Group Delay Dispersion
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Definition: the frequency dependency of the group delay, or (quantitatively) the corresponding derivative with respect to angular frequency
The group delay dispersion (also sometimes called second-order dispersion) of an optical element is the derivative of the group delay with respect to the angular frequency, or the second derivative of the change in spectral phase:
It is usually specified in fs2 or ps2. Positive (negative) values correspond to normal (anomalous) chromatic dispersion. For example, the group delay dispersion of a 1-mm thick silica plate is +35 fs2 at 800 nm (normal dispersion) or −26 fs2 at 1500 nm (anomalous dispersion). Another example is given in Figure 1.
Note that the group delay dispersion (GDD) always refers to some optical element or to some given length of a medium (e.g. an optical fiber). The GDD per unit length (in units of s2/m) is the group velocity dispersion (GVD).