Information transmission using radial carpet beams

Gaussian beam diffraction from radial structures: detailed study on the diffraction from sinusoidal amplitude radial gratings

In this work, we report a comprehensive theoretical investigation on the diffraction of a Gaussian beam from structured radial apertures. In particular, the study of near- and far-field diffraction of a Gaussian beam from an amplitude radial grating having a sinusoidal profile provides new theoretical insights and possible applications. We observe a high self-healing feature at far-field for the Gaussian beam in the diffraction from amplitude radial structures. It is also shown that by increasing the spokes number of the grating, the strength of the self-healing decreases, and reforming of the diffracted pattern into a Gaussian beam occurs at longer propagation distances. The energy flow towards the central lobe of the diffraction pattern and its dependence on the propagation distance are also investigated. In the near-field regime, the diffraction pattern is very similar to the intensity distribution in the central area of the radial carpet beams generated in the diffraction of a plane wave from the same grating. It is shown that by optimally choosing the waist radius of the Gaussian beam, in the near-field regime, it is possible to have a petal-like diffraction pattern, which has been experimentally used in multiple-particle trapping. Compared to radial carpet beams, since in this case there is no energy in the geometric shadow of the radial spokes of the grating, the main part of the power of the incident Gaussian beam is transferred to the main intensity spots of the petal-like pattern, which significantly increases the multi-particle trapping efficiency. We also show that regardless of the grating spokes number, at the far field, the diffraction pattern becomes a Gaussian beam, and its power share reaches 2/3 of the total power passed through the grating.

1024-ary composite OAM shift keying for free-space optical communication system decoded by a two-step neural network

The demand for high-dimensional encoding techniques for communication systems is increasing. Vortex beams carrying orbital angular momentum (OAM) provide new degrees of freedom for optical communication. In this study, we propose an approach for increasing the channel capacity of free-space optical communication systems by integrating superimposed orbital angular momentum (OAM) states and deep learning techniques. We generate composite vortex beams with topological charges ranging from -4 to 8 and radial coefficients ranging from 0 to 3. A phase difference among each OAM state is introduced to significantly increase the number of available superimposed states, achieving up to 1024-ary codes with distinct features. To accurately decode the high-dimensional codes, we propose a two-step convolutional neural network (CNN). The first step is to make a coarse classification of the codes, while the second step is to finely identify the code and achieve decoding. Our proposed method demonstrates 100% accuracy achieved for the coarse classification after 7 epochs, 100% accuracy achieved for the fine identification after 12 epochs, and 99.84% accuracy achieved for testing, which is much faster and more accurate than one-step decoding. To demonstrate the feasibility of our method, we successfully transmitted a 24-bit true-color Peppers image once with a resolution of 64 × 64 in the laboratory, yielding a bit error rate of 0.

Gear-like rotatable optical trapping with radial carpet beams

Optical tweezers have become a powerful tool in the fields of biology, soft condensed matter physics, and nanotechnology. Here, we report the use of recently introduced radial carpet beams (RCBs) in the optical tweezers setup to trap multiple particles. An RCB is produced by diffraction of a plane or Gaussian beam from an amplitude radial grating. Because of the radial symmetry of the grating, all the diffraction orders are propagated along the optical axis and are used for trapping. Based on the number of grating spokes, the produced RCB has a definite number of high-intensity spots on the transverse plane located over a circular ring. These high-intensity spots of the beam provide multi-traps when it passes through an objective lens and have enough gradient force to trap polystyrene and silica particles. Moreover, the diffracted light from the grating has this property to transfer the angular momentum. We show that the multi-trapped birefringent particles could rotate in their own traps when polarization of the trapping RCB to be circular. In addition, the orbital rotation of the particles is simply executable by manually rotating the grating in its plane around the optical axis.

Colorful radial Talbot carpet at the transverse plane

In this work we theoretically and experimentally investigate the diffraction of spatially coherent and collimated white light beam from radial amplitude gratings. Theoretical part of the work is resolved with the Fresnel-Kirchhoff integral. In the experimental part, a collimated wave-front of a white light beam emitting from a halogen lamp is transmitted through a radial amplitude grating. We digitally record the diffraction pattern in various distances from the grating using a CCD camera. The resulted diffraction pattern that we call it "Colorful radial Talbot carpet at the transverse plane" has a shape-invariant form under propagation. The other significant aspects of this pattern are the existence of a quite patternless dark area located around the optical axis and an intense rainbow-like ring in the vicinity of the patternless area. The rainbow color changes radially from the violet in the vicinity of the patternless area to red by increasing the radius in which the purity of the colors in the inner side is dominant. We call this phenomena "diffraction-based rainbow". In addition, the transverse plane Talbot carpet pattern consists colorful self-images of the grating's spokes at the larger radii. The theoretical calculations and experimental results verify each other completely. The introduced diffraction-based rainbow can be utilized in spectrometry.