Yufeng Tan

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.

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.

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.

Optical sparse aperture imaging with faint objects using improved spatial modulation diversity

The next generation of optical sparse aperture systems will provide high angular resolution for astronomical observations. Spatial modulation diversity (SMD) is a newly developed post-processing technique for such telescopes, faced with challenges of imaging faint objects, which are very attractive for astronomers but always make raw diversity images suffer serious photon noise. In this paper, we propose an improved SMD with denoising reprocessing embedded to address the problem. The blocking-matching and 3D filtering algorithm, a state-of-the-art denoising technique, is first employed to process the diversity images with low photon intensities generated by spatial modulation, specifically switching off each sub-aperture sequentially. SMD algorithm then can be applied to estimate wavefront and digitally restore images. It is demonstrated by both simulations and experiments that the proposed method outperforms the previous SMD in terms of reconstructions of wavefront and imagery from the raw images of faint objects corrupted seriously by photon noise. The reported method may provide an alternative approach to acquire high-quality images of faint objects for astronomical observations of the future segmented mirrors or telescope arrays.

Active sparse aperture imaging using independent transmitter modulation with improved incoherent Fourier ptychographic algorithm

Optical sparse aperture imaging shows great promise for the next generation of high resolution systems. In this paper, we propose and demonstrate an active sparse aperture imaging approach using independent transmitter modulation to digitally overcome phasing errors, correct aberrations, and further improve resolution. The reported imaging scheme consists of a general sparse aperture system and an active illumination unit, specifically an independent pattern projector. A series of raw images are captured with the projector scanned to illuminate the object. Based on the acquired data set, the improved incoherent Fourier ptychographic algorithm is utilized to reconstruct sparse aperture images with distortions removed and contrast enhanced. Furthermore, thanks to illumination pattern modulation, higher resolution beyond the diffraction limit of the synthetic aperture system is gained as a benefit. Good-quality and higher-resolution sparse aperture imagery obtained by employing our proposed technique in both simulation and experiment demonstrates the effectiveness. The reported approach may provide new insights to address the phasing and image restoration problems of sparse aperture systems in the transmitting path rather than only in the receiving path.

Performance analysis of multi-Gaussian beams steered by rotational Risley-prism-array

Rotational Risley-prism-array system is an effective way to realize high power and high beam quality of deflecting laser output. In order to reveal the quality performance of deflecting beam, the beam compression in the direction of deflection and far field energy centrality of a hexagonal-distributed 7-Gaussian beam array based on rotational Risley-prism-array were studied in detail in this paper. The analytic formulae of the pointing position for the outgoing beam based on the prisms’ rotational angles are calculated by using nonparaxial ray tracing method. Then, the analytical expression for intensity propagation was derived based on the extended Huygens-Fresnel principle. From the irradiance distribution and PIB curve in the focal plane, the quantitatively simulation shows that the beam compression will be more significant as the deflecting angle of emergent increases. The energy centrality will decrease as the propagation distance increases, the fill factor decreases and the deviation angle increases. The mathematical model and calculation results can offer a reference for optical engineering application.

Thermal blooming on laser propagation in an aspirating pipe

Thermal blooming effect of gas on laser propagation can seriously degrade performance of far-field beam quality and energy distribution. Numerical simulation is carried out to study the influences of thermal blooming on laser propagation in line pipes. A physical model of thermal blooming effect of gas on laser propagation in an aspirating pipe is established. Axial flow and suction in the outlet are used to attenuate the thermal blooming effect. Based on the computational fluid dynamics (CFD) software, stable calculation of flow field is carried out first, then the optical field and the fluent field is coupling calculated by means of user defined function (UDF). The results show that radial flow is enhanced in the aspirating pipe and the index of refraction gradient caused by thermal blooming effect is decreased. It is indicated that the beam quality of the outlet is improved compared with the pipe model without aspirating. The optical path difference (OPD) distribution of the outlet is analyzed and decomposed by Zernike polynomials. It is shown that the defocus item of 4m aspirating pipe is decreased more than an order of magnitude compared with the 4m pipe without aspirating.

Structural design of 3m class ground based telescope elevation ring

In order to improve the dynamic stiffness of telescope mount, the accuracy of aiming and stability of optical system, a topology optimization method based on the theory of variable density and taking maximum stiffness as objective function is studied. In the topology optimization analysis of elevation ring, one of the most important members of the telescope mount, two kinds of structural are designed: one is a traditional plate welding structure and the other is a combination of plate welding and truss welding. Furthermore, the stiffness and modal performance of the elevation ring in different performances are analyzed and compared. The results show that in meeting the strength and stiffness of the premise, the mass of elevation ring with plate welding and truss welding is 7.00T and the moment of inertia is 11.94 t•m2. What`s more, the total deformation in the horizontal direction and the zenith direction are 6.70μm and 55.86μm, respectively; the stress is within stress range of materials promise; the modal is 105.9Hz.Compared with the traditional structural with plate welding, this new structural design approached to ensure the dynamic stiffness while effectively reducing its own weight with reduction rate 10.7% and moments of inertia with reduction rate 12.3%. This new structural of plate welding and truss welding has obvious advantages in lightweight design. This new design method based on topology optimization will provide efficient help to later components design of the telescope mount.