Kerr Lens Mode Locking
Kerr lens mode locking is a technique of passive mode locking a laser, using an artificial saturable absorber based on Kerr lensing in the gain medium. The latter effect causes a reduction in the beam size for high optical intensities. Via two different mechanisms, this can effectively act like a fast saturable absorber:
- In the case of hard aperture KLM, the Kerr lens reduces the optical losses at an aperture which the beam must pass in each resonator round trip.
- In the case of soft aperture KLM, the Kerr lens leads to a better overlap of laser and pump beam, and thus to a higher gain for the peak of the pulse. That increase of gain has a similar effect as a decrease of losses; both effects increase the net round-trip gain.
The article on passive mode locking explains how a saturable absorber leads to mode locking.
Kerr lens mode locking has enabled the generation of the shortest pulses with durations down to ≈ 5 fs in Ti:sapphire lasers. Its strength lies in the very fast response and the fact that no special saturable absorber medium is required. The main disadvantage is the need to operate the laser close to a stability limit of its resonator, because otherwise the Kerr lensing effect is too weak. As a consequence, long-term stable operation is difficult to achieve, and the resonator design is a difficult task. Also, reliable self-starting mode locking is often not achieved. Often such lasers start in a noisy operation mode, not producing ultrashort pulses, after being turned on, and switch to mode-locked operation only after an external trigger, e.g. when a resonator mirror is manually tapped in order to stimulate power fluctuations.
KLM is sometimes called self mode locking because it does not require a visible saturable absorber device. Its first observation , where that term was introduced, has not yet been explained with the influence of nonlinear focusing based on the Kerr effect; that was provided by others shortly after that first report .
A kind of KLM has been applied to vertical external-cavity surface-emitting lasers (VECSELs) . Their gain medium does not exhibit a true Kerr nonlinearity, but a similar effect based on gain saturation and the dependence of refractive index on the carrier density. This typically leads to a negative index change due to gain saturation, but not with an index change in proportion to the momentary optical intensity.
Comprehensive modeling of Kerr lens mode locking is difficult due to the complicated spatio-temporal dynamics; note that the beam radius varies during the temporal pulse shape. Simplified models can at least roughly predict the achieved modulation depth and saturation power, and thus assist in finding a suitable resonator design.
A possible alternative to KLM is passive mode locking with a real saturable absorber, e.g. with a SESAM. It is also possible to combine KLM and a SESAM with particularly broad reflection bandwidth to achieve self-starting mode locking and very short pulses.
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