6 research outputs found
Recommended from our members
Finite element modelling of photonic crystal fibres
Photonic crystal fibre (PCF), a new kind of optical fibre, has many air-holes in their cross-section and has potential applications to new optical communication systems. The main objective of this research is the modelling of photonic crystal fibre to identify the fundamental and higher order quasi-TE and TM modes with square, ,rectangular and circular air holes in a square and hexagonal matrix, by using a rigorous full-vectorial H-field based finite element method (FEM). Besides the modal solutions of the effective indices, mode field profiles, spot sizes, modal hybridness, polarization beat length and group velocity dispersion values for equal and unequal air holes; research was carried out to optimize and design highly birefringent PCF. The variation of modal birefringence is shown through the effect of hole diameters, air hole arrangement, structural asymmetry, operating wavelength, and pitch-distance. Birefringence was enhanced by breaking the structural symmetry and this was verified by using unequal air holes. The diameter of two air holes and four air holes in the first ring was changed to break the rotational symmetry and a comparison between the two designs is made in this work. In this work, highly birefringent PCF is designed with higher operating wavelength, larger d2/A value, lower pitch length for a given structural asymmetry. It is identified that birefringence value increases rapidly when d2 is much larger than d. At lower pitch value, one of the highest birefringence values reported so far at wavelertgth of 1.55 J.Jm for an asymmetric PCF using circular air holes. A single polarization guide PCF structure is also achieved. In this study, it has been identified that for fixed d/A and d2/A value, as operating wavelength is increased, birefringence increases significantly. It can also be identified that for higher d/A values, birefringence changes rapidly with A as their corresponding cutoff condition also approaches. One important validation of this work is the existence of modal birefringence for PCF with six-fold rotational symmetry. It is shown that birefringence value of a simple PCF incorporating circular holes but of different diameters is high compared to polarization maintaining Panda or Bow-tie fibres. This research also aims to investigate the modal leakage losses of PCF, by using a semi-vectorial beam propagation method (BPM) based on the versatile FEM. The robust perfectly matched layer (PML) boundary condition has been introduced to the modal solution approach. The effects of d2/A, operating wavelength and number of air holes have been thoroughly detailed and explained. In this study, it has been identified that the confinement loss decreases significantly with the increased number of rings, lower operating wavelength and lower d2/A value. For special case, PCF with large spot-size provides higher leakage los
Recommended from our members
Finite element modelling of polarization issues in optoelectronic systems
Many integrated optical subsystems incorporate guided wave photonic devices and connecting optical waveguides with two-dimensional confinement and a high index contrast between the core and the cladding. The modes present in such waveguides are not purely of the TE or TM type but are hybrid in nature, with all six components of the electric and magnetic fields being present, which makes the overall system to be polarization dependent. In present high- performance photonic components and optoelectronic subsystems, this polarization issue is a major issue to be tackled with for further enhancement of their performances.
This thesis describes the design, analysis and optimisation of such polarization-based waveguide devices such as polarization rotators, polarization splitters and polarization controllers. To characterize the polarization properties of such devices a fully vectorial approach is necessary. In this work, the most versatile and accurate full vectorial H-field based finite element method (FEM) is used to simulate complex waveguide structures in order to optimize and evaluate novel devices, prior to fabrication. This method can accurately calculate the propagation constants of both polarized modes and consequently these are used to calculate the half-beat length, which is an important parameter when designing waveguide devices involving polarization issues. Many important photonic devices, such as polarization splitters, polarization rotators and polarization controllers may be fabricated by combining several butt-coupled uniform waveguide sections. The least squares boundary residual (LSBR) method is used to obtain both the transmission and the reflection coefficients of all the polarized modes by considering both the guided and radiated modes.
