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The evolution of circular Pearcey Gaussian vortex beams (CPGVBs) in strongly nonlocal nonlinear media is investigated numerically. The breathers-like balance states of CPGVBs are obtained directly by adjusting the light field structure parameters of CPGVBs and the nonlinear factor. In particular, as the nonlinear factor increases, the number of CPGVBs foci increases and the distance between the foci decreases. As intensity of beam is increased the nonlinearity becomes stronger and this leads to increase of foci number with corresponding decrease of distance between foci. Adjustment of the nonlinearity and diffraction leads to formation of the spatial optical quasi-soliton states. We also investigate the stability of quasi-solitons in the presence of disturbances. In this propagation regime the light field parameters have a significant impact on the configuration of quasi-solitons. • We can change the shape of the focus and the distribution of the beam during transmission by adjusting the structural parameters of the circular Pearcey Gaussian vortex beams (CPGVBs). • CPGVB has rich controllability and excellent transmission results under strong nonlocal nonlinear conditions, bringing more possibilities for particle manipulation and capture, as well as the implementation of spatial optical solitons in CPGVB. • We have conducted a study on the stability of quasi-optical solitons generated by Pierce beams, taking into account actual disturbance scenarios. [ABSTRACT FROM AUTHOR]

Metalens can expand optical imaging capabilities due to the ability of metasurfaces to manipulate light propagation. Multi-focal metalens have been widely used in multi-imaging systems, detectors, and laser printing. This study presents the design of a UV metalens composed of alumina nanopillars. We investigate the focusing properties of representative UVA (343 nm) at multiple focal points on the in-plane axis, off-axis, and within a single plane. In addition, we extend the focusing ability of the metalens with multiple focal planes. We design the metalens with multiple imaging capabilities and produce focuses with different shapes in four focal planes. Simulation results show that the focal points of the planes have different shapes, the location of the maximum light intensity in the xz plane is in good agreement with our designed focal length, and the average focusing efficiency is greater than 60% in each plane. To evaluate the feasibility of planar spacing detection, we calculate the error-tolerant rate (Etr) of the device to be more than 90%. This work has important implications for the integrated miniaturization of UV photonic devices and the study of optical tomography. • The all-dielectric material used in the ultraviolet band has a low loss and a transmittance of more than 90%, which ensures a good focusing effect of the metalens. At the same time, it is also conducive to the processing and integration of subsequent devices. • Based on the arbitrary manipulation of on-axis and off-axis focal points, four-focus and six-focus patterns within a single focal plane are realized. • By expanding the focal plane and taking advantage of the smaller focal depth and higher resolution of UV, the bifocal plane with 2 μm plane spacing and the quadfocal plane with 5 μm plane spacing are designed. It can be used in many fields such as multi-layer imaging, and has certain research significance for the integrated miniaturization of UV photonic devices. • The proposed metalens can accurately control the focal plane spacing according to the requirements, and the error tolerance rate of the calculated control focal plane spacing is more than 90%. [ABSTRACT FROM AUTHOR]

3D printing has been widely used in the micromachining fields due to its flexibility in design and fabrication. However, due to the presence of undulations on the surface of the finished part, the 3D printing has hardly been employed in the optics manufacturing. In this study, we propose a manufacturing method for optical splitters based on 3D printing and microfluidic abrasive machining. We simulate the effect of microfluidic abrasive machining on microchannels, that helps mitigate the impact of undulations. The 3D printed optical splitter using microfluidic abrasive machining presents high efficiency in optical propagation and high uniformity. We believe that the 3D printing accompanied with microfluidic abrasive micromachining would be a promising technique in the optics manufacturing fields. [ABSTRACT FROM AUTHOR]

A Bragg grating based on a hybrid plasmonic waveguide (HPW) is proposed and investigated. This HPW's structure consists of an air gap sandwiched between a silicon waveguide and a silver grating with its reflective ridges towards the silicon waveguide. The width of the air gap varies periodically because of the ridges and grooves of the silver grating, which results in the periodical modulation of the HPW's effective refractive index along the light propagation direction. A Bragg grating is shown with an extinction ratio of the transmission spectrum 24 d B , a transmittance of 85 % , and a resonance peak 3 d B bandwidth 19 n m in the HPW consisting of a silicon waveguide with a cross-section of dimensions 350 n m × 350 n m , and a silver grating of ridge height 50 n m , period 325 n m , and length 40 μ m. The relation between the resonance peak central wavelength and the period of the grating is found to be nonlinear and is formulated. There should be a tradeoff between the high extinction ratio of the transmission spectrum and low loss in the optimization of the HPW parameters. • A Bragg grating, consisting of an air gap sandwiched between a silicon waveguide and a silver grating, is proposed. • It has an extinction ratio of the transmission spectrum 24 dB, a transmittance of 85%, and 3 dB bandwidth 19 nm. • Its nonlinear relation between the resonance peak central wavelength and the period of the grating is formulated. [ABSTRACT FROM AUTHOR]

The disappearance of the permittivity of epsilon-near-zero (ENZ) materials can significantly enhance the optical nonlinear absorption, which provides a feasible way to realize broadband optoelectronic devices. Here, we used titanium nitride (TiN) film with ENZ wavelength at 540 nm as the saturable absorber (SA), and hexagonal gold nanolayer as the reverse saturable absorber (RSA), which are arranged in tandem to fabricate a broadband nonreciprocal passive photonic diode, and the maximum nonreciprocity factor of 4.8 dB was obtained experimentally in the ENZ region. Our results show the potential applications of TiN in nonlinear optical devices, and may make inroads towards the ENZ-based high-performance broadband optoelectronic devices. [ABSTRACT FROM AUTHOR]

