Agilent Technologies 86038B, User Manual

Page: 63 / 350

Agilent 86038B Photonic Dispersion and Loss Analyzer, Second Edition
63
• Avoidance of phase wrapping when measuring highly
dispersive devices.
• Avoidance of aliasing when measuring ripple.
Equation 5 warns us not to reduce modulation frequency more
than necessary.
Relationship of setup parameters to phase wrapping
Equation 4 implies that the selection of modulation frequency
and wavelength increment play a part in the potential problem
of phase wrapping. The phase detectors used in the Agilent
86038B have a range of
\ 180-degrees. Exceeding the range of
the phase detector causes a phase wrapping error. The wrap-
free range is expressed by:
Equation 6
Consider this limitation in the context of the stepped mode of
operation. After each wavelength step, the phase detector is
automatically zeroed to make the entire \ 180-degree range
available for the next step. If the next wavelength step produces
a phase change larger than
\ 180 degrees, the phase detector
loses the history of the actual phase change and gives us only an
apparent phase change. For example, if a wavelength step
produces an actual phase change of positive 185 degrees, it will
be interpreted by the phase detector as a change of negative 175
degrees. This is the phase wrapping error. The phase detector is
zeroed mathematically, not physically, so there is no noise
contribution, regardless of the wavelength step size. Use
Equation 6 to check your selections of wavelength increment
step and modulation frequency, to assure that the \ 180 degree
limit is not exceeded. For convenience, the Sens/Res Setup
Panel displays a calculation of the maximum measurable CD for
the chosen parameters.
Phase wrapping errors can also occur in the swept mode,
though they are less likely to occur when using small wavelength
increments (large numbers of amplitude and phase samples
across the measurement wavelength range).
Δφ
360
°
D f
m
Δλ ⋅ ⋅ ⋅ =
Δφ
180
° ± <