EXFO FTB-5500B PMD Analyzer - Manual (Page 12)

Introducing the FTB-5500B Polarization Mode Dispersion Analyzer
4 FTB-5500B
Basic Polarization Mode Dispersion Theory
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,
ω
, 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 δτ is Maxwellian and, by definition, PMD
is its rms value, that is:
PMD =
√<
DGD2
>
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
ω
, PMD is stable in time, provided that the
averaging window is sufficiently large (
Δωδτ
>> 1).