Structural design study for performance improvement of focus-tunable liquid lens arrays

Abstract

Autotereoscopic 3D display is designed to solve the inconvenience caused by the polarization and shutter glasses used in the conventional glasses type 3D displays. Discomfort such as headache, dizziness, and visual fatigue are caused by the fact that only the binocular parallax is used as a reason for feeling 3D information. The autostereoscopic 3D displays use a monocular focus as well as a binocular disparity to make the 3D look more natural. For this reason, there have been a lot of researches on the multi-view and integral imaging which are the autostereoscopic 3D displays.

Focus-tunable lens arrays can play a large role in implementing autostereoscopic three-dimensional imaging. In addition to solving 2D / 3D issues that are currently an issue in multi-view, it also extends the range of narrow depths that are a problem in integral imaging. In order to realize these functions, liquid crystal method, EAP (electro-active polymer) method, hydraulic method, and electrowetting method have been proposed, and these studies are for the application of the above-mentioned autostereoscopic method. However, there are advantages and disadvantages to each method. The liquid crystal has slow operating speed and low optical power. EAP has very high operating voltage and very slow response time. Electrowetting and hydraulic pressure methods have more advantages than the above two cases. In the case of electrowetting, it has fast response speed and very low power consumption, but it has a difficulty on process, has higher optical power than LC but lower optical power than other methods. The hydraulic system has high optical power and fast response speed, and it is advantageous for a lens array of high fill factor in structure. However, it is disadvantageous in that the optical aberration is severe due to the difficulty of surface curvature control due to the elastic material and distortion of the surface thin film.

In this paper, firstly, we applied the special structure to the EWOD method which requires high speed and low power to improve the optical power, which is the biggest disadvantage, and to make the display device capable of 2D / 3D conversion by increasing the NA. In addition, in order to solve the problem of the fill factor which is the biggest disadvantage in the lens array capable of changing the focus, the most advantageous hydraulic method is used to eliminate the boundary of the lens and to eliminate the optical aberration caused thereby.

Electrowetting phenomenon was first discovered by Gabriel Lippmann in 1875, and Varioptic manufactures and sells a single liquid lens using it. In this paper, attempted to fabricate a variable lenticular lens using this phenomenon. The production of an electro-wetted liquid lenticular array has begun for mobile use, and by taking advantage of the fact that it has a faster response speed and shorter focal distance than conventional technologies for 2D / 3D conversion, The thickness of the lens is about 2mm, which is similar to that of the solid model lenticular lens. A bi-convex structure using ETPTA was devised to solve the problem of low NA, which is a problem of existing researches. The slanted-deposition method using evaporation was devised and electrode patterning was performed. In the case of the dielectric layer, a multi-layered dielectric structure using Al2O3 and TiO2 was used to have high dielectric constant and dielectric strength. For the uniform application of liquids, a silicon etching process has been developed to fabricate the open-structure with shared end portions, which enables uniform application of the liquid. Electro-wetting liquid To control the leakage that can occur because the lens is liquid, 3M tape, UV adhesive and polycarbonate gasket were used to block the leak and complete the sample. A high NA of about 0.399 was achieved using a liquid with a low refractive index and a low refractive index of the chamber. The distance between the lens and the display was reduced to about 0.3 mm, and the thickness of the entire sample was as thin as 2 mm, achieving a very fast operating speed of 1.66 ms. This resulted a crosstalk reduction to 14.3 percent.

In the case of liquid lenses to be used for integral imaging, the electrowetting method, which has considerable disadvantages in terms of fill-factor, is difficult to use. For this reason, the hydraulic system was adopted. A triangular, square, or hexagonal array should be used instead of a circular array to achieve fill-factor performance close to 100%. However, in a typical hydraulic approach, these polygonal lenses cause severe aberrations. Because of this, most lens arrays are of circular type and can only achieve a low fill factor. Considering this point, a special structure consisting of thin membrane, walls and columns was devised and it was predicted that the aberration caused by the polygon could be removed. Through the ANSYS simulation, we measured how the surface morphology changes according to the thickness of the walls, and the optimum solid model was derived. The thin membrane, walls, and columns were all made of PDMS elastic membrane, and ethylene glycol aqueous solution, which has no chemically reactivity with PDMS, was used for refractive index matching. In this study, we design a system to detect the symmetrical distortion of the optical axis by using a rectangular lens array as a target and to designate the index according to how the distortion of diagonal direction and vertical and horizontal direction occurs. The thin membrane was fabricated with a parabolic surface using a commercial lens, which minimized distortion in the initial state. Next, the distortion of the diagonal direction and the distortion of the vertical and horizontal directions were measured as the thickness of the walls was changed. It was confirmed that the polygonal aberration was minimized when the walls having a thickness of about 1.4 mm was present. The distortion of about 15.17% was minimized to 1.28%. For more precise measurement, we set up a Shack-Hartmann wavefront sensing system using a 4-f system and measured the quadrafoil aberration at the square aperture through the wavefront passing through. $2.77, -1.96\lambda$, which occurred when the proposed special structure was not used, could be minimized to about $0.39\lambda$. This proved the utility of this structure.