This article introduces the production process of fiber bundles from fiber being, fiber drawing, fiber arrangement to assembly.

One way to make coherent fiber bundles. Start by looping a single long, thin fiber many times around a spool, glue the fibers together in one spot and remove them from the spool. Then cut through the glued region. This gives a flexible bundle, with fibers loose in the middle and fixed on both ends. Because the two ends were originally adjacent to each other, the fibers are all in the same positions.
This approach is simple in concept and dates back to the mid-1950s, when it was used to make the first flexible fiber bundle. However, it is a demanding process and is difficult when using very thin fibers, which are likely to break.

An alternative approach is to draw many fibers simultaneously to finer and finer diameters in a series of stages. The first step is to draw a step-index fiber with a diameter about 2.5 mm. These fibers are easy to handle, and a group of them— typically 37 to 169— are
grouped together, heated until they soften, and stretched out into a rigid multifiber about 2 mm in diameter, as shown in Figure 30.2. Then a number of multifibers (typically 61 to 271) are packed together, heated, and drawn again to produce a rigid fiber bundle, containing many thousands of fibers. Each fiber in the final bundle is about 3 to 20 pm in diameter. The number of fibers drawn together in each step is chosen to make patterns that pack together well.

The fused fiber bundle process can be used to make flexible bundles, with a few important changes. And you will note that the large core is surrounded by two rings of cladding. One is the conventional low-index cladding that confines light to the core in all fibers. The composition of the other depends on the type of bundle being made.
For rigid bundles, that outer layer is a dark absorptive glass that keeps light from leaking between the cores in the bundle. A certain amount of light always leaks into the fiber cladding. Usually this stays in the inner part of the cladding, but for imaging bundles the
cladding is quite thin. If the claddings were all fused together— as they would be without the dark glass— the light could freely disperse through the whole bundle within the fused cladding glass. Then it could leak back into the cores and degrade the image.
For flexible bundles, that outer layer is a glass that is soluble in acid. Manufacturers cover the ends of the rigid rod and then dip the whole fused bundle into an acid that dissolves away that leachable layer in the middle of the rod, leaving a flexible bundle of many thin fibers, which are arranged so their ends are aligned for imaging.
Individual fibers in a flexible coherent bundle can be small, but not quite as small as in a fused bundle. Some performance limits of flexible bundles are comparable to those of rigid bundles. When flexible bundles are used, an added concern is breakage of individual
fibers, which does not occur in fused bundles. Each fiber break prevents light transmission from one spot on the input face. The loss of a single fiber is not critical, but as more fibers break, the transmitted light level drops and resolution can decline as well. Eventually breakage reaches a point where the image-transmitting bundle is no longer usable. Plastic fibers can reduce the breakage problem, but have other limitations.

Randomly aligned bundles are made by collecting many fibers into a bundle, much like collecting strands of spaghetti. This would be very difficult if the fibers were as thin as those in imaging bundles, but such fine fibers are not needed because the resolution does not
matter; random bundles serve purely as “light pipes.” Typically, random bundles are made of fibers with diameters in the 100-pm range, which are flexible enough to bend freely with minimum fiber breakage.