power point presentation on Analysis and Reduction of Polarization Mode dispersion

8/13/2019 power point presentation on Analysis and Reduction of Polarization Mode dispersion

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Analysis and Reduction of

Polarization Mode

Dispersion in an Optical Fiber

D N PREM (11BEC0527)

D DINAKAR (11BEC0313)

S SIVA SAI VAMSI (11BEC

T VENKATA SAI PAVAN (

Submitted to :

Prof Sugumaran s

Slot: B1


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Abstract In this project we deal with the effects and compensation of polarisation mode

dispersion (PDM).

Polarization mode dispersion(PMD) is a form of modal

dispersion where two different polarizations of light in a waveguide,which normally travel at the same speed, travel at different speeds dueto random imperfections and asymmetries, causing random spreadingof optical pulses.

It has found to be a considerable attention in high data rate systems , now a days.

In general a small amount of PMD is to be considered in high data rate systems since itlimits the data rate.

PMD is a time varying and unstable phenomenon.

In this project we analyse the PMD by varying the length and birefringence of the fiberand compensate the effect of PMD using optsim simulation for 10 and 40 giga bits persecond(Gb/s) transmission systems.

Here the compensation of PMD is done by optical compensation method calledDeterministic Differential Group Delay (DDGD).


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POLARISATION :

Polarization, also called wave polarization, is an expression of the orientationof the lines of electric flux in an electromagnetic field ( EM field ). Polarizationcan be constant that is, existing in a particular orientation at all times, or it canrotate with each wave cycle.

Polarizing angle or Brewster's angle: This angle is the angle of incidence

where the reflected light is linearly polarized. Here's the equation:

MODE :

Modes is a mathematical and physical concept describing the propagation ofelectromagnetic waves through media.

In case of fiber, a mode is simply a path that a light ray can follow in travelling

down a fiber. Modes are the possible solutions of the Helmholtz equation for waves, which is

obtained by combining Maxwell's equations and the boundary conditions.

Waves can have same mode but have different frequencies.

There are two modes in optical fiber :1.single mode fiber

2. multimode fiber


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A single-mode optical fiber (SMF) is an optical fiber designed to carry only asingle ray of light (mode).

Fibers that support many propagation paths or transverse modes are called multi-mode fibers(MMF).

In an optical fiber, the normalized frequency, V (also called the V number), isgiven by

where a is the core radius,

is the wavelength in vacuum,

n1 is the maximum refractive index of the core,

n2 is the refractive index of the homogeneous cladding.

In multimode operation of an optical fiber the approximate number of modes isgiven by

where g is the profile parameter, and

V is the normalized frequency, which must be greater than 5 for

the approximation to be valid.


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Dispersion

Dispersion is the spreading of light pulse as it travels down the length of an optical

fiber .

Dispersion is the phenomenon in which the phase velocity of a wave depends on

its frequency or alternatively when the group velocity depends on the frequency. Dispersion limits the bandwidth or information carrying capacity of a fiber.

There are three main types of linear dispersion in a fiber:

1. Modal dispersion

2.Material dispersion

3. waveguide dispersion

Modal dispersion arises because rays follow different paths through the fiberand consequently arrive at the other end of the fiber at different times.

It occurs only in multimode fibers.


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Material dispersion is a type of dispersion that occurs in the optical fiber due to theinteraction of various wavelengths with the physical matter in the crystallinestructure of the glass.

The refractive index of the glass varies according to the wavelength of the opticalsignal i.e. different wavelengths travel at different speeds in the medium.

Waveguide dispersion occurs because optical energy travels in both the core andcladding, which have slightly different refractive indices.

It occurs most significantly in single mode fiber.

There are two types of nonlinear dispersions in a fiber:

1. Polarisation mode dispersion

2.chromatic dispersion

Chromatic dispersion is caused by delay differences among the group velocities ofthe different wavelengths composing the source spectrum.

The consequence of the chromatic dispersion is a broadening of the transmittedimpulses.

It is caused essentially due to Material and Waveguide dispersions.


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Problem def ini tion Increase in PMD causes pulse broadening, inter symbol interference and severely

degrades the system performance.

The PMD of the fiber fluctuates with temperature and stress changes.

It normally increases the bit error rate in long distance communication, since it is time

varying.


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Causes of PMD :A. BIREFRINGENCE:

When the temperature increases, the refractive index varies randomly whichresults in varying speeds.

Thus the refractive index will have a different value across the horizontal and

vertical axis of the fiber core.

This difference in the refractive index will result in two orthogonal states ofpolarization.

This will cause birefringence in which the light gets split up into fast axis (nx)and slow axis (ny), when a ray of light enters a fiber. This phenomenon is also

known as double refraction.


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B. DIF FERENTIAL GROUP DELAY :

The DGD is the difference in propagation times between the two polarizationstates.

This differential time delay between the propagation modes is called as firstorder PMD.

This delay depends on the frequency and varies over the bandwidth.

This will cause further pulse spreading resulting in second order PMD.

Further increase in PMD over long distance transmission and high data rate

transmission systems, higher order PMD will occur which limits the data rate.


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PROPOSED METHOD :Deterministic Differential

Group Delay (DDGD)

This method is based on concept of splitting up of the optical signals into fast

and slow polarization components.

Here the input optical signal gets split up into two branches and sent to the

two polarisers POL1 and POL2. These two polarisers are complementary to

each other.

An optical delay element is inserted to POL1 whereas the signal exiting from

POL2 is not delayed and set to the value orthogonal to POL1.

Finally the two signals are summed up. As a result the twocomponents experience a differential delay.

Since the delay set up is deterministic, it is called as Deterministic

Differential Group Delay (DDGD).


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PMD compensation after DDGD :


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Works to be done : Implementation of problem using OPTSIM.

Analysing the effects of PMD for different data rates

Compensation using DDGD and comparing results

References:

Indian Journal of Science and Technology-Supplementary Article on

Analysis of Polarization Mode Dispersion in Fibers and its Mitigation using an

Optical Compensation Technique.

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power point presentation on Analysis and Reduction of Polarization Mode dispersion