Integrated optical devices based on liquid crystals embedded in polydimethylsiloxane flexible substrates

The contribution of this thesis is to find possible solutions for the creation of interconnections and optical switches to be used in microoptofluidic systems in the frame of the research activities of the Optoelectronic laboratory of the Department of Information Engineering, Electronics and Telecommunications (DIET). The main goal is to explore a new technology for integrated optic based on a low cost technology to produce low driving power devices. Optofluidics is the science which links the field of photonics with microfluidics, for the creation of innovative and state-of-the-art devices. Liquid crystals (LC) can be used for optofluidic applications because they have the possibility to change without external mechanical actions, the average direction of the molecules through the application of electric fields, reorienting the crystal molecules in such a way as to alter their optical properties [1-2]. The research on LC is more than a century old, but only since the ‘80s of the past century these materials were employed in various fields, from flat panel displays used for televisions, tablets, and smartphones, to biomedical and telecommunication applications [3-5]. The results reported in this thesis include simulation, design and preliminary fabrication of optofluidic prototypes based on LC embedded in polydimethylsiloxane (PDMS) channels, defined as LC:PDMS, with co-planar electrodes to control LC molecular orientation and light propagation. Fabrication techniques which were used include microelectronic processes such as lithography, sputtering, evaporation, and electroplating. The simulations were performed through the combined use of COMSOL Multiphysics® and BeamPROP®. I used COMSOL Multiphysics® to determine the positioning of the molecules in a LC:PDMS waveguide. The LC are the core through which light propagates in a PDMS structure. In addition to these simulations, I used COMSOL Multiphysics® to determine the orientation of the LC under the effect of an electric field [6-7] to create low-power optofluidic devices [8], [11]. I used BeamPROP® to explore the optical propagation of various optical devices such as: optical couplers, the zero gap optical coupler, and a multimodal interferometer. All these devices have been simulated through various combinations of geometries which will be extensively explained in the following chapters. The fabrication of prototypes was made in the Microelectronic Technologies laboratory of DIET. The optofluidic prototypes that I designed could be used in interconnection systems on biosensing devices for chemical or biological applications [10-11], wearable [12], or lab on chips [13], which are increasingly being applied in many research fields [14]. Many of these devices need to interface with electronics for processing signals coming from the interaction between the device with molecules, liquids or other biological substances. Moreover it is necessary to create flexible and biocompatible interfaces, whose features are not guaranteed in classic metal tracks. As it will be clear in the first chapter, metal interconnections must be designed with spatial, energy and throughput restrictions. To develop the optofluidic prototypes, I chose to use a combination of two materials for their commercial availability and ease of use: E7 and 5CB LC produced by Merck® as the transmissive medium and PDMS Sylgard 184 produced by Dow Corning® for the cladding [15-16]. The molecules of the LC are anisotropic, whose shape is elongated like that of a cigar. Under appropriate temperature conditions these molecules retain a state of aggregation in which, while retaining some mechanical properties of the fluids, they have the characteristics of crystals such as birefringence or x-ray reflection. These properties are due to two factors that characterize the various phases of LC: the orientational and positional order that vary according to the temperature. E7 was used in its nematic mesophase. The material used for the cladding of my prototypes was PDMS, a thermosetting polymer, flexible, biocompatible, economical, easy to work, and suitable for the creation of optical and optofluidic devices due to its transparency. The thesis is organized in six chapters whose contents are briefly outlined below: • In the first chapter there is a brief description of optofluidics and the transport phenomena of the liquids in the microchannels. The essential parameters for a correct interpretation of the behavior of the materials in the devices will be defined. Some examples of microfluidic devices, Optofluidic Optical Components (OOC) will be mentioned. • In the second chapter, LC’s will be presented, along with their general characteristics and their behavior in the presence of electric fields. An overview of integrated optic devices based on LC will be reported. • In the third chapter the experimental results will be presented concerning the fabrications and the technologies used to obtain electro-optical LC:PDMS waveguides. • The fourth chapter will be dedicated to a brief description of COMSOL Multiphysics® and BeamPROP® simulators, and the implementation of the model of LC channels in PDMS both in 2D and 3D. Also a brief description of Monte Carlo simulations based on Lebwohl-Lasher potential will be mentioned. • In the fifth chapter an LC:PDMS optical directional coupler and the most significant results will be described. • The sixth chapter is dedicated to the multimodal interferometer and its field of application, the theory behind this device and the results obtained from the simulations using the BeamPROP® • In the conclusion, a brief recap of the results obtained in this thesis and future developments will be presented

