Generic dark hollow beams using negative cones chemically etched in fiber tips

Abstract

We report here the characteristics of dark hollow beams (DHBs) generated using negative micro-axicons fabricated via selective chemical etching process in the tip of optical fibers. The DHB output from the etched fibers were further manipulated to generate single and multiple period optical bottle beams, 0^th- and 1^st- order Bessel beams and astigmatic Bessel beams and beams with helical wavefront. Selective excitation of the guided modes in single and multimode optical fibers with V-numbers ranging from 2.405 - 5.69 are etched to different negative cone dimensions and are used to generate in a controllable way DHBs and related optical beams. Negative cones are etched in the fiber tips via selective chemical etching process wherein the different etch rates of the fiber core and cladding dopants and dopant concentration results in different cone angles and cone depths. Input laser beam guided through the optical fiber diffracts and refracts at the tip generating beams with one or many bright rings surrounding the dark central spot. By positioning the DHB in the front focal plane of a bi-convex lens we generate Bessel beams whose characteristics are compared with that generated using positive cone etched in the fiber tip and bulk axicon. Under certain excitation conditions we also observe helical wavefront DHBs with phase dislocation embedded in the beam.