We present an efficient method for nonparaxial (i.e., wide angle) and bidirectional wave propagation in optical fibers and fiber components. The method is applicable to all structures such as tapers and fiber gratings which are azimuthally symmetric in respect of the refractive index distribution and the geometry. We use the collocation method to numerically solve the Helmholtz equation as this method allows to take advantage of the symmetry of the structure. This results in reduction of a 3-D propagation problem to a 2-D propagation problem giving a computational advantage by orders of magnitude. The method can also be used to obtain modes with similar computational advantage. The method works for fields of all azimuthal symmetries, i.e. , one can treat all L P l m modes. In order to show the applicability of the method we have included examples on obtaining modes (LP and OAM) in step-index and ring core fibers (RCFs), propagation through a tapered RCF and computation of reflection spectra of few-mode fiber Bragg gratings (FM-FBGs). • An efficient method for propagating waves of any azimuthal order through azimuthally symmetric fiber structures is presented. • Method can be used to propagate non-paraxial waves to obtain bidirectional propagations, e.g., to model fiber Bragg gratings. • Method can also be used to propagate paraxial waves for unidirectional propagation such as in fiber tapers. • The method reduces a propagation problem from 3-D to a 2-D, thereby giving a computational advantage by orders of magnitude. • The method can be used to model orbital angular momentum (OAM) modes for space division pultiplexing. [ABSTRACT FROM AUTHOR]

Even in free space, which is commonly considered of as a flat space–time in most settings, the degree of polarization of a partially spatially coherent light beam changes as it travels. Similarly, the polarization degree would change when a partially spatially coherent light beam propagates in a curved space–time. The difference of the polarization degree between the curved space and flat space can reveal the essential structure of the curved space. In this work, we consider a simplest case of curved space known as Schwarzschild spacetime. We can simulate the Schwarzschild space–time as an optical material with an effective refractive index. The difference of the polarization degree of a light beam propagating in curved space and flat space can be achieved up to 5%, which is detectable in practical measurement. In addition, we have found that the partially spatially coherent light source is necessary for obtaining significant changes in polarization degree. Our results provide an alternative method to estimate the Schwarzschild radius of a massive object with the optical polarization degree measurement. • We consider the partially spatially coherent light in a three-dimensional curved space with the Schwarzschild space–time structure. • The polarization degree change is about 5 % away from the case of light propagation in a flat space, and this change is detectable in practical measurements. [ABSTRACT FROM AUTHOR]

Turbulence effects can seriously affect the propagation of structured light, especially for orbital angular momentum (OAM) modes, causing spread and crosstalk. We derive an analytical solution to the mode spectrum of a Laguerre-Gaussian beam (LGB) propagating in weak compressible turbulence. The influence of the radial number and turbulence strength in the mode spectrum on the probability of LGB propagation is mainly considered. The results show that the weak compressible turbulence interferes strongly with the LGB, and this effect is mainly reflected in the sharp drop of the signal mode energy at short transmission distance. We find that the pure radial mode has a higher signal mode energy than the pure OAM mode, which means that the pure radial mode is more resilient to compressible turbulence than the pure OAM mode. In addition, the increase of radial number can improve the performance of the OAM dimension, but on the other hand, it reduces the probability of the radial dimension. Finally, we discussed the propagation properties of higher-order modes. Our research clarifies the interaction of OAM mode and radial mode in turbulence, and provides a theoretical basis for applications based on radial modes in the field of free-space communication. • The analytical solution of the mode spectrum of Laguerre-Gauss beam (LGB) carrying radial indices in weak compressible turbulence is obtained. • The impact of compressible turbulence on the propagation characteristics of LGB mode was clarified, and the correlation between radial number and OAM number was established. [ABSTRACT FROM AUTHOR]

The propagation of light underwater is affected by water absorption and scattering, resulting in poor visibility of obtained underwater images, and exhibiting degradation phenomena such as color deviation and low contrast. To improve the quality and visibility of underwater images, a method of underwater image enhancement based on Golden Jackal optimization is proposed. The algorithm consists of two processing stages. The first stage is mainly processed in the RGB color space. Based on the attenuation characteristics of underwater color channels, a constructed color compensation and correction strategy is used to eliminate color deviation caused by water absorption. In second stage, it is mainly processed in the Lab color space, using the Golden Jackal optimization algorithm and fitness function to search the optimal parameters of transformation function to improve the image contrast by optimizing the grey intensity of L component and its histogram distribution. The feasibility and effectiveness of the proposed method have been verified through testing and analysis of much underwater degraded image data. In subjective and objective analysis with multiple state-of-the-art underwater image clarity methods, the proposed method performs excellent in color restoration and detail texture enhancement. Further experiments have shown that the proposed method has strong robustness and universality. It is not only suitable for enhancing various underwater scene images, but also for enhancing environmental images with fog, low lighting, and sandstorms. Meanwhile, the proposed method can effectively improve the performance of underwater image segmentation, keypoint detection, and saliency detection. • Proposed an underwater image enhancement algorithm based on Golden Jackal. • Solve the adaptation problem of fixed parameter settings in physical methods. • Adaptively compensates for attenuation according to each color channel. • The proposed algorithm shows strong robustness and generalization performance. • Also suitable for enhancing images with low illumination, fog, and sandstorms. [ABSTRACT FROM AUTHOR]

Random fluctuation of refractive index distorts the wavefront of light propagation in the atmosphere, which destroys the coherence of the light waves, as well as has an impact on optical detection and communication, etc. In this paper, a phase-based measurement method for synchronously temporal and spatial distributions of atmospheric refractive-index structure parameter is applied. First of all, the depended relation between phase and refractive-index structure parameter is established, based on which an experiment is adopted. In experiment, 3600 frames of moiré fringes were obtained within an hour. And then, the phase and deflection angles distributions of moiré fringes are obtained by Fourier analysis and multi-grid method. Finally, the temporal and spatial distributions of refractive-index structure parameter are yielded according to the deduced theoretical relationship. The results show that both the temporal and spatial distributions refractive-index structure parameter could be measured by moiré deflectometry, and they are in good agreement with the existing methods. In a word, the related results could provide some references for studying the refractive-index structure parameter and the influence of atmosphere on optical detection and communication. • A moiré measurement method for the refractive-index structure parameter is presented. • The proposed method could realize the simultaneous measurements of temporal and spatial distributions. • The proposed method achieves simultaneous measurements with high precision. [ABSTRACT FROM AUTHOR]

