What is a Fiber Bundle?
This article introduces the types, characteristics, structure and applications of fiber optic bundles.
Optical fibers were invented for imaging and were soon applied to illumination as well. Imaging requires a bundle of fibers, one to carry each point on the image. For imaging, the bundle must be coherent, which in this case means that the ends of the fibers must be arranged in the same way on both ends of the bundle.
Project an image onto one end of a coherent bundle, and the same image appears on the other end.
To visualize how a coherent bundle works, start with a handful of drinking straws all
the same length. With a little care, you can hold the straws so they are aligned parallel
to each other. Look through the straws at a printed page, and you’ll see the words through the array of little pipes. The smaller the straws, the smaller the bit of the page you see through each one. Individual fibers are like individual straws, but they are much thinner and far more flexible. Fibers guide light by total internal reflection, but straws only transmit light straight along their axes.
Two basic families of fiber bundles
There are two basic families of fiber bundles, each developed for distinct applications.
Long, thin flexible bundles of loose fibers are used to examine or deliver light to otherwise inaccessible places. Important examples are the flexible endoscope threaded down a patients throat to examine the stomach, and the flexible colonoscope used to examine the colon. Industrial counterparts are used to inspect the interiors of engines. Imaging requires coherent bundles, but illumination normally is done with bundles in which the fibers are randomly aligned. For most imaging and illumination applications, flexibility is important.
A second family is rigid fiber bundles in which the fibers have been fused together to make a solid block. Processing retains the light-guiding structure of the individual fibers that are aligned end to end in the bundle. Usually they are shorter and fatter than flexible
bundles. These fused bundles can be used as optical devices for transmitting or magnifying images piece by piece, as well as for some types of inspection and for some other optical applications.
Both types of fiber bundles are based on step-index multimode fibers with thick cores and thin claddings. This structure means that light reaching the input face of the bundle is most likely to fall on a core, so it is transmitted to the other end of the bundle. Individual
fibers may be drawn quite thin, but the ratio of core to cladding thickness remains unchanged. The difference between core and cladding index is larger than in communication fibers, so the cores can be drawn finer and still transmit multiple modes of light.
Single-mode transmission is not desirable in imaging or illumination fibers because it limits how much light they can collect at the face of the bundle.