Editing Laser (section)

==== Mode locking ====
{{Main|Mode locking}}

A mode-locked laser is capable of emitting extremely short pulses on the order of tens of [[picosecond]]s down to less than 10&nbsp;[[femtoseconds]]. These pulses repeat at the round-trip time, that is, the time that it takes light to complete one round trip between the mirrors comprising the resonator. Due to the [[Fourier uncertainty principle|Fourier limit]] (also known as energy–time [[Uncertainty principle|uncertainty]]), a pulse of such short temporal length has a spectrum spread over a considerable bandwidth. Thus such a gain medium must have a gain bandwidth sufficiently broad to amplify those frequencies. An example of a suitable material is [[titanium]]-doped, artificially grown [[sapphire]] ([[Ti-sapphire laser|Ti:sapphire]]), which has a very wide gain bandwidth and can thus produce pulses of only a few femtoseconds duration.

Such mode-locked lasers are a most versatile tool for researching processes occurring on extremely short time scales (known as femtosecond physics, [[femtosecond chemistry]] and [[ultrafast science]]), for maximizing the effect of [[nonlinear optics|nonlinearity]] in optical materials (e.g. in [[second-harmonic generation]], [[parametric down-conversion]], [[optical parametric oscillator]]s and the like). Unlike the giant pulse of a Q-switched laser, consecutive pulses from a mode-locked laser are phase-coherent; that is, the pulses (and not just their [[Envelope (waves)|envelopes]]) are identical and perfectly periodic. For this reason, and the extremely large peak powers attained by such short pulses, such lasers are invaluable in certain areas of research.