Yuan Zhao

Growth and properties of Zn doped lithium niobate crystal

Abstract By adopting proper temperature field, growth parameters and pulling–annealing technique a series of high quality ZnO doped lithium niobate (LN) crystals were grown by Czochralski method from the congruent melt. The defect structure of Znxa0:xa0LN crystals was studied by X-ray analyses, IR and Raman spectrum. It is suggested that Zn ions first occupied Nb ions on Li sites, then Zn ions occupied the ordinary Li sites. Its photo-refraction threshold indicated crystal containing 6xa0mol% Znxa0:xa0LN has the highest resistance to optical damage, similar to Mg doped LN (5xa0mol% MgO), having increased two orders of magnitude.

Salient target detection based on pseudo-Wigner-Ville distribution and Rényi entropy

We present what we believe to be a novel method based on pseudo-Wigner-Ville distribution (PWVD) and Rényi entropy for salient targets detection. In the foundation of studying the statistical property of Rényi entropy via PWVD, the residual entropy-based saliency map of an input image can be obtained. From the saliency map, target detection is completed by the simple and convenient threshold segmentation. Experimental results demonstrate the proposed method can detect targets effectively in complex ground scenes.

Range accuracy limitation of pulse ranging systems based on Geiger mode single-photon detectors

Geiger mode single-photon detectors have been used in pulse ranging system and three-dimensional imaging systems due to high sensitivity and easy integration. The ranging accuracy and precision is influenced by many factors. Based on statistical theory, five main factors are discussed in this article, namely, echo signal intensity, pulse width, detector quantum efficiency, target position, and background noise. An analytical relationship among the ranging accuracy, precision, and these factors is obtained for a Q-switched laser pulse. Through this relationship, it is shown that the echo signal intensity and pulse width are more important than other factors and higher echo signal intensity and narrower pulse width can result in better accuracy and precision.

Gain-modulated three-dimensional active imaging with depth-independent depth accuracy

We present a three-dimensional imaging method using a pulsed laser as a flood illuminating source and an intensified camera as the receiver with exponentially modulated gain. The depth map of a scene is obtained from two intensity images and the depth accuracy is independent of the depth of the target in the scene. We demonstrate a depth-independent depth accuracy of 0.32 m in an indoor experiment and obtain a depth map of an outdoor scene ranging from 150 to 180 m under a lower signal to noise ratio condition.

Growth and properties of RE:Zn:LiNbO3 crystals

Double-doped Nd:Zn:LN and Er:ZN:LN single crystals were grown by Czochralski method from a congruent melt. The optical absorption and fluorescence spectra of RE:Zn:LN were measured. The pumping wavelengths and the laser oscillation wavelengths of RE:Zn:LN were confirmed at the first time. The laser oscillation wavelength of Nd:Zn:LN is 1090 nm, which is longer by 30 nm than that of Nd:YAG and Nd:Mg:LN

Improvement of range accuracy of range-gating laser radar using the centroid method.

An improved processing approach based on the relation between range accuracy and slicing number is proposed to improve the range accuracy of range-gating laser radar. The sequence of time-slice images is segmented according to their optimal slicing number and processed in segments to achieve the range information of objects. Experimental results indicate that the slicing number has a significant impact on range accuracy, and the highest range accuracy can be achieved when the systems work with an optimal slicing number.

Scannerless three-dimensional imaging using a pulsed laser and an intensified charge-coupled device with linearly modulated gain

We have developed a scannerless three-dimensional imaging approach that uses a pulsed laser to illuminate a target and an intensified charge-coupled device as the receiver with linearly modulated gain in gated time. The target range is extracted by processing an intensity image with linearly modulated gain and an intensity image with constant gain. Feasibility of the approach is demonstrated by an indoor experiment. Our prototype system can produce 256x256 pixel range images at a 20 Hz frame rate in an outdoor test. The test results indicate that range accuracy is related to the slope coefficient and initial value of the modulated gain.

Varied threshold with laser flight time in scannerless range-gated ladar

We present a method based on maximum-likelihood estimation to calculate the threshold of the number of converted photoelectrons according to Poisson and Gaussian statistics. The calculations for detection and false-alarm probabilities are given including a proposal of error probability. An efficient approach of setting threshold according to laser flight time is discussed as the laser power decreases in propagation. Comparison of the fixed threshold and the varied threshold in Poisson and Gaussian statistics is provided. The varied threshold approach tends to keep the error probability at the lowest level. This varied threshold approach is found to have a similar effect on ranging accuracy as a threshold-fixed approach.

Multipulse gate-delayed range gating imaging lidar

We present a technique to reconstruct a higher resolution of depth map of range gating imaging lidar by applying the delays of the gates to a typical range gating lidar system during the detection of each returned laser pulse with the encoding of the returned signal. With the consequent delays of the gate, the depth of the scene is extended accordingly. A multipulse gate-delayed range gating lidar system is designed to prove the resolution improvement from 6 to 1.5 m. The unchanged peak power of the laser, the widths of the laser pulse and the sampling period result in a simple structure of the lidar system.

Hopfield neural network-based image restoration with adaptive mixed-norm regularization

To overcome the shortcomings of traditional image restoration model and total variation image restoration model, we propose a novel Hopfield neural network-based image restoration algorithm with adaptive mixed-norm regularization. The new error function of image restoration combines the L2-norm and L1-norm regularization types. A method of calculating the adaptive scale control parameter is introduced. Experimental results demonstrate that the proposed algorithm is better than other algorithms with single norm regularization in the improvement of signal-to-noise ratio (ISNR) and vision effect.