Fiber has the ability to propagate light launched into it with very low loss. In addition, virtually none of the light escapes the fiber along its walls, provided the recommended minimum bending radius specifications are not violated.

Fiber optics are generally constructed from a very pure form of glass, although some forms of low loss plastic fiber are currently under development. Signals used for transmission on fiber are generally in the red/infra-red range of the spectrum. This is because the materials currently being used for fiber manufacturing exhibit lowest losses in this range of wavelengths.

The ability of fiber to contain and propagate the light signal comes about from Snell’s Law of Refraction. Snell’s Law basically states that total internal reflection occurs for light travelling in a medium when it encounters a boundary to a medium of lower refractive index, at an angle greater than what is known as the critical angle. Fiber optic is manufactured in such a manner that the refractive index of the material is greatest in the center of the core and decreases towards the edges. Hence light travelling inside is totally reflected whenever it approaches the edges of the fiber.

The importance of not exceeding the fiber’s minimum recommended bending radius specifications can be seen. If this bending radius is exceeded, the critical angle criterion will not be met and some of the light will escape. This will manifest itself as a loss in the circuit. On long underground routes, it is important to ensure that if the fiber cable is directly buried, it is not placed under excessive stress by the fill material. Pressure from the fill material can cause localized sharp bends in the fiber cores. This can cause un-budgeted loss increases due to escaping light, to a point where the cable becomes unworkable. It then becomes a very expensive excercise to dig it up again to rectify it.