Achromatic doublet electrowetting prism array for beam steering device in foveated display.

A foveated display is a technology that can solve the problem of insufficient angular resolution (relative to the human eye) for near-eye display. In a high-resolution foveated display, a beam steering element is required to track the human gaze. An electrowetting prism array is a transmissive non-mechanical beam steering device, that allows a light and compact optical system to be configured and a large aperture possible. However, the view is obstructed by the sidewall of the prism array. When the size of the cell prism is 7mm, the prism array has an 87% fill-factor. To push the fill-factor to 100%, the cell prisms were magnified using a lens array. Image processing was performed such that the image produced by the lens array was identical to the original. Beam steering by refraction is accompanied by chromatic dispersion, which causes chromatic aberration, making colors appear blurry. The refractive index condition to reduce chromatic dispersion was obtained using the doublet structure of the electrowetting prism. The chromatic dispersion was reduced by 70% on average.

Simple method of acquiring high-quality light fields based on the chromatic aberration of only one defocused image pair.

Direct light field acquisition method using a lens array requires a complex system and has a low resolution. On the other hand, the light fields can be also acquired indirectly by back-projection of the focal stack images without lens array, providing a resolution as high as the sensor resolution. However, it also requires the bulky optical system design to fix field-of-view (FOV) between the focal stacks, and an additional device for sensor shifting. Also, the reconstructed light field is texture-dependent and low-quality because it uses either a high-pass filter or a guided filter for back-projection. This paper presents a simple light field acquisition method based on chromatic aberration of only one defocused image pair. An image with chromatic aberration has a different defocus distribution for each R, G, and B channel. Thus, the focal stack can be synthesized with structural similarity (SSIM) 0.96 from only one defocused image pair. Then this image pair is also used to estimate the depth map by depth-from-defocus (DFD) using chromatic aberration (chromatic DFD). The depth map obtained by chromatic DFD is used for high-quality light field reconstruction. Compared to existing light field indirect acquisition, the proposed method requires only one pair of defocused images and can clearly reconstruct light field images with Blind/Referenceless Image Spatial Quality Evaluator (BRISQUE) scores lowered by 17%-38% and with Perception-Based Image Quality Evaluator (PIQE) scores lowered by 19%-45%. A defocused image pair is acquired by our customized compact optical system consisting of only three lenses, including a varifocal lens. Image processing and image quality evaluation are all performed using MATLAB.

Compact and fast depth sensor based on a liquid lens using chromatic aberration to improve accuracy.

Depth from defocus (DFD) obtains depth information using two defocused images, making it possible to obtain a depth map with high resolution equal to that of the RGB image. However, it is difficult to change the focus mechanically in real-time applications, and the depth range is narrow because it is inversely proportional to the depth accuracy. This paper presents a compact DFD system based on a liquid lens that uses chromatic aberration for real-time application and depth accuracy improvement. The electrical focus changing of a liquid lens greatly shortens the image-capturing time, making it suitable for real-time applications as well as helping with compact lens design. Depth accuracy can be improved by dividing the depth range into three channels using chromatic aberration. This work demonstrated the improvement of depth accuracy through theory and simulation and verified it through DFD system design and depth measurement experiments of real 3D objects. Our depth measurement system showed a root mean square error (RMSE) of 0.7 mm to 4.98 mm compared to 2.275 mm to 12.3 mm in the conventional method, for the depth measurement range of 30 cm to 70 cm. Only three lenses are required in the total optical system. The response time of changing focus by the liquid lens is 10 ms, so two defocused images for DFD can be acquired within a single frame period of real-time operations. Lens design and image processing were conducted using Zemax and MATLAB, respectively.

Image stitching using an electrowetting-based liquid prism with a fabrication method.

In this paper, we propose a new method for image stitching using an electrowetting-based liquid prism. Several images were obtained by adjusting the voltages applied to four sidewalls of the liquid prism, and a panoramic image was achieved through an image stitching algorithm. The relationship between the tilting angle of the liquid prism and the normal vector of the liquid-liquid interface was presented. Novel fabrication method has been proposed to improve the performance of the liquid prism, including the addition of a new structure to prevent oil isolation, plastic chamber material, plastic laser cutting, and oil selection. The fabricated liquid prism has a size of 5 × 5 × 8 mm, a maximum beam steering angle of ±10.5 °, a response time of 19.1 ms, and a resolution of 14.25 lp/mm. The required number of images according to the overlapping area was presented through the simulation, and the image stitching using two or three images was demonstrated.

Histogram compensation algorithm for an efficient phase-only hologram calculation.

