Beam wander relieved orbital angular momentum communication in turbulent atmosphere using Bessel beams

Stability of optical knots in atmospheric turbulence

Topology advances the understanding of many branches of science and technology, from elementary particle physics to condensed matter physics. While the topological stability of mathematical knots implies robustness to perturbations and suggests their potential as information carriers, the behavior of optical knots in perturbative environments is largely unexplored. Here, we experimentally and theoretically investigate the effects of atmospheric turbulence on optical knot stability and demonstrate that their topological invariant can be preserved in the weak turbulence regime but may not be conserved in the stronger turbulence conditions, despite their topological nature. Such topology transitions occur through reconnection events, where the additional optical modes resulting from the interactions with the turbulent medium change the vortex lines in space. Additionally, we propose an optimization algorithm to maximize the distance between the phase singularities at each longitudinal plane, facilitating measurements of optical knots and improving their performance in the presence of turbulence.

Generation of the stable propagation Bessel beam and the axial multifoci beam with pure phase elements

A recently proposed method is upgraded to convert two amplitude phase modulation systems (APMSs) to pure phase elements (PPEs), for generating the stable propagation Bessel beam and the axial multifoci beam, respectively. Phase functions of the PPEs are presented analytically. Numerical simulations by the complete Rayleigh-Sommerfeld method demonstrate that the converted PPE has implemented the same optical functionalities as the corresponding APMS, in either the longitudinal or the transverse direction. Compared with the traditional APMS, the converted PPE possesses many advantages such as fabrication process simplification, system complexity reduction, production cost conservation, alignment error avoidance, and experimental precision enhancement. These inherent advantages position the PPE as an ideal choice and driving force behind further advancements in optical system technology.

Scattering characteristics of a terahertz Bessel vortex beam by 3D dielectric-coated targets

In order to explore the application of terahertz (THz) vortex beams in 3D dielectric-coated target detection and imaging, scattering characteristics of a THz Bessel vortex beam by 3D dielectric-coated conducting targets are investigated in terms of the combination of planewave angular spectrum expansion and a physics optics method in this paper. The accuracy of the proposed method is verified by comparing with the results of FEKO software. The scattering characteristics of a THz Bessel vortex beam by several typical 3D dielectric-coated targets are analyzed in detail. The effects of the beam parameters (topological charge, half-cone angle, incident angle and frequency) are discussed. The results show that with an increase of topological charge, the magnitude of the radar cross section (RCS) decreases, and the maximum value moves away from the incident direction gradually; the distribution of the RCS does not keep symmetry as the incident angle increases, and the corresponding orbital angular momentum state distribution of the far-scattered field is distorted remarkably.

The Effect of Air Turbulence on Vortex Beams in Nonlinear Propagation

Vortex beams with orthogonality can be widely used in atmospheric applications. We numerically analyzed the statistical regularities of vortex beams propagating through a lens or an axicon with different series of turbulent air phase screens. The simulative results revealed that the distortion of the transverse intensity was sensitive to the location and the structure constant of the turbulence screen. In addition, the axicon can be regarded as a very useful optical device, since it can not only suppress the turbulence but also maintain a stable beam pattern. We further confirmed that a vortex beam with a large topological charge can suppress the influence of air turbulence. Our outcomes are valuable for many applications in the atmospheric air, especially for optical communication and remote sensing.

Terahertz structured light: nonparaxial Airy imaging using silicon diffractive optics

