Micro optical pattern shaping for tailored light emission from organic LEDs.

The application of large area OLEDs for lighting and signage purposes potentially requires essential changes of the common Lambert-like emission pattern. We demonstrate an array based micro optical approach for pattern shaping of area light sources based on distorted Fourier imaging of an aperture array with a micro lens array. Narrow angular emission patterns of ± 35° and ± 18° FWHM obtained experimentally demonstrate the pattern shaping with low stray light levels. The internal recycling of initially rejected photons yields intensity enhancements exceeding a factor two in forward direction that is still well below the theoretical limits due to limited reflectivity.

Ultraslim fixed pattern projectors with inherent homogenization of illumination.

For a given illumination source brightness, the transmitted flux of common single-aperture projection optics scales with all three system dimensions, thus preventing the realization of slim devices along with a high lumen output. In this article we introduce a multichannel approach, called "array projector," which breaks this constraint, thus enabling the realization of ultraslim but high flux systems with inherent homogenization for still image content. The concept is based on regular two-dimensional arrangements of absorbing object structures and projective microlenses superposing their individual images on the screen. After deriving first-order scaling laws for the multichannel projector in contrast to common single-aperture optics, specification of system parameters is shown considering aberrations of a single-channel and collective effects of the array. The technological realization of a sample system is shown and characterized in terms of modulation transfer, homogeneity, depth of focus and flux.

Optical Cluster Eye fabricated on wafer-level.

Wafer-level optics is considered as a cost-effective approach to miniaturized cameras, because fabrication and assembly are carried out for thousands of lenses in parallel. However, in most cases the micro-optical fabrication process is not mature enough to reach the required accuracy of the optical elements, which may have complex profiles and sags in the mm-scale. Contrary, the creation of microlens arrays is well controllable so that we propose a multi aperture system called "Optical Cluster Eye" which is based on conventional micro-optical fabrication techniques. The proposed multi aperture camera consists of many optical channels each transmitting a segment of the whole field of view. The design of the system provides the stitching of the partial images, so that a seamless image is formed and a commercially available image sensor can be used. The system can be fabricated on wafer-level with high yield due to small aperture diameters and low sags. The realized optics has a lateral size of 2.2 × 2.9 mm2, a total track length of 1.86 mm, and captures images at VGA video resolution.

Thin wafer-level camera lenses inspired by insect compound eyes.

We propose a microoptical approach to ultra-compact optics for real-time vision systems that are inspired by the compound eyes of insects. The demonstrated module achieves approx. VGA resolution with a total track length of 1.4 mm which is about two times shorter than comparable single-aperture optics on images sensors of the same pixel pitch. The partial images that are separately recorded in different optical channels are stitched together to form a final image of the whole field of view by means of image processing. A software correction is applied to each partial image so that the final image is made free of distortion. The microlens arrays are realized by state of the art microoptical fabrication techniques on wafer-level which are suitable for a potential application in high volume e.g. for consumer electronic products.

The Gabor superlens as an alternative wafer-level camera approach inspired by superposition compound eyes of nocturnal insects.

We present the microoptical adaption of the natural superposition compound eye, which is termed "Gabor superlens". Enabled by state-of-the-art microoptics technology, this well known principle has been adapted for ultra-compact imaging systems for the first time. By numerical ray tracing optimization, and by adding diaphragm layers and a field lens array, the optical performance of the Gabor superlens is potentially comparable to miniaturized conventional lens modules, such as currently integrated in mobile phones. However, in contrast to those, the Gabor superlens is fabricated using a standard microlens array technology with low sag heights and small diameter microlenses. Hence, there is no need for complex diamond turning for the generation of the master structures. This results in a simple and well controllable lens manufacturing process with the potential to high yield.

Chirped fiber Bragg gratings written with ultrashort pulses and a tunable phase mask.

We report a fabrication technique for chirped fiber Bragg gratings (CFBGs) using a flexible setup based on a poly(methyl-methacrylate) phase mask. The period of the phase mask can be thermally tuned during the inscription process, allowing the grating period of uniform fiber Bragg gratings to be shifted about 7 nm by a temperature change of 74 K. In addition, CFBGs with bandwidths up to 2 nm are demonstrated in non-photosensitive fibers by IR femtosecond inscription.

Artificial compound eye zoom camera.

We demonstrate a highly compact image capturing system with variable field of view but without any mechanically moving parts. The camera combines an ultra-thin artificial apposition compound eye with one variable focal length liquid lens. The change of optical power of the liquid lens when applying a voltage results in a change of the magnification of the microlens array imaging system. However, its effect on focusing of the individual microlenses can be neglected due to their small focal length.

Design and fabrication of stacked, computer generated holograms for multicolor image generation.

We present a diffractive optical element consisting of computer-generated holograms and dielectric multilayer mirrors in a stratified setup. Illuminated with a white laser beam, consisting of three single lasers with wavelengths of 635 nm, 543 nm, and 473 nm, this element enables the far field projection of arbitrary, multicolor images. Certain advantages of holographic image generation, e.g., the possibility of a large depth of focus and a very easy optical setup, are maintained with the new element.