In this work, a combination of the FEM and LSBR methods has been extensively used to obtain the TE to TM or TM to TE polarized power transfer efficiency in semiconductor waveguides and the polarization crosstalk in high index contrast silica waveguides. When designing polarization rotators or identifying the possible polarization crosstalk, it is necessary to calculate the modal ‘hybridity’ of a mode. In this study, it is identified that when the waveguide lacks structural symmetry, the modal hybridity is enhanced, and thereby a considerably high polarization conversion is expected. A compact 400pm long passive polarization rotator (PR) with cascaded asymmetrical waveguide sections is designed with a very low insertion loss of 0.2dB. A more compact 320pm long, much improved PR is designed and analysed by using only a single slanted walled rib waveguide section. Furthermore, an extensive study of fabrication tolerances of a compact, single-stage PR is undertaken. A 99.8% polarization conversion is achieved with a very low crosstalk value of- 29dB. The design of a compact 1.6mm long single-section polarization splitter in a deeply etched semiconductor MMI waveguide is also presented. An extensive analysis of polarization crosstalk is carried out using high index contrast planar silica waveguides. The origin of polarization crosstalk in silica waveguides is explained and it is shown that a significant polarization conversion can take place in a long, high index contrast silica waveguide. It is also learnt that non-verticality of the sidewalls causes the significant polarization crosstalk in many silica-based components. Finally a novel design concept of an active polarization controller is presented using twin electrodes with both biasing and controlling signals. The asymmetry is introduced by incorporating a non-symmetric modulating electric field in order to control the polarization conversion. Both the phase matching and polarization conversion are achieved simultaneously
New Photonic devices based on NLO(non-linear optical) crystalline waveguides
El RbTiOPO4 és un cristall de òptica no lineal amb alts coeficients electró-òptics i un llindar de dany òptic elevat, això el converteix en un material potencial per aplicacions electro-òptiques. Actualment hi ha un interès en el desenvolupament de components òptics basats en materials dielèctrics, identificat com un tema de recerca punter per Europa Horitzó 2020. La finalitat d’aquesta tesis és explorar el RTP com a plataforma dielèctrica per dispositius fotònics, que tenen aplicacions en les telecomunicacions i en el sensat biològic.
En aquesta tesis s’han crescut materials monocristal•lins en volum de RTP, K:RTP i Na:KTP pel mètode de Top seeded solution growth. Els cristalls obtinguts són òptims per ser utilitzats com a plataforma per fabricar guies d’ona i com a substrats pel creixement de capes epitaxials. Capes epitaxials de (Yb,Nb):RTP sobre RTP(001), RTP sobre K:RTP(001) i K.:RTP(100), i KTP sobre Na:KTP(001) s’han crescut per la metodologia de liquid phase epitaxy. Aquesta metodologia ha permès obtenir capes monocristal•lines amb una interfase d’alta qualitat cristal•lina
La fabricació de guies d’ona ha esta realitzada per RIE i ICP-RIE. Es reporta en aquesta tesis, un avanç en el coneixement del procés de etching del RTP. El mètode d’intercanvi iònic, amb Cs+ com ió, s’ha utilitzat per produir guies rectes, corbes i MZ. Degut a l’alta conductivitat iònica del RTP al llarg de la direcció c cristal•logrà fica, l’ intercanvi iònic és altament factible i gairebé unidireccional.
S’ha obtingut exitosament el procés de guiat de llum en totes les guies d’ona fabricades. Pels Y-splitters i els MZ fabricats sobre els cristalls RTP/(Yb,Nb):RTP/RTP(001) estructurats amb RIE sobre la capa activa o bé el substrat, la guia obtinguda és monomode amb polarització TM a 1550 nm. Les pèrdues de propagació són de 3.5 dB/cm. Per les guies d’ona rectes fabricades sobre RTP/(Yb,Nb):RTP/RTP(001) per estructuració del recobriment per ICP-RIE, les pèrdues de propagació són 0.376 dB/cm a 1550 nm.El RbTiOPO4 es un cristal de óptica no-lineal con altos coeficientes electro ópticos y un lÃmite de daño óptico elevado, eso lo convierte en una potencial material para aplicaciones electrópticas. Actualmente existe un gran interés en el desarrollo de componentes ópticos basados en materiales dieléctricos, esto ha sido identificado como un tema puntero de investigación por Europa Horizonte 2020. La finalidad de esta tesis es explorar el RTP cómo plataforma dieléctrica para dispositivos fotónicos, que tienen aplicaciones en les telecomunicaciones y en el sensado biológico.
En esta tesis, se han crecido materiales monocristalinos en volumen de RTP, K:RTP y Na:KTP por el método de Top seeded solution growth. Los cristales crecidos son óptimos para ser utilizados como plataforma para fabricar guÃas de onda y como sustratos para el crecimiento de capas epitaxiales. Capas epitaxiales de (Yb,Nb):RTP sobre RTP(001), RTP sobre K:RTP(001) yK.:RTP(100), i KTP sobre Na:KTP(001) se han crecido mediante la metodologÃa de liquid phase epitaxy. Esta metodologÃa ha permitido obtener capes monocristalinas con una interfase de alta calidad cristalina.
