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Understanding the Attributes Being Measured
Agilent 86038A Optical Dispersion Analyzer, Third Edition
Polarization Dependent Loss (PDL)
Polarization dependent loss (PDL) is the peak-to-peak variation in the loss
of a test device over all polarization states. If the optical input level to the
test device is constant over the measurement time, the PDL is also equal
to the peak-to-peak variation of output power over all polarizations.
The mechanisms that produce PDL include material absorption, oblique
optical reflections, fiber bends and various aspects of optical circuit
design. In cases of passive components exhibiting pure PDL, the
polarization states corresponding to maximum and minimum loss are
diametrically opposite one another on the Poincare sphere.
Higher-order device attributes such as center wavelength and bandwidth
of narrowband filters are also polarization dependent. However, it is not
always possible to extract the polarization dependence of these attributes
from measurements of the PDL at a series of wavelengths; the polarization
state that produces the minimum loss at one wavelength may not produce
the minimum at all other wavelengths. For this reason, measurement of
the polarization dependence of such higher order attributes may require
multiple measurements of the device response across its wavelength
range, with each complete measurement performed using a different fixed
input state of polarization. This produces a set of, for example, 64 device
responses at 64 randomly selected states of polarization. The center
wavelength and bandwidth are computed for each case, and the peak-to-
peak variation in each of these attributes expresses their polarization
dependence. The 86038A does not provide a function for measuring these
higher order polarization dependencies.
Tips on Measuring Polarization Dependent Loss (PDL)
• When making PDL or DGD measurements, the polarization scanner runs at a
fixed speed and produces PDL or DGD changes with the majority of spectral
content around 8 Hz. This limits the minimum IF bandwidth when scanning to
70 Hz, which avoids attenuating the 8 Hz components. Using a wider IF band-
width allows a faster measurement, but raises the noise in the measurement
and may slightly bias DGD and PDL toward larger values.
• During PDA normalization, 70 Hz IF bandwidth is used to characterize noise
components with the scanner off, for both GD and Magnitude. These noise
measurements provide the best correction of DGD and PDL when measuring in
a 70 Hz IF bandwidth.
Refer to "Phase Dynamic Accuracy" on page
• PDL and DGD may indicate negative values, which are clearly not possible. This
results from noise in the measurement. Negative values are useful when
smoothing across wavelength sample points to obtain an unbiased estimate of
the average DGD or average PDL.
Measurement Concepts
108.
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