Interpreting Pdl Data; Characterizing Specific Devices; Single-Output Devices - JDS Uniphase SWS15100 User Manual

Interpreting PDL Data

PDL values calculated by the SWS are obtained from linear combinations of the insertion losses
at four polarization states that are well defined, relative to one another. The computation of PDL
is accurate to within the level of system noise. At a low IL value, this level is ±0.025 dB, giving an
uncertainty in the PDL of 0.05 dB. However, when insertion losses are high, the noise also
increases. For example, at 40 dB insertion loss, the noise is ±0.25 dB; therefore, the uncertainty
in the PDL measurement increases to ±0.5 dB. Care must be taken when interpreting PDL
measurements at high insertion losses.
The effects of system noise on PDL measurements can be reduced in two ways: averaging and
smoothing. The number of traces to average can be selected. With averaging over four traces,
the noise can be reduced by a factor of two. Smoothing applies a triangular filter to the data. If
the half size chosen for smoothing is n, the power at any point is:
where P(k) is the power at point k. The sum is for values of r, from -n to +n, for a half filter size of
n.
Improvement in device measurement can be achieved by averaging and smoothing the
reference in the same way as is done for the traces. As a precaution, do the averaging in PDL
mode if a polarization controller is present, even if the measurements are in insertion loss mode.
The effect of smoothing is to reduce the noise over a small wavelength region. Smoothing also
has the effect of degrading the wavelength resolution. When smoothing is used, the half-size of
the triangular filter must be chosen with the characteristics of the device considered. Care must
also be used while interpreting the results. When studying the characteristics of a device in
regions where the IL varies rapidly with wavelength, it can be preferable to average over several
references and scans to improve the measurement.

Characterizing Specific Devices

Single-Output Devices

Simple passive optical devices that can be characterized using SWS include:
•
Attenuators
•
Bandpass filters
•
Isolators
•
Fiber Bragg gratings (in transmission)
•
Etalons
•
Optical filters
94 – Operating and Maintenance Instructions
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∑
P(k+r) (n+1-|r|)
P(k) =
2
(n+1)