Ge Ren

Synthetic aperture imaging by using spatial modulation diversity technology with stochastic parallel gradient descent algorithm.

In this paper, we propose and demonstrate the synthetic aperture imaging by using spatial modulation diversity technology with stochastic parallel gradient descent (SPGD) algorithm. Instead of creating diversity images by means of focus adjustments, the technology, proposed in this paper, creates diversity images by modulating the transmittance of individual sub-aperture of multi-aperture system, respectively. Specifically, spatial modulation is realized by switching off the transmittance of each sub-aperture with electrical shutters, alternately. Based on these multi diversity images, SPGD algorithm is used for adaptively optimizing the coefficients of Zernike polynomials to reconstruct the real phase distortions of multi-aperture system and to restore the near-diffraction-limited image of object. Numerical simulation and experimental results show that this technology can be used for joint estimation of both pupil aberrations and an high resolution image of the object, successfully. The technology proposed in this paper can have wide applications in segmented and multi-aperture imaging systems.

MEMS Inertial Sensors-Based Multi-Loop Control Enhanced by Disturbance Observation and Compensation for Fast Steering Mirror System

In this paper, an approach to improve the disturbance suppression performance of a fast steering mirror (FSM) tracking control system based on a charge-coupled device (CCD) and micro-electro-mechanical system (MEMS) inertial sensors is proposed. The disturbance observation and compensation (DOC) control method is recommended to enhance the classical multi-loop feedback control (MFC) for line-of-sight (LOS) stabilization in the FSM system. MEMS accelerometers and gyroscopes have been used in the FSM system tentatively to implement MFC instead of fiber-optic gyroscopes (FOG) because of its smaller, lighter, cheaper features and gradually improved performance. However, the stabilization performance of FSM is still suffering a large number of mechanical resonances and time delay induced by a low CCD sampling rate, which causes insufficient error attenuation when suffering uncertain disturbances. Thus, in order to make further improvements on the stabilization performance, a cascaded MFC enhanced by DOC method is proposed. The sensitivity of this method shows the significant improvement of the conventional MFC system. Simultaneously, the analysis of stabilization accuracy is also presented. A series of comparative experimental results demonstrate the disturbance suppression performance of the FSM control system based on the MEMS inertial sensors can be effectively improved by the proposed approach.

Restoration of Degraded Images Using Pupil-Size Diversity Technology With Stochastic Parallel Gradient Descent Algorithm

The performance of imaging systems is inevitably degraded by aberrations of optical systems. Furthermore, images detected by long-distance imaging schemes also suffer blurring induced by atmospheric turbulence. To address this problem, we propose and demonstrate an aberration-free imaging procedure in this paper, which is termed pupil-size diversity technology. With no additional optical element, the reported technique first acquires several intensity images only by simply resizing the pupil of an imaging system. The spatial difference of pupil areas generates pupil diversity. Then, based on the nonlinear optimization method, a high-quality image eliminating distortions can be reconstructed by processing the multiple diversity images with the stochastic parallel gradient descent algorithm. Comparative results of simulations and experiments, for correcting inner and external aberrations, respectively, verify the validity. The proposed technology in this paper may provide an alternative for adaptive optics systems and find wide applications in computational photography and remote sensing.

Enhanced Disturbance Observer Based on Acceleration Measurement for Fast Steering Mirror Systems

In this paper, a modified disturbance observer (DOB) for fast steering mirror (FSM) optical system based on a charge-coupled device (CCD) and inertial sensors is proposed. Combining a DOB with the classical cascaded multiloop feedback control, including position loop, velocity loop, and acceleration loop, that the disturbance suppression performance of line-of-sight in an FSM system can be significant improved. However, due to the quadratic differential in the FSM acceleration open-loop response, in fact, it is very difficult to realize an integral algorithm to compensate a quadratic differential in practical application. Thus, the conventional DOB controller has to be simplified further to make a concession, which eventuates in still insufficient disturbance compensation, particularly at low frequency. To solve this problem, an enhanced DOB control structure, which changes the compensation plant to be the acceleration open-loop and avoids the saturation of double integration skillfully, is proposed. The recommended method optimizes the controller design, which is conducive to controller fulfillment in practical systems. A series of comparative experimental results demonstrate that the disturbance suppression performance of the FSM control system can be effectively improved by the proposed approach.

Restoration of Sparse Aperture Images Using Spatial Modulation Diversity Technology Based on a Binocular Telescope Testbed

In this paper, our recently developed spatial modulation diversity technology (SMDT) is first used to correct the blurring and restore the lost midband contrast of synthetic images based upon an actual sparsely filled aperture system, specifically a binocular telescope testbed consisting of two horizontally arranged collector telescopes with 127 mm diameter. Synthetic images are obtained with the testbed cophased by an optimization algorithm. Diversity images are generated by using electronic shutter to modulate the transmittance of each subaperture, alternately. We capture two independent datasets corresponding to different objects regarded as the reference of each other. Then an improved algorithm of SMDT for real sparse aperture systems is proposed to restore the synthetic imagery by processing the datasets. The experimental results that the reconstructed images present high quality and contrast and that the repeatable wavefront measurements show a good agreement demonstrate the availability of SMDT for image restoration in actual multiaperture systems.

