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.