Fabrication of single-mode waveguide structure in optical multimode fluoride fibers using self-channeled plasma filaments excited by a femtosecond laser

A permanent structure of a single-mode waveguide in optical multimode fluoride fibers was first fabricated using a self-channeled plasma filament excited by a femtosecond (110-fs) Ti:sapphire laser (_p=800 nm). The photoinduced refractive-index modification in a multimode step-index fluoride glass (ZBLAN) fiber with a 100/110-m core/cladding diameter reached a length of approximately 12–15 mm from the input surface of the optical fiber, with the diameters ranging from 5 to 8 m at input intensities more than 1.0×10¹² W/cm². The graded refractive-index profiles were fabricated to have a symmetric form from the center of a multimode fluoride fiber and a maximum value of the refractive-index change (n) was measured to be 1.3×10^-2. The beam profile of the output beam transmitted through the modified multimode fibers showed that the photoinduced refractive-index modification produced a permanent structure of a single-mode waveguide. PACS 42.55.-f; 42.65.Jx; 42.81.Wg