JEOS:RP - Journal of the European Optical Society Rapid publications
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Analysis of hybrid dielectric-plasmonic slot waveguide structures with 3D Fourier Modal Methods
Get PDFRecently, plasmonic waveguides have been intensively studied as promising basic building blocks for the construction of extremely compact photonic devices with subwavelength characteristic dimensions. A number of different types of plasmonic waveguide structures have been recently proposed, theoretically analyzed, and their properties experimentally verified. The fundamental trade-off in the design of plasmonic waveguides for potential application in information technologies lies in the contradiction between their mode field confinement and propagation loss: the higher confinement, the higher loss, and vice versa. Various definitions of figures of merit of plasmonic waveguides have been also introduced for the characterization of their properties with a single quantity. In this contribution, we theoretically analyze one specific type of a plasmonic waveguide – the hybrid dielectric-loaded plasmonic waveguide, or – as we call it in this paper – the hybrid dielectric-plasmonic slot waveguide, which exhibits very strong field confinement combined with acceptable losses allowing their application in some integrated plasmonic devices. In contrast to the structures analyzed previously, our structure makes use of a single low-index dielectric only. We first define the effective area of this waveguide type, and using waveguide parameters close to the optimum we analyze several waveguide devices as directional couplers, multimode interference couplers (MMI), and the Mach-Zehnder interferometer based on the MMI couplers. For the full-vector 3D analysis of these structures, we use modelling tools developed in-house on the basis of the Fourier Modal Method (FMM). Our results thus serve to a dual purpose: they confirm that (i) these structures represent promising building blocks of plasmonic devices, and (ii) our FMM codes are capable of efficient 3D vector modelling of plasmonic waveguide devices
Optical contrast of oil dispersed in seawater under windy conditions
Get PDFOil pollution of natural waters represents a serious threat for aquatic ecosystems, and the assessment of the degree of pollution requiresmeasurement strategies including remote sensing and modeling. While surface oil pollution is relatively easy to detect with radar or opticalremote sensing, underwater oil emulsions can only be detected using visible light as an information carrier. Here the Michelson contrast ofsea area polluted by an oil-in-water emulsion under various wind conditions is simulated through Monte-Carlo modeling. The results showthe premise for optimal direction of observations in which the possibility of detecting the emulsified oil is maximized
Novel trends in optical non-destructive testing methods
Get PDFNon-destructive testing (NDT) describes a wide range of methods for measuring and comparing physical quantities against a nominal condition. In this paper we describe and compare different optical NdT (ONDT)-methods with respect to their characteristics and capability for different measurement tasks. ONDT may be specified in two categories, passive and active. The NDT principles of the first category just use a measurement method like view inspection, elipsometry or reflectometry to detect defects which are easily accessible. The principles of the second category use an excitation force, such as heat or mechanical vibration introduced by transducers to detect hidden defects. This category can be specified into two subcategories. The first subcategory "time-/depth-resolved" includes measurement methods delivering detailed information of the geometric features of a hidden defect. Therefore the excitation of the material and the detection of the reaction have to provide a time step which enables depth-solved measurements. Phase-resolved thermography and laser ultrasound are examples for this category. The second subcategory "Integrating" includes measurement technique coupled with an excitation that enables detection of defects but not evaluation of their geometric features. Examples for these measurement techniques are shearography, reflectometry, vibrometry and thermography coupled with excitation method like simple heating or loading with a constant force. We demonstrate experimental results obtained using methods developed in our institute and highlight directions of further development
Passive high ratio sunlight concentration configurations
Get PDFDuring the day the sun is moving along curved 1-D trajectory across the sky. This is essential a priori information that can be used in order to design passive optics to perform efficient collection of sunlight without tracking after sun’s location in the sky. Despite the movement of the sun along its trajectory the optics will collect the energy to the same spatial location where the photo-voltaic cell is positioned. Two novel designs are proposed while the first one is based on waveguides and the second on prisms with high concentration ratio of above 10X. The proposed configurations are validated numerically and experimentally
Correction of misalignment introduced aberration in non-null test measurements of free-form surfaces
