Turbulence aberration correction for vector vortex beams using deep neural networks on experimental data.

The vector vortex beams (VVB) possessing non-separable states of light, in which polarization and orbital angular momentum (OAM) are coupled, have attracted more and more attentions in science and technology, due to the unique nature of the light field. However, atmospheric transmission distortion is a recurring challenge hampering the practical application, such as communication and imaging. In this work, we built a deep learning based adaptive optics system to compensate the turbulence aberrations of the vector vortex mode in terms of phase distribution and mode purity. A turbulence aberration correction convolutional neural network (TACCNN) model, which can learn the mapping relationship of intensity profile of the distorted vector vortex modes and the turbulence phase generated by first 20 Zernike modes, is well designed. After supervised learning plentiful experimental samples, the TACCNN model compensates turbulence aberration for VVB quickly and accurately. For the first time, experimental results show that through correction, the mode purity of the distorted VVB improves from 19% to 70% under the turbulence strength of D/r0 = 5.28 with correction time 100 ms. Furthermore, both spatial modes and the light intensity distribution can be well compensated in different atmospheric turbulence.

Experimental demonstration of free-space multi-state orbital angular momentum shift keying.

Orbital angular momentum (OAM), a new dimension of photons, has potentials in lots of domains as high-dimensional data coding/decoding. Here we experimentally demonstrate a free-space data transmission system based on 8 bits multi-state OAM shift keying, where multiplexed optical vortices containing 8 various OAM states are employed to constitute 8 bits binary symbols. In the transmitter, the data coding of OAM shift keying is realized by switching a series of special-designed holograms. And in the receiver, the decoding is done by a single Dammann vortex grating along with image processing. We experimentally transmit data, including a gray-scale image, in free-space for 10 meters, showing zero bit-error-rate. The demonstrated results indicate a wide prospect for the future high-dimensional large data rate optical security communications.

Demonstration of free-space one-to-many multicasting link from orbital angular momentum encoding.

Multicasting is necessary when distributing signals between multiple users. In this Letter, we demonstrate an orbital angular momentum (OAM) encoding-based free-space one-to-many multicasting link, where digital signals are encoded into a series of time-varying OAM states and transmitted from one transmitter to multiple receivers with various locations. Moreover, encoding N various signals simultaneously in one transmitter and sending them at the same time to N various receivers separately, is also demonstrated. As a proof-of-concept, four different gray images are coded by one transmitter simultaneously and multicast to four various receivers separately. The favorable decoding results returned by the four receivers show good multicasting performance.

Detection of angular acceleration based on optical rotational Doppler effect.

The angular acceleration of a spinning object can be estimated by probing the object with Laguerre-Gauss (LG) beams and analyzing the rotational Doppler frequency shift of returned signals. The frequency shift is time dependent because of the change of the rotational angular velocity over time. The detection system is built to collect the beating signals of LG beams back-scattered from a non-uniform spinning body. Then a time-frequency analysis method is proposed to study the evolution of the angular velocity in time. The experimental results of different angular accelerations of the rotator are consistent with expectations. The measurement errors of different probe beams with various topological charges from l = ± 10 to l = ± 100 are also investigated.

Orbital angular momentum channel monitoring of coaxially multiplexed vortices by diffraction pattern analysis.

We theoretically and experimentally demonstrate a scheme to monitor the weight of a single orbital angular momentum (OAM) channel for coaxial multiplexed optical vortices with large mode spacing. A specially designed holographic grating is illuminated by the incident multiplexed vortices first. Then the weight of each single OAM channel is obtained after analyzing the captured diffraction patterns. This work will find applications in domains where multiplexed optical vortices are of interest, such as the OAM-based data-transmission system, and so on.

Non-diffractive Bessel-Gauss beams for the detection of rotating object free of obstructions.

Bessel-Gauss beams carrying orbital angular momentum are widely known for their non-diffractive or self-reconstructing performance, and have been applied in lots of domains. Here we demonstrate that, by illuminating a rotating object with high-order Bessel-Gauss beams, a frequency shift proportional to the rotating speed and the topological charge is observed. Moreover, the frequency shift is still present once an obstacle exists in the path, in spite of the decreasing of received signals. Our work indicates the feasibility of detecting rotating objects free of obstructions, and has potential as obstruction-immune rotation sensors in engine monitoring, aerological sounding, and so on.

Simultaneous generation of multiple perfect polarization vortices with selective spatial states in various diffraction orders.

We demonstrate an approach to generate multiple perfect polarization vortices (PPVs) with selective spatial polarization distribution in various diffraction orders. The key is the design of a hologram with an anisotropic polarization diffraction grating. In the experiment, a setup consisting of two spatial light modulators is built. By encoding the specially designed holograms, PPVs with various states are obtained in different diffraction orders simultaneously, for instance, in radial and azimuthal polarized states. These PPVs may have applications in laser processing, optical tweezers, and so on.

Time Circular Birefringence in Time-Dependent Magnetoelectric Media.

Light traveling in time-dependent media has many extraordinary properties which can be utilized to convert frequency, achieve temporal cloaking, and simulate cosmological phenomena. In this paper, we focus on time-dependent axion-type magnetoelectric (ME) media, and prove that light in these media always has two degenerate modes with opposite circular polarizations corresponding to one wave vector , and name this effect "time circular birefringence" (TCB). By interchanging the status of space and time, the pair of TCB modes can appear simultaneously via "time refraction" and "time reflection" of a linear polarized incident wave at a time interface of ME media. The superposition of the two TCB modes causes the "time Faraday effect", namely the globally unified polarization axes rotate with time. A circularly polarized Gaussian pulse traversing a time interface is also studied. If the wave-vector spectrum of a pulse mainly concentrates in the non-traveling-wave band, the pulse will be trapped with nearly fixed center while its intensity will grow rapidly. In addition, we propose an experimental scheme of using molecular fluid with external time-varying electric and magnetic fields both parallel to the direction of light to realize these phenomena in practice.