We theoretically examine the temporal behavior of novel Airy beams, their interactions, and their dependence on thermal energy associated with diffusive nonlinearity in photorefractive media. For the first time, it is examined under the time-dependent diffusive-saturable nonlinear Schrödinger equation (DSNLSE). The results show that optical Airy beam propagation is effectively controlled by illumination time. In long propagation distance, the significant illumination time of the thermal energy-independent optical Airy beams associated with diffusive nonlinearity lead to unstable self-deflection soliton beams over the increasingly intense competition between photorefractive nonlinearity and diffusive nonlinearity. Meanwhile, in short propagation distance, optical Airy beams were found to maintain their acceleration over a certain distance as time increased. The temporal behavior of the interaction of optical Airy beams shows that in phase, they are affected by illumination time, while out of phase, they show almost no significant changes. We find that fusion solitons eventually emerge in phases. On the other hand, we also find that this self-trapped wave packets self-bend during propagation with an acceleration rate that depends on the thermal energy associated with diffusive nonlinearity. Finally, we found that the study results agreed well with previous experiments and studies. • Temporal behavior of diffusion-trapped Airy beams examined in photorefractive media. • The explicitly time-dependent diffusive-saturable nonlinear Schrödinger equation derived as models. • Illumination time effectively control the propagation of optical Airy beams. • Optical Airy beams exhibit self-deflection soliton beam. [ABSTRACT FROM AUTHOR]

Multiple-input single-output (MISO) have extraordinary propagation properties, making it possible to further improve the performance of underwater wireless optical communication (UWOC). In this paper, a tilted MISO-UWOC system consists of a beam array and an aperture averaged detector, academically called multiple-input aperture-average (MIAA) systems, are proposed. Different from the original approach where beams are parallel to each other, the tilted approach has a series of multiple beam transmitters individually aimed at the receiver. The MIAA systems are extensively compared to single-input single-output (SISO) systems on scintillation index and SNR for various oceanic turbulence conditions and system configurations by wave optics simulation (WOS). Our results reveal that tilted MIAA systems show tremendous advantages over the parallel MIAA systems including the reduced scintillation and the improved signal to noise ratio (SNR) under the same configuration. The obtained results can be useful for the UWOC system design. [ABSTRACT FROM AUTHOR]

The asymmetric or unidirectional light propagation has attracted enormous research interest in optical communication system. Most of these waveguides are static and cannot be modulated. In order to achieve a dynamic and tunable asymmetric waveguide, we propose a two-dimensional photonic crystal waveguide with a localized PT-symmetry. With the band coalesce, the structure achieves a unique waveguide mode band. With an excitation of line source the light in the waveguide undergoes an asymmetric or symmetric propagation in the two opposite directions of the waveguide, dependent on the source position and frequency. For a frequency in the waveguide mode band, the contrast of the field amplitudes in the two directions change periodically with the displacement of the source; whereas for the fixed source, the contrast can be modulated by the source frequency. • A photonic crystal waveguide with a localized PT-symmetry is proposed. • The band coalesce achieve a unique asymmetric propagation. • The asymmetric propagation can be adjusted by the frequency and position of the source. [ABSTRACT FROM AUTHOR]

Abstract This paper studies the characteristics of infrared waves in an antiferromagnetic waveguide. It is found that a power criterion and a frequency criterion should be satisfied for the incident waves propagating in this nonlinear guiding structure, that is, the incident waves satisfying both the frequency threshold and power threshold can steadily propagate in this gyromagnetic waveguide. The analysis shows that the peak field of the nonlinear magnetic wave will not remain at the interface. It initially moves away from the interface during the course of power increase, quickly reaches the furthest position in the waveguide, and then gradually moves back towards the interface. The maximum distance of the peak field varies typically in the range from tens to hundreds of microns, depending on its incident frequency and power. We expect that these theoretical findings revealing the interesting properties of a nonlinear antiferromagnetic waveguide can provide ideas for possible applications, and be verified in future experiments. [ABSTRACT FROM AUTHOR]

Abstract The light beam propagating characteristics of an open-type photonic crystal power splitter are investigated through modulating the structure's surfaces. The light transmittance can be improved by optimizing the radii of air holes on the input surface. The propagating directions of the outgoing branch beams can be adjusted continuously by altering the period's length of air holes on the output surface, and influence of the alteration of those air holes' locations to the splitting phenomena is also studied. Furthermore, keeping the air holes' arrangement on the output surface unchanged, the splitting behavior can also be adjusted by changing the arrangement of the air holes adjacent to the last row near the output surface. Comparing the light beam propagating behaviors along two opposite directions at normal incidence, it is shown that both the light transmittance and the light splitting characteristics can be significantly different. Highlights • Period's length and holes' location on output surface determine splitting behaviors. • Splitting direction determined by holes' period is explained by light grating theory. • Light propagation is quite different at the two cases of opposite forward directions. [ABSTRACT FROM AUTHOR]

