Optical tweezers for trapping in a microfluidic environment.

Optical tweezers use the force from a light beam to implement a precise gripping tool. Based purely on an optical principle, it works without any bodily contact with the object. In this paper we describe an optical tweezers that targets an application within the framework of nuclear magnetic resonance (NMR) spectroscopy of small objects, which are embedded inside a microfluidic channel that will be integrated in a micro-NMR detector. In the project's final stages, the whole system will be installed within the wide bore of a superconducting magnet. The aim is to precisely maintain the position of the object to be measured, without the use of susceptibility disturbing materials or geometries. In this contribution we focus on the design and construction of the tweezers. For the optical force simulation of the system we used a geometrical optics approach, which we combined with a ray fan description of the output beam of an optical system. By embedding both techniques within an iterative design process, we were able to design efficient optical tweezers that met the numerous constraints. Based on details of the constraints and requirements given by the application, different system concepts were derived and studied. Next, a highly adapted and efficient optical trapping system was designed and manufactured. After the components were characterized using vertical scanning interferometry, the system was assembled to achieve a monolithic optical component. The proper function of the optical tweezers was successfully tested by optical trapping of fused silica particles.

Freeform characterization based on nanostructured diffraction gratings.

The in-line characterization of freeform optical elements during the production cycle is challenging. Recently, we presented a compact sensor setup for the characterization of the wavefront generated by freeform optical elements in transmission. The sensor is based on a common-path interferometer consisting of diffractive components and Fourier filtering being adapted to the subsequent numerical post processing. Additionally, it offers several degrees of freedom for enlarging the measurement range of the wavefront gradients. In this contribution, we propose an advanced sensor setup for the measurement of wavefronts generated by freeform elements in reflection. The main advantage is the uni-axial illumination of the test object and the measuring system without the need for conventional beamsplitters. Due to this uni-axial arrangement, the main challenge is to avoid the effect of stray light and back reflections on the measurement signal-to-noise ratio. This is achieved by implementing a highly absorbing amplitude grating based on nanostructured silicon. We demonstrate the experimentally realized measurement system and compare its performance to a commercial Shack-Hartmann sensor.

Ultrathin Alvarez lens system actuated by artificial muscles.

A key feature of Alvarez lenses is that they may be tuned in focal length using lateral rather than axial translation, thus reducing the overall length of a focus-tunable optical system. Nevertheless the bulk of classical microsystems actuators limits further miniaturization. We present here a new, ultrathin focus-tunable Alvarez lens fabricated using molding techniques and actuated using liquid crystal elastomer (LCE) artificial muscle actuators. The large deformation generated by the LCE actuators permits the integration of the actuators in-plane with the mechanical and optical system and thus reduces the device thickness to only 1.6 mm. Movement of the Alvarez lens pair of 178 µm results in a focal length change of 3.3 mm, based on an initial focal length of 28.4 mm. This design is of considerable interest for realization of ultraflat focus-tunable and zoom systems.

Integration of an opto-chemical detector based on group III-nitride nanowire heterostructures.

The photoluminescence intensity of group III nitrides, nanowires, and heterostructures (NWHs) strongly depends on the environmental H(2) and O(2) concentration. We used this opto-chemical transducer principle for the realization of a gas detector. To make this technology prospectively available to commercial gas-monitoring applications, a large-scale laboratory setup was miniaturized. To this end the gas-sensitive NWHs were integrated with electro-optical components for optical addressing and read out within a compact and robust sensor system. This paper covers the entire realization process of the device from its conceptual draft and optical design to its fabrication and assembly. The applied approaches are verified with intermediate results of profilometric characterizations and optical performance measurements of subsystems. Finally the gas-sensing capabilities of the integrated detector are experimentally proven and optimized.

Optical system for trapping particles in air.

An innovative optical system for trapping particles in air is presented. We demonstrate an optical system specifically optimized for high precision positioning of objects with a size of several micrometers within a nanopositioning and nanomeasuring machine (NPMM). Based on a specification sheet, an initial system design was calculated and optimized in an iterative design process. By combining optical design software with optical force simulation tools, a highly efficient optical system was developed. Both components of the system, which include a refractive double axicon and a parabolic ring mirror, were fabricated by ultra-precision turning. The characterization of the optical elements and the whole system, especially the force simulations based on caustic measurements, represent an important interim result for the subsequently performed trapping experiments. The caustic of the trapping beam produced by the system was visualized with the help of image processing techniques. Finally, we demonstrated the unique efficiency of the configuration by reproducibly trapping fused silica spheres with a diameter of 10 µm at a distance of 2.05 mm from the final optical surface.

