IAN LANG ELECTRONICS
Attenuation and Distortion of Signals.
Optical fibers are commonly made of plastics (for very short runs) and silica glass for long ones. Each has imperfections due to the materials and manufacturing processes, and we shall study these effects in the following pages.
Attenuation is the loss of power in a signal between the transmitter and receiver and can be measured in decibels per kilometre (dB/km). The decibel is a logarithmic unit and in power calculations 3dB means a doubling of the power if there is a gain, a halving if there is a loss. A loss can also be written as -3dB.
The first cause of attenuation in optical fiber is absorption. Light can be considered as an electromagnetic wave and like all such it has a wavelength. In communications technology we tend to use the infra-red portion of the spectrum in bands A and B, and standardisation is upon the 850, 1300 and 1550 nm "windows". The latter is dominant and reaches from 1530 to 1560nm. This leads to our first topic:
The graph on the left shows the attenuation experienced at different wavelengths in a silica glass cable. It can be seen that the decibel loss generally drops with an increase in the wavelength, until 1600 nm when it begins to rise again. This is intrinsic absorption and it is caused by the vibration of silicon oxide (SI-O) bonds interacting with the magnetic field of the optical signal thus transferring light energy to the atomic bond. Intrinsic absorption therefore generally lessens as the wavelength increases until about 1500nm.
As can be seen the 1300 and 1550 nm "windows" are much less prone to loss than is the 850nm, and for this reason the two former are used for long-distance communications and the latter for medium. 850nm systems have a usual practical range of perhaps 10km, but the advantage is that systems using this wavelength are easier and cheaper to make and install.