Light scattering due to interface and coating imperfections is a significant concern for optical components, while on the other hand, scattered light contains valuable information about its source. This turns scattering based techniques into excellent tools for the characterization of surfaces and thin film coatings.
At Fraunhofer IOF, angle resolved light scattering techniques are developed and used for the characterization of optical surfaces, coatings, and components for a broad range of applications. Examples will be shown, such as the analysis of ultra-low optical losses of an ultra-high reflecting mirror. Beyond that, the non-contact, fast, and robust measurement approach makes the technique even suitable for integration into fabrication processes or test environments. We show approaches for integration of a light scattering sensor into a roll-to-roll process for fabrication of colorshift foil by evaporation, as well as the sensor integration into even a magnetron sputtering coating system.
Even high-end optical components exhibit small amounts of imperfections, which can easily limit the performance of optical systems with respect to imaging contrast, optical throughput, imaging ghosts, and increased light scattering. Characterizing the scattering properties of optical components is thus an important step during the development of sophisticated optical systems as well as to identify and steadily improve materials as well as manufacturing and assembling steps. This is illustrated for different optical components as well as optical systems. Furthermore, different characterization concepts are discussed, which allow overcoming typical limits for angles resolved light scattering measurements, such as scattering very close to the specular beam directions (off specular scattering angles < 0.1°) or measurements in retro-reflection, which are important for gratings used in Littrow configuration or optical mirrors for laser-based communication
Subsurface damage (SSD) in optical components is almost unavoidably caused by mechanical forces involved during grinding and polishing and can be a limiting factor, particularly for applications that require high laser powers. In this contribution, non-destructive characterization techniques are evaluated with respect to their capability to determine SSD in fused silica. For this, differently polished surfaces with different SSD levels have been prepared. An initial destructive analysis using etching in hydrofluoric acid in combination with white light interferometry revealed a high amount of SSD in one of the sample types compared to a very low amount of SSD in a second one. It is shown that nondestructive absorption as well as scattering measurements are sensitive towards SSD related differences in the samples. Finally, laser-induced damage tests proved a significant impact of SSD on the laser stability by determining a reduced damage threshold of 31 ± 3 J/cm² for the sample with high amount of SSD compared to 45 ± 5 J/cm² for the high-quality polished sample.
Droplet based microfluidic technology is a miniaturized platform for microbial analysis on picoliter scale. With its costefficiency, high-throughput and feasibility of complex handling protocols, droplet microfluidics is a favorable platform for applications such as microorganism screening or synthetic biology. Scattered-light-based microbial detection, in comparison to the widely used fluorescent-label-based approach, provides a contact-free and label-free, yet sensitive measuring solution. The angular dependency of scattered light delivers an elaborate information about the morphology and the physical properties, e.g. size and refractive index, of microbial samples. Due to the complexity and ambiguity of the droplet contents, an angle resolved scattered light detection system could provide powerful method for a label-free identification and quantification of the microbes in droplets. In this paper, a novel approach of light scattering measurement in Polydimethylsiloxane (PDMS) microfluidic chips is presented, engaging optical fibers for a light-scattering-based on-chip microbial detection. Optical fibers, with their fast readout and compact size, are very suitable for easier system integration towards flexible and versatile lab-on-a-chip applications.
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