Agilent Technologies 86038B User Manual page 55

Agilent 86038B Photonic Dispersion and Loss Analyzer, Second Edition
with slow PSP allows the total DGD to be reduced. This degree
of alignment along a fiber typically depends on wavelength and
also varies with time, due to temperature and stress changes.
Therefore the DGD at a given time for a certain wavelength may
not be the same as obtained in an earlier measurement. But an
average over many measurement points can be used to
determine how the DGD will behave statistically over time. In
particular, the probability that a certain DGD limit is exceeded
can be determined. This average DGD value is known as
polarization mode dispersion, PMD, which is thus a useful
parameter for optical fiber and other complex optical paths.
The statistical way that DGD accumulates in a fiber span means
that it increases proportional to the square-root of the fiber
length. Thus, the PMD coefficient, is defined as the PMD divided
by the square root of the fiber length, expressed in
The causes of PMD are several. Even the simplest optical device
usually exhibits a slight dependence of group delay on input
polarization. The behavior may be produced by anisotropy of the
index of refraction in directions perpendicular (transverse) to
the direction of propagation. Differential group delay is also
observed in devices in which the light is split into two paths
according to polarization and later recombined. In this
situation, any difference in the parallel path lengths produces a
differential group delay. In optical integrated circuits a common
cause of PMD is the asymmetry in the cross sectional profile of
the waveguides. In optical fiber, the main cause of differential
group delay is non-circularity of the optical core. The problem
was extremely severe in some of the older installed fibers.
Tips on Measuring Differential Group Delay (DGD)
• Good DGD measurements with the 86038B depend on
making low-noise GD measurements and on stability of the
DUT during the measurement. This means that high
modulation frequency and low IF bandwidth are beneficial.
Especially for measurements of long fibers, the drift
correction feature of the 86038B dramatically reduces the
sensitivity of the swept-wavelength DGD measurements to
DUT instability, especially temperature drift, by remeasuring
the GD spectrum at the first polarization state at the end of
the measurement. When making precision DGD
measurements, use the Real Time Port Monitor to verify GD
drift before normalizing or measuring. Refer to
Port Monitor" on page 101.
"Real-Time
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