Integrated all-fiber structures for generating doughnut beam arrays and hollow Bessel-like beams

Two all-fiber structures for generating doughnut beam arrays and hollow Bessel-like beams are demonstrated experimentally based on mode-selective couplers at the visible band. The mode-selective couplers are acted as mode converters to produce doughnut beams, which have the properties of high mode conversion efficiency (94%), low insertion loss (<0.5 dB) and wide bandwidth (∼100 nm). A compact and efficient approach is presented through integrating a tapered multicore fiber with a mode-selective coupler, and a doughnut beam array with nearly equal power distribution is obtained based on the mechanism of coupled-power theory in the tapered multicore fiber. The hollow Bessel-like beams with different cylindrical vector modes and orbital angular momentum are achieved by fabricating a polymer microtip at the facet of the few-mode fiber output port of a mode-selective coupler. The self-healing characteristics of a Bessel-like beam are investigated in detail and the application in an optical tweezer system of a hollow Bessel-like beam exhibits a longer focal depth than that of a Gaussian beam. Both methods used for generating all-fiber-based doughnut beam arrays and hollow Bessel-like beams have the advantages of compactness, flexibility, and easy integration with free-space optical systems. Generated two kinds of beams show huge potentials for the application fields of super-resolution microscopy, laser material process and optical trapping and so on.