End-emitting Fiber End-emitting fiber is generally a step-index multimode fiber with a large transparent core that transmits the light and thin transparent cladding that traps the light in the core in an optical process called [total internal reflection." The core is large in comparison to the thin cladding as that makes it more efficient in coupling light from the illuminator. The cladding does not transmit light, so any light coupled into the cladding will not be transmitted by the fiber. End-emitting fibers are generally made from plastic as it can be made in larger sizes than glass and is less expensive and easier to install. Plastic Optic Fiber (POF) is made in sizes from 0.1 to 20 mm in diameter. Glass fibers are generally made in much smaller sizes (hair thin, about 50-150 microns or 0.05 to 0.15 mm) and bundled together to make larger diameter cables. The choice of core and cladding materials determines the angle of light rays accepted from sources and transmitted by the fiber (called modes), defined by a specification called numerical aperture(NA). The light will exit the fiber in a cone that shows the size of the NA with larger NA having a wider output cone of illumination. Higher NA fibers also couple light from sources more efficiently, as it will capture light at higher angles emitted by the source. Typical fibers have acceptance cones of 30-60 degrees, corresponding to NAs of 0.3-0.6. When optics are used to focus the light emitted from the fiber, the NA of the fiber must be known to choose appropriate optics.
Side emitting optical Fiber Side-emitting fiber is basically similar to end-emitting fiber except the core/cladding boundary is designed to be slightly inefficient. Instead of trapping all the light in the core, the boundary is rough and some light is scattered into the cladding where it becomes visible. By careful design, the fiber can have a smooth glow that looks much like a neon light tube. Smaller edge-emitting fibers have been woven into tapes that emit light in a band. Since much of the light is lost by the edge-emission along the fiber, edge-emitting fiber has high attenuation. This may limit the lengths of edge-emitting fiber that can be used. This can be alleviated by illuminating the fiber from both ends by using two Illuminators or looping the fiber back around to the same illuminator, or using reflective end caps to send excess light back up the fiber from the far end.
Fiber Optic Light
