fiber. Therefore, this technique is unsuitable for determining the diameter of the primary coating layer after the secondary coating layer has been applied. Also, the carbon coating does not correspond to the primary or secondary coating layers of a coated optical fiber, because these layers are comprised of a polymer material. Rather, the carbon coating layer is a coating layer that is applied before the primary coating layer has been applied. Furthermore, the technique utilizes light scattering effects caused by light scattered by the carbon coating to generate the intensity distribution. It does not utilize data relating to light transmitted through the coated optical fiber. Therefore, this technique could not be used to obtain information relating to a coating layer disposed underneath another coating layer.
Accordingly, a need exists for a method and an apparatus for measuring the diameter of the primary coating layer of a coated optical fiber and/or for determining the eccentricity of the primary coating layer.
SUMMARY OF THE INVENTION
The present invention provides an optical detection system for determining the diameter of a primary coating layer that has been applied to an optical fiber and/or for determining the eccentricity of the primary coating layer. The s
Jakobsen Christian
Pedersen Flemming
Fitel USA Corp.
Nguyen Tu T.
Thomas Kayden Horstemeyer & Risley LLP
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