Fundamental Mode Locking
Fundamental mode locking is mode locking of a laser with a single pulse circulating in the laser resonator. The converse term is harmonic mode locking, with multiple pulses in the resonator. Advantages of fundamental mode locking are that possible instabilities of harmonic mode locking are avoided, and that the laser setup is usually more compact. On the other hand, harmonically mode-locked lasers have a potential for lower laser noise.
In the pulse repetition rate regime of many gigahertz, as often required for optical data transmission, fundamental mode locking requires a fairly short laser resonator. This can be achieved e.g. with monolithic diode lasers, which compete with harmonically mode-locked external-cavity diode lasers and fiber lasers.
For passively mode-locked bulk lasers, an additional challenge for fundamental mode locking with high repetition rates is the tendency for Q-switching instabilities. Nevertheless, such lasers have been developed which can be operated at pulse repetition rates of tens of gigahertz or even up to 160 GHz .
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