Beam homogenizers based on chirped microlens arrays.

Lens array arrangements are commonly used for the homogenization of highly coherent laser beams. These fly's eye condenser configurations can be used to shape almost arbitrary input intensity distributions into a top hat. Due to the periodic structure of regular arrays the output intensity distribution is modulated by equidistant sharp intensity peaks which are disturbing the homogeneity. As a new approach we apply chirped microlens arrays to the beam shaping system. These are non-regular arrays consisting of individually shaped lenses defined by a parametric description which can be derived completely from analytical functions. The advantages of the new concept and design rules are presented.

Artificial neural superposition eye.

We propose an ultra-thin imaging system which is based on the neural superposition compound eye of insects. Multiple light sensitive pixels in the footprint of each lenslet of this multi-channel configuration enable the parallel imaging of the individual object points. Together with the digital superposition of related signals this multiple sampling enables advanced functionalities for artificial compound eyes. Using this technique, color imaging and a circumvention for the trade-off between resolution and sensitivity of ultra-compact camera devices have been demonstrated in this article. The optical design and layout of such a system is discussed in detail. Experimental results are shown which indicate the attractiveness of microoptical artificial compound eyes for applications in the field of machine vision, surveillance or automotive imaging.

Thin compound-eye camera.

An artificial compound-eye objective fabricated by micro-optics technology is adapted and attached to a CMOS sensor array. The novel optical sensor system with an optics thickness of only 0.2 mm is examined with respect to resolution and sensitivity. An optical resolution of 60 x 60 pixels is determined from captured images. The scaling behavior of artificial compound-eye imaging systems is analyzed. Cross talk between channels fabricated by different technologies is evaluated, and the influence on an extension of the field of view by addition of a (Fresnel) diverging lens is discussed. The lithographic generation of opaque walls between channels for optical isolation is experimentally demonstrated.

Chirped arrays of refractive ellipsoidal microlenses for aberration correction under oblique incidence.

Improvements of the resolution homogeneity of an ultra-thin artificial apposition compound eye objective are accomplished by the use of a chirped array of ellipsoidal micro-lenses. The array contains 130x130 individually shaped ellipsoidal lenses for channel-wise correction of astigmastism and field curvature occurring under oblique incidence. We present an analytical approach for designing anamorphic micro-lenses for such purpose based on Gullstrands equations and experimentally validate the improvement. Considerations for the design of the photolithographical masks for the micro-lens array fabrication by melting of photoresist cylinders with ellipsoidal basis are presented. Measurements of the optically performance are proceed on first realized artificial compound eye prototypes showing a significant improvement of angular resolution homogeneity over the complete field of view of 64.3?.

Implementation of field lens arrays in beam-deflecting microlens array telescopes.

Laterally displaceable microlens array telescopes allow for variable and fast beam deflection. The generation of spurious light usually leads to a reduction of transfer efficiency with increasing displacement. We present the introduction of an array of field lenses on the back side of a recollimating microlens array that results in a reduced deflection angle dependency of transfer efficiency. A paraxial matrix formalism is used to prove the theoretical elimination of spurious light by use of a field lens array. The fabrication of well-aligned double-sided lens arrays by UV replication is discussed. Measurements of transfer efficiency with and without the use of field lens arrays are compared with the results of numerical wave-optic simulations.

Artificial apposition compound eye fabricated by micro-optics technology.

By exploring micro-optical design principles and technology, we have developed an artificial apposition compound eye. The overall thickness of the imaging system is only 320 microm, the diagonal field of view is 21 degrees, and the f-number is 2.6. The monolithic device consists of an UV-replicated microlens array upon a thin silica substrate with a pinhole array in a metal layer on the back side. The pitch of the pinholes differs from that of the lens array to provide individual viewing angle for each channel. Theoretical limitations of resolution and sensitivity are discussed as well as fabrication issues and compared with experimental results. A method to generate nontransparent walls between optical channels to prevent cross talk is proposed.

Formation of micro-optical structures by self-writing processes in photosensitive polymers.

A local exposure of UV-sensitive polymers leads to a local curing. This corresponds to a saturable and irreversible nonlinear change of the refractive index, which evidently leads to a filamentation of the hardening polymer. This paper investigates the physical background of these effects and analyzes how the different influencing factors could be used for a steered, partly self-written formation of micro-optical structures. The structure formation is simulated on the basis of an iterative beam propagation method with consideration of a set of process parameters, e.g., the photoinitiator concentration or the exposure intensity. It is shown theoretically as well as experimentally that a variation of material- and exposure-specific process parameters gives opportunities for a controlled structure formation. The experimental realization of a configuration by use of a beam shaper within a UV contact exposure process is presented by means of the preparation of high-aspect-ratio conic structures.