Fourier-transform ghost imaging with super-Rayleigh speckles.

Ghost imaging based on the high-order correlation of optical field has developed rapidly and has been extended to the x-ray region. However, the limited flux leads to severe image deterioration. Here, an approach of Fourier-transform ghost imaging with super-Rayleigh speckles is proposed to realize high quality ghost imaging at low photon flux level. The super-Rayleigh speckles are designed by optimizing binary modulating screens based on the direct binary search algorithm. The experimental results show that the speckle contrast can be greatly enhanced and high visibility Fourier-transform diffraction pattern of the sample can be obtained. The sample's image in spatial domain is successfully achieved even if the detected photon level decreases to 0.1 photons/pixel. This method is of great importance for high-resolution imaging in the photon-limited scenarios, especially for laboratory x-ray systems.

Path sampling and integration method to calculate speckle patterns.

A stable speckle pattern is generated when a coherent beam illuminates a stationary scattering medium that contains numerous scatterers with fixed positions. To date, there has been no valid method to the best of our knowledge for calculating the speckle pattern of a macro medium with a large number of scatterers. Here, a new method based on possible path sampling with corresponding weights and coherent superposition is presented for the simulation of optical field propagation in a scattering medium and output speckle patterns. In this method, a photon is launched onto a medium with fixed scatterers. It propagates in one direction; upon collision with a scatterer, its direction is updated. The procedure is repeated until it exits the medium. A sampled path is obtained in this manner. By repeatedly launching photons, numerous independent optical paths can be sampled. A speckle pattern, corresponding to the probability density of the photon, is formed by the coherent superposition of sufficiently sampled path lengths ending on a receiving screen. This method can be used in sophisticated studies of the influences of medium parameters, motion of scatterers, sample distortions on speckle distributions, and morphological appearances. It can be used for micro-examination of optical fields in scattering media and may inspire new methods and techniques for non-invasive precision detection and diagnosis of scattering media.

Underwater ghost imaging with detection distance up to 9.3 attenuation lengths.

Underwater ghost imaging LiDAR is an effective method of underwater detection. In this research, theoretical and experimental investigations were conducted on underwater ghost imaging, combining the underwater optical field transmission model with the inherent optical parameters of a water body. In addition, the Wells model and the approximate Sahu-Shanmugam scattering phase function were used to create a model for underwater optical transmission. The second-order Glauber function of the optical field was then employed to analyze the scattering field degradation during the transmission process. The simulation and experimental results verified that the proposed underwater model could better reveal the degrading effect of a water body on ghost imaging. A further series of experiments comparing underwater ghost imaging at different detection distances was also conducted. In the experimental system, gated photomultiplier tube (PMT) was used to filter out the peak of backscattering, allowing a larger gain to be set for longer-range detection of the target. The laser with a central wavelength of 532 nm was operated at a frequency of 2 KHz, with a single pulse energy of 2 mJ, a pulse width of 10 ns. High-reflective targets were imaged up to 65.2 m (9.3 attenuation lengths (ALs), attenuation coefficient c = 0.1426 m-1, and scattering coefficient b = 0.052 m-1) and diffuse-reflection targets up to 41.2 m (6.4 ALs, c = 0.1569 m-1, and b = 0.081 m-1). For the Jerlov-I (c = 0.048 m-1 and b = 0.002 m-1) water body, the experimentally obtained maximum detection distance of 9.3 ALs can be equivalent to 193.7 m under the same optical system conditions.

Temporal characterization of electron dynamics in attosecond XUV and infrared laser fields.

We use a Wigner distribution-like function based on the strong field approximation theory to obtain the time-energy distributions and the ionization time distributions of electrons ionized by an XUV pulse alone and in the presence of an infrared (IR) pulse. In the case of a single XUV pulse, although the overall shape of the ionization time distribution resembles the XUV-envelope, its detail shows dependence on the emission direction of the electron and the carrier-envelope phase of the pulse, which mainly results from the low-energy interference structure. It is further found that the electron from the counter-rotating term plays an important role in the interference. In the case of the two-color pulse, both the time-energy distributions and the ionization time distributions change with varying IR field. Our analysis demonstrates that the IR field not only modifies the final electron kinetic energy but also changes the electron's emission time, which is attributed to the change of the electric field induced by the IR pulse. Moreover, the ionization time distributions of the photoelectrons emitted from atoms with higher ionization energy are also given, which show less impact of the IR field on the electron dynamics.

