An optical fiber for producing laser radiation at a characteristic
wavelength includes a first multimode core region and an active region
embedded within the core region for producing radiation at the
characteristic wavelength when pumped by pump radiation. The core region
is adapted for guiding the laser radiation in a longitudinal direction of
the fiber and is adapted for guiding pump radiation. The active region
has a sufficiently small transverse dimension such that radiation
produced in the active region is not confined to the active region.