Optical vortex and correlation image sensor for networked deformation sensing of infrastructures

We present a model based on the Fresnel diffraction scheme for the spatial coherence function of random fields created by scattering optical vortex and perfect vortex beams. By using the spatial coherence function we showed analytically, numerically, and experimentally the dependence and independence of the speckle size of an optical vortex and perfect optical vortex (POV) with a topological charge, respectively. We also showed in both cases the linear dependence of speckle size on the distance of propagation. Furthermore, we describe a regime in which the spatial coherence function is nonevolving for the optical vortex beam and the POV beam with the propagation distance.

Optical vortex beams are profiled as helical wavefronts with a phase singularity carrying an orbital angular momentum (OAM) associated with their spatial distribution. The transverse intensity distribution of a conventional optical vortex has a strong dependence on the carried topological charge. However, perfect optical vortex (POV) beams have their transverse intensity distribution independent of their charge. Such “size-invariant” POV beams have found exciting applications in optical manipulation, imaging, and communication. In this paper, we investigate the use of POV modes in the efficient generation of high-dimensional quantum states of light. We generate heralded single photons carrying OAM using spontaneous parametric downconversion (SPDC) of POV beams. We show that the heralding efficiency of the SPDC single photons generated with a POV pump is greater than that with normal optical vortex beams. The dimensionality of the two-photon OAM states is increased with POV modes in the pump and projective measurements using Bessel–Gaussian vortex modes that give POV, instead of Laguerre–Gaussian modes.

This work presents an investigation of tight focusing of optical vortices, which produce regions where the Poynting vector is directed opposite to the beam's propagation direction-known as areas of reverse energy flow. We study both phase optical vortices and polarization optical vortices (cylindrical vector beams). For phase optical vortices, it is shown that the emergence of reverse energy flow as well as splitting of the initial optical vortex depends on the handedness of the circular polarization and the phase of the initial optical vortex. An optical vortex with an initial topological charge of $m\; \le \; - {3}$ splits into three optical vortices with topological charges $m + {2}$, ${-}{1}$, and ${-}{1}$ when the polarization is right circular. In contrast, under left-hand circular polarization, a vortex with the topological charge $m$ is preserved at the focus. The reverse-flow regions are located between the centers of phase singularities (screw dislocation points). As the topological charge of the phase vortex increases, the region of reverse flow along the radial coordinate widens, and the distance from the vortex center to the optical axis also increases. In the focusing of cylindrical vector beams (polarization optical vortices), the reverse energy flow region is also observed, but screw dislocations are absent.

Topological reactions of optical correlation vortices

Space-division multiplexing (SDM) has emerged as a promising technology for both classical and quantum communications. Twisted light, particularly in the form of orthogonal optical vortex beams, offers significant potential for SDM by enhancing the capacity and performance of optical communication systems. A critical component in such systems is a twisted light (de)multiplexer, which enables scalable and efficient (de)multiplexing of twisted light to meet the demands of high-capacity optical communications. In this work, we propose and experimentally demonstrate quasi-wavelet conformal mapping microstructures for efficient twisted light demultiplexing. These microstructures, specifically designed and fabricated for this experiment, successfully demultiplex 11 optical vortex beams with a low crosstalk below −20.9 dB at the target wavelength. Moreover, the microstructures exhibit a broad operational bandwidth (1525–1570 nm), maintaining crosstalk levels under −13 dB. To further validate their performance, the microstructures are also tested in the demultiplexing of various configurations of multiplexed two-channel optical vortex beams. The presented quasi-wavelet conformal mapping microstructures with compact size and exceptional demultiplexing performance hold great promise for applications in high-capacity optical communication systems using twisted light.

Given by the Shannon theorem, the data rate in a single mode fiber is approaching the capacity limit of 100 Tbit/s, which even applies to all existing wavelength division multiplexing and advanced modulation formatting techniques. Optical vortex beams, including orbital angular momentum (OAM) beams with phase singularities and cylindrical vector beams (CVBs) with polarization singularities, are orthogonally structured light beams providing new degrees of freedom for multiplexing optical communication, for which the multiplexer is the key component. Although there are various OAM detection approaches such as the optical geometric transformation and vortex grating, CVB sorting with high efficiency and large dynamic range has not been demonstrated before. In this work, we propose and demonstrate an efficient approach for multiple coaxial CVB sorting based on the spin-dependent optical geometric transformation using the Pancharatnam–Berry optical element device fabricated with the photoaligned liquid crystal....

Coherent Beam Combining (CBC) can scale the overall output power while maintaining high beam quality. In recent years, this technique has also been employed to generate the high-power Optical Vortex Beam (OVB). In this work, we designed a coherent phased array, and an Optical Vortex Beam Array (OVBA) was generated via the CBC technique. Numerical simulation was studied, and an experimental setup was set up. The results showed that the OVBA could be generated in the far field. Meanwhile, the phase detection results indicated that the OVBA included several phase singularities. This work can present a new idea for manipulating the high power structured optical fields.

A full comparative analysis of the chain of phase singularities generated when a quasi-plane wave and a Gaussian beam pass a double-phase-ramp (DPR) converter is presented based on the theoretical and experimental data. The overall output beam structure includes a system of interrelated optical vortices (OVs) whose linear trajectories form a threedimensional singular skeleton which can be applied for the trapping and guiding of microparticles. An internal structure of each individual phase singularity is characterized by the OV topological charge and by the morphology parameters of the equal intensity ellipses in the OV-core area. The rectilinear equidistant OV trajectories form a chain in the transverse cross section, and their identical morphology parameters can be useful for the applications to metrology and micromanipulation. As a separate result, we consider the DPR-induced transformations of the incident Laguerre-Gaussian beams of the lowest orders and show that the incident multicharged OVs are transformed into small sub-chains of the OVs located in the near-axial region.

