Unlike copper wire, which can carry electrical signals of any frequency from DC to many megahertz, fiber optic cable is designed to carry light in a range of optical wavelengths. These wavelengths are typically from around 800 to over 1500 nanometers (nm). Operating characteristics of a particular optical fiber are normally specified at discrete wavelengths, which correspond to the output wavelengths of common light-emitting diode (LED) and semiconductor laser emitters/transmitters. Multimode fiber is normally used at 850 nm or at 1300 nm, while single-mode fiber is used at 1310 nm or 1550 nm. For any given installation, only one mode and one wavelength are used. The fiber cable should be characterized for operation at that mode and wavelength.
Optical fiber is rather like a waveguide for light. While RF waveguides are hollow with metal sides, optical fiber is solid. During manufacture, the refractive index of the glass is made to vary with the diameter of the fiber core. A difference in refractive index is what makes light actually bend between air and water, for example. The light is sent down the fiber and literally bounces off the “walls” formed by the step in the refractive index. In a single-mode fiber, the transmitting emitter is a laser diode, chosen so that the light entering the fiber will be coherent and straight along the axis of the fiber. A multimode fiber, on the other hand, often uses a graded refractive index. Light entering from an LED (not a laser) enters at all angles, but is coached down the fiber by the gradient of refraction. Multimode fiber is also available in stepped index, but the step occurs much nearer the outer diameter than with single-mode fiber.
there is characteristics of Optical fiber that can carry very large bandwidths of information at low attenuation. Multimode fiber can be used at distances exceeding 3000 m, although other constraints, such as transmitted bandwidth, may limit operation to less distance.