Ghost imaging lidar system for remote imaging.

Research towards practical applications of ghost imaging lidar system especially in longer sensing distance has been urgent in recent years. In this paper we develop a ghost imaging lidar system to boost an extension of remote imaging, where the transmission distance of the collimated pseudo-thermal beam can be improved hugely over long range and just shifting the adjustable lens assembly generates wide field of view suiting for short-range imaging. Based on the proposed lidar system, the changing tendency of illuminating field of view, energy density, and reconstructed images is analyzed and verified experimentally. Some considerations on the improvement of this lidar system are also discussed.

Fast focusing method in ghost imaging with a tracking trajectory.

The imaging environment is unstable for trembling disturbance, which is detrimental to object reconstruction. In this Letter, we experimentally investigated ghost imaging (GI) under a temporal trembling disturbance. The fast-focusing method based on imaging with small sampling measurements is proposed, and the theoretical model and algorithm are validated. It is demonstrated that the proposed method is effective to obtain a better-resolution image of the object under the strong trembling disturbance including a laboratory and a real trembling environment. The results provide a promising approach to deal with image degradation caused by an unstable environment and can find potential applications for ghost imaging in remote sensing.

Controlling beam dynamics with spectral quadratic phase modulation in the fractional Schrödinger equation.

The propagation dynamics of Gaussian beams and finite energy Airy beams with spectral quadratic phase modulation (QPM) modeled by the fractional Schrödinger equation (FSE) are numerically investigated. Compared with beam propagation in the standard Schrödinger equation, the focusing property of beams under FSE is influenced by the QPM coefficient and the Lévy index. For symmetric Gaussian beams, the focusing position increases and the focusing intensity decreases for the larger QPM coefficient or smaller Lévy index. For asymmetric Airy beams, multiple focusing positions occur, and the tendency of focusing intensity is opposite to that of Gaussian beams. Our results show the promising application of the FSE system for optical manipulation and optical splitting by controlling the QPM.

BER performance of an FSOC system over atmospheric turbulence channels based on computational temporal ghost imaging.

In a free space optical communication (FSOC) system, atmospheric turbulence will increase the bit error ratio (BER) and impair FSOC link reliability. Since computational temporal ghost imaging (CTGI) has anti-interference, we present an FSOC system over atmospheric turbulence based on CTGI. The simulation results show that the BER performance of CTGI is better than on-off keying under different atmospheric turbulence regimes. To improve the performance of the CTGI scheme, the influence of the number of transmission samples and code length is analyzed. It is shown that BER performance improves with the increment of the number of samples, while code length has no impact. This scheme provides an idea for reliable communication over atmospheric turbulence and an important reference for improving wireless optical communication in an extreme environment.

Highly accurate OAM mode detection network for ring Airy Gaussian vortex beams disturbed by atmospheric turbulence based on interferometry.

Atmospheric turbulence reduces the detection accuracy of orbital angular momentum (OAM) modes, which affects the performance of OAM optical communication. In this paper, we propose a method based on interferometry and a residual network (ResNet) to detect the OAM modes of ring Airy Gaussian vortex beams (RAGVBs) disturbed by atmospheric turbulence. The RAGVBs first interfere with spherical waves to obtain the sign features of the OAM modes, and then ResNet is employed to recognize OAM modes from the interferograms. The results demonstrate that the detection accuracy is higher than that of the OAM spectrum method under different turbulence strengths. The detection accuracy can even reach over 99% under strong fluctuations. Our research provides a reference for improving the performance of OAM optical communication through atmospheric turbulence.

Propagation dynamics and crosstalk of orbital angular momentum beams influenced by a supersonic wind-induced environmental disturbance.

Near field airflow induced by wind is an important factor influencing vortex beams propagation under airborne optical communication, and the cross-talk among different orbital angular momentum (OAM) modes occurs in OAM-based optical communication. In this paper, the propagation of vortex beams through a supersonic wind-induced random environment is investigated. The wind-induced phase model is firstly validated by wind tunnel experiment, with the phase model, vortex beams propagation under supersonic wind condition is analyzed, and the spiral spectrum distortion is discussed in detail. It is demonstrated that the larger wind velocity and boundary-layer thickness leads to the enhanced distortion and spiral spectrum expansion of OAM beams. The behavior of multiplexed vortex beams influenced by supersonic wind is also studied, and the effect of the topological charge interval is given. Our results may provide a powerful tool to estimate the effect of a random airflow environment on OAM-based communication performance under airborne condition.

Enhancing critical resolution of a ghost imaging system by using a vortex beam.

In an imaging system, resolution and signal-to-noise ratio (SNR) are two important indexes to characterize imaging quality. Ghost imaging is a novel imaging method whose imaging resolution and SNR are affected by the speckle size. In this paper, the relation between speckle size and resolution as well as that between speckle size and SNR in the GI system is analyzed in detail. It is shown that the critical resolution, resolvable minimum-separation between two adjacent objects, is approximately equal to the speckle size (speckle diameter). There exists an optimum SNR when the speckle size is larger than the object size. Based on our conclusion, we propose a scheme to enhance the critical resolution of the GI system by using a vortex beam, and the enhancement ability under different topological charges is clearly presented, which can be quantized by a simple formula.

Propagation dynamics of Laguerre-Gaussian beams in the fractional Schrödinger equation with noise disturbance.

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.

Analysis of the allowable maximum amplitude of random jitter in computational ghost imaging.

