Zhiyong Lu

Laboratory demonstration of static-mode down-looking synthetic aperture imaging ladar

A static-mode synthetic aperture imaging ladar (SAIL) in which the target and carrying platform are kept still during the collection process is proposed and demonstrated. A target point of 0.5 mm× 0.5 mm and a two-dimensional (2D) object are reconstructed in the experiments, in which an optical collimator with a focal length of 10 m is used to simulate the far-field condition. The achieved imaging resolution is in agreement with the theoretical design. The static-mode down-looking SAIL has the capability to eliminate the influence from the atmospheric turbulence and can be conveniently operated outdoors. OCIS codes: 280.6730, 280.3640, 100.2000, 100.3010, 110.0110. doi: 10.3788/COL201513.042801.

Optical image processing for synthetic-aperture imaging ladar based on two-dimensional Fourier transform

A two-dimensional (2D) Fourier transform algorithm for the image reconstruction of synthetic-aperture imaging ladar (SAIL) collected data is suggested. This algorithm consists of quadratic phase compensation in azimuth direction and 2D fast Fourier transform. Based on this algorithm and the parallel 2D Fourier transform capability of spherical lens, an optical principle scheme that processes the SAIL data is proposed. The basic principle, design equations, and necessary analysis are presented. To verify this principle scheme, an experimental optical SAIL processor setup is constructed. The imaging results of SAIL data obtained by our SAIL demonstrator are presented. The optical processor is compact, lightweight, and consumes low power. This optical processor can also provide inherent parallel and speed-of-light computing capability, and thus has potential applications in on-board and satellite-borne SAIL systems.

Speckle reduction of synthetic aperture imaging ladar based on wavelength characteristics

We propose a speckle-reduction method based on wavelength characteristics of speckle effect in synthetic aperture imaging ladar (SAIL). The return signal, which is the back scattering field with speckle effect from the rough surface of target, can be integrated over N chirp periods and heterodyne detected with a local-oscillator signal. After performing image processing respectively, the final image can be regarded as the incoherent superposition of the N sub-images. Numerical simulations indicate the effectiveness of this method. Our research may facilitate practical applications of SAIL.

Imaging process and signal-to-noise ratio improvement of enhanced self-heterodyne synthetic aperture imaging ladar

This Letter gives the general construction of an enhanced self-heterodyne synthetic aperture imaging ladar (SAIL) system, and proposes the principle of image processing. A point target is reconstructed in the enhanced self-heterodyne SAIL as well as in down-looking SAIL experiments, and the achieved imaging resolution of the enhanced self-heterodyne SAIL is analyzed. The signal-to-noise ratio (SNR) of the point target final image in the enhanced self-heterodyne SAIL is higher than that in the down-looking SAIL. The enhanced self-heterodyne SAIL can improve the SNR of the target image in far-distance imaging, with practicality.

Resampling technique in the orthogonal direction for down-looking Synthetic Aperture Imaging Ladar

The implementation of down-looking Synthetic Aperture Imaging Ladar(SAIL) uses quadratic phase history reconstruction in the travel direction and linear phase modulation reconstruction in the orthogonal direction. And the linear phase modulation in the orthogonal direction is generated by the shift of two cylindrical lenses in the two polarization-orthogonal beams. Therefore, the fast-moving of two cylindrical lenses is necessary for airborne down-looking SAIL to match the aircraft flight speed and to realize the compression of the orthogonal direction, but the quick start and the quick stop of the cylindrical lenses must greatly damage the motor and make the motion trail non-uniform. To reduce the damage and get relatively well trajectory, we make the motor move like a sinusoidal curve to make it more realistic movement, and through a resampling interpolation imaging algorithm, we can transform the nonlinear phase to linear phase, and get good reconstruction results of point target and area target in laboratory. The influences on imaging quality in different sampling positions when the motor make a sinusoidal motion and the necessity of the algorithm are analyzed. At last, we perform a comparison of the results of two cases in resolution.

Ghost image generated by relative movement of target and lidar platform in enhanced self-heterodyne SAIL

Enhanced self-heterodyne synthetic aperture imaging ladar (SAIL) is based on the down-looking SAIL, with a transmitter of two coaxial polarization-orthogonal beams of spatial parabolic phase difference and a receiver of heterodyne detection. Enhanced self-heterodyne SAIL has the capability to eliminate the influence from the atmospheric turbulence, has a high advantage in the detection of weak signals. However, the relative movement of target and lidar platform will generate ghost image in enhanced self-heterodyne SAIL. The image process of ghost image is mathematically detailed and simulated, the results image as the theoretical analysis. The enhanced self-heterodyne SAIL is necessarily work in spotlight mode in order to eliminate ghost image for far-distance lidar imaging.

3D coherent imaging ladar based on FMCW technology

3D coherent imaging ladar based on frequency modulated continuous wave (FMCW) technology is proposed. The triangle wave is used to modulate chirped laser. The distance information is detected by optical heterodyne technique. Meanwhile, the beam scanning is realized by rotation double-prism. And the 3D target image is obtained. The 3D coherent imaging ladar has advantages of anti-background interference, high sensitivity and long-detected distance. Under the condition of the strongly sunlight background, the targets outdoor are scanned and high resolution images are achieved. This technique has a great potential for applications in extensive imaging ladar fields.

Performance of DPSK free-space optical communication with spatial diversity

The performance of satellite-to-ground downlink optical communications over Gamma-Gamma distributed atmospheric turbulence are discussed for a direct detection differential phase shift keying system with spatial diversity, which combines the beam coherently before demodulation. Bit-error rate (BER) performances for various numbers of apertures are analyzed and compared for different zenith angle. We also consider the effect of the fiber coupling efficiency and loss ratio of beam coupler to the final BER. The results of numerical simulation shows that the advantage increases with the number of aperture. All the numerical results are verified by Monte-Carlo simulations.

Influence of polarization features of target reflection on synthetic aperture imaging ladar

Synthetic aperture imaging ladar (SAIL) is one of the most possible optical active imaging methods to break the diffraction limit and achieve super-resolution in a long distance. Nevertheless, two-dimensional reconstructed images of the natural targets have not been achieved. Polarization state change of the backscattered light, which is always determined by the interaction of the light and the materials on the target plane, will affect the imaging of SAIL. The Mueller matrices can describe the complex polarization features of the target reflection and treat this interaction. In this paper, a measurement of the Mueller matrices for different target materials will be designed, and the influences of polarization characteristic of targets on resolution element imaging in side-looking and down-looking SAILs will be theoretically analyzed.

Imaging processing with bidirectional modulation in down-looking synthetic aperture imaging ladar

Down-looking synthetic aperture imaging ladar have bidirectional (positive and negative direction) modulation in the orthogonal direction of travel during phase modulation. The return signal can be also collected in the two directions. The imaging processing with bidirectional modulation is used and demonstrated. The signal-to-noise ratio can be enhanced in this mode synthetic aperture imaging ladar. Meanwhile, the velocity of carrying-platform can be faster. In the experiment, the return signals with bidirectional modulation are stacked and rebuilt. Compared to the unidirectional modulation imaging, the faster and clearer imaging is realized with bidirectional modulation. This technique has a great potential for applications in extensive synthetic aperture imaging ladar fields.