Teledyne Lecroy IQS25, Руководство оператора

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Operator’s Manual
63
Polarization De-multiplexing
The term polarization refers to the directional plane that the Electromagnetic (EM) wave (carrier wave)
oscillates in.
Two EM waves can be made to propagate along the same fiber by combining them with a 90 degree
difference in polarization (orthogonal polarization states or modes, A
X
and A
Y
). Each of these orthogonal
polarization states can carry data (e.g. QPSK, QAM) that is independent of the other, effectively doubling
the amount of information transmitted over the same fiber. A single mode fiber (SMF) can carry two
orthogonal optical modes (A
x
and A
y
) which may rotate as it propagates (while maintaining their
orthogonality). As these modes travel through SMF, they rotate and experience different propagation
delays through the fiber due to fiber birefringence. These effects cannot easily be predicted since they are
dependent on stresses and strains on different parts of the fiber.
NOTE: A specially designed type of fiber known as polarization maintaining (PM) fiber does not allow the
polarization to rotate so that the light exits fiber at a known polarization state.
One of the most common Polarization De-multiplexing Algorithm is Constant Modulus Algorithm
(CMA).The algorithm takes advantage of the fact that all QPSK symbols have same amplitude (constant
modulus) when they are polarization de-multiplexed. It is an iterative algorithm that uses time-domain
taps to bring back signal components shifted in time due to PMD, (PMD correction) while re-distributing
the mixed X and Y signal components. Here the Gradient controls the speed and accuracy of the
adaptation and Symbols is the length of training set and Num Taps refer to the number of taps in the
equalization filter.
) 2 ( ) 2 (
) ( ) ( ) (
ft kz i
Y
ft kz i
X
e t A e t A t E
π π
− −
+ =
 