Performance analysis of a noncontact plastic fiber optical fiber displacement sensor with compensation of target reflectivity

peer-reviewedAn inexpensive fiber-based noncontact distance sensor specific for monitoring short-range displacements in micromachining applications is presented. To keep the overall costs low, the sensor uses plastic optical fibers and an intensiometric approach based on the received light intensity after the reflection from the target whose displacement has to be measured. A suitable target reflectivity compensation technique is implemented to mitigate the effects due to target surface nonuniformity or ageing.The performances of the sensor are first evaluated for different fiber configurations and target reflectivity profiles and positions using a numerical method based on Monte Carlo simulations. Then, experimental validations on a configuration designed to work up to 1.5mm have been conducted. The results have confirmed the validity of the proposed sensor architecture, which demonstrated excellent compensation capabilities, with errors below 0.04mm in the (0-1)mm range regardless the color and misalignment of the target

On the performance of M-QAM optical signals in ROADM based optical networks

The optical network physical layer limitations, in particular, the crosstalk due to imperfect isolation of optical network switching components, such as Reconfigurable Optical Add/Drop Multiplexers (ROADMs), can become a limiting factor in the performance of these networks. In this work, the impact of in-band crosstalk due to M-ary quadrature amplitude modulation (M-QAM) interferers on the performance of 4-QAM and 16-QAM coherent receivers in ROADM based networks is analysed, using Monte Carlo (MC) simulation. Several Wavelength Select Switch (WSS) models, a ROADM component, were studied, and each WSS model had a different optical filter profile. Two crosstalk metrics, unweighted crosstalk and weighted crosstalk, are used to measure the system performance degradation regarding the Optical Signal-to-Noise Ratio (OSNR) penalty at the coherent receiver. The difference between the two, is the fact that weighted crosstalk takes into account that the spectrum content closer to the center of the channel bandwidth has more impact than spectral content closer to the edges of the channel bandwidth. Using unweighted crosstalk metric, it can be concluded that optical filters with a wider rejection bandwidth in the center of the channel, have a better performance in terms of OSNR penalty at the coherent receiver. With the weighted crosstalk metric, it was observed that regardless of the WSS filter profile, the OSNR penalty performances of each WSS were similar. It can be concluded that the weighted crosstalk metric is very reliable at predicting system performances independently of the filter shape present in the WSS.As limitações do nivel físico das redes ópticas, nomeadamente a diafonia (crosstalk) devido ao isolamento imperfeito de elementos de redes de fibra óptica, como os Multiplexadores Ópticos de Adição/Extracção Reconfiguráveis (ROADMs), podem constituir um factor limitativo no desempenho das redes ópticas. Nesta dissertação, foi estudado através de simulação de Monte-Carlo (MC), o impacto do crosstalk homódino devido a sinais interferentes com o formato de modulação em amplitude e em quadratura (M-QAM), no desempenho dos receptores coerentes de sinais 4-QAM e 16-QAM. Foram também estudados vários Selectores de Comprimento de Onda (WSS), um componente dos ROADMs, e em que cada WSS tem um filtro óptico com um perfil diferente. Duas métricas de crosstalk, unweighted crosstalk e weighted crosstalk, foram usadas para estudar a degradação do desempenho do sistema de comunicações ópticas referente à relação de sinal-ruído óptica (OSNR), no receptor coerente. A diferença entre estas duas métricas, é o facto de o weighted crosstalk levar em consideração que o conteúdo espectral perto do centro da largura de banda do canal, tem um impacto maior do que o conteúdo espectral perto das margens da largura de banda do canal. Analisando os resultados com a métrica unweighted crosstalk, foi possível concluir que WSSs com filtros ópticos com a banda de rejeição maior e mais centrada com a largura de banda do canal, obtiveram melhor desempenho em relação à penalidade de OSNR medida no receptor coerente. Usando o weighted crosstalk como métrica, foi observado que todos os WSSs registaram desempenhos semelhantes em termos de penalidade de OSNR. Podemos concluir, que o weighted crosstalk constitui uma métrica bastante fiável a prever desempenhos de sistemas de comunicações ópticos, independentemente do perfil dos filtros usados em cada WSS

Photonic integrated circuit design in a foundry+fabless ecosystem

A foundry-based photonic ecosystem is expected to become necessary with increasing demand and adoption of photonics for commercial products. To make foundry-enabled photonics a real success, the photonic circuit design flow should adopt known concepts from analog and mixed signal electronics. Based on the similarities and differences between the existing photonic and the standardized electronics design flow, we project the needs and evolution of the photonic design flow, such as schematic driven design, accurate behavioral models, and yield prediction in the presence of fabrication variability

Multiphysics modelling of high-speed optoelectronic devices for silicon photonics platforms

L'abstract è presente nell'allegato / the abstract is in the attachmen

Investigating the effect detector geometry has on heterodyne to non-heterodyne signal ratio (HNHR) in a low-coherence tissue imaging interferometer

