Polarization mode dispersionPolarization mode dispersion

At any point along a fiber, a polarized light pulse can be decomposed into componentsaligned with two local, orthogonal axes of the fiber: a fast axis and a slow axis.

In the real world of cabled fiber, the orientation of these axes and the relative differencein propagation speed corresponding to each axis (directly related to the magnitude ofthe local birefringence) changes along the optical path.

PMD is affected by physical stress, temperature, and fiber imperfections

The different sections of the fiber will have different orientations of the localbirefringence axes.(The change in orientation of the local principal axesis known as a mode-coupling event.)

In each segment, a time delay will be introduced between that portions of the lightaligned with the local fast versus the local slow birefringence axes.

Since the relative orientation of these axes in adjoining segments is different, the pulsewill experience a statistical spreading over time.

The two mutually orthogonal input states of polarization exist are known as the inputprincipal states of polarization, one of which corresponds to the fastest and the other tothe slowest pulse propagation time through the fiber.

The difference in these two propagation times is known as the differential group delay(DGD) corresponding to that wavelength, and the PMD is defined as the wavelength-averaged value of the DGD.

Transmission bits (0, 1) get wider along the fiber so that 0 and 1 are undetectable

Because the individual factors that cause PMD cannot be measured as a constantlychanging, unstable stochastic process.

This random process results in a broadening of the pulses earning information, whichcan impair the ability of a receiver to decode them correctly.

PMD is thus a critical phenomenon that limits the transmission rate.

The total PMD of a number of spans in a network is given by a root mean squaresummation:

PMD = [N (PMDN)2]1/2

Example of PMD measured with the interferometric method

The average DGD over the wavelength range gives the mean fiber PMD

The impact of PMD in a system is particularly sensitive to:

a) an increase in the channel bit rate (one of the most important criteria)b)an increase in the number of spans (equivalent to an increase in the length of the fiberlink)c) an increase in the number of channels (since the more channels there are, the higherthe probability of one of them being affected by a larger instantaneous value of thedifferential group delay than its mean value).

The impact of the use of chromatic dispersion compensation techniques on PMD isunclear, but research into this issue is underway.Soliton-based transmission systems using return-to-zero (RZ) bit coding have beenshown to be less susceptible to PMD impairment than non-return-to-zero (NRZ) coding

.

Figure presents PMD criteria in a different way. For each of three common bit rates, itshows the sensitivity-penalty at the receiver that will be introduced by a givendifferential group delay (i.e., the difference between the group velocities of the twopropagating principal states of polarization).

Imapct of PMD on transmission qualityBit rate 10 Gbit/sLengrth of G. 652 fiber 100 kmPMD 10 ps/km-1/2 and 50 ps/km-1/2

Chromatic dispersion 0 ps/nmkm

Input

Outpu PMD=10 ps/km-1/2

Output PMD=50 ps/km-1/2

PMD compensator

PMD compensator

DOF – Degree of Freedom

I n t r a - b i t P o l a r i z a t i o n M o d u l a t i o n D i v e r s i t y

oś szybka światłowodu

oś wolna światłowodu

czastrwania bitu

różnicoweopóźnienie grupowe

analizator

połowa bitu