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.
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