A Monte Carlo simulation is used to simulate the emergent light distribution from a turbid media sample placed in the probe beam arm of a scanning low-coherence interferometer that is used to build-up voxelated images in three-dimensions. The sample is a simple three-layered model with an embedded cylinder which simulates the physical and optical properties of skin for near-infrared light passing through the sample. Coherent light from an input probe beam of variable profile and incidence angle is traced through the sample by means of ray-tracing, and any emergent beams are collected in a sample surface array which also logs any emergent Monte Carlo generated photons reaching the surface. At the surface, the heterodyne to nonheterodyne signal ratio (HNHR) measured by a user-defined variable geometry detector is calculated for each point upon a linear scan of the detector position across the sample surface. A coherence gate is set to image a voxel located at the uppermost point of the blood vessel-simulating cylinder. Monte Carlo simulations are performed for various input probe beam angles and probe beam profiles, and the HNHR analysed for various detector geometries by varying: the detector area, central detector axis angle and the acceptance angle at the detector. The Monte Carlo results confirm the benefit of confocal detection, and indicate that angular decoupling of the light delivery and detection systems can improve differential HNHR measurements at scan extremities by 25 to 30 dB provided that stratum comeum is either removed or the sample index matched to the imaging system. Also presented is the description of a prototype experimental low-coherence interferometer system and some of the results derived from it: at its best this system was able to measure reflected coherent signals with a dynamic range approximating 130 dB

Optical Communication Systems

A receiver, transmitter, and photon counting detector for use in an optical communication link are disclosed. Also disclosed are methods of communicating using the transmitter, the receiver, and the photon detector

Impact of in-band crosstalk in an optical network based on multi-degree CDC ROADMs

he most common optical networks nodes are known as reconfigurable optical add/drop multiplexers (ROADMs). The architecture and components of these nodes have evolved over the time to become more flexible and dynamic. Particularly, the wavelength add/drop structures of these nodes have become more complex and with new features such as colorless, directionless and contentionless (CDC). One of the main limitations of the optical networks physical layer, the in-band crosstalk, is mainly due to the imperfect isolation of the components inside these nodes. This crosstalk is enhanced, when an optical signal traverses a cascade of ROADM nodes. In this work, the impact of in-band crosstalk, optical filtering and amplified spontaneous emission (ASE) noise on the performance of an optical communication network based on a cascade of CDC ROADMs with coherent detection and the modulation format quadrature phase-shift keying with polarization-division multiplexing (PDM-QPSK) at 100-Gb/s is studied through Monte-Carlo simulation. Two architectures, broadcast and select (B&S) and route and select (R&S), and two possible implementations for the add/drop structures, the multicast switches (MCSs) and the wavelength selective switches (WSSs), were considered. The degradation of the optical communication network performance due to in-band crosstalk is assessed through the optical-signal-to-noise ratio (OSNR) calculation. In particular, an OSNR penalty of 1 dB due to in-band crosstalk is observed when the signal passes through a cascade of 19 CDC ROADMs with 16-degree, based on a R&S architecture and with add/drop structures implemented with WSSsOs nós das redes de comunicação ótica mais comuns são os multiplexadores óticos de inserção/extração reconfiguráveis (ROADMs – acrónimo anglo-saxónico de reconfigurable optical add/drop multiplexers). A arquitetura e componentes destes nós têm evoluído ao longo do tempo no sentido de se tornarem mais flexíveis e dinâmicos. Em particular, as estruturas de adição/extração destes nós, tornaram-se mais complexas e detêm novas características que oferecem as funcionalidades CDC (acrónimo anglo- -saxónico de colorless, directionless e contentionless). Uma das principais limitações do nível físico das redes óticas, o crosstalk homódino, deve-se principalmente ao isolamento imperfeito dos componentes presentes dentro destes nós. Este tipo de crosstalk tem um impacto ainda mais significativo quando o sinal ótico atravessa uma cadeia de nós baseados em ROADMs. Nesta dissertação, o impacto do crosstalk homódino, filtragem ótica e ruído ASE (acrónimo anglo-saxónico de amplified spontaneous emission) no desempenho de uma rede de comunicação ótica baseada numa cadeia de CDC ROADMs com deteção coerente e usando o formato de modulação PDM-QPSK (acrónimo anglo-saxónico de polarization-division multiplexing quadrature phase-shift keying) a um ritmo binário de 100-Gb/s é investigado através de simulação Monte-Carlo. Consideraram-se duas arquiteturas, B&S e R&S (acrónimos anglo-saxónicos para broadcast and select e route and select), e duas possíveis implementações para a estruturas de inserção/extração, os MCSs e os WSSs (acrónimos anglo-saxónicos de multicast switches e wavelengh selective switches). A degradação do desempenho da rede ótica devido ao crosstalk homódino foi obtida através do cálculo da relação sinal-ruído ótica. Em particular, obteve-se uma penalidade de 1 dB para esta relação devido ao crosstalk homódino quando o sinal percorre uma cadeia de 19 CDC ROADMs com grau 16, uma arquitetura R&S e estruturas de inserção/extração baseadas em WSSs