Roman Kleindienst

Hybrid optical (freeform) components—functionalization of nonplanar optical surfaces by direct picosecond laser ablation

The performance of optical systems is typically improved by increasing the number of conventionally fabricated optical components (spheres, aspheres, and gratings). This approach is automatically connected to a system enlargement, as well as potentially higher assembly and maintenance costs. Hybrid optical freeform components can help to overcome this trade-off. They merge several optical functions within fewer but more complex optical surfaces, e.g., elements comprising shallow refractive/reflective and high-frequency diffractive structures. However, providing the flexibility and precision essential for their realization is one of the major challenges in the field of optical component fabrication. In this article we present tailored integrated machining techniques suitable for rapid prototyping as well as the fabrication of molding tools for low-cost mass replication of hybrid optical freeform components. To produce the different feature sizes with optical surface quality, we successively combine mechanical machining modes (ultraprecision micromilling and fly cutting) with precisely aligned direct picosecond laser ablation in an integrated fabrication approach. The fabrication accuracy and surface quality achieved by our integrated fabrication approach are demonstrated with profilometric measurements and experimental investigations of the optical performance.

Computer-aided manufacturing for freeform optical elements by ultraprecision micromilling

The successful fabrication of several freeform optical elements by ultraprecision micromilling is presented in this article. We discuss in detail the generation of the tool paths using different variations of a computer-aided manufacturing (CAM) process. Following a classical CAM approach, a reflective beam shaper was fabricated. The approach is based on a solid model calculated by optical design software. As no analytical description of the surface is needed, this procedure is the most general solution for the programming of the tool paths. A second approach is based on the same design data. But instead of a solid model, a higher order polynomial was fitted to the data using computational methods. Taking advantage of the direct programming capabilities of state-of-the-art computerized numerical control units, the mathematics to calculate the polynomial based tool paths on-the-fly during the machining process are implemented in a highly flexible CNC code. As another example for this programming method, the fabrication of a biconic lens from a closed analytical description directly derived from the optical design is shown. We provide details about the different programming methods and the fabrication processes as well as the results of characterizations concerning surface quality and shape accuracy of the freeform optical elements.

Parallelized chromatic confocal sensor systems

In this paper we present chromatic confocal distance sensors for the parallelized evaluation at several lateral positions. The multi-point measurements are performed using either one- or two-dimensional detector arrays. The first sensor combines the concepts of confocal matrix sensing and snapshot hyperspectral imaging to image a two-dimensional array of laterally separated points with one single shot. In contrast to chromatic confocal matrix sensors which use an RGB detector our system works independently from the spectral reflectivity of the surface under test and requires no object-specific calibration. Our discussion of this sensor principle is supported by experimental results. The second sensor is a multipoint line sensor aimed at high speed applications with frame rates of several thousand frames per second. To reach this evaluation speed a one-dimensional detector is employed. We use spectral multiplexing to transfer the information from different measurement points through a single fiber and evaluate the spectral distribution with a conventional spectrometer. The working principle of the second sensor type is demonstrated for the example of a three-point sensor.

Highly efficient refractive Gaussian-to-tophat beam shaper for compact terahertz imager

We describe an efficient and compact terahertz (THz) illumination system as part of an active THz imager. The design and fabrication methods are verified by comparing measurements with wave optical simulations that form a basis for the design of a Gaussian-to-tophat beam shaper. Introducing realistic alignment tolerances in the simulations led to an improved agreement with the experimental data. Furthermore, we propose a method to reduce the alignment effort for THz beam shaping elements by correlating measured results with simulated data of a misaligned system.

Spectrally multiplexed chromatic confocal multipoint sensing

We present a concept for chromatic confocal distance sensing that employs two levels of spectral multiplexing for the parallelized evaluation of multiple lateral measurement points; at the first level, the chromatic confocal principle is used to encode distance information within the spectral distribution of the sensor signal. For lateral multiplexing, the total spectral bandwidth of the sensor is split into bands. Each band is assigned to a different lateral measurement point by a segmented diffractive element. Based on this concept, we experimentally demonstrate a chromatic confocal three-point sensor that is suitable for harsh production environments, since it works with a single-point spectrometer and does not require scanning functionality. The experimental system has a working distance of more than 50 mm, a measurement range of 9 mm, and an axial resolution of 50 μm.

Integrated microsystems for optical sensing and imaging applications

Compact optical systems generally form the backbone of integrated optoelectronic microsystems. Miniaturization as well as integration requirements result in system configurations with folded optical axis such as in planar integrated freespace optics. For optimum performance in such systems geometries, the surface profiles of the corresponding optical elements deviate from classical spherical or aspherical shapes. Optimized plane-symmetric or freeform optical elements are required instead. We discuss design, fabrication and characterization of freeform optical elements for the integration of optical microsystems. The systems performance is demonstrated for imaging as well as sensor applications.

Thermographischer Detektor basierend auf einem neuartigen Mikro-Spiegel Sensor

Zusammenfassung Wir berichten über die Entwicklung eines Wärmebildgeräts, das auf einem neuartigen, patentierten Sensor basiert. Der Sensor besteht aus einer Matrix aus Mikro-Spiegeln für deren Betreib keine Energiezufuhr in Form von Strom oder Spannung und darüber hinaus keine Kühlung notwendig ist. Die Mikro-Spiegel verbiegen sich proportional zur aufgenommen Wärmestrahlung (Infrarot-Intensität), vergleichbar mit einem bi-Metall-Thermometer. Die Verbiegung des einzelnen Mikro-Spiegels wird durch Auslenkung eines, an dem Spiegel reflektierten Kaltlichtstrahles von einem konventionellen CCD-Sensor erfasst und in Echtzeit-Wärmebilder umgewandelt. Das System besteht aus optischen Standardkomponenten. Durch Dünnschichttechnologien und CMOS-Kompatibilität des Sensors besitzt diese Innovation klare Kostenvorteile gegenüber aktuellen Infrarotdetektoren.

Iterative design process for highly efficient optical trapping systems

The development of an optical system for trapping particles in air is presented. Based on optics design as well as optical force simulations an efficient optical system is designed, manufactured and characterized.

Three-dimensional laser ablation for functionalization of non-planar optical surfaces

Efficient laser beam shaping is one of the keys to achieve maximum process performance and to exploit new application fields for lasers. However, with increasingly more complex beam shaping tasks typically the challenges related to system size, weight and reliability scale up as more and more conventional optical components like spherical lenses, gratings, prisms etc. are involved. To overcome this traditional tradeoff several optical functions can be integrated in a single hybrid [1] [2] (reflective/diffractive, refractive/ diffractive [3]) optical freeform component enabling miniaturized [4] and robust beam shaping solutions. For the fabrication of these complex optical surfaces we use integrated fabrication techniques (section 2), which combine micromachining of continuous reflective freeform surfaces [5] and precise laser ablation of high frequency diffractive structures efficiently [6]. In this article we focus on the generation of high quality diffractive structures in copper using 3-dimensional pic...

Synthetic design and integrated fabrication of multifunctional hybrid beam shapers

The performance of optical systems is typically improved by adding conventional optical components which is automatically connected to an increasing system size and weight. Hybrid optical freeform components can help to overcome this traditional tradeoff by designing a single complex optical surface that performs several optical functions at once. In this article we present the synthetic design and integrated fabrication of a reflective hybrid beam shaper offering beam deflection, transformation and splitting capabilities. The shape accuracy and surface quality of the component are demonstrated with profilometric measurements. Experimental investigations of the optical performance verify the suitability of the applied fabrication methods and design approach.