Adaptive optics
In the Laboratory for Microactuators, we approach adaptive optics from the point of view of the actuation, of course together with the optical performance and requirements. Our research portfolio addresses most optical elements, such as various types of tunable lenses, axicon mirrors and lenses, freeform optical elements, apertures, and gratings. For light sensitive high-resolution applications, pure micro optics often do not provide sufficient aperture, so we focus more on the millimetre rather than the micrometre scale. The materials that we use include elastomers such as silicones (PDMS) but also for example flexible glass membranes and fluids. To integrate the actuation intrinsically into our devices, we frequently use piezo ceramics as they tend to be fast and efficient. We have developed extensive know-how to process these materials and integrate them into the fabrication. In particular, we frequently combine classical and rapid-prototyping MEMS and precision mechanics techniques, for example UV-laser structuring, micro-moulding but also dedicated process solutions.
Adaptive lenses | Dr. Matthias Wapler, Florian Lemke
Tunable lenses are a long-running core topic in our research on adaptive optics. The program started initially with piezo-driven PDMS-membrane lenses, which also triggered our research into piezo actuators and silicone material properties, chemical compatibility and processing. Since then, we have optimized and simplified the design and fabrication and extended the concept into zoom lenses and apertures and added elastic biomimetic functional surface nanostructures.
One of our recent innovations was the application of ultra-thin glass as a membrane material. On the one hand, this gives greater chemical stability and mechanical robustness, allowing for new refractive media and mobile applications. On the other hand, the stiff membrane allows for new actuation concepts. Our active glass-piezo composite lens membranes enable ultra-compact lenses with sub-millisecond response times and aspherical aberration correction. Current research includes the optimization of the glass membrane lenses with elastomer and fluid refractive media for higher speed, quality and fabrication reproducibility, higher order aberration correction and greater robustness.
The characteristic advantages of the lenses are reflected in the applications, where we have several cooperation projects with academia and industry on MR-compatible optics, microscopy, high-speed scanning and industry 4.0.
MR-compatible microscopy | Dr. Matthias Wapler
MR microscopy has two major limiting factors: On the one hand, the resolution of conventional techniques is limited to around 10µm and on the other hand, scans may take several hours. The latter is a major problem when studying living cells or tissue or even growth but may be compensated by real-time motion correction. Another problem is the clear identification of structures in the MR image. All these issues may be solved by concurrent observation with an optical microscope. This, however, causes severe technical challenges as the MR scanners have limited space, a large static magnetic field and strong electromagnetic fields. Hence, a usual microscope would not operate properly inside the scanner – or even destroy it. On the other hand, the MR imaging requires an extremely homogeneous magnetic field and low electromagnetic background.
We have developed a fully MR-compatible multi-purpose microscope with flexible illumination and magnification. Fitting in the just 72mm diameter of a 9.4T “small animal scanner”, we obtained a sub-micrometer resolution and a magnetic distortion by just 19 parts per billion. A key solution is the focusing with a custom-built adaptive lens, in addition to the optical and mechanical design and the material selection. Currently, we are developing a miniaturized microscope with just 10mm outer diameter that can operate in an 11.7T MR-spectrometer.
Adaptive Bessel beams for the neurosciences | Dr. Angelina Müller
The University’s cluster of excellence BrainLinks-BrainTools copes with the development of technical interfaces to the brain for various kinds of stimulation. Within that cluster we aim to develop an implantable device for optogenetic stimulation of cortical areas in freely behaving small animals. Optogenetics is a neuromodulation technique to control the activity of individual neurons in living tissue and to precisely measure the response in real-time.
We are developing small devices that enable the independent stimulation of multiple channels using an integrated system of laser diodes and micro-optical elements in one small package. In contrast to existing systems, our device will not penetrate the brain mechanically, but can be implanted above the dura mater into the skull. In particular, we will not only create a 2D array that allows to stimulate different regions, but we also try to stimulate selectively different depths, allowing for full 3D stimulation. This will allow biologists to study the neurological response of cortical networks non-invasively with high spatial and temporal resolution.
One core technology is the combination of laser diodes combined with refractive conically shaped lenses (axicons) to shape light beams or light columns with an extended focal zone. These so-called Bessel beams proved to have enhanced penetration depth into scattering tissue because of their -as sometimes referred to- self healing properties. For this purpose, we are developing a novel fabrication process to cast and mold transmissive aspherical and axicon lens arrays with mm-sized elements.
Tunable mirrors with free-form surfaces | Binal Bruno
Since four years we are working on new concepts for tunable conical mirrors and lenses, called axicons. We have presented for the first time axicon mirrors with a tunable angle which are used for beam shaping of ultra-short pulse lasers. This research is part of an interdisciplinary collaboration with the “Max-Born-Institute for Short Pulse Spectroscopy (MBI)” in Berlin.
In addition rotational symmetric axicons we have also demonstrated elliptical tunable axicons as well as spiral phase plates for the adaptive shaping of hollow beams with tunable topological charge.
Further research in this field focusses on the development of Fresnel Mirrors which are linear axicon elements. They can be used to generate interference patterns with the pattern size depending on the wavelength of the incident light and the tilting angle of the mirror.
Switchable gratings | Dr. Matthias Wapler
Optical gratings can be used for the dispersion of light, for example for optical spectroscopy. Various principles exist for tuning effective grating constants, however, there are only few approaches for tuning grating efficiencies, i.e. dynamically adjusting the intensity in the diffraction orders. This behaviour can be useful in hyperspectral imaging systems, where there is a need to switch between a lateral and spectral mode, but could also be used for variable optical attenuators, fibre switches or projection systems.
We are investigating several working principles of such efficiency adaptive gratings. One of these is the compression of elastic gratings which we produce by replication processes ourselves. The mechanical deformation of the grating teeth which we realize with a piezo stack actuator leads to an adjustable diffraction efficiency. Another concepts is the variation of the refractive index of the surrounding medium. The actuation can be achieved for example by pumping a fluid over the grating. Our latest approach is the use of moiré patterns, that gives large effects from small displacements.
Lenses for Industry 4.0 | Dr. Matthias Wapler
Tunable lenses are a good choice for adjusting industrial vision sensors, smart cameras, etc. to various working distances and fields of view because of their compact size and fast response times. An issue which needs to be addressed in this context is the robustness of the devices, especially long-term, temperature and vibration stability. We have already proven long-term stability for our silicone-membrane lenses which are still working after actuating them over 100 million cycles in one year.
For a current research project we aim on cycle-reproducible characteristics which can be achieved by controlling the lenses with a feedback signal of a lens-integrated sensor, like pressure or strain. Therefore, a distinct focal power can be approached by the lens.
At the moment, we are working in a collaborative project together with Hahn-Schickard Gesellschaft to build a smart camera platform that features a variable focus lens with integrated control and power management.