Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 10

Years: MAY 2015

Attenuation of light signal as it propagates along a fiber is an important consideration in the design of an optical communication system, since it plays a major role in determining the maximum transmission distance between a transmitter and a receiver or in-line amplifier.

The longer the fiber is and the farther the light has to travel, the more the optical signal is attenuated. Consequently, attenuation is measured and reported in decibels per kilometer (dB/Km) also known as attenuation rate or attenuation coefficient.

Attenuation varies depending upon the fiber type and the operating wavelength.

• Figure below shows three optical windows which offer minimum signal attenuation and also relationship between attenuation and wavelength.

• The first optical window is defined from 800-900nm, where the minimum signal loss is 4dB/km. In early 1970’s this window was used for operation of optical sources and detectors.

• By reducing the concentration of hydroxyl ions and metallic impurities in the fiber material, in 1980’s manufacturers were able to fabricate optical fibers with very low loss in the 1100-1600nm region. This spectral band is called long wavelength region.

• The second optical window is centered at 1310nm also called O-band, which offers 0.5dB/km.

• The third optical window is centered at 1550nm also called C-band, which gives the loss of 0.2dB/km.

• Hence while designing optical systems for long distance applications the 1550nm wavelength is preferred because loss offered at this wavelength is minimum than any other wavelength.

For silica based optical fibers, single mode fibers have lower attenuation than multimode fibers.The higher the wavelength the lower is the attenuation. This is true over the typically 800-1600nm operating wavelength range for conventional datacom and telecom optical fibers.

Causes of Attenuation:

The basic attenuation mechanism in a fiber is;

Absorption: This is related to fiber material.

Scattering: It is associated with both the fiber material with structural imperfections in optical waveguide.

Bending (radiative losses): Attenuation owing to radiative effects originates from perturbation in fiber geometry (both microbending and macrobending)

Dispersion: Due to the modes.