Nonlinear propagation effects on high-power optical-vortex pulses in anisotropic crystals is reported. Nonlinear effects on optical-vortex pulse generation through the spin-orbit interaction and its propagation in an anisotropic crystal is investigated.The beam of the left-circularly-polarized optical vortex pulse is focused on one of the surfaces of a LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> or rutile crystal (NC) by a convex lens L1. After propagating along the optic axis of the crystal, the beam is collimated by another convex lens L2.

An audio coding method based on multi-ring optical perfect vortex beam (MR-OPVB) is proposed, which uses its orbital angular momentum (OAM) as the information carrier, and realizes efficient signal coding through MR-OPVB. The coaxial interference method of fundamental-mode Gaussian beams was used to extract OAM information, focusing on the interference effects between perfect vortex waves of different radii and their impact on OAM recognition, as well as strategies to mitigate such interference disturbances. Convolutional neural network was used to recognize the mode of MR-OPVB affected by atmospheric turbulence, which can achieve 99.8% recognition accuracy in moderate turbulence conditions. The trained resnet-18 model was utilized to identify and decode the MR-OPVB affected by atmospheric turbulence, and the distortion rate of the recovered audio signal was 0.954%. This study offers a coding concept for optical perfect vortex beam communication and validates that machine learning can effectively enhance the quality of free space optical communication.

We show both theoretically and numerically that if an optical vortex beam has a symmetric or almost symmetric angular harmonics spectrum [orbital angular momentum (OAM) spectrum], then the order of the central harmonic in the OAM spectrum equals the normalized-to-power OAM of the beam. This means that an optical vortex beam with a symmetric OAM spectrum has the same topological charge and the normalized-to-power OAM has an optical vortex with only one central angular harmonic. For light fields with a symmetric OAM spectrum, we give a general expression in the form of a series. We also study two examples of form-invariant (structurally stable) vortex beams with their topological charges being infinite, while the normalized-to-power OAM is approximately equal to the topological charge of the central angular harmonic, contributing the most to the OAM of the entire beam.

Conventional correlation-based tracking methods are limited by the correlation window size and a lack in ability to track individual speckles or particles. Wang et al.[1] in their definition of Optical Vortex Metrology (OVM) propose that phase singularities are optimal encoders for position marking and can be localized with accuracy. OVM is widely used to process real-life and pseudo optical vortices [1, 2, 3], but so far the accuracy of the method has not been determined. Optical Vortex Metrology (OVM) uses phase singularities, produced in the complex signal generated by a Laguerre-Gauss filter operation applied to an image, for displacement measurements. Each phase singularity is essentially a pseudo-phase vortex, which can be uniquely identified from its structure, enabling it to be tracked between images. OVM is of interest here as not only the technique has potential to provide accurate position tracking but also pointwise rotation measurement.

Optical vortices, having doughnut-shaped spatial profile with zero intensity around the phase singularity, are of great interest due to its wide range of applications in variety of fields in science and technology. Two spatially separated entwined vortices with opposite topological charge forms a vortex dipoles. Due to their entwining nature, the vortex dipoles are created and annihilated in pairs of opposite charge, and carry a very stable topological structures. In recent times, vortex dipoles have created a lot of interest in studying in propagation dynamics [1] and optical vortex metrology [2]. Typically, optical vortex dipoles are created using holographic techniques. Recently, a new technique has been demonstrated to generate vortex dipole through coupling between the amplitude and phase differences of the superposing beams [3]. Here we report, for the first time to the best of our knowledge, on the generation of tunable vortex dipole directly from an optical parametric oscillator (OPO). Pumping a continuous wave (cw), doubly resonant OPO with Gaussian beam and carefully adjustment of the cavity mirrors, we have generated vortex dipole of power as much as 84 mW tunable across 968 nm-1181 nm. The generic scheme can used to generate vortex dipole at any wavelength and at all time scales.

In this paper the concept of optical vortex scanning microscope (OVSM) is presented. In the OVSM a sample is scanned by the focused laser beam with optical vortex. The beam possessing an optical vortex contains a line along which the phase is undetermined. At image plane this line is seen as the single point. The position of such special singular point (vortex points) changes when the beam is diffracted by a sample. The vortex point sensitivity is higher than the sensitivity of the whole focused beam. In the OVSM the position of vortex point is traced while scanning the sample. In this paper the behavior of focused vortex beam, when scanning the phase step is studied. This simple example illustrates both the OVSM idea and the method for data analysis.

When a phase singularity is suddenly imprinted on the axis of an ordinary Gaussian beam, an optical vortex appears and starts to grow radially, by effect of diffraction. This radial growth and the subsequent evolution of the optical vortex under focusing or imaging can be well described in general within the recently introduced theory of circular beams, which generalize the hypergeometric-Gaussian beams and which obey novel kinds of ABCD rules. Here, we investigate experimentally these vortex propagation phenomena and test the validity of circular-beam theory. Moreover, we analyze the difference in radial structure between the newly generated optical vortex and the vortex obtained in the image plane, where perfect imaging would lead to complete closure of the vortex core.