La fabricación de guÃas de onda se ha hecho por RIE y ICP-RIE: Se reporta en esta tesis un avance en el conocimiento del proceso de etching en el RTP. El método de intercambio iónico, con Cs+ como ion, se ha utilizado para producir guÃas de onda rectas, curvas y MZ. Debido a la alta conductividad iónica del RTP a lo largo de la
dirección c cristalográfica, el intercambio iónico es altamente factible y casi unidireccional.
Se ha obtenido el guiado con éxito en todas las guÃas de onda fabricadas. En los Y-Splitters y MZ fabricados sobre los cristales RTP/(Yb,Nb):RTP/RTP(001) estructurados con RIE sobre la capa activa o bien el sustrato, la guÃa obtenida es monomodo con la polarización TM a 1550 nm. Las pérdidas de propagación son de 3.5 dB/cm. Para las guÃas de onda rectes fabricadas sobre RTP/(Yb,Nb):RTP/RTP(001) por estructuración del recubrimiento por ICP-RIE, las pérdidas por propagación son de 0.376 dB/cm a 1550 nm.RbTiOPO4 is a non-linear optical crystal with high electro-optic coefficients and high optical damage threshold, which makes it suitable for electro-optic applications. There’s a current interest in developing dielectric based photonic components for integrated optics, identified as a topic of research by the Europe Horizon 2020. The aim of this thesis is to explore RTP for dielectric based photonic platforms, which have applications in telecommunications and biosensing.
In this thesis is reported the successful grow of bulk single crystals of RTP, K:RTP and Na:RTP by Top Seeded Solution Growth technique. The crystals obtained are suitable to be used as platforms to fabricate optical waveguides and for substrates for growth of epitaxial layers. Epitaxial layers of (Yb,Nb):RTP were grown on RTP(001), RTP was grown on K:RTP(001) and K:RTP(100) and KTP was grown on Na:KTP(001) by Liquid phase epitaxy. This methodology allows obtaining a single crystalline layer, with high quality crystalline interface.
Waveguide fabrication was performed by RIE and ICP-RIE. Advancement in this etching process on RTP is reported in this thesis. Cs+ ion exchange method was used to produce straight, bends and MZ waveguides. Due to the RTP high ionic conductivity along the c crystallographic direction, ion exchange is highly feasible and almost unidirectional.
Waveguiding of the fabricated channel waveguides has been successful. For the Y-Splitter and MZ waveguides fabricated on the RTP/(Yb,Nb):RTP/RTP(001) crystals, by structuring the active layer or the substrate by RIE, the waveguides obtained were single mode in TM polarization at 1550 nm. The propagation loss was 3.5 dB/cm. For straight waveguides fabricated on the RTP/(Yb,Nb):RTP/RTP(001), by structuring the cladding by ICP-RIE, the propagation losses were 0.376 dB/cm at 1550 nm. The waveguides fabricated by Cs+ ion exchange have larger losses due to inhomogeneity on the Cs exchange among different ferroelectric domains present in the structure
Recommended from our members
Finite element characterisation of plasmonic waveguides in terahertz and optical frequencies
In recent years plasmonic devices have become an interesting area of research due to the sub-wavelength confinement and propagation of radiation, allowing the design of very compact structures. Compact structures are necessary to make smaller integrated optical circuits. Due to the use of metals, plasmonic guides usually show more losses compared to the conventional dielectric guides. Therefore, plasmonic waveguides are not normally used for long distance transmission. However, they are promising for inter-chip or intra-chip communication and also have seen a lot of sensor applications.
There has been considerable interest in exploiting the frequency bands in the terahertz regime to open up new frontiers of research across a diverse range of applications. An array of opportunities for creating novel technologies using this frequency band had remained largely unexplored and undeveloped for a considerable period of time due to the lack of suitable sources, as well as lack of guiding and detecting devices.
This thesis describes the design, analysis and optimisation of plasmonic devices in optical and terahertz frequencies. A fully vectorial H-field based finite element method has been used in the research reported in this thesis to reveal the modal characteristics of different plasmonic structures.
A six layer planar contra-directional nano-coupler has been analysed at optical frequency. Three different modes of propagation were considered to study the characteristics of different properties of the structure, including the coupling length. A design approach has been proposed to make the coupler low loss as well as smaller in length.
For the terahertz plasmonics, a rectangular metallic hollow core guide was considered at terahertz frequency. Several modes were considered for the modal analysis of the structure. Modal analysis was performed by changing metal, introducing different dielectric coating in the hollow core, changing the thickness of the metal and dielectric layers and changing the dimensions of the guide. A dispersion analysis was also performed. The criteria for designing very low loss, compact and low dispersion guide have been presented for the structure at the end of the study