Agilent Technologies 86038B User Manual page 60

assure extremely high accuracy over a very wide length range,
from less than a millimeter to 50km and beyond. The phase
samples for the length measurement are made at two orthogonal
states of optical polarization so that the result is polarization
averaged.
The Optical Phase trace is calculated as the optical-frequency
integral of the GD. Since the GD trace displays relative GD
values, the measured or specified length value is used to provide
an absolute scale for the GD. Thus the slope of Optical Phase
does correspond to absolute GD when the length is correctly
determined. The Optical Phase trace is relative and begins at
zero.
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Agilent 86038B Photonic Dispersion and Loss Analyzer, Second Edition