Orbital angular momentum (OAM) of an optical vortex has attracted great interest from the scientific community due to its significant values in high-capacity optical communications such as mode or wavelength division multiplexer/demultiplexer. Although several configurations have been developed to demultiplex an optical vortex, the multiwavelength high-order optical vortex (HOOV) demultiplexer remains elusive due to lack of effective control technologies. In this study, we present the design, fabrication, and test of metasurface optical elements for multiwavelength HOOV demultiplexing based on optical gyrator transformation transformations in the visible light range. Its realization in a metasurface form enables the combined measurement of OAM, the radial index p, and wavelength using a single optical component. Each wavelength channel HOOV can be independently converted to a high-order Hermitian-Gaussian beam mode, and each of the OAM beams is demultiplexed at the converter output. Furthermore, we extend the scheme to realize encoding of the three-digit gray code by controlling the wavelength or polarization state. Experimental results obtained at three wavelengths in the visible band exhibit good agreement with the numerical modeling. With the merits of ultracompact device size, simple optical configuration, and HOOV recognition ability, our approach may provide great potential applications in photonic integrated devices and systems for high-capacity and demultiplex-channel OAM communication.
Based on the classical spectral representation method of simulating turbulent wind speed fluctuation, a harmonic superposition algorithm was introduced in detail to calculate the homogeneous turbulence wind field simulation in space. From the view of the validity of the numerical simulation results in MATLAB and the simulation efficiency, this paper discussed the reason for the bias existing between three types of turbulence intensity involved in the whole simulation process: simulated turbulence intensity, setting reference turbulence intensity, and theoretical turbulence intensity. Therefore, a novel spectral correction method of a standard deviation compensation coefficient was proposed. The simulation verification of the correction method was carried out based on the Kaimal spectrum recommended by IEC61400-1 by simulating the uniform turbulent wind field in one-dimensional space at the height of the hub of a 15 MW wind turbine and in two-dimensional space in the rotor swept area. The results showed that the spectral correction method proposed in this paper can effectively optimize the turbulence intensity of the simulated wind field, generate more effective simulation points, and significantly improve the simulation efficiency.
Long-wave infrared imaging systems are widely used in the field of environmental monitoring and imaging guidance. As the core components, the long-wave infrared lenses suffer the conditions of less available materials, difficult processing, large volume and mass. Metalens composed of sub-wavelength structures is one of the most potential candidates to achieve a lightweight and planar optical imaging systems. Meanwhile, it is essential to obtain large-aperture infrared lenses with high power and high resolution. However, it is difficult to use the finite-difference time-domain method to simulate a large-aperture metalens with the diameter of 201 mm due to the large amount of computational memory and computational time required. Here, to solve the mentioned problem, we firstly propose a simulation method for designing a large-aperture metalens, which combines the finite-difference time-domain algorithm and diffraction integration. The finite-difference time-domain algorithm is used to simulate the meta-atom’s transmitted complex amplitude and the one-dimensional simplification of the diffraction integral is to calculate the focused field distributions of the designed metalens. Furthermore, the meta-atom spatial multiplexing is applied to design the all-silicon metalenses with the aperture of 201 mm to realize dual-wavelength (10 μm and 11 μm) achromatic focusing, super anomalous dispersion focusing and super normal dispersion focusing. The designed metalenses are numerically confirmed, which reveal the feasibility of all-silicon sub-wavelength structures to accomplish the multiwavelength dispersion control. The designed all-silicon metalenses have the advantage of lightweight and compact. The proposed method is effective for the development of large-aperture imaging systems in the long-wave infrared.
Traditional infrared lenses cannot meet the requirements of planarization and lightweight of infrared optical systems due to their large volume and mass. An infrared metasurface with almost zero thickness can control the incident beam’s amplitude, phase and polarization arbitrarily, which make it possible to circumvent these limitations. However, no metasurface has been designed to realize sub-diffraction focusing in the long-wave infrared band. In this article, a longwave infrared meta spiral zone plate (LWIR-MSZP) is designed, which converts the incident linearly polarized beam to an azimuthally polarized beam and focuses the latter into a sub-diffraction solid spot. The designed LWIR-MSZP works at a wavelength of 10.6 μm and has a diameter of 480 μm and numerical aperture (NA) of 0.8. The simulated full width at half maximum (FWHM) and depth of focus (DOF) of the focal spot are 0.6λ and 2.24λ, respectively. The simulated efficiency is 24.04%. The proposed design procedure greatly simplifies the long-wave infrared sub-diffractive focusing optical system and complements the technical gap to achieve sub-diffractive focusing in long-wave infrared using a metasurface. To make it further, the all-silicon meta-atom employed in this work has the advantage of low cost once semiconductor fabrication techniques are introduced. We believe that this result can be applied to the related fields of super-resolution imaging and laser processing in long-wave infrared band.
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