Structured light - electromagnetic waves with a strong spatial inhomogeneity of amplitude, phase, and polarization - has occupied far-reaching positions in both optical research and applications. Terahertz (THz) waves, due to recent innovations in photonics and nanotechnology, became so robust that it was not only implemented in a wide variety of applications such as communications, spectroscopic analysis, and non-destructive imaging, but also served as a low-cost and easily implementable experimental platform for novel concept illustration. In this work, we show that structured nonparaxial THz light in the form of Airy, Bessel, and Gaussian beams can be generated in a compact way using exclusively silicon diffractive optics prepared by femtosecond laser ablation technology. The accelerating nature of the generated structured light is demonstrated via THz imaging of objects partially obscured by an opaque beam block. Unlike conventional paraxial approaches, when a combination of a lens and a cubic phase (or amplitude) mask creates a nondiffracting Airy beam, we demonstrate simultaneous lensless nonparaxial THz Airy beam generation and its application in imaging system. Images of single objects, imaging with a controllable placed obstacle, and imaging of stacked graphene layers are presented, revealing hence potential of the approach to inspect quality of 2D materials. Structured nonparaxial THz illumination is investigated both theoretically and experimentally with appropriate extensive benchmarks. The structured THz illumination consistently outperforms the conventional one in resolution and contrast, thus opening new frontiers of structured light applications in imaging and inverse scattering problems, as it enables sophisticated estimates of optical properties of the investigated structures.

Orbital angular momentum spectrum of pin-like optical vortex beams in turbulent atmosphere

The analytical formula of the probability density of a single orbital angular momentum (OAM) mode for pin-like optical vortex beams (POVBs) in turbulent atmosphere is derived. Its OAM spectrum in the receiving plane is obtained by the numerical calculation. For comparison, the OAM spectrum of commonly Gaussian vortex beams is showed, too. Those results show that POVBs show good performance on resisting the cross talk of the OAM mode induced by the turbulence in some cases, such as smaller radius of the receiving aperture, longer propagation distance, and stronger turbulence. Our finding has application in free-space optical communication based on the OAM mode.

Steady optical beam propagating through turbulent environment

A steady optical beam (SOB) propagating stably in a disorder medium is constructed by using a specially designed aspherical lens. Our theoretical and experimental results show that the generated SOB exhibits much better propagation features with small divergence and long Rayleigh length, as well as weak deformation through turbulent environment as compared with a conventional Gaussian beam. The beam parameter product of the SOB reaches 49.40% of the Gaussian beam by multiple measurements within a certain distance range. The SOB may find applications in optical communications and optical detection in turbulent transmission conditions.

Orbital Angular Momentum Mode Sensing Technology Based on Intensity Interrogation

A novel optical fiber sensing technology based on intensity distribution change in orbital angular momentum (OAM) mode is proposed and implemented herein. The technology utilizes a chiral long-period fiber grating (CLPFG) to directly excite the 1st-order OAM (OAM₁) mode. The intensity changes in the coherent superposition state between the fundamental mode and the OAM₁ mode at the non-resonant wavelength of the CLPFG is tracked in order to sense the external parameters applied to the grating area. Applying this technology to temperature measurement, the intensity distribution change has a good linear relationship with respect to temperature in the range of 30 °C to 100 °C. When the intensity was denoted by the number of pixels with a gray value of one after binarization of collected images, the sensitivity was 103 px/°C and the corresponding resolution was 0.0097 °C. Meanwhile, theoretical and experimental results show that the sensitivity and resolution can be further improved via changing the area of the collected image. Compared with sensing methods based on spiral interference pattern rotation in previous work, this sensing technology has the advantage of exquisite structure, easy realization, and good stability, thus making it a potential application in practices.

Simultaneous measurement of orbital angular momentum spectra in a turbulent atmosphere without probe beam compensation

In free-space optical (FSO) communications, the orbital angular momentum (OAM) multiplexing/demultiplexing of Bessel beams perturbed by atmospheric turbulence is of great significance. We used the Gerchberg-Saxton algorithm without a beacon beam to compensate for the aberrant helical phase of the Bessel beam distorted by the turbulent atmosphere. The optical vortex Dammann axicon grating was applied for the simultaneous measurement of the intensities of the demodulated spectra of the OAM modes of the Bessel beams disturbed by atmospheric turbulence. The experimental results demonstrate that the distorted phase of the Bessel beam can be compensated and the mode purity of the target OAM mode is enhanced from 0.85 to 0.92 in case of weak turbulence. Our results will improve the quality of the OAM modes of Bessel beam (de)multiplexing in FSO communication systems.