A fast computer-generated phase-only hologram (POH) calculation is needed to realize a holographic display. Existing iterative methods require many iterations and thus require speed improvement, and noniterative methods add constraints to the image, resulting in restrictions on quality or range of expression. In this paper, we propose an efficient iteration algorithm without using a feedback process. For the several initial iterations among the entire iteration process, the image constraint was replaced by a histogram adjusted image. This pre-iteration can compensate for the loss of bright and dark pixels in the image reconstructed by the POH, resulting in a more accurate POH. The quality of image reconstructed by the proposed method was compared with that of existing algorithms by computer simulation and optical regeneration. Compared to the previous method, a 1.17dB increase in peak signal-to-noise ratio was obtained, and the number of iterations to achieve the same image quality has been reduced by 23%. The proposed method hardly increased the amount of computation and could be used simultaneously with the existing feedback methods.

Dynamics of optical injection in an external cavity based FP-LD for wide tunable microwave signal generation.

In this paper, we analyze the dynamics of optical injection in an external cavity based Fabry-Pérot laser diode (ECFP-LD) for wide tunable microwave signal generation. The ECFP-LD is a specially designed FP-LD that has a self-locked single dominant mode. The injected beam power is varied to analyze the dynamics of optical beam injection on the ECFP-LD. The ECFP-LD shows the interesting behavior of red-shift followed by hopping to another self-injected mode equivalent to FP-LD external and internal cavity modes separation, which provides the fine and coarse tuning of the self-injected mode. The optical beating of the injected beam and the self-injected mode, whether it is the fine red-shifted self-injected mode or the hopped self-injected mode equivalent to the external or internal cavity mode separation, provides a wide tunable range of microwave generation. We obtained fine tuning of 3 GHz for every self-injected mode, and coarse tuning of 15 GHz and 150 GHz, which is equivalent to the free spacing tuning of the external cavity (0.12 nm) and internal FP-LD cavity (1.17 nm), with change in the power of the injected beam. The maximum coarse tuning range is about 3.72 nm, and the corresponding beating frequency tuning range is 330 GHz. Hence, a wide tunable microwave frequency can be obtained by optical beating of the shifted/hopped self-injected mode and the injected beam.

Nonmechanical three-dimensional beam steering using electrowetting-based liquid lens and liquid prism.

In this paper, we present a system for nonmechanical three-dimensional beam steering using an electrowetting based liquid lens and liquid prism. The optical design of the presented system was modeled with Zemax and three-dimensional beam steering was simulated by changing the ROC of the lens and the apex angle of the prism. The liquid lens from Corning-Varioptic was used and the liquid prism was fabricated and these were combined. The liquid lens and liquid prism were filled with two immiscible liquids whose densities are the same. The liquid lens provides variable focal lengths as the applied voltage is changed. The diopter range of the liquid lens is from -3.9 D to 14.5 D. Beam steering on the x-axis, y-axis, and xy-axis was achieved by applying different voltages to four sidewalls of the liquid prism. The liquid prism has a beam steering angle of up to 11.6 °, 12 °, and 11.8 ° on x-axis, y-axis, and xy-axis, respectively. By combining the electrowetting actuated liquid lens and liquid prism, three-dimensional beam steering control including the z-axis direction was demonstrated.

Low-noise high-efficiency double-phase hologram by multiplying a weight factor.

This Letter describes a method of generating a phase-only hologram, which improves the lack of diffraction efficiency and peripheral noise phenomenon, which is a problem of the existing double-phase hologram (DPH) method. The proposed weight factor DPH method can be obtained by a simple matrix operation, so there is no need for additional computing power. The optimal weight factor value depends on the image size, so we have found the optimal value for each image size. The numerical simulation and optical experiments were conducted to compare the images generated using the proposed weigh factor DPH method, the existing DPH method, and the random phase method. The peak signal-to-noise ratio of the images obtained through the proposed weight factor is improved by 14.6% compared to the existing DPH.

Method to reduce the aberration of a polygonal aperture focus-tunable lens array for high fill factor.

Focus-tunable liquid lens arrays have been used to improve depth in glass-free three-dimensional devices where high fill factor is required. For high fill factors, polygonal apertures should be used, but due to the difficulty of controlling aberrations during focus tuning, polygonal aperture focus-tunable lenses have rarely been studied. This Letter proposes a new structure with walls at the lens boundaries using a square parabolic lens model to control the quadrafoil aberration induced by the square aperture. ANSYS simulation confirmed that the lens surface was closest to the parabolic lens model when the wall thickness was 1.4 mm. A fabricated lens array demonstrated the quadrafoil inhibitory effect and confirmed the utility of the new structure.

Enhanced see-through near-eye display using time-division multiplexing of a Maxwellian-view and holographic display.

