Optical-fiber splicing: the mechanical option

Once considered suitable only as a temporary repair measure, mechanical fiber-optic splicing has come into its own based on positive results after more than 10 years of field use. Today, this type of splice can offer fast, dependable results with minimal capital-equipment outlay and only basic training requirements, and involves no compromise in splice quality or signal integrity.

Mechanical fiber-optic splicing is attractive because of its simplicity and portability. Very little training and experience are required for successful use, the necessary equipment is substantially less costly than traditional fusion-splicing systems, and mechanical splicing can be done successfully under rugged field conditions.

Fusion splicing is most cost-effective for use in construction projects that have many thousands of splices. Here, the cost-per-splice of fusion-splicing equipment can be justified. In contrast, mechanical splicing is most cost-effective for projects involving a few thousand splices or less. The low cost of mechanical tooling makes it affordable to purchase multiple sets of splicing equipment to equip field crews.

Mechanical splicing is a relatively simple task. The optical-fiber ends are stripped, cleaned, and cleaved, placed in the splice, and assembled with a tool that closes the splice and ensures accurate fiber tension and alignment.

The key to mechanical-splice performance, both upon mating and over time, is an index-matching gel surrounding the fiber ends inside the splice that effectively closes the mechanical gap. This gel has the same refractive index as the glass fiber, and thus provides for good signal transmission while excluding moisture and other contaminants. The index-matching gel maintains optical clarity and contributes to mechanical-splice performance over years of operation, even under difficult conditions. It serves as the optical equivalent of solder in a copper-to-copper joint.

In cases where multiple fibers are joined, it is now possible to reduce splicing time while preserving splice quality. Components are available that allow for ribbonizing of discrete fibers in a multifiber splice process, with up to 12 fibers at a time. Ribbonizing is important to construction efficiency, because it offers greater splicing efficiency and reduces per-fiber splice costs. Fusion splicers are available for ribbon splicing, but their performance is generally not as good as can be achieved using mechanical multifiber splicing.

A key difference between fusion and mechanically spliced optical fibers is remateability. A properly designed mechanical splice can be opened and closed again to allow repositioning of fields or field changes. Mechanical splice remateability increases yield and reduces material cost.

Laboratory tests confirm that the best mechanical splices can maintain low signal loss and low reflectance over time, even under difficult operating conditions. The results of these tests show that mechanical splices are capable of delivering performance equal to fusion splicing, both in terms of dependability and in maintaining low insertion loss and reflectance values over time.

Field experience with mechanical splices confirms laboratory testing. Mechanically joined optical splices in a vault at the airport in St. Louis were immersed in water for more than 30 days during severe flooding in 1993. Components were exposed directly to waterborne contaminants that included silt, raw sewage and jet fuel. Despite all this, the mechanical splices in the vault provided uninterrupted service throughout the disaster.

As the economical advantages and performance capabilities of mechanical splicing become more widely known, telcos, contractors, and cable-TV operators are beginning to use these splices as connectors. Some of the new fiber distribution units on the market make use of mechanical splices instead of connectors by providing high density and low cost in crossconnect and interconnect configurations.

Larry Sellers is a technical-service engineer at 3M Telecom Systems Div. (Austin, TX).

This article is reprinted courtesy of Cable Foreman, now OSP Engineering and Construction, a PennWell publication.

For more information, see the "Splices and Splicing" product comparison table in this issue.

More in Optical Tech