Anwendung von MHz-fs-Lasern in der Ophthalmologie und Erarbeitung eines Therapiekonzeptes für die laserassistierte Behandlung der Alterssichtigkeit

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Optical coherence tomography for laser transmission joining processes in polymers and semiconductors

Laser transmission welding applications become wide spread technology in joining transparent and turbid materials. The requirement of higher throughputs and lesser rejects support the need of alternative methods to commonly used machine vision or temperature measurements. Optical coherence tomography delivers the opportunity to qualitatively assess transmission welding seems and welding spots even in three dimensions. Fourier Domain Mode Lock Lasers (FDMLs) used in OCT enable high throughput by high sweep rates. Preliminary results of OCT used for qualitatively assessment of laser transmission welded polymers and fiber reinforced plastics are shown as well as first approaches of using OCT in a silicon chip bonding process

Hydrogels for light delivery in in vivo optogenetic applications

Biocompatible hydrogels present interesting opportunities for in vivo waveguiding for optogenetic or photomedical applications. Here, we investigate the applicability of poly(ethylene glycol) diacrylate hydrogels in combination with scattering particles as optical diffusors. Gel characteristics and bioactivity can be tuned to achieve controlled light distribution and tissue interaction

Wavefront sensorless adaptive optics for optical coherence tomography guided femtosecond laser surgery in the posterior eye

Surgery with fs-laser in the posterior part of the eye could be useful for separation of tractional epiretinal membrane and vitreous floaters treatment. However, focus degradation occurs near the retina due to induced aberrations by cornea and lens. To overcome this issue, adaptive optics with wavefront sensor and wavefront modulator can be utilized. We demonstrate an alternative concept for image guided femto second laser (fs-laser) surgery in the posterior eye with wavefront sensorless adaptive optics (WFSLAO). Our laboratory setup consists of an 800 nm fs-laser and a superluminescent diode (SLD) with 897.2 nm central wavelength. The SLD is used for optical coherence tomography (OCT) whereby the light for the OCT sample arm and the fs-laser share the same optical path which contains a deformable mirror, scanner and focusing optics. Energy calibrated photodiodes are used to measure the threshold energy for a laser induced optical breakdown inside a water filled chamber that acts as simple eye model. OCT image based metrics were used to determine a set of Zernike polynomials that describe a near optimal deformable mirror state. Such a mirror state improved OCT resolution and at the same time lowered the required fs-laser energy for a laser induced optical breakdown inside the eye model substantially. © COPYRIGHT SPIE

Nonlinear laser scanning microscopy of oral multispecies-biofilms: Fixative induced fluorescence as a fast and economical in vitro screening method

In this letter we report a fast and easy method which could be used for initial screening of multispecies-biofilm development on putative new dental implant materials. Most staining methods require numerous washing steps that can result in detachment of loosely bound biofilms and therefore falsify the results. Thus, we used glutaraldehyde fixation, which induces autofluorescence through bacterial membrane protein cross-linking and concurrently stabilizes the biofilm structure. We analyzed the biofilms with nonlinear laser scanning microscopy and were able to (I) evaluate the multispecies-biofilm growth and (II) distinguish between bacterial species based on different two-photon autofluorescence intensities. © 2016 by De Gruyter

Gold nanoparticle-mediated laser stimulation causes a complex stress signal in neuronal cells

Gold nanoparticle mediated laser stimulation of neuronal cells allows for cell activation on a single-cell level. It could therefore be considered an alternative to classical electric neurostimulation. The physiological impact of this new approach has not been intensively studied so far. Here, we investigate the targeted cell's reaction to a laser stimulus based on its calcium response. A complex cellular reaction involving multiple sources has been revealed. © 2017 SPIE-OSA

Finite element study of the accommodation behaviour of the crystalline lens after fs-laser treatment

