Abstract: There is provided a gain medium capable of radiating spectral lines differing in intensity as a function of position along a selected, gain-medium axis. A first resonator is arranged along a first optical axis that crosses the gain medium in a first region along the selected, gain-medium axis; the first resonator is adapted to establish a first optical mode that selectively lases one group of the spectral lines of the gain medium, namely a group thereof having high intensity within the volume in which the first mode is established. A second resonator is arranged along a second optical axis that crosses the gain medium in a second region along the selected, gain-medium axis; the second resonator is adapted to establish a second optical mode that lases a second group of spectral lines differing at least in part, from the first group. The second group of spectral lines has high intensity within the volume in which the second mode is established.