Abstract Moiré tomography has been considered as powerful diagnostic tool to flow fields with advantages of noncontact, compact optical structure, strong anti-vibration capability and large dynamic range. However, the refractive index reconstructions in the existing moiré tomography are based on the straight ray assumption, by which the light propagation effects caused by the inhomogeneities of flow fields are not taken into account. In this paper, a spatial-domain filtered back-propagation (FBPP) algorithm for moiré tomography is studied from the wave equation in inhomogeneous media. Three advancements can be achieved by the algorithm: (1) the weak scattering effect caused by the inhomogeneities of flow field is considered; (2) the first-order derivative of the projected phase measured by moiré deflectometry can be directly employed to quantitatively reconstruct the refractive index; (3) the reconstruction is calculated in pure spatial domain, which can reduce the errors resulted by the conversions in frequency domain in the conventional FBPP algorithm. The feasibility and accuracy of the algorithm is checked by numerical simulations. Finally, the refractive index of an axisymmetric propane-air premixed flame is measured by moiré tomography with the proposed reconstruction algorithm and the temperature distribution is deduced. [ABSTRACT FROM AUTHOR]

Abstract At present three main mechanisms of optical propagation with periodically focused modes in the chains of dielectric spherical particles with a refractive index near n = 1.59 are considered in the literature: evanescent whispering gallery mode coupling, photonic nanojet-induced modes, and the propagation of transverse magnetic modes under conditions close to Brewster angle geometry. In this paper we show that quasi-periodical focusing modes exist in a range of near unity refractive indices for chains of dielectric particles in both spherical and cubic forms. We also show that such a medium can be considered a gradient index waveguide with a parabolic law of refractive index variation. Highlights • We report the quasi-periodical focusing modes exist in a range of near unity refractive indices. • The photonic nanojet-induced modes are studied by finite-difference time-domain calculations. • Such medium may be considered as a gradient index waveguide. [ABSTRACT FROM AUTHOR]

Abstract Point spread function defines the imaging property of a linear optical imaging system. Here Fermat spiral is introduced into Greek-ladder sieves to generate multiple different point spread functions along light propagation and further to realize multiplanar anisotropic imaging. Theoretical analysis and experimental results show that this kind of diffractive lenses has trifocal spots on the optical axis. The first and third foci are asymmetrical, meaning that they can provide different spatial resolution in different directions. The second focus is a vertex focal spot, which can be used for phase-contrast imaging. Due to amplitude-only diffractive lenses, they will have great potential in X-ray holography, X-ray microscopy, phase-contrast bio-imaging and optical trapping for the next generation of synchrotron radiation and free electron laser in the future. © 2018 Elsevier Ltd. All rights reserved. [ABSTRACT FROM AUTHOR]

Tissue turbulence and the effects of tissue turbulence on the propagation of optical waves are reviewed. After the introduction of a survey on the reported research in this area, various topics are elaborated. These topics include the spectrum of tissue turbulence, propagation of light in the tissue, average intensity, beam spread that occur at the receiver plane in the tissue. Other entities examined are the signal to noise ratio (SNR), intensity correlation, beam wander, mutual coherence function and the spectral degree of coherence. Furthermore, spectral change, cross spectral density, spectral correlation function, scintillation, bit error rate (BER), coupling efficiency in tissue turbulence are investigated. The refractive index structure of tissues, imaging in the presence of turbulence in the tissue, scattering, absorption and polarization aspects in tissues are reported. Finally, optical coherence tomography applications in turbulent tissues are reviewed. • Tissue turbulence and its effects on optical waves are reviewed. • Various optical entities of the propagating beam are presented in detail. • Absorption and scattering within biological tissues are covered in the review. [ABSTRACT FROM AUTHOR]

This study investigates the nonlinear dynamics and hysteresis phenomena in a figure-of-9 fiber laser. The laser cavity consists of an Erbium doped nonlinear amplifying loop mirror (NALM) containing a Dispersion Compensation Fiber (DCF) segment, coupled to a nonlinear optical loop mirror (NOLM). A lumped-element model is developed for the laser cavity accounting for bi-directional light propagation. The model is based on the modified nonlinear Schrödinger equation (NLSE) with coefficients directly related to the parameters of each cavity segment. Gain saturation is taken into consideration for stable laser operation. The numerical results show that self-starting passive mode-locking and self-structuration of soliton lattices occur in the cavity of the laser. The soliton structuring is based on the concept of dissipative solitons, which requires a balance between dispersion-nonlinearity and gain-loss. The gain-loss parameters influence both the temporal pulse interactions and the NALM's transmission, leading to a large variety of single and multiple breathing dissipative soliton dynamics. In that context, riveting studies of the transition dynamics from breathing single pulses to high-order multiple breathing dissipative solitons and hysteresis with respect to unsaturated amplifier gain in a figure-of-9 all-fiber laser are explored. [Display omitted] • Investigates nonlinear dynamics and hysteresis in figure-of-9 fiber laser. • lumped-element model used based on the modified nonlinear Schrödinger equation. • Model considers bi-directional light propagation and gain saturation. • Studies transition from single to high-order multiple breathing dissipative solitons. • Explores hysteresis with respect to unsaturated amplifier gain. [ABSTRACT FROM AUTHOR]

Improved resolutions of synthetic aperture radar (SAR) have complicated real-time high-accuracy imaging. To reduce the time and power required by digital processing algorithms, we propose a novel idea of photoelectric combination and design a SAR optical imaging system. Range matched filtering is realized in the digital domain, whereas azimuth autofocusing is realized through free propagation in the optical domain, with range cell migration correction at the speed of light. To adapt to changeable SAR parameters, a compound lens is formed by combining spatial light modulators. The system is simple, compact, efficient, and adaptable, with nano-second processing delays. [ABSTRACT FROM AUTHOR]

