Fiber-coating standards

Fiber-coating standards

WILLIAM B. GARDNER, AT&T

The polymer coating applied to optical fibers during the production draw process protects the glass from abrasion. In addition, application of the polymer coating increases the fiber cable diameter from 125 microns to 245 microns. The diameter is increased by another 5 microns when a color-coded layer is added over the coating. Colors enable the identification of individual fibers within a cable.

However, color-coded combinations (dashes) can be difficult to apply and sometimes cause small increases in fiber loss. Consequently, the last two dashes have been replaced by the solid colors rose and aqua. These choices originated in International Electrotechnical Commission document IEC-304.

The Telecommunications Industry Association, or TIA, Working Group FO-6.7.10 on Color Coding, chaired by Rolf Frant¥of Bell Communications Research, has fine-tuned the definition of aqua. Moreover, an optimized definition of the color rose is expected to be balloted soon.

The 12 colors approved for fiber-coating coding are defined in TIA/Electronic Industries Association standard 598-A document Optical Fiber Cable Color Coding, which was re-issued in May 1995. The colors are specified in terms of the Munsell Color System (for example, see ASTM D1535-1989).

Interpretation using these color charts is subjective, and the fiber`s small size further complicates the process. A colorimeter instrument can quantify colors objectively, but the process requires a multilayer matrix of fibers. Applying the colorimeter to the windings on a fiber spool, however, might prove a workable solution.

In addition to meeting hue, value and chroma color requirements, the 5-micron thick coloring layer must be sufficiently translucent or easily removable for local injection and detection devices to measure splice losses and identify fibers. The TIA has published Test Method FOTP-106, which uses a spectrophotometer to measure the infrared absorbency of coating materials that have been formed into thin films. A procedure for using this test method to create a local injection and detection transparency specification has not yet been derived.

The TIA Working Group FO-6.6.7 on fiber coatings, chaired by Eric Urruti of Corning Inc., has completed a coating geometry round robin. The coordinator, Tim Drapela of the National Institute of Standards and Technology, or NIST, will distribute results on coating diameter, non-circularity and coating-cladding concentricity error at the TIA Meeting in Sarasota, FL, January 22 to 26, 1996.

A previous round robin on fiber diameter demonstrated the importance of calibrating with fibers whose diameters had been measured by a precision micrometer at NIST (see Lightwave, March 1994, page 10). The micrometer could not be applied to a coated fiber, but Corning made glass fibers with a 245-micron nominal diameter to use as coating calibration standards.

Matt Young of NIST measured these fibers with the precision micrometer, while John Baines of the National Physical Laboratory in the United Kingdom corroborated the measurements with the side-view microscope technique described in TIA FOTP-119. These artifacts for calibration are expected to be available for purchase from NIST some time in 1996.

Coatings must adhere to the fiber and yet be readily strippable by mechanical means. There has been no success to date in specifying this tradeoff. Balloting on TIA FOTP-178, Measurements of Strip Force for Mechanically Removing Coatings from Optical Fibers, ended in May. A draft of a similar method for ribbon strip force (FOTP-118) is being worked jointly between FO-6.6.7 and FO-6.7. Unfortunately, the generation of a specification for strip force is complicated because strip force depends on multiple variables, such as the size and shape of the tool`s aperture, the sharpness and temperature of the blades, the length of fiber stripped and the stripping rate. q

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