A hollow beam from a holey fiber

A high-quality spectrally isolated hollow beam is produced through a nonlinear-optical transformation of Ti: sapphire laser pulses in a higher order mode of a photonic-crystal fiber (PCF). Instead of a doughnut shape, typical of hollow beams produced by other methods, the far-field image of the hollow-beam PCF output features perfect sixth-order rotation symmetry, dictated by the symmetry of the PCF structure. The frequency of the PCF-generated hollow beam can be tuned by varying the input beam parameters, making a few-mode PCF a convenient and flexible tool for the guiding and trapping of atoms and creation of all-fiber optical tweezers.