Branched flow is a form of transient wave propagation event that occurs in extreme circumstances. We demonstrate branched propagation of light in a silica-based active disordered photonic lattices. The test-bed proposed here can be completely tailored thus providing us ample scope to study various aspects of the branched flow and explore a better insight of the formation of branches. It is observed that by applying gain above a threshold level, the branches are formed which travel through the lattice resulting in an intense collimated beam that might lead to the development of a new type of nano-laser in the near future. • A new scheme for achieving branched flow of light in a disordered photonic lattice as a testbed. • Photons undergo Lévy walks leading to heavytailed distribution of transverse intensity profile. • quantitative studies of branched flow of light to understand intriguing physics of the phenomenon, • Evidence of achieving long and stable branches of light has been revealed. • Identified gain in the bulk as a crucial tuning parameter for controlling the branched flow of light. [ABSTRACT FROM AUTHOR]

Classical nonlinear waves exhibit, as a general rule, an irreversible process of thermalization toward the Rayleigh–Jeans equilibrium distribution. On the other hand, several recent experiments revealed a remarkable effect of spatial organization of an optical beam that propagates through a graded-index multimode optical fiber (MMF), a phenomenon termed beam self-cleaning. Our aim here is to evidence the qualitative impact of disorder (weak random mode coupling) on the process of Rayleigh–Jeans thermalization by considering two different experimental configurations. In a first experiment, we launch speckle beams in a relatively long MMF. Our results report a clear and definite experimental demonstration of Rayleigh–Jeans thermalization through light propagation in MMFs, over a broad range of kinetic energy (i.e., degree of spatial coherence) of the injected speckle beam. In particular, the property of energy equipartition among the modes is clearly observed in the condensed regime. The experimental results also evidence the double turbulence cascade process: while the power flows toward the fundamental mode (inverse cascade), the energy flows toward the higher-order modes (direct cascade). In a 2nd experiment, a coherent laser beam is launched into a relatively short MMF length. It reveals an effect of beam cleaning driven by an incipient process of Rayleigh–Jeans thermalization. As discussed through numerical simulations, the fast process of Rayleigh–Jeans thermalization observed in the 1st experiment can be attributed due to a random phase dynamics among the modes, which is favored by the injection of a speckle beam and the increased impact of disorder in the long fiber system. • We compare Rayleigh–Jeans thermalization and beam self-cleaning in multimode fibers. • Beam cleaning occurs through an incipient process of Rayleigh–Jeans thermalization. • Increasing the impact of disorder, complete Rayleigh–Jeans thermalization is observed. • Numerical simulations corroborate the experimental observations. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a new method of using graphene coating to enhance the sensitivity of the cylindrically-symmetric refractive index (RI) sensors. Previously, it was implicitly considered that graphene coating can only enhance the sensitivity of RI sensors in the polarization differentiating configuration. Many of the conventional RI sensors, including the cylindrical symmetric ones, are incapable of differentiating between the polarization modes. To address this issue, the enhancement in RI sensitivity of a graphene-coated cylindrically-symmetric configuration is studied by modifying the Fresnel coefficients. The theoretical calculations show the possibility of improving the cylindrical symmetric sensors with graphene coating. The proposed coating is quite simple and can be used for the improvement of a major fraction of conventional RI sensors. [ABSTRACT FROM AUTHOR]

Abstract By solving the normalized dimensionless Snyder–Mitchell model, the analytical expression of the Airy Gaussian vortex (AiGV) beams propagating in strongly nonlocal nonlinear media (SNNM) is specifically derived for the first time. We analytically and numerically study the AiGV beams propagating in SNNM. The influence of the changes in the input energy parameter and decay factor on the wave nodes of the propagation trajectory of the AiGV beams is described in detail. The difference in the effect of the wave nodes on the Poynting vector and the angular momentum is shown. The input energy parameter and distribution factor are discussed for the influence of the diffraction of the AiGV beams, the distribution area of Poynting vector, angular momentum, and gradient force, the peak value of the maximum scattering force, and the period of fluctuation. Our research results provide many interesting properties related to the AiGV beams in SNNM and also broaden the research areas of the AiGV beams and may promote the potential application of the AiGV beams in unique SNNM in physics or other related fields. [ABSTRACT FROM AUTHOR]

A realizable concept on the generation and propagation of phase singular optical beams in high refractive index As 2 S 3 Chalcogenide five ring hexagonal triangular lattice microstructure optical fiber (MOF) is presented. The hexagonal lattice structure with missing air holes in the first ring allows the generation of optical vortices which has been investigated. Variations in fiber mode characteristics along the length of the MOF are shown and it is demonstrated that a vortex mode repeats itself after an optimized fiber length. Numerical analysis has also been carried out for other optical properties such as the effective refractive index, V-parameter, total dispersion and loss to investigate the effect of fiber design parameters on the critical characteristics. The design freedom of the MOF enabled to achieve a low value of total dispersion variation ( < 40 nm/ps km) over a range of 650 nm optical bandwidth. Moreover, the vortex mode has a small confinement loss ( < 0.029 dB/m) up to the range of 1800 nm. Our results pave the way to applications in active and passive MOF fiber based sources and devices, optimized tweezers high-resolution imaging and sensing application. [ABSTRACT FROM AUTHOR]

Because elliptical beams have many special properties and applications, an elliptical Airy beam with and without a vortex phase has been proposed. Based on the definition of the elliptical Airy beam, an experimental setup for the generation is given, and the formula for the evolution of the intensity is derived. Meanwhile, from the propagation of the Airy beam in vacuum, the abruptly autofocusing length is derived. Based on the formula of the abruptly autofocusing length, the intensity distributions at some special locations are analyzed and discussed, especially in the autofocusing point. Study shows that some interesting intensity profiles are reformed due to the asymmetry intensity distribution of the elliptical Airy beam and abrupt autofocusing of the circular Airy beam during propagation. The different eccentricity of the elliptical Airy beam has similar properties, such as the beam split with the propagation distance and the reformed beam shaped like a ”light knife” (very narrow beam) at some positions due to the convergence process out of synchronization along the x- and y-axis. When we impart a vortex on the elliptical Airy beam, the intensity distribution will rotate and split with the propagation distance. [ABSTRACT FROM AUTHOR]

