Design and optical characterization of specialty optical fibers for the stable transmission of vector-vortex beams

Abstract

In recent years, significant research has been dedicated towards exploring complex structured light beams such as optical vortices, a class of light beams that demonstrate doughnut shape intensity profiles and characterized by dark central hole, formed either due to phase singularities (known as orbital angular momentum (OAM) beams) or polarization singularities (cylindrical vector beams (CVBs)). Owning to such unique optical properties, vortex beams have demonstrated promising advances in numerous applications such as: quantum optics, optical microscopy, particle manipulation, sensing, optical trapping, and space division multiplexing. For example, vortex beams (with doughnut shaped intensity profile) are well-established in applications related to stimulated emission depletion (STED) super resolution microscopy, where annular beams enable breaching the diffraction limit. Another application is associated to using optical vortices in unique light-matter interactions (with possible applications in molecular spectroscopy), which could lead to new physical phenomenon not observable with conventional Gaussian beams. These captivating applications of vortex beams have inspired more research work in probing these beams in both free space and guided fiber optics. Moreover, further innovation in currently existing technology is vital in order to realize the high purity generation and stable transmission of these exotic beams. Consequently, researchers and engineers have been on the forefront to explore different generation techniques such as: spiral phase plate, spatial light modulator (SLM), and nanostructured metamaterials, to name few. In parallel, numerous novel fiber designs have been proposed and developed towards the stable transmission of optical vortex. This thesis examines specific designs, optimization and characterization of optical fibers tailored for the propagation of optical vortex beams. The principal objective is to develop an optical fiber suitable for the stable transmission of optical vortex, which maintains high mode purity. In this thesis, we propose a novel fiber design that demonstrates a new guiding regime termed endlessly mono-radial, where the fiber supports only guided modes of the fundamental radial order over a very large wavelength range. The latter property is important for the stable and broadband transmission of mono-radial CV and OAM beams. A finite element method (FEM) based model was developed for studying ring shaped (i.e. annular) core photonic crystal fiber. Moreover a systematic parameter investigation allows one to find new guiding regimes and evaluate fiber properties desirable for vortex beams. The simulation results are compared with a theoretical analytical model based on the exact solution of Helmholtz equation. This is followed by a demonstration of numerical simulation for nonlinear supercontinuum generation of vortex beams using 835 nm femtosecond pulse laser employing optimized fiber design parameters. In the second part of thesis, fiber theoretical designed and simulated in first part, is later fabricated and experimentally characterized for transmission of optical vortex, which displays higher mode stability and purity in fiber. In this experiment, OAM and CV beams are generated using S-plate (commercially available spiral phase plate) in free space and launched in fabricated fiber for analyzing mode purity and stability upon propagating through the fiber. This experimental characterization allows a deeper understanding of the CV and OAM beams transmission in fiber. In the last part of thesis, we proposed a radically new design: the radially anisotropic ring-core fiber for novel guiding characteristic. We show that the proposed fiber could in principle allow the stable co-propagation (with lower intermodal coupling) of different kind of OAM beams formed by the superposition of both the radially (TE) and azimuthally (TM) polarized modes, and coherent combination of hybrid (HE / EH) modes. This feat is possible because the proposed fiber enables to bridge the non-degeneracy of the TE and TM modes, without affecting the degeneracy of hybrid (HE / EH) modes. In addition to this, this fiber design could unlock further unconventional waveguiding regime, where either the doughnut-shaped radially or azimuthally polarized mode becomes the fundamental mode of the fiber in place of usual Gaussian-like HE11 mode. We hope that these findings will help in further development of next generation optical sensing and communication systems.

Date	22 Jun 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Bora Ung (Supervisor)