Basic Principles of Optical Fibers and Cables

Optical fiber transmission is based on the principle of total internal reflection, which occurs when light encounters the interface between two different media. In a step-index optical fiber-where n1 represents the refractive index of the core medium and n2 represents the refractive index of the cladding medium, with n1 > n2-total internal reflection occurs when light entering the core strikes the core-cladding interface at an angle of incidence greater than the critical angle (θc). Under these conditions, no light energy escapes the core, and the incident light is able to propagate forward through the interface via countless total internal reflections.

However, when an optical fiber is bent, the normal to the interface shifts direction, resulting in smaller angles of incidence. Consequently, the angle of incidence for a portion of the light rays falls below θc, preventing them from undergoing total internal reflection. Nevertheless, those rays with initially larger angles of incidence can still undergo total internal reflection; thus, light transmission persists even when the fiber is bent, though this bending does result in energy loss. Typically, when the bending radius exceeds 50 to 100 millimeters, the resulting loss is negligible. Conversely, minute bends can cause severe "microbending loss."

Electromagnetic wave theory is frequently employed to further investigate the mechanisms of optical fiber transmission, specifically by solving the wave equations subject to the boundary conditions of the optical fiber waveguide medium. The light propagating within an optical fiber comprises numerous "modes," each representing a specific distribution of the electromagnetic field and corresponding to a particular light ray as described in geometric optics. The specific guided modes present within an optical fiber depend on the fiber's normalized frequency (V-number).
In this context, NA denotes the Numerical Aperture, a parameter related to the refractive indices of the core and cladding media; *a* represents the radius of the fiber core; and λ represents the wavelength of the transmitted light. When an optical fiber is bent, mode coupling occurs; a portion of the energy shifts from the guided modes into radiation modes, thereby escaping the core and being lost.
Performance Characteristics: The primary parameters characterizing an optical fiber include attenuation, bandwidth, and others.