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(Acronym: KLM)
Definition: a technique for mode locking a laser, exploiting nonlinear self-focusing
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. This can effectively act like a fast saturable absorber when it leads to a better overlap of laser and pump beam, or when it reduces the optical losses at some aperture.
KLM 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.
The modeling of Kerr lens mode locking is very 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 to achieve self-starting mode locking and very short pulses.
Bibliography
| [1] | D. E. Spence, P. N. Kean, W. Sibbett, "60-fsec pulse generation from a self-mode-locked Ti:sapphire laser", Opt. Lett. 16 (1), 42 (1991) |
| [2] | F. Salin et al., "Modelocking of Ti:sapphire lasers and self-focusing: a Gaussian approximation", Opt. Lett. 16 (21), 1674 (1991) |
| [3] | T. Brabec et al., "Kerr lens mode locking", Opt. Lett. 17 (18), 1292 (1992) |
| [4] | Piché et al., "Self-mode locking of solid-state lasers without apertures" (soft aperture mode locking), Opt. Lett. 18 (13), 1041 (1993) |
| [5] | J. Herrmann, "Theory of Kerr-lens mode locking: role of self-focusing and radially varying gain", J. Opt. Soc. Am. B 11 (3), 498 (1994) |
| [6] | Y. Chou et al., "Measurements of the self-starting threshold of Kerr-lens mode-locking lasers", Opt. Lett. 19 (8), 566 (1994) |
| [7] | G. Cerullo et al., "Self-starting Kerr-lens mode locking of a Ti:sapphire laser", Opt. Lett. 19 (14), 1040 (1994) |
| [8] | I. P. Christov et al., "Mode locking with a compensated space-time astigmatism", Opt. Lett. 20 (20), 2111 (1995) |
| [9] | D. H. Sutter et al., "Semiconductor saturable-absorber mirror-assisted Kerr lens modelocked Ti:sapphire laser producing pulses in the two-cycle regime", Opt. Lett. 24 (9), 631 (1999) |
| [10] | U. Morgner et al., "Sub-two cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser", Opt. Lett. 24 (6), 411 (1999) |
See also: Kerr lens, Kerr effect, mode locking, mode-locked lasers, ultrafast lasers, self-starting mode locking, titanium-sapphire lasers