The light beam transmission characteristics of the two-dimensional photonic crystal structures consisting of a square lattice of rectangular dielectric rods are studied. Through optimizing the parameters of the structure, the self-collimated light propagation within a large frequency region is achieved, and the corresponding light propagation direction can be altered through rotating the rectangular rods in the square lattice, which is explained by the analysis of the equi-frequency surface contours. A compound structure composed of rectangular rods with different rotating angles is proposed in this paper, which has different kind of functions depending on the incident light beam’s spatial width and incident location. The single-channel collimating propagation with different forward directions, two-branch-channel splitting with adjustable energy ratio, three-branch-channel splitting with different output beams’ propagation directions can all be achieved. These kinds of devices can help expand the application of energy beam splitters in the optical circuits system and realize miniaturization integration. [ABSTRACT FROM AUTHOR]

The nonlinear Schrödinger equation is the basis of the traditional stability analysis of nonstationary guided waves in a nonlinear three-layer slab structure. The stationary (independent of the propagation distance) solutions of the nonlinear Schrödinger equation are used as ”initial data” in this analysis. In the present paper, we propose a method to investigate the dependence of these solutions on the experimental parameters and discuss their stability with respect to the parameters. The method is based on the phase diagram condition (PDC) and compact representation (in terms of Weierstrass’ elliptic function and its derivative) of the dispersion relation (DR). The problem’s parameters are constrained to certain regions in parameter space by the PDC. Dispersion curves inside (or at boundaries) of these regions correspond to possible physical solutions of Maxwell’s equations as ”start” solutions for a traditional stability analysis. Numerical evaluations of the PDC, DR, and power flow including their parameter dependence are presented. [ABSTRACT FROM AUTHOR]

Analysis of the guard-time interval impact on bit error rate (BER) of a linear SOA-based space switch aiming the guard-time reduction is presented, including effects of fiber propagation. The characterization employed feed-forward filtering and optical switching techniques, through several types and lengths of fibers (with and without dispersion compensation). The Wiener filter (with 2 taps) consistently showed a switching performance improvement, with a guard-time (GT) reduction enough to achieve the BER pre-FEC limit. In most configurations the average GT was of 429 ps (3 bits GT at 7 Gbps ). The overall best GT result achieved was using 100 mA of SOA bias current and applying the Wiener Filter, resulting in a GT of 286 ps (2 bits GT at 7 Gbps ). The space-switch operating parameters with focus on the bias current and switching technique configuration were also analyzed, aiming at reducing the SOA-based electro-optical space switch guard-time, allowing the achievement of high commutation rates. [ABSTRACT FROM AUTHOR]

We study soliton-modes of optical pulses and their stabilities in a nonlocal nonlinear fused dual-core planar waveguide. Taking the different parameters, we find the symmetric, antisymmetric and asymmetric bright soliton pairs and dipole soliton pairs. During the long-distance propagation, the amplitudes of some solitons adding the white noise exhibit periodic oscillations and the solitons except asymmetric soliton can propagate steadily. We also find that the phases and profiles of solitons in the nonlocal coupler are determined by the propagation constant, the amplitude of the optical field and the region of the fused segment. [ABSTRACT FROM AUTHOR]

This paper presents an optical 90° hybrid based on a general 4 × 4 multimode interferometer (MMI), which is realized on hybrid plasmonic waveguides (HPW). First, we analyze the propagation characteristics of multi-order hybrid plasmonic modes in the HPW-MMI, and reveal that the optimal positions of self-images would shift from the theoretical ones. By optimizing the thickness of the dielectric interlayer of the HPW structure, the width and length of the multimode section, and especially by adjusting the position of the input and output waveguides, this HPW hybrid shows a maximum phase error of 0.7°, a transmission imbalance of 0.6 dB, and CMRRs of below −20 dB at the wavelength of 1550 nm. [ABSTRACT FROM AUTHOR]

Based on the extended Huygens–Fresnel integral, the analytical formulas for the average intensity of the rotating elliptical chirped Gaussian vortex beam (RECGVB) in the oceanic turbulence are derived in this paper. Meanwhile, the evolutions of the intensity, the effective beam widths in the x - and y -directions and the dynamic center of gravity for the RECGVB propagating through the oceanic turbulence are explored by numerical examples. Results show that the intensity pattern of the RECGVB which presents two light spots at the beginning deforms and spins within a certain propagation range. Moreover, the position of the dynamic center of gravity can be modulated by the first-order chirped parameters while the profile of the intensity pattern can be affected by the second-order chirped parameters. In addition, the effects of the oceanic turbulence parameters and the initial beam width on the normalized intensity and the effective beam width of the RECGVB are discussed in detail. [ABSTRACT FROM AUTHOR]

In recent years, we have seen proliferation in the presence of surveillance devices such as cameras and commercial unmanned aircraft vehicles (UAVs). These devices have posed ever-increasing security threats. This paper proposes an active laser detection system to counter surveillance components by exploiting cat-eye effect. We first establish a cat-eye target detection model and the calculation results show that our proposals are capable of achieving results comparable to those generated by thermal detection systems but more cost effective. We also investigate the properties of reflected spots generated by cat-eye targets, and formulate corresponding recognition criteria. Using our hardware platform and the proposed algorithms, our experiments produce the recognition of static cat-eye targets at a wide range of operating distances, as well as the real-time identification of moving cat-eye targets. These results have not been achieved before, to the best of our knowledge. The entire system can be applied not only for the detection of a wide categories of stationary monitoring targets with cat-eye configurations including cameras and optical sights, but also for tracking and confronting moving surveillance devices such as UAVs. [ABSTRACT FROM AUTHOR]

