The quickly increasing data transfer load requires an urgent revolution in current optical communication. Orbital angular momentum (OAM) multiplexing is a potential candidate with its ability to considerably enhance the capacity of communication. However, the lack of a compact, efficient, and integrated OAM (de)multiplexer prevents it from being widely applied. By attaching vortex gratings onto the facets of a few-mode fiber, we demonstrate an integrated fiber-based OAM (de)multiplexer. A vortex grating fabricated on the fiber facet enables the direct multiplexing of OAM states at one port and the demultiplexing of OAM states at the other port. The measured bit error rate of the carrier signal after propagating through a 5-km few-mode fiber confirms the validity and effectiveness of the proposed approach. The scheme offers advantages in future high-capacity OAM communication based on optical fiber.
Optical beam wander is one of the most important issues for free-space optical (FSO) communication. We theoretically derive a beam wander model for Bessel beams propagating in turbulent atmosphere. The calculated beam wander of high order Bessel beams with different turbulence strengths are consistent with experimental measurements. Both theoretical and experimental results reveal that high order Bessel beams are less influenced by the turbulent atmosphere. We also demonstrate the Bessel beams based orbital angular momentum (OAM) multiplexing/demultiplexing in FSO communication with atmospheric turbulence. Under the same atmospheric turbulence condition, the bit error rates of transmitted signals carried by high order Bessel beams show smaller values and fluctuations, which indicates that the high order Bessel beams have an advantage of mitigating the beam wander in OAM multiplexing FSO communication.
We propose an effective all-fiber method to generate a high-order optical vortex (OV) via twisting a strong modulated long-period fiber grating (LPFG) written in a four-mode fiber (4MF). With a special design and optimization of the procedures of CO-laser irradiation, an LPFG with strong period deformation is achieved in the 4MF. Based on this LPFG, we can directly convert the linear polarization (LP) fiber fundamental mode (LP) to the high-order LP core mode (LP) with efficiency of 99.7% and then transform the LP mode into a high-order OV mode (±2 order). This is the first time, to the best of our knowledge, that ±2-order OV modes have been experimentally generated with just one fiber grating in an all-fiber-system.
Metasurfaces are planarized and miniaturized versions of conventional optical elements. Subwavelength‐thick single‐layer metalenses have diffraction limited resolution for on‐axis imaging but relatively low resolution for off‐axis imaging due to off‐axial aberrations. The aberrations of planar single‐layer metalenses have been corrected by patterning two metasurfaces on both sides of a substrate to form metalens doublets with a thickness of hundreds of micrometers to millimeters. The multilevel diffractive lenses are demonstrated to achieve wide angle imaging with a thickness of several micrometers, however, the off‐axial aberrations are not compensated. Here, an epsilon‐greedy algorithm‐based scheme for achieving a planar wavelength‐thick single‐layer aberration‐compensated (SLAC) flat lens consisting of dielectric nanoring structures fabricated by 3D printing are proposed. The scheme is experimentally validated via a SLAC flat lens. This SLAC flat lens has a thickness of 1 µm, a numerical aperture of 0.45, a focal length of 1 mm, a full field of view (FOV) of 32° that enables aberration‐compensated imaging along the focal plane and monochromatic microscopic imaging with resolution better than 2.2 µm at a wavelength of 633 nm. This scheme can lead to ultrathin wide‐FOV flat lens designs as well as low‐cost mass production, which has various applications in miniscopes, mobile camera modules, and machine vision.
Cylindrical vector beams (CVBs), including radial, azimuthal, and hybrid polarization vortex states, are based on the polarization singularity and regarded as the eigenmodes of fiber. In this Letter, we propose and demonstrate CVB (de)multiplexing communication in a few-mode fiber (FMF). We simulate the eight CVB modes, including the ±1 orders and the ±2 orders supported by the FMF. We measure the mode purities of the ±1-order and the ±2-order CVBs, which can reach 67.87%, 69.26%, 73.84%, and 71.95% after transmission in the 5 km FMF. The mode cross talk between the CVBs and their adjacent orders is less than -7.54 dB. In the experiment, we demonstrate four modes of coaxial CVB multiplexing communication with the orders of -2 and spatially orthogonal polarization states. The mode cross talk is less than -23.35 dB for all channels. CVB modes carrying 10 Gbit/s on-off keying signals are transmitted in the 5 km FMF and achieve bit-error rates below the forward error correction threshold of 3.8×10.
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