Group Delay; Polarization Mode Dispersion/Differential Group Delay - Agilent Technologies 86038B User Manual

Polarization Mode Dispersion/Differential Group Delay

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• When making precision GD measurements, use the Real Time Port
Monitor to verify DUT stability before normalizing or measuring. The
length and thus GD of long fibers changes significantly with
temperature, so rapid temperature changes should be avoided. Fiber
connectors can also be a source of instability and should be
cleaned and secured carefully. Fibers require protection from
normal room air currents and need to be secured and
allowed time to settle after handling. Refer to
101. Especially for stepped-wavelength
Monitor" on page
measurements, which take more time, the drift correction feature of
the 86038B dramatically reduces sensitivity of the GD measurements
to DUT instability by referencing each GD measurement point to a
reference wavelength measurement.
• When measuring ripple in either GD or IL spectra, the step
size and modulation frequencyare each important. If the step
size is too large, there is the risk of aliasing, giving the wrong
ripple period. Even when the step size is sufficiently small to
avoid aliasing, a too high modulation frequency could
increase the signal linewidth and thus resolution so as to
falsely reduce the apparent peak to peak amplitude of the
ripple. The ripple can be displayed as "Deviation" from the
Curve Fit function.
Tips on Measuring Chromatic Dispersion (CD)
• The combination of moderate smoothing of both the GD trace
and the CD trace may lower the CD trace noise and preserve
the CD wavelength resolution better than smoothing the CD
trace alone. Refer to
The group delay of an optical element can depend on the
polarization state of thelight. The differential group delay, DGD,
is the difference between the GD values at the two principal
states of polarization, which are orthogonal to each other. In a
simple element like a waveplate, the PSP correspond to the fast
and slow axes. The DGD at a particular wavelength thus gives
the magnitude of the GD dependence on polarization and is
usually expressed in ps. The DGD at the wavelength of the signal
determines the amount of signal degradation.
Optical fiber can have nonzero DGD due to imperfect cross-
section or due to external stress. Each segment of a long fiber
can have a different set of PSP, randomly oriented from segment
to segment. When the fast PSPs are closely aligned then the
DGD values of segments will be added, while alignment of fast
Agilent 86038B Photonic Dispersion and Loss Analyzer, Second Edition
"Smooth" on page 125.
"Real-Time Port