Suggested Reading - Agilent Technologies 86038B User Manual

After viewing the measurement results, you may wish to speed
up the measurement by taking larger wavelength steps or by
moving the slider controls. If phase wrapping occurs, you can
identify it by the change in the shape of the group delay curve
compared to the earlier conditions.
A second approach to choosing appropriate setup parameters is
to check your values using the equations and graphs presented
earlier in this section. For example, if the device is expected to
exhibit fine group delay ripple of a particular wavelength
period, select a wavelength step and modulation frequency from
Figure 15.

Suggested reading:

Fiber Optic Test and Measurement (Derickson Ed.)
T.Niemi et al., "Limitations of the phase shift method in
measuring dense group delay ripple in fiber Bragg gratings",
IEEE Photonics Technology Leltters, vol.13, no.12, p.1334,
December 2001.
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Agilent 86038B Photonic Dispersion and Loss Analyzer, Second Edition