Image transmission with binary coding for free space optical communications in the presence of atmospheric turbulence

Understanding the influence of atmospheric turbulence on optical information transmission is important for free space optical communication. In this paper, the image transmission through a 1 km horizontal turbulent channel has been numerically investigated, and a simulation model including the process of image pixels encoding and decoding is given. The peak signal-to-noise ratio of the received image is evaluated, and the influences of the channel factors and detector noise are discussed in detail. The critical value of noise level and turbulence strength is given. Our results provide a simulation model for image transmission in a turbulent channel along with insight into the impacts of turbulence parameters and detector noise, which are useful for applications in optical communication.

Receiving sensitivity improvement by wide-spectrum OAM with OTDM and polarization multiplexing in weak atmospheric turbulence

In this paper, a wide-spectrum orbital angular momentum (OAM) system with a polarization and optical time division multiplexing (OTDM) free-space transmission system is experimentally demonstrated. To enhance the system transmission performance in atmospheric turbulent channel, a wide-spectrum laser and an OAM beam are used. The wide-spectrum laser can be generated by utilizing pumped laser to pump nonlinear fiber, and OAM can be generated with a special light modulator. Furthermore, OTDM and polarization multiplexing methods are used to enhance the communication rate from 4 Gbit/s to 32 Gbit/s. With the use of the wide-spectrum laser and the OAM beam, the receiving scintillation index (SI) can be reduced, and detection sensitivity can be improved. It is the first time a wide-spectrum OAM communication system performance has been studied. It is shown that under weak atmospheric turbulence condition, the SI can be reduced by 38% and the receiving sensitivity can be improved by 3.18 dB via wide-spectrum OAM beams.

Beam wander in wireless optical communications between misaligned transceivers in oceanic turbulence

In this paper, we investigate the beam wander in oceanic turbulence taking into account the misaligned displacements caused by slight changes in the position and attitude of the underwater platform. First, we derive the longitudinal distance and radial distance or the misaligned displacements according to the relationship between the misaligned transceivers. Then, we formulate the beam wander variance of a Gaussian beam propagating through oceanic turbulence. Finally, we obtain the beam wander variance in underwater wireless optical communications between the misaligned transceivers according to longitudinal distance. In addition, we express the mean pointing error displacements.

Lossy wavefront sensing and correction of distorted laser beams

The art of rectifying a laser beam carrying amplitude and phase distortions has been demonstrated through several competing methods. Both wavefront sensor and wavefront sensor-less approaches show that the closed-loop correction of a laser beam can be accomplished by exploiting high-resolution sampling of the beam distortion in its spatial or time domain, respectively. Moreover, machine-learning-based wavefront sensing has emerged recently, and uses training data on an arbitrary sensing architecture to map observed data to reasonable wavefront reconstructions. This offers additional options for beam correction and optical signal decoding in atmospheric or underwater propagation. Ideally, wavefront sensing can be achieved through any resolution in spatial samples, provided that more frequent sampling in the time domain can be achieved for a reduced number of spatial samples. However, such trade-offs have not been comprehensively studied or demonstrated experimentally. We present a fundamental study of lossy wavefront sensing that reduces the number of effective spatial samples to the number of actuators in a deformable mirror for a balanced performance of dynamic wavefront corrections. As a result, we show that lossy wavefront sensing can both simplify the design of wavefront sensors and remain effective for beam correction. In application, this concept provides ultimate freedom of hardware choices from sensor to sensorless approaches in wavefront reconstruction, which is beneficial to the frontier of study in free-space optical communication, lidar, and directed energy.

Effective tensor approach for simulating the propagation of partially coherent Hermite-sinh-Gaussian beams through an ABCD optical system in turbulent atmosphere

An efficient tensor approach is used to simulate the propagation of partially coherent Hermite-sinh-Gaussian (H-ShG) beams through an ABCD optical system in a turbulent atmosphere. Analytical expressions for the average intensity are derived. The properties of the average intensity of the beam are studied numerically. It is found that the average intensity distribution of the beam is almost independent of the structure constant of turbulence when the spatial coherence length is very small and the beam with larger spatial coherence length is less affected by the turbulent atmosphere. Moreover, the transition from an H-ShG beam into a Gaussian beam becomes quicker with smaller beam order and smaller Sh parameters.

