Understanding Fiber Optics


WHAT IS AN OPTICAL FIBER ? Fiber structure and fiber types - An optical fiber is made of 3 concentric layers as described on the following sketch:

• core: This central section, made of silica, is the light transmitting region of the fiber. • cladding: It is the first layer around the core. It is also made of silica, but not with the same composition as the core. This creates an optical waveguide which confines the light in the core by total reflection at the core-cladding interface. • coating: It is the first non-optical layer around the cladding. The coating typically consists of one or more layers of a polymer that protect the silica structure against physical or environmental damage.

core cladding coating

• Multimode fiber

• Singlemode fiber

This type of fiber is called “multimode” because light rays travel through the fiber following different paths called “modes”.

In that case, the fiber is called “singlemode” because only one mode is propagated. It travels “straight” through the fiber. The core diameter is typically 9 µm.

Multimode fiber

Singlemode fiber 9 / 125

50 / 125 or 62,5 / 125

cladding diameter in microns ( µ m)

cladding diameter in microns ( µ m)

core diameter in microns ( µ m)

core diameter in microns ( µ m)


Attenuation and wavelength Light is gradually attenuated when it is propagated along the fiber. The attenuation value is expressed in dB/km (decibel per kilometer). It is a function of the wavelength ( λ ), i.e. of the color (frequency) of the light. That means that the operating wavelength to transmit a signal in an optical fiber is not any wavelength.It corresponds to a minimum of attenuation. The following graph gives the linear attenuation as a function of the wavelength:

The operating wavelengths, which light sources have been developed for, are 850 nm (nanometers) and 1300 nm in multimode, and 1310 nm and 1550 nm in singlemode. Example: For a 850 nm operating wavelength, there is a 3 dB light attenuation after 1 km propagation (according to the graph). 3 dB mean that 50% of the light has been lost.

1 2 3 4 5

wavelength (nm)

850 nm

1300 nm

1550 nm


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