Kerr-lens modelocking

Kerr-lens modelocking is a method of modelocking lasers via a nonlinear optical process known as the optical Kerr effect. This method allows the generation of pulses of light with a duration as short as a few femtoseconds.

The optical Kerr effect is a process which results from the nonlinear response of an optical medium to the electric field of an electromagnetic wave. The refractive index of the medium is dependent on the field strength .

Because of the non-uniform power density distribution in a Gaussian beam (as found in laser resonators) the refractive index changes across the beam profile; the refractive index experienced by the beam is greater in the centre of the beam than at the edge. Therefore a rod of an active Kerr medium works like a lens for high intensity light. In the laser cavity short bursts of light will then be focused differently to continuous waves (cw). By aligning the cavity in a way such that the resonator is more lossy for cw light than for pulses, the pulsed regime is favored and the laser will turn to pulsed operation (modelocked). The favoring of different regimes and pulse lengths can be achieved by the cavity design (stability of the cavity), but is often supported by an aperture, that simply cuts off (hard aperture) or attenuates (soft aperture) the cw beam at the focal region of the pulsed beam.

Since Kerr-lens modelocking is an effect that directly reacts on the electric field, the response time is high enough to produce light pulses in the visible and near infrared with lengths of less then 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).

Starting a Kerr-lens modelocked laser

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 cw operation, which often is favored 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 cw-operation to pulsed operation while changing the power density by kicking the end mirror of the resonator cavity. 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 puselength 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.

Laser media for ultrashort pulses (e.g. Ti:Sapphire) Dispersion management with prism sequences. Chirped mirror technology allows to compensate timing mismatch of different wavelengths inside the cavity due to material dispersion while keeping the stability high and the losses low.