When many cavity modes oscillate with fixed phase relationships, they interfere to form a single pulse circulating in the cavity, emitted once per round trip at a repetition rate of c divided by 2L, typically tens of MHz to GHz. Pulse length shrinks as the locked bandwidth grows, reaching picoseconds or femtoseconds. Active locking uses a modulator; passive locking uses a saturable absorber, a SESAM or the Kerr lens effect.
Mode locking looks solid, spectrum's 60 nm wide and smooth, and the photodiode shows a clean 80 MHz train with no CW spike.
Also heard as
- modelocking
- ML
Do not mix it up with
- Q-switching: Q-switching releases stored energy as one nanosecond pulse; mode locking makes a continuous train of much shorter pulses.
Related terms
Longitudinal mode
One of the discrete frequencies at which a standing wave fits a whole number of half-wavelengths into the resonator.
Saturable absorber
A material whose absorption drops at high intensity, used to start and sustain Q-switching or mode locking passively.
Kick
To start a Kerr-lens mode-locked laser by briefly disturbing the cavity so a pulse can form.
Rep rate
Clipped form of repetition rate: how many pulses per second a pulsed laser emits.
Lock
The stable locked state of a laser, either mode-locked pulsing or frequency-locked to a reference.