Fabrication of double cladding structure in optical multimode fibers using plasma channeling excited by a high-intensity femtosecond laser

Refractive index modification in multimode optical fibers was first demonstrated using the plasma channeling excited by a high-intensity femtosecond (110 fs) Ti: sapphire laser (lambda(p) = 790 nm). The refractive index modification induced in a pure silica multimode step-index fiber with a 100/110 mu m core/cladding diameter reached a length of approximately 9 similar to 10 mm from the input surface of the optical fiber with the diameters ranging from 5 to 8 mu m at input intensities more than 1.5 X 10(12) W/cm(2). The graded refractive index profiles were fabricated to be a symmetric form from the center of a multimode fiber and a maximum value of refractive index change (Delta n) was measured to be 2.1 x 10(-2). According to the electron spin resonance spectroscopic measurement, it was found that the defect concentration of SiE' center increased significantly in the modified region in relation to that of the region without modification. The plasma self-channeling would induce the refractive index modification with the defects. The intensity profile of the output beam transmitted through the modified multimode fibers showed that the bulk modification produced a double cladding structure. The fabrication method of the double cladding structure in optical fibers can be a useful tool for a variety of applications such as mode converters and single-mode connectors in the fields of optical communication and optical sensor. (C) 1999 Elsevier Science B.V. All rights reserved.