Compact near-eye display system using a superlens-based microlens array magnifier

In this paper, we present an array of tunable microlenses on polydimethylsiloxane (PDMS) microfluidic chips as a convenient platform for the study of cell responses to mechanical strains. This array of microlenses is an array of microwells covered with a thin PDMS membrane that are connected by microfluidic channels. The deformation of the PDMS membrane on the microlenses is tuned by external hydraulic pressure that is introduced through the microchannels. The degree of deformation of the PDMS membrane can be estimated from the focal length of the microlens, and the corresponding mechanical strain that is caused can be calculated from finite element simulation. We demonstrate the capability of this array of microlenses as a general platform for studying the influence of mechanical strain on adherent cells by using NIH 3T3 fibroblasts and HeLa cells as our models.

We demonstrate the depth measurement method of holographic images using integral imaging. The depth information of holographic images can be obtained with a single capture by conventional integral imaging pickup system composed of a micro lens array (MLA) and an image sensor. In order to verify the feasibility of our proposed method, an elemental image set of holographic images formed by a MLA was generated by a computer, and then refocused images at different planes were reconstructed numerically using computational integral imaging reconstruction (CIIR) technique for depth measurement. Note that we set the distance between MLA and image sensor as focal length of micro lens for large depth of focus. From the numerical results, we can measure the depth representation of holographic images successfully. However, refocused images from an optically captured elemental image set provide poor depth discrimination due to expected error in distance between MLA and image sensor. Only an object in a particular narrow depth range can be focused clearly when the image sensor is placed out of the MLA focal plane. The simulated results in this condition matched reasonably with the experiment result.

Plasticized polymer gel has emerged as a new material for the fabrication of microlenses with variable focal lengths. To explore a non-toxic and non-irritating gel, we prepared a polyvinyl alcohol (PVA)/glycerol gel, which is sensitive to external direct-current (DC) voltage and exhibits a strong polarization effect upon application of an electric field. Due to the polarization effect, PVA/glycerol gel bends toward cathode like the polyelectrolyte gel. To fabricate the microlens array, PVA/glycerol gel was spin-coated on a glass substrate with diamond Indium Tin Oxides electrode. Upon application of the DC electric field, the surface of the gel membrane followed a cosine wave for creeping motion of PVA particles, hence the membrane exhibited the property of a microlens array. The experiments for measuring imaging quality and focal length of PVA/glycerol-based lenticular microlens array are performed. Our microlens array allows tunable focal length with lower driving voltage compared to the microlens array with polyvinyl chloride gel.

Immersive virtual and augmented reality systems (VR/AR) are entering the consumer market and have the potential to profoundly impact our society. Applications of these systems range from communication, entertainment, education, collaborative work, simulation and training to telesurgery, phobia treatment, and basic vision research. In every immersive experience, the primary interface between the user and the digital world is the near-eye display. Thus, developing near-eye display systems that provide a high-quality user experience is of the utmost importance. Many characteristics of near-eye displays that define the quality of an experience, such as resolution, refresh rate, contrast, and field of view, have been significantly improved in recent years. However, a significant source of visual discomfort prevails: the vergence-accommodation conflict (VAC). This visual conflict results from the fact that vergence cues, but not focus cues, are simulated in near-eye display systems. Indeed, natural focus cues are not supported by any existing near-eye display. Afforded by focus-tunable optics, we explore unprecedented display modes that tackle this issue in multiple ways with the goal of increasing visual comfort and providing more realistic visual experiences.

An artificial neural network and genetic algorithm were used to achieve a high quality microlens array fabrication using a UV proximity printing process in this study. The UV proximity printing process can precisely control the geometric profile of the microlens array in the fabrication process without thermal reflow. The major objective in using the robust design is to reduce the variations in the focal length of the microlens array, allowing improved focus and enhanced illumination brightness. The artificial neural network was used first to characterize the nonlinear relationship between the manufacturing parameters and the properties derived from experimental data. The manufacturing parameters which affect microlens array uniformity include: (1) geometrical ratio, (2) printing gap, (3) spin coating revolution speed, (4) exposure time and (5) developing time. It is very important to control these parameters to decrease the sensitivity to noise. The L18 orthogonal array was used as the learning data for the artificial neural network to construct a system model that could predict the focal length for arbitrary setting of parameters. Then, the genetic algorithm was applied to obtain the robust setting of parameters. The results showed that the microlens array quality could be significantly improved in comparison with the original design.

In order to determine light aberrations, Shack-Hartmann optical wavefront sensors make use of microlens arrays (MLA) to divide the incident light into small parts and focus them onto image planes. In this paper, we present the design and fabrication of long focal length MLA with various shapes and arrangements based on a double layer structure for optical wavefront sensing applications. A longer focal length MLA could provide high sensitivity in determining the average slope across each microlens under a given wavefront, and spatial resolution of a wavefront sensor is increased by numbers of microlenses across a detector. In order to extend focal length, we used polydimethysiloxane (PDMS) above MLA on a glass substrate. Because of small refractive index difference between PDMS and MLA interface (UV-resin), the incident light is less refracted and focused in further distance. Other specific focal lengths could also be realized by modifying the refractive index difference without changing the MLA size. Thus, the wavefront sensor could be improved with better sensitivity and higher spatial resolution.

