Atlantic Fibre Optics

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Optical description

Optical fiber is a cylindrical structure that transmits light along its axis. The fiber consists of a core surrounded by a cladding layer. Like other glasses, the glass used in optical fiber has a refractive index of about 1.5. For the fiber to guide the optical signal the refractive index of the core must be slightly higher than that of the cladding, though typically the difference is less than one per cent. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber.

A diagram which illustrates the propagation of light through a multi-mode optical fiber.

Fiber with large core diameter, called multi-mode fiber (from the electromagnetic analysis, see below), may be analyzed by geometric optics. In a step-index fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary) are completely reflected. The minimum angle for total internal reflection is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, where they are not useful for conveying light along the fiber. In this way, the minimum angle for total internal reflection determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture makes it easier to efficiently couple a transmitter or receiver to the fiber. However, by allowing light to propagate down the fiber in rays both close to the axis and at various angles, a high numerical aperture also increases the amount of multi-path spreading, or dispersion, that affects light pulses in the fiber.

In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This reduces multi-path dispersion because the high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. Index grading also causes light rays to bend smoothly as they approach the cladding, rather than reflect abruptly from the core-cladding boundary. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.

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Fiber with a core diameter narrower than a few wavelengths of the light carried, is analyzed as an electromagnetic structure. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. The optical fiber is seen as a cylindrical dielectric wave guide. This wave guide supports one or more confined transverse modes by which light can propagate along its axis. Fiber supporting only one mode is called single-mode or mono-mode fiber, while fiber that supports more than one mode is called multi-mode fiber. By the waveguide analysis, it is seen that the light energy in the fiber is not completely confined in the core, but, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.