Measurement Method - Agilent Technologies 86038A User Manual

Measurement Concepts Description of the 86038A

Measurement Method

In the group delay/chromatic dispersion measurement, the wavelength is
stepped or swept and the change in recovered RF modulation phase is
recorded. The change of phase is converted to an equivalent change in the
group delay according to the formula shown in Equation 1. The derivative
of group delay with respect to wavelength gives chromatic dispersion. See
Equation 2.
The loss or gain measurement proceeds in the same way, but the
measurement is based on the amplitude of the recovered modulation
envelope.
Both the group delay and the loss/gain measurement require an initial
normalization measurement of a through path. After normalization, the
test devices is inserted in the through path and the group delay and
loss/gain is measured in comparison with the normalization data.
PDL and PMD measurements are performed in the step mode with the
polarization scanner running. At each wavelength the polarization is
scanned for a few seconds. The DGD at each wavelength is calculated
using the minimum to maximum phase change in the same formula used
to calculate group delay, but in this case the stimulus is polarization rather
than wavelength.
The PDL information is obtained from the minimum to maximum variation
in amplitude of the recovered modulation envelope, where the stimulus is
polarization.
The fiber length measurement is based on the variation of modulation
phase as the RF modulation frequency is changed. Intuitively, the length of
the test device can be expressed in terms of an equivalent number of
modulation periods. At sufficiently low modulation frequencies, even the
longest test device is less than a single RF cycle in length. As the RF
modulation frequency is increased, the test device becomes more and
more RF cycles in length. This increase is more rapid when the test device
is long. This is the basis for the length measurement performed by the
86038A. The wavelength is fixed and the stimulus is the RF modulation
frequency.
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Agilent 86038A Optical Dispersion Analyzer, Third Edition