JEOS:RP - Journal of the European Optical Society Rapid publications
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Characterization of a waveguide Mach-Zehnder interferometer using PDMS as a cover layer
Full text linkA Mach-Zehnder interferometer made with shallow rib waveguides is studied experimentally and using simulations. The rib-height giving single-mode guidance is found as function of core thickness and polarization. Devices have been made using shallow rib waveguides (5 nm rib height) in silicon nitride. The sensitivity and the limit of detection (LOD) is studied experimentally regarding the length of the sensing window and for two cover media: water with hydrochloric acid (HCl) and polydimethylsiloxane (PDMS). For HCl-solution, which is the standard method for testing Mach-Zehnder interferometers, the measured sensitivity and LOD was 13000Ï€ rad/RIU and 1.1x10^-7Ï€ RIU, respectively, for a 2 cm long sensing window. This is comparable to the best results reported previously. With PDMS as the cover medium, the temperature of the device was increased in order to measure the sensitivity. This is a new approach that makes it possible to measure the sensitivity with a solid cover medium which has a relatively high temperature coefficient for the refractive index. Measured sensitivity and LOD was 15200Ï€ rad/RIU and 1.3x10^-7Ï€ RIU, respectively, again for a 2 cm long sensing window. Measured sensitivities agreed with simulations and increased linearly with sensing length as expected. However, the LOD showed a minimum for 2 cm sensing length. This was mainly due to increased noise for 3 cm sensing length, both for HCl-solution and PDMS. With higher sensitivity and similar LOD for PDMS compared to HCl-solution, it is concluded that using the temperature dependence of PDMS is a good alternative for testing Mach-Zehnder interferometers
Kinematic evaluation of the whiffletree lateral support setup and its application for the mirror support of the TMT tertiary mirror
Full text linkWith increasing size of the reflecting mirror apertures in telescopes, the lateral support is becoming more and more important. In order to find a setup satisfying the requirements for the TMT tertiary mirror, based on the classic three-point flexure support setup, a twelve-point lateral support setup was investigated its basic internal relations and properties were investigated. The virtual equivalent circle concept is then proposed to evaluate different potential setups. By studying the relationship between the virtual equivalent circle radius and the thermal deformation and resonant frequency, it can be concluded that a larger virtual equivalent circle radius results in a smaller the thermal deformation and a higher resonant frequency. Preliminary research results suggested that the TMT tertiary mirror surface figure error and thermal deformation requirements can easily be met. However, it was more difficult to obtain resonant frequencies higher than 15 Hz. Based on the virtual equivalent circle concept, the TMT tertiary mirror support setup was optimized, resulting in a resonant frequency of 17.7 Hz, which satisfies the requirements. The whiffletree lateral support setup can be applied to similar reflecting mirror support structures
Photopolymer-based volume holographic optical elements: design and possible applications
Full text linkIn this paper, Volume Holographic Optical Elements (V-HOEs), such as holographic gratings and spherical lenses, are designed and fabricated by using a prototype of photopolymer. The recording process of V-HOEs and their appropriate characterization are described. Moreover, V-HOEs possible applications as solar concentrator are investigated and results are discussed. Finally, a system that allows passive solar tracking is proposed and preliminary results are reported
Near-infrared light absorption and scattering based on a mono-layer of gold nanoparticles
Full text linkWe report fabrication and characterization of large-area ultrathin near-infrared light absorbers and scatterers based on a mono-layer of gold nanoparticles laying on top of a dielectric spacer and an aluminum reflector. The nanoparticles are formed through thermal annealing of an evaporated continuous gold film. Through optimization of initial gold-film thickness, spacer thickness, as well as annealing temperature we obtained samples that exhibit very low (~2%) broadband specular reflectance at near-infrared (NIR) wavelength range. By considering also diffuse reflection, we identify that the low specular reflectance can be due to either relatively high light absorption (~70%) or high light scattering (over 60%), with the latter achieved for samples having relatively sparse gold nanoparticles. Both strong absorption and scattering of NIR light are not inherent properties of the bulk materials used for fabricating the samples. Such composite optical surfaces can potentially be integrated to solar-energy harvesting and LED devices
Optical amplification and stability of spiroquaterphenyl compounds and blends
