The methods of mode locking can be divided into two categories:
- Active.
- Passive.
Active :
Active mode locking is a mode locking method in which the mode locking element is driven by an external source.
It has different types:
- AM – which is induced by an amplitude modulator.
- FM – which is induced by a phase modulator.
- Synchronous pumping – which is induced by a periodic modulation of the laser gain at a repetition rate equal to the fundamental cavity frequency.
Passive :
A mode locking in which the element that induces the mode locking is not driven externally instead exploits some non-linear optical effect is called passive mode locking.
It has different types:
- Fast saturable absorber- which is uses saturation properties of a suitable absorber (eg: a dye molecule or a semi conductor).
- Ken lens mode locking (KLM)- which exploits the self focusing property of a suitable, transparent non-linear optical medium.
- Slow saturable absorber- which uses the dynamic saturation of gain medium.
- Additive pulse – which exploits the self phase modulation induced in a suitable non-linear optical medium inserted in an auxiliary cavity and coupled to a main cavity of identical length.
