Design of a laser guide star for applications to adaptive optics

Laser guide stars (LGSs) are fundamental elements of adaptive optics. They indeed allow to form an artificial star in the sky, which is used as a reference for the compensation of the effect of atmospheric perturbation on wave fronts. This paper describes the design principle of laser guide stars and in particular show the advantage of using a combination of two afocal systems. Indeed, the first afocal can be used, with controlled defocus, to tune the size and position of the beam waist at the entrance of the second afocal. This allows, for example, to decrease the ultimate size of the artificial star which can be achieved. Moreover, it also allows to set parameters so that tolerances of the system are released.