Studies of advanced integrated nano-photonic devices in silicon

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 267-285).Electronic-photonic integrated circuits (EPICs) are a promising technology for overcoming bandwidth and power-consumption bottlenecks of traditional integrated circuits. Silicon is a good candidate for building such devices, due to its high-index contrast and low propagation loss at telecom wavelengths. The current thesis presents recent advances in demonstrating discrete components built in silicon-on-insulator (SOI) platforms, around 1550 nm, that can be used as building blocks for future EPIC systems. The first part of this thesis investigates electro-optic modulators based on one-dimensional photonic crystal microcavities, with femtojoule switching energies, as well as on-chip optical interconnects using the super-collimation effect in two-dimensional photonic crystals, both in hole- and rod-based configurations. The second part focuses on microring-based structures, demonstrating wide thermal tunability and hitless operation of single-ring filters, as well as three more advanced categories of devices suitable for wavelength-division multiplexing (WDM) applications. These are twenty-channel second-order tunable filterbanks (both in dual- and counter-propagating configurations), reconfigurable optical add-drop multiplexers (ROADMs) with telecom-grade specifications, and a dynamical slow light cell for delay lines and optical memory elements. All the devices demonstrated in this thesis can be integrated on the same chip. The small device footprints and the use of the SOI platform are ideal for integration with a standard CMOS process, enabling the fabrication of novel electronic-photonic integrated circuits. These new EPIC systems may one day play an important role in the scaling of current computing systems and taking advantage of the WDM capability to increase operational bandwidth, while keeping the power consumption at low levels.by Marcus Dahlem.Ph.D

Novel structures and applications of leaky thin-ridge silicon waveguides

The ability to utilize signals at optical frequencies, as opposed to say microwave frequencies, provides much more bandwidth and signal transmission speed to meet the increasing telecommunication demands in today's world. The ability to integrate optical circuits in the same manner as in electronic integrated circuits means that optical devices can be miniaturized and can even complement today's complex electronic circuits and devices. Silicon nanophotonics is a highly attractive platform for emerging integrated optical solutions in areas including optical signal transmission, signal processing, optical sensing and optical computing. This is primarily because the silicon platform is compatible with CMOS fabrication processes, which through significant investment have developed and matured over many years to serve the electronics industry. Transitioning into an optical platform that can exploit this vast electronics manufacturing industry is viable particularly for enabling low cost mass manufacturing of integrated photonic circuits. High refractive index contrast silicon waveguide platforms such as silicon-on-insulator (SOI) enable strong confinement of light in sub-micron waveguides as well as the sharp bending of waveguides with minimal loss. The SOI platform has therefore attracted research interest into the development of compact integrated silicon photonic circuits. Thin-ridge SOI waveguides are particularly promising because they minimize signal transmission loss by significantly reducing the waveguide etch-depth and therefore reducing scattering losses due to sidewall roughness. However, a consequence of the reduced etch-depth is the possibility for TM guided modes to couple to highly coherent TE radiation in the adjacent slab. This TM-TE coupling phenomenon, named lateral leakage radiation, is the subject of this thesis. The main aim of this thesis is to investigate the possible exploitation of this inherent TE-TM coupling relationship. The novel structures presented herein could have potential applications which include optical biosensing, polarization rotation and resonant optical filtering. The main contributions of this research work include first and foremost the discovery of a resonant TE-TM coupling effect in thin-ridge waveguides. This resonance effect has a canonical Lorentzian response and the quality-factor can be controlled by adjusting the waveguide dimensions. It is also shown that several such resonator waveguides can be cascaded in a coupled resonator topology to realize higher order Chebyshev filter responses. Another contribution in this thesis is that a holographic-based grating structure exploiting the TM-TE coupling in thin-ridge waveguides can be used to efficiently convert a Gaussian TE slab beam into a collimated TM slab beam. It is shown that an apodized grating is the most suitable design for achieving this goal. Lastly, it is also shown through simulation that the lateral leakage effect can be utilized as a biosensor to measure refractive index changes at the surface of a thin-ridge waveguide caused by the deposition of biomolecules. A tapered thin-ridge waveguide in tandem with a planar lens structure is proposed as a potential sensor topology for evanescent field biosensing. In summary, it has been shown that lateral leakage in thin ridge waveguides can be enhanced using unique waveguide structures and exploited for integrated optical applications