In this paper, we suggest the improved integration of a holographic display and a Maxwellian-view display using time-division multiplexing and describe an image rendering process for the proposed system. In general, the holographic displays have a resolution limit when used to represent a virtual 3D scene. In the proposed system, the holographic display processed relatively few layers of the virtual 3D scene, while the remaining objects were processed with a Maxwellian-view display to which was applied a Gaussian smoothing filter. Hence, we obtained the retaining holographic image quality, expanding the field of view, and reducing the computation time of the proposed system. The holographic display of the proposed system had an image size of 28 mm × 28 mm with a field of view of 1.02° and a 10.8 mm eye box. The Maxwellian-view display had an image size of 230 mm × 230 mm with a field of view of 22.6 ° and a 0.9 mm eye box diameter. Each display was integrated in time-division multiplexing of 40 Hz, and the proposed system was successfully verified.

Novel biconvex structure electrowetting liquid lenticular lens for 2D/3D convertible display.

Recently, a planoconvex structure electrowetting lenticular lens capable of 2D/3D conversion through a varifocal property by an electrowetting phenomenon has been developed. However, even though it has a similar planoconvex structure to that of a commercial solid lenticular lens, comparable 3D performance could not be realized because the refractive index difference between nonconductive liquid and conductive liquid was not large. Therefore, the goal of the present study is to obtain better 3D performance compared to the conventional planoconvex structure by introducing a novel biconvex structure using ETPTA. The newly developed biconvex structure electrowetting lenticular lens showed greatly improved characteristics compared to the planoconvex structure: dioptric power (171.69D → 1,982.56D), viewing angle (26degrees → 46degrees), and crosstalk ratio (27.27% → 16.18%). Thanks to these improvements, a fine 3D image and a natural motion parallax could be observed with the biconvex structure electrowetting lenticular lens. In addition, the novel biconvex structure electrowetting lenticular lens was designed to achieve a plane lens state with a no voltage applied condition, and as such it could show a clean 2D image at 0 V. In conclusion, a novel biconvex structure electrowetting lenticular lens showed 2D/3D switchable operation as well as excellent 3D performance compared to a solid lenticular lens.

See-through display combined with holographic display and Maxwellian display using switchable holographic optical element based on liquid lens.

We report a switchable holographic optical element based on a liquid lens for a see-through display. For the switchable holographic optical element, we recorded two optical components in the holographic film in two steps. A numerical simulation was also done to define the recording and reconstruction conditions. After the recording process, the entire system was changed from 4f optics to Maxwellian optics by changing wavefront of the reference wave using a liquid lens. The diffraction efficiency was 0.46 for a single element recording and around 0.14 for a double element recording. The holographic display and the Maxwellian display were successfully switched without any crosstalk. The field of view and eye box of the holographic display were 1° and 4.36 mm, respectively, and the field of view and the eye box of the Maxwellian display were 3.8° and 23.2 um, respectively. In the proposed system, spatial frequency filtering by the liquid lens and image shape distortion seriously affected the hologram image. However, we successfully verified the feasibility of our proposed switchable holographic optical element using a liquid lens.

Electro-wetting lenticular lens with improved diopter for 2D and 3D conversion using lens-shaped ETPTA chamber.

In this paper, we introduce a method for improving the lens diopter of 2D/3D convertible devices using electro-wetting. For stable operation, an electro-wetting device requires high dioptric performance and this was achieved using bi-convex liquid-liquid-solid phases. 1-Chloronaphthalene with a refractive index of 1.633 was used as an oil phase to achieve high diopters. ETPTA (trimethylolpropane ethoxylate triacrylate), a UV-sensitive material with low chemical reactivity to the 1-Chloronaphthalene, was used as a chamber material. This resulted in a diopter of 3030D for high quality multi-view images without unstable oil movement or trembling. The ETPTA was molded on a 0.3mm thick glass substrate that was coated with UV adhesive (NOA 81). The maximum diopter capable of stable operation was 3425D. 2D and 3D conversion and parallax motion were demonstrated.

Time multiplexing technique of holographic view and Maxwellian view using a liquid lens in the optical see-through head mounted display.

We report a liquid lens based optical see-through head mounted display that can simultaneously display both a maxwellian view and a hologram. Holograms are reconstructed by an angular spectrum layer based synthesis method. A hologram and Maxwellian view are simultaneously displayed by focusing the liquid lens from 0 D to 20 D with 60 Hz. The hologram is reconstructed at a position 1.5 m from the eye, and it is confirmed that the Maxwellian view is clear, even if the focus of the eye changes from 50 cm to 1.7 m. In the proposed system, the liquid lens acts as a low-pass filter. Since the PSNR is about 23 dB in the currently used 10 mm diameter liquid lens, the image quality is not adequate. However, we successfully verify the feasibility of our proposed system. In addition, if a large diameter liquid lens of 30 mm or more is applied, excellent image quality of 30 dB or more can be realized.

Effect of oil on an electrowetting lenticular lens and related optical characteristics.

