Group Delay & Group Delay Dispersion (GD & GDD)

Basics

The velocity of each spectral component of a laser pulse is a function of the refractive index n at the wavelength λ of that component.

Fig.2:Example Sapphire

Fig.3:Normal dispersion

In the example above, longer wavelengths are faster than shorter ones. This is called “normal dispersion”.

The propagation of a compressed laser pulse through a dispersive medium (gasses, crystals, glass,…) will change the temporal pulse shape.

Fig.4:Visualisation of GD and GDD

GDD > 0 (Normal Dispersion)
→ Shorter wavelengths propagate slower than longer wavelengths (“red is faster than blue”)

GDD < 0 (Anomalous Dispersion)
→ Shorter wavelengths propagate faster than longer wavelengths (“blue is faster than red”)