1,720,962 research outputs found
Polarization-insensitive directional couplers based on SOI wire waveguides
No full textOptical directional couplers based on SOI-wire waveguides have been modelled by a semi-analytical approach based on the Coupled Mode Theory and Finite Element Method. The modelling is used to obtain analytically optical power at the parallel and cross ports by utilizing numerically calculated coupling coefficients. Geometrical dimensions of the couplers have been optimized to obtain a polarization-independent behaviour. The influence of non-vertical sidewalls on the coupler performance has also been addressed
Design rules for single-mode and polarization-independent silicon-on-insulator rib waveguides using stress engineering
Full text linkThere is a trend towards miniaturization of silicon photonic circuits due to superior performance and small cost. Design rules that must be imposed on the geometry of optical waveguides to make them behave as polarization-independent and singlemode devices are well known for waveguides with relatively large cross sections and for some small cross-sectional rib waveguides with vertical sidewalls and an air top cladding. The influence of the top oxide cover on waveguide birefringence was analyzed recently, but only for relatively large cross-sectional waveguides. This paper reports simulations for both single-mode and polarization-independent behavior for small cross-sectional waveguides with variable rib width, etch depth, top oxide cover thickness, and sidewall angle. The results show that the stress-induced effects must be taken into account to satisfy both requirements. Design rules to maintain birefringence-free operation and to satisfy single-mode behavior for small rib silicon-on-insulator (SOI) waveguides are presented
Auto-regressive optical filters in silicon-on-insulator waveguides.
No full textThe subject of the thesis is the modelling, design, fabrication and characterisation of single-stage and multi-stage resonators on Silicon-on-Insulator (SOI) strip and rib waveguides. The devices have been investigated with the aim to produce small and efficient wavelength selective elements that could one day be used in multiplexers, filters and other components of integrated optical circuits. Due to the complexity of devices and very often requirements of advanced simulation packages, most of the devices have been modelled (lambda = 1.55mum) through separate analyses of the components forming the filters. The study starts with the modelling of rib and strip waveguides aiming at the single-mode and zero-birefringent regime of operation, followed by the analysis of a directional coupler. The modelling suggests that the cross-sectional rectangular area of a strip waveguide should be smaller than 0.10mum2. Similarly, rib waveguides with a height of 1.35mum, and a waveguide width of 0.8mum or 1.0mum, could be used as basic single-mode and zero-birefringent elements for building relatively large rib waveguide based devices. The analysis of a directional coupler on strip waveguides has shown that a near-polarisation-independence regime is possible for waveguide separations below 0.20mum and waveguide widths in range 0.29 - 0.40mum, when a waveguide height is chosen to be 0.29mum or 0.34mum. Simplified z-transform models of filters have been employed to calculate values of the most relevant figures of merit such as Free Spectral Range (FSR), Full Width at Half Maximum (FWHM), Finesse (F) and Q-factor, and also to quantify the sensitivity of the transfer function to the changes of geometric parameters, coupling issues and thermal effects. Based on the modelling and information from test chips of previous students, 4 main designs grouped in 6 test chips have been proposed and fabricated in collaboration with the Intel Corporation Photonics research groups from San Jose and Jerusalem. Two designs were based on rib waveguide type devices and two on strip waveguide type devices. The goals in all cases were; polarisation insensitivity, single-mode behaviour, improvement of the FSR, shaping response by using various geometries, the possibility of tuning response by thermal means etc. Experimental results have shown improvement in the FWHM and FSR as expected for both strip and rib designs. An additional stage of multi-level, serially coupled racetrack resonator in rib waveguides has resulted in a decrease of the FWHM by more than 30% (6pm). Polarisation independence by using identical multiple serial-coupled rib racetracks has also been demonstrated. The FSR above 60nm have been reported for small strip resonators (radius of l.5mum) with good polarisation characteristics for rings which radius is near 3mum. To the author's knowledge this is the largest FSR yet reported for a silicon based ring resonator. There is also improvement of the spectral response of multiple Vernier rings, which, with some corrections in terms of side lobes appearing in the spectrum, may be used for designing devices with the FSR as large as 70nm
Auto-regressive optical filters in silicon-on-insulator waveguides.
