5: Active Mode Locking
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For simplicity, we assume, that the laser operates in the transverse fundamental modes and, therefore, we only have to treat the longitudinal modes of the laser similar to a simple plane parallel Fabry-Perot resonator (Figure: 5.1). We consider one polarization of the field only, however, as we will say later for some mode-locked laser polarization dynamics will become important.
The task of mode-locking is to get as many of the longitudinal modes lasing in a phase synchronous fashion, such that the superposition of all modes represents a pulse with a spatial extent much shorter than the cavity. The pulse will then propagate at the group velocity corresponding to the center frequency of the pulse.

- 5.3: Active Mode-Locking by Phase Modulation
- Side bands can also be generated by a phase modulator instead of an amplitude modulator. However, the generated sidebands are out of phase with the carrier, which leads to a chirp on the steady state pulse. We can again use the master equation to study this type of modelocking.