While there are many ways to realize autostereoscopic 2D/3D switchable displays, the electrowetting lenticular lens is superior due to the high optical efficiency and short response time. In this paper, we propose a more stable electrowetting lenticular lens by controlling the quantity of oil. With a large amount of oil, the oil layer was broken and the lenticular lens was damaged at relatively low voltage. Therefore, controlling the amount of oil is crucial to obtain the required dioptric power with stability. We proposed a new structure to evenly adjust the volume of oil and the dioptric power was measured by varying the volume of oil. Furthermore, the optical characteristics were finally analyzed in the electrowetting lenticular lens array with a proper amount of oil.

Optically tunable microwave, millimeter-wave and submillimeter-wave utilizing single-mode Fabry-Pérot laser diode subject to optical feedback.

In this paper, we use optical feedback injection technique to generate tunable microwave, millimeter-wave and submillimeter-wave signals using single-mode Fabry-Pérot laser diode. The beat frequency of the proposed generator ranges from 30.4 GHz to 3.40 THz. The peak power ratio between two resonating modes at the output spectrum of can be less than 0.5 dB by judiciously selecting feedback wavelength. In the stabilization test, the peak fluctuation of photonic signal is as low as 0.19 dB within half hour. Aside from locking regions, where the laser is easily locked by the injection beam, the side-mode suppression ratio is well over 25 dB with the maximum value of 36.6 dB at 30.4 GHz beat frequency. In addition, the minimum beat frequency interval between two adjacent photonic signals is as low as 10 GHz.

Improving the performance of an electrowetting lenticular lens array by using a thin polycarbonate chamber.

In this paper, we used a thin polycarbonate (PC) chamber to improve the performance of an electrowetting lenticular lens array. The polycarbonate chamber changed the radius of curvature (ROC) of the oil acting as a lens, which increased the dioptric power of the liquid lens to 1666.7D. The increase in dioptric power required a reduction in the distance between the optical center of the lens and the display pixels under the chamber, which was accomplished by reducing the thickness of the chamber. The optimal thickness of the chamber was determined to be 0.5mm. Using this thin PC chamber, transmittance and viewing angle were measured and compared with an electrowetting lenticular lens with a conventional 1mm poly methyl methacrylate (PMMA) chamber was done. Crosstalk which degrades clear 3D images, is an inevitable factor in lenticular lens type multi-view systems. With the 0.5mm PC chamber, the viewing zone was expanded and the ratio of the crosstalk area was reduced, which resulted in a clear 3D image. The new method of depositing the electrode layer also ensured the uniform operation of the liquid lens array.

Depth plane adaptive integral imaging using a varifocal liquid lens array.

This paper proposes an enhanced integral imaging system with an electrically controllable image plane to address the issue of the limited depth problem in integral imaging. For implementation of the variable image plane, a varifocal liquid lens array and driving device are adopted instead of an ordinary solid lens array. The position of the central depth plane is varied by adjusting the focal length of the lens array. The proposed system enables matching between the object position and depth plane electrically, and thus an object moving from 5.15 to 11.72 cm is clearly displayed with this method.

Short-pulse controlled optical switch using external cavity based single mode Fabry-Pérot laser diode.

We propose and demonstrate a novel scheme for short pulse controlled all-optical switch using external cavity based single mode Fabry- Pérot laser diode (SMFP-LD). The proposed scheme consists of control unit and switching unit as two essential blocks. The basic principle of the proposed scheme is the optical bistability property of SMFP-LD for the control unit and the suppression of the dominant beam of SMFP-LD with injection locking for the switching unit. We also present the analysis of hysteresis width and rising/falling time with change in wavelength detuning which helps to find the optimum wavelength detuning value and power of light beams at different stages of the proposed scheme that gives wide input dynamic power range, high ON/OFF contrast ratio, and low rising/falling time. Input data of 10 Gb/s Non Return to Zero (NRZ) signal is switched at output ports depending upon the control signal generated by the control unit, which comprises of optical SR latch. Output waveforms, clear eye diagrams with extinction ratio of about 11 dB, rising/falling time of about 30 ps and 40 ps, and bit error rate (BER) are measured to validate proposed scheme. No noise floor is observed at output ports up to BER of 10-(12) and the maximum power penalty recorded is about 1.7 dB at a BER of 10-(9) which shows good performance of the proposed short pulse controlled optical switch using SMFP-LDs.

Adaptive double-sided fluidic lens of polydimethylsiloxane membranes of matching thickness.

A fluidic lens with double-sided polydimethylsiloxane membranes of different thicknesses that complement each other to reduce spherical aberration was fabricated. It is operated with magnetic repulsion induced by the current in the voice coil. The optical power of the proposed lens was observed to be considerably higher compared to that of a typical convex glass lens. The focal lengths of rays passing through the optical axis and the edge of the lens was measured and compared with the simulation data. The spherical aberration of the proposed lens was observed to be negligibly small compared to that of a plano-convex lens.