Band-Pass and OH-Suppression Filters for the E-ELT - Design and Prototyping

Optical filters are used for a variety of purposes at astronomical telescopes. In the near infrared region, from 0.8 to 2.5 um, bandpass and edge filters are used to separate the different astronomical channels, such as the J, H, and K bands. However, in the same wavelength range light emission generated in the earth's atmosphere is superimposed on the stellar radiation. Therefore, ground based astronomical instruments measure, in addition to the stellar light, also unwanted contributions from the earth's atmosphere. The characteristic lines of this OH emission are extremely narrow and distributed over the complete NIR spectral range. The sensitivity of future telescopes, like the European Extreme Large Telescope (E-ELT) which is currently being designed by ESO, can be dramatically improved if the atmospheric emission lines are effectively suppressed while the stellar radiation is efficiently transferred to the detector systems. For this task, new types of optical filters have to be developed. In this framework new design concepts and algorithms must be used, combining the measurement needs with practical restrictions. Certainly, the selected deposition process plays the key role in the manufacturing process. Precise and highly stable deposition systems are necessary to realise such filter systems with an appropriate homogeneity. Moreover, the production control techniques must be adapted to match the high level of precision required in the NIR range. Finally, the characterisation set-ups for such filters systems have to be provided. The manufacturing of such a filter system for a feasibility study of an E-ELT instrument is presented. The design development, the deposition with adapted Ion Beam Sputtering deposition plants, and the characterisation of such filters in the J-Band is described.Comment: contribution to SPIE 8168 on 'Advances in Optical Thin Films IV', Marseille 201

Dynamic liquefaction of shear zones in intact loess during simulated earthquake loading

The 2010-2011 Canterbury earthquake sequence in New Zealand exposed loess-mantled slopes in the area to very high levels of seismic excitation (locally measured as >2 g). Few loess slopes showed permanent local downslope deformation, and most of these showed only limited accumulated displacement. A series of innovative dynamic back pressured shear-box tests were undertaken on intact and remoulded loess samples collected from one of the recently active slopes replicating field conditions under different simplified horizontal seismic excitations. During each test, the strength reduction and excess pore water pressures generated were measured as the sample failed. Test results suggest that although dynamic liquefaction could have occurred, a key factor was likely to have been that the loess was largely unsaturated at the times of the large earthquake events. The failure of intact loess samples in the tests was complex and variable due to the highly variable geotechnical characteristics of the material. Some loess samples failed rapidly as a result of dynamic liquefaction as seismic excitation generated an increase in pore-water pressure, triggering rapid loss of strength and thus of shear resistance. Following initial failure, pore pressure dissipated with continued seismic excitation and the sample consolidated, resulting in partial shear-strength recovery. Once excess pore-water pressures had dissipated, deformation continued in a critical effective stress state with no further change in volume. Remoulded and weaker samples, however, did not liquefy, and instead immediately reduced in volume with an accompanying slower and more sustained increase in pore pressure as the sample consolidated. Thereafter excess pressures dissipated and deformation continued at a critical state. The complex behaviour explained why, despite exceptionally strong ground shaking, there was only limited displacement and lack of run-out: dynamic liquefaction was unlikely to occur in the freely draining slopes. Dynamic liquefaction however remained a plausible mechanism to explain loess failure in some of the low-angle toe slopes, where a permanent water table was present in the loess

Measurement of the nonlinear refractive index in optical thin films

Based on the z-scan method, an interferometric set-up for measuring the optical Kerr-effect was engineered and optimized. Utilizing a Mach-Zehnder configuration, the wave front deformation caused by the Kerr induced selffocusing is monitored. Fitting this deformation to a theoretical approach basing on a beam propagation model, the nonlinear refractive index is obtained. The procedure can be applied to measure the nonlinear refractive index of both, the substrate material as well as the deposited dielectric layer on top of the substrate. The nonlinear refractive index of a layer specially deposited for this purpose as well as for several substrate materials was measured and the results presented. © 2017 SPIE