Multipath effect in communication system is important to the quality of signals. The influence of the multipath effect on the performance of ghost imaging is investigated numerically and experimentally. The reflected light from ground glass is used as the light source to trigger the multipath effect, and we compare it with the traditional ghost imaging using transmitted light from ground glass as the light source. To compare the influence of the multipath effect, ghost imaging using reflected light from the aluminized ground glass under different distances is presented. The multipath effect is affected by the complexity of mutual interference, and it has both constructive and destructive influence on the quality of ghost imaging. The strong multipath effect increases the resolution, but it also reduces the signal-to-noise ratio of the reconstructed images. [ABSTRACT FROM AUTHOR]

Phase retrieval using single farfield image data is the key to many applications. However, this method easily falls into stagnation because of the multiple solutions. A wavefront reconstruction algorithm based on the hybrid focal lengths Fresnel zone plane is proposed in this paper. By using the proposed method the partially focused and defocused information of wavefront can be captured on just one farfield image. This method can effectively solve the phase ambiguity, and the optical structure is very simple and easy to implement. Simulation results show that the proposed method can reconstruct wavefront aberration very precisely with just one far field. [ABSTRACT FROM AUTHOR]

Here, a modified W 1 photonic crystal waveguide consisting of silicon rods arranged in a hexagonal lattice in the air is investigated for the aim of obtaining highly flattened defect band under the light line. This condition results in achieving ultra-low group velocity and suppresses the out of plane scatterings, simultaneously. The presented waveguide which is composed of the periodic coupled cavities inside the channel, provided high time delay with group index of n g ∼ 647 and improved normalized delay bandwidth product of ∼ 0.54 for the propagating wave. A group velocity of about ∼ 0.001545c is achieved by immersing the rods with the optimized radius and periodicity of r = 0 . 2 a and 3 a into the center of the channel, respectively. The PWE and the FDTD results with slight discrepancy, present the feasibility of ultra-slow light phenomenon in the presented structure. [ABSTRACT FROM AUTHOR]

We developed a silica optical-waveguide-type red–green–blue (RGB) multiplexer called an RGB coupler. It is a two-stage color multiplexer with a novel mode converter. In successive stages green and red light are combined with blue light accumulatively almost without leakage of recoupling. The mode converter has an additional multimode waveguide that is wider than the single-mode waveguides and is sandwiched between the single-mode waveguides of a conventional directional coupler. It converts the propagation mode of the single-mode waveguide into a higher-order mode of the additional multimode waveguide. Using a higher-order mode enhances the isolation of colors and enables us to reduce the circuit length. In this study we fabricated an RGB coupler about 2.5-mm long and 1-mm thick, and for each color the loss, excluding that of input/output routing circuits, is less than 1.2 dB. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the propagation dynamics of circular Airy beams (CAB) in a medium with radial parabolic potential both analytically and numerically. Our results show that CAB will exhibit periodic abruptly autofocusing behavior under the action of radial parabolic potentials. Within every period, we interestingly observe a novel autodefocusing behavior, which manifests that optical peak intensity abruptly decreases by several orders of magnitude. In particular, such unique “autofocusing-to-autodefocusing” effect can lead to the formation of a series of elegant periodic optical bottles having paraboloidal shapes during the whole propagation process. Our findings indicate that CAB is expected to provide a unique tool in optical micromanipulation and optical trapping. [ABSTRACT FROM AUTHOR]

Bi-directional light propagation is expected to enable enhanced functionality of all-optical signal processing operations compared to unidirectional approaches. In this work, we report on compensation of slow gain recovery in semiconductor optical amplifiers (SOAs) in a double-pass condition. The unsaturated gain of the employed SOA is increased by 12.6 dB, and the corresponding normalized conjugate output of four-wave mixing is enhanced by 16.3 dB. The theory attributes this efficiency improvement to the unsaturated gain enlargement which, in turn, compensates for the inherently long carrier lifetimes of SOAs by 50%. The saturation output power remains virtually unchanged. [ABSTRACT FROM AUTHOR]

The use of Bessel light beams, as well as Bessel acoustic beams, substantially enhances the capabilities of acousto-optic methods for control of optical field. We present a theoretical study of the process of optical Bessel beams conversion by means of backward acousto-optic scattering on a Bessel acoustic field in a transversely isotropic crystal. It is shown that, with an appropriate choice of Bessel beams parameters, the backscattering in visible spectral range can be realized at relatively low acoustic frequencies less than one gigahertz. Under conditions of phase matching and transverse spatial synchronism, the efficiency of backscattering is sufficiently high, which is interesting, for example, for construction of acousto-optic spectral analyzers. [ABSTRACT FROM AUTHOR]

Unidirectional reflectionless propagation in a non-Hermitian metamaterial is obtained based on phase coupling between two resonators. The unidirectional reflectionless propagation can be obtained at exceptional point by adjusting polarization angle θ and distance d between two resonators. Moreover, coherent prefect absorptions are obtained near exceptional point with the high absorbance of ∼ 0.99 and high quality factor of ∼ 83. [ABSTRACT FROM AUTHOR]

Airy beams have called attention of many researchers due to their non-diffracting, self-healing and transverse accelerating properties. A key issue in research of Airy beams and its applications is how to evaluate their nondiffracting propagation distance. In this paper, the critical transverse extent of physically realizable Airy beams is analyzed under the local spatial frequency methodology. The maximum nondiffracting propagation distance of aperture-truncated Airy beams is formulated and analyzed based on their local spatial frequency. The validity of the formula is verified by comparing the maximum nondiffracting propagation distance of an aperture-truncated ideal Airy beam, aperture-truncated exponentially decaying Airy beam and exponentially decaying Airy beam. Results show that the formula can be used to evaluate accurately the maximum nondiffracting propagation distance of an aperture-truncated ideal Airy beam. Therefore, it can guide us to select appropriate parameters to generate Airy beams with long nondiffracting propagation distance that have potential application in the fields of laser weapons or optical communications. [ABSTRACT FROM AUTHOR]

