The self-channeling of extremely high power laser beams permits the ignition of plasma filaments in dielectrics, such as air and glasses. If no constraints are imposed by the geometry of the material, the plasma appears as a straight bright line. Here, we show that plasma filaments may be ignited in the vicinity of the interface between two materials in optical fibers, i.e., either the core-cladding or the cladding-air interface. In the latter case, helical plasma filaments lead to the emission of rainbow spiral beams. In perspective, our results may pave the way to a novel approach for micro-structuring optical fibers, and for generating light beams with orbital angular momentum.
We study multiphoton absorption-induced damages to standard silica multimode optical fibers, induced by means of femtosecond infrared laser beams. During the damaging process, the dynamics of beam propagation turns out to non-trivially evolve over a time scale of several hours. Such a long term evolution produces an irreversible drop of the optical transmission, which is accompanied by a drastic change of the output supercontinuum spectrum. A microscopic analysis of the damages was carried out by means of both optical microscopy and absorption-contrast computed X-ray tomography. This has permitted us to obtain information about the sign of the refractive index variation which is induced by the optical breakdown. Our results will find application in a wide array of emerging technologies employing high-power fiber optic beams, such as fiber lasers and micromachining.
We analyzed the nonlinear dynamics of pulsed beam self-cleaning in nonlinear tapered Ytterbium doped and Erbium-Ytterbium codoped graded-index multimode optical fibers, with quasi-uniform doping distribution in the core cross-section. By increasing the net gain when operating in active configuration we observed that the output spatial intensity distribution changed from a speckled into a high-quality and bell-shaped beam. By launching pulses in the normal dispersion regime of the taper, from the wider into the smaller core diameter, we generated a supercontinuum emission between 520 nm and 2600 nm. When the laser pulses were launched into the small core side of the tapered fiber or in the Erbium-Ytterbium fiber, self-cleaning was obtained without any self-phase modulation-induced spectral broadening or frequency conversion.
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