|
Bessel beams (BBs) were first introduced by Durnin in 1987 and have a transverse intensity distribution dictated by the Bessel function. BBs are characterized by their diffraction-free propagation and self-healing nature. The family of BBs is categorized into two sets - zeroth-order Bessel beams (ZOBBs) with narrow high-intensity center and high-order Bessel beams (HOBBs) with phase singularity and dark center. HOBBs are vortex beams as they carry orbital angular momentum (OAM). Several approaches to generate BBs have been devised; some of them include transforming a narrow annular beam with a lens, using an axicon, or using spatial light modulators. Nevertheless, these techniques involve space-consuming and expensive table-top diffractive optical elements. In recent years, the on-fiber generation of BBs has gained prominence as it offers miniaturized optical probes that can find exciting applications in different fields, ranging from bio-imaging to communications. Here, we present on-fiber 3D printed complex photonic structures that convert the Gaussian-like mode from single-mode fibers into BBs of various orders. Remarkably, we report for the first time the generation of HOBBs from optical fibers. Our technique is inspired by Durnin's approach of generating BBs due to the transformation of an annular beam through a lens. Our novel design has three sections; the first and second sections contain photonic crystal waveguides that convert the input Gaussian-like mode into an annular beam of arbitrary radius and width, which is then transformed into BBs with the help of a micro-lens. To generate HOBBs, we also integrated a spiral phase plate in the stacked structure. We compared the experimentally generated BB parameters with what predicted from theory and found an excellent match. For HOBBs, we performed modal decomposition to confirm the existence of OAM. Overall, we showcase the results of various BBs with orders up to 20.
|