A new method, named step simulation, is proposed to fabricate refractive 128/spl times/128 element Si microlens (MLS) arrays for hybrid integration with PtSi IR Focal Plane Arrays (FPAs). By the method, the focal length of rectangle-based multilevel Si MLS arrays is over 1 millimeter, much longer than typical focal length of MLS array fabricated by conventional binary optics Fresnel technique. Si MLS arrays and IR FPAs are coupled with infrared glue. The IR responsivity of the hybrid device is improved significantly.

The near-eye display feature in emerging spatial computing systems produces a distinctive visual effect of mixing virtual and real worlds. However, its application for all-day wear is greatly limited by the bulky structure, energy expenditure, and continuous battery heating. Here, we propose a lightweight holographic near-eye display system that takes advantage of solar energy for self-charging. To guarantee the collection of solar energy and near-eye display without crosstalk, we implement holographic optical elements (HOEs) to diffract sunlight and signal light into a common waveguide. Then, small-area solar cells convert the collected solar energy and power the system. Compact power supply components replace heavy batteries, thus contributing to the lightweight design. The simple acquisition and management of solar energy provide the system with sustainable self-charging capability. We believe that the lightweight design and continuous energy input solution will significantly promote the popularity of near-eye display in our daily lives.

The thermal reflow process is widely used in microlens array fabrication. However, the resulting arrays are commonly criticized for their low fill factor. In this work, caulking is applied to fill the gaps between adjacent lenses. The experimental results prove that a gapless microlens array with a 100% fill factor could be successfully produced and the caulking time precisely controlled. Furthermore, an artificial neural network and genetic algorithm are used to achieve high quality using the thermal reflow and caulking. The L18 orthogonal array is used as the learning data for the artificial neural network to construct a system model that could predict the results (e.g., S/N, focal length, and roughness) for arbitrary parameter settings. The genetic algorithm is then applied to obtain the optimal parameter settings. The major objectives in using the optimal design are to reduce the variation in the focal length and the surface roughness for a microlens array. This allows improved focus and enhanced illumination brightness. The results show that microlens array quality could be significantly improved in comparison with the original design.

Inspired by the structure of natural compound eyes, a liquid tunable-focus microlens array omnidirectionally arranged on a transparent hemispherical dome is demonstrated. Benefiting from the dome structure, this microlens array exceeds traditional planar ones with wide field of view (FOV). Each microlens in this microlens array was formed by a curved water-oil meniscus. The focal length of the microlenses could be individually tuned by a thermo-responsive hydrogel microactuator. Planar and hemispherical microlens arrays were tested and the focal length of the hemispherical microlens array varied from 3 mm to 32 mm. Focused images from the microlens arrays were recorded by a CCD-coupled stereoscope.

We propose an experimental method consisting of a standard Fizeau interferometer with transmission sphere (TS) for the determination of the focal length of microlens array (MLA) by spherical wavefronts. The TS is axially translated to produce a spherical wavefront of different curvatures with respect to the MLA. The align mode provision of the interferometer helps to monitor the tilt of the MLA with respect to the spherical wavefront. The focal length is determined from the measured distance of adjacent image spots for various spherical wavefronts at the focal plane of the MLA. Error analysis and experimental demonstration with an off-the-shelf MLA are addressed here.

Because the near eye display (NED) device mainly displays its imaging effect with a unique virtual image, in essence, compared with the traditional display device, the measurement requirements and methods of brightness, light leakage ratio and other related parameters have also changed. By analyzing the imaging characteristics of optical waveguide AR display equipment, a comparative test scheme for brightness and light leakage ratio is proposed. The same type of arrayed optical waveguide NED module is experimentally measured by using equipment with different test principles, and the measurement results are compared and analyzed. The research results show that the measurement results of the ordinary aiming point luminance meter are not the luminance values in the real display. For the measurement of a certain illumination degree of the equipment, the luminance value and light leakage measured by the aiming point luminance meter have lower deviation than that measured by the two-dimensional imaging luminance meter. Therefore, in the process of testing NED, it is necessary to correctly select the measuring instrument according to its imaging characteristics, the size of the area to be tested and the test scene.

3D light field camera has been provided as a valid method to capture 3D surface topography with a microlens array. Through inverting the propagation direction, 3D virtual image projection can also be achieved with conventional optics and a microlens array. By compressing the 3D virtual image from the microlens array with an additional lens, microscale 3D virtual image can be achieved. We propose that microscale 3D virtual image can be valuable in microscale 3D fabrication. In this study, we provide a theoretical basis to combine femtosecond laser with the above mentioned 3D virtual image reconstruction scheme to cure photopolymers. With multiphoton effects induced by femtosecond laser, we expect 3D patterning of regions inside the photopolymer can be possible. Compared with the demonstration of 3D patterning with continuous near UV light and the proposed 3D light field projection scheme, curing of photopolymer can be limited to focal points (i.e., voxel) of the projected microscale 3D surface with two photon absorption effect when femtosecond light with high enough intensity is applied. The required femtosecond light intensity is also determined in this study. We expect this new 3D photolithography method can be valuable in fast and high precision patterning of micro to macro structures in materials that are photoactive.

Soft mold of microlens arrays fabricated by surface self-assembly

Strain tunable optics of elastomeric microlens array