Get PDFIn interferometric testing of surfaces a major task is to avoid the introduction of aberrations due to misalignment of the surface under test. An automated method for the positioning of aspheric and free-form surfaces in a non-null test interferometer, as well as a method for the distinction between alignment introduced aberrations and surface errors is presented. A combination of both methods allows for a fully automated alignment with low requirements to the accuracy of the positioning stage. Further, the misalignment introduced uncertainties to the measurement are estimated. Simulation results as well as experimental results showing the feasibility of the method are presented
Self-tuning laser speckle contrast analysis based on multiple exposure times with enhanced temporal resolution
Get PDFLaser Speckle Contrast Analysis (LASCA) was introduced in 1981. Since then, several enhancements were applied to it. Nowadays, thetechnique can provide relatively high accuracy as well as high temporal and spatial resolution during the examination of ocular or cerebraltissues. However, in the case of skin, the results are highly affected by the intensive scattering on the skin surface, as the scattering onthe non-moving parts of the sample lead to the detrimental decrease of the accuracy. We present a LASCA method based on the use ofmultiple exposure times, combined with the switching-mode control of the light intensity and a special sampling technique to achieve nearto real-time measurement of the skin perfusion. The system based on our method is able to automatically handle the destructive effect ofthe skin surface and re-tune itself according to the changes of the sample, while it provides full-field perfusion maps with high accuracy,without the need of any precalibrations
Direct machining of curved trenches in silicon with femtosecond accelerating beams
Get PDFControl of the longitudinal profile of ablated structures during laser processing is a key technological requirement. We report here on the direct machining of trenches in silicon with circular profiles using femtosecond accelerating beams. We describe the ablation process based on an intensity threshold model, and show how the depth of the trenches can be predicted in the framework of a caustic description of the beam
Performance analysis of wavelength multiplexed SAC OCDMA codes in beat noise mitigation in SAC OCDMA systems
Get PDFIn this paper we investigate the use of wavelength multiplexed spectral amplitude coding (WM SAC) codes in beat noise mitigation in coherent source SAC OCDMA systems. A WM SAC code is a low weight SAC code, where the whole code structure is repeated diagonally (once or more) in the wavelength domain to achieve the same cardinality as a higher weight SAC code. Results show that for highly populated networks, the WM SAC codes provide better performance than SAC codes. However, for small number of active users the situation is reversed. Apart from their promising improvement in performance, these codes are more flexible and impose less complexity on the system design than their SAC counterparts
Qualifying parabolic mirrors with deflectometry
Get PDFPhase-measuring deflectometry is a full-field gradient technique that lends itself very well to testing reflective optical surfaces. In the past, the industry’s interest has been focussed mainly on the detection of defects and ripples, since it is easy to achieve sensitivity in the nm range. On the other hand, attempts to reconstruct the absolute surface shape from the gradient map have been plagued by systematic errors that accumulate to unacceptable uncertainties during data integration. Recently, thanks to improved measurement and evaluation techniques, the state of the art in absolute surface measurement has reached a level of maturity that allows its practical usage in precision optical manufacturing and qualification systems. We demonstrate the techniques, and the progress, by way of results from mirrors for telescopes, solar concentrators, and precision laboratory assemblies
Low-loss multilayer compatible a-Si:H optical thin films for photonic applications
Get PDFThis paper reports about hydrogenated amorphous silicon which can be employed as low-loss optical material for small footprint and cost-effective photonic integrated circuits. Basic waveguides, photonic wire based couplers, Mach-Zehnder interferometers, ring resonators and Mach-Zehnder assisted ring resonators were designed, fabricated, and optically characterised. The propagation loss of rib and photonic wire waveguides were determined to be 2 dB/cm and 5.3 dB/cm, respectively. The 90° bending losses of 5 µm curved photonic wires were determined to be 0.025 dB/90°. Three-dimensional tapers, which were fabricated without additional etching steps and were deposited on top of the fabricated photonic wires showed a net coupling loss of 4 dB/port. Multimode 3 dB-splitters were systematically investigated resulting in 49-51% splitting ratios. Mach-Zehnder interferometers that were realised with these splitters showed interference fringe depths of up to 25 dB for both polarisations. Compact ring resonators with 10 µm radius implemented as notch filters and in Mach-Zehnder coupled configurations provided extinction ratios of ≥20 dB and Q-factors up to 7500