Sapphire-GaN-based planar integrated free-space optical system.

We report on the design and fabrication of a planar integrated free-space optical system working on the basis of binary phase diffractive optical elements (DOEs) realized in GaN on a sapphire substrate. Group III-nitride/sapphire substrates enable the parallel monolithic integration of passive microoptical elements like lenses and gratings as demonstrated here and optoelectronic devices like light emitters and photodetectors on a single wafer. We present an approach for the simultaneous optimization of the efficiency of transmissive and reflective diffractive optical elements processed in a single lithographic etching step.

Tuneable planar integrated optical systems.

Planar integrated free-space optical systems are well suited for a variety of applications, such as optical interconnects and security devices. Here, we demonstrate for the first time dynamic functionality of such microoptical systems by the integration of adaptive liquid-crystal-devices.

Practical realization of massively parallel fiber-free-space optical interconnects.

We propose a novel approach to realizing massively parallel optical interconnects based on commercially available multifiber ribbons with MT-type connectors and custom-designed planar-integrated free-space components. It combines the advantages of fiber optics, that is, a long range and convenient and flexible installation, with those of (planar-integrated) free-space optics, that is, a wide range of implementable functions and a high potential for integration and parallelization. For the interface between fibers and free-space optical systems a low-cost practical solution is presented. It consists of using a metal connector plate that was manufactured on a computer-controlled milling machine. Channel densities are of the order of 100/mm(2) between optoelectronic VLSI chips and the free-space optical systems and 1/mm(2) between the free-space optical systems and MT-type fiber connectors. Experiments in combination with specially designed planar-integrated test systems prove that multiple one-to-one and one-to-many interconnects can be established with not more than 10% uniformity error.

Compact planar-integrated optical correlator for spatially incoherent signals.

A new, to our knowledge, approach for the planar integration of optical correlators is demonstrated. A VanderLugt-type architecture was used to allow the processing of the spatially incoherent signals of active optoelectronic smart-pixel-device arrays. In a folded optical system all passive components were implemented as a single multiple-phase-level element. The relations among the spatial resolution, the light efficiency, and the system design parameters are derived. High signal quality and low noise levels were achieved experimentally.

Planar-integrated optical vector-matrix multiplier.

We present the design of a planar-integrated optoelectronic vector-matrix multiplier. The inherent parallel-processing potential is fully exploited by optical implementation of multiplications and summations. Planar integration makes the free-space optical system compatible with electronic VLSI technologies. It is composed of phase-only diffractive optical elements, which implement lens and multiple-beam-splitter functions. A demonstrator version of the optical system for a matrix of size 10 x 10 was fabricated on quartz glass by means of multimask lithography and reactive ion etching. It shows low cross talk and good uniformity of the signals.

Iterative optimization of phase-only diffractive optical elements based on a lenslet array.

We describe the design of Fourier-type phase-only array generators. The numerical optimization employs the Fienup algorithm, where the parageometric design of the phase retardation profile, with the form of a lenslet array, is used as the initial guess of the optimization process. This approach provides designs with high performance that can be obtained with comparatively low computing effort. This is particularly true for elements generating large spot arrays. For symmetric reconstruction fields, the optimized phase profile typically has the same symmetry as that for the reconstruction field and can be easily unwrapped.

Efficient detour-phase encoding of one-dimensional multilevel phase diffractive elements.

We show that detour-phase encoding with a multilevel blaze structure as a carrier grating is especially suited to the implementation of diffractive elements with relatively high complexity in one axis. For our proposal the carrier grating is aligned perpendicularly to this axis. In this way the element can be encoded with a high space-bandwidth product and a high phase resolution by use of a moderate carrier frequency. Moreover, this frequency can be adjusted to isolate the reconstructed field from the noise resulting from high diffraction orders of the carrier grating or caused by etching errors during fabrication.

Integrated micro-optical imaging system with a high interconnection capacity fabricated in planar optics.

