Fabrication study of double-cladding structure in optical fibers using plasma channeling induced by a femtosecond laser

Abstract

Refractive index modification in multimode optical fibers is first demonstrated using the plasma channeling excited by a high-intensity femtosecond (110 fs) Ti:sapphire laser ((lambda) <SUB>p</SUB> equals 790 nm). The refractive index modification induced in a pure silica multimode step-index fiber with a 100/110 micrometers core/cladding diameter reaches the length of approximately 9 approximately 10 mm from the input face of the optical fiber with the diameters ranging from 5 to 8 micrometers at input intensities more than 1.5 X 10<SUP>12</SUP> W/cm<SUP>2</SUP>. The graded refractive index profiles are fabricated to be a symmetric form from the center of a multimode fiber and a maximum value of refractive index change ((Delta) n) is measured to be 2.1 X 10<SUP>-2</SUP>. According to the electron spin resonance spectroscopic measurement, it is 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 induced the refractive index modification with the defects. The intensity profile of the output beam transmitted through the modified multimode fibers shows that the bulk modification produces 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.