No full textThe subject of the thesis is the modelling, design, fabrication and characterisation of single-stage and multi-stage resonators on Silicon-on-Insulator (SOI) strip and rib waveguides. The devices have been investigated with the aim to produce small and efficient wavelength selective elements that could one day be used in multiplexers, filters and other components of integrated optical circuits. Due to the complexity of devices and very often requirements of advanced simulation packages, most of the devices have been modelled (lambda = 1.55mum) through separate analyses of the components forming the filters. The study starts with the modelling of rib and strip waveguides aiming at the single-mode and zero-birefringent regime of operation, followed by the analysis of a directional coupler. The modelling suggests that the cross-sectional rectangular area of a strip waveguide should be smaller than 0.10mum2. Similarly, rib waveguides with a height of 1.35mum, and a waveguide width of 0.8mum or 1.0mum, could be used as basic single-mode and zero-birefringent elements for building relatively large rib waveguide based devices. The analysis of a directional coupler on strip waveguides has shown that a near-polarisation-independence regime is possible for waveguide separations below 0.20mum and waveguide widths in range 0.29 - 0.40mum, when a waveguide height is chosen to be 0.29mum or 0.34mum. Simplified z-transform models of filters have been employed to calculate values of the most relevant figures of merit such as Free Spectral Range (FSR), Full Width at Half Maximum (FWHM), Finesse (F) and Q-factor, and also to quantify the sensitivity of the transfer function to the changes of geometric parameters, coupling issues and thermal effects. Based on the modelling and information from test chips of previous students, 4 main designs grouped in 6 test chips have been proposed and fabricated in collaboration with the Intel Corporation Photonics research groups from San Jose and Jerusalem. Two designs were based on rib waveguide type devices and two on strip waveguide type devices. The goals in all cases were; polarisation insensitivity, single-mode behaviour, improvement of the FSR, shaping response by using various geometries, the possibility of tuning response by thermal means etc. Experimental results have shown improvement in the FWHM and FSR as expected for both strip and rib designs. An additional stage of multi-level, serially coupled racetrack resonator in rib waveguides has resulted in a decrease of the FWHM by more than 30% (6pm). Polarisation independence by using identical multiple serial-coupled rib racetracks has also been demonstrated. The FSR above 60nm have been reported for small strip resonators (radius of l.5mum) with good polarisation characteristics for rings which radius is near 3mum. To the author's knowledge this is the largest FSR yet reported for a silicon based ring resonator. There is also improvement of the spectral response of multiple Vernier rings, which, with some corrections in terms of side lobes appearing in the spectrum, may be used for designing devices with the FSR as large as 70nm
Auto-regressive optical filters in silicon-on-insulator waveguides.
Full text linkThe subject of the thesis is the modelling, design, fabrication and characterisation of single-stage and multi-stage resonators on Silicon-on-Insulator (SOI) strip and rib waveguides. The devices have been investigated with the aim to produce small and efficient wavelength selective elements that could one day be used in multiplexers, filters and other components of integrated optical circuits. Due to the complexity of devices and very often requirements of advanced simulation packages, most of the devices have been modelled (lambda = 1.55mum) through separate analyses of the components forming the filters. The study starts with the modelling of rib and strip waveguides aiming at the single-mode and zero-birefringent regime of operation, followed by the analysis of a directional coupler. The modelling suggests that the cross-sectional rectangular area of a strip waveguide should be smaller than 0.10mum2. Similarly, rib waveguides with a height of 1.35mum, and a waveguide width of 0.8mum or 1.0mum, could be used as basic single-mode and zero-birefringent elements for building relatively large rib waveguide based devices. The analysis of a directional coupler on strip waveguides has shown that a near-polarisation-independence regime is possible for waveguide separations below 0.20mum and waveguide widths in range 0.29 - 0.40mum, when a waveguide height is chosen to be 0.29mum or 0.34mum. Simplified z-transform models of filters have been employed to calculate values of the most relevant figures of merit such as Free Spectral Range (FSR), Full Width at Half Maximum (FWHM), Finesse (F) and Q-factor, and also to quantify the sensitivity of the transfer function to the changes of geometric parameters, coupling issues and thermal effects. Based on the modelling and information from test chips of previous students, 4 main designs grouped in 6 test chips have been proposed and fabricated in collaboration with the Intel Corporation Photonics research groups from San Jose and Jerusalem. Two designs were based on rib waveguide type devices and two on strip waveguide type devices. The goals in all cases were; polarisation insensitivity, single-mode behaviour, improvement of the FSR, shaping response by using various geometries, the possibility of tuning response by thermal means etc. Experimental results have shown improvement in the FWHM and FSR as expected for both strip and rib designs. An additional stage of multi-level, serially coupled racetrack resonator in rib waveguides has resulted in a decrease of the FWHM by more than 30% (6pm). Polarisation independence by using identical multiple serial-coupled rib racetracks has also been demonstrated. The FSR above 60nm have been reported for small strip resonators (radius of l.5mum) with good polarisation characteristics for rings which radius is near 3mum. To the author's knowledge this is the largest FSR yet reported for a silicon based ring resonator. There is also improvement of the spectral response of multiple Vernier rings, which, with some corrections in terms of side lobes appearing in the spectrum, may be used for designing devices with the FSR as large as 70nm
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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