Optical Fibers are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances.

    The Optical Fiber used in communication Systems consists of a glass core and a glass cladding. The outer most layer is a plastic wear resisting coating. The whole fiber is cylindrical. The typical structure of Optical Fiber and Cable is shown in figers.

Core - Thin glass center of the fiber where the light travels.

Cladding - Outer optical material surrounding the core that reflects the light back into the core.

Outer Jacket - Plastic coating that protects the fiber from damage and moisture

    There are basically two modes of a transmission fiber.

Single mode fiber has a light ray in only one direction.

Multimode fiber has a number of paths in which light ray may travel.

    It is internationally agreed that DWDM systems will only utilize single mode fiber as transmission media. Because the single mode fiber has advantages of low internal attenuation, large bandwidth, easy upgrade and capacity expansion and low cast. At present, ITU-T has defined four types of single mode optical fiber with different design recommendations G.652, G.653, G.654, G.655.

    G.652 is for extensive use, called 1310nm property optimal single mode fiber and also called dispersion un shifted fiber.

    G.653 is called as dispersion shifted fiber or 1550nm property optimal fiber. Used for ultra high speed and ultra long distance optical transmission.

    G.654 fiber is cut-off wavelength shifted single mode fiber. It is mainly designed to reduce the attenuation at 1550nm . Its zero dispersion point is still near to 1310nm.The dispersion at 1550nm is relatively high, up to 18ps/(nm.km). Mainly used for submarine optical fiber communication with very long regenerator section distance.

    G.655 fiber, a nonzero dispersion shifted single mode optical fiber, is similar to G.653 fiber and preserves certain dispersion near 1550nm to avoid four wave mixing phenomenon in DWDM transmission. It is suitable for DWDM system application.

Attenuation in Optical Fiber

    Attenuation in optical fiber is mainly determined by three types of loss: absorption loss, scattering loss and bend loss.

       Absorption loss is caused by the fiber material where excessive metal impurity and OH-ion absorb the light to result in loss.

        Scattering loss is often caused in the case that a part of optical power is scattered outside the fiber when uneven refractive index distribution local area emerges within the fiber and causes light scattering because of the micro change in fiber material density and uneven density of composition such as SiO2, GeO2 and P2O5. It can also occur when some bubbles or gas scabs are remained at the core cladding boundary.

        Bend loss:- Bend of the optical fiber will cause radiation loss. Bend may occur during the laying of optical fiber or in other case bend may be caused during production of the fiber and the cable, called as microbend.

The bend loss of G.652 fiber should not be larger than 1dB at 1550nm window.

The bend loss of G.655 fiber should not be larger than 0.5dB at 1550nm window.

Dispersion in Optical Fiber

        It refers to physical phenomenon of signal distortion caused when various modes carrying signal energy or different frequencies of the signal have different group velocity and disperse from each other during propagation. There are three kinds of dispersion in optical fiber

a) Modal dispersion:- This is caused when the fiber carries multiple modes of the same frequency signal energy and different mode has different time delay during transmission.

b) Material dispersion:- Since refractive index of the fiber core material is a function of the frequency, signal components of different frequency propagate at different velocities along the fiber, causes dispersion known as Material dispersion.

c) Waveguide dispersion:- It occurs in the fibers of carrying different frequencies in the same mode causes dispersion, because of different group velocities during propagation.

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