Xiquan Fu

The vortex generated in the gas–liquid interface may appear in the propagation path, and shows severe disturbance to light propagation, thus affects the final imaging. In this Letter, the influence from the vortex on underwater ghost imaging is investigated experimentally. We find a counter-intuitive phenomenon that imaging quality becomes worse when the vortex is close to the imaging target, which is absolutely different from the typical channel disturbances, such as atmospheric turbulence and scattering. This phenomenon can be explained by the influence mechanism of speckle variation and the effective data distribution on the bucket detector. Meanwhile, a modified imaging method based on the effective bucket samples is proposed to enhance imaging quality and verified in experiment.

The evolution of Laguerre–Gaussian (LG) beams in the fractional Schrödinger equation (FSE) with Gaussian noise disturbance is numerically investigated. Without noise disturbance, the peak intensity of LG beams increases with the increment of radial or azimuthal indices, and the turning point of the peak intensity between different radial indices exists. As propagation distance gets longer, the intensity of the outermost sub-lobe exceeds that of the main lobe. When Gaussian noise is added, for a given noise level, the stability of peak intensity is enhanced as the Lévy index increases, while the center of gravity shows the opposite phenomenon. Moreover, the increment of the radial index can weaken the stability of the center of gravity. We also investigate the stability of the peak intensity of Airy beams in the FSE, and generally, the stability of LG beams is better than that of Airy beams. All these properties show that LG beams modeled by the FSE have potential applications in optical manipulation and communications.

Near field airflow induced by wind is an important factor influencing imaging quality when the imaging system is placed on a moving platform with high speed, such as airborne imaging. In this Letter, ghost imaging through an airflow environment is experimentally and numerically investigated. The experiment is performed with a wind tunnel, and imaging quality decreases with wind velocity. The simulation model of ghost imaging through this kind of environment is proposed, and simulation results match well with experiments. With the model, imaging results are extended into the supersonic wind region with the effects of airflow factors discussed in detail, and a comparison between airflow and atmosphere turbulence is presented. The results can find potential applications in optical imaging and may be a powerful tool to estimate the effect of airflow on performance of the imaging system.

Hypersonic turbulence, causing the environment disturbance, is an important factor influencing optical propagation and optical communication. Based on the extended Huygens-Fresnel principle and second-order moments of the Wigner distribution function, we have derived the analytical expressions of the average intensity, the normalized propagation factor, and the normalized mean-squared beam width of Gaussian Schell-model beams in anisotropic hypersonic turbulence. The evolution properties demonstrate the severe beam expansion and beam quality degradation, and the influences of the source and turbulent parameters are discussed in detail. The comparison between anisotropic hypersonic turbulence and atmosphere turbulence is given, and it is proved that the influence of anisotropic hypersonic turbulence is much stronger. Our results play a guiding role in optical signal transmission in the turbulent environment induced by hypersonic aircraft.

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.

Based on the extended Huygens-Fresnel principle, we have derived the analytical expression of the average intensity of optical coherence lattices (OCLs) in oceanic turbulence with anisotropy, and then the beam quality parameters including the Strehl ratio (S_R) and the power-in-the-bucket (PIB) are obtained. One can find that the OCLs will eventually evolve into Gaussian shape with the periodicity reciprocity gradually breaking down when propagating through the anisotropic ocean water, and that the trend of evolving into Gaussian can be accelerated for increasing the ratio of temperature and salinity contributions to the refractive index spectrum ω, the lattice constant a and the rate of dissipation of mean square temperature χ_T or decreasing the anisotropic factor ξ and the rate of dissipation of turbulent kinetic energy per unit mass of fluid ε. Further, the S_R and PIB in the target plane under the effects of oceanic parameters are discussed in detail, and the S_R and PIB can be increased for the larger ξ and ε or the smaller χ_T and ω, namely, the beam quality becomes better. Our results can find potential application in the future optical communication system in an oceanic environment.

The dynamics of Airy beams modeled by the fractional Schrödinger equation (FSE) with the potential barrier are numerically investigated. Adjusting the Lévy index provides a convenient way to control the diffraction of Airy beams which presents the non-diffraction and splitting property. It has been found that the total reflection of beams occurs when the depth of the single potential barrier exceeds the threshold, and that the number of reflected waves is influenced by the Lévy index and the location of potential barrier. However, the periodic self-imaging phenomenon of Airy beams is shown under a symmetric potential barrier when the Lévy index is equal to one, and the self-imaging period of the asymmetric Airy beams is analytically demonstrated and is as twice as that of symmetric Gaussian beams, moreover, the chaoticon of light field is formed during propagation as the Lévy index increases. All the properties of Airy beams modeled by FSE confirm the potential application in optical manipulation.

We have numerically investigated the propagation properties of ring Airy beams modeled by a fractional Schrödinger equation (FSE). Beam splitting is exhibited and clarified from the view of group delay. Compared with the case of the condition in free space, the focal intensity is greatly enhanced and presents a trend from increasing to decreasing with the decrement of the Lévy index, while the focal length largely increases; in addition, the parabolic trajectory is distorted with the Bessel pattern gradually disappearing. However, both the focal intensity and its length get decreased due to the absence of the initial phase of ring Airy beams; simultaneously, the size of the focal spot decreases for a larger Lévy index. The changes of focusing properties are clearly clarified. Moreover, without initial phase, the ratio of focal intensity under the Lévy index to the condition in free space is given to show the faster growth of focal intensity when the Lévy index is lower than 1.25; otherwise, the trend gets reversed. The autofocusing properties of ring Airy beams modeled by FSE can find applications in optical trapping and manipulation.

The influence of detector noise on ghost imaging (GI) is investigated at low light levels. Based on the characteristics of the additive detector noise, we establish the analytical model and display the ghost images through numerical and experimental demonstrations. It is shown that the contrast-to-noise ratio and visibility of reconstructed images are sharply affected by the detector noise. Following the increase of the ratio of average signal intensity to the average noise, the quality of reconstructions is enhanced. To reduce the measurement numbers and, thus, shorten the consuming time without sacrificing the imaging quality, we propose a sorting technique in the traditional GI algorithm for a high quality image reconstruction. The results demonstrated here will be favorable to the applications of low-light-level imaging.