Wen-Kai Yu

Adaptive compressive ghost imaging based on wavelet trees and sparse representation.

Compressed sensing is a theory which can reconstruct an image almost perfectly with only a few measurements by finding its sparsest representation. However, the computation time consumed for large images may be a few hours or more. In this work, we both theoretically and experimentally demonstrate a method that combines the advantages of both adaptive computational ghost imaging and compressed sensing, which we call adaptive compressive ghost imaging, whereby both the reconstruction time and measurements required for any image size can be significantly reduced. The technique can be used to improve the performance of all computational ghost imaging protocols, especially when measuring ultra-weak or noisy signals, and can be extended to imaging applications at any wavelength.

Lensless ghost imaging with sunlight.

An experiment demonstrating lensless ghost imaging (GI) with sunlight has been performed. A narrow spectral line is first filtered out and its intensity correlation measured. With this true thermal light source, an object consisting of two holes is imaged. The realization of lensless GI with sunlight is a step forward toward the practical application of GI with ordinary daylight as the source of illumination.

Three-dimensional single-pixel compressive reflectivity imaging based on complementary modulation

A three-dimensional (3D) imager with a single-pixel detector and complementary intensity modulation of a digital micromirror device (DMD) array, which does not rely on scene raster scanning as in light detection and ranging (LIDAR) or on a two-dimensional array of sensors as used in time-of-flight (TOF) cameras, can not only capture full-color, high-quality images of real-life objects, but also recover the depth information and 3D reflectivity of the scene, reducing the required measurement dimension as well as the complexity, and cutting the cost of the detector array down to a single unit. The imager achieves spatial resolution using compressed sensing to exploit the sparsity of the signal. The disparity maps of the scene are reconstructed using sum of absolute or squared differences to reveal the depth information. This nonscanning, low-complexity 3D reflectivity imaging prototype may be of considerable value to various computer vision applications.

High-speed secure key distribution over an optical network based on computational correlation imaging

We present a protocol for an optical key distribution network based on computational correlation imaging, which can simultaneously realize privacy amplification and multiparty distribution. With current technology, the key distribution rate could reach hundreds of Mbit/s with suitable choice of parameters. The setup is simple and inexpensive, and may be employed in real networks where high-speed long-distance secure communication is required.

Protocol based on compressed sensing for high-speed authentication and cryptographic key distribution over a multiparty optical network

We present a protocol for the amplification and distribution of a one-time-pad cryptographic key over a point-to-multipoint optical network based on computational ghost imaging (GI) and compressed sensing (CS). It is shown experimentally that CS imaging can perform faster authentication and increase the key generation rate by an order of magnitude compared with the scheme using computational GI alone. The protocol is applicable for any number of legitimate user, thus, the scheme could be used in real intercity networks where high speed and high security are crucial.

Iterative denoising of ghost imaging

We present a new technique to denoise ghost imaging (GI) in which conventional intensity correlation GI and an iteration process have been combined to give an accurate estimate of the actual noise affecting image quality. The blurring influence of the speckle areas in the beam is reduced in the iteration by setting a threshold. It is shown that with an appropriate choice of threshold value, the quality of the iterative GI reconstructed image is much better than that of differential GI for the same number of measurements. This denoising method thus offers a very effective approach to promote the implementation of GI in real applications.

High-visibility ghost imaging from artificially generated non-Gaussian intensity fluctuations

The image quality in ghost imaging is vital in practical applications. Through theoretical analysis, we find that for thermal light the average intensity as well as the fluctuations of an arbitrary incident field can greatly influence the image quality. Based on this, we suggest an easily realizable scheme to improve the visibility by generating speckles of non-Gaussian intensity distributions with a spatial light modulator. Numerical simulation demonstrates that this method can significantly improve the visibility, and the effect on the imaging resolution is also discussed. This method may thus be helpful in promoting the implementation of ghost imaging in real applications.

Thermal stability of cubic GaN film grown by molecular-beam epitaxy on GaAs(001)

The thermal stability of cubic-phase GaN (c-GaN) film grown by molecular-beam epitaxy was investigated by Raman scattering spectroscopy and X-ray scattering. The results of Raman scattering shows that, after annealing at 1000degreesC, the intensity of transverse (TO) and longitudinal (LO) optical peaks from cubic phase obviously decreases while the intensity of TOb peak from the boundary effect slightly decreases, but the transformation of the hexagonal phase (alpha-GaN) can not be detected due to a little of alpha-GaN inclusion. X-ray reflectivity measurements indicate that there is a high-electron-density layer between the substrate and the GaN film, and it becomes uniform and much thinner after high-temperature annealing, counting for the Raman results of the intensity change of the TOb peak. The results of high-angle X-ray diffraction and X-ray reciprocal space mapping revealed that the relative content of alpha-GaN obviously increases after annealing at 1000degreesC, and (10 (1) over bar1) is the most stable diffraction lattice of the alpha-GaN hexagonal phase

Evaluation criterion of thermal light ghost imaging based on the receiver operating characteristic analysis

The performances of different thermal ghost imaging (GI) algorithms are compared in an experiment of computational GI using a digital micromirror device. Here we present a rather different evaluation criterion named receiver operating characteristic (ROC) analysis that serves as the performance of merit for the quantitative comparison. A ROC curve is created by plotting the true positive rate against the false positive rate at various threshold settings. Both theoretical analysis and experimental results demonstrate that the ROC curve and the area under the curve are better and more intuitive indicators of the performance of the GI, compared with conventional evaluation methods. Additionally, for examining gray-scale objects, the calculation of the volume under the ROC surface is analyzed and serves as a performance metric. Our scheme should attract general interest and open exciting prospects for ROC analysis in thermal GI.

Compressive moving target tracking with thermal light based on complementary sampling

We present a method to directly reconstruct background subtracted images using compressive sensing (CS) theory and complementary modulation technique. Since the moving objects of interest only occupy a small portion of the field of view, i.e., they are sparse in the spatial domain, and the complementary modulation strategy makes the values of patterns in the suitable range for CS reconstructions, thus, with this method, we can retrieve object silhouettes and trajectory with high image quality and strong robustness against the changing illumination and the noise. To demonstrate the performance of the proposed protocol, we make a comparison of our strategy with traditional methods. This protocol may attract general interest and be instructive for the fields of surveillance systems, teleconferencing, and even search and rescue missions.