Abstract
We report the first experimental observation of laser-induced self-channeled plasma formation and bulk modification in multimode optical fibers during excitation by a high-intensity (above 10<SUP>11</SUP> W/cm<SUP>2</SUP>) femtosecond (110fs) Ti:sapphire laser. The solid-density plasma formation at various input intensities and the corresponding variation of anti-Stokes spectra were observed simultaneously. The threshold for plasma formation in the optical fiber was found to be 8 X 10<SUP>11</SUP> W/cm<SUP>2</SUP>. When the input intensity exceeded 1.5 X 10<SUP>12</SUP> W/cm<SUP>2</SUP>, self-channeled plasma formation was observed with a length of 9 - 10 mm from the input end of an optical fiber. It was observed that the bulk modification, after 5 minutes of irradiation at 1.7 X 10<SUP>12</SUP> W/cm<SUP>2</SUP>, resulted in a 5 micrometer diameter and approximately 6 mm length, in two types of multimode optical fiber with core/cladding diameters of 100/110 micrometer and 200/220 micrometer.
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