<title>Incorporation of diffractive structures on side-polished fiber arrays</title>

Abstract

This paper investigates methods to launch high modes propagating along helix paths into a graded index fiber. These techniques may be particularly useful to avoid the central dip problem due to defects located in the center of the index profile of multimode fibers. Light was coupled into a graded index multimode fiber using the flat surface of side-polished or D-shaped fiber. The fibers were polished down to a few microns close to the core on a distance of about 3mm. The first method, active, is based on coupling the light from a D-shaped single mode fiber to a D-shaped graded index fiber. The two fibers are in contact through a film of index matching fluid and the coupling may be thought as the leakage from a fiber to a slab waveguide. The launching angle of the skew rays in the multimode fiber is controlled by the tilt between the fibers. Simulations using beam propagation method are presented. Analytical equations for the path of the skew rays in graded index fiber are also used for a parabolic index profile. The second method, passive, consists of etching a tilted grating on a side polished multimode fiber by means of focused-ion beam (FIB) technology. We discuss the fabrication of such a grating and the possibility of using FIB technology to etch diffractive elements on fused silica waveguides.