86,988 research outputs found
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
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
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
Il problema è come pensiamo al problema: Gestire le tensioni con il pensiero paradossale
No full textBioengineering the ovary to preserve and reestablish female fertility
No full textDifferent bioengineering strategies can be presently adopted and have been shown to have great potential in the treatment of female infertility and ovarian dysfunction deriving from chemotherapy, congenital malformations, massive adhesions as well as aging and lifestyle. One option is transplantation of fresh or cryopreserved organs/fragments into the patient. A further possibility uses tissue engineering approaches that involve a combination of cells, biomaterials and factors that stimulate local ability to regenerate/ repair the reproductive organ. Organ transplant has shown promising results in large animal models. However, the source of the organ needs to be identified and the immunogenic effects of allografts remain still to be solved before the technology may enter the clinical practice. Decellularization/ repopulation of ovary with autologous cells or follicles could represent an interesting, still very experimental alternative. Here we summarize the recent advancements in the bioengineering strategies applied to the ovary, we present the principles for these systems and discuss the advantages of these emerging opportunities to preserve or improve female fertility
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
Some properties of proteolytic enzymes and storage proteins in recalcitrant and orthodox seeds of Araucaria
No full textIncertezza e tensioni nel cambiamento organizzativo: Implicazioni per la pratica manageriale da un caso nel settore del parcel delivery
No full textDeveloping social leadership: Cultural and technological influences
No full textThe development of social leadership is triggered by the adoption of social media for internal purposes by organizations. It involves dynamics of collaborative and mutual influence between leaders and followers, and among followers themselves, mediated by social media. The aim of this paper is to analyze individual attitudes to social leadership as a function of cultural and technological factors. Using data collected from a sample (N=178) of employees, we developed and adapted multiple item scales from a wide range of sources in the literature to assess organizational culture, information technology and social media adoption, in addition to other individual and organizational variables. Regression analysis was used to investigate the relationship between these variables. What emerged was a three-dimensional model of social leadership as a positive attitude towards: shared responsibility, social media usage and direction. In particular, organizational culture strongly influenced two of these three dimensions, while the use of social media for personal purposes predicted attitudes to the use of social media in organizations
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