Passive saturable absorbers for short-pulse lasers

By delivering extremely high peak powers, from a few kilowatts to a few tens of petawatts, in an extremely short time, from a few femtoseconds to a few tens of nanoseconds, short-pulse lasers enable a multitude of applications in fields as diverse as :

• marking, drilling, cutting and welding materials on a micrometric scale;

• refractive eye and cataract surgery ;

• X-ray production and particle acceleration for imaging and treatment of cancerous tumors;

• the study of chemical reactions;

• hot plasma physics ;

• thermonuclear fusion by inertial confinement ;

• etc.

It is therefore a very active and constantly evolving field of research and development, motivating the work of many researchers and industrialists.

Depending on the field of application, the most suitable laser source will have to deliver pulses of varying length, energy (in Joules) and/or intensity (in W/cm ² ), at the most appropriate rate and wavelength.

In fact, different techniques and different types of material will often have to be considered to produce the laser best suited to a given application, and the choice will be made not only on the basis of the efficiency of the laser source in question, but also on its ease of use and cost.

This article focuses on "passive" mode-locking and triggering techniques using saturable absorption materials. These techniques and materials enable us to produce laser sources emitting from the visible to the mid-infrared range, at rates ranging from a few Hz to a few GHz.

To this end, the presentation will be divided into three parts:

• the first part will be devoted to a brief, general description of how short-pulse lasers work and the various techniques that can be used;

• the second part is devoted to a detailed description of saturable absorber materials themselves. First, the spectroscopic, dynamic and energetic parameters that characterize them will be discussed, followed by their conditions of use and operation in laser cavities, then the techniques used to measure the various parameters, and finally the models and formulations to be used depending on whether a "fast" or "slow" saturable absorber is involved;

• the third part of the presentation will review the saturable absorber materials that have led or could lead to the most interesting results. These include crystals doped with...