An integrated free-space optical interconnection system with 2500 parallel data channels is demonstrated. The design is based on a combination of microchannel imaging and conventional imaging. A modification of the hybrid imaging configuration allows one to achieve optimized image quality over large image fields.

High-efficiency detour-phase holograms.

We suggest a new approach to the fabrication of detour-phase holograms. By combining the concept of the detour phase with highly efficient carrier gratings it is possible to overcome the major drawback of detour-phase holograms, i.e., their low efficiency. We call the new hologram type a kinoform detour-phase hologram (KDPH). KDPH's facilitate the implementation of diffractive elements with high phase complexity. Especially if off-axis reconstruction is desired, the number of degrees of freedom for the design of the phase component is increased significantly.

Graphic codes for computer holography.

Several aspects of graphic codes of computer-generated holograms are discussed. This is in contrast to the algorithmic studies that have dominated research in the field of computer holography in recent years. We study the graphic cells of binary cell-oriented holograms under the aspects of specificfabrication and performance problems such as the pen-width problem and zero-order scattering. Asresult, we present experiments with new or modified cell structures that avoid some of these problems.

Transition between diffractive and refractive micro-optical components.

Optical components are usually classified into diffractive and refractive elements. In this classification, refractive components are defined as elements that are sufficiently described by geometrical optics. For micro-optics this distinction is very often not applicable. Our goal is to understand which parameters control the transition from elements that can be interpreted as refractive to those elements that are called diffractive. We investigate the linear blazed grating and focus on the wavelength dependence of its properties. For this we adopt an approach well known from the theory of echelette gratings. Our results can easily be transferred to other blazed components, such as Fresnel lenses.

Evaluation of microlens properties in the presence of high spherical aberration.

Microlenses can be generated with various fabrication technologies. Some of these technologies cause large spherical aberrations in the resulting microlenses. We describe an algorithm based on Rayleigh's quarter-wave criterion, which allows the evaluation of lens parameters for those microlenses. Specifically, we investigate numerical aperture, focal length, and space-bandwidth product with respect to applications in optical microsystems. We apply our algorithm to different types of microlenses, three gradient-index lenses, and one surface-relief lens. The experimental results demonstrate that our algorithm provides a helpful characterization method for microlenses with large aberrations.

Astigmatic gradient-index elements for laser-diode collimation and beam shaping.

For the conversion of light from edge-emitting laser diodes into symmetric laser beams two main tasks have to be performed: collimation and beam shaping. Generally these two jobs are performed separately. Because of the inherently different divergence angles of the emitted light, collimation with astigmatic lenses generally results in a beam with an elliptically shaped amplitude distribution. This asymmetry has to be compensated for by an anamorphic imaging step to obtain the desired spherical beam profile. It can be advantageous to combine both jobs in one element. We demonstrate the design, the fabrication, and the application of refractive gradient-index elements, which allow one to perform both jobs with a single element. Our astigmatic lenses were fabricated by silver-sodium ion exchange in glass.

Fresnel ping-pong algorithm for two-plane computer-generated hologram display.

Image formation of three-dimensional objects suffers from out-of-focus noise if the light is coherent. Nevertheless, it is possible to generate two noiseless image intensities (I(A)and I(B)) at two depth locations by means of a single computer hologram. The phases related to I(A) and I(B) provide design freedom. To accomplish this goal, we use a ping-pong algorithm that bounces back and fourth between the two planes. Our ping-pong algorithm is the fourth member of a family of algorithms. The first member is known as the Gerchberg-Saxton algorithm. Basic considerations and experimental results are presented. Details of the algorithm are explained in Appendix A.

Index-distributed planar microlenses for three-dimensional micro-optics fabricated by silver-sodium ion exchange in BGG35 substrates.

Fabrication of planar microlens arrays by silver-sodium ion exchange is possible by using a new glass type, optimized for this technology. Because of its nonlinear diffusion response it is well suited to the fabrication of microlens arrays. We show that the diffusion coefficient can be described theoretically by an exponential concentration dependence. The parameters of the planar microlenses are measured interferometrically and by imaging experiments. Because of the specific index distribution, new evaluation techniques for the determination of lens parameters from interferometric measurements have been applied. We also present a simple model that relates the achievable lens parameters to the diffusion conditions.