Rigorous Analysis on Ring-Doped-Core Fibers for Generating Cylindrical Vector Beams

Abstract

We propose a novel active fiber design for selectively generating cylindrical vector beams (CVBs) or cylindrical vector modes (CVMs) which can be applied to conventional fiber lasers. A fiber is designed to have a ring-shaped core refractive index profile which can lead to the best overlap between the active dopant distribution profile and the lowest-order CVM (LCVM) field profile. Therefore, the overlap factor (OVF) of the LCVM becomes even higher than that of the fundamental mode. We emphasize that this condition cannot be satisfied by a conventional step-index core fiber (SICF) but by the ring-doped core fiber (RDCF). Because the lasing threshold is inversely proportional to the OVF, the LCVM can predominantly be stimulated even without going through special procedures to impose extra loss mechanisms to the fundamental mode. We numerically verify that the OVF of the LCVM with the doped ions can significantly exceed that of the fundamental mode if the proposed fiber design is applied. In addition, an RDCF of the proposed fiber design can also operate in a regime containing no higher-order modes besides the LCVM, so that it can selectively and efficiently generate the LCVM without being disrupted by the parasitic lasing of the higher-order modes. We highlight that an optimized RDCF can lead to a >30 % higher OVF ratio than a SICF having the same doped area. The proposed model is expected to be useful for enhancing the efficiency of generating CVBs in an all-fiber format.

Sub-wavelength annular-slit-assisted superoscillatory lens for longitudinally-polarized super-resolution focusing

“…We perform FEM numerical simulations via COMSOL to analyze the characteristics of SOL for RP, AP, and combined input. Note that circular optical waveguides have second order degenerate modes with both RP and AP components 26 . The SOL used on the simulation is the lens from ref.…”

“…Despite these many benefits, LP foci are not widely used in practical applications, primarily because the necessary RP input is difficult to generate and align. For linearly-or circularly-polarized inputs, additional free-space optical components are required to convert the polarization state, which complicates the whole system 26 . For waveguide inputs, such as optical fibre modes, one can prepare cylindrical vector states -which are composed of a superposition of radial, azimuthal, and hybridized degenerate modes -by means of donut shaped active fibre, or long-period-grating mode couplers [27][28][29] .…”

“…The most obvious light source for SAS-SOL would be mode-selected cylindrical waveguides, for example optical fibre, which generate both RP and AP components 26 . If a free-space focusing system is preferred, then prepared RP beam would be focused correctly by the SAS-SOL.…”

Metallic triangular pillar grating arrays for high transmission polarizers for air:glass interfaces

“…22) The design parameters are the period, height of the structure and the duty ratio at the bottom face of the metallic structure. The wavelength is selected as 1064 nm in the calculations (as appropriate for the Yb doped fiber laser [23][24][25] ). The base material is assumed to be silica 26) and the metal gold.…”