Fiber-optic connector issues under study

Fiber-optic connector issues under study

bruce g. LeFEVRE, AT&T

Approximately 12 fiber-optic connector designs that have gained acceptance in telephony, cable TV and private networks are covered by U.S. or international standards or specifications. These standards typically address one or more of the following parameters, depending on the type and scope of the document--interface dimensions, performance requirements, qualification approval and quality control. Most of the standards are currently in balloting or revision, with some details yet to be resolved. However, there is general agreement on the main issues.

Connector types BFOC/2.5 (also known as ST) and SC have emerged as two of the major components used in many cabling applications. The international documents under development to cover these components illustrate the important connector issues being studied, and the structure and function of the various standards organizations.

International Electrotechnical Commission Subcommittee 86B, or IEC SC 86B, on fiber-optic interconnecting devices and passive components deals with connectors on a worldwide basis. Three working groups within this subcommittee develop standards dealing with test and measurement methods (Working Group 4), reliability (Working Group 5) and interface dimensions, performance requirements and qualification approval (Working Group 6). The corresponding organizations within the Telecommunications Industry Association, or TIA, in the United States maintain consistency wherever possible with the IEC.

Many work projects undertaken in IEC SC 86B originate from interactions with other standards bodies. For example, the liaison between SC 86B WG6 and ISO/IEC JTC1/SC25/WG3 for customer premises cabling resulted in a set of Detail Specifications for ST and SC connectors for use in multimode and singlemode fiber installations. The specifications contain detailed drawings and information for mechanical intermateability, performance requirements and qualification approval/quality conformance schedules.

Some issues raised during balloting of the ST and SC connector detail specifications pose fundamental questions about the functions of the SC 86B documents. One issue concerns the inclusion of certain ferrule tip geometry details (tip radius and fiber undercut) as drawing requirements, in addition to the corresponding performance requirement (reflectance). Including both issues could be redundant and restrictive in certain situations.

Drawing requirements

Some drawing requirements need to be specified together with performance. For angled PC-connector designs, both the angle and the keying tolerance must be specified to ensure performance and compatibility of randomly mated plugs. The key issues center on what value, tolerance and measurement method to accept as standard. The present SC drafts for angled PC connectors specify both 8 and 9 degrees for 1300-nanometer minimum dispersion fiber, and 12 degrees for dispersion-shifted fiber (for a 60-decibel reflectance capability).

Detail specifications are required only if the user requires qualification approval of the product within the quality assurance system. The document structure evolving in SC 86B provides flexibility for situations where this may or may not be the objective. Basic Interface Standards are being drafted to accommodate existing designs, as are Performance Standards for particular applications and environments. Measurement Standards exist and are updated on an on-going basis. Reliability Standards designating failures-in-time rates (derived from accelerated tests) are in the planning stages. q

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