Influence of structural length-scale sensitivities on Hermite–Gaussian rectangular vortex beam propagation in biological tissues

The orbital angular momentum (OAM) states are used for OAM modulation and OAM multiplexing in free-space optical (FSO) communication systems to enhance the transmission quality and capacity. Although a few impactful methods had been developed to measure static OAM states, the fast and accurate recognition of dynamic OAM states in FSO communications remains a challenge due to the OAM crosstalk easily induced by the atmospheric turbulence. In this paper, we present a new simple statistical recognition method realized by utilizing the turbulence-induced crosstalk distribution, other than the routine way to use the spatial intensity patterns. The proposed method is based on acquiring the priori OAM crosstalk distribution of the training optical data, and then using similarity analysis to recognize the OAM states of the information optical data. Its availability and accuracy are validated through the computer simulations. The recognition accuracy under relatively strong turbulence conditions can be improved by using wavefront compensation and specific distance calculation.

In the free space optical communication, the information can be encoded as the orbital angular momentum (OAM) state of light, which is called OAM shift keying (OAM-SK). This paper has proposed a communication system with OAM-SK, in which an image has been delivered from the transmitter to the receiver successfully in the simulation environment. Specifically, we have carefully designed and implemented the phase holograms used at the transmitter and the receiver for multiplexing and de-multiplexing the OAM states, respectively. At the transmitter, the multiplexing phase hologram designed by the modified Lin's algorithm is loaded on the spatial light modulator 1 (SLM1) to generate the multiplexing vortex beam, which is a superposition of multiple vortex beams with different OAM states. Correspondingly, at the receiver, a novel phase hologram is designed and loaded on the SLM2 to effectively de-multiplex the multiplexing vortex beam in different directions. In our phase hologram used at the receiver, the detected power of each OAM state can be controlled by adjusting the weight coefficient by the modified Lin's algorithm. This way, the incident power can be concentrated to the target OAM states, from which the target OAM states can be detected more effectively than conventional fork grating.

On the basis of vector plane wave spectrum expansion, the reflection and transmission characteristics of an arbitrarily polarized terahertz Bessel vortex beam by an inhomogeneous unmagnetized plasma slab are investigated. Magnitude profiles of electric field components in the transverse plane, which is perpendicular to the central axes of the reflected and transmitted beams, as well as the weight distributions of orbital angular momentum (OAM) states, are displayed for both homogeneous and inhomogeneous unmagnetized plasma slabs. The results indicate that the magnitude profiles of electric field components for both the reflected and transmitted beams have a serious distortion which is more distinct for the inhomogeneous plasma slab, resulting in diverse patterns which are different from those of the incident beam. Compared with the incident beam, many adjacent OAM states are derived in both the reflected and transmitted beams. For the incidences of both the x- and circular-polarizations, the weight of the predominant OAM state in the reflected beam for the homogeneous plasma slab is greater than that for the inhomogeneous plasma slab but contrary to this conclusion for the transmitted beam. The deformation of OAM states cannot be reflected by the patterns of the field distribution directly. Polarization has few effects on the weight of OAM states of the transmitted beams to the same plasma slab. The weight of the predominant OAM state in the transmitted beam rises with an increase in the incident angle, which is contrary to the conclusion in the reflected beam.

Light beams carrying multiple orbital angular momentum (OAM) states, which can be realized by the structured media with phase singularities, have attracted great attentions in the fields of high dimensional optical information processing. Alternatively, a simple uniaxial crystal can be used to simultaneously generate four OAM states of light through the second harmonic generation and cascaded optical spin–orbit interaction (SOI) processes. However, two of the OAM states realized in the crystal are very weak and limit the practical applications. Here, we aim to circumvent this constraint by using the sequential optical SOI processes in two crystals with threefold rotational symmetry. Four angular momentum states of the fundamental waves are prepared after the first crystal and then are utilized to generate the corresponding second harmonic waves (SHWs) with opposite spin and doubled OAM in the second crystal. Further through a sequential SOI process, totally eight angular momentum states of the SHWs with nearly equal energy are experimentally observed. The proposed methodology may find potential applications in optical communications, parallel optical computing, optical manipulation and so on.

