My introduction to fiber optics was a guided tour of the Bell Labs section where they were developing the technology being used for the first commercial networks. What I remember most was the office of Jack Cook, the engineer developing the Biconic connector. The door to Jack’s office looked like it had a beaded curtain, but it was not beads; it was made of Biconic connector patchcords.
Jack’s office was tiny, and in the back, he had a small plastic molding machine where he was developing the molding process for fiber optic connectors. Because he needed such tight tolerances to mold the fiber optic connectors, he could not use regular tooling. He was molding the connectors around the fiber itself. As the connectors cooled, he hung them on a rod across the top of his doorway. After they cooled, he would polish the fibers on a table in his office.
Other manufacturers machined their connectors from metal. The SMA connector used a stainless-steel ferrule machined to micron tolerances with a hole drilled down the center. The fiber was also epoxied into the hole, and the connector was polished.
The big breakthrough came in the mid-1980s with the invention of the ceramic ferrule for connectors. It was molded with micron tolerances, was compatible with the glass fiber and epoxies, and was much easier to polish than plastic or metal ferrules.
For decades, this was the termination method for fiber optics—glue the fiber into the connector and polish it. That still is how connectors are installed in factories making patchcords today. With the advent of the ceramic ferrule, most of the experimentation focused on the glue—heat-cured epoxy, fast-curing anaerobic or hot-melt adhesives—trying to speed up the termination process.
The adoption of single-mode fiber for outside plant networks in the mid-1980s made field termination much more difficult. Single-mode connectors required tolerances much tighter than multimode to get low loss and a different polishing technique to get low reflectance. Field-polished adhesive/polish connectors could not achieve the performance necessary for single-mode terminations. As a result, installers started using fusion-splicing to attach factory-made pigtails, a practice that remains in use today.
In the late 1990s, prepolished/splice connectors appeared. They were factory-polished connectors with a mechanical splice in the connector body. Rather than gluing and polishing, you cleaved the fiber, inserted it into the connector, crimped it, and then you were done.
Prepolished/splice connectors had several problems, including loss, yield and cost. The big problem was the cleaving of the fiber to get low loss in the mechanical splice. In those days, termination kits for prepolished/splice connectors included a fiber cleaver that looked like an office stapler. If you did not have the touch to use it, it was just about as good as a stapler at cleaving fiber—which is to say awful! I recommended that users of these connectors invest in a good cleaver for fusion-splicing that would produce quality results.
Manufacturers knew this problem, but solutions came slowly. Kits started having quality cleavers and tools that simplified assembly. More recently, prepolished/splice connector kits have gotten quite good and easy to use but are still quite expensive. Connectors remain expensive because they are hard to make.
Recently, a new connector termination method appeared. Like prepolished/splice connector, these connectors were factory-assembled and polished. Instead of a mechanical splice in the connector, it had a short fiber pigtail, already cleaved and ready to fusion-splice to a fiber. Special fixtures for splicing machines from several manufacturers made assembly of these connectors as simple as splicing two fibers. Splice protection was part of the connector, so splice closures were not needed. The simple design also dropped the price to a fraction of the cost of the older prepolished/splice connectors.
Talk about good timing! These connectors came along about the same time that fiber to the home was really taking off, and data centers and passive optical LANs started using single-mode fibers in premises applications. A data center can have hundreds of thousands of terminations, so installation speed and cost are of the utmost importance.
Even better, a number of low-cost, portable fusion splicers appeared. The cost of these splicers was only little more than a typical termination kit for prepolished/splice connectors. Along with the lower connector cost, the appeal to contractors and installers was obvious.
These connectors, now known as splice-on connectors, may well become the dominant field-termination method. They are faster, cheaper and easier with better performance—what more could you ask for?