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Extra loss due to Fano resonances in inhibited coupling fibers based on a lattice of tubes
Full text linkConfinement loss of inhibited coupling fibers with a cladding composed of a lattice of tubes of various shapes is theoretically and numerically investigated. Both solid core and hollow core are taken into account. It is shown that in case of polygonal shaped tubes, confinement loss is affected by extra loss due to Fano resonances between core modes and cladding modes with high spatial dependence. This explains why hollow core Kagome fibers exhibit much higher confinement loss with respect to tube lattice fibers and why hypocycloid core cladding interfaces significantly reduce fiber loss. Moreover it is shown that tube deformations, due for example to fabrication process, affect fiber performances. A relationship between the number of polygon sides and the spectral position of the extra loss is found. This suggests general guide lines for the design and fabrication of fibers free of Fano resonance in the spectral range of interest
Fibre a nucleo cavo per applicazioni nella gamma dei terahertz
No full textLo scopo di questa tesi è la modellizazione e la realizzazione di una fibra ottica flessibile, a basse perdite e a larga banda per applicazioni nella gamma dei terahertz (0.1THz-10THz). Negli ultimi anni la tecnologia impiegata per la generazione/ricezione di radiazioni THz ha conosciuto un notevole sviluppo, creando i presupposti per numerose applicazioni. La mancanza di una guida a basse perdite, tuttavia, rappresenta una delle principali cause che ne limitano l'utilizzo. A causa della limitata conduttività dei metalli e l'alto assorbimento dei materiali dielettrici nella gamma THz, le guide classiche non possono essere utilizzate. Poichè l'aria rimane uno dei materiali più trasparenti per la radiazione THz, questo ha spinto alla ricerca di nuove guide d'onda con design innovativi al fine di massimizzare la percentuale di campo propagante in aria a discapito di quella nel dielettrico. In tal senso le guide a nucleo cavo offrono un'interessante soluzione concentrando la maggior parte della potenza nella parte centrale della fibra che è fatta d'aria.
La fibra proposta in questa tesi presenta un cladding costituito da un reticolo di tubi dielettrici mentre il core è cavo. Dato che un singolo giro di tubi attorno al core è sufficiente per garantire un buon cofinamento della radiazione, la struttura proposta è al contempo compatta, flessibile e di facile realizzazione.
La prima parte della tesi si concentra sullo studio teorico delle caratteristiche transissive delle fibra al fine di migliorarne le capacità di confinamento e la larghezza di banda. A tal fine viene proposto un modello teorico che è in grado di descrivere le proprietà dell'intera struttura partendo da quelle del singolo tubo del cladding. Il notevole vantaggio risiede nel fatto che il singolo tubo può essere studiato analiticamente: in questo modo si riesce a stabilire quali siano gli effetti delle dimensioni fisiche e geometriche del cladding sulle prestazioni della fibra, definendo così le principali linee guida per il design.
Successivamente vengono analizzati anche gli effetti delle perturbazioni della struttura ideale sulle performance. Siccome i tubi del cladding sono spesso ottenuti attraverso processi di filatura, in primo luogo si studia l'impatto della loro forma sul meccanismo di confinamento. Si dimostra che la fibra è piuttosto sensibile a questo tipo di alterazioni e solo tubi sufficientemente circolari sono in grado di garantire basse perdite e larga banda. Gli altri tipi di perturbazione che vengono considerati riguardano l'ellitticità del core e la curvatura della fibra, poichè entrambi impattano direttamente sull'utilizzo della fibra hollow core in sistemi THz. Grazie ad un'accurata analisi numerica, si dimostra che la fibra proposta è in realtà fortemente robusta contro questi tipi di alterazioni.
Infine si mostrano anche le tecniche di assemblaggio e caratterizzazione per due diversi modelli di fibre hollow core: il primo basato su tubi di polimetilmetacrilato, mentre il secondo su tubi di Zeonex. In entrambi i casi le fibre vengono assemblate a mano dimostrando come, essendo strutture autosostenute, non siano necessarie ulteriori fasi di filatura o incollatura. Come jacket delle fibre viene poi utilizzato un tubo di termorestringente al fine di preservarne la flessibilità. I dati sperimentali confermano la notevole capacità di queste fibre di ridurre le perdite di propagazione ben al di sotto dell'assorbimento causato dal materiale del cladding. Per la fibra di PMMA, per esempio, si ottengono riduzioni di 31 e 272 volte rispettivamente a 0.375 e 0.828 THz, con perdite dell'ordine degli 0.3 dB/cm e 0.16 dB/cm. I test sulla curvatura, inoltre, confermano che la fibra proposta può essere effettivamente utilizzata per la realizzazione di sistemi THz flessibili dato che anche curvature di poche decine di centrimetri producono solo effetti marginali sulle caratteristiche trasmissive della fibra.This thesis concerns with the development of a low loss, broadband and flexible waveguide for terahertz (0.1THz-10THz) applications. In recent years THz sources and detectors have undergone a great development thus suggesting an increasing number of possible applications. However, the lack of a broadband low-loss waveguide preclude real implementations. Classical metal or solid core waveguides can not be used for this purpose, owing to the limited conductivity of metals and the prohibitively high absorption of dielectrics in the THz range. Since air is one of the most transparent dielectrics for the THz radiation, many innovative fibers designs have been proposed in the literature with the goal to maximize the amount of field that propagates in air at the expense of that in the dielectric. In this sense hollow core waveguides offer an interesting solution because a high percentage of the THz radiation is confined in an airy core.
The fiber proposed in this thesis is an hollow core fiber whose cladding is based on a lattice of dielectric tubes. Since only one turns of tubes around the core is enough to give reasonably low leakage loss, the whole structure is also very compact, flexible and easy to assemble: no stack and draw process is required and the whole waveguide can be assembled manually starting from the stand alone cladding tubes.