A tunable dual-ring microstructure fiber that can support stable transmission for different orbital angular momentum (OAM) states and possess ultrahigh dispersion coefficients and low confinement losses is proposed and theoretically investigated. The proposed fiber is composed of two high-refractive-index rings and a double-cladding structure. Owing to the central air core and outer cladding, the dual-ring structure can support stable transmission for the OAM states. The mode fields of different OAM states in the inner ring can spread to the outer ring under certain conditions, which leads to high absolute values of dispersion around the coupling wavelengths. By tuning the refractive indices of the dual rings, the proposed fiber can achieve dispersion control for different OAM modes. Moreover, the specially designed two-layer air holes in the inner cladding can affect the mode-coupling coefficients, which are characterized by the effective mode areas and the overlap integral of the electric fields between the resonant ring modes. Therefore, the dispersion curves and operating wavelengths of the OAM modes can be modulated by regulating the physical parameters (the radius of the two-layer air holes or the infiltrated functional materials) of the inner cladding. We built a theoretical model and analyzed the modulation method and mechanism of the dispersion curves based on the coupled mode theory. The theoretical results indicate that the proposed fiber is flexible and has potential dispersion-compensating applications in fiber OAM systems.

We derive analytical expressions of the probabilities for the signal and the crosstalk orbital angular momentum (OAM) states of a high order Bessel–Gaussian beam used in underwater optical communications. Our numerical results show that the probabilities of the signal and crosstalk OAM states are affected by the propagation distance and the parameters of the oceanic turbulence. The receiver aperture can be used to reduce the probabilities of the crosstalk OAM states and the probability of the detected signal OAM states keeps almost unchanged. Our results may improve the quality of optical communications systems.

Bessel vortex beams are widely studied, since their intensity is independent of the propagation distance, and also their original field intensity distribution can be reconstructed after passing through an obstacle. Therefore, such beams are more advantageous for long-distance communication, optical imaging, and other potential applications. Based on the expansion of electromagnetic fields in terms of vector wave functions, in this investigation a method has been proposed to study the reflection and transmission of an incident circularly polarized Bessel vortex beam by a uniaxial anisotropic slab. The expansion coefficients of a circularly polarized Bessel vortex beam are derived by use of the cylindrical vector wave functions. The magnitude profiles of the electric field amplitude and phase, as well as the distribution of orbital angular momentum (OAM) states for both the reflected and transmitted beams, are numerically simulated and discussed. The effects of dielectric tensor and incident angle on the propagation characteristics of a circularly polarized Bessel vortex beam are analyzed. The observed results indicate that the contours of the electric field components cannot retain circularly symmetric structures; the distortion of phase distribution is obvious, and except for the predominant OAM state, other OAM states are derived, particularly for the reflected beam. Although few OAM states emerge in the transmitted beam, the predominant OAM state is still the same as that of the incident Bessel vortex beam.

The transmission model of partially coherent elegant Laguerre–Gaussian (ELG) beams in oceanic turbulence is established by employing the spatial power spectrum of the refractive index of sea-water and the optics approximation. The mathematical expressions of mode probability, energy content, and orbital angular momentum (OAM) state detection probability for partially coherent ELG beams are obtained. The influences of beam and turbulence parameters on the propagation properties of light beam are illustrated in detail with numerical simulations. Results indicate that partially coherent ELG beams with smaller azimuthal mode, smaller OAM state difference, longer blue–green incident wavelength, and appropriate beam waist are less affected by turbulent ocean. Moreover, the evolution characteristics of laser beam may be able to keep a better quality under the condition of lower rate of dissipation of mean-square temperature, higher rate of dissipation of turbulent kinetic energy per unit mass of fluid, and smaller ratio of temperature to salinity for maritime environment. In addition, the partially coherent ELG beams exhibit more robust resistance to turbulence than the partially coherent standard Laguerre–Gaussian beams. These results are expected to be potential helpful in understanding the optical transmission process in turbulent media and designing the optical vortex communication systems.

Optical vortices are a new kind of laser beam and receiving more and more attention currently.The complex amplitudes of optical vortices comprise a helical term exp (il),with l being the topological charge and the azimuthal angle.Each photon in optical vortices carries the orbital angular momentum (OAM) with a value of lħ,where ħ is the Planck's constant divided by 2.The topological charge l is the eigenvalue of optical vortices,and determines the helical wavefront distribution,thus also known as OAM state.Moreover,such an OAM state can be an infinite integer state. And vortices with various OAM states are orthogonal to each other,making it possible to be employed in high capacity data-transmission system.In addition,the above unique features contribute to their widely applications in lots of areas such as optical tweezers and spanners,rotation detection,quantum entanglement,etc.In these applications,detecting OAM states is basic,and greatly significant.Recently researchers have developed lots of approaches to detecting the OAM states,including the methods of interference,diffraction gratings,metasurface,etc.Of such approaches,the scheme of diffraction gratings is the simplest and most widely used,where one or more diffraction gratings are employed. When optical vortices propagate through such gratings,the OAM states are acquired immediately through capturing and analyzing the distinct OAM-related diffraction patterns.In this review,we focus on the techniques of detecting OAM states through diffraction gratings,which have been demonstrated by our group and other researchers.Some of the main detection gratings,including double-slit,triangle aperture and slit,angular slit,cylindrical lens,graduallychanging-period grating,annular grating,are introduced.In addition,schemes like composite fork grating,Dammann vortex grating and integrated Dammann grating,are presented to detect the OAM state for coaxial multiplexed vortices. Besides diagnosing OAM state,measuring the intensity proportion of each OAM channel,known as OAM spectrum, in multiplexed vortices is also necessary in some cases.Therefore we also introduce the ways to measure the OAM spectrum,e.g.the OAM mode sorter,the gray-scale algorithm.

