Technical and Application Notes

A Reference Guide to Optical Fibers and Light Guides

Bundle Construction

Fiber optic bundles are groups of fibers bound together, typically at the ends only, and encased in a flexible protective jacket. The inside diameter of the protective jacket of a bundle is typically larger than the minimum outside diameter of the combined fibers. This allows the fibers to move freely inside of the jacket.

By leaving the center section of the fiber unbound by epoxy and free to move inside of the protective jacket, the bundle maintains a minimum bend radius close to that of the individual fibers making it up. The protective jacket is rigidly attached to the end adapters or ferrules of the bundle. This configuration removes any tensile strain from the fibers. It also reduces the risk of damage to the fibers from over-twisting the bundle.

Bundle Ends

The ends of a fiber bundle can be arranged into an infinite number of 2-dimensional shapes and configurations. Typically, a ferrule or end adapter of some kind is used to hold the fibers in their desired shape prior to bonding. An epoxy or some other bonding agent is then injected into the fixture. After the epoxy sets, the entire end face is optically polished.

Because of this final finishing step, it is typical to use an epoxy or bonding agent with a set hardness that is close to the hardness of the fibers. This will provide a more rigid, uniform end face, reducing individual fiber breakage during the polishing operation.

If the application is for UV fluorescent excitation, remember to specify a (relatively) non-fluorescent cladding and binder or epoxy. This will prevent auto-fluorescence, in which case the fluorescent emissions from the binder leak into the fiber core, and is transmitted through along with the excitation light.

All fiber optic bundles posses an inherent coupling loss in addition to the losses associated with the individual fibers. When packing fibers into a bundle, space will always be present between the fibers themselves. There will also be space between the outermost fibers and the ferrule or adapter that is containing them.

These spaces are filled in by the epoxy or bonding agent. Light does not get transmitted by the bonding agent, so these spaces do not transmit through the bundle. They are dead space, and a cause of decreased transmission. The percentage of useable area in a bundle is the total fiber core area divided by the area of the end face exposed to the source.

Most fibers that are useable in bundles are available in a wide range of diameters. You can usually work out a good quantity and fiber diameter arrangement that will give the highest useable area for a given bundle size and shape.

 

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