Full text linkIn this contribution, we present a systematic investigation on a series of spiroquaterphenyl compounds optimised for solid state lasing in the near ultraviolet (UV). Amplified spontaneous emission (ASE) thresholds in the order of 1 μJ/cm^2 are obtained in neat (undiluted) films and blends, with emission peaks at 390±1 nm for unsubstituted and meta-substituted quaterphenyls and 400±4 nm for para-ether substituted quaterphenyls. Mixing with a transparent matrix retains a low threshold, shifts the emission to lower wavelengths and allows a better access to modes having their intensity maximum deeper in the film. Chemical design and blending allow an independent tuning of optical and processing properties such as the glass transition
Point diffraction interferometry to measure local curvatures and caustics of noisy wave fronts: Application for determining optical properties of fish lenses
Full text linkThe study of caustics is important because they contain information about the image formation properties of optical systems. In this work we use the concept of caustic as a set of focal points, and we have developed a second order approach theory to determine local slopes and curvatures of a wavefront emerging from an optical system. The method is based on the use of a point diffraction interferometer, and the analysis of the interferograms allows us to compute the focal region. Experimental results obtained with a plano-convex lens demonstrate the accuracy of the combined theoretical-experimental method here developed. Application to noisy wavefronts such as those produced by biological samples, specifically in crystalline lenses of fish eyes, are also exposed
Comparative theoretical analysis between parallel and perpendicular geometries for 2D particle patterning in photovoltaic ferroelectric substrates
Full text linkThis paper describes the dielectrophoretic potential created by the evanescent electric field acting on a particle near a photovoltaic crystalsurface depending on the crystal cut. This electric field is obtained from the steady state solution of the Kukhtarev equations for thephotovoltaic effect, where the diffusion term has been disregarded. First, the space charge field generated by a small, square, light spotwhere d <<Â l (being d a side of the square and l the crystal thickness) is studied. The surface charge density generated in both geometriesis calculated and compared as their relation determines the different properties of the dielectrophoretic potential for both cuts. The shapeof the dielectrophoretic potential is obtained and compared for several distances to the sample. Afterwards other light patterns are studiedby the superposition of square spots, and the resulting trapping profiles are analysed. Finally the surface charge densities and trappingprofiles for different d/l relations are studied
Automated characterization and quantification of hydrocarbon seeps based on frontal illuminated video observations
Full text linkHydrocarbon releases, either natural or due to anthropogenic activities, are of major relevance for the marine environment. In this work we specify our approach to quantify these seeps by subsea imaging utilizing camera systems and frontal illumination setups on board remotely operated vehicles. This work showcases, based on a campaign in the region west of Svalbard, improved methodological guidelines for the seep quantification operation together with a novel automated post-mission evaluation. The comparison of automated quantification with manual information extraction illustrates the efficiency of this new method while processing comparable estimates of seep characteristics
Spectrofluorometric characteristics of fluorescent dissolved organic matter in a surface microlayer in the Southern Baltic coastal waters
Full text linkThis paper presents results of characterization of Dissolved Organic Matter (DOM) using fluorescence spectroscopy in the surface microlayers (SML) and subsurface layers (SS) in the Baltic Sea. Samples for spectroscopic measurements were collected during five research cruises in April/May and October 2013 and 2014 in a surface microlayer and a subsurface layer at a depth of 1 m along two transects from the river outlets to the open sea. The first transect was located from the Vistula River outlet to the Gdansk Deep and the second transect was located ´ from the Åeba River outlet to SÅ‚upsk Furrow. Results indicated that DOM fluorescence intensity in the SML is higher by 20% compared to the SS. The Humification Index, HIX values were lower in SML than SS by 13%. That indicates that SML is depleted in molecules with high molecular weight and higher aromaticy. The inverse relationship of fluorescence intensity of dominant peaks with salinity both in SML and SS suggests that FDOM variability is regulated mostly by terrestrial DOM input
Speckle reduction in double-pass retinal images using variable-focus lenses
Full text linkSpeckle arises in double-pass images when coherent light is scattered by the retina. Since this noise degrades the images that are used to characterize the eye, there is special attention in reducing speckle when working with instruments based on retina reflections. In this work, we present a method for speckle reduction in double-pass retinal images by producing minor periodic variations in the vergence of the beam entering the eye with a variable-focus lens during image recording. Measurements in an artificial and a real eye following the implementation of the method corroborate the speckle reduction