Dispersion in Optical Fiber / Notes on Dispersion

Dispersion in Optical Fiber

Various wavelengths of the signal have different propagation velocities. Dispersion is the spreading out of a light pulse in time as it propagates down the fiber. Dispersion limits the information carrying capacity of a fiber. It is measured as ps/nm.km

Types of Dispersion

–Intermodal dispersion
–Material dispersion
–Waveguide dispersion.
( Also read about losses in optical fiber )

Intermodal Dispersion:

•Each mode enters the fiber at a different angle, take different path, travel different distance and arrive at different time at the fiber output.
•Multimode fibers have many different light modes since they have much larger core size.
•The light pulse spreads out in time which can cause signal overlapping.

Material Dispersion

•Dependence of refractive index of the material on wavelength.
•The velocity of light through a fiber depends on its wavelength.
•Different wavelengths travel at different velocity due to refractive index, hence the different propagation causes material dispersion.
•Material dispersion at 1300nm for silica is zero.

Waveguide Dispersion

•Distribution of light between core and cladding is a function of the wavelength travelling in waveguide.
•Light ray that travels in the cladding travels faster than that in the core causes waveguide dispersion.
•Single mode fiber suffers from waveguide dispersion as it has small core diameter

Chromatic Dispersion

•Combined effect of Material and Waveguide Dispersion

Polarization Mode Dispersion (PMD)

•Asymmetry, due to Mechanical and thermal stresses during fiber manufacturing introduces small index of refraction differences for the two polarization states.
•Polarization Mode Dispersion (PMD) a form of modal dispersion, causes broadening of the input pulse due to a phase delay between input polarization states.

Effect of Dispersion

•Spreading of pulses which can cause Inter Symbol Interference ‘ISI.
•Detection of individual pulse is not easy at receiver.
•Poor BER performance.
•Limit the communication distance.
•Limit the transmission rate.

Losses in Optical Fiber

Signal Degradation in Optical Fiber :

• Attenuation and dispersion determine the maximum distance an optical signal can be transmitted before the receiver is unable to detect it.
• The attenuation and dispersion of a fiber are wavelength dependant.

Transmission Losses in Fibers:

Transmission loss or attenuation of the signal in an optical fiber is measured in dB/km.

Types of Losses

–Material Absorption

–Rayleigh scattering

–Waveguide imperfections

–Radiative loss

Material Absorption

•Intrinsic Absorption:

–Intrinsic absorption losses correspond to absorption by fused silica.

–intrinsic material absorption for silica in the wavelength range 0.8~1.6um is below 0.1dB/km.

•Extrinsic Absorption:

–Extrinsic absorption is related to losses caused by impurities within silica.

–The main source of extrinsic absorption silica fibers is the presence of water vapors.

Rayleigh Scattering

•Silica molecules move randomly in the molten state and freeze in place during fiber fabrication.

•Density fluctuation lead to random fluctuations of the refractive index.

•Light scattering in such a medium is known as Rayleigh scattering.

Waveguide Imperfections

•Imperfections at the core-cladding interface, such as random core-radius variations, can lead to losses.

•This has been taken good care of in optical fiber manufacturing and the core radius is made sure not to vary significantly along the fiber length.

Radiative Losses

•Radiative losses occur whenever an optical fiber undergoes a bend of finite radius of curvature.

•Fibers can be subjected to two types of bends.

–Macroscopic bending: 

Bends having diameter smaller than the specified by manufacturer e.g. when a fiber cable turns a corner.

–Microscopic bending: 

Random microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables, e.g. deformation of axis.

Gap- Loss :

•Gap-loss happens when there is a space, breakage, between fiber connection.

•Light can cross this gap, but spreads out and is weakened and diffused