Fraunhofer-Institut für Optronik, Systemtechnik und Bildauswertung IOSB

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  • Publication
    Confocal fluorescence microscopy with high-NA diffractive lens arrays
    ( 2022)
    Li, Zheng
    ;
    Taphanel, Miro
    ;
    Längle, Thomas
    ;
    Beyerer, Jürgen
    Traditionally, there is a trade-off between the numerical aperture and field of view for a microscope objective. Diffractive lens arrays (DLAs) with overlapping apertures are used to overcome such a problem. A spot array with an NA up to 0.83 and a pitch of 75 m is produced by the proposed DLA at a wavelength of 488 nm. By measurement of the fluorescence beads, the DLA-based confocal setup shows the capability of high-resolution measurement over an area of 3mm 3mm with a 2.5 0.07 NA objective. Further, the proposed fluorescence microscope is insensitive to optical aberrations, which has been demonstrated by imaging with a simple doublet lens.
  • Publication
    3.5 W Broadband PM Hybrid Amplifier at 2051 nm with Holmium- and Thulium-Doped Single-Clad Fibers
    ( 2021)
    Tench, Robert E.
    ;
    Amavigan, Alexandre
    ;
    Romano, Clément
    ;
    Traore, Daniya
    ;
    Delavaux, Jean-Marc
    ;
    Robin, Thierry
    ;
    Cadier, Benoît
    ;
    Laurent, Arnaud
    ;
    Crochet, Patrice
    We report the design and performance of a packaged broadband PM hybrid HDFA/TDFA in the 2000-2100 nm band using all-single-clad doped fibers. Internal small signal G = 49.1 dB, small signal NF = 6.5 dB, and P out = 3.54 W are achieved at l s = 2051 nm. The simulated 3 dB output power bandwidth is 98 nm. Comparisons of experimental data and simulations show good agreement. We investigate through simulations an all-Holmium two stage PM optical amplifier design for comparison to the hybrid PM HDFA/TDFA.
  • Publication
    Influence of bandwidth error on the performance of adaptive optics systems for uncooperative beacons
    ( 2021)
    Toselli, Italo
    ;
    Gladysz, Szymon
    We discuss the capability of adaptive optics to increase the performance of laser systems operating in atmospheric turbulence. Our approach is based on the Zernike filter functions, Taylor's frozen-flow hypothesis, and bandwidth limitations of a realistic servo control system. System performance is analyzed in terms of the Strehl ratio on target. Our results for plane-wave geometry indicate that adaptive optics can be effective even when engaging fast moving targets and that moderate closed-loop bandwidths of ∼100Hz would suffice for most analyzed scenarios. Applications of interest are beam delivery systems and free-space optical communications.
  • Publication
    Fraunhofer Institute of Optronics, System Technologies and Image Exploitation: Introduction to the focus issue
    ( 2021)
    Gladysz, S.
    ;
    Beyerer, J.
    ;
    Eichhorn, M.
    This focus issue offers a glimpse into the breadth and depth of research in optics and image processing at the Fraunhofer Institute of Optronics, System Technologies and Image Exploitation (abbreviated Fraunhofer IOSB). The institute belongs to the Fraunhofer Society, which is Europe's largest organization for applied research. The society comprises 72 institutes spread throughout Germany as well as subsidiaries in several European countries, and the USA, UK, Chile and Singapore, each focusing on different fields of applied science. It is named after the Bavarian physicist Joseph von Fraunhofer (1787-1826), the inventor of the spectrometer, optical lens manufacturer and an entrepreneur who exemplified the goals of the society, i.e.: a focus on research excellence, innovation, and commitment to market and customer-oriented research.
  • Publication
    Impact of threshold assessment methods in laser-induced damage measurements using the examples of CCD, CMOS, and DMD
    ( 2021)
    Schwarz, Bastian
    ;
    Ritt, Gunnar
    ;
    Eberle, Bernd
    Based on our earlier investigations, we continued and intensified our effort on the assessment of laser-induced damage effects in the visible range on a digital micromirror device (DMD) in comparison to different electro-optical imaging sensors such as complementary metal-oxide-semiconductors (CMOS) and charge-coupled devices (CCD). The main two objectives of our current work are: i) to fill the gap for the damage threshold regarding the time scale of picosecond pulses (527 nm) for CCD and CMOS devices and ii) evaluate the performance of a new device, the DMD, with both nanosecond pulses (532 nm) and picosecond pulses (527 nm) and compare the results with those of the CCD/CMOS. In the course of this research, we improved the experimental setup. Furthermore, we characterized the damage caused by laser pulse energies exceeding the laser-induced damage threshold (LIDT). For both the CMOS and CCD cameras, we received damage thresholds of about 10mJ/cm2 (picosecond pulses). For the DMD, we obtained LIDT values of 130mJ/cm2 (nanosecond laser pulses) and 1500mJ/cm2 (picosecond laser pulses). In case of the CMOS devices, we additionally compared the appearance of the damage obtained from the output signal of the camera under test and the microscope images of the surface of the camera. The first visible changes on the surface of the sensor occurred at energy densities that are an order of magnitude higher than the threshold values related to the output signal.
