Basic Characteristics

Geometry ( Figure 11.12 ). An optical fiber is a cylindrical dielectric structure that can guide a light beam over distances ranging from tens of meters to tens of kilometers. It mainly consists of a cylindrical core (radius a, refractive index n ₁) surrounded by a cladding of slightly lower refractive index n ₂.

Figure 11.12: Optical fiber geometry.

Refractive Index Profiles. The refractive index distribution in the transverse direction is given by:

(11.41a)

(11.41b)

where the index contrast ? is defined as:

(11.42)

Optical fibers are classified according to the value of the parameter g, which can be varied to get different profiles: are presently used for short-distance applications (less than 100 m) due to their higher losses. Two typical loss spectra are represented in Figures 11.13 and 11.14 for silica fiber and plastic fiber, respectively.

Figure 11.13: Spectral attenuation in silica fiber.

Figure 11.14: Spectral attenuation in plastic fiber.

• Step index fiber ( g ?)

• Graded index fiber: g = 1 (triangular profile), g = 2 (parabolic profile), etc.

Spectral Loss. Depending on the application, two kinds of dielectric material are currently used to manufacture optical fibers: silica (with dopants) and plastic. The fibers with core and cladding made from silica exhibit low loss and are systematically used for long-distance applications. Plastic-clad fibers and...