Daniel Pätz

Rigorous analysis of shadowing effects in blazed transmission gratings

Blazed transmission gratings have become crucial components in many hybrid optical systems. Shadowing effects are known to occur at their passive blaze facets, which may impair the systems efficiency performance. For optical designs, it is desirable to have a simple but accurate description of this phenomenon. We show that the efficiency reduction in low diffraction orders is dominated by a linear dependence on the ratio of wavelength to grating period rather than a quadratic dependence as proposed in extended scalar theory. The strength of the electromagnetic shadowing will be determined using rigorous diffraction methods and discussed with respect to imaging optical components. Results are compared to existing ray-optical models.

Tunable compound eye cameras

We present design and realization concepts for thin compound eye cameras with enhanced optical functionality. The systems are based on facets with individually tunable focus lengths and viewing angles for scanning of the object space. The active lens elements are made of aluminum nitride (AlN)/nanocrystalline diamond (NCD) membranes. This material system allows slow thermally actuated elements with a large deformation range as well as fast piezoelectric elements with a smaller deformation range. Due to the extreme mechanical stability of these materials, we are able to realize microoptical components with optimum surface qualities as well as an excellent long-term stability. We use facets of microlenses with 1 mm in diameter and a tunable focusing power to compensate for the focus shift for different viewing angles during the scanning procedure. The beam deflection for scanning is realized either by laterally shifting spherical elements or by a tunable microprism with reduced aberrations. For both actuators we present a design, fabrication concept and first experimental results.

Analysis of the influence of the passive facet of blazed transmission gratings in the intermediate diffraction regime.

Blazed diffraction gratings are of enormous practical importance for imaging and analyzing hybrid optical systems. The intermediate diffraction regime is characterized by the transition from the scalar to the rigorous electromagnetic theory. An effect known as shadowing occurs and reduces the diffraction efficiency. Based on rigorous calculations for optimized sawtooth-shaped and binary-multilevel blaze profiles, we deduce a semianalytical model describing the shadowing phenomenon for the general case of oblique incidence. We discuss illumination both from air and from the substrate. Though a multilevel blaze possesses a discrete substructure, our shadowing model remains valid, if substructural effects are neglected. We find that electromagnetic effects due to the passive blaze facet lead to the efficiency reduction, and the blazing efficiency shows a linear dependence on the ratio of blaze wavelength to grating period. Our shadowing model is applied to predict the performance of a Littrow-like blazing condition in transmission geometry as, e.g., for a diffractive solid immersion lens.

Depth of focus analysis of optical systems using tunable aperture stops with a moderate level of absorption

The size of the aperture stop of a lens is a major parameter to define, e.g., the depth of focus of an optical imaging system. In conventional systems, totally absorbing apertures are generally assumed. Their optical performance can be easily described by a geometric ray model. We propose an extended model to estimate the depth of focus with respect to a nontotally absorbing circular aperture, which may correspond to new concepts for tunable apertures, in particular for micro-optical systems. We present specifications to analyze and optimize the performance of those systems and verify the theoretical model by experimental depth of focus measurements with a partly transparent aperture.

Tunable multisegment Si x N y /AlN piezo lenses for wavefront correction

A combined layer structure of aluminum nitride (AlN) and silicon nitride (SixNy) enables the fabrication of multisegment piezo-actuated micro lenses with a precise control of the lens surface. It is demonstrated that these micro lenses offer free aspheric deformation of the lens surface and can operate at high repetition rates along with reproducible and precise tunability. The AlN/SixNy micro lenses are highly advantageous to be used as wave front filter or for fast focus correction.

Tunable anamorphotic imaging system based on fluidic cylindrical lenses

We present an anamorphotic imaging system with a separately tunable magnification in horizontal and vertical direction in order to change the aspect ratio of the image. The design is based on cylindrical membrane lenses made of aluminum nitride with tunable focal power to realize a vario system without moving elements. We demonstrate the design concept of cylindrical lenses with a very compact geometry, the optical system as well as experimental results.

Multifunctional LTCC Substrates for Thermal Actuation of Tunable Micro-Lenses Made of Aluminum Nitride Membranes

Thin membranes are widely used for tunable micro-lenses, where the membrane, usually made from polymers, defines the surface of a subjacent liquid. If the liquid is pressurized, the membrane deflects and forms a lens. In many cases, a macroscopic pump is used to generate the pressure. Here, we use a multifunctional LTCC substrate, which consists of two cavities. A micro-fluidic network allows their independent filling. For the actuation, the actuating cavity is filled with air and the optical cavity is filled with immersion oil. If the actuating cavity is heated by screen-printed resistors, the fluid expands and a pressure is transmitted to the optical cavity via a micro-fluidic channel. The optical cavity is covered with a glass chip and a membrane chip, respectively. We use 500 nm thick membranes of aluminum nitride (AlN), which have a 3 mm diameter and are fabricated using technologies of silicon based micro-electro-mechanical systems. With 6 W electrical heating power a temperature increase of 100 K w...

Diamond/AlN Thin Films for Optical Applications

In this work we report on membranes made of nanocrystalline diamond (NCD) and AlN for the use in tunable micro‐optics. For the growth of the AlN and NCD thin films, magnetron sputtering and chemical vapor deposition techniques have been used, respectively. A chemical‐mechanical polishing process of NCD layers has been introduced, which is crucial for the growth of c‐oriented, fiber textured AlN films. AlN layers deposited on as grown and polished nanocrystalline diamond along with free standing membranes have been compared by studying microstructure, surface morphology, piezoelectrical response as well as optical properties.

Design considerations for integrated microoptical systems combining refractive and diffractive optical components

Planar integrated microoptical systems have been demonstrated for a variety of applications such as optical interconnects, sensing and security applications. Diffractive optical elements provide the necessary design freedom to optimize the optical performance of such systems along the folded optical axis. For enhanced optical efficiency it is necessary to combine diffractive and refractive elements within such systems. Hereby the refractive components provide most of the optical power while the diffractive elements are used as correction elements for optimized system performance. The integration of refractive components has significant consequences on the geometry of planar integrated optical systems as well as on the optical systems design. Based on this approach we present various designs for efficient planar-optical (phase-contrast) imaging systems. We compare various possibilities for the simulation of diffractive and holographic optical components and their integration in the design of planar microoptical systems. To this end we apply commercial design software (e.g. ZemaxTM, ASAPTM) as well as self programmed tools.