Integrated photonic analog-to-digital converters

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 161-172).Accurate conversion of wideband multi-GHz analog signals into the digital domain has long been a target of analog-to-digital converter (ADC) developers, driven by applications in radar systems, software radio, medical imaging, and communication systems. Aperture jitter has been a major bottleneck on the way towards higher speeds and better accuracy. Photonic ADCs, which perform sampling using ultra-stable optical pulse trains generated by mode-locked lasers, have been investigated as a promising approach to overcome the jitter problem and bring ADC performance to new levels. This work demonstrates that the photonic approach can deliver on its promise by digitizing a 41 GHz signal with 7.0 effective bits and 52 dBc spur-free dynamic range (SFDR) using a discrete-component photonic ADC. This corresponds to 15 fs jitter, a 4-5 times improvement over the jitter of the best electronic ADCs, and an order of magnitude improvement over the jitter of electronic ADCs operating above 10 GHz. The feasibility of a practical photonic ADC is demonstrated by creating an integrated ADC with a modulator, filters, and photodetectors fabricated on a single silicon chip and using it to sample a 10 GHz signal with 3.5 effective bits and 39 dBc SFDR. In both experiments, a sample rate of 2.1 GSa/s was obtained by interleaving two 1.05 GSa/s channels; higher sample rates can be achieved by increasing the channel count. A key component of a multi-channel ADC - a dual multi-channel high-performance filter bank - is successfully implemented. A concept for broadband linearization of the silicon modulator, which is another critical component of the photonic ADC, is proposed. Nonlinear phenomena in silicon microring filters and their impact on ADC performance are analyzed, and methods to reduce this impact are proposed. The results presented in the thesis suggest that a practical integrated photonic ADC, which successfully overcomes the electronic jitter bottleneck, is possible today.by Anatol Khilo.Ph.D