Separation of spin and orbital angular momentum states from cylindrical vector beams

With heralded pair coherent states (HPCS), orbital angular momentum (OAM) states and pulse position modulation (PPM) technology, a decoy-state measurement-device-independent quantum key distribution (MDI-QKD) protocol is proposed. OAM states and PPM technology are used to realize the coding of the signal states in the HPCS light source. The use of HPCS light source, OAM coding and PPM coding cannot only reduce the error rate but also improve the key generation rate and communication distance. The new MDI-QKD protocol also employs three-intensity decoy states to avoid the attacks against the light source. By calculating the error rate and key generation rate, the performance of the MDI-QKD protocol is analyzed. Numerical simulation shows that the protocol has very low error rate and very high key generation rate. Moreover, the maximum communication distance can reach 455 km.

Free-space information transfer using the elliptic vortex beam with fractional topological charge

Twisted ion-acoustic waves with finite orbital angular momentum (OAM) states are numerically investigated in a plasma containing electrons, positrons and dynamical ions. For this purpose, the well-known Vlasov–Poisson equations are solved together under the paraxial approximation. The perturbed quantities are assumed to follow the Laguerre–Gaussian type solutions and a generalized plasma response function for finite OAM states is derived. The generalized response function is numerically analyzed to investigate dispersion characteristics of the twisted ion-acoustic waves. Consequently, a computer code is developed which is based on the very famous numerical technique known as the Newton–Raphson method. The dependence of the (real) wave oscillation frequency and damping rate involving the twisted ion-acoustic waves is highlighted with different plasma parameters. It is found that the dynamics of ion-acoustic waves are strongly influenced by the twist parameter owing to OAM states. The results obtained in the present study are useful in the understanding of the particle transport and trapping phenomena.

Optical states characterized by an electromagnetic field with a spiral azimuthal phase are known as orbital angular momentum (OAM) states. Examples of structured light carrying OAM states include Laguerre-Gauss (LG) beams, which can be transformed into Hermite-Gauss (HG) beams through astigmatic transformations using cylindrical or spherical lenses. In this study, we explore the dynamics of OAM conservation by combining two noncommutative operations: astigmatic transformation, achieved with a tilted spherical lens, and an up-conversion process facilitated by a crystal. In this experiment, we observe that the intensity distribution in the second harmonic (SH) beam undergoes a separation process resembling the stages of mitosis, a biological process where a single cell divides into two genetically identical daughter cells. The transformations in the SH beam closely mirror key mitotic phases, including metaphase, anaphase, and telophase. To validate these similarities, we analyze the intensity, phase, and electric field profiles obtained from numerical simulations. These findings are further supported by experimental results and enhanced through phase retrieval techniques.

The orbital angular momentum state (OAM) of the photon can be theoretically evaluated from minus infinity to infinity, and the different orbital angular momentum states are orthogonal to each other. Therefore, in recent years, it has attracted wide attention from the academic circle and the industry. The purpose of this paper is to study the characteristics of single photon orbital angular momentum transfer based on information processing technology and to provide Suggestions for its technical optimization. Based on the characteristics of the angular momentum state of photon orbit, this paper studies its application and related problems in information processing and transmission. In this paper, quantum multiuser communication technology is studied, and a multiuser communication scheme based on photon orbital angular momentum state is proposed. The scheme takes advantage of the orthogonality of different orbital angular momentum states of the photon and assumes that the sender and receiver share a pair of entangled orbital angular momentum states. On this basis, the spatial light modulator is used to modulate it to the entangled photon of the sender, so that the sending photon carries all the user’s information at the same time. Finally, the receiver accurately extracts the information of each sender by measuring the coincidence count and according to the specific receiving rules. Numerical simulation and experimental results show that the coincidence rate of different coding schemes is between 0 and 0.66, which proves the feasibility of the scheme.