Velocity selection of ultra-cold atoms with Fabry-Perot laser devices: improvements and limits

We discuss a method to select the velocities of ultra-cold atoms with a modified Fabry-Perot type of device made of two effective barriers and a well, created, respectively, by blue- and red-detuned lasers. The laser parameters may be used to select the peak and width of the transmitted velocity window. In particular, lowering the central well provides a peak arbitrarily close to zero velocity having a minimum but finite width. The low-energy atomic scattering off this laser device is parametrized and approximate formulae are found to describe and explain its behaviour and limits. We illustrate the velocity-selection method with an atomic wave packet prepared as a Bose-Einstein condensate