Wavefront Sensing and Image Restoration With Spatially Overlapping Diversity Technology

We propose and demonstrate a method of spatially overlapping diversity technology for wavefront sensing and image restoration with undersampled systems, which can handle extended objects illuminated by spatially incoherent light. The technique first scans a smaller subaperture across the large pupil of undersampled systems to generate several properly sampled images. Then, a nonlinear optimization method is used for joint reconstruction of wavefront and object. Simulations performed with different phase distributions initially validate the technique. Furthermore, an experimental configuration is established to measure the loaded defocus and wavefront transmitted through a plano-convex lens. A detailed analysis of experimental results is presented to demonstrate the effectiveness of this technique.

Coregistration based on stochastic parallel gradient descent algorithm for SAR interferometry

The coregistration of complex image pairs is a very important step in interferometric synthetic aperture radar (InSAR) data processing. This article proposes a coregistration method based on the stochastic parallel gradient descent (SPGD) algorithm. Stochastic parallel perturbations are imposed on the translation coefficients of the polynomial coregistration model to make the performance evaluation function converge to a global extremum, which allows the translation coefficients to be obtained, and then the coregistration is achieved after resampling. Data processing of images from Kashgar and Mount Etna show that the proposed method is effective and robust. Furthermore, a series of experiments is designed to evaluate the convergence characteristics of the proposed method, which indicates that it has a stable convergence process and good robustness.

Propagation of rotational Risley-prism-array-based Gaussian beams in turbulent atmosphere

Limited by the size and weight of prism and optical assembling, Rotational Risley-prism-array system is a simple but effective way to realize high power and superior beam quality of deflecting laser output. In this paper, the propagation of the rotational Risley-prism-array-based Gaussian beam array in atmospheric turbulence is studied in detail. An analytical expression for the average intensity distribution at the receiving plane is derived based on nonparaxial ray tracing method and extended Huygens-Fresnel principle. Power in the diffraction-limited bucket is chosen to evaluate beam quality. The effect of deviation angle, propagation distance and intensity of turbulence on beam quality is studied in detail by quantitative simulation. It reveals that with the propagation distance increasing, the intensity distribution gradually evolves from multiple-petal-like shape into the pattern that contains one main-lobe in the center with multiple side-lobes in weak turbulence. The beam quality of rotational Risley-prism-array-based Gaussian beam array with lower deviation angle is better than its counterpart with higher deviation angle when propagating in weak and medium turbulent (i.e. Cn2 < 10-13m-2/3), the beam quality of higher deviation angle arrays degrades faster as the intensity of turbulence gets stronger. In the case of propagating in strong turbulence, the long propagation distance (i.e. z > 10km ) and deviation angle have no influence on beam quality.

Structure and mechanical design for a large-aperture telescope

For a better understanding and forecasting of the universe, the high resolution observations are needed. The largeaperture telescope is an integrated success with a combination of material, mechanics, optics and electronics. The telescope is a classic Cassegrain configuration with open structure, alt-azimuth mount, and retractable dome. The instrumentation has a rotating mass of approximately 52 tons and stands over 9 m tall. The 3-m aperture primary mirror is a honeycomb lightweighted mirror with fused silica material and active cooling. The paper will address preliminary design and development of the telescope mount structure, axes drive system, encoder mount and primary mirror system. The structure must have the best performance of stiffness and stability to demand an acceptable image quality. As the largest optical element of the telescope, primary mirror must be well controlled and protected both during operational and non-operational periods. An active cooling system of primary mirror is provided by a flushing subsystem at the front side and sucking subsystem on the central hole to keep the temperature of the facesheet close to that of ambient air. A two-layer mirror cover mounted on the elevation ring is proposed to protect the optical elements and inner beam tube from dust, dirt and debris. Furthermore, the latest plans for future upgrades will be also described.

Error-Based Observer of a Charge Couple Device Tracking Loop for Fast Steering Mirror

The charge couple device (CCD) tracking loop of a fast steering mirror (FSM) is usually used to stabilize line of sight (LOS). High closed-loop bandwidth facilitates good performance. However, low-rate sample and time delay of the CCD greatly limit the high control bandwidth. This paper proposes an error-based observer (EBO) to improve the low-frequency performance of the CCD tracking system. The basic idea is by combining LOS error from the CCD and the controller output to produce the high-gain observer, forwarding into the originally closed-loop control system. This proposed EBO can improve the system both in target tracking and disturbance suppression due to LOS error from the CCD’s sensing of the two signals. From a practical engineering view, the closed-loop stability and robustness of the EBO system are investigated on the condition of gain margin and phase margin of the open-loop transfer function. Two simulations of CCD experiments are provided to verify the benefits of the proposed algorithm.