In this paper, a general theory of coherent population oscillation effect in an Er 3 + -doped fiber under the dual-frequency pumping laser with counter-propagation and co-propagation at room temperature is presented. Using the numerical simulation, in case of dual frequency light waves (1480 nm and 980 nm) with co-propagation and counter-propagation, we analyze the effect of the pump optical power ratio (M) on the group speed of light. The group velocity of light can be varied with the change of M. We research the time delay and fractional delay in an Er 3 + -doped fiber under the dual-frequency pumping laser with counter-propagation and co-propagation. Compared to the methods of the single pumping, the larger time delay can be got by using the technique of dual-frequency laser pumped fiber with co-propagation and counter-propagation. [ABSTRACT FROM AUTHOR]

Using the method of Laplace transform the field amplitude in the paraxial approximation is found in the two-dimensional free space using initial values of the amplitude specified on an arbitrary shaped monotonic curve. The obtained amplitude depends on one a priori unknown function, which can be found from a Volterra first kind integral equation. In a special case of field amplitude specified on a concave parabolic curve the exact solution is derived. Both solutions can be used to study the light propagation from arbitrary surfaces including grazing incidence X-ray mirrors. They can find applications in the analysis of coherent imaging problems of X-ray optics, in phase retrieval algorithms as well as in inverse problems in the cases when the initial field amplitude is sought on a curved surface. [ABSTRACT FROM AUTHOR]

Optical setups for measuring the sequence of ultrafast optical wavefronts using a chirped pulse as a reference wave in digital holography are proposed and analyzed. In this method, multiple ultrafast object pulses are used to probe the temporal evolution of ultrafast phenomena and they are interfered with a chirped reference wave to record a digital hologram. Wavefronts at different times can be reconstructed separately from the recorded hologram when the reference pulse can be treated as a quasi-monochromatic wave during the pulse width of each object pulse. The feasibility of this method is demonstrated by numerical simulation. [ABSTRACT FROM AUTHOR]

We study the propagation of light in kagomé hollow core photonic crystal fibres (HC-PCFs) filled with three different noble gases, namely, helium, xenon and argon. Various properties, including the guided modes, the group-velocity dispersion, and the nonlinear parameter were determined. The zero dispersion wavelength and the nonlinear parameter vary with the gas pressure which may be used to tune the generation of new frequencies using the same pump laser and the same fibre. In the case of the kagomé HC-PCF filled with xenon, the zero dispersion wavelength shifts from 693 to 1973 nm when the pressure is increased from 1 to 150bar, while the effective Kerr nonlinearity becomes comparable to that of silica. We have simulated the propagation of femtosecond pulses launched at 790 nm in order to study the generation of supercontinuum and UV light in kagomé HC-PCFs filled with the noble gases. [ABSTRACT FROM AUTHOR]

We propose an efficient scheme for generating and controlling beating stationary light pulses in a five-level atomic sample driven into electromagnetically induced transparency condition. This scheme relies on an asymmetrical procedure of light storage and retrieval tuned by two counter-propagating control fields where an additional coupling field, such as the microwave field, is introduced in the retrieval stage. A quantum probe field, incident upon such an atomic sample, is first transformed into spin coherence excitation of the atoms and then retrieved as beating stationary light pulses exhibiting a series of maxima and minima in intensity due to the alternative constructive and destructive interference. It is convenient to control the beating stationary light pulses just by manipulating the intensity and detuning of the additional microwave field. This interesting phenomenon involves in fact the coherent manipulation of dark-state polaritons and could be explored to achieve the efficient temporal splitting of stationary light pulses and accurate measurement of the microwave intensity. [ABSTRACT FROM AUTHOR]

Under the trend of miniaturization and reduction of system complexity, conventional bulky photonic elements are expected to be replaced by new compact and ultrathin dielectric metasurface elements. In this letter, we propose an α T i O 2 dielectric metasurface (DM) platform that could be exploited to design high efficiency wave-front control devices at visible wavelength. Combining with fundamental principles and full wave simulations (Lumerical FDTD 3D solver ® ), we successfully realize four DM devices, such as anomalous beam deflectors, polarization insensitive metalens, wave plates and polarization beam splitters. All these devices can achieve high transmission efficiencies (larger than 80%). Among them, the anomalous refraction beam deflectors can bend light propagation to any desired directions; the polarization insensitive metalens maintains diffraction limited focus (focal spot as small as 0.67 λ ); the quarter-wave and half-wave plates have broadband working wavelengths from 550 to 1000 nm; and the polarization beam splitter can split an arbitrarily polarized incident beam into two orthogonally polarized beams, the TM components is deflected to the right side, and the TE components is deflected to the left side. These devices may find applications in the areas of imaging, polarization control, spectroscopy, and on-chip optoelectronic systems etc., and our studies may richen the design of all-dielectric optical elements at visible wavelength. [ABSTRACT FROM AUTHOR]

We present a tunable output-frequency filter (TOF) algorithm to reconstruct the object from noisy experimental data under low-power partially coherent illumination, such as LED, when imaging through scattering media. In the iterative algorithm, we employ Gaussian functions with different filter windows at different stages of iteration process to reduce corruption from experimental noise to search for a global minimum in the reconstruction. In comparison with the conventional iterative phase retrieval algorithm, we demonstrate that the proposed TOF algorithm achieves consistent and reliable reconstruction in the presence of experimental noise. Moreover, the spatial resolution and distinctive features are retained in the reconstruction since the filter is applied only to the region outside the object. The feasibility of the proposed method is proved by experimental results. [ABSTRACT FROM AUTHOR]