Exfo FastReporter 2 Manuel d'utilisateur télécharger pdf (Page 372)

Measuring Polarization Mode Dispersion: Theory

362 FastReporter 2

In a long fiber, numerous mode-coupling events occur along the fiber

length, so that light emerging from the output end is the superposition of a

number of pulses with different delays. Nonetheless, it turns out that for

any given optical frequency, , one can always find two orthogonal input

principal states of polarization (PSPs) such that a light pulse with the same

input SOP as the input PSP, undergoes no spreading. For a single wave

plate, the PSPs are the two birefringent axes, whereas for a concatenation

of wave plates, neither the input nor the output PSPs correspond to the

alignment of the birefringent axes anywhere.

Contrary to the case of a wave plate, the DGD and PSPs of a long fiber are

dependent on wavelength and fluctuate in time as a result of

environmental variations such as temperature, external mechanical

constraints, etc. Their behavior is random, both as a function of wavelength

at a given time and as a function of time at a given wavelength. Fortunately,

this behavior can be characterized statistically. It can be demonstrated that

the probability density function of  is Maxwellian and, by definition, PMD

is its rms value, that is:

Note: PMD is sometimes defined as the mean value of the DGD, which for a

Maxwellian distribution yields a value 17 % lower than the rms definition.

If the average is calculated over , PMD is stable in time, provided that the

averaging window is sufficiently large (Dwdt >> 1).

PMD DGD

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Exfo FastReporter 2 Manuel d'utilisateur Page 372