In order to improve the fibers performances in terms of low propagation loss and wide transmission bandwidths, a theoretical analysis of the confinement is given first. A new theoretical model able to predict fiber's transmission properties is proposed. It is based on the observation that the spectral properties of the microstructured fiber can be derived from those of the cladding tubes. Since tube waveguides can be described analytically, this model allows to determine how the geometrical and physical features of the microstructured cladding affect the spectral behavior of the whole waveguide. General guidelines for the design of this kind of fibers are then given.
Perturbations to the ideal structure are then investigated in order to evaluate their effect on the performance of the fiber. Since very often cladding tubes are obtained from a previous drawing process, shape alterations are considered first. It is shown that the circular case allows to obtain wider transmission bandwidths and lower losses, suggesting that the drawing process of the tubes must be carefully controlled. Other perturbations, such as elliptical core shapes or bending are also taken into account. The former is of key importance for most of the proposed THz applications since they are based on the propagation of pulses with unknown polarizations. Core's ellipticity may cause undesired echoes in the system thus compromising performances. Also bending loss plays a central role in most of the THz setups since flexible waveguides are required. Through a detailed numerical analysis it is shown that the proposed fiber is actually strongly robust against both of these kind of perturbations.
Finally, manufacturing and characterization of two different kind of microstructured waveguides is shown, starting from polymethylmethacrylate and Zeonex tubes, respectively. The whole structure is assembled manually and, thanks to the its self sustain feature, no gluing or further drawing steps are required. An heat-shrink tube is used for the jacket in order to preserve the flexibility of the fiber. Experimental data for the straight case confirm that propagation loss is strongly reduced with respect to the absorption of the bulk material used in the cladding. For example, a 31 and 272 times reduction at 0.375 and 0.828 THz respectively is shown for the PMMA case, giving propagation loss of 0.3 dB/cm and 0.16 dB/cm. Bending characterization confirms that this waveguide can be used in a real flexible THz setup with bending radii down to few tenth of centimeters without any visible reduction of the transmission spectrum
Confinement Loss in Kagome and Tube Lattice Fibers: Comparison and Analysis
No full textIn this paper, a thorough numerical analysis of the confinement loss in kagome and tube lattice fibers is presented. The results show that the confinement loss strongly depends on the shape of the struts composing the core boundary and the cladding. This explains why confinement loss in kagome fibers is much higher than in tube lattice ones. In fact, the closer to a perfectly circular arc the struts, the lower the confinement loss. For this reason, struts shape must be carefully controlled during the fabrication process
Elliptical hollow core tube lattice fibers for terahertz applications
No full textHollow core microstructured fibers whose cladding is composed by an elliptical arrangement of circular dielectric tubes are numerically investigated. Birefringence, differential group delay, and polarization dependent loss are evaluated for different values of ellipticity in order to quantify its effect on the two polarizations of the fundamental core mode. All these parameters have a minimum at the center of each transmission window and they assume maximum values at the edges. In particular, the birefringence goes always to zero irrespective of ellipticity. This proves that tube lattice fibers are strongly immune against unwanted core shape deformation, in particular when the working frequency is close to the center of the transmission windows. On the other hand they are not suitable to obtain polarization maintaining fibers
Birefringence in Elliptical Tube Fibers
No full textThe dispersion and loss properties of elliptical hollow core tube fibres are analyzed
Propagating and radiating properties of Broadband Hollow Core fibers in Terahertz spectral region
No full textIn this paper Hollow Core Microstructured Fibers consisting of a regular arrangement of dielectric tubes of Teflon are numerically investigated in order to obtain efficient and highly directive aperture antennas for Terahertz applications. The good aperture field distribution allow to obtain high power coupling coefficient with freely propagationg gaussian beams, narrow beamwidth and low side lobe level over a wide range of frequencies as wide as 1.5THz centered at 2.25 THz. Antenna performances are not affected by fiber dimension allowing a more flexible design in order to better meet the requirements of a particular application. An array configuration of fibers arranged in a hexagonal lattice have been also investigated
Fibre ottiche a nucleo cavo con simmetria ottagonale per applicazioni Terahertz
No full textA new hollow core fiber for THz applications consisting of dielectric tubes arranged with a octagonal symmetry is presented. Numerical results show that this fiber exhibits an effectively single mode propagation with lower propagation loss with respect to fiber with hexagonal symmetry previously proposed
Terahertz Tube Lattice Fibers With Octagonal Symmetry
No full textA new hollow core fiber for terahertz applications consisting of dielectric tubes arranged with an octagonal symmetry is presented. Simulation results have shown that this fiber exhibits an effectively single-mode propagation with lower propagation loss with respect to hollow core fibers with a hexagonal symmetry. The suppression of higher order modes is due to a better phase-matching condition with cladding modes propagating into the hollow core of tubes. In particular, by using tubes with 44-μm thickness and outer diameters as low as 0.5 mm, a rejection of high order modes higher than 20 dB/m and fundamental mode propagation loss lower than 5 dB/m from 2.30 to 2.85 THz can be obtained
Analysis of the Waveguiding Mechanism in Tube Lattice Fibers
No full textIn this paper, waveguiding mechanism and modal characteristics of hollow core ber made of a regular arrangement of dielectric tubes is investigated. A simply analytical model to predict dispersion curves and low loss regions is proposed and numerically validated
Analysis of the Confinement Loss in Kagome Fibers
No full textHollow-core Inhibited Coupling Fibers (ICFs) have been widely analyzed in these years in order to minimize their Con ̄nement Loss (CL)
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