There are many differnt types of fiber optic cable; as shown in Fig.1, it is possible to package multiple fibers into a single unibody cable or into a ribbon or zipcord structure. Bundles of fibers whose ends are bound to gether, ground, and polished can form flexible light pipes. Of course, it is possible to bundle the fibers in such a way that there is no fixed relationshiip between the location of an input fiber and an output fiber; the principle purpose of such structures is to conduct light from one location to another, for illumination as an example; these are sometimes referred to as incother, for illumination as an example; these are sometimes referred to as incoherent bundles, although they have little to do with optical coherence theory. Amore interesting case is when the fibers are carefully arranged so that they occupy the same relative positions at both ends of the bundle; such bundles are said to be coherent. A coherent bundle of single-mode fiber is capable of conducting a high-quality image even when the bundle is made highly flexible; such fiber arrays have many applications in remote vision systems, and are used in fiber optic endoscopes for medical applications. Not all fiber arrays are made flexible; fused, rigid bundles or mosaics can be used to replace low-resolution sheet glass in cathode ray tubes. Mosaics consisting of hundreds to millions of individual fibers with their claddings fused together have mechanical properties very much l ike homogeneous glass. Another common application of mosaics is as a field flattener.
If the image formed by a lens system falls on a curved surface, it is often desirable to reshape it into a plane, for example to match a photographic film plate. A mosaic can be grund and polished on one end surface to correspond with the contours of the image, and on the other surface to match the configuration of the detector. Similarly, a sheet of fused tapered fibers can be used to either magnify an image or miniaturize an image, depending on whetther the light enters the smaller or larger end of the fibers.
Many simple devices such as Fiber Optic Splitters, couplers, and combiners have been manufacured; the most common techniques include fiber taaperingtapering. Other fabrication techniques can also be used, including micro-optics and integrated optical components; however, optical fiber devices are particularly useful because they can be inserted into existing networks as just another piece of cable. One of the most common devices is a tapered fiber optic power splitter, often implemented in single-mode fiber. In this process, two glass fibers with their protective jackets removed are brought close to gether and parallel to each other, then fused and stretched using a torch or similar heat source. Light that is initiallylaunched into only one fiber will be partially coupled into the adjacent fiber as it propagates through the tapered region. Light propagating in the single-mode fiber is not confined to the core but extends into the surrounding cladding. In the case of a fiber taper it has been shown that light propagating through the input fiber core is initially transferred to the cladding interface as it enters the tapered region, then to the core-cladding mode of the adjacent fiber. The light transfers back to the core modes as it exits the tapered reion. This is known as a cladding mode coupling device. Light that is transferred to a higher order mode of the core-cladding structure is readily stripped away by the gigher refractive index of the fiber coating, resulting in excess attenuation. The simplest case of light coupling from the cladding of one fiber into another through a fused taper can be described to a good approximation by the scalar wave equation and first-order perturbation theory; if light is propagating along the axis, then the exchange of optical power, p, is given by
Where is propagation distance and cladding, material properties, and the overlap distance between the two fibers. Although this is only an approximation and neglects higher order terms, it does reflect the sinusoidal dependence of coupled power on wavelength and the dependence of power transfer on cladding diameter and other effects. Tapered couplers can be used to separate wavelengths using this dependence; by proper choice of the device length and taper ratio, two wavelengths can be made to emerge from two differnt output ports. Some applications include filters for wavelength division multiplexing (WDM) systems, or multiplexing signal and pump beams in an erbium-doped fiber amplifier. In some cases, such as Fiber Splicer, it is more desirable to remove the dependence of coupled power on wavelength; acromatic couplers can be fabricated by using two fibers with different propagation constants. These are known as dissimilar fibers; in most cases, fibers are made dissimilar by changing their cladding diameters or cladding indices. In this case, the preceding equation for coupled power must be modified and the power vs. distance is not simply sinusoidal, but becomes much more comlex.
Other approaches are also possible, such as tapering the device such that the modes expand well beyond the cladding boundaries, or encapsulating the fibers in a third material with a different refractive index. Often, it is desirable to third material with a different refactive index. Often, it is desirable to taper multiple fibers together so that an input signal is split between many output fibers. Typically, a single input is split into outputs, where the configuration of fibers in the tapered region affects the output power distribution; care must be taken to achieve a u niform optical power distribution among the output fibers. The optical power coupled from one fiber into another can also be changed by bending the tapered by bending the tapered device at its midpoint; into another can also be changed by bending the tapered device at its midpoint; this frustrates coupled power transfer. For example, displacing one end of a 1 cm long taper by only 1 mm can change the coupled power by over. Applications for this effect include variable optical attenuators and optical switches.