The method used to terminate a fiber optic component dictates the final quality, its performance, and its limitations in application. The overview below describes four different methods and their associated cost impact.
GENERAL EPOXY
The most common method of terminating a fiber bundle employs a ferrule (hollow metal or plastic block or tube to contain fibers) and epoxy. The epoxy keeps the fibers contained in the ferrule and serves 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 technician will be successful grinding and polishing the fiber face. A properly ground and polished face will create a high quality, low loss termination. Epoxy also prevents liquid, dust and debris from entering the assembly, which are contributing factors of fiber deterioration.
Several different types of epoxy are available. Each type has different working characteristics which effect the cost and sometimes, performance 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 semi-transparent color when cured, a light absorbing and dispersing characteristic not good for use with some applications.
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. Maximum working temperature can match the softening point point of the glass fiber (1100F)
MECHANICAL
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 allow glass fiber optic components to be used in applications exposed to the softening point of the fiber material (Borosilicate glass – 914F; Plastic – 158F; Polyimide buffered silica – 500F). See below for further insight.
FUSING
Because the ends of the fiber bundle are softened, squeezed together and fused, the interstitial spaces between individual fibers have been eliminated, reducing the active diameter of the bundle. The amount of diameter reduction is 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 raw material.
Because the fiber bundle diameter is reduced, some believe it’s possible to improve coupling efficiency, and maintain the original bundle diameter, by adding more fiber. 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, because fibers at the circumference are broken (crushed) during the fusing process.
Fusing offers the best potential to polish. A fused bundle will not burn, melt, or crater. 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.
Please note: due to the higher melting point of quartz (silica fiber), typical fusing is not an standard option with components made from this material… If fusing is required for a quartz application, it can be done with special fiber construction.
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.
SWAGING
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. Additionally, Pull resistance is significantly compromised; there is no chemical bond between the ferrule and fibers.
HOT KNIFE
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 or cleave the fiber at 90º to the ferrule. When fiber is not finished at 90 degrees to the ferrule, a skewed end tip is created, which will reduces emitting accuracy and coupling efficiency.
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.