Reducing the cross-talk among different orbital angular momentum modes in turbulent atmosphere by using a focusing mirror

The cross-talk among different orbital angular momentum (OAM) modes induced by the turbulent atmosphere is a challenging effect commonly occurring in OAM-based free-space optical (FSO) communication. The aim of this study is to propose a simple method to reduce the crosstalk and demonstrate its effect by analytical derivation and numerical simulation. It is found that the crosstalk is largely reduced by using a focusing mirror. Our results will be useful in free-space optical communication.

Investigation on the behavior of a laser propagating through a random environment induced by wind

Wind is an important factor in environment disturbance, and the inhomogeneous distribution of wind velocity leads to random airflow, which severely affects beam propagation. In this paper, numerical and experimental studies have been performed to investigate the behavior of a laser propagating through a random environment induced by wind, and the main focus is the beam deflection evolution under the effect of the wind velocity. The experiment is performed with the wind tunnel, and the beam deviates from the center during propagation, a process in which the average beam deflection presents an increasing trend for the larger wind velocity and the airflow interval. The simulation model of beam propagation through this kind of environment is proposed, and the numerical simulation agrees with the experimental results. With the model, the average beam deflection results are extended into the high-speed region, and the comparison between the airflow and turbulence environment is also presented. The results can find potential applications in optical propagation and communication between two moving platforms with high speed.

Self-healing high-dimensional quantum key distribution using hybrid spin-orbit Bessel states

Using spatial modes for quantum key distribution (QKD) has become highly topical due to their infinite dimensionality, promising high information capacity per photon. However, spatial distortions reduce the feasible secret key rates and compromise the security of a quantum channel. In an extreme form such a distortion might be a physical obstacle, impeding line-of-sight for free-space channels. Here, by controlling the radial degree of freedom of a photon's spatial mode, we are able to demonstrate hybrid high-dimensional QKD through obstacles with self-reconstructing single photons. We construct high-dimensional mutually unbiased bases using spin-orbit hybrid states that are radially modulated with a non-diffracting Bessel-Gaussian (BG) profile, and show secure transmission through partially obstructed quantum links. Using a prepare-measure protocol we report higher quantum state self-reconstruction and information retention for the non-diffracting BG modes as compared to Laguerre-Gaussian modes, obtaining a quantum bit error rate (QBER) that is up to 3× lower. This work highlights the importance of controlling the radial mode of single photons in quantum information processing and communication as well as the advantages of QKD with hybrid states.

Optimization of the probability of orbital angular momentum for Laguerre-Gaussian beam in Kolmogorov and non-Kolmogorov turbulence

We derive the probabilities of the signal OAM state and crosstalk OAM state for a Laguerre-Gaussian (LG) beam propagating through Kolmogorov and Non-Kolmogorov turbulence, and derive the accurate analytical function of the probability for the received OAM state modulated by an arbitrary receiver aperture. The probability of the detected OAM state with a receiver aperture for different values of the radius is demonstrated numerically. Our numerical results show that the probability of the signal OAM state remains almost invariant when the radius of the receiver aperture varies. The probability of the crosstalk OAM state decreases with the decrease of the radius of the receiver aperture, thus it can be optimized by choosing a suitable value of the radius of the receiver aperture. Our results will be useful in free-space optical communications.

Beam wander of a partially coherent Airy beam in oceanic turbulence

The beam wander of a partially coherent Airy beam in oceanic turbulence is investigated with the help of the extended Huygens-Fresnel integral formula. Analytical expression for the second-order moment and the beam wander of a partially coherent Airy beam propagating in oceanic turbulence is derived. From the numerical results based on the analytical formula, we find that increasing the dissipation rate of turbulent kinetic energy or decreasing the dissipation rate of mean-square temperature and relative strength of temperature and salinity fluctuations of oceanic turbulence tends to decrease the wander effect of a partially coherent Airy beam. Moreover, it is found that increasing the transverse scale factor and wavelength or decreasing the coherent length and exponential truncation factor of a partially coherent Airy beam decreases beam wander in oceanic turbulence. Our results will be useful in optical underwater communications and laser defense.