Mode-locked lasers generate short optical pulses by establishing a fixed phase relationship between all the different lasing longitudinal modes of the laser cavity. To achieve this requires a mechanism that results in higher net gain for pulsed operation than for CW operation.

A Kerr-lens mode-locked laser uses the non-linear index of refraction (Kerr effect) of some material (e.g. the laser crystal itself) to provide the mode-locking mechanism. A laser beam with a Gaussian transverse intensity profile creates a Gaussian refractive index profile in the non-linear medium. This acts as a focussing lens, with the focussing effect increasing with optical intensity. When combined with an effective aperture in the laser cavity (which again could be the non-linear medium itself), the Kerr lens can act as a saturable absorber. Short pulses, which have higher peak intensities, are focussed more tightly and are transmitted through the aperture, whilst lower intensities experience greater losses.

Most commonly used laser crystals exhibit ‘normal’ dispersion, i.e. long wavelengths travel faster than shorter ones. This leads to a temporal spreading of the pulse each time it passes through the crystal. To produce short pulses therefore requires a source of ‘anomalous’ dispersion in the laser cavity to compensate for the normal dispersion in the laser crystal.

One method of providing anomalous dispersion is to place a pair of prisms in one arm of the cavity, arranged so that the longer wavelength components have to travel through more glass than the shorter wavelength components. If the right material is chosen, both group velocity dispersion and third order dispersion can be minimized in this way.

Alternatively, negative group velocity dispersion or ‘chirped’ mirrors can be used to provide dispersion compensation. These can provide more control over higher order dispersion, and also allow shorter laser cavities to be constructed.

Ti:sapphire is the laser medium most commonly used in Kerr-lens mode-locked lasers. One of the main reasons for this is its large gain bandwidth, extending from 700 nm to more than 1000 nm, which is necessary for ultrashort pulses to be generated. In these lasers, the Kerr medium is the Ti:sapphire crystal itself.

At NPL we have two Ti:sapphire mode-locked lasers for optical frequency metrology in the visible and near-infrared regions of the spectrum. The first laser uses prisms for dispersion compensation, the second uses negative GVD mirrors.