Kerr lens mode locking (original) (raw)

Acronym: KLM

Definition: a technique for mode locking a laser, exploiting nonlinear self-focusing

Categories: light pulses, methods

• mode locking
  • passive mode locking
    * Kerr lens mode locking
    * additive-pulse mode locking

Related: Kerr lensKerr effectmode lockingmode-locked lasersultrafast lasersself-starting mode lockingtitanium–sapphire lasers

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DOI: 10.61835/pus Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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What is 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.
• A modified version of the second method exploits nonlinear self-focusing in a separate passive Kerr medium [21]. The advantage of that approach is that the requirements on the radiance (brightness) of the pump beam are then much lower. In some cases, that allows one to realize lasers with substantially higher average output power [24].

The article on passive mode locking explains how a saturable absorber leads to mode locking.

KLM is sometimes called self mode locking because it does not require a visible saturable absorber device. Its first observation [2], where that term was introduced, was not yet explained with the influence of nonlinear focusing based on the Kerr effect; that was provided by others shortly after that first report [3]. The basic idea behind the mechanism was already presented by Lariontsev and Serkin in 1975 [1].

Kerr lens mode locking has enabled the generation of the shortest pulses with durations down to ≈ 5 fs in Ti:sapphire lasers. Its main advantages are the following:

• Its strength lies in the very fast response, suitable for generating the shortest light pulses.
• No special saturable absorber medium is required; the technique can thus be applied in different spectral regions without special components.

However, there are also some disadvantages:

• One generally needs 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.
• Reliable self-starting mode locking is often not achieved. That is a negative consequence of the fast absorber response; slow absorbers are better in terms of self-starting. 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 to stimulate power fluctuations.
• Accurate modeling is difficult due to the complicated spatio–temporal dynamics and the uncertainties related to how close one is to the resonator's stability limit. Simplified models can at least roughly predict the achieved modulation depth and saturation power, and thus assist in finding a suitable resonator design. However, accurate predictions are difficult.
• Depending on the application, it may also be disturbing that the laser beam radius may change during the pulse.
• The power conversion efficiency of KLM lasers is often relatively low, e.g. due to non-ideal overlap between laser and pump beam and critical resonator alignment. However, quite high efficiency has been achieved in some cases [30].

A modified kind of KLM has been applied to vertical external-cavity surface-emitting lasers (VECSELs) [18]. 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.

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; this is sometimes called SESAM-assisted Kerr lens mode locking.

Frequently Asked Questions

What is Kerr lens mode locking (KLM)?

How does Kerr lens mode locking work?

KLM works via two main mechanisms. In hard-aperture KLM, the intensity-induced focusing reduces losses at an aperture. In soft-aperture KLM, it improves the overlap between the laser and pump beams, thus increasing the optical gain for the pulse peak.

What are the main advantages of KLM?

The primary advantages of Kerr lens mode locking are its very fast response, which enables the generation of extremely short pulses (down to a few femtoseconds), and that it doesn't require a special saturable absorber component.

What are the challenges or disadvantages of KLM?

KLM lasers often need to operate near a stability limit, making the resonator design difficult and long-term stability a challenge. Also, reliable self-starting mode locking is often not achieved, and the power conversion efficiency can be low.

Is KLM self-starting?

Often, it is not. A negative consequence of the fast absorber response is that KLM lasers may not spontaneously switch to mode-locked operation when turned on and can require an external trigger, such as a manual tap on a mirror.

In which lasers is the use of Kerr lens mode locking most common?

Kerr lens mode locking is most often used in Ti:sapphire lasers, where it has enabled the generation of some of the shortest pulses ever produced, with durations down to approximately 5 fs.

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