Integrated Optical Filters for Microwave Photonic Applications

[EN] Microwave photonics (MWP) is a well-established research field that investigates the use of photonic technologies to generate, distribute, process and analyze RF waveforms in the optical domain. Despite its great potential to solve long-standing problems faced by both the microwave and electronics industries, MWP systems are bulky, expensive and consume a lot of power. Integrated microwave photonics (IMWP) is an emerging area of research that promises to alleviate most of these drawbacks through the use of photonic integrated circuits (PIC). In this work, we have aimed at further closing the gap between the worlds of MWP and integrated optics. In particular, we have focused on the design and experimental characterization of PICs with reconfigurable, ring-assisted Mach-Zehnder interferometer filters (RAMZI), and demonstrated its potential use in different IMWP applications. These filters consist of a symmetric MZI loaded with ring resonators, which are coupled to the MZI branches by different optical couplers. The contributions of this thesis can be split into two sections. In the first one, we demonstrate integrated optical couplers and reflectors with variable power splitting and reflections ratios. These exploit the well-known properties of tapered multimode interference couplers (MMI), and their inherent robustness makes them highly suitable for the implementation of both RAMZI and reflective filters. Besides, we study in detail the impact of manufacturing deviations in the performance of a 4x4 MMI-based 90º hybrid, which is a fundamental building block in coherent optical communication systems. In the second section, we demonstrate the use of integrated RAMZI filters for three different IMWP applications, including instantaneous frequency measurement (IFM), direct detection of frequency-modulated signals in a MWP link, as well as in tunable, coherent MWP filters. A theoretical analysis of the limits and trade-offs that exist in photonics-based IFM systems is also provided. Even though these are early proof-of-concept experiments, we hope that further technological developments in the field will finally turn MWP into a commercial reality.[ES] La fotónica de microondas (MWP) es un campo de investigación que estudia el uso de tecnologías ópticas para generar, distribuir, procesar y analizar señales de RF. A pesar de su gran potencial para resolver algunos de los problemas a los que se enfrentan las industrias electrónica y de microondas, estos sistemas son voluminosos, caros y consumen mucha potencia. La fotónica de microondas integrada (IMWP) es un área emergente que promete solucionar todos estos inconvenientes a través de la utilización de circuitos ópticos integrados (PIC). En esta tesis, hemos pretendido avanzar un poco más en el acercamiento entre estas dos disciplinas. En concreto, nos hemos centrado en el diseño y caracterización experimental de PICs con filtros reconfigurables basados en interferómetros Mach-Zehnder cargados con anillos (RAMZI), y demostrado su potencial uso en diferentes aplicaciones de IMWP. Los filtros RAMZI están hecho básicamente de un MZI simétrico cargado con anillos, los cuales a su vez se acoplan a las ramas del interferómetro a través de distintos acopladores ópticos. Las contribuciones de este trabajo se pueden dividir en dos partes. En la primera, hemos demostrado acopladores y reflectores ópticos integrados con coeficientes de acoplo y reflexión variables. Éstos explotan las propiedades de los acopladores por interferencia multimodal (MMI), y su robustez les hace muy atractivos para la implementación de filtros RAMZI y de tipo reflectivo. Además, hemos analizado el impacto que las tolerancias de fabricación tienen en el rendimiento de un híbrido óptico de 90º basado en un MMI 4x4, el cual es un elemento fundamental en los sistemas de comunicaciones ópticas coherentes. En la segunda parte, hemos demostrado el uso de filtros RAMZI en tres aplicaciones distintas de IMWP. En concreto, hemos utilizado dichos filtros para implementar sistemas de medida de frecuencia instantánea (IFM), detección directa de señales moduladas en frecuencia para enlaces fotónicos, así como en filtros coherentes y sintonizables de MWP. También hemos desarrollado un análisis teórico de las limitaciones y problemas que existen en los sistemas IFM. A pesar de que todos los experimentos realizados han consistido en prototipos para una prueba de concepto, esperamos que futuros avances tecnológicos permitan que la fotónica de microondas se convierta algún día en una realidad comercial.[CA] La fotònica de microones (MWP) és un camp d'investigació que estudia l'ús de tecnologies òptiques per a generar, distribuir, processar y analitzar senyals de radiofreqüència. A pesar del seu gran potencial per a resoldre alguns dels problemes als que s'enfronten les indústries electrònica i de microones, estos sistemes son voluminosos, cars i consumixen molta potència. La fotònica de microones integrada (IMWP) és un àrea emergent que promet solucionar tots estos inconvenients a través de la utilització de circuits òptics integrats (PIC). En esta tesi, hem pretés avançar un poc més en l'acostament entre estes dos disciplines. En concret, ens hem centrat en el disseny i caracterització experimental de PICs amb filtres reconfigurables basats en interferòmetres Mach-Zehnder carregats amb anells (RAMZI), i demostrat el seu potencial en diferents aplicacions d' IMWP. Els filtres RAMZI estan fets bàsicament d'un MZI simètric carregat amb anells, els quals, al seu torn, s'acoblen a les branques del interferòmetre a través de distints acobladors òptics. Les contribucions d'este treball es poden dividir en dos parts. En la primera, hem demostrat acobladors i reflectors òptics integrats amb coeficients de transmissió i reflexió variables. Estos exploten les propietats dels acobladors per interferència multimodal (MMI), i la seua robustesa els fa molt atractius per a la implementació de filtres RAMZI i de tipo reflectiu. A més a més, hem analitzat l'impacte que les toleràncies de fabricació tenen en el rendiment d'un híbrid òptic de 90 graus basat en un MMI 4x4, el qual és un element fonamental en els sistemes de comunicacions òptiques coherents. En la segona part, hem demostrat l'ús de filtres RAMZI en tres aplicacions diferents de IMWP. En concret, hem utilitzat estos filtres per a implementar sistemes de mesura de freqüència instantània (IFM), detecció directa de senyals modulades en freqüència per a enllaços fotònics, així com en filtres coherents i sintonitzables de MWP. També hem desenvolupat una anàlisi teòrica de les limitacions i problemes que existixen en els sistemes IFM. A pesar de que tots els experiments realitzats han consistit en prototips per a una prova de concepte, esperem que futurs avanços tecnològics permeten que la fotònica de microones es convertisca algun dia en una realitat comercial.Sánchez Fandiño, JA. (2016). Integrated Optical Filters for Microwave Photonic Applications [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/67690TESI