  • Publication
    Mid-infrared optical parametric oscillator pumped by a high-pulse-energy, Q-switched Ho3+:YAG laser
    ( 2021)
    Griesbeck, Michael
    ;
    Büker, Hendrik
    ;
    Eitner, Madeleine
    ;
    Goth, Katharina
    ;
    Braesicke, Peter
    ;
    Eichhorn, Marc
    ;
    Kieleck, Christelle
    A high-pulse-energy mid-infrared light source is presented, based on a zinc-germanium-phosphide optical parametric oscillator (ZGP OPO) pumped by an actively Q-switched high-pulse-energy Ho3+:YAG laser. TheHo3+:YAG pump laser source is capable of generating a pulse energy of 15 mJ from a single Ho3+:YAG rod at room temperature at a pulse repetition frequency (PRF) of 700 Hz. A maximum power of 20.1 W at a central wavelength of 2090 nm can be obtained in continuous operation, with a slope efficiency of 45.1%. A good beam quality with an M2 better than 1.3 was achieved in Q-switched operation. The presented laser architecture was used as a suitable pump source for a ZGP-based OPO. Operated at a PRF of 2 kHz and pumped with a pulse energy of 8 mJ, a low conversion threshold of 1.5 W and a maximum total output power of 6.3 W could be obtained in a linear ZGP based OPO. At maximum power, the peak power of the generated mid-infrared radiation exceeded 120 kW, while the beam quality was affected by the strong gain lens building inside the nonlinear material as a consequence of the high-energy pump pulses.
  • Publication
    Laser safety assessments supported by analyses of reflections from metallic targets irradiated by high-power laser light
    ( 2021)
    Henrichsen, Michael
    ;
    Schwarz, Bastian
    ;
    Ritt, Gunnar
    ;
    Azarian, Adrian
    ;
    Eberle, Bernd
    When using kilowatt-class lasers in outdoor environments, ensuring laser safety turns out to be a complex issue due to the large safety areas that must be respected. For the special cases of collimated or focused laser radiation reflected from ideally flat but naturally rough metallic surfaces, the classical laser hazard analysis is deemed insufficient. In order to investigate the corresponding hazard areas for the aforementioned cases, we performed experiments on laser-matter interactions. Using high-power laser radiation, we studied the spatial and temporal reflection characteristics from four different metallic samples. For the evaluation of total reflection characteristics, we performed curve-fitting methods comprising Gaussian-like specular components, diffuse scattering components according to the ABg-scatter model and Lambertian components. For the investigation of occurring caustics, we developed a dedicated model in order to assess the divergence of the contained structures as a function of distance. Our evaluations have shown that the majority of the reflected power is scattered and based on these findings, that resulting nominal optical hazard distance values, even under worst-case assumptions, are significantly smaller than those of the non-reflected laser beam.
  • Publication
    High pulse energy ZnGeP2 OPO directly pumped by a Q-switched Tm3+-doped single-oscillator fiber laser
    ( 2021)
    Schneider, Julian
    ;
    Forster, Patrick
    ;
    Romano, Clément
    ;
    Eichhorn, Marc
    ;
    Kieleck, Christelle
    A mid-infrared ZnGeP2 optical parametric oscillator pumped by a Tm3+-doped fiber laser is reported, providing pulse energies of 230µJ, pulse widths of 40 ns, and peak powers of ∼6kW with excellent efficiency and beam quality. The pump source is an actively 𝑄-switched single oscillator optimized to generate high pulse energies.
  • Publication
    Straightness metric for measurement of turbulence-induced distortion in long-range imaging
    ( 2021)
    Hofmann, Julia
    ;
    Goelzer, Rilene
    ;
    Wegner, Daniel
    ;
    Gladysz, Szymon
    ;
    Stein, Karin
    Algorithms used for mitigation of the effects of atmospheric turbulence on video sequences often rely on a process for creating a reference image to register all of the frames. Because such a pristine image is generally not available, no-reference image quality metrics can be used to identify frames in a sequence that have minimum distortion. Here, we propose a metric that quantifies image warping by measuring image straightness based on line detection. The average length of straight lines in a frame is used to select best frames in a sequence and to generate a reference frame for a subsequent dewarping algorithm. We perform tests with this metric on simulated data that exhibits varying degrees of distortion and blur and spans normalized turbulence strengths between 0.75 and 4.5. We show, through these simulations, that the metric can differentiate between weak and moderate turbulence effects. We also show in simulations that the optical flow that uses a reference frame generated by this metric produces consistently improved image quality. This improvement is even higher when we employ the metric to guide optical flow that is applied to three real video sequences taken over a 7 km path.
  • Publication
    Classification of airborne 3D point clouds regarding separation of vegetation in complex environments
    ( 2021)
    Bulatov, Dimitri
    ;
    Stütz, Dominik
    ;
    Hacker, Jorg
    ;
    Weinmann, Martin
    Classification of outdoor point clouds is an intensely studied topic, particularly with respect to the separation of vegetation from the terrain and manmade structures. In the presence of many overhanging and vertical structures, the (relative) height is no longer a reliable criterion for such a separation. An alternative would be to apply supervised classification; however, thousands of examples are typically required for appropriate training. In this paper, an unsupervised and rotation-invariant method is presented and evaluated for three datasets with very different characteristics. The method allows us to detect planar patches by filtering and clustering so-called superpoints, whereby the well-known but suitably modified random sampling and consensus (RANSAC) approach plays a key role for plane estimation in outlier-rich data. The performance of our method is compared to that produced by supervised classifiers common for remote sensing settings: random forest as learner and feature sets for point cloud processing, like covariance-based features or point descriptors. It is shown that for point clouds resulting from airborne laser scans, the detection accuracy of the proposed method is over 96% and, as such, higher than that of standard supervised classification approaches. Because of artifacts caused by interpolation during 3D stereo matching, the overall accuracy was lower for photogrammetric point clouds (74-77%). However, using additional salient features, such as the normalized green-red difference index, the results became more accurate and less dependent on the data source.