Depth of field extension and objective space depth measurement based on wavefront imaging

LC-based lightfield camera prototype for rapidly creating target images optimized by finely adjusting several key coefficients and a LC-guided refocusing-rendering

A lightfield camera prototype is constructed by directly coupling a liquid-crystal (LC) microlens array with an arrayed photosensitive sensor for performing a LC-guided refocusing-rendering imaging attached by computing disparity map and extracting featured contours of targets. The proposed camera prototype presents a capability of efficiently selecting the imaging clarity value of the electronic targets interested. Two coefficients of the calibration coefficient k and the rendering coefficient C are defined for quantitively adjusting LC-guided refocusing-rendering operations about the images acquired. A parameter Dp is also introduced for exactly expressing the local disparity of the electronic patterns selected. A parallel computing architecture based on common GPU through the OpenCL platform is adopted for improving the real-time performance of the imaging algorithms proposed, which can effectively be used to extract the pixel-leveled disparity and the featured target contours. In the proposed lightfield imaging strategy, the focusing plane can be easily selected and/or further adjusted by loading and/or varying the signal voltage applied over the LC microlenses for realizing a rapid or even intelligent autofocusing. The research lays a solid foundation for continuously developing or upgrading current lightfield imaging approaches.

Electrically addressed focal stack plenoptic camera based on a liquid-crystal microlens array for all-in-focus imaging

Focal stack cameras are capable of capturing a stack of images focused at different spatial distance, which can be further integrated to present a depth of field (DoF) effect beyond the range restriction of conventional camera's optics. To date, all of the proposed focal stack cameras are essentially 2D imaging architecture to shape 2D focal stacks with several selected focal lengths corresponding to limited objective distance range. In this paper, a new type of electrically addressed focal stack plenoptic camera (EAFSPC) based on a functional liquid-crystal microlens array for all-in-focus imaging is proposed. As a 3D focal stack camera, a sequence of raw light-field images can be rapidly manipulated through rapidly shaping a 3D focal stack. The electrically addressed focal stack strategy relies on the electric tuning of the focal length of the liquid-crystal microlens array by efficiently selecting or adjusting or jumping the signal voltage applied over the microlenses. An algorithm based on the Laplacian operator is utilized to composite the electrically addressed focal stack leading to raw light-field images with an extended DoF and then the all-in-focus refocused images. The proposed strategy does not require any macroscopic movement of the optical apparatus, so as to thoroughly avoid the registration of different image sequence. Experiments demonstrate that the DoF of the refocused images can be significantly extended into the entire tomography depth of the EAFSPC, which means a significant step for an all-in-focus imaging based on the electrically controlled 3D focal stack. Moreover, the proposed approach also establishes a high correlation between the voltage signal and the depth of in-focus plane, so as to construct a technical basis for a new type of 3D light-field imaging with an obvious intelligent feature.

Depth-of-Field-Extended Plenoptic Camera Based on Tunable Multi-Focus Liquid-Crystal Microlens Array

Plenoptic cameras have received a wide range of research interest because it can record the 4D plenoptic function or radiance including the radiation power and ray direction. One of its important applications is digital refocusing, which can obtain 2D images focused at different depths. To achieve digital refocusing in a wide range, a large depth of field (DOF) is needed, but there are fundamental optical limitations to this. In this paper, we proposed a plenoptic camera with an extended DOF by integrating a main lens, a tunable multi-focus liquid-crystal microlens array (TMF-LCMLA), and a complementary metal oxide semiconductor (CMOS) sensor together. The TMF-LCMLA was fabricated by traditional photolithography and standard microelectronic techniques, and its optical characteristics including interference patterns, focal lengths, and point spread functions (PSFs) were experimentally analyzed. Experiments demonstrated that the proposed plenoptic camera has a wider range of digital refocusing compared to the plenoptic camera based on a conventional liquid-crystal microlens array (LCMLA) with only one corresponding focal length at a certain voltage, which is equivalent to the extension of DOF. In addition, it also has a 2D/3D switchable function, which is not available with conventional plenoptic cameras.

