Kerr-lens modelocking

Initiation of Kerr-lens modelocking depends on the strength of the nonlinear effect involved. If the laser field builds up in a cavity the laser has to overcome the region of continuous-wave operation, which often is favoured by the pumping mechanism. This can be achieved by a very strong Kerr-lensing that is strong enough to modelock due to small changes of the laser field strength (laser field build-up or stochastic fluctuations). Modelocking can also be started by shifting the optimum focus from the continuous-wave operation to pulsed operation while changing the power density by kicking the end mirror of the resonator cavity (though a piezo mounted, synchronous oscillating end-mirror would be more 'turn key'). Other principles involve different nonlinear effects like saturable absorbers and saturable Bragg reflectors, which induce pulses short enough to initiate the Kerr-lensing process. == Modelocking – evolution of the pulse ==

Intensity changes with lengths of nanoseconds are amplified by the Kerr-lensing process and the pulselength further shrinks to achieve higher field strengths in the center of the pulse. This sharpening process is only limited by the bandwidth achievable with the laser material and the cavity-mirrors as well as the dispersion of the cavity. The shortest pulse achievable with a given spectrum is called the bandwidth-limited pulse. Chirped mirror technology allows to compensate for timing mismatch of different wavelengths inside the cavity due to material dispersion while keeping the stability high and the losses low. The Kerr effect leads to the Kerr-lens and Self-phase modulation at the same time. To a first approximation it is possible to consider them as independent effects. == Applications ==

Since Kerr-lens modelocking is an effect that directly reacts on the electric field, the response time is fast enough to produce light pulses in the visible and near infrared with lengths of less than 5 femtoseconds. Due to the high electrical field strength focused ultrashort laser beams can overcome the threshold of 1014 W cm−2, which surpasses the field strength of the electron-ion bond in atoms. These short pulses open the new field of ultrafast optics, which is a field of nonlinear optics that gives access to a completely new class of phenomena like measurement of electron movements in an atom (attosecond phenomena), coherent broadband light generation (ultrabroad lasers) and thereby gives rise to many new applications in optical sensing (e.g. coherent laser radar, ultrahigh resolution optical coherence tomography), material processing and other fields like metrology (extremely exact frequency and time measurements). == References and notes ==

• D. E. Spence, P. N. Kean, and W. Sibbett, Opt. Lett. 16, 42(1991). • M. Piche, Opt. Commun. 86, 156(1991). • B. Proctor, E. Westwig, and F. Wise, Opt. Lett. 18, 1654(1993). • V. Magni, G. Cerullo, and S. De Silvestri, Opt. Commun. 101, 365(1993).