Polarization Convertion In Optical Fiber Based On Metasurface

Polarization conversion?PC?in optical fiber is a very important operation in practice,such as fiber sensors.Up to now,however,the PC in fiber is usually achieved by coiling the fiber to realize geometric phase accumulation,coupling an external bulk optical element,or using the birefringent effect generating from mechanical squeezing.The electrical field in an optical structure is determined by amplitude,phase and polarization,so we can reshape the beam profile by controlling these three components.In recent years,the metasurface has become a hot research focus and based on it,many works on controlling the optical wavefront,especially the control of PC in free space,have been reported.In this paper,we propose a distinct approach for PC in optical fiber by introducing an all-dielectric metasurface in it,which is proved to be compact,efficient and robust.Based on this approach,nearly perfect polarization conversions can be achieved.The metasurface of our design is composed of series of cylindroids which are arranged into hexagonal lattices.In order to realize full polarization control,the geometric parameters and the arrangement of cylindroids must be appropriate.In this paper,the LP₀₁^x mode is changed to LP₀₁^ymode with the 74.6% transmission effiency;The LP₀₁^xmode is changed to left/right-handed circular polarization mode with the 93.1% transmission effiency;The LP₀₁^x mode is changed to vector modes,radial polarization mode TM₀₁ with the 84.1% transmission effiency and azimuthal polarization mode TE01 with the 82.2% transmission effiency.In order toevaluate the responses for fabrication errors,which can influence the polarization conversion,the structure parameters of metasurface and the refractive index of the fiber are numerically calculated.In addition,as many metasurfaces in free space have a narrow bandwidth,the all-dielectric metasurface designed in this paper has a relatively broad of 60 nm.Compared to traditional polarization converters,the all-dielectric metasurface is ultra-thin and sub-wavelength scale.The fabrication of this all dielectric metasurface can rely on the mature semiconductor manufacture technologies,so our work may have some potential applications in optical fiber systems and optical integrated circuits.