With aging the ability of the crystalline lens to adapt to different viewing distances decreases. Until now there is no satisfying treatment available. It is possible to influence the deformability of the crystalline lens by inducing cuts using femtosecond (fs)-laser. To test the influence of several different cutting geometries is expensive and a huge amount of crystalline lenses is needed. Finit Element Method offers the possibility to test the influence of different cutting geometries on the flexibility. Therefore, it is necessary to develop a method to simulate the cuts in an adequate way. In first simulations cuts were assumed to be isotropic inter layers. Later on, effective material properties for the inter layer were calculated with a homogenization technique. The results confirm the influence of the cuts on the flexibility.DFG/BE3990-

Tile-Based Two-Dimensional Phase Unwrapping for Digital Holography Using a Modular Framework

A variety of physical and biomedical imaging techniques, such as digital holography, interferometric synthetic aperture radar (InSAR), or magnetic resonance imaging (MRI) enable measurement of the phase of a physical quantity additionally to its amplitude. However, the phase can commonly only be measured modulo 2π, as a so called wrapped phase map. Phase unwrapping is the process of obtaining the underlying physical phase map from the wrapped phase. Tile-based phase unwrapping algorithms operate by first tessellating the phase map, then unwrapping individual tiles, and finally merging them to a continuous phase map. They can be implemented computationally efficiently and are robust to noise. However, they are prone to failure in the presence of phase residues or erroneous unwraps of single tiles. We tried to overcome these shortcomings by creating novel tile unwrapping and merging algorithms as well as creating a framework that allows to combine them in modular fashion. To increase the robustness of the tile unwrapping step, we implemented a model-based algorithm that makes efficient use of linear algebra to unwrap individual tiles. Furthermore, we adapted an established pixel-based unwrapping algorithm to create a quality guided tile merger. These original algorithms as well as previously existing ones were implemented in a modular phase unwrapping C++ framework. By examining different combinations of unwrapping and merging algorithms we compared our method to existing approaches. We could show that the appropriate choice of unwrapping and merging algorithms can significantly improve the unwrapped result in the presence of phase residues and noise. Beyond that, our modular framework allows for efficient design and test of new tile-based phase unwrapping algorithms. The software developed in this study is freely available

Femtosecond laser induced step-like structures inside transparent hydrogel due to laser induced threshold reduction

In the area of laser material processing, versatile applications for cutting glasses and transparent polymers exist. However, parasitic effects such as the creation of step-like structures appear when laser cutting inside a transparent material. To date, these structures were only described empirically. This work establishes the physical and chemical mechanisms behind the observed effects and describes the influence of process and material parameters onto the creation of step-like structures in hydrogel, Dihydroxyethylmethacrylat (HEMA). By focusing laser pulses in HEMA, reduced pulse separation distance below 50 nm and rise in pulse energy enhances the creation of unintended step-like structures. Spatial resolved Raman-spectroscopy was used to measure the laser induced chemical modification, which results into a reduced breakdown threshold. The reduction in threshold influences the position of optical breakdown for the succeeding laser pulses and consequently leads to the step-like structures. Additionally, the experimental findings were supplemented with numerical simulations of the influence of reduced damage threshold onto the position of optical breakdown. In summary, chemical material change was defined as cause of the step-like structures. Furthermore, the parameters to avoid these structures were identified

Hydrogels for targeted waveguiding and light diffusion

Advances in photomedicine and optogenetics have defined the problem of efficient light delivery in vivo. Recently, hydrogels have been proposed as alternatives to glass or polymer fibers. These materials provide remarkable versatility, biocompatibility and easy fabrication protocols. Here, we investigate the usability of waveguides from poly(ethylene glycol) dimethacrylate for targeted light delivery and diffusion. Different hydrogel compositions were characterized with regard to water content, chemical stability, elasticity, refractive index and optical losses. Differences in refractive index were introduced to achieve targeted light delivery, and scattering polystyrene particles were dispersed in the hydrogel samples to diffuse the incident light. Complex constructs were produced to demonstrate the versatility of hydrogel waveguides. © 2019 Optical Society of America