Spectral polarization camera based on ghost imaging via sparsity constraints.

A spectral polarization camera based on ghost imaging via sparsity constraints (GISC) is presented. The proposed imager modulates three-dimensional spatial and spectral information of the target into two-dimensional speckle patterns using a spatial random phase modulator and then acquires the speckle patterns at four linear polarization channels through a polarized CCD. The experimental results verify the feasibility of the system structure and reconstruction algorithm. The GISC spectral polarization camera, which has a simple structure and achieves compressive sampling during the imaging acquisition process, provides a simple scheme for obtaining multi-dimensional information of the light field.

Ghost imaging based on Y-net: a dynamic coding and decoding approach.

Ghost imaging incorporating deep learning technology has recently attracted much attention in the optical imaging field. However, deterministic illumination and multiple exposure are still essential in most scenarios. Here we propose a ghost imaging scheme based on a novel dynamic decoding deep learning framework (Y-net), which works well under both deterministic and indeterministic illumination. Benefited from the end-to-end characteristic of our network, the image of a sample can be achieved directly from the data collected by the detector. The sample is illuminated only once in the experiment, and the spatial distribution of the speckle encoding the sample in the experiment can be completely different from that of the simulation speckle in training, as long as the statistical characteristics of the speckle remain unchanged. This approach is particularly important to high-resolution x-ray ghost imaging applications due to its potential for improving image quality and reducing radiation damage.

Experimental investigation of chirped amplitude modulation heterodyne ghost imaging.

We have constructed a chirped amplitude modulation heterodyne ghost imaging (CAM-HGI) experimental system that demonstrates a robust ability against background light in experiments. In the experiments, the background light is simulated by irradiating a spatiotemporal random modulated light field onto the target. The effects of background light, modulation depth and modulation duration of the signal light source on CAM-HGI are investigated experimentally. The results show that the quality of CAM-HGI can be improved by increasing the modulation depth and the modulation duration of the signal light source, and more importantly, an image with a good signal-to-noise ratio (SNR) can be achieved even when the irradiation SNR is lower than -30 dB. This technique of CAM-HGI has an important application prospect for laser imaging in strong background light environments.

Focal-plane three-dimensional imaging method based on temporal ghost imaging: a proof of concept simulation.

A new focal-plane three-dimensional (3D) imaging method based on temporal ghost imaging is proposed and demonstrated. By exploiting the advantages of temporal ghost imaging, this method enables the utilization of slow integrating cameras and facilitates 3D surface imaging within the framework of sequential flood-illumination and focal-plane detection. The depth information is achieved by a temporal correlation between received and reference signals with multiple-shot, and the reflectivity information is achieved by flash imaging with a single-shot. The feasibility and performance of this focal-plane 3D imaging method have been verified through theoretical analysis and numerical experiments.

Optimization of light fields in ghost imaging using dictionary learning.

Ghost imaging (GI) is a novel imaging technique based on the second-order correlation of light fields. Due to limited number of samplings in practice, traditional GI methods often reconstruct objects with unsatisfactory quality. To improve the imaging results, many reconstruction methods have been developed, yet the reconstruction quality is still fundamentally restricted by the modulated light fields. In this paper, we propose to improve the imaging quality of GI by optimizing the light fields, which is realized via matrix optimization for a learned dictionary incorporating the sparsity prior of objects. A closed-form solution of the sampling matrix, which enables successive sampling, is derived. Through simulation and experimental results, it is shown that the proposed scheme leads to better imaging quality compared to the state-of-the-art optimization methods for light fields, especially at a low sampling rate.

Non-locally coded Fourier-transform ghost imaging.

