OBSERVATION OF SELF-CHANNELED PLASMA FORMATION AND BULK MODIFICATION IN OPTICAL FIBERS USING HIGH-INTENSITY FEMTOSECOND LASER

We report the first experimental observation of laser-induced self-cha nneled plasma formation and bulk modification in multimode optical fib ers during excitation by a high-intensity (more than 10(11) W/cm(2)) f emtosecond (110 fs) Ti: sapphire laser (780 nm). Solid-density plasma formation at various input intensities and the corresponding variation .of anti-Stokes spectra were observed simultaneously. The threshold in tensity for plasma formation in the optical fiber was found to be 8 x 10(11) W/cm(2). When the input intensity reached 1.5 x 10(12) W/cm(2), self-channeled plasma formation was observed at a distance of 9-10 mm from the input end of an optical fiber. After 5 min of irradiation at 1.7 x 10(12) W/cm(2), bulk modification, 5 mu m in diameter and appro ximately 6 mm in length was observered in two types of multimode optic al fibers with core/cladding diameters of 100/110 mu m and 200/220 mu m.