Determination of the laser-induced damage threshold of polymer optical fibers

Investigating the properties of manufactured polymer optical fibers is essential to determine proper areas of application. Using pulsed laser radiation, especially with respect to laser activity in optical fibers, the maximum acceptable transmittable energy without inducing damage is of particular interest. Therefore, this work is related to laser-induced damage in polymer optical fibers at a wavelength of 532 nm and a pulse duration of 7.3 ns. In particular, the influence of the coupling condition on the transmittable pulse energy and the damage behavior applying an R-on-1 test procedure are analyzed in this study. The obtained results give information about the long-Term behavior and will be used to optimize the manufacturing process. © COPYRIGHT SPI

Determination of the laser-induced damage threshold of polymer optical fibers

Investigating the properties of manufactured polymer optical fibers is essential to determine proper areas of application. Using pulsed laser radiation, especially with respect to laser activity in optical fibers, the maximum acceptable transmittable energy without inducing damage is of particular interest. Therefore, this work is related to laser-induced damage in polymer optical fibers at a wavelength of 532 nm and a pulse duration of 7.3 ns. In particular, the influence of the coupling condition on the transmittable pulse energy and the damage behavior applying an R-on-1 test procedure are analyzed in this study. The obtained results give information about the long-Term behavior and will be used to optimize the manufacturing process. © COPYRIGHT SPI

Laser-induced pit formation in UV-Antireflective coatings

Previous studies have shown that nanometer scale defects can lead to the formation of submicrometer craters, if located in coatings with a relatively small thickness. Due to the small size, such damages are challenging to detect in the online and offline damage detection and may therefore lead to an overestimation of the LIDT for the tested optical component. However, the influence of these nanopits on the optical properties and the impact on the initiation of catastrophic damage was not investigated in detail in the past. In order to study the correlation between nanopits, optical properties and catastrophic damage, samples with an AR-coating were fabricated by means of ion beam sputtering (IBS) and tested for their laser resistance by LIDT raster scans in the nanosecond regime at 355 nm. The generation and morphology changes of the nanopits were monitored for different pulse numbers and in dependence of the starting fluence. In addition to the inspection with an optical microscope in differential interference contrast (DIC) mode as prescribed by ISO 21254, alternative inspection methods, for example, dark field microscopy and scanning electron microscopy (SEM), were used to detect the nanopits. The damage test revealed that nanopits occur rarely in standard AR-coatings and possess only a small relevance for the LIDT. The typical damage morphology observed consisted of micrometer-sized pits which exhibited a stable size over a large fluence range and no growth after repeated irradiation. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only

Laser-induced pit formation in UV-Antireflective coatings

Previous studies have shown that nanometer scale defects can lead to the formation of submicrometer craters, if located in coatings with a relatively small thickness. Due to the small size, such damages are challenging to detect in the online and offline damage detection and may therefore lead to an overestimation of the LIDT for the tested optical component. However, the influence of these nanopits on the optical properties and the impact on the initiation of catastrophic damage was not investigated in detail in the past. In order to study the correlation between nanopits, optical properties and catastrophic damage, samples with an AR-coating were fabricated by means of ion beam sputtering (IBS) and tested for their laser resistance by LIDT raster scans in the nanosecond regime at 355 nm. The generation and morphology changes of the nanopits were monitored for different pulse numbers and in dependence of the starting fluence. In addition to the inspection with an optical microscope in differential interference contrast (DIC) mode as prescribed by ISO 21254, alternative inspection methods, for example, dark field microscopy and scanning electron microscopy (SEM), were used to detect the nanopits. The damage test revealed that nanopits occur rarely in standard AR-coatings and possess only a small relevance for the LIDT. The typical damage morphology observed consisted of micrometer-sized pits which exhibited a stable size over a large fluence range and no growth after repeated irradiation. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only