1,721,080 research outputs found
Time delay distribution in Bragg gratings
No full textA layer-by-layer analysis of the time delay of both reflected and transmitted light in one-dimensional photonic band-gap structures is developed and applied to uniform Bragg gratings. An effective Fabry-Pérot cavity is associated with every layer along the Bragg grating, multiple paths with a well defined layer traversal time are identified, and the average time is computed, introducing an appropriate weighting factor that accounts for interference between different paths. The analysis presented leads directly to a complex-valued time delay whose real part is shown to be equivalent to the classic phase time delay. Physical meaning is also given to the imaginary part. The local dwell time, interpreted as the average time spent by light in the layer independently of the final (transmitted or reflected) state, is proved analytically to be related to the energy density distribution when small index change gratings are considered. The time delay evolution is derived at different wavelengths and the nonuniform distribution along the grating is discussed. Nonintuitive features such as superluminal transmission time delay for propagation inside the band gap and negative reflection time delay close to transmission resonances are addressed. Finally, the effect of introducing a small perturbation in the structure is shown to be directly related to the local time delay and is proposed as a possible experimental measurement scheme for both its real and imaginary parts
BER and total throughput of asynchronous DS-OCDMA/WDM systems with multiple user interference
No full textThe BER and throughput of Direct-Sequence OCDMA/WDM systems based on quadripolar codes and superstructured fiber Bragg gratings are statistically derived under asynchronous operation, intensity detection, and Multiple User Interference. Performance improvements with Forward Error Correction are included
Fundamental properties of Bragg gratings and their application to the design of advanced structures
No full textThis thesis presents the analysis of the local properties of Bragg gratings and their application to the improvement of standard designs and advanced structures. The time spent by light inside each grating section is derived in terms of complex-valued quantities, and clear meaning is given to both the real and imaginary parts. Improved physical understanding of propagation and energy distributions inside periodic structures is obtained. Local properties also explain in a more intuitive way well understood features of different gratings, which improves intuition of new complex designs. The analysis of the effect of perturbations is immediate using this approach and has important practical applications. Independent confirmation of the theory is obtained, and experimental measurement of the imaginary part of the local time delay is given. Phase errors affect the grating writing techniques, and the related sensitivity is analysed in detail. The robustness of different designs is discussed with respect to such manufacturing errors. Fine tuning of standard or advanced grating designs by means of suitable error distributions is also proposed, and optimised characteristics either in the reflectivity or in the dispersive response are obtained. This method is integrated with inverse scattering designs to further boost their performances. Improved complex designs are also proposed in case losses affect propagation in the grating. Cladding mode losses are compensated using an iterative layer-peeling algorithm. The design of the first wide-band dispersion-compensating grating realised with a standard single mode fibre is shown. Background losses and UV-induced losses in gratings are also compensated using a modified layer-peeling method. The physical limitations related to grating design in lossy media are explained using the derived understanding of local properties. New advanced designs are also considered that fully exploit the theoretical potentialities and manufacturing capabilities of Bragg gratings. The performance of code-division multiple access systems based on superstructured gratings is improved by combining encoding, bandwidth filtering, and dispersion compensation in the same high reflectivity grating
Non-destructive characterisation of fibre couplers
No full textA technique for the non-destructive characterisation of couplers is proposed. A CO2 laser beam is scanned along the coupler length inducing a local perturbation to the coupler eigenmodes. Asymmetric and symmetric perturbations can give accurate mapping of power-evolution and coupler-waist shape
Inverse scattering design of fiber Bragg gratings with cladding mode losses compensation
No full textCladding mode losses are included in Bragg gratings' design using a fast, discrete transfer matrix method and an iterative inverse scattering algorithm. Simulations show perfect equalization in a linear dispersion compensating grating written on a standard SMF fiber
Nondestructive characterization of fiber couplers by a local perturbation method
Full text linkA technique for the nondestructive characterization of fiber couplers is demonstrated. A CO2 laser beam is scanned along the coupler length inducing a local perturbation to the coupler eigenmodes. Information about the power evolution and the coupling-constant variation along the coupler length can be obtained by applying an asymmetric and symmetric perturbation, respectively. Experimental results of the characterization of a half- and a full-cycle fiber coupler using this technique are presented
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
Effect of phase shift perturbations and complex local time delay in fiber Bragg gratings
Full text linkThe effect of introducing a phase shift inside a fiber Bragg grating is addressed and related to the time spent in the defect position by light. The impact on phase errors characterization in gratings is 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
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