Non-Hermitian effects in exciton-polariton systems

In this thesis we study non-Hermitian aspects of exciton-polariton Bose-Einstein condensates. Exciton-polaritons are hybrid matter-light quasiparticles created when microcavity photons are strongly coupled to quantum well excitons, which are bound electron-hole pairs. Being composite bosons with a very low effective mass, exciton-polaritons can undergo Bose-Einstein condensation at relatively high temperatures - from cryogenic to as high as room temperature in some semiconductors. Exciton-polariton condensates are an experimentally attractive system due to the high transition temperature and ease of in-situ diagnostics. They are also fundamentally non-Hermitian because they exist in a balanced landscape of loss and gain, where excitation by a pump laser counteracts the radiative decay of polaritons. Because of their hybrid light-matter nature exciton-polariton condensates are also an ideal platform for designing new optoelectronic devices, and non-Hermitian effects may be useful to this end. Hermiticity is posited as an axiom of quantum mechanics in order to ensure that energies are real. However in recent decades it has been shown that a class of non-Hermitian Hamiltonians which adhere to a weaker condition of symmetry under simultaneous spatial and time reversal (PT symmetry) can still have real energies. Many of the essential features of Hermitian quantum mechanics can be reproduced with such Hamiltonians. In general PT symmetric systems exhibit two phases, one in which eigenvalues are real, and another in which the eigenvectors spontaneously break the PT symmetry and eigenvalues are complex. The transition occurs at an exceptional point, a non-Hermitian degeneracy where eigenstates coincide as well as eigenvalues. EPs can also be observed in non-Hermitian systems lacking PT symmetry. This has led to a collection of interesting experiments in optical and other systems that provide analogues of non-Hermitian quantum mechanics because loss and gain are represented by an imaginary potential. In these systems PT symmetry breaking has allowed for enhanced sensing, loss-induced transparency, gain-induced suppression of lasing, and sensitive switching. Exciton-polaritons condensates are inherently non-Hermitian as they experience loss and gain. However this aspect has been largely overlooked, apart from a few experiments which demonstrate EPs. Experiments in optical and other systems suggest that non-Hermitian effects in polaritons may be harnessed to design optoelectronic devices. In addition, the demonstration of PT symmetry breaking in yet another system is of inherent intellectual interest. We aim to provide theoretical guidance for current and future experiments that exploit the non-Hermiticity of polariton condensates. One chapter focuses on a very simple PT symmetric system - a PT symmetric square well for polaritons. Ths system is simple enough to be analytically tractable, but also exhibits interesting and subtle behaviour. We show how a nearly-PT symmetric square well can be implemented for polaritons using established trapping techniques. We further show that unavoidable PT asymmetry removes the PT symmetry breaking transition, but that most of this behaviour can easily be restored. In support of recent experiments, another part of the work focuses on whispering gallery modes (WGMs) of polariton condensates in a shallow circular trap. We show that an interesting experimental effect - a robust blueshift of half a free spectral range under certain pumping conditions - can be attributed to coupling with a non-Hermitian resonator. We also discuss the viability of various schemes for reaching EPs of polariton WGMs, and present preliminary numerical results which show that some of these schemes are viable. The research presented in this thesis provides a road map for future experimental and theoretical work that will harness non-Hermitian effects beyond the observation of EPs in polariton condensates

Integration of Arsenic Trisulfide and Titanium Diffused Lithium Niobate Waveguides

A chalcogenide glass (arsenic-trisulfide, As2S3) optical waveguide is vertically integrated onto titanium-diffused lithium-niobate (Ti:LiNbO3) waveguides to add optical feedback paths and to create more compact optical circuits. Lithium-niobate waveguides are commonly used as building blocks for phase and amplitude modulators in high speed fiber communication networks due to its high electrooptic coefficient and low mode coupling loss to single-mode optical fibers. Although it can easily be modulated using an RF signal to create optical modulators, it lacks the intrinsic trait to create optical feedback loops due to its low core-to-cladding index contrast. Ring resonators are main building blocks of many chip-scale optical filters that require these feedback loops and are already demonstrated with other material systems. We have, for the first time, incorporated As2S3 as a guiding material on Ti:LiNbO3 and fabricated s-bends and ring resonators. We have examined As2S3-on-Ti:LiNbO3 waveguides at simulation, microfabrication, and optical characterization levels