Kerr-lens modelocking

The standard cavity with flat mirrors and a thermal lens in the laser crystal has the smallest beam width on the end-mirrors.The continuous-wave light exits the crystal end face with a larger beam width and slight divergence.Because of the divergence the light is effectively coming from a point farther apart and leads to more convergence after the concave mirror.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.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.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.