Optical properties of electrically controlled arc-electrode liquid-crystal microlens array for wavefront measurement and adjustment

An electrically controlled arc-electrode liquid-crystal microlens array (AE-LCMLA), with tuning and swing focus, is proposed, which can be utilized to replace the traditional mechanically controlled microlenses and also cooperate with photosensitive arrays to solve the problems of measuring and further adjusting a strong distortion wavefront. The top patterned electrode of a single LC microlens is composed of three arc-electrodes distributed symmetrically around a central microhole for constructing the key controlling structures of the LC cavity in the AE-LCMLA. All the arc-electrodes are individually controlled, and then the focal spot of each microlens can be moved freely in a three-dimensional fashion including along the optical axial direction and over the focal plane by simply adjusting the driving signal voltage applied over each arc-electrode, independently. The featured performances of the AE-LCMLA in a wavelength range of ∼501-561  nm are the driving signal voltage being relatively low (less than ∼11  V_rms), the focal length tuning range being from ∼2.54  mm to ∼3.50  mm, the maximum focus swing distance being ∼52.92  μm, and the focus swing ratio K being ∼20‰.

Electrically controlled liquid-crystal microlens matrix with a nested electrode array for efficiently tuning and swinging focus

A new type of electrically controlled liquid-crystal microlens matrix (EC-LCMM) with a nested electrode array for efficiently tuning and swinging focus, which means that the focus position can be adjusted in three dimensions, is proposed. The EC-LCMM is constructed by a 10 × 10 arrayed annular-sector-shaped aluminum electrode with a central microhole of 140μm diameter and three annular-sectors of 210μm external diameter and the period length of 280μm. To the arrangement of the patterned electrode, both the 10 × 10 LC microlens array based on the annular-sector-shaped aluminum electrode and the 9 × 9 LC microlens array based on an arrayed quasi-quadrilateral-ring-shaped electrode can be obtained. The 9 × 9 LC microlens array is formed by matching adjacent four annular-sector-shaped sub-electrodes in the 10 × 10 LC microlenses. The developed EC-LCMM can be used to electrically tune focus along the optical axis and also swing focus over a focal plane selected. The typical performances include: electrically tunable focusing in a driving voltage range of 3~7V_rms, the focal length in a range of 2~0.6mm, and the maximum focus swing distance being 16μm. For effectively describing the focus swing efficiency, the parameters of SF and SA are defined, which are the ratios between the focus swinging distance and the current focal length along the optical axis, and between the focus swinging extent and the external diameter of a single annular-sector-shaped aluminum electrode, respectively. The SF and SA of the EC-LCMM are ~16‰ and ~7.6%, respectively. It can be expected that the complex wavefront can be more efficiently measured and adjusted according to the EC-LCMM-based Shack-Hartmann wavefront measuring and adjusting architecture.

Scalable and ultrafast epitaxial growth of single-crystal graphene wafers for electrically tunable liquid-crystal microlens arrays

The scalable growth of wafer-sized single-crystal graphene in an energy-efficient manner and compatible with wafer process is critical for the killer applications of graphene in high-performance electronics and optoelectronics. Here, ultrafast epitaxial growth of single-crystal graphene wafers is realized on single-crystal Cu₉₀Ni₁₀(1 1 1) thin films fabricated by a tailored two-step magnetron sputtering and recrystallization process. The minor nickel (Ni) content greatly enhances the catalytic activity of Cu, rendering the growth of a 4 in. single-crystal monolayer graphene wafer in 10 min on Cu₉₀Ni₁₀(1 1 1), 50 folds faster than graphene growth on Cu(1 1 1). Through the carbon isotope labeling experiments, graphene growth on Cu₉₀Ni₁₀(1 1 1) is proved to be exclusively surface-reaction dominated, which is ascribed to the Cu surface enrichment in the CuNi alloy, as indicated by element in-depth profile. One of the best benefits of our protocol is the compatibility with wafer process and excellent scalability. A pilot-scale chemical vapor deposition (CVD) system is designed and built for the mass production of single-crystal graphene wafers, with productivity of 25 pieces in one process cycle. Furthermore, we demonstrate the application of single-crystal graphene in electrically controlled liquid-crystal microlens arrays (LCMLA), which exhibit highly tunable focal lengths near 2 mm under small driving voltages. By integration of the graphene based LCMLA and a CMOS sensor, a prototype camera is proposed that is available for simultaneous light-field and light intensity imaging. The single-crystal graphene wafers could hold great promising for high-performance electronics and optoelectronics that are compatible with wafer process.