Flexible transparent fiber devices, sometimes called lightguides, used for either image or information transmission, in which light is propagated by total internal reflection. In simplest form, the optical fiber or lightguide consists of a core of material with a refractive index higher than the surrounding cladding. The optical fiber properties and requirements for image transfer, in which information is continuously transmitted over relatively short distances, are quite different than those for information transmission, where typically digital encoding of information into on-off pulses of light (on = 1, off = 0) is used to transmit audio, video, or data over much longer distances at high bit rates. Another application for optical fibers is in sensors, where a change in light transmission properties is used to sense or detect a change in some property, such as temperature, pressure, or magnetic field. See Reflection of electromagnetic radiation, Refraction of waves
There are three basic types of optical fibers. Propagation in these lightguides is most easily understood by ray optics, although the wave or modal description must be used for an exact description. In a multimode, stepped-refractive-index-profile fiber (illus. a), the number of rays or modes of light which are guided, and thus the amount of light power coupled into the lightguide, is determined by the core size and the core-cladding refractive index difference. Such fibers, used for conventional image transfer, are limited to short distances for information transmission due to pulse broadening. An initially sharp pulse made up of many modes broadens as it travels long distances in the fiber, since high-angle modes have a longer distance to travel relative to the low-angle modes. This limits the bit rate and distance because it determines how closely input pulses can be spaced without overlap at the output end.
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A graded-index multimode fiber (illus. b), where the core refractive index varies across the core diameter, is used to minimize pulse broadening due to intermodal dispersion. Since light travels more slowly in the high-index region of the fiber relative to the low-index region, significant equalization of the transit time for the various modes can be achieved to reduce pulse broadening. This type of fiber is suitable for intermediate-distance, intermediate-bit-rate transmission systems. For both fiber types, light from a laser or light-emitting diode can be effectively coupled into the fiber. See Laser, Light-emitting diode
A single-mode fiber (illus. c) is designed with a core diameter and refractive index distribution such that only one fundamental mode is guided, thus eliminating intermodal pulse-broadening effects. Material and waveguide dispersion effects cause some pulse broadening, which increases with the spectral width of the light source. These fibers are best suited for use with a laser source in order to efficiently couple light into the small core of the lightguide and to enable information transmission over long distances at very high bit rates.
A special class of single-mode fibers comprises polarization-preserving fibers. In an ideal, perfectly circular single-mode fiber core, the polarization state of the propagating light is preserved, but in a real fiber various imperfections can cause birefringence; that is, the two orthogonally polarized modes of the fundamental mode travel at different speeds. For applications such as sensors, where controlling the polarization is important, polarization-maintaining fibers can be designed that deliberately introduce a polarization. This is typically accomplished by using noncircular cores (shape birefringence) or by introducing asymmetric stresses (stress-induced birefringence) on the core. See Birefringence, Polarized light
The attenuation or loss of light intensity is an important property of the lightguide since it limits the achievable transmission distance, and is caused by light absorption and scattering. Optical fibers based on silica glass have an intrinsic transmission window at near-infrared wavelengths with extremely low losses. Glass fibers, intrinsically brittle, are coated with a protective plastic to preserve their strength. See Optical materials
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