The imaging blur caused by the relative motion between an imaging system and an imaging target can be eliminated by a compensation operation based on the motion law, although it is invalid when the movement is random. In this paper, the influence of the random jitter on imaging resolution is investigated quantitatively in a computational ghost imaging system, and the maximum of random-jitter amplitude that the system can allow is analyzed. The numerical and experimental results verify our conclusion, and the error between the experiment and simulation is less than 12%. The main reason for the error is the disturbance of experimental noise. By using ghost imaging algorithms to reduce the corresponding impact, this error can be further reduced to 6%.

Improving the signal-to-noise ratio of computational ghost imaging of a reflective object with a rough surface by Hadamard modulated light field.

We propose an optimization scheme to improve the reconstruction quality of computational ghost imaging (GI) of a reflective target with a rough surface by using the Hadamard modulation light field (HCGI). By comparison with computational GI with a traditional Gaussian light field (GCGI), the signal-to-noise ratio of GCGI is quite bad, and it is difficult to distinguish the imaging signal from the background when the surface roughness of the object is higher, while a ghost image with better quality can be obtained by HCGI. The difference is explained by comparing the distribution of the correlation coefficient. Additionally, it is found that HCGI has better noise robustness in comparison with GI with other random coded patterns.

Ghost imaging influenced by a supersonic wind-induced random environment.

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.

Ghost imaging for detecting trembling with random temporal changing.

The imaging environment can be destabilized for moving objects and imaging platforms, thus leading the detection to be random trembling, which is detrimental to object reconstruction. In this Letter, we experimentally investigate ghost imaging for detecting trembling with random temporal changing, and an improvement method based on the temporal property of the imaging process is proposed. It is demonstrated that this method is effective in addressing image degradation due to the trembling disturbance and obtaining a higher-quality image of the object. The results provide a promising approach to deal with image degradation caused by an unstable environment and can find potential applications for ghost imaging in remote sensing.

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

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

Imaging quality improvement of ghost imaging in scattering medium based on Hadamard modulated light field.

The scattering medium in the imaging path can affect the imaging quality of traditional ghost imaging. We propose to substitute a Gaussian light source with a Hadamard modulated light field to reduce correlated noise due to the occurrence of scattering medium and to improve the corresponding signal-to-noise ratio (SNR). In the simulation, scattering media with different intensities are applied on the test arm, and the result shows that Hadamard modulated light, with the particular orthogonality, is superior to Gaussian light. By substituting Gaussian light with Hadamard mudulated light, a ghost image with higher SNR can be obtained under both weak and strong scattering.

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

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

Effect of the collection range of a bucket detector on ghost imaging through turbulent atmosphere.

A model of the collection range of a bucket detector is proposed to investigate ghost imaging through turbulent atmosphere. The influence of the collection range of a bucket detector on the quality of ghost imaging is discussed under different turbulence intensities. The numerical results show that the quality of ghost imaging increases with increment in the collection range of the bucket detector and with a decrease in turbulence intensity. Specifically, we give a critical collection range of a bucket detector, which is the minimum collection range for ghost imaging at different turbulence intensities. Additionally, a comparison of ghost imaging through turbulent atmosphere with ghost imaging through vacuum atmosphere is provided to show the effect of turbulent atmosphere on ghost imaging.

Average intensity and beam quality of optical coherence lattices in oceanic turbulence with anisotropy.

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 (SR) 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 χT or decreasing the anisotropic factor ξ and the rate of dissipation of turbulent kinetic energy per unit mass of fluid ε. Further, the SR and PIB in the target plane under the effects of oceanic parameters are discussed in detail, and the SR and PIB can be increased for the larger ξ and ε or the smaller χT and ω, namely, the beam quality becomes better. Our results can find potential application in the future optical communication system in an oceanic environment.

Influence of transversely inhomogeneous pseudo-thermal light source on lensless ghost imaging.

The influence of a transversely inhomogeneous pseudo-thermal light source on lensless ghost imaging is investigated theoretically and experimentally. Based on classical optical theory, a model of lensless ghost imaging with an inside inclined light source is analyzed. We use the optical path difference between different transverse positions of the light beam to estimate the degree of inhomogeneity. The results indicate that the transversely inhomogeneous light source decreases the visibility and signal-to-noise ratio of the reconstructed image. Finally, we implement experiments to verify our results using inclined ground glass.

Reduction of the defocusing effect in lensless ghost imaging and ghost diffraction with cosh-Gaussian modulated incoherent sources.

The inhibition effect from the cosh-Gaussian modulated incoherent source on the defocusing effect is investigated theoretically in lensless ghost imaging (LGI) and ghost diffraction (LGD) systems. The corresponding numerical simulations are presented to show the influence of the cosh-Gaussian incoherent source on the defocusing effect in LGI and LGD. Compared with the widely used Gaussian incoherent source, it is shown that the defocusing effect in LGI and LGD can be greatly weakened by properly adjusting the modulation parameter ω of the cosh-Gaussian source. To explain this phenomenon, the analytical expression for point spread function of the LGI system with the cosh-Gaussian source is derived.

Plasma optical modulation for lasers based on the plasma induced by femtosecond pulses.

We present a theoretical and experimental study of plasma optical modulation for probe lasers based on the plasma induced by pump pulses. This concept relies on two co-propagating laser pulses in carbon disulfide, where a drive laser pulse first excites plasma channels while a following carrier laser pulse is modulated by the plasma. The modulation on the probe beam can be conveniently adjusted through electron density, plasma width, propagation distance of plasma, the power of pump lasers, or the pump beam's profile. The experimental results and theoretical solutions are very consistent, which fully illustrates that this method for plasma optical modulation is reasonable. This pump-probe method is also a potential measurement technique for inferring the on-axis plasma density shape.