A non-locally coded Fourier-transform ghost imaging (FGI) scheme and relevant coded phase retrieval method have been proposed to improve the image quality in ghost imaging. By inserting masks in the reference beam, the sample in the test beam is non-locally modulated, and coded Fourier-transform diffraction patterns of the sample are obtained via intensity correlation calculations between the two beams. Encoding and decoding procedures are incorporated in the phase retrieval process based on traditional hybrid input-output algorithm. Simulation and experiment results show that the spatial information of samples is successfully recovered from the coded diffraction patterns obtained using three masks, and the image quality is improved remarkably. If promoting this approach to x-ray FGI systems, it may realize high-resolution x-ray microscopy without increasing the sample's radiation damage.

Ghost imaging LiDAR via sparsity constraints using push-broom scanning.

Ghost imaging LiDAR via sparsity constraints using push-broom scanning is proposed. It can image the stationary target scene continuously along the scanning direction by taking advantage of the relative movement between the platform and the target scene. Compared to conventional ghost imaging LiDAR that requires multiple speckle patterns staring the target, ghost imaging LiDAR via sparsity constraints using push-broom scanning not only simplifies the imaging system, but also reduces the sampling number. Numerical simulations and experiments have demonstrated its efficiency.

Expansion of the FOV in speckle autocorrelation imaging by spatial filtering.

Optical imaging through inhomogeneous media based on autocorrelations suffers from a limited field of view (FOV), since the optical memory effect (ME) of a scattering medium has its inherent angular extent. Here we successfully expand the angular ME range by exploiting a spatial filtering technique to select low-frequency components, mainly ballistic light and less scattered light, thereby increasing the FOV of the speckle autocorrelation imaging. Both a simulation and experimental verifications are presented. This technique, which is not limited to the discussed 4f structure, can provide a guideline for the design of an optical system to image through scattering media.

Bidirectional image transmission through physically thick scattering media using digital optical phase conjugation.

We demonstrate the feasibility of bidirectional image transmission through a physically thick scattering medium within its memory effect range by digital optical phase conjugation. We show the bidirectional transmission is not simply the consequence of optical reciprocity. We observe that when the spatial light modulator (the device performing the digital optical phase conjugation) is relayed to the middle plane of the medium, the memory effect will be fully exploited and thus the transmitted images will have maximum field of view (FOV). Furthermore, we show that the FOV can be expanded n times by performing n times wavefront measurements.

Spatial multiplexing reconstruction for Fourier-transform ghost imaging via sparsity constraints.

A spatial multiplexing reconstruction method has been proposed to improve the sampling efficiency and image quality of Fourier-transform ghost imaging. In this method, the sensing equation of Fourier-transform ghost imaging is established based on recombination and reutilization of the correlated intensity distributions of light fields. It is theoretically proved that the scale of the sensing matrix in the sensing equation can be greatly reduced, and spatial multiplexing combined with this matrix reduction provides the feasibility of ghost imaging with just a few measurements. Experimental results show better visibility and signal-to-noise ratio in the Fourier spectrums reconstructed via spatial multiplexing compared with previous methods. The transmittance of an object is also recovered in spatial domain with better image quality based on its spectrum of spatial multiplexing reconstruction. This method is especially important to x-ray ghost imaging applications due to its potential for reducing radiation damage and achieving high quality images in x-ray microscopy.

Hyperspectral ghost imaging camera based on a flat-field grating.

A spectral camera based on ghost imaging via sparsity constraints (GISC) acquires a three-dimensional (3D) spatial-spectral data cube of the target through a two-dimensional (2D) detector in a single snapshot. However, the spectral and spatial resolution are interrelated because both of them are modulated by the same spatial random phase modulator. In this paper, we theoretically and experimentally demonstrate a system by equipping the GISC spectral camera with a flat-field grating to disperse the light fields before the spatial random phase modulator, hence consequently decoupling the spatial and spectral resolution. By theoretical derivation of the imaging process we obtain the spectral resolution 1nm and spatial resolution 50µm about the new system which are verified by the experiment. The new system can not only modulate the spatial and spectral resolution separately, but also provide a possibility of optimizing the light field fluctuations of different wavelengths according to the imaging scene.

Optical phase conjugation of diffused light with infinite gain by using gated two-color photorefractive crystal LiNbO3:Cu:Ce.

Light focusing in multiple scattering circumstances is important in biomedical imaging, manipulation, and therapy. Until now, many traditional photorefractive crystals have been used to generate an optical phase conjugated wavefront in an analogue time-reversed optical focusing technology. However, owing to erasure of a volume hologram during a reading procedure, the optical energy gain can never reach unity, limiting its application in delivering more energy into a target area. In this work, we investigated a gated two-color photorefractive crystal LiNbO3:Cu:Ce to generate optical phase conjugation of diffused light with infinite gain.

Effect of two-center interference on molecular ionization in multiphoton ionization regime.

Using solution of the full three-dimensional time-dependent Schrödinger equation (TDSE) in prolate spheroidal coordinates, we investigate the orientation dependence of ionization of H2+ in near-infrared laser fields. It is found that, the ionization probability decreases as a function of the alignment angle in tunneling ionization regime, while it ascends with the increase of orientation angle in multiphoton ionization regime for the internuclear distance R=2 a.u. Furthermore, the result obtained by the length gauge strong-field approximation theory is in qualitative agreement with that calculated by the TDSE but the radiation gauge strong-field approximation and molecular ADK theories fail to reproduce the TDSE result. Analysis indicates that the above intriguing feature can be ascribed to the interference between the partial electron wave packets emitted from different molecular cores, which becomes evident at low laser intensity due to increased width of the initial mechanical momentum of the photoelectron at ionization moment. In addition, when the internuclear distance increases to R=4 a.u., the ionization yields decrease vs alignment angle in both tunneling and multiphoton regimes since the electron wavefunction of the 1σg orbit is more concentrated in the molecular axis than that of R=2 a.u.

Quantum temporal imaging by four-wave mixing.

We investigate temporal imaging of broadband squeezed light by four-wave-mixing. We consider two possible imaging configurations: phase-conjugating (PC) and phase-preserving (PP). Both of these configurations have been successfully used for temporal imaging of classical temporal waveforms. We demonstrate that for quantum temporal imaging, precisely, temporal imaging of broadband squeezed light, these two schemes have very different behavior: the PC configuration deteriorates squeezing, while the PP configuration leaves it intact. These results are very important for the applications of temporal imaging for quantum communications and quantum information processing.

Pulse-compression ghost imaging lidar via coherent detection.

Ghost imaging (GI) lidar, as a novel remote sensing technique, has been receiving increasing interest in recent years. By combining pulse-compression technique and coherent detection with GI, we propose a new lidar system called pulse-compression GI lidar. Our analytical results, which are backed up by numerical simulations, demonstrate that pulse-compression GI lidar can obtain the target's spatial intensity distribution, range and moving velocity. Compared with conventional pulsed GI lidar system, pulse-compression GI lidar, without decreasing the range resolution, is easy to obtain high single pulse energy with the use of a long pulse, and the mechanism of coherent detection can eliminate the influence of the stray light, which is helpful to improve the detection sensitivity and detection range.

Fourier-Transform Ghost Imaging with Hard X Rays.

Knowledge gained through x-ray crystallography fostered structural determination of materials and greatly facilitated the development of modern science and technology in the past century. However, it is only applied to crystalline structures and cannot resolve noncrystalline materials. Here we demonstrate a novel lensless Fourier-transform ghost imaging method with pseudothermal hard x rays that extends x-ray crystallography to noncrystalline samples. By measuring the second-order intensity correlation function of the light, Fourier-transform diffraction pattern of a complex amplitude sample is achieved at the Fresnel region in our experiment and the amplitude and phase distributions of the sample in the spatial domain are retrieved successfully. For the first time, ghost imaging is experimentally realized with x rays. Since a highly coherent x-ray source is not required, the method can be implemented with laboratory x-ray sources and it also provides a potential solution for lensless diffraction imaging with fermions, such as neutrons and electrons where intensive coherent sources usually are not available.