<title>Modification in optical fibers using high-intensity femtosecond lasers</title>

Abstract

Refractive index modification in optical multimode fibers was firstly demonstrated, as a fabrication method of double cladding structure, using plasma channeling excited by a high intensity femtosecond Ti:Sapphire laser. The induced refractive index modification in a pure silica multimode fiber with 100/110 micrometers core/cladding diameter was reached to the length of approximately 9-10 mm from the end of optical fiber with the diameters ranging form 5 to 8 micrometers at more than input intensity of 1.5 X 10<SUP>12</SUP> W/cm<SUP>2</SUP>. It had graded refractive index profile that was a symmetric form from the center of a multimode fiber and the maximum values of refractive index change was 1.6 X 10<SUP>-2</SUP>. According to the ESR spectroscopic measurement, it was observed that the concentration of defect of SiE center was heavily increased in refractive index modification refer to that of non-modification region. It was suggested that the defect was induced by the multiphoton absorption process through plasma channeling. Near-field pattern and beam intensity profile of a modified multimode fiber showed that bulk modification played a role as a double cladding structure. The fabrication method of double cladding structure in the end of multimode fiber can be useful tool for mode converter and single-mode connector in the fields of optical communication and optical sensor.