The method used to terminate a fiber optic component dictates the final quality of that part, its performance, and its limitations in application. The overview below describes four different methods and their associated cost impact.
The most common method of terminating a fiber bundle employs a ferrule (hollow metal or plastic tube to contain fibers) and epoxy to keep the fibers contained in the ferrule and serve as a grinding and polishing fixture. As long as the ID of the “tube” is sized for the required bundle, and the epoxy is mixed, applied, and cured properly, the resulting combination allows the technician to grind and polish the end, creating a high quality, high transmission termination. This method keeps the fibers secure, prevents dust and debris from entering the assembly or acting as a heat sync, all contributing factors to fiber deterioration.
Several different types of epoxy are available. Each has different working characteristics that may effect the final cost of your part. The two most common types used in general construction are selected for workability and resistance to heat. The epoxy used for higher temperature resistance has a limit of 600ºF continuous exposure. While this high temperature termination has good heat resistance, it darkens to a deep red color when cured, a light absorbing characteristic not good for use with some fibers.
There is minimal if any increase in cost using the common high temperature epoxy.
Heat resistance can be further increased using very special epoxies. However the method of application, the care required during application, and the cost of material all contribute to increase the labor and material cost of the part.
Because high temperatures in some applications will melt epoxy, Terminations made without epoxy are available. Mechanical swaging or fusing can be used for these applications; but each method has its pros and cons. Both methods permit the glass fiber optic component to be used in applications exposed to the melting point of Borosilicate glass (900ºF).
Please note: due to the higher melting point of quartz (silica fiber), typical fusing is not an option with components made from this material.
Conversely, plastic fiber can be easily fused, but fusing plastic fiber offers no advantage in high temperature applications above 70ºC (the melting point of plastic fiber). Sometimes, plastic fusing will be employed to improve coupling efficiency, as the technique removes the interstitial spaces between the fibers in a bundle.
Finally, Hot Knifing, a field termination technique for plastic fiber, will be reviewed. FTI does not recommend hot knifing as a viable method of termination.
Because the ends of the fiber bundle are softened, squeezed together and fused, the active diameter of the bundle is reduced, because the interstitial spaces between individual fibers have been eliminated. The amount of reduction is about 10-14% depending on the pack of the fibers and their starting size. Once fused, fiber ends can withstand temperatures up to the softening point of the glass combination.
Because the fiber bundle diameter is reduced, by adding more fiber, some believe it's possible to improve coupling efficiency, and maintain the original bundle diameter. The bundle starts out larger than required, and is then fused to the finished size. While there is more fiber per area than a non-fused version of the same part, light transmission is about the same, as fibers are broken during the fusing process.
Because the fiber ends are fused together, this mechanical method offers the best potential to polish. However, due to slow processing time and high capital investment, fusing is also the most expensive mechanical terminating process, adding significant cost to small run orders.
Swaging does less physical damage than fusing, but this technique limits the ability to affect a good final polish, which reduces transmission when compared to the same bundle with polished end. Swaging requires special finishing techniques which increase the cost of the finished part, but not to the extent of the price increase incurred by fusing.
Parts terminated using a swaging process have the same temperature resistance as fusing, however, extra care should be taken in the application, as interstitial voids remain in the bundle face, which could trap or hold dirt and moisture.
Hot knife termination is a field terminating technique common in commercial lighting applications with plastic fiber only. This simple technique requires plastic fibers to be hand gathered in the ferrule tube, then cut flush with a heated razor knife. Recognized for its convenience and speed, this termination method is popular with installers and contractors who custom make the fiber harness in the field.
However, because the termination happens at the jobsite, great care must be taken to keep the termination clean. It’s also very difficult to achieve an optimal pack and cleave the fiber at 90º to the ferrule, creating the same effect as skew.
Hot knifed terminations are not polished, which further compromises transmission efficiency. Therefore this technique creates a marginal termination, with the greatest potential for early failure. When at all possible, all fiber optic manufacturers recommend investing the extra effort to plan and order pre-finished plastic fiber harnesses for long life and high transmission performance.
For industrial fiber optic applications, discuss the application with the sales engineer to develop the optimum termination method for your application.
For commercial lighting, take care when terminating in the field. Cutting on a bias and the introduction